MIMIC Total Precipitable Water fields for the 24 hours ending 1800 UTC on 21 October 2024, above, show a stream of rich moisture pulled northward into Alaska ahead of a strong storm over western Russia. The result was large snowfall rates over much of central and northern Alaska, as shown in the snowfall rate from this website (here is a Quick Guide on MIRS Snowfall rate) animation below. Two principle bands of snow are indicated, one over eastern Alaska, oriented south-southeast to north-northwest, and one over north-central Alaska, oriented by southeast to northwest.

There are NOAA-20 and NOAA-21 overpasses in the animation above, and data (infrared and microwave) from those satellites can be used to create vertical profiles of temperature, that is, NUCAPS profiles. Those profiles can be gridded to show swaths of thermodynamic variables, such as the 850-mb temperature field shown below. The colorbar has been edited in AWIPS so that values near 0^oC (i.e., the melting point), are black. However, many regions show temperatures at 850 mb colder than 0^oC, but with rain observed at the surface! A possible reason in this discrepancy is that the data being used in the gridding in central AK is from soundings that did not converge, or from soundings that include only microwave data (that is, the infrared retrieval did not converge to a solution). See the image at the bottom, showing ‘Data Quality’; yellow shows profiles for which the infrared retrieval failed (but the microwave retrieval did not) red are regions where both infrared and microwave retrievals failed. It’s also possible that some of these points are below ground! There are many things to consider when interpreting NUCAPS temperature fields.

Just how unusually moist is this airmass over Alaska? Percent of Normal fields, from this site, show values in excess of 200% of normal!

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VIIRS infrared imagery (I05 imagery at 11.45 µm) from the NOAA-21 (mislabeled, alas, at Suomi NPP) show high-resolution infrared imagery at about 90-minute time-steps. Cold Cloud tops associated with a low pressure system bringing light snow to northern Alaska are apparent — but during the gap from 00-08 UTC on 16 October when NOAA-21 does view Alaska, the satellite signature of the system degrades significantly. GOES-18 ABI data is consistently viewing Alaska (and, indeed, Mesoscale Domain #2 from GOES-18 by default gives 1-minute imagery over Alaska), and those scenes time-matched to the VIIRS imagery above (shown below; one could show an animation with much finer time resolution with ABI to show exactly when the organization of the system changed most profoundly.) also show the change in organization to the system over Alaska. This animation (from this website) shows a surface analysis at 12-h time steps on the 15th and 16th.

The two animations above have different scaling and different enhancements, although they have the same endpoints. The ABI imagery is uniformly colder, however, even though the resolution is much finer for VIIRS data. One reason for the difference is limb-cooling effects. The plot below shows how ABI Brightness Temperatures vary as a function of view angle. A very slanted view of the surface, or of a cloud, means a path from the near-surface to the satellite that passes through a lot of the cold upper tropopshere, leading to brightness temperatures.

NOAA-20, NOAA-21 and Metop-C satellites all carry infrared and microwave sounders; data from those instruments can be used to create vertical profiles (NUCAPS Profiles) of temperature and humidity, and those profiles can be gridded to provide horizontal swaths of thermodynamic variables (i.e., ‘Gridded NUCAP’ fields). The animation shows estimates of 850-mb temperatures in swaths over/near Alaska between 1600 UTC on 15 October 2024 and 1400 UTC 16 October 2024. Temperatures colder than -15^oC are widespread over northern Alaska and the Arctic Ocean.

Gridded NUCAPS fields are available outside of AWIPS at this site, and also here.

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CSPP Geosphere imagery, above (link), shows a band of cumulus clouds over central Lake Michigan. The still image, below, from 1600 UTC shows cool air and surface winds converging into this band.

The toggle from 1201 UTC, below, shows the band in the Night Microphysics RGB with NEXRAD reflectivity overlain, and also surface and 850-mb observations. Note the 850-mb temperatures at Green Bay (-2^o C) and at Detroit (-1^o C) and at Gaylord MI (-5^o C).

The temperatures at 850-mb are below zero (Celsius) and Lake Surface Temperatures are in the 17-18^oC range (below, source). That difference is greater than the ca. 13 Celsius degrees needed to support Lake Effect bands.

The Lake Effect season has begun!

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Parts of northern Alaska were under a Winter Storm Warning for much of the weekend, as shown below by the red regions to the north of Fairbanks. (The red regions around the Seward Peninsula were regions where a High Wind Warning had been cancelled after strong winds weakened. This toggle of MetopB and MetopC ASCAT winds from 0530-0630 UTC on 13 October (source) shows the very strong winds offshore). The Airmass RGB animation (created using Geo2Grid software) above shows the abundant mid-level moisture streaming north and the development of a storm system by 1200 UTC on 13 October as evidence by the comma cloud.

What products can be used to highlight where snow might be falling over northern Alaska, where no radars are present, and where webcams are scarce. Microwave Integrated Retrieval System products include snowfall rate. This product is computed from Microwave sounders on board many Low-Earth-Orbit (LEO) satellites including Suomi-NPP, NOAA-20/NOAA-21, and MetopB/C. The animation below shows values (computed from data downloaded at the GINA Direct Broadcast antenna and processed using CSPP software; created imagery is here). The heaviest snowfall is diagnosed to the south of the Brooks Range through about 1400 UTC (as shown in this image from 0452 UTC, for example), at which point maximum snowfall shifts to the north.

Polar2Grid software can be used to create MIRS snowfall rate imagery with data downloaded from (for example) Amazon Webservice websites that hold Suomi NPP or NOAA-20 data. The toggles below compare the computed Snowfall rates and the airmass RGBs. As is often the case over Alaska, the GOES animations give the evolution of the atmosphere and LEO data gives important smaller-scale information, in this case snowfall rates. The heaviest snow is diagnosed to occur right in the comma head over the near the Brooks Range from 1130-1331 UTC. By 2300 UTC, the heaviest snowfall rates have diminished. MiRS snowfall rates are available in Alaska Region AWIPS machines for a simple comparison to other satellite imagery.

Microwave Snow Fall Rate imagery is also available at this website. The Snow Fall Rate Quick Guide is here.

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