I’ve had some opportunity to view the NESDIS Snowfall Rate (SFR) Products today, in particular, to see how it performs during the central Plains/Midwest snowstorm. These products are being delivered by SPoRT to several collaborative offices in the CONUS and Alaska for evaluation during the current winter.
Background info: the Merged SFR product contains NSSL Multi-Radar Multi-Sensor (MRMS) precipitation data with insertions of polar-orbiter derived precipitation rate data when those are available. The precipitation rate data from the polar-orbiters is available in AWIPS in individual swaths or contained within this merged product (in the merged product, the MRMS data replace the polar-orbiter data). The data are available in AWIPS as liquid equivalent values or a snowfall rate with three distinct snowfall-to-liquid ratios: 10:1, 18:1, 35:1. To learn more about this product, you may click here to see training material provided by researchers at NESDIS and SPoRT.
So, let’s take a quick look at some of the data today and I’ll share a few comments and thoughts. This first image is the Merged SFR product valid at 1130 UTC with METAR plots (yellow) at 12 UTC.
Without any polar orbiting data available at this time, this image contains only the MRMS precipitation data. In the image (Image 1), notice the band of heavier precipitation stretching roughly west-east across southern Nebraska and Iowa, and the relatively tight precipitation gradient in southern Iowa. At the time of this image, notice no snowfall was occurring at the Des Moines location, per the SFR product or the 12 UTC METAR. Pay particular attention to the discrepancy in times between the METARs and the SFR product at this point…there is a 30-minute offset. Now, let’s look shortly later as a swath of polar orbiter data became available.
I have layered the imagery so that the polar imagery swath data are laid atop the Merged SFR product. Notice that the polar orbiter derived data indicate a band of relatively heavier precipitation spreading northward into Nebraska and Iowa. This is important because the polar orbiters observe precipitation within the clouds on average ~30 minutes before it manifests at the surface. In this image (Image 2), notice that this band of heavier precipitation has now spread northward to include Des Moines and points to the west of there, where little to no precipitation was occurring earlier. So, the NESDIS polar data suggested significant snowfall production was translating northward within the mid/upper cloud layer. Knowing the data typically offer about a 30 minute lead time for observations at the surface, a forecaster could have surmised something about precipitation production aloft, intensity and overall storm evolution while obtaining more data about timing to impacts at a metro area.
The next image shows the timing of the arrival of the precipitation at Des Moines per the merged SFR product and the Des Moines surface observation (Image 3).
In image 2, remember that the SFR swath data indicated high snowfall rates, >1 inch/hr (per the 10:1 ratio…which may be understimated) directly over Des Moines and surrounding areas at 1145 UTC, while the Merged SFR above (Image 3) shows precipitation finally entering the city and the observation site at ~1230 UTC. Notice that the Des Moines METAR showed light snow during the 1300 UTC observation (Image 3).
Let me point out something important here. In the Merged SFR product, the satellite derived data are purposely delayed 30 minutes for insertion into the official delivered product. This was decided as the configuration of the official product since precipitation in the satellite derived data typically precede the arrival of precipitation at the surface by about 30 minutes. The thinking being that this apparent discrepancy would be observed between the MRMS data and the satellite derived data, and would lead to forecaster confusion. That is understandable, especially for this latest experimental iteration of the SFR product. However, after viewing these data in a few cases, I think it is advantageous that the satellite derived data contain important information about the evolution of snowfall and precipitation production aloft, well before it manifests at the surface. The fact that satellite derived observations of precipitation rates precede the occurrence of snowfall at the surface by about 30 minutes, and if you noticed, by about one hour in this case, makes these satellite derived swath data operationally relevant and important.