Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 032347

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
647 PM CDT Fri Apr 3 2020

202 PM CDT

Through Saturday night...

The lake breeze is positioned across the area from NW to SE,
fighting a stronger southwest low level wind where inland
temperatures have surged into the upper 60s inland. Meanwhile, a
concentrated area of precipitation is located along a fairly sharp
cold front across Iowa and extending into the southern Plains.
Precipitation upstream of the local area has not just been rain
but snow and freezing rain as the initially shallow/sloped cold
frontal boundary undercuts the warmer air aloft. The upper forcing
is not all that strong, and radar loops depict occasional
weakening/narrowing of the precip shield occasionally augmented by
periods of frontal induced strengthening at times.

Fortunately our temperatures have warmed considerably (as have
pavement temps) today to make this unlikely in northern Illinois
and northwest Indiana. There is some weak isentropic lift ahead of
front which will lead to some spottier pre-frontal showers this
evening. The upper trough and associated surface front will move
into the region tonight and will expand the showers across the
area. The upper trough is progged to lift north as it moves east,
with progressively weaker large scale ascent with eastward extent,
so should see some decrease in precipitation intensity and
perhaps coverage as rain moves east overnight into Saturday
morning, though some occasional frontogenesis will allow some
local reinvigoration.

In wake of rain late tonight into Saturday look for skies to
clear out from the west, with a significantly cooler air mass in
place in the wake of the cold front. The surface high will move
into the western Great Lakes region and set the stage for another
healthy lake breeze Saturday afternoon and evening, with much
cooler temps (40s) expected near the lake.



315 PM CDT

Sunday through Friday...

The main concern is the threat for thunderstorms at times Monday
PM through Tuesday evening/night. While it is several days out,
the large scale pattern on Tuesday PM may be supportive of a
strong to severe thunderstorm risk in the region.

After a chilly start to Sunday, the afternoon will have a nice
recovery inland back into the upper 50s to around 60 under mostly
sunny skies. High pressure centered over Lake Michigan will bring
onshore flow through the day, keeping lakeside areas in the mid
to upper 40s.

The unsettled stretch of weather Monday through Tuesday night will
bring the threat for a couple rounds of showers and thunderstorms.
At the surface, high pressure will exit east, yielding developing
southerly winds. True warm front with better moisture surge will
not lift north until Monday night into Tuesday morning.

On Monday, low amplitude mid and upper ridging over the Midwest
may be topped by a lead short-wave in westerly flow aloft. The
guidance indicates that a plume of fairly steep mid-level lapse
rates will be advected eastward, which should yield sufficient
MUCAPE for a threat for scattered embedded thunderstorms Monday
PM-night. There is timing uncertainty with this lead wave.
Temperatures Monday afternoon will further moderate well into the
60s, though presence of clouds and precip. may temper them a bit.
Also, the Illinois shore will likely hold onto onshore flow,
especially with northward extent, to keep highs in the 50s.

Looking ahead to Tuesday, as mentioned, a warm front will be
lifting north, carrying dew points into the lower 60s for many
areas. A strong EML with mid-level lapse rates approaching or
possibly exceeding 8C/km will be over the area, which would likely
present a cap to thunderstorm development for part of the daytime
as things stand now. The mid-level pattern will trend to northwest
flow. While the guidance has been waffling on the location of a
surface low pressure system, the trend today was a track from the
northern Plains to the northern Lakes, tied to a mid-level short-
wave. This wave would then potentially approach toward early
evening, providing height falls and cooling aloft to erode the
cap, with low level convergence from surface cold front. Assuming
we are capped and dry out for a portion of the day with some sun,
temps could warm well into the 70s per 850 mb temps in the mid
teens C.

Aforementioned set-up with temps in 70s and lower 60s dew points
could yield moderate instability of 1000-1500+ j/kg MLCAPE. This
is concerning as strengthening northwest flow with approaching
wave later in the day/early evening may support 35-50 kt of deep
layer bulk shear and a strongly veering profile. This period will
be one to watch for northwest flow strong to severe thunderstorm
risk Tuesday PM. The above described wind profiles would potentially
be supportive of supercells depending on overall evolution and
cold front timing.

Initial cold front will knock temps down only slightly for Wednesday
into the 60s. A secondary cold front Wednesday evening/night will
bring a healthier surge of cooler air. Precipitation with this
secondary front will depend on magnitude of large scale forcing.
Official forecast has a dry forecast for now this far out, but
note that 12z ECMWF and GFS do indicate precip potential. Thursday
will have breezy-west northwest winds and temps only in 50s
followed by lighter winds under high pressure Friday, when
afternoon temps may struggle to reach 50 degrees.



For the 00Z TAFs...

There are several aviation weather concerns this period which

-Northwest wind shift with an incoming cold front
-Development of MVFR and spotty IFR cigs
-Period of -SHRA with reduced vsbys
-Potential for brief period of -DZ towards early Saturday morning
-Low potential for temperatures to approach freezing as
 precipitation ends at RFD
-Lake breeze push Saturday afternoon

Cold front is currently pushing through RFD at this time, and will
be making steady progress towards the Chicago-area terminals
through the mid-late evening and have nudged the timing of FROPA
up just a bit from the previous forecast. Have also moved the
mention of showers prevailing with the expectation that coverage
will be on the increase late this evening and overnight. Based on
upstream observations, some occasional IFR cigs/vsbys appear
possible as the frontal inversion continues to cool and saturate.
Did contemplate adding a brief period of -DZ into the TAFs as the
main area of showers pulls away, but confidence was not quite high
enough to do so. However, have added a mention at GYY as
cool/moist flow looks to continue off the lake through the morning
hours on Saturday, and it`s possible that cigs/vsbys are still a
bit too optimistic in this current TAF.

Regarding the RFD TAF: some forecast guidance indicates
temperatures may get close to the freezing mark as precipitation
ends late tonight. At this time, think that dry air will make
sufficient inroads to shut any precipitation of consequence off
before temperatures fall to within a degree or two of the freezing
mark and have left any frozen precipitation mention out of the
TAF. Should -DZ be more expansive than currently anticipated,
there could be a low potential for a brief period of -FZDZ at RFD
towards 10-13z or so, but this again appears to be on the lower
end of the probabilistic spectrum.

MVFR cigs should scatter through the morning/early afternoon, with
a lake breeze likely to push through the Chicago-area terminals
sometime during the 18-20z timeframe.





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