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THE CALM BEFORE THE SNOW...


THE CALM BEFORE THE SNOW

After a mild but wet day Tuesday, the Midwest is situated between storms Wednesday. Winds have turned to the northeast drawing in the colder air that will lead to our next system being a snow producer. Tuesday evening's satellite imagery shows the energy approaching the 4 corners region while Tuesday's storm lifts northeast.

As I write this, the NWS has a winter storm watch in effect for late Wednesday night through the day Wednesday. At some point Wednesday depending on new data, the watches will likely be upgraded to warnings where the heavier snow is expected, to winter weather advisories where amounts will be lighter further southeast

The question on my mind currently is the amplification of the storm and the impacts of dry air on the NW fringe. As for amplification, the energy coming out of the southwest is shearing some as the closed 500mb low attempst to break into an open wave. That indicates little in the way of intensification (or deepening) of the surface low as it travels northeast. That keeps the storm from really getting wrapped up and might allow for a slight southeastward shift in its track.


Something else to keep an eye on is the dry air entering the northern flank of the storm. (It's coming in on the NE winds behind Tuesday's storm). Not only will this create a sharp cut-off to the north edge of the snow band, it could cut into totals in my far NW counties and nudge the maximum snow band a hair southeast. You can see the dry air in place over northern Iowa Wednesday evening. It may be tough to displace.

At their best, PWAT's (available water vapor) never get much higher that .55". That's much lower than last weeks storm which maxed out close to 1.40" and dumped 9" of snow in Dubuque. What I'm saying is this is not an overly moist system.

On a positive note for snow, temperatures will be lowering during the event allowing snow ratios to reach 12:1 before the snow ends. Last weeks ratio's were more like 7:1 making for a super heavy wet snow. This one should be drier, especially the second half of it allowing it to fluff up more.


Another plus in favor of a healthy snow band is the 850 low the 3k NAM tracks through southeast Iowa. Just northwest of its path is the sweet spot for frontogenic forcing and banded snow. This is a critical feature to watch in any heavy snow event.

SO WHAT'S THE VERDICT?


Going back to the latest satellite. I like the impressive display of energy over the southwest. That looks formidible with good inflow and ventilation. It implies that the 3k NAM and GFS with its well defined 850 circulation over SE Iowa Thursday morning could have merit. Unfortunately, the EURO came in less organized and further south with the 850 circulation. That shifts the heavier snow band (which is not as impressive on the Euro) about 75 miles further southeast. A concern I mentioned earlier

Let's go to the models and thier latest runs. While these are not specific forecasts, thier trends give us important clues as to where the road leads us. This is the latest data avaiable to me. I will comment below.


THE GFS

The 3k NAM

The 12k NAM

The EURO

The Canadian

All the evidence on the table tonight suggests a solid 3-6 inch snowfall for areas near and NW of a line from about Mt. Pleasant to the Quad Cities and on to Sterling/Rock Falls. Where banding occurs, some places "could" reach 8 inches but the axis of such a swath remains a bit in doubt. The latest EURO (see above) has the heavier amounts (6" plus) from the Quad Cities towards Rockford. I am thinking that may be the way to go. Southeast of the Quad Cities precipitation may briefly start as some freezing rain or sleet before going over to snow. This area looks good for 1-3 inches with the lower amounts the further southeast you go. There is still some time for alterations and the fact the EURO is further SE than the U.S. based models gives me pause. I really hate to bet against the EURO this late in the game. We shall know more and be able to fine tune totals as Wednesday unfolds.


As for timing, snow or mixed precip. does not begin until late Wednesday night and should taper of and end in all areas by early evening. Northeast winds could exceed 30 mph at time causing blowing and drifting in reduced visibility, especially in open areas.


A COLD START TO THE WEEKEND

Behind the system the weekend starts cold. Lows Friday morning will be in the single digits north of I-80.

Wind chiils should fit into the range of 5 below north to 5 above south.

Highs Friday will remain in the low to mid 20s but the core of the cold quickly retreats and temperatures moderate into the 30s in snow covered areas to the 40s in the far southeast where little exists.


Long term, this active pattern looks to be in place for at least another week with a ridge over the east and a trough anchored over the west that will regularly eject energy into the pattern. The potential is there for well above normal precipitation and depending on track, some of these systems could deliver snow but recent guidance suggests the brunt of that stays just northwest. Overall, temperatures after Friday look to be near or even a bit above normal. Stay tuned for updates on the coming storm Thursday. Roll weather...TS



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