Wednesday, November 18, 2015

November 20-22 Winter Storm

A winter storm is expected to impact portions of the Plains, Midwest and Great Lakes over the November 20-22 timeframe.

Tropical Tidbits
The GFS model is insistent on a strong, yet compact low pressure system bringing accumulating snowfall to the North US. Using a 10:1 snow-to-liquid ratio, the 12z run of this model drops 4-8" of snow in northeast Nebraska and central Iowa, with 2-5" spread across southern South Dakota, central Nebraska, and the northern and southern sides of Iowa. Amounts in the 6-10" range are then forecasted in northern Illinois, northern Indiana, and southwest Michigan, with lesser amounts into southern Wisconsin, central Illinois, central Indiana, and central Michigan. Some lake effect snow after the storm could enhance totals on the eastern side of Lake Michigan.

Tropical Tidbits
The GEM model has a slightly different scenario. It paints a swath of 5-10" across northeast Nebraska into much of Iowa. Southern Iowa could see amounts near 12" according to this model, before amounts back in the 5-10" range are forecasted in north Illinois, north and central Indiana, and southern Michigan. This model is notorious for over-doing snowfall amounts, and has historically not performed well compared to other model guidance with respect to snowfall amounts, storm strength, and storm track.

It does appear the GFS and GEM models are in a snowier 'camp' together, with both of them laying down a swath of significant snowfall. The NAM model is against this snowier forecast. Since the system responsible for this threat will not be onshore until Thursday evening, and these early-season storms can have low ratio snows, it would be wise to stick with lower-end guidance for the time being. These concerns are also reflected in the Area Forecast Discussion from the NWS office in Chicago:

SOME FACTORS UNDER CONSIDERATION IS
THAT THE MODELS ARE TRENDING TOO HIGH WITH RESPECT TO SNOW TO LIQUID
RATIOS WHILE SFC AND GROUND TEMPERATURES WILL HAVE A BIG IMPACT ON
ACCUMULATION RATES.

The Chicagoland office expects 4 to 5 inches of snow for that area, using 7:1-8:1 to 9:1-12:1 ratios across their forecast office. I do believe this is a reasonable forecast, given the low-ratio nature of the system.

To summarize:

- A winter storm is expected to impact the area over the November 20-22 timeframe.
- Snowfall amounts generally in the 2-6" range can be expected in portions of the Plains, Midwest, and Great Lakes.

Andrew

2 comments:

Anonymous said...

Thank you Andrew for keeping us safe! We really appreciate you & all you do!

I do prey it's a miss though!! I'm wanting 70's & dry for the entire year 2015 & 2016!!

Here's to wishing :)

bree

Christopher Ebie said...

Thanks for the report, Andrew. It looks like the snow could go either way in south central Michigan.