Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131813

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1213 PM CST Tue Feb 13 2018

227 AM CST

Through Tonight...

The second straight day of quiet weather will unfold with little
of concern through tonight.

Quite a bit of high cloudiness will be over the area through
today per upstream satellite imagery, which adds a challenge for
temperatures (a theme through this week). Yesterday the deep
fresh snow cover and its much higher albedo drove sharp
temperature contrasts (e.g. 12 degree difference in highs from
Sugar Grove to Midway Airport). Today this contrast should be
much more negligible due to clouds although the uncertainty
exists in how warm conditions will get. While 925mb temperatures
would support mid-upper 30s, cannot see that happening outside of
the southern CWA given clouds and a snow pack with a temperature
below 23 degrees (per NOHRSC data). Parts of north central
Illinois likely will only achieve the mid 20s. Even though a light
south wind tonight, temperatures could dip sharply in the evening
with the forecast cloud departure.



227 AM CST

Wednesday through Monday...

The time period of interest for the rest of this week is
Wednesday evening through Thursday, with the main impacting
weather to be fog, and certainly potential for that to be dense.
With marginal temperatures with respect to freezing in northern
locations, there could be a brief risk of some slick elevated
surfaces Thursday morning. It still is a non-zero chance of light
freezing precipitation Thursday morning, although the far greater
likelihood is for fog.

Low pressure tracking across the Upper Midwest on Wednesday will
increase southerly flow and low-level warm and moist advection.
Temperatures will again be challenging as a majority of model
members want to try to push a good part of the CWA into the lower
to mid 40s. While that is certainly likely in the south, have
gone closer to some cooler raw (non-MOS) guidance for northern
and central CWA locations keeping them just shy of 40. Early-mid
February climatology for the forecast 850 and 925mb temperatures
offer very few cases with this deep of snow cover, but those that
do have Chicago peaking around 40 as an average.

Low-level isentropic surfaces from the suite of guidance support
rapid stratus spread in during Wednesday afternoon and some
drizzle or patchy light rain possibility continuing into the
evening. Temperatures are likely to at least hold steady if not
inch up in the evening with dew points increasing to the mid-upper
30s (10-20 degrees higher than the snowpack temperature). By
overnight, guidance shows low-level flow off the deck turning more
westerly and pushing some of the low stratus away. While initial
fog may be more advection character, this stratus shove may
actually help with some radiational component into the overnight.
Do think the overall signal for dense fog is fairly strong and
continues into Thursday morning when a secondary low pressure
develops over the central Plains and approaches. Temperatures for
Thursday morning, which tie into just how warm we get Wednesday
afternoon and evening, look to be at least around freezing in
north central Illinois. GFS Ensemble (GEFS) have 60% of their
members for RFD with below freezing temperatures for instance. So
some potential for freezing on elevated surfaces with the fog will
exist, if indeed the fog and temperatures pan out as forecast.

The signal for precipitation really does not come back until mid-
late morning Thursday and more so afternoon as the aforementioned
low pressure nears and 850mb moisture transport returns. The good
news with this later morning time is that it may limit or nix the
light freezing rain threat, although it is still in the envelope
of possibility given the uncertainty on temperatures. Winds will
turn northerly across the northern CWA during the afternoon,
though the advection will not strengthen until the evening behind
the low. Any rain amounts looks fairly minimal, with the deeper
saturation for heavier rain staying to our southeast, which is
good news with the amount of snow cover. For any snow Thursday
night, that signal continues to weaken, with very limited
wraparound moisture and deformation development, so if it is
realized it should be light and probably brief.

Cold advection will continue into Friday and temperatures could
quite possibly fall into the teens during the morning before
minimal recovery. After high pressure prevails into Saturday,
attention then turns to what could be an active first half of next
week. Global guidance solutions, with teleconnection support, do
indicate a western U.S. long wave trough with a highly anomalous
ridge offshore of the southeast U.S. The ECmwf is more amplified
with this through Monday than the GFS, bringing us three systems
each day from Sunday-Tuesday. The GFS holds back some of its
energy until midweek, spacing out the action. There would be
concerns for some heavier rain, or at least cumulative rain
totals on melting snow cover, as well as a threat of some mixed
precipitation too. The uncertainty in temperatures is not
surprisingly very large as captured by the GEFS temperature plumes
forecast for the local area. We will just have to wait for
further detail opportunity as we get later this week into the



For the 18Z TAFs...

There are minimal aviation concerns in the near term. Fog and low
clouds will eventually become a concern, possibly during the
latter portions of the ORD/MDW 30 hr TAF, but more likely beyond
this time frame.

High pressure will move east through tonight, and this will allow
SSE winds to shift to SW later today and tonight. High clouds will
dissipate. Tomorrow lower clouds will shift towards the region,
likely impacting areas across Central Illinois during the day. At
this point confidence is low as to if any lower clouds (MVFR/IFR)
will get into the terminals tomorrow afternoon, as guidance is
mixed. With the potential there, have included a SCT low MVFR
deck tomorrow afternoon. This will be the period to watch for the
arrival of lower clouds which could be IFR. SW winds will increase
above 10 kt tomorrow also.




523 am...High pressure over the eastern lakes will move to New
England today. The gradient will tighten today with southwest
winds increasing to 30kts and these wind speeds will continue into
Thursday morning. There could be some periodic gale gusts mainly
over northern portions of Lake Michigan including tonight into
Wednesday morning. Low pressure will move across the upper lakes
Wednesday night into Thursday. A trailing cold front from this low
will move south across the lake on Thursday with winds shifting
northerly and increasing to 30kts. Another strong high pressure
will build across the northern Plains Thursday night and then move
east across the southern lakes region Friday night. cms





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