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Dec 29-Jan 2 potential storm event


Brian D
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The HRRR is pretty to look at, but it's counting on colder thermals keeping the sleet farther south.  Other models are easily surging sleet up through east-central Iowa.  I'm not going to bet against the warm air aloft.  4-8" seems reasonable for Cedar Rapids.

The Friday wave is trending back nw again, but, coming from the gulf, it's too darn warm.

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Wanted to take a little break from the model bombardment. Our players are about halfway on the field now. The first system has given LA an early AM round of thunderstorms. 

That ridge (zigzag) off the West Coast pumps back up quickly after the next trough passes through and then digs into Mexico (similar to the Friday wave), thanks largely to the persistent upper low to the southeast of Hawaii. All of that teleconnects to ridging along or just off the East Coast.

There's a lot of convection in the tropics/sub-tropics playing into the pattern, more than what can be shown on this satellite. Once the Friday wave digs into Mexico, the ridging ahead of it (over the eastern Gulf, Florida and the Bahamas) will likely be beefed up by convection developing over the warm Gulf of Mexico. That process is partially what allows the warm air aloft to flood north ahead of the upper low, bringing the risk for mixed/freezing precip as high pressure to the north continually feeds lower level cold/dry air. 

WV_Pac.thumb.png.ab156ec4454769817847a61a4ba07e63.png

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Get it Chicago Storm. That LOT morning AFD can sure fire a snow starved weenie up. Pretty sure its finally happening again.  A meaningful  Bears/Pack game and a Border Battle for the Snow in the same week.  Feeling naively optimistic Geo's magnet and SE trend can team up to Save the Palm Tree's and drop a nice surprise further south.

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our buddy with and update

National Weather Service Chicago/Romeoville, IL
1032 AM CST Mon Dec 28 2020

.UPDATE...
1032 AM CST

No big changes to the forecast for tomorrow-tomorrow night`s
wintry precip event. In addition to a decision on the Winter
Storm Watch for Winnebago, Boone, and Ogle Counties, will be
needing a Winter Weather Advisory for a good chunk of the
remainder of the CWA, likely including the entire Chicago metro.
Will defer these decisions to the full afternoon forecast package
issuance.

Looking at the 12z suite thus far, noting a tendency for pretty
rapid top-down saturation of the initially dry cP air mass in the
wake of departing high pressure, so envision a bit earlier onset
than in current official forecast. Ingredients discussed in the
short term discussion from the overnight shift are certainly
there, with their longest duration and impacts generally where the
watch is in effect. Forecast soundings indicate that the snow
will likely fall at a moderate to even heavy clip along and north
of the Kankakee River (shorter duration south of I-80), with strong
omega well aligned with the DGZ into the early to mid evening.

Onset time of the snow will be just prior to and during the
afternoon commute hours, so travel impacts are likely. With the
strong LLJ driving strong WAA, eventually a warm nose will surge
in aloft during the mid to late evening, changing precip over to
sleet and then freezing rain, latest in the far north where snow
should hold on a bit longer. Given the accumulating snow, followed
by sleet and then up to to 1/10-2/10" ice, hazardous travel can
be expected, warranting expected advisory issuance and perhaps
upgrade to warning where a watch is in effect.

Castro
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