VIIRS True-Color (above) and False-Color (below) imagery (available as an LDM pull from CIMSS) over the western Great Lakes at 1753 UTC (NOAA-20), 1844 UTC (Suomi-NPP) and 1937 UTC (NOAA-20, again) show the recent expansion of late-winter snowcover into southern Wisconsin and northern Iowa. Additionally, ice over the lake (more easily differentiated from clouds in False-Color imagery because the use of the 2.25 µm band in the ‘red’ component of that RGB means that ice/snow acquire a cyan or blue coloring, whereas non-glaciated clouds are white) is apparent, especially near the shoreline.

True Color imagery exploits the presence of the 0.55 µm band (A true ‘green’ band) on VIIRS. Thus, relationships between red, blue and ‘veggie’ bands are not needed for the creation of a simulated green band to make a true color imagery.

---

VIIRS True- and False-color imagery is also available at the VIIRS Today website.

View only this post Read Less

VIIRS True Color and False Color RGB images from Suomi-NPP and NOAA-20 (above) revealed narrow swaths of lake effect snow cover — which appeared as shades of cyan in the False Color images — downwind of Milford Lake (located NW of Junction City) and Turtle Creek Lake (located NNW of Manhattan) in northeastern Kansas on 22 February 2022.

A sequence of Tulsa radar 0.5-degree base reflectivity images at 30-minute intervals (from 0700-1800 UTC) showed the lake effect bands streaming southeastward from those 2 lakes — followed by a toggle between Suomi-NPP VIIRS True Color and False Color RGB images at 1903 UTC (below). A northwesterly flow of cold arctic air (in the teens to single digits F) across the still-unfrozen water of the lakes picked up enough moisture to create narrow cloud bands that produced the accumulating snowfall.

View only this post Read Less

NUCAPS profiles from NOAA-20 provide a widespread swath of thermodynamic information that occurs (over CONUS) when normal radiosondes are not available (for example, around 1800 UTC as above). In addition, NUCAPS provides information in regions away from the sparse radiosonde network. The image above shows NUCAPS sounding availability points over Wisconsin; southern Wisconsin is forecast to receive freezing rain and ice pellets overnight. What do NUCAPS soundings, and gridded NUCAPS fields show?

Gridded NUCAPS temperature fields (at 850 mb and 925 mb) are shown below. The color enhancement has been altered so that values around the melting point (0^o C) are black. The NUCAPS fields are capably capturing the strength and location of the low-level inversion: note how much farther north the warm air is at 850 mb compared to 925 mb! This information might be useful is diagnosing regions most at risk to accumulating freezing rain. (One might look at gridded dewpoint depressions fields as well to determine where evaporative cooling might occur).

And individual NUCAPS profile near 43 N, 89 W, shown below, shows a very strong surface inversion.

How does the NUCAPS profiles compare with a forecast profile? The GFS 6-h forecast profile, taken from the tropicaltidbits website, shows that NUCAPS is viewing a somewhat dryer airmass than is present within the model. NUCAPS profiles give information about moisture that might be missing (or mis-timed) in numerical model output. The horizontal extent of the dry air in NUCAPS is large, as shown in the gridded NUCAPS 850-500 mb layer relative humidity analysis shown at bottom.

---

Added, 22 February 2022, 2:22 PM: NUCAPS provided another view of the thermodynamics over southern Wisconsin after noon, as shown below. Thick clouds over eastern Wisconsin prevented the retrieval from converging to a solution there (hence the red points), but three green points near/over eastern Dane County provided useful information. The sounding on the Dane County/Jefferson County border at 43^o N, 89^o W (shown here) continued to show warm air aloft that would allow for continued freezing rain/drizzle.

What did the gridded NUCAPS Temperature field at 850 mb look like? As shown below, it continues to show a warm tongue into southern Wisconsin; however, caution in interpreting this field is warranted. Many of the points that were used to create the field were from ‘yellow’ retrievals (i.e., microwave retrievals that might be unable to resolve a low-level inversion) and ‘red’ retrievals. Thus the gridded field might not represent the true values there.

View only this post Read Less

1-minute Mesoscale Domain Sector GOES-16 (GOES-East) Day Cloud Phase Distinction RGB images (above) displayed a narrow band of glaciated (brighter shades of green) clouds that produced snow squalls as it moved rapidly eastward across Pennsylvania on 19 February 2022. Due to the fast motion of the snow squall band, resultant snowfall amounts were generally 1 inch or less at most locations — but the visibility often quickly dropped to near zero due to intense snowfall rates and gusty winds. For example, as the snow squall moved through State College just after 16 UTC, the surface visibility went from 10 miles to less than 1/4 mile and then back to 10 miles within 30 minutes (below).

Here’s a timelapse of the snow squall as it passed through State College, shot from our office window #PAwx pic.twitter.com/cJfpjXTCxm

— NWS State College (@NWSStateCollege) February 19, 2022

In far eastern Pennsylvania, snow squalls caused a 50-vehicle pileup on Interstate 81, closing the section between the McAdoo and Hazelton exits for about 6 hours (media report) — the accident occurred as the snow squall band was moving through that area around 18 UTC, as seen in a closer view of GOES-16 Day Cloud Phase Distinction RGB images (below) .

In a before/after toggle between GOES-16 Day Cloud Phase Distinction RGB images at 1724 UTC and 1900 UTC below), the dark blue signature of bare ground in the Hazelton area prior to the arrival of the snow squall was replaced by a darker green signature of snow-covered ground.

As was seen with the snow squall passage at State College, a brief but rapid drop in surface visibility occurred at Hazelton (below) as the squall passed through around 18 UTC.

View only this post Read Less