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.
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.
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.
A shortwave trough digging into northern Mexico on 5 Feb 2020 brought gusty winds to the surface, leading to areas of lofted blowing dust, primarily from sources marked at points A and B (Fig 1). The lofted dust had traveled as far as Houston, TX, per media reports of dust being deposited at the surface.
There were two regions in particular from which a significant amount of surface material was lofted and subsequently carried a long distance. These locations, marked in Fig 2 and 3, include red earth from central Zacatecas (point A), and sand from southwest Coahuila (point B), where many sand dunes are present.
GOES-East 5-min CONUS imagery captured the onset of lofted dust from the aforementioned regions, along with it’s evolution as it wrapped around the southeast portion of the trough and was carried northeast into south Texas. The high spatial resolution afforded by the 500 m 0.64 um visible band provides the most detailed look of the lofted dust as it elaves it’s source, particularly toward sunset (Fig 4). Figure 5 provides a zoomed in look at lofted dust from the red earth (point A) region.
The Geocolor product developed at CIRA combines Channels 1, 2, and 3 and additional computations (making up for the lack of a green channel) to create pseudo-true color imagery during the day. In this case, the daytime geocolor imagery captured the dust plumes quite well, and differentiated the lofted red earth (shades of red) from the red earth region in the south and lofted sand (tan/gray) from the sand dune region to the north (Fig 6). The two source regions also appear red and tan, respectively.
Infrared imagery can also be used to capture lofted dust. The 10.3 – 12.3 um split window difference, previously discussed here for detecting dust, provides a very clear dust signature (negative difference values, dark gray to black; Fig 7).
Including the split window difference in the Dust RGB, along with the SO2 difference and 10.3 um IR window band, allows for dust detection (bright magenta to pink) along with cloud classification (Fig 8).
A daytime SNPP VIIRS pass at 1926 UTC over the area provided high resolution still images of the lofted dust from the red earth region shortly after onset. Similar products are available as above, but with better spatial resolution and slightly difference spectral specifications (Figs 9-13). Recall, the I bands provide the highest spatial resolution (375 m), while M band imagery is 750 m.
Continuing the GOES-East IR-based detection animations into the overnight hours (SWD in Fig 14, Dust RGB in Fig 15), detection becomes difficult as the lofted dust becomes increasingly dispersed, and cloud cover increases. However, careful analysis of the imagery allows one to diagnose the plumes extending northeast across southern Texas through the evening, with the faint remnants of the southern plume making it well up the Texas Gulf Coast.
Feature relative animations (such as when using feature following zoom in AWIPS) provide an intriguing alternative for viewing features in satellite imagery. Such features may include thunderstorms, boundaries, snow bands, dust, and smoke. Closed cellular convection is yet another feature for which a feature relative animation allows for a clearer picture of relevant processes. From the example on 4 Feb 2020 over the eastern Pacific, the divergence from the center of each cell is obvious, painting a picture of the implied rising air in the center of each cell (higher reflectance areas), and sinking air around the edges (low reflectance).
The same features are highlighted in Figure 2, but over a static region.
A zoomed out view of the region shows the development of the above closed cellular convection (in the middle of the scene) within a broader eastern Pacific Ocean anti-cyclone (Fig 3). Open cellualr covnection is also present in this scene, further to the southeast.
A longer animation (Sunday – Tuesday) shows the full evolution of the features behind the early week US trough and surge of cooler air and within the building eastern Pacific anticyclone (Fig 4).
A mid-upper level trough brought a variety of weather to the US during the first week of Feb 2020.
On 31 Jan, GOES-West WV imagery captured the early evolution of the trough over the central Pacific while a ridge was well established over the western US (Fig 1). Blending the satellite imagery with a model forecast not only allows one to guage model performance, but also provides a visualization of how features apparent in the imagery may evolve into the forecast period.
By 3 Feb, The ridge had eroded and the trough had advanced into the Great Basin, developing into a closed low (Fig 2). The periodic image degradation is due to the GOES-17 cooling system anomaly and and approaching vernal equinox.
The trough had sent a cold front south down the high plains during the overnight hours of the 2-3 Feb, with associated gravity wave perturbations evident in water vapor imagery (Fig 3). Combined with surface obs and RAP surface analyses, rapid pressure rises are apparent in the wake of the front, in addition to winds becoming northerly and then easterly and temperatures dropping considerable.
Nighttime Microphysics RGB imagery from GOES-West shows the rapid development of low clouds (dull yellow – green) across the high plains as temperatures dropped behind the front and the low levels became saturated. Surface obs also show winds becoming easterly upslope through the evening (Fig 4). Snowfall across the eastern plains to this point had been purely stratiform due to a saturated low layer and easterly upslope flow, per analysis of the RGB imagery.
After sunrise, cloud analysis is best done using the Day Cloud Phase Distinction RGB (Fig 5). The widespread low stratus deck (cyan) is obvious across the scene and contrasts with snow cover (green), high clouds (red), and clear ground (dark blue). By the end of this period, the upper low had advanced into northeast Utah, spreading stronger large scale forcing east across the Rockies, leading to increasing coverage of convective snow showers (textured reds).
A vigorous shortwave trough dug southeast out of Canada starting the evening of 19 Jan, continued across the middle of the US from the 20th to 21st, and across and east of Florida from the 21st to the 22nd. A long loop of GOES-East upper level water vapor imagery highlights the evolution and the shortwave as it dives southeast, with sinking and drying air (warm colors) on it’s western periphery, and rising and moistening air (white to green colors) to it’s east (Fig 1).
Zooming in to the southeast US as the storm moved offshore during the night of the 21st to the morning of the 22nd, significant strengthening of the shortwave is observed as thunderstorms in the ascending region and drying in the descending region both become more pronounced (Fig 2). Relevant large scale features are highlighted at the end of the period.
An alternative view of the strengthening is provided in the Airmass RGB imagery from GOES-16. The shades of red becoming more apparent on the western side of the storm represents sinking/drying/higher PV air into the upper troposphere (Fig 3).
The upper trough was accompanied by anomalously dry air and cold temperatures for the southeast US. Cooling temperatures can be visualized by the GOES-16 Land Surface Skin Temperature product, with skin temperatures across south Florida falling from the 70s during the day of the 21st to 30s during the evening (Fig 4).
The dry air is simialrly captured in the GOES-16 TPW product, with values across much of south Florida dropping to near or below 0.3″ (Fig 5).
This drying was also captured in radiosonde data, with the 12Z sounding from Key West measuring 0.3″ of TPW (Fig 6), which is well below average (1.2″) and actually is a new daily min for that location (Fig 7).
A surface low and intense and nearly stationary convection associated with the shortwave was captured in GOES-16 visible imagery and GLM Flash Extend Density data during the day on the 22nd (Fig 8).