WINNER, WINNER, SNOW FOR DINNER...
After some very erratic runs, the models eliminated much of the noise Friday and settled on a solution that does indeed bring snow to much my of my area Sunday night and Monday. There was never a doubt about a storm, but the GFS and EURO went round and round determining the thermal profiles. To their credit, the Canadian GEM and 12K NAM have been locked in on the snow threat and now everybody is seated at the table and snow is on the menu.
This is all tied to an active storm track that forces energy to cut beneath a block in SC Canada. In the 500mb jet stream depiction you can see not only the Sunday night Monday system, but another that comes right behind it in a similar fashion the middle of next week.
Both systems are what we call cut-off lows which tend to be dynamic and slow moving. The allows them time to ingest moisture and often makes them significant precipitation produces. As wave lengths shorten this time of year we start seeing cut-offs on a regular basis which look like bowling balls as they roll across the lanes of the nation. In fact, I like to say we are now entering into bowling season. Get your shoes on. Note all the precipitation that stretches across the center of the country in the pattern.
With the upper level block in place over Canada you can also see how the cut-offs prefer to run west to east, taking the path of least resistance instead of cutting northeast into the block (a more typical storm track earlier in winter). In the graphic below the 7 day precip. departures show dryness in Canada and the southern U.S. as the bowling balls play over the center of the nation.
GETTING BACK TO THE STORM
Back to the topic of the Sunday night Monday storm, it appears that in my region, the area near and north of I-80 will see the greatest accumulations, something I anticipated the past couple of days. Locations near and north of HWY 30 stand the best chance of seeing up to 6 inches of heavy wet snow, locally a narrow band of 7-8" is possible where the strongest forcing is maximized. Most likely that is close to HWY 20, especially in Iowa. At some point I expect there is a good chance a winter storm watch may be issued for this area, at the very least a winter weather advisory. That's an NWS call that should be made Saturday.
The critical element in this event is evaporative cooling. How much and how soon will determine snowfall amounts. Initially, as precipitation develops temperatures will be warm enough aloft and at the surface for rain to start Sunday night. As the energy approaches and lift and precipitation increases, the air will cool from the top down as colder air is drawn into the system on easterly winds. This will happen last in my southern counties so amounts will be lighter here. This area could also see a period of freezing rain but confidence on that is low.
So now, I present you with what you've all been waiting for, the model snowfall forecasts. Keep in mind this is just raw model guidance, and are not official forecasts, just trends that forecasts will be derived from closer to the actual event. Consistency is what you want to see.
The EURO
The GFS
The experimental GFS parallel
The Canadian GEM
The 12K NAM
The GFS is still the furthest north but until this latest run it was even further north and a clear outlier. Now it's getting in the game ( I think it will still come a bit further south) and I'm sure you can see we are starting to experience good confidence develop in the overall solution. I hope that continues!
Right now the two factors that could cause problems are intensity and dynamic cooling. As you can see the snow totals are higher in Iowa than Illinois because the disturbance fills and slowly weakens as it comes east. That reduces the amount of liquid precipitation and snowfall. If the system weakens faster that could have implications for lower amounts, especially in Illinois.
The other issue with a slightly weaker system is the inability for evaporative cooling to be as significant and that could cut into amounts as well. So far, models are not indicating these issues but as a forecaster you look for problems and I'll be watching those trends the next 48 hours.
Last but not least, the midweek system that follows also looks strong and has more of a classic look with a well organized surface low attending the upper air support. The GFS shows another dynamic cooling set-up that changes rain to snow NW of the surface low track.
It also shows some additional snow of a significant nature. See below.
Confidence is low as to how this ends up with other models such as the EURO not nearly as excited about the intensity and snow threat. For now this system is on the back burner until Monday's passes to the east. That's where things stand late on a Friday night. Updates later Saturday...perhaps a winter storm watch for parts of the area? Roll weather...TS
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BOOK ENDORSEMENT.
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Jolene Kronschnabel-Director of Hawkins Memorial Library, La Porte City, Iowa
