Severe thunderstorms developed across the southeast US ahead of a potent shortwave trough during the day/evening of 19 April 2020. Water vapor imagery captured the strengthening shortwave as it accelerated east across the southern plains and into the southeast (Fig 1). RAP analysis overlay helps to quantify the shortwave, revealing the sharpening 500 mb trough in the height field, increasing 500 mb wind field ahead of the wave (including wind speeds over 70 knots), and the vorticity max.

By the late evening hours, the primary region of thunderstorms had shifted east into Alabama, Georgia, and the Florida Panhandle. Unfortunately, NWS TAE required emergency backup during a period covering at least 0545 – 0745 UTC, resulting in NWS Houston taking over warning issuance. To make matters worse, data from area radars was unavailable/intermittent to forecasters, primarily during the ~0600 UTC to 0700 UTC timeframe (Fig 2). Therefore, the warning forecaster was required to rely on satellite and lightning data for warning decisions.

The primary tools used following the loss of radar data was GOES-East IR imagery (Fig 3) and total lightning data from Earth Networks (not shown in examples), and later, GLM (Fig 4) as well. Cloud top trends in the IR imagery were monitored closely, including brightness temperature trends and overshooting tops. Persistence of overshooting tops and cold temperatures for storms that had previously appeared strong/severe in radar (and were warned on) aided confidence in continued warning issuance, as did rapid increases in lightning activity. Similarly, warming of storm tops, loss of OTs, and decreases in lightning density provided confidence in letting warnings expire.

Earth Networks lightning data was also used to account for parallax in the satellite imagery, helping a forecaster determine more accurately where the updraft core was located geographically. In this case, storm tops would have been displaced roughly 10-14 km to the NNW in the GOES-East imagery and GLM data.

***AWIPS display/procedures shown in this blog post were created after the event, and are similar but not exactly that used by the warning forecaster***

Bill Line (NESDIS and CIRA) and Sean Luchs (NWS HGX)

A hail storm moved south-southeast across portions of southwest Kansas during the late afternoon hours of 19 April 2020. This storm produced copious amounts of hail along a path of about 15 miles.

The evolution of the cu field across central Kansas can be analyzed in detail using the the GOES-East Day Cloud Phase Distinction RGB. The cu field evolves from flat, liquid clouds (cyan), to a more agitated cu field with clumping clouds and enhanced vertical growth and glaciation (green; Fig 1). With convective initiation, vertical growth is rapid, and cloud tops transition to yellows and reds with cooling and continued glaciation of cloud tops. A glimpse of the hail swath (green) is seen in the wake of thunderstorms, east of Kinsley.

The sky cleared off just enough prior to sunset such that an enhanced version of the GOES-East Day Cloud Phase Distinction RGB product could remotely sense the hail swath (Fig 2).

Numerous pictures of significant hail accumulation were received by NWS Dodge City on social media, some of which showing 4 to 5 inches of accumulation (Fig 3).

The hail swath stuck around through the cool evening hours, and was still apparent in GOES-East imagery the following morning (Fig 4, Fig 5).

GOES-East provides a fairly high resolution confirmation as to where exactly accumulating hail fell following passage of a thunderstorm and clearing of clouds. While MRMS provides a good estimate to where hail fell and how large it was, it doesn’t lend much detail about hail accumulation. The satellite imagery provides an observation of the hail swath, or where the more significant accumulating hail occurred. In this case, the swath diagnosed in GOES imagery matched up well with the MRMS MESH track, but the actual length of the accumulation of hail did not extend as far south into Kiowa County as what may have been suggested by MRMS MESH.

Mike Umscheid (NWS Dodge City, KS) and Bill Line (NESDIS and CIRA)

Flooding within the Red River of the North basin is practically a yearly rite of passage for those living there signaling the exit of winter and coming of summer. This area straddling the North Dakota and Minnesota border is well prepared for river and overland flooding because of its frequency. Spring of 2020 is proving to be no different with major flooding occurring over much of the Red River and its tributaries due to seasonal snow melt.

It can be difficult to keep a pulse on how impactful flooding really is with a lack of comprehensive reports in an ever-evolving and fluid situation. Plus with extensive flood mitigation systems in place, the majority of floods that occur in this area are nothing more than just an inconvenience for most people. Still for the minority that have property impacted by flooding, it can be quite notable and our services can be tailored to them, or at least towards the emergency services aiding them. Additionally, a threat to life can come to fruition if one finds themselves in very cold floodwaters (an example could be a car sliding off the road into floodwaters). This is ultimately our mission as National Weather Service meteorologists: to protect life and property.

This post will explore the satellite imagery that aided NWS Grand Forks in flood operations in mid-April 2020. In this case, flooding was due to springtime snow melt, but the following procedures and workflows may be applicable to warm season convection as well.

On April 10, 2020, moderate to major flooding was occurring along the Red River and its tributaries within the central and northern basin. Wondering if there were any areas experiencing impactful flooding outside of current flooding warnings, forecasters initially turned to GOES-R ABI River Flood Products for help in highlighting areas of observed floodwater coverage. Overlaying flood warning polygons, forecasters then looked for areas highlighted by the ABI River Flood Extent product not encompassed by polygons. This was the case in western Polk and Marshall counties, Minnesota, as noted in Figure 1. Values higher than 60% (orange and red coloring) were of particular interest as lower values into the 30-50% range were believed to be that of non-impactful standing meltwater in agricultural fields. While the ABI River Flood Products are updated every hour, the ABI’s spatial resolution of 1 km can smooth out the spatial extent of potentially impactful flood waters. VIIRS offers this same imagery at a finer resolution of 375 m, but at the expense of one or two images per day which require a daytime, cloud-free sky to provide useful information.

Luckily this area of interest was mostly cloud-free during one of the VIIRS passes. It confirmed higher percentage values in the same areas of interest, particularly in northwest Polk County, Minnesota (Fig 2).

There is additional satellite imagery available to further provide details on floodwaters and its potential impacts. Higher resolution satellite imagery down to 10 m from Sentinel-2 and Landsat 8 has recently become available on the web and already processed for quick viewing at sites like Sentinel-Hub and Remote Pixel. A timely, cloud-free pass from the Sentinel-2 satellite over the area of interest was available to forecasters for interrogation (Fig 3). It revealed extensive break out water from the Red River north of Grand Forks, North Dakota, and surrounding Oslo, Minnesota, although these areas were well within the flood warning polygons. What about the other areas of expansive meltwater in agricultural plots between Alvarado, East Grand Forks, and Tabor, Minnesota? Sentinel-2 imagery hinted that some of these floodwaters might be over roads and surrounded farmsteads. These areas of adjacent flooded plots of land correlated nicely with higher percentages within the ABI and to an extent VIIRS River Flood Extent products, thus confirming impactful flooding would possible here.

Forecasters took this approach of using satellite imagery to hone in on targeted areas for intel gathering of flood impacts. Looking for road closures on state Department of Transportation and county websites, data mining on social media, and calling emergency managers confirmed impactful flooding in these areas. Thus, these areas could warrant a flood headline. Ultimately the decision was made to not issue an Aerial Flood Warning as the majority of these areas fell just within current flood warning polygons.

So does the application of this imagery stop here? Not quite. ABI and VIIRS River Flood Extent products highlight nicely the extent of observed floodwater. In this case it was used as a source for the graphic in Figure 4. The imagery answers the “where” and “when” of flooding within the graphic while the photo depicts the impact, capped off with safety messaging. This imagery also yielded high confidence in overland flooding impacts which was messaged via the Hydro section within the Area Forecast Discussion from NWS Grand Forks. Additional imagery analysis was conveyed to the North-Central River Forecast Center which was useful in assessing the current state of snowpack, river and ditch ice, and floodwater expanse.

In summation, satellite imagery from ABI and VIIRS River Flood Products as well as from Sentinel-2 provided excellent details in gauging flooding impacts from river and overland flooding. Starting with the most coarse spatial resolution, yet highest temporal resolution imagery like ABI was a good starting point in searching for floodwaters over a broad area. Then, incrementally honing in on highest flood extent signals at higher spatial resolution imagery proved to be a good workflow in pinpointing areas of interest to target intel gathering of current impacts from flooding. Furthermore, this imagery was useful in messaging through graphics, discussions, and collaboration.

Honorable mentions:

Did you know Nighttime Microphysics RGB can by used to diagnose floodwaters (Fig 5)? The highest contribution of sensed floodwater comes from the Split Window Difference product. In this case, there was high contrast between lower values of liquid bodies of water and higher values of land. However, due to dependence on sensed infrared energy, seasonal and air mass differences can change the appearance of floodwaters within the RGB. You can adjust the RGB to help draw out this floodwater signal from the Split Window Difference as well.

Besides high resolution imagery from Sentinel-2 and Landsat 7/8, cloud penetrating Sentinel-1 offers similar resolution imagery while capable of detecting liquid water versus snow and bare ground (Fig 6). Although, it does take a more careful eye in interpretation of the imagery, especially when looking at “waterlogged” snowpack which can give a false signal for liquid water. This type of imagery can be extremely useful when pesky clouds prevent imaging radiometers from sensing the ground.

Resources:
NOAA Global Flood Product Quick Guide
VIIRS Flood Areal Extent Quick Guide
VIIRS Flood Detection Map Quick Guide
AWIPS Instructions Document

Carl Jones
Meteorologist
NWS Grand Forks

Bill Line, NESDIS and CIRA

GOES-East upper level (6.2 um) water vapor imagery with GLM and RAP analysis overlay depicted the Friday night – Sunday morning evolution of a storm system that brought severe thunderstorms to the southern US (Figure 1). A closed upper low initially centered over southern California on Friday accelerated east across northern Mexico Saturday as it evolved into an open wave, and eventually progressed ENE into the southern plains and southeast Sunday into Monday morning. Water vapor imagery shows the drying/warming descending air (warm colors) wrapping around the southern portion of the strengthening shortwave in conjunction with the intense upper level jet. Strong ascent is apparent east/northeast of the upper energy below the exit region of the upper jet represented as regions of cooling (cool colors and white) and developing thunderstorms. The RAP overlays help one to conceptualize what is being observed in the more detailed (temporally and spatially) water vapor imagery.

Additionally in Figure 1, a shortwave trough is diagnosed digging southeast across the northwest US Saturday and then east across Colorado into the central high plains on Sunday as it swings around the base of a broad upper low slowly sinking south into the far north-central US. This system brought a swath of snowfall to the Rockies, central high plains, and into the Midwest, which is apparent in the GOES-East Day Cloud Phase Distnction RGB as green (Fig 2).

Visible/GLM FED Sandwich combo imagery with NWS warning polygons during the daytime shows the development and evolution of thunderstorms throughout the day, and the relationship between lightning activity and strong (warned) storms (Fig 3). GLM is effective in highlighting the location of updraft cores and updraft trends within a messy cloud field and long flashes extending outward from the main updrafts and into the anvils. The image combination includes quantitative information from GLM without sacrificing the vital texture information from the VIS.

Bill Line, NESDIS and CIRA

A significant winter storm moved through the Northern Plains and Upper Midwest April 1-3, 2020, bringing over a foot of heavy snow, more than a quarter inch of icing, thunder–sleet and -snow, gusty winds, and moderate rainfall over an area already experiencing seasonal snowmelt flooding. The combination of ice, snow, and flooding made for particularly hazardous travel conditions as well.

It was known ahead of the event that mixed precipitation of sleet and freezing rain would cause ice accumulation on the “warm” side of a slow moving front and eventual surface low. Regional NWS offices messaged this threat ahead of time, although there remained some uncertainty of how much and exactly where ice accretion would take place. The FRAM (Freezing Rain Accumulation Model) from SPC’s HREF as well as NBM’s ice accretion percentiles offered useful guidance to forecasters, although painted a very broad threat area including the entire state of Minnesota.

So how did it pan out?

Well, we can turn to the ABI on board GOES-East to help fulfill this answer, specifically the 1.61 um channel known as the Snow/Ice Band. At this wavelength, solar radiation energy is strongly absorbed by snow and ice with little reflected energy travelling back towards the sensor. This is why snow and ice show up relatively dark in the 1.61 um channel compared to snow-free ground and liquid phased clouds. While both are efficient at absorbing in this wavelength, ice still more strongly absorbs radiation than snow, allowing ice to appear even darker than snow. Additionally, wide areas of liquid water are even more strongly absorbed in the 1.61 um. This makes flood waters including major flooding along the Red River of the North and other ice-free lakes very dark. It is possible that darker swaths in southern Minnesota into Iowa may be enhanced from higher soil moisture from rain prior to a wintry mix producing ice.

With a mostly clear sky in place on April 4, we can use these properties to get an idea of where ice accretion occurred. While we are at it, let’s go down the multispectral imagery path and look at the Day Snow-Fog RGB which includes the 1.61 um channel.

Notice a dark red swath extending down the spine of Minnesota, into eastern South Dakota and into Iowa. There is also another area in southeastern Minnesota, again into Iowa, and western Wisconsin. These are areas where ice from freezing rain and sleet accrued. The streak-like nature of this signature within Iowa and southern Minnesota points to convective elements and showery activity leading to widely varied accretions and likely associated impacts telling us this was a very difficult forecast to pinpoint. Overall, guidance from HREF and NBM did well highlighting the general area, although they likely smoothed out these high spatially varied accretions within their ensemble systems.

The Day Snow-Fog RGB utilizes the 1.61 um channel as it’s green component. Less reflectance of snow and ice in this channel leads to lesser green values added to the overall combination. Within the RGB’s red component, the visible-like 0.87 um channel, snow is much more reflective with little or no reflectance from ice accrued areas. These differences make for easily noticeable contrasts between snow and ice, making it easier for forecasters to diagnose areas that experienced overall more ice than snow.

While the darker signature of icing helped forecasters see where icing may have been more prevalent than snowfall, it does not mean this was the only area of icing. Just to the west of the dark red strip in western Minnesota significant icing still occurred as depicted in this USGS photo of ice accrual on a river gage near Fargo disrupting data transmission. However, a transition from wintry mix later to accumulating snowfall lead to accumulated snow hiding the icing signature. The same could be said for patch around southeastern Minnesota.

As the power of an April sun warmed ground temperatures in the Upper Midwest, ice and thinner snowpack quickly disappears on the Day Snow/Fog RGB and CIRA’s Snow/Cloud Layers RGB.

So how is this information useful after the storm? Not only does it increase a forecaster’s situational awareness of potentially highest impacted areas, it can be used as an Impact Decision Support Service tool for illustrating to partners where exactly these conditions occurred. Forecasters at the National Weather Service in Grand Forks, North Dakota, and hydrologists at the North Central River Forecast Center used this imagery to gauge important spatiotemporal characteristics of the associated liquid water equivalent leading to conclusions of which locations would first see this water move into area rivers already undergoing flooding from winter snowpack melt. Additionally, satellite imagery including the Day Snow Fog RGB image above was used in a hydrologic briefing to core partners within the Red River of the North basin, giving them an idea of these important characteristics while using imagery to justify some aspect of updated river forecasts.

Carl Jones
Meteorologist
NWS Grand Forks

A low pressure system deepened rapidly over the Atlantic just off the Carolina coast during the day on 01 April 2020. GOES-East water vapor imagery and RAP surface analysis provide a great visualization of the strengthening low as it progressed east away from the coast (Fig 1). Dry/warming descending air (warm colors) is evident wrapping around the low from the south, while ascending/cooling air (including deep moist convection) is obvious further east, north, and west of the low.

As the low strengthened, very gusty winds developed at the surface, prompting the issuance of a Hurricane Force Wind Warning by the NWS. HRRR model analyses indicate widespread wind gusts in excess of 50 knots wrapping around the southern and eastern portion of the low (Fig 2).

GOES-East low-level DMWs, while not abundant nearest the center of circulation where cloud streets developed and winds were likely strongest, did produce several wind vectors over 50 knots near the low center, indicating strong flow just above the surface.

METOP-B ASCAT observed surface winds in excess of 50 knots wrapping around the western portion of the low between 1500 and 1600 UTC (Fig 4a). Later between 1800 and 1900 UTC, AMSR2 measured surface wind speeds between 50-60 knots on the western portion of the low, and 40-50 knots wrapping around the southern quadrant, between 18-19 UTC (Fig 4b).

An area of clearing was present adjacent to deep moist convection near the low center during the morning hours after sunrise. GOES-East visible (0.64 um) imagery captured rough seas (white caps) and associated/implied sea spray under the clear skies, confirming gusty winds reaching the surface during that period (Fig 5). The white caps/sea spray is diagnosed by regions of higher reflectance compared to nearby calmer seas. A modified gray-scale colortable is utilized in order to best highlight the white cap/lofted sea spray signature.

A zoomed in feature following animation provides an alternative means for viewing the evolution of the white caps through the morning (Fig 6). The phenomenon is most prolific extending south and east from the region of thunderstorms in the center of the scene.

The longer wavelength 0.865 um imagery provides better contrast for detecting the white caps (compared to clear sky calmer seas) given less influence of atmospheric aerosols compared to at the shorter wavelengths (Fig 7). However, spatial resolution is degraded by 4x compared to the 0.64 um channel with ABI, reducing clarity of the feature.

The white caps can be diagnosed in the snow-cloud RGB as regions of bright blue (Fig 8).

Finally, the milky appearance of the white caps was easily apparent in the Geocolor (true color) imagery (Fig 9).

A longer feature following animation shows white caps and sea spray continued along the southern portion of the low center into the afternoon and early evening hours (Fig 10).

Bill Line, NESDIS and CIRA

A remarkable shortwave trough brought widespread strong to severe thunderstorms to the Mississippi River Valley region during the day/evening of 28 March 2020.

GOES-East 6.2 um water vapor imagery with RAP field overlays provides an excellent view of the synoptic setup and evolution during this event (Fig 1). Features such as the positions of the low and upper level jet core as depicted in model analyses (such as the RAP here) are confirmed/corrected through analysis of the water vapor imagery.

Focusing in on the Midwest, convection developed along a dryline and warm front, with thunderstorms eventually producing large hail and tornadoes. The GOES-East Split Window Difference (SWD) and Infrared Window Combo imagery (available on the STOR) captures the evolution of the dry air north and east around the southeast portion of the deepening cyclone, and convective initiation along it’s leading edge (Fig 2). The GOES imagery provides better horizontal spatial and temporal resolution when compared to surface observations and surface analyses (hourly RAP surface equivalent potential temperature shown here). The simple gray scale color table of the SWD shows relatively dry low-level air as darker gray, with relatively moist regions lighter gray. An overlay of IR window is provided for cold brightness temperatures (clouds) as non-gray colors). The dry air is diagnosed surging northeast through eastern Kansas into northern Missouri and southern Iowa. Convection develops along the dryline, as diagnosed in the SWD imagery, across Iowa.

While the GOES-East TPW derived product also captured the punch of drier air northward and dryline boundary evolution (at lower spatial res), the CAPE product only ever shows values less than 500 j/kg (Figs 3 and 4). This is lower than what was observed by radiosondes and what was computed by model analyses and the SPC mesoanalysis.

One-minute imagery from GOES-East was available across the region during this event. Day Cloud Phase Distinction 1-min imagery showed deepening cumulus clouds along the dryline, with a transition from cyan to green indicating glaciation and imminent convective initiation (Fig 5). Continued vertical growth and transition to yellow and red colors indicates further glaciation and cooling of cloud tops, and that convective initiation has occurred. Shortly thereafter, the first lightning flashes occurred with these storms per GLM FED data. FED values then increase quickly leading up to the first reported tornado. All of this is apparent in real-time given the very low latency (<1-min) of the 1-min imagery.

Polar passes from SNPP (x2) and NOAA-20 meant three VIIRS images within about a 1.5 hour timeframe. This imagery provided higher spatial detail in the cloud field as convection began to initiate along the dryline. Day Cloud Phase Distinction RGB imagery created from the VIIRS 375 m I bands allows for the diagnosis of highly detailed glaciation trends in the cloud field (Fig 6).

Further south, strong thunderstorms developed within the broader cloud shield over northeast Arkansas, with one storm producing a tornado that caused damage and injuries across Jonesboro, AR. GOES-East 1-min visible imagery with GLM semi-transparent overlay shows increasing visible texture through the broad cirrus shield as the storm approaches Jonesboro from the southwest (Fig 7). FED values increase quickly as well just prior to/during the development of a tornado, with MRMS low-level rotation tracks confirming significant rotation as the storm advanced through the town.

Bill Line, NESDIS and CIRA

Low clouds quickly expanded across the central high plains during the overnight hours of 25-26 March 2020 as low-level easterly upslope flow associated with a surface lee trough drove moisture into the region (Fig. 1). GOES-East Nighttime Microphysics RGB imagery highlighted the westward expansion of low clouds (cyan) through the evening, along with the evolution of other cloud layers such as high cirrus clouds (red or black), and mid level clouds (green and dark yellow). This RGB was modified slightly to account for the colder airmass (reduce warm end of the blue IR component).

The animation transitions to the Day Cloud Phase Distinction RGB after sunrise to allow for continued cloud classification. The transition procedure can be found on the STOR VLAB page. The clouds still appear as cyan, with high level cirrus clouds shades of yellow and red, surface snow is green, and bare ground dark blue. This RGB was also modified slightly to account for the colder airmass (reduce warm end of red IR component) and low light conditions (reduce high end of VIS and NIR components). The low clouds progressively erode during the morning, and completely dissipate by early afternoon.

Bill Line, NESDIS and CIRA

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.

Bill Line, NESDIS and CIRA