A narrow, but intense snow band formed over an area stretching from central Iowa to southwestern Wisconsin over the the night of 19 February into the morning of 20 February. Localized accumulations reached a foot in parts of northeast Iowa. This map from the La Crosse, WI, National Weather Service office shows the extent of this snowfall. (A lengthier discussion of this event from NWS La Crosse is available here).

A week later, this narrow corridor of snow is still around and still exerting influence on local weather conditions. Take a look at the following plot of surface temperatures from AWIPS in the early afternoon of 27 February 2026. North-central Iowa is experiencing temperatures in the lower 60s while parts of eastern Iowa are even reaching 70. However, there’s a cooler alley in northeastern iowa where temperatures are in the low-to-mid 40s.

Without any context, an analyst might opt to put some sort of synoptic-scale feature in northeast Iowa and assume that larger-scale flow is causing this pool of cooler temperatures. However, this being the CIMSS Satellite Blog, we’re going to recommend that you check out contemporaneous satellite imagery to verify what might be happening. Here’s the same figure, this time with the 0.64 micron highest resolution satellite imagery included as the base layer.

It’s clear that the surface snow cover is exerting a downward influence on temperatures. But why? There are two main reasons. The first is that snow has a high shortwave albedo: it reflects a substantial fraction of the light that shines on it. The albedo of new fallen snow can be as high as 90%, thoough for older snow such as this it’s much closer to 40 or 50%. Still, that is a substantial fraction of the incoming solar heating that is redirected away from the surface and cannot help to change the temperature. The other impact of snow is through phase changes. The solar energy that isn’t reflected from by the snow goes into changing its phase through melting or sublimation.

Here’s a pair of animations captured from AWIPS (thus the color dithering is not ideal). The first shows the true color RGB with the surface temperatures, while the second depicts the Day Cloud Phase Distinction RGB. Both are useful for identifying the location of the snow band surrounded by bare ground, while the latter animation helps discriminate between the snow (green) and ice clouds aloft (pink).

Finally, we should also take a look at the impact of the snow band on nighttime temperatures. While the earth’s surface generally has very high infrared emissivities, surface snow has a near perfect emissivity up to 0.99. This means that the snow is a very effective emitter of infrared radiation, more so than other surface types. Thus, we expect snow-covered ground to be colder at night than uncovered ground. This, of course, is on top of the fact that the snow-covered ground didn’t get as warm during the day as the other locations did, so it’s already starting from a lower temperature. This image shows the Band 13 (10.35 micron) imagery for 12:40 AM on the 27th. The color table has been adjusted to enhance contrast. Recall that for this window channel, in the absence of clouds we’re reading the brightness temperature of the surface. That surface is colder where the snow is, and the surface temperature observations back that up.

View only this post Read Less

5-minute CONUS Sector GOES-19 (GOES-East) True Color RGB images created using Geo2Grid (above) showed aircraft glaciation trails drifting east-southeastward over southern Lake Michigan on 25 February 2026. As aircraft ascended/descended through a thin cloud layer composed of supercooled water droplets, additional cooling from wake turbulence (reference) — and/or particles from jet engine exhaust acting as ice condensation nuclei — caused the small supercooled water cloud droplets to transform into larger ice crystals (many of which then fell from the cloud layer as snow).

GOES-19 Day Cloud Phase Distinction RGB images (below) revealed that the cloud layer which the aircraft penetrated was composed of supercooled water droplets (shades of violet to purple) — while the ice crystal aircraft glaciation trails exhibited shades of green.

Cursor samples of the GOES-19 Cloud Top Temperature (CTT) derived product at 2 locations along the undisturbed supercooled water droplet cloud between aircraft glaciation trails (below) showed that while those CTT values were quite cold, they were still a few degrees warmer than the -40ºC temperature required to assure spontaneous freezing via homogeneous nucleation.

In a comparison of GOES-19 Shortwave Infrared brightness temperatures over an undisturbed portion of the supercooled water droplet cloud vs within an aircraft glaciation trail (below), enhanced solar reflection off the small spherical supercooled droplets produced a 3.9 µm temperature that was about 11ºC warmer than that of the ice crystals within a glaciation trail.

View only this post Read Less

The early morning hours of 21 February 2026 brought some intense tropical convection to the vicinity of the Samoan Islands. Radar coverage is lacking in this part of the world so satellites remain the best way to monitor storms in this region for potential safety hazards. A good first stop is checking the NUCAPS vertical profiles for a location to gauge the potential for convective instability. Since this post was written a few days after the event in question, our standard sources for NUCAPS profiles no longer have the relevant data, but the NOAA OSPO HEAP site allows us to go back 20 days to review the satellite-observed skew-T profiles. American Samoa is in the UTC –11 time zone (almost as far away from UTC as one can get) so this profile is around 1:20 AM local time. NUCAPS profile retrievals are applied to collocated infrared and microwave sounder observations. The microwave (MW) observations are able to see through the clouds but lack the vertical resolution of the infrared (IR) observations. The highest-quality profiles include both the MW and IR, but if the clouds are too thick, the retrieval algorithm will default to the MW only. The following plot shows both the MW-only (blue) and the MW+IR (maroon) profiles.

It’s interesting to see how the two different methodologies result in two very different levels of instability. The MW only retrieval has a CAPE of 3643 J/kg while the MW+IR retrieval brings it down all the way to 0. The biggest difference between the two appears to be differences in near-surface temperatures and the presence of a mid-level inversion between 600 and 800 hPa. However, those low-level temperatures appear to be an underestimate, especially for the MW+IR retrieval. At 1200 UTC, the surface temperature and dew point at the Pago Pago, AS, airport were 30/25 C respectively. That’s much warmer than NUCAPS estimated. If you append those values to the bottom of the sounding (the mean surface level pressure was 1005 hPa, and since the elevation of the airport in Pago Pago is just a few meters above sea level, the station pressure is basically the same as MSLP) you’ll see that the profile becomes much more positively buoyant. Here’s a hand-drawn illustration of that, with the new temperature and dew point added in red and the corresponding area of positive buoyancy shaded in orange. At the very least, it is likely that the CAPE value measured by the microwave-only retrieval is closer to the truth than the 0 value returned by the MW+IR retrieval.

LightningCast captured the potential for significant lightning from this storm. The following animation shows the LightningCast contours overlaid on top of the Band 13 infrared imagery from GOES-18. The occasional blue patches represent the GLM-observed flash density, helping to verify LightningCast’s predictions.

The deep reds of the Night Microphysics RGB also help indicate the presence of deep, thick convective clouds.

Together, the thermodynamic observations from the polar-orbiting satellite and the continuous, time resolved imagery from the geostationary perspective help offer a comprehensive portrait of the intensity of this event.

View only this post Read Less

5-minute CONUS Sector GOES-19 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) showed the evolution of a large and intense Nor’easter that produced as much as 37.9 inches of snowfall in Rhode Island (a preliminary State record) and wind gusts as high as 84 mph in New York (WPC Storm Summary) during the 22 February – 23 February 2026 period. The combination of high snowfall rates (up to 3 inches per hour) and strong winds produced widespread blizzard conditions.

A slightly closer view using 1-minute Mesoscale Domain Sector GOES-19 Mid-level Water Vapor images (below) included overlays of GLM Flash Extent Density and GLM Flash Points, along with plots of surface weather symbols and peak wind gusts (note that there was a brief outage of GOES-19 imagery on 23 February). Isolated inland GLM signatures suggested that some brief thundersnow may have occurred (0335 UTC | 0529 UTC | 1036 UTC | 1057 UTC) — in fact, regarding the 0529 UTC GLM signature, thundersnow was reported at nearby Teterboro, New Jersey (KTEB) and Manhattan, New York (KJRB) 3-9 minutes later (in their 5-minute METARs).

5-minute GOES-19 daytime True Color RGB + Nighttime Microphysics RGB images from the CSPP GeoSphere site (below) displayed the Nor’easter during the 22-24 February period.

View only this post Read Less