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).
A fast-moving shortwave trough brought a quick shot of wintry precip to parts of the Upper Midwest during the early part of 15 Jan. With an associated surface low pressure departing into the Ohio Valley, and high pressure building in from the west, northwest winds gusting to around 30 mph developed across the region.
The gusty northwest winds lofted snow off of the ground, causing areas of blowing snow and leading to reduced visiblities under otherwise clear skies across southern Minnesota and northern Iowa during the afternoon and early evening of 15 Jan. Bands reminiscent of cloud streets associated with the areas of blowing snow could be diagnosed in GOES-16 imagery. First analyzing the 0.64 um VIS, the bands of lofted snow area not obvious, but shadowing, especially later in the day, and the high (500 m) resolution of the band allowed for detection of the features (Fig 1).
The 1.6 um “snow/ice” band has been shown to effectively highlight areas of lofted snow (blowing snow) from the clear sky. The reflectance of the lofted tiny ice particles is typically between that of snow/ice on the surface (lower reflectance) and low liquid clouds (higher reflectance). Therefore, the lofted ice (over snow) can be more easily identified in the 1.6 m band vs the 0.64 um band, despite having a lower (1 km) spatial resolution.
Combining the aforementioned two bands with the 10.3 um IR window band in the Day Cloud Phase Distinction RGB, we get an even better picture of the blowing snow. The lofted ice is identified as a dull cyan above the surface snow (green) and adjacent to the low liquid water clouds (bright blue).
The Day-Snow-Fog RGB, which also includes the 1.6 um band (along with 0.87 um and 3.9 – 10.3 um difference), has been discussed as an effective means of identifying and tracking blowing snow. The contribution from 1.6 um band is the main driver behind the detection, with slight contribution from the other two components. For blowing snow detection, we replace the 0.87 um component with the higher resolution 0.64 um visible channel in order to capture finer details in the blowing snow bands.
Slight modifications were made to the RGBs in order to better highlight the feature of interest. The recipes used in this post are shown in figure 5.
Finally, a SNPP overpass during the early afternoon allowed for high resolution VIIRS imagery of the blowing snow bands to be collected. The 375 m 1.61 um band (I3) provided a great depiction of the blowing snow “streets” across the region.
A compact shortwave trough brought widespread severe weather to the southern US 10-11 January 2020. A Moderate Risk for severe storms was issued by the Storm Prediction Center for 10 January for a significant severe wind gust threat, with secondary threats of large hail and tornadoes. The threat on the 10th was centered over Texas, Oklahoma, Arkansas, and Louisiana, shifting east to Mississippi/Alabama on the 11th.
GOES-West upper-level water vapor imagery during the 48 hours leading up to convective development on the 10th show the evolution of the shortwave trough from off the northwest US coast, south along the west coast, and then east across the southwest US and into Texas (Fig 1).
The potent system drove enhanced low-level moisture into the region along with strong mid-upper level winds. This VISIT blog post highlights the utility of layer PW products for tracking the evolution of moisture and the EML leading up to the convective event.
Convection was ongoing/developing across Oklahoma during the morning/early afternoon on the 10th. Further south into Texas, convection developed a little later during the afternoon as the upper trough and associated strong forcing shifted closer, and inversion associated with the EML eroded.
By the late morning, GOES-16 derived motion winds indicated a swath of 60-70 knot SSW winds around 6 km AGL out ahead of the trough across north Texas (prior to CI here). Using the satellite-derived winds in combination with surface obs below to compute a vector difference, we achieve 0-6 km bulk shear values of around 55 knots, supporting the development of supercell thunderstorms (Fig 2). Further, the strong unidirectional deep layer flow is parallel to the surface/low-level boundary as diagnosed in visible imagery and surface obs (SSW to NNE). This orientation along with the strong shortwave forcing imply a likelihood of linear thunderstorm mode and wind threat quickly upon development.
GOES-16 1-min Day Cloud Phase Distinction RGB imagery showed widespread low stratus clouds (cyan) and showers (darker green indicating ice in cloud) across north Texas by early afternoon (Fig 3). During this period, convection rapidly develops southward across the region, clear in the 1-min RGB imagery per the vertical growth and transition to reds and yellows (result of cooling in the IR and increasing reflectance in the VIS as convection deepens).
Now looking at 1-min VIS/IR sandwich imagery for the following hour, convection continues to grow quickly southward across north Texas (Fig 4). The sandwich imagery maintains the high resolution/detail available in the 500 m VIS, while also representing the degree of cooling via the IR. Important storm to features are also easily detectable, including overshooting tops and above anvil cirrus plumes.
During this same period, VIS/GLM FED sandwich imagery showed lightning jumps with the strongest storms embedded in the broader line during early development (Fig 5). The north Texas storm for which a severe thunderstorm and tornado warning was issued saw lightning activity increasing from around 20 fl/5-min to around 90 fl/5-min within a ~15-minute period leading up to warning issuance. This rapid increase in flash density signifies a rapidly intensifying updraft. This particular storm had 1″ hail reported with it.
Analyzing 1-min visible imagery even further south, storms continued to develop south along a narrow line. The low sun angle allows for easy diagnosis of vertical growth as well as relevant storm top features such as overshooting tops and eventual above anvil cirrus plumes. Ahead of the developing storms, billow cloud formations indicate the continued presence of a stable layer inhibiting surface base convection.
A series of strong upper jets and related large scale subsidence helped to bring gusty winds to northeast Colorado for an extended period from 5-7 January 2020. Winds across the eastern plains generally gusted as high as 30-50 mph, while winds in the foothills and mountains peaked above 60 mph, with some reports of winds in excess of 80 mph.
During the afternoon of the 6th, gusty winds across the plains in the presence of dry, dormant fields resulted in widespread blowing dust. Recall the 10.3 – 12.3 um “split window difference” is able to highlight lofted dust quite well given the absorption of radiation at 10.3 um by silicate particles present in the dust. When lofted dust is present in the low levels, therefore, the 10.3 um band will sense slightly higher in the atmosphere, or cooler temperatures, compared to the 12.3 um band, resulting in near 0 to slightly negative SWD. In clear sky regions, the SWD will typically be positive since the 12.3 um channel will sense higher in the atmosphere (cooler temperatures) compared to at 10.3 um given the weak sensitivity of the 12.3 um “dirty window” channel to atmospheric moisture.
Similarly, one could view a 10.3 minus 11.2 um channel difference to identify lofted dust. While the difference may be greater in areas of dust, the clear sky difference will be less, reducing contrast between lofted dust and clear sky regions.
For this event, the lofted dust is readily apparent in the SWD across NE Colorado as darker shades of gray (Fig 1). I prefer a linear gray scale color table when using the SWD, whether it be for detecting lofted dust or moisture gradients. For this case, the range of the gray scale was adjusted to -2 to 5 to better highlight the lofted dust. It is recommended and simple to adjust the range on the fly to make the feature of interest stand out. However, a default value from around -3 to 10 will at least prove useful for initial detection.
The SWD is combined with the 10.3 um IR-Window channel and 11.2 µm – 8.4 µm Split Cloud Phase Difference (SCPD) to give us the Dust RGB, which can also be used for cloud classification. Similar to the SWD, the SCPD will highlight low-level lofted dust (small positive values). For this case, the dust is highlighted in the default RGB, as pink (Fig 2).
A slight modification to the RGB increases contrast and allows for easier identification of the dust plume. Instead of pink, the dust now appears as a dark blue above a brighter cyan/blue background (Fig 3). Low clouds appear as bright green, very cold surface as slightly darker green, and warmer surface as brighter cyan. High clouds are red.
The difference in RGB recipe is shown in Figure 4.
There is a GOES-R derived product for dust detection. In this case, it appears to capture the most opaque region of the dust plume (Fig 5).
That night, strong winds continued across the front range, extending into the adjacent plains. Surface observations are unable to capture the spatial intricacies of the gap winds as they seep into the lower elevations and I-25 corridor. Overnight GOES IR imagery showed the extent of the gusty winds via the presence of the warm anomalies due to the strong sinking/warming flow from west to east off of the high terrain (Fig 6).
In this example the warmer brightness temperatures are represented by darker shades of gray, while the cooler temperatures are lighter shades of gray. Gusty westerly winds per the surface obs match up with the warmer brightness temperatures per the satellite IR. The eastern slopes/downsloping region of the foothills appears warmer, while areas further west in the mountains are colder. Near the I-25 corridor, including the Denver area, localized areas of warming/gusty winds are diagnosed extending from west to east out of the mountain gaps. For example, early in the loop, Denver obs within a warmer plume indicate temperatures in the low 40s with winds gusting near 40 mph. Nearby areas with cooler IR brightness temperatures have surface obs in the low 20s and light winds. A broader area of gusty westerly downsloping winds and resulting warmer temperatures is measured over southern Wyoming advancing southeast into far northern Colorado, while much colder temperatures and lighter winds under the inversion are present over much of the rest of eastern Colorado.
An IR animation with RAP model temperature analysis highlights the broader surface temperature pattern, but fails to capture the smaller scale warmer gap flow regions (Fig 7).
The hourly Land Surface Temperature (LST) derived product form GOES-16 captures the smaller-scale warm features (lighter blue) quite well as they emanate from the foothills (Fig 8).
This more detailed (spatially/temporally) information available from GOES could aid mountain area forecasters in making their short-term forecast updates for wind and temperature during overnight wind events.