A widespread sea stratus event evolved across the Gulf of Alaska and into adjacent inner channels from 3/15 – 3/16 as broad high pressure established itself over the region above favorable low level moisture. Forecasters at the NWS WFO Juneau office noted their use of GOES and VIIRS imagery together to aid in tracking the evolution of low clouds during this event, along with an associated drizzle threat at the surface beneath the stratus.
GOES-West full disk water vapor imagery revealed an omega block setup over the Gulf of Alaska, with low pressure on either side of the Gulf of Alaska high pressure (Fig 1).
Both GOES-17 imagery and VIIRS imagery were used by forecasters in decisions of whether or not to include lower CIGs/VIS conditions in the 18Z TAFs. These decisions impacted local pilots whose ability to fly depended on the extent of the lower cloud bases. Forecasters also used GOES and VIIRS imagery in combination with other datasets to provide DSS to core partners regarding low cloud evolution. For example, Forest Service called the office inquiring about if and when the low clouds were going to lift in a certain area as they needed to take a helicopter to a mountain top to service infrastructure. Forecasters were able to give them some guidance on if it would lift and what the ceiling could be if it did by using a combination of area cameras, recent trends in satellite data, and model data.
Analysis of GOES-West full disk Nighttime Microphysics RGB imagery at night transitioning to Day Cloud Phase distinction RGB imagery during the day on the 16th reveals the wide swath of low cloud cover over the Gulf, and expansion of clouds east into the inner channels (Fig 2). The IR components to the RGBs were modified slightly to account for the cooler airmass (lower the warm end by 10-20 C). At nadir, the ABI bands in the nighttime RGB have 2 km spatial resolution, while the Day RGB components have 0.5, 1, and 2 km resolutions. However, at the latitude of the Gulf of Alaska, pixel size is approximately 3-4x larger.
Overnight 375 m I band VIIRS fog difference (11.4 um minus 3.7 um) imagery provides a much higher resolution (spatially) of the low clouds, with three subsequent passes showing expansion of the low clouds east into the inner channels (Fig 3). Cloud edges and smaller scale cloud features are more easily diagnosed in the more detailed VIIRS imagery compared to GOES. During this 1.5 hour period of time, low stratus spread around PAGS and into PASI and PAGN weather observation sites. Recall the VIIRS I bands (0.64 um, 0.86 um, 1.6 um, 3.7 um, 11.4 um) and associated multispectral products provide the highest resolution (375 m), while the M bands and associated products provide a lower 750 m.
Day cloud Phase Distinction RGB imagery from VIIRS provides a similar higher resolution look at the extent of the low clouds during the day (Fig 4). Localized low cloud cover is diagnosed spreading south over PAPG during this 1.5 hour time frame. This RGB utilizes three I bands, so provides 375 m resolution.
Forecasters specifically noted the value of the periodic high resolution and low parallax VIIRS imagery for this type of event in order to get a better representation of cloud type. In AWIPS, they will view the GOES imagery with VIIRS overlaid, taking advantage of the strengths of both data sources.
Bill Line (NESDIS and CIRA) and Aaron Jacobs (NWS WFO Juneau)
A Kona Low established itself west of Kauai on 16 Mar 2020, driving anomalously high levels of tropical moisture (TPW of 1.5″ to 2.0″) into the region. GOES-West full disk water vapor imagery showed the tightly wrapped low set up west of the Hawaiian Islands and only slowly moving east from late on the 15th through the 16th (Fig 1).
The Advected Layer PW product combines temperature and moisture information from multiple polar-orbiting satellites to provide a 4D structure of moisture in the atmosphere. In this case, the blended product shows deep moisture from the tropics wrapping around the low and over Hawaii in all layers (Fig 2).
The increased moisture and forcing associated with the low resulted in the development of persistent showers thunderstorms over/near the islands during the previous evening through the day. These storms produced heavy rain and gusty winds, leading to the issuance of a Flash Flood Watch for the state, a Flash Flood Warning for the island of Kauai, and multiple Special Marine Warnings for gusty winds.
The development and evolution of deep convection near the islands around the sunrise period is shown in an IR to VIS/IR Sandwich transition loop (Fig 3). Prior to sunrise, the animation shows IR alone, while after sunrise, the animation includes the high texture of the VIS in combination with the IR. The most impressive convection is diagnosed developing near and northeast of Kauai.
Visible imagery combined with semi-transparent GLM after sunrise reveals periodic lightning flashes associated with the convection, but with relatively low density (Fig 4). Surface obs indicated measured peak wind gusts of 38 knots associated with these thunderstorms. Hawaii is in the southwest corner of the GOES-West PACUS sector, meaning 5-min imagery is always available over the islands.
Substituting visible imagery for the Day Cloud Phase Distinction RGB provides more insight into cloud makeup with this event (Fig 5). It provides a contrast between low liquid clouds (cyan) and high ice clouds (red and yellow), with convective cores (textured red/yellow) still apparent due to the contribution of texture from the 500 m VIS.
The Kona Low remained in place west of Hawaii on the 17th, continuing to drive moisture northward and resulting in persistant thunderstorm activity over and near the state. Given the continued thunderstorm flash flood threat, WFO Honolulu requested and was granted a long-duration (36 hours) GOES-West meso sector (2) to provide 1-min satellite imagery over the region. Ninety-minutes of 1-minute visible imagery from the morning of the 17th, with semi-transparent GLM FED overlay, shows the most robust thunderstorm activity developing south of the islands (Fig 6). The very high temporal resolution imagery with very low latency provides forecasters a valuable tool for diagnosing newly developing updrafts and tracking their evolution, particularly over the ocean far from radar coverage.
The Kona Low stuck around west of Hawaii through Thursday night, when it finally lifted to the northeast and exited the region as a broad upper trough/closed low approached from the west. Hourly GOES-West Water Vapor imagery from Sunday morning thorough Friday morning shows the evolution of the upper low and associated lightning activity (GLM FED) through the week (Fig 7). The continued flux of tropical moisture and development of convection near/over Hawaii is apparent in the imagery. As the late week trough approaches from the west, the Kona low lifts northeast within the increasing southwesterly flow.
A potent upper level low pressure system traversed across northern Africa on 12 March 2020, causing the development of strong winds at the surface. Water Vapor imagery from EUMETSAT Meteosat-8 shows the compact low advancing east across Egypt on the 12th, with gravity waves emanating southward across bordering countries (Fig 1).
The windy conditions at the surface resulted in a broad area of blowing dust across much of the northern half of the continent. The EUMETSAT Meteosat-8 satellite captured the lofted dust as it was carried south and west away from the low and over a long distance. The thick layer of lofted dust was captured well in both the 0.6 um VIS (Fig 2) and 10.8 um IR Window (Fig 3) single-band imagery.
Combining multiple bands to make multispectral imagery allowed for similar detection of dust across Africa. The dust was diagnosed in the split window difference and IR Window image combo, Geocolor, and Dust RGB imagery.
A closer look at region of optically thick blowing dust across Sudan is provided in Figure 7.
In this FDTD GOES Applications Webinar, the Huntsville NWS office discussed the history of total lightning data usage at the HUN NWS Office, and described the usage of GLM data during a couple of recent severe weather cases. On December 16, 2019, the GLM data were used to diagnose severe convection in a moderately unstable environment. On January 11, 2020, GLM data factored heavily into warning decisions for severe convection, including tornado warnings, when their primary radar (KHTX) failed.
Below is an example of how GLM data were used in messaging during one of the events:
“Once the storms entered Marshall county, we really started to see an uptick in lightning activity, the strongest that we had seen that day. So we got on the radio and social media, and said, ‘we are really confident this storm is rapidly intensifying, something can happen here, … people in the path really need to be taking shelter right now.'”
FDTD Satellite Applications Webinars are peer-to-peer learning; staff from WFOs, National Centers, CWSUs, RFCs lead the presentations. The presentations are short (less than 30 minutes) and recorded for on-demand viewing.
March 2020 started off rather windy and unstable in the Northern Plains leading to convection during a month that still sees predominately wintertime stratiform precipitation. There were several instances of showers producing lightning, graupel/snow, and convectively forced severe wind gusts in addition to synoptically driven high winds. Designating convective cells that may be more impactful than others in the cool season (late fall, winter, and early spring) can be difficult using remote sensing instruments like satellite and radar due to their shallow heights and relatively warm cloud tops. Satellite appearance of shallow convection tends to be unimpressive in the infrared using default color schemes made for taller, colder cloud topped warm season convection. Also, due to the shallow depth of the cloud layer, radar can easily overshoot this layer composed of what tends to be marginal returns. Lastly, lightning activity tends to be much lower than that in the warm season, if present at all.
But there are some ways to make subtle features of shallow convection more easily tracked by adjusting the satellite display. Let’s explore..
First let’s take a look at an event on March 4, 2020, that produced several convectively induced high wind gusts at times exceeding 75 mph. The Day Snow-Fog RGB has proven useful in monitoring wintertime precipitation, mainly because of its use of near-infrared bands to help delineate snow cover, bare ground, and cloud top microphysical composition. It is also well known that using RGB’s capable of tracking cloud features while color sorting aforementioned properties (looking at you Day Cloud Phase Distinction RGB) can be quite useful for tracking forcing mechanisms, areas of potential convective initiation, and stage in convective life cycle.
Figure 1 shows convective clouds moving through eastern Montana into the Dakotas and northern Wyoming. Convection is already ongoing in the start of the loop with a line of showers attached to a cold front extending from near Great Falls to Glasgow, Montana. The cold front can be tracked on satellite propagating south and east undercutting additional convection in the “warm sector” ahead of the front. Ahead of the front are additional clusters of growing cumulus clouds as the convective temperature was reached, aided by ample insolation over a snow free surface beneath a relatively cool column of air (focus of cloud growth probably had some influence from differential heating and convergence augmented by variations in terrain). These cumulus clouds initially exhibit a highly textured, bubble-like appearance with whiter coloring indicating non-glaciated clouds. They very quickly become bright pink while maintaining their texture signaling glaciation of clouds through ascent. Finally they turn from cumulonimbus to stratocirrus losing texture, acquire more purple coloring, and move west to east in broader upper flow as parcels detrain from their initial paths of low level ascent, thus indicating the majority of additional ascent is likely over. The ability to track these mesoscale forcing mechanisms and stages in storm growth can be important to forecasters as this may give lead time to potential initiation or advection of impactful convection, as well as convective decay.
Now let’s focus on the tristate region of the Dakotas and Montana, an area where several severe wind gusts were measured. Using Rapid City’s observed sounding as representative of the environment, this area was primed to transfer not only cloud layer winds of around 50 kt towards the surface, but any evaporation of descending precipitation cores may help accelerate these winds from aloft towards the surface (DCAPE ~700 J/kg, inverted V sounding).
We can utilize procedures meant for tracking severe convection in the warm season while making some minor adjustments to help gear towards a cool season environment. Figure 2 is an example of one such procedure. The range of the Day Cloud Phase Distinction RGB’s red component (10.3 um, Band 13, “Clean IR”) was edited from [Min: 7.5, Max: -53.5] to [Min: -10, Max -70] in an effort to desaturate red coloring by shifting this thermal sensing band towards the colder, better representing the temperatures typical in the cool season. Convective evolution is represented by a transition of colors from cyan to green to yellow/red as clouds glaciate and then cool. The inclusion of the visible spectrum within the Day Cloud Phase Distinction RGB also gave clues on storm development featuring more turbulent cloud texture near the upshear side of continually developing anvils, indicative of sustained updrafts reaching their equilibrium level. A separate, standalone look at Band 13 offered a way to track coldest cloud tops, although adjustments were also made here shortening the range from [Min: 55.0, Max: -109.0] to [Min: 0, Max -70] while making all values warmer than -25 C transparent. This allowed robust convection to be more easily tracked in an area of poor radar coverage with more red colored tops corresponding to relatively taller, stronger updrafts. One particular cluster of showers moved through Glendive, MT, with its AWOS measuring a 47 kt (54 mph) wind gust 18:56 UTC followed by Dickinson, ND, reporting 66 kt (76 mph) wind gust 19:56 UTC. This cluster exhibited relatively rapid cloud top cooling around 40 minutes prior to reaching Glendive and another prior to reaching Dickinson. Lastly, lightning activity can be analogous to updraft strength. Adjustments to the GLM FED appearance were made to shorten the default range of 1-260 to 1-20 helping draw out meager lightning rates. While overall lightning activity was low (as can be typical in cool season convection in the Northern Plains), this adjustment did help bring attention to cells that had relatively high activity compared to neighboring cells. One cell that stands out emanates from the same cluster that moved through Glendive and Dickinson but on its southern flank. Looking at MRMS isothermal reflectivity at -20C, the cell holding highest lightning activity has reflectivity exceeding 40 dBZ. All of these signals, upstream reported severe wind gusts, and our knowledge of the thermodynamic and kinematic environment can steer the forecaster into thinking this particular cell is strong and chances of producing severe downdraft wind gusts are high. This prompted NWS Bismarck’s to issue its earliest severe thunderstorm warning with an eventual 81 mph wind gust being reported at a mesonet site in Hettinger County, North Dakota.
A lightning-producing cell precipitating rain, snow, and graupel formed west of Bismarck, North Dakota, moving southeast into northeast South Dakota, during the evening of March 3, 2020. Using a similar procedure as displayed before, Figure 3 shows relatively rapid cooling of an expanding cloud top/anvil as noted in Band 13 (default color scheme shown). Values exceeding 35 dBZ in MRMS isothermal reflectivity at -20 C correlated nicely with the appearance of sensed lightning by GLM and ground based networks.
A graupel/snow depositing shower moved through Fargo, North Dakota, near sunrise March 3, 2020. This along with other showers left a trail of accumulated graupel/snow apparent on the Day Snow-Fog RGB (Figure 4). This is seen as more orange shaded streaks atop the background darker red coloring of the older snowpack. The shower moved through the Fargo-Moorhead metro right as sunlight was becoming available. Thus, it was beneficial to adjust the near infrared components of the RGB to account for little visible light to work with. This helped brighten the image revealing the metro was right on the edge of new accumulation (which lined up nicely with radar).
Later in the day, clouds peeled away revealing additional showers had deposited new frozen precip accumulation throughout eastern North Dakota into northwest Minnesota (Figure 5).
The main takeaway of this post is to encourage forecasters to become more comfortable with adjusting satellite products to help draw out convective features. Such adjustments may be especially necessary during the cool season. That’s not to say just blindly make adjustments, rather forecasters need to understand what is being adjusted and how that would affect the appearance and meaning of edited products. After determining an environment is possible for impactful convection, forecasters should think about adjusting satellite display to make subtle convective features typical in the cool season more easily tracked.
A hyperactive period of intense, extratropical cyclone development unfolded last month across the North Atlantic. A series of four winter storms, each generating hurricane force winds, developed in rapid succession. No minimum central pressure records were broken, but all four storms underwent rapid cyclogenesis or bombogenesis. One of the storms, Storm Dennis (named by the UK Met Office), attained the second lowest pressure reading behind Storm Braer in 1993. Three of these four cyclones managed to achieve remarkable pressures in the 920-930 mb range.
Below is a table summarizing what we know about each of these four storms.
Next, we describe each system using a combination of satellite and surface analyses.
The first storm, which we are calling the “Greenland Bomb,” deepened explosively to 930 mb by February 8. In the surface analysis below, the storm – designated as a hurricane-force low – is located just east of the tip of Greenland. The extreme cyclonic wind field helped propel the next storm in the series, shown over Newfoundland as a 966 mb low, into Europe.
The low reached the British Isles as a 945 mb storm, causing widespread flooding and wind damage. It was named Storm Ciara by the UK Met Office, and is shown on the synoptic surface chart below.
On 12 February 2020, a third storm was brewing in the North Atlantic Ocean that was forecast to rapidly intensify into an strong extratropical cyclone that would develop hurricane-force winds. The system’s pressure dropped from 1005 mb on 12 February at 00 UTC to 962 mb on 13 February at 00 UTC, a 43 mb decrease in pressure in just 24 hours that classified this system as a bomb cyclone. We are referring to this third cyclone as the “Iceland Bomb.”
Stratospheric air intrusions are a known contributor to rapid intensification of extratropical cyclones via the advection of potential vorticity from the stratosphere into the troposphere. In the case of the Iceland Bomb, there was a strong signal of stratospheric air in the troposphere seen in GOES-16 Airmass RGB imagery, indicated by deep red/magenta in the dry slot of the cyclone.
The wave pattern across the North Atlantic was extremely active, as seen in the Airmass RGB imagery above and Atlantic surface analysis below. A storm force low preceded the Iceland Bomb, and Storm Dennis closely following.
In the image below, the RGB imagery of the Iceland Bomb does a remarkable job of delineating “textbook” conveyor belts, or the basic 3D circulation branches of a wintertime, marine cyclone. The dry conveyor belt descends from the stratosphere into the southwestern quadrant of the storm. The warm conveyor belt – with its moisture-laden, high cloud shield originating in the warm sector – stands out in stark contrast to the cold conveyor belt. The cold conveyor belt undercuts the warm conveyor belt from the east, wrapping into the comma-head of low and middle layer clouds.
The Iceland Bomb deepened further as it moved across the Atlantic and reached its minimum central pressure of 929 mb on 14 February at 06 UTC. Hurricane-force winds were sustained for 48 hours by this extratropical cyclone, slamming Iceland with winds of historic strength and producing phenomenal wave heights up to 64 ft. Although terrain enhanced, the highest wind gust recorded in Iceland was 159 mph in Hafnarfjall.
Almost like dejavu, another cyclone closely followed the Iceland Bomb. The low pressure system, which would eventually receive the name of Storm Dennis, originated in the same location as the Iceland Bomb and took nearly the same path. The system deepened from 996 mb on 13 February at 12 UTC to 956 mb on 14 February at 12 UTC, a 40 mb drop in 24 hours, classifying this system as another bomb cyclone.
Unlike the Iceland Bomb, this cyclone moved a little more to the east, impacting Ireland and the United Kingdom with intense hurricane-force winds. A wind gust of 118 mph was recorded in the Scottish Highlands, and a gust of 107 mph was recorded in Brocken, Germany, the strongest gust recorded outside of the United Kingdom (WNEP).
Other impacts included intense waves with heights up to 52 ft, severe flooding produced by prolonged intense rainfall, and structural damage caused by falling trees. All of this occurred just a week after the region was hit by storm Ciara, which exacerbated the impacts.
Rapid intensification of Storm Dennis was aided by a stratospheric air intrusion that originated in a trough located over the eastern United States that can be seen in the Airmass RGB imagery above. Deep red/majenta coloring indicates dry upper levels and high ozone concentrations associated with stratospheric air in the troposphere. Orange coloring indicates potential vorticity in the Airmass RGB product, and it trails all the way from the trough towards Europe, feeding into storm Dennis. Also pictured above is a massive atmospheric river stretching across the entire North Atlantic that provided ample moisture for the development of Storm Dennis.
The low pressure centers from the Iceland Bomb and Storm Dennis met up just south of Iceland where they did a Fujiwara dance, rotating around each other and then merging into one low pressure center. The dance can be seen in the Atlantic surface analysis above and the Airmass RGB imagery below. The Fujiwara effect is most commonly observed when two tropical cyclones pass close to one another. The combined system continued to move northeastward and dissipated by 18 February at 00 UTC.
Broader Perspectives Of The Hyperactive Storm Period
Having bomb cyclones in February is not unusual, as the peak season for these events in the North Atlantic and North Pacific is September through April, but having FOUR bomb cyclones in rapid succession of this intensity is unique.
This winter has featured a persistent, strongly positive arctic oscillation (AO) index, as shown below. Such a strong, positive index is driven by intense low pressure over the polar North Atlantic and high pressure ridging in the subtropical Atlantic.
The extreme pressure difference or gradient, in turn, has ratcheted up the intensity of the zonally-oriented polar jet stream coursing across the Atlantic Basin – at times reaching 240 mph. The dynamics of such an intense jet are a key reason for the sequence of very intense North Atlantic cyclones.
When one compares this winter’s East Coast snow activity (thus far, one of the least snowiest winters on record for portions of the Mid Atlantic), with the winter of 2009-2010 (record-breaking seasonal snowfall), the contrast in persistent North Atlantic Oscillation phase is striking (image below). Since early December, nor’easters have failed to materialize along the typical breeding ground of Cape Hatteras.
The intensity of the Iceland Bomb and Storm Dennis was aided by a massive atmospheric river spanning the entire width of the Atlantic Ocean. The atmospheric river funneled moisture from the tropical Caribbean all the way to Europe, following along a 5,000 mile long surface cold front and an intense upper air jet, which aided Dennis in producing an impressive 6.2 inches of rain over 48 hours in Cray Reservoir in South Wales as well as high rain totals across the region (WNEP).
Shown below is the integrated water vapor (IVP) product at 12 UTC on 16 February, showing extensive plume of Caribbean moisture entering into the large circulation of Storm Dennis (top panel). Also below, the Atlantic surface analysis the day prior illustrates the lengthy cold front, bisecting the entire Atlantic basin, that helped concentrate this atmospheric river.
This stormy period will be remembered for the incredible low pressures, verified hurricane-force winds (using ASCAT), and significant wave activity. The North Atlantic has remained stormy in early March, but pressures have been higher, though hurricane-force wind events continue.
Thank you for reading!
Deirdre Dolan (U. of MD-College Park), Jeffrey Halverson (U. of MD-Baltimore County), and Michael Folmer (NWS/NCEP/OPC)
The 4 March 2020 FDTD Satellite Applications Webinar was a tag-team discussion of a high profile snow squall event in PA/NY led by Dave Radell (SOO at New York City, NY) and Mike Jurewicz (SOO at State College, PA). In this webinar Mike and Dave shared several tips and tricks for fellow forecasters who may face this type of phenomenon in their own offices in the future. Here is one example of how satellite data was used to give a more complete picture as the event unfolded:
“A product that’s gaining a lot of popularity is the Day Cloud Phase, it’s an RGB product […] I would highly recommend the RGB product not only for these snow squall scenarios but for convection in general. […] although you get some idea in the traditional visible of the cloud enhancements, it really jumps out at you on the RGB loop. As these cloud features are growing taller and you’re getting more glaciation at the higher levels. With the ingredients that go into the RGB loop you can see that red and orange tint so it really jumps out at you nicely what the developing convective features are versus the lighter blue shading which is the flatter more laminar stratocumulus features. Again it could be another clue as to what really are the features of interest.”
“These snow squalls can be pretty robust convection in the winter time but in the grand scheme they’re fairly shallow features so you’re going to have issues with radar overshooting. But if you can combine your radar analysis with satellite you can get a more complete picture of how things are evolving.”
FDTD Satellite Applications Webinars are peer-to-peer learning; staff from WFOs, National Centers, CWSUs, RFCs lead the presentations. The presentations are short (less than 30 minutes) and recorded for on-demand viewing.
A rapidly intensifying low pressure system made its way into the Atlantic on Tuesday, 11 February 2020 and quickly grew into a powerful extratropical cyclone producing hurricane force winds by Thursday, 13 February 2020. By 0600 UTC on 14 February 2020, the low bottomed out with a minimum low pressure of 929mb. This system deepened by more than 40 mb in 24 hours during its rapid intensification phase, classifying it as a “bomb” cyclone. It tracked north towards Iceland where it caused hurricane force wind gusts, the highest gust, although terrain enhanced, reached 159 mph (https://www.severe-weather.eu/recent-events/near-record-wind-gusts-255kmh-hafnarfjall-iceland-mk/). These gusts were recorded on the leading edge of the cyclone where the cold conveyor belt north of the occluded front in the N-NE quadrants played a role.
This intense extratropical cyclone was closely followed by another cyclone (named Dennis by the UKMet office) rapidly intensified during the day on 14 February 2020 deepening by 40 mb in 24 hours, classifying it as another bomb cyclone. This system is following a similar path as the previous cyclone, capitalizing on the favorable baroclinic environment left in the wake of the first cyclone.
In the RGB Airmass imagery (above) from GOES-16, it is clear that there is also a large potential vorticity anomaly (red shading) upstream of the system, originating from a trough over the eastern United States. This inflow of potential vorticity into the storm is aiding the rapid intensification of the system. This system deepened to 920 mb as of the Ocean Prediction Center (OPC) 1800 UTC analysis on 15 February 2020. Hurricane force winds have been sampled by ASCAT scatterometers and by aircraft in the early morning hours of 15 February 2020 with maximum winds of 94 kt!
The above ASCAT and Altimeter images from ~1700-1745 UTC on 15 February 2020 show winds in the primary 920 mb low (south of Iceland) still at hurricane-force (>65 kt) with significant wave heights near 42 ft in the southeast quadrant. Meanwhile the older, lee-side low that was part of the 13 February 2020 storm is still exhibiting winds of 50-60 kt, aided by a barrier jet in eastern Greenland and a tip jet near the southern tip of Greenland. The altimeter readings near this latter storm were 30-42 ft (note that it’s possible higher waves (~50+ ft) were in the vicinity of both storms at this time).
These latest ASCAT and Altimeter passes show the 922 mb low (as of the 0000 UTC 16 February 2020 OPC analysis) has started to fill (weaken slightly) with plenty of storm-force wind barbs and significant waves still over 40+ ft, though higher winds of 65+ kt and waves over 50 ft are most likely not sampled.
We will have a more detailed recap of this past week’s stormy north Atlantic in the coming days, so please stay tuned.
On February 12, 2020, a very strong arctic cold front swept through the upper tier of the central CONUS bringing blizzard conditions, dangerous wind chills colder than -50 F, wind gusts exceeding 60 mph, and crashing temperatures to portions of the Dakotas, Minnesota, Iowa, and Nebraska. Despite little snowfall expected with this front (generally between 1-2 inches), blowing snow and significant visibility reductions appeared likely should the existing snow pack be susceptible to being lofted and blown around. But prior to this impactful frontal passage, forecasters were left with a difficult decision: just how susceptible is the existing snow pack to become lofted into blowing snow?
One tool that could help forecasters answer this question is close examination of snow pack appearance on the 1.61 um Snow/Ice band offered on GOES-R ABI. It’s has been shown that the Snow/Ice band is sensitive to the amount of liquid water in a volume of snow and ice, i.e. its “water to ice crystal ratio” (CIMSS Blog example). We can apply this in operations by looking for “old, crusted over snow” from “new, fresh and fluffy snow.” In Figure 1, a swath of newly felled snowfall across eastern SD, southern MN, northern IA, into WI and MI can be seen as a lighter shade of grey compared to darker northern neighbors (you need to wait for bright white clouds to pull away to reveal the dark snow pack below). This lighter shaded area is where forecasters can more confidently delineate a fresher snow pack that may be more susceptible to being lofted and blown around. Lastly, this loop applies adjustment to the display range of the Snow/Ice band in order to more easily draw out this area, changing from the AWIPS default Min-Max of 0-100 to an adjusted Min-Max of 3-30.
As the cold front encroached upon Canadian border states of the Northern Plains overnight, close examination of the GOES-East Nighttime Microphysics RGB (Figure 2) revealed the exact location of the arctic cold front by way of rapidly advancing, arching area of low stratus towards the south embedded or underneath mid-upper level clouds moving northeastward. Higher clouds were associated with a weak mid level and surface wave moving east along the strong baroclinic zone charging southward.
Figure 3 annotates the location of the arctic cold front as well as METAR observations with the Nighttime Microphysics RGB. Closely scrutinizing satellite imagery for any subtle details can help forecasters latch onto synoptic and mesoscale features, particularly those that bring hazardous weather like this arctic front whose impacts started immediately after frontal passage. Rapid temporal tracking of this feature offered by GOES-East could give forecasters details like timing of onset to impacts, something very important to IDSS.
As the sun rose on February 12, 2020, forecasters at NWS Grand Forks were anxious to see if blowing snow could be viewed on satellite imagery as area observations, reports, webcams, and radar suggested (as it turned out the crust on snow pack might have been broken north of the aforementioned area of newly felled snow due to very gusty winds exceeding 60 mph aiding to the production of blowing snow). Luckily a GOES-East mesoscale sector was over the FGF area (thanks DMX!) during this time allowing 1 minute imagery to provide the most up to date satellite view available. A look at the Day Snow-Fog RGB (proven to be useful in monitoring blowing snow during the day) gave indications of horizontal convective rolls associated with blowing snow, evident moving out of southern Manitoba into northern North Dakota, but perhaps not as quite obvious for operational usefulness (Figure 4).
While the blowing snow plumes were somewhat noticeable in Figure 4, the default Day Snow-Fog RGB composite ranges within AWIPS doesn’t show features well in times of low light, i.e. near sunrise/sunset and near poleward locations in winter. Since all three of the components (0.86 um, 1.61 um, 3.9-10.7um) that make up the Day Snow-Fog RGB are sensitive to solar reflectance, we can adjust the composite ranges of this RGB to become more representative of the little solar reflectance available just after sunrise in this high latitude location. Shrinking the RGB composite ranges from the default (R: 0-100, G: 0-70, B: 0-30) to (R: 0-20, G: 0-15, B: 0-25) allows the forecaster to more clearly see swaths of horizontal convective rolls that make up blowing snow (Figures 5 & 6). Forecasters can adjust RGB ranges on the fly like this to make features easier to track, making it more operationally useful. Just don’t forget to readjust the range as more sunlight becomes available!
Figure 7 displays how blowing snow could be tracked on satellite throughout the day as dry arctic air scoured away clouds moving south and east near the front (stratus colored white/lavender as well as some higher cirrus as orange/red). Blowing snow extended all along the Dakotas and Minnesota border with blowing snow seemingly influenced by not only the Red River Valley in eastern North Dakota and northwest Minnesota (Figure 8), but also by the Coteau De Prairie/Sisseton Hills in eastern South Dakota and southwestern Minnesota (Figure 9). Significantly reduced visibility as noted by METARs can be matched with the blowing snow plumes seen funneling down the lower elevation of the Red River Valley (Figure 8) and Buffalo River Valley (Figure 9). Also, cessation of blowing snow can be noted within the Red River Valley as the horizontal convective rolls dissipated north to south in Figure 7. Lastly, blowing snow plumes could be noted advecting southward out of southeastern South Dakota into the relatively snow-free far northeast Nebraska towards the end of the loop in Figure 9.
Analysis of GOES-East imagery during this event provided information on antecedent conditions as well as precise tracking of the arctic cold front and blowing snow causing blizzard conditions behind the front. This information was used directly in operations helping refine the boundary of blizzard warnings and winter weather advisories, social media messaging, as well as IDSS support to core partners of the National Weather Service.
Ideal conditions for the development of heavy freezing spray developed across the coastal waters of the northern Gulf of Alaska, including Cook inlet, on the back edge of a low pressure system during the afternoon/evening of 10 Feb 2020. Analysis of GOES-West water vapor imagery reveals the associated upper level trough exiting to the east as the next system approaches from the west (Fig 1).
Given the expected surface conditions, NWS Anchorage issued a Heavy Freezing Spray Warning for the associated offshore waters zone (Fig 2).
During the late afternoon and early evening of the 10th, surface observations indicated the development of gusty winds, temperatures well below freezing, and rough seas; all conditions necessary for the development of freezing (salt water) spray. Gusty winds and temperatures well below freezing were reported at Stations AUGA2 and AMAA2 (Fig 3), with wave heights of 10-15 ft reported at 46080. This is the region within which heavy freezing spray was expected and likely occurred based on analysis of VIIRS and ABI satellite imagery.
Given the relatively high latitude of the region, three VIIRS passes (one from NOAA-20, two from SNPP) were available during the day within 2.5 hours of each other. The five 375 m I bands from SNPP VIIRS for 2123 UTC are shown in Fig 4, centered over the region of freezing spray. A modified gray scale color table was created to focus on the reflectance values of the spray. The higher spatial resolution of the VIIRS imagery (vs GOES) captured the phenomenon in enhanced detail, allowing for easier diagnosis as to where freezing spray was occurring at that moment. The spray is observed as a region of relatively high reflectance (lighter gray) vs lower reflectance open sea extending from station AUGA2 through Cook Inlet and station AMAA2 into western portions of the broader Gulf of Alaska. Viewing VIIRS 375 m channels I1 – I3, it is obvious that the spray is most apparent in channel I2 given the relatively high reflectance of the lofted sea particles over the very low reflectance ocean surface.
Now viewing I4 and I5, the spray has a higher brightness temperature (darker gray) in the I4 (3.9 um) channel vs I5 (11.45 um) channel as a result of added reflectance component during the daytime due to scattering of the airborne particles. The brightness temperatures are similar in areas of clear sky with no spray over the ocean. Taking the difference between the two channels provides a clear view of where the spray is occurring. Sea current patterns are also apparent in bands I4 and I5, and are differentiated from the sea spray, particularly in band I4 where the spray has a higher brightness temperature, and currents have a lower brightness temperature.
Figure 4: 2123 UTC 10 Feb 2020 SNPP VIIRS I bands 1-5 and band 4 minus 5 difference (left to right, top to bottom). Higher res: I1 (0.64 um), I2 (0.865 um), I3 (1.61 um), I4(3.74 um), I5 (11.45 um), I4 – I5. Color table: low reflectance and warmer brightness temperatures is dark gray, high reflectance and cooler brightness temperatures is light gray. For the I4-I5 difference, dark gray represents a greater positive difference, light gray is near 0 difference.
Imagery from the three daytime VIIRS passes (2038 UTC from NOAA-20, 2123 UTC and 2306 UTC from SNPP) provides a sense of evolution of the spray during the day at high spatial resolution (Fig 5). Clouds appear to have developed within the region of spray by 2306 UTC.
GOES-West Full Disk sector provided high temporal resolution imagery over the region of lofted sea particles. At such a high latitude, the imagery spatial resolution is degraded (pixel area increases by roughly 4x), but still useful for detecting the spray in a broad sense. Given our analysis of the VIIRS imagery, we take a look at the 1 km (at nadir) 0.86 um band from GOES-West, and are able to track the evolution of the sea spray through the afternoon (Fig 6). A similar color table is used to that presented with the VIIRS imagery.
The CIRA Snow/Cloud product was very effective at capturing the spatial extent of the sea spray without needing to make any modifications (Fig 7). The integrity of the RGB for tracking other features is, therefore, maintained.
The ability to detect and track sea spray could be useful to NWS forecasters in verifying forecast products such as a Heavy Freezing Spray Warning, and for issuing new forecast products.