Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 312000

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
300 PM CDT Tue Mar 31 2020

226 PM CDT

Through Wednesday night...

Cloudy and seasonably cool conditions are in place across the area
this afternoon. Earlier this morning we had a few isolated
showers move in off the lake across northeastern IL. These have
since dissipated. However, with a weak mid-level impulse
currently shifting overhead, per the latest GOES 16 water vapor
imagery, I cannot rule out a the possibility for a few more brief
isolated sprinkles or light showers this afternoon. Most areas
will remain dry, and any showers that do develop will dissipate
this evening.

We may see few breaks in the lower level cloud cover this
evening, before more low-level cloud cover looks to move in over
northeastern IL overnight and into early Wednesday morning. It
does appear that this cloud cover could begin to scatter out some
Wednesday afternoon, so some sunshine is possible. Temperatures
also should begin to rebound back into the low 50s, especially for
inland areas. Light north-northeasterly winds are expected over
the area during the day as a surface ridge of high pressure shifts
over the area. This should promote a better lake push of east-
northeasterly winds Wednesday afternoon, thus supporting cooler
conditions (in the 40s) near the lake.



300 PM CDT

Thursday through Tuesday...

Generally above normal temperatures are in store through early
next week, with the exception of a cooler interlude on Saturday.
There will be periodic chances for showers, centered on later
Friday PM into Saturday and then next work week, with Saturday
night into Sunday probable to be dry. Our next decent threat for
thunderstorms is not until next work week.

A temporarily blocky mid and upper level pattern will featured a
sharp, amplified ridge centered over our longitude on Thursday.
Longwave trough over northwest Canada and northwest 1/3 of the
CONUS will slowly break down the ridging into Friday. At the
surface, high pressure will be centered roughly over Lake Michigan
into mid day Thursday. Warm advection will help temperatures warm
to around 60/lower 60s inland (isol`d mid 60s far west), though
there could be a fair amount of mid and high cloud cover
preventing even milder temps. Fairly early lake breeze with weak
pressure gradient over the region will keep Illinois shore in
upper 40s and bring afternoon cooling to Indiana shore.

On Thursday night into Friday morning, a weak mid-level wave with
no upper jet support could produce some very spotty showers or
sprinkles into parts of northwest and north central Illinois. With
dry low and mid levels on forecast soundings, most areas should
stay dry. A cold front will slowly approach Friday PM and night.
Gulf moisture trajectories will never fully be established and
upper level support will continue to be lacking, with primary
lift from lower and mid level convergence. Think that Friday
afternoon should be primarily dry, except perhaps far
west/northwest under mid and high clouds. Mid 60s are likely for
highs, except mid to upper 40s for Illinois shore due to southeast

While there is some uncertainty in exact timing, latest trends
suggest increasing shower coverage Friday night into Saturday
morning. With main mid level and jet forcing passing well north,
wonder if that keeps shower coverage more scattered than
widespread, though likely PoPs in grids appear reasonable. If
frontal timing is fast like shown on some guidance for parts of
northern Illinois, Saturday could be mostly dry. With the stronger
signal pointing toward a good chunk of northern Illinois drying
out Saturday afternoon, cut back on NBM initialized PoPs northwest
of I-55. Saturday will be 5-10 degrees cooler than Friday, but the
cooling will be short-lived.

Temperatures inland on Sunday should be back into the lower 60s,
though high pressure over the region will bring lakeside cooling
(upper 40s-lower 50s). Also cut back low NBM PoPs on Sunday to go
with a dry forecast. Sunday night into next work week, we go back
to a ridging east and troughing west pattern, which should open up
moisture trajectories and yield potentially much above normal
temps. At this time, next Tuesday could be into the 70s area wide.
Periodic chance PoPs from NBM appear reasonable in this pattern,
with thunder mention also reasonable.



For the 18Z TAFs...

Low-level cloud cover trends continues to be main aviation
weather concern through the period.

MVFR CIGs around 1500-2000 FT AGL continue across the region early
this afternoon. Some gradual improvement is expected with these
CIGs this afternoon and evening. However, there are strong signs
that low CIGs will return to at least portions of the area late
tonight into Wednesday morning. For this reason, I have included a
mention of IFR CIGs at the Chicago area terminals from late
tonight through Wednesday morning. CIGs should improve by early
Wednesday afternoon.

Otherwise, expect northeasterly winds through much of the period,
with the strongest winds, up in the 10-13 KT range, expected this





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