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.

Bill Line, NESDIS/CIRA

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.

GOES-16 1-min visible imagery depicts rapidly developing narrow line of strong convection across central Texas by late afternoon on Jan 10. #txwx pic.twitter.com/KmU0P01Kvd

— Bill Line (@bill_line) January 10, 2020

Bill Line, NESDIS/CIRA

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 a slightly negative SWD. 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.

Bill Line, NESDIS/CIRA

A snow squall developed off of Lake Ontario during the morning of 18 Dec 2019 ahead of a southward surging surface cold front and related upper trough digging into the northeast US (Fig 1). As has been noted, these localized bands of heavier snowfall can be diagnosed in the Day Cloud Phase Distinction (DCPD) RGB from ABI. This can be useful to forecasters as a supplement to radar, particularly for developing snow bands and over lakes, as well as when radar data are unavailable, for use in conjunction with surface observations and webcams.

In this case, the DCPD RGB highlighted the development of the band over Lake Ontario, and it’s evolution southeast into New York (Fig 2). The 1-min imagery allowed for real-time analysis of the band as it evolved. The bright green areas indicate the likely areas of heavier snow, resulting from the relatively high reflectance in the 0.64 um band and cooler brightness temperatures in 10.3 um band (taller/convective clouds), but low reflectance in 1.61 um band (ice at cloud top). Convective cloud elements are also detectable along the snow band in the DCPD RGB owing to the high (500 m) resolution from the 0.64 um band component. The initial snow squall warnings were issued at the end of this animation. The Rochester ob indicates heavy snowfall and significantly reduced visibility at the end of the loop as the snow band moved overhead. GOES-16 DMWs in the SFC-900 mb layer over the lake indicated low-level cloud motion of 25-30 knots, comparing well to winds observed at the surface. A meso-low is also apparent over the far eastern portion of the lake.

It should be noted that the default settings for this RGB were modified to better highlight the features of interest for this case given the relatively cool airmass and low light. The ranges used were (Red: -63.5, -2.5…Green: 60, 0…Blue: 45, 1), while the gamma’s all remained at 1.

For the final time-step in the above animation, a qualitative comparison between the DCPD RGB and MRMS composite refelctivity reveals a correlation between the bright green areas and highest reflectivity (Fig 3) across New York and Pennsylvania. Additional snow squall warnings were issued for the bands in northeast Pennsylvania.

The DCPD RGB and radar imagery are compared again in a two panel for the hour immediately following the animation in Figure 2 (Fig 4).

Figure 5 includes a long radar loop with the aforementioned NWS Snow Squall Warning polygon.

Ground truth from a Rochester webcam revealed rapidly deteriorating conditions associated with the snow squall (Fig 6).

Bill Line, NESDIS

A strong and extensive upper-level jet brought widespread weather impacts to the United States in mid-December 2019. Water vapor imagery from GOES-West on 12 Dec showed the jet extending from north of Hawaii to the US Rockies (Fig 1). The feature is identified in water vapor imagery by the sharp poleward transition from cooler to warmer brightness temperatures, along with localized fast moving moisture/cloud features. The more significant vorticity maxima/shortwaves are diagnosed in the water vapor imagery as well.

Airmass RGB imagery makes clearer the transition from a warmer/more moist airmass south of the jet (greens) to the cooler/drier airmass to the north (Fig 2). Shades of red near the jet and shortwaves are also apparent, indicating the descent of drier, higher PV air.

By 13 Dec, the jet exit region had dug into the southern US, helping to deepen a trough (Fig 3). The jet exit region, trough and associated deepening surface low would continue east and develop into a nor’easter as it progressed up the east coast. The far southeast US was highlighted in a Slight Risk for Severe by the SPC on the 13th in association with the deepening trough/jet. Further west, the jet core remained over the US Rockies, with GOES-East Derived Winds product indicating speeds as high as 180 knots around 300 mb! The strong jet was driving elevated amounts of pacific moisture into the western US. As a result of the enhanced moisture content and strong orographic forcing, heavy snow was ongoing across much of the Rockies, resulting in numerous winter weather highlights. Across the Colorado Rockies, several feet of snow was forecast to fall by the end of the weekend.

The Airmass RGB product on the 13th continued to depict the contrast between the two airmass poleward across the jet (Fig 4).

The operational Blended TPW product, which blends moisture information from multiple instruments, clearly shows the plume of moisture associated with the pacific jet driving east across the eastern pacific and western US (Fig 5). TPW values were indicated to be well above normal, (200% of normal in many areas; Fig 6). The prolonged period of such anomalously high TPW values lends confidence to significant snow potential over high elevations where orographics are favorable.

Analysis of the Layer PW product shows that the lengthy plume of moisture extended vertically from the surface to the upper levels (Fig 7).

Bill Line, NESDIS

Snow bands accompanied by gusty winds led to localized and brief periods of significantly reduced visibility across upstate New York during the afternoon of 05 Dec 2019. A few of the snowbands advancing south of Syracuse prompted the issuance of Snow Squall Warnings by NWS BGM. Area webcams confirmed the hazardous driving conditions (see tweets here and here). The first warning and radar imagery associated with one of the earlier snow squalls is shown in Fig 1.

Rare snow squall warning south of Syracuse. This is almost like a severe thunderstorm warning in the sense that it is a relatively short lived, and intense burst or line of snow, much like a thunderstorm is. pic.twitter.com/mrvDW0cE5h

— Stacey Pensgen (@WHEC_SPensgen) December 5, 2019

The Day Cloud Phase Distinction RGB can be useful in identifying bands of potentially heavier snowfall early in their development, as well as in tracking said bands in time with radar imagery (or in the absence of adequate radar imagery). One-min GOES-16 imagery was available over the region for this event, providing near-real-time satellite information.

GOES-16 Day Cloud Phase Distinction RGB imagery highlighted clouds associated with earlier snow squalls accelerating southeast away from Lake Ontario between Rochester and Syracuse (Fig 2). Most of the scene appears as cyan color, signifying primarily low-level, liquid cloud tops given near equal moderate-high contributions (higher reflectance) from 0.64 um (green) and 1.6 um (blue), with low contribution (relatively warm brightness temperatures) from the IR window (red). The snow squall, however, appears as a localized area of bright green given a lack of contribution from the 1.6 um (blue) component, due to ice at the cloud top, in contrast to the surrounding stratus deck. The region associated with the snow squall also appears to have weak convective elements per the high spatial detail contribution from the 500 m 0.64 um band. While light snow was observed to be falling across areas under the low stratus (cyan areas), the area with obvious glaciation and convective elements had the heaviest observed snowfall and gusty winds, and resulting reduced visibility.

Also plotted in Fig 2 are GOES-16 Derived Motion Wind vectors, capturing a 30-40 knot progression of the squall. These speeds are similar to the observed wind speeds at the surface.

Underlaying the Cloud Top Phase derived product from GOES-16 allows for a cloud top phase determination to be output along with the RGB information, when sampling in AWIPS (Fig 3). In this case, the derived product is indeed indicating ice where we expected based on the RGB interpretation, and supercooled liquid droplets in the cyan zones.

A loop of the Cloud Top Phase product shows the progression of the “ice” area (red), though maybe not the full areal extent as can be diagnosed from the RGB (Fig 4).

Later in the afternoon, another area of apparent significant cloud top glaciation is observed developing over Lake Ontario, expanding as it accelerates southeast and onshore (Fig 5). Subsidence in the immediate vicinity of this weak area of convection is also observed via cloud clearing.

The CTP product again also captures the area of increasing cloud top glaciation (Fig 6).

Bill Line, NESDIS

A mid November Rocky Mountain low pressure system and series of associated disturbances brought rain and snow to parts of the Rockies and adjacent plains 20-22 Nov 2019.

By the morning of 21 Nov, analysis of GOES-16 upper-level water vapor imagery revealed the broad upper low centered over southern Nevada, with a more potent shortwave trough rounding the base of the broad low, lifting northeast across southeast Arizona (Fig 1). An associated jet was also rounding the base of the low with it’s entrance region poking northeast into New Mexico. A separate, localized jet was analyzed across eastern Utah. A cold front associated with the previous day’s shortwave trough was diagnosed in the imagery pushing south across E New Mexico, Texas, and Oklahoma through the morning. Finally, trailing west-east oriented energy associated with the northern plains shortwave trough is apparent in water vapor imagery slowly sagging south across the northern Rockies/High Plains, marking the transition from moist atmosphere (south) to very dry atmosphere (north).

Of particular interest was the apparent “fanning out” of cold (high-level) clouds over Utah/Colorado/Wyoming in the northeast quadrant of the broad low and interface with the northern energy. Overlaying the upper-level (above 350 mb) GOES-16 Derived Motion Winds (DMWs) in Fig 1, a GOES-R baseline product, this “diffluence” pattern becomes quite obvious in the wind field, with apparent divergence in wind direction and convergence in wind speed northward (>60 knots to <30 knots from south to north across Colorado). Areas of upper level diffluence such as that in this example don’t necessarily represent vertical motion in either direction in the atmosphere below it given competing convergence/divergence aloft considering geostrophic balance. This particular region was experiencing a relative lull in precipitation coverage and intensity during this period between stronger large scale forcing mechanisms. Isolated light to moderate precipitation was still observed over the higher terrain given favorable moisture and orographics, along with possible larger scale lift near the Utah jet steak.

Visible imagery during the morning across the region was quite difficult to interpret given recent snowfall and various cloud layers all presenting similar reflectance.

By combing the 0.64 um, 1.6 um, and 10.3 um bands into the Day Cloud Phase Distinction RGB, we can easily distinguish the bare surface (dark blue), snow (green), low clouds (cyan), and high clouds (red). Widespread low cloud cover is apparent over the high plains in the present of easterly upslope surface flow in the wake of the cold front.

Bill Line, NESDIS