Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220832

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
332 AM CDT Sun May 22 2022

Issued at 215 AM CDT Sun May 22 2022

Through Monday...

There are no significant weather concerns through Monday as dry
air builds in with the arrival of a 1030 mb high. Lingering spotty
showers across parts of northwest Indiana will come to an end over
the next few hours as the main upper vort max continues its
eastward trek into Michigan. A reinforcing cold front is making
inroads into our forecast area early this morning. Relatively deep
mixing behind this feature this morning and afternoon will foster
the development of northwesterly breezes into the afternoon with
occasional gusts towards 25 mph or so. With upstream surface
observations indicating widespread low to mid 30 degree dewpoints
and 800 mb Tds near -9 C, cut afternoon dewpoints a bit which
yields minimum relative humidity values now towards 30 percent.
We`ll likely build into diurnal cumulus and will additionally
pick up additional high-level cloud cover through the day.

Winds will ease this evening and overnight and while skies likely
won`t be totally clear owing to continued tufts of high cloud
cover, conditions look favorable for temperatures to cool pretty
readily into the low 40s away from the immediate lake influences.
Wouldn`t be surprised to see a smattering of upper 30s across some
of our typical cool spots near the Fox and Rock River valleys.
Temperatures weren`t quite cool enough to warrant inclusion of
frost in the weather grids at this point, however. Either way,
looks like a chilly/crisp night.

While the weather looks dry to start the week, freshening easterly
winds and increasing mid and high cloud cover through the
afternoon will continue to hold highs in the uppers/lower 60s near
the lake with mid and upper 60s inland.



Issued at 325 AM CDT Sun May 22 2022

Monday night through Saturday...

Main highlights:

* Soaking rain with embedded thunderstorms Tuesday night into
  Wednesday morning

* Conditional low CAPE/high shear severe threat Wednesday PM

* Warming up for the Memorial Day holiday weekend with low PoPs

On Monday night into Tuesday morning, unsurprisingly with the
lingering influence of unseasonably strong and dry high pressure
over the Great Lakes, the global guidance has trended primarily
dry. There may be a few sprinkles that break through the stout dry
layer between 700 and 800 mb for locales mainly near and west of
I-39. Skies becoming cloudy will result in Monday night overnight
lows about 5-10F warmer than the chilly Sunday night readings.

The feature of interest for the Tuesday night into Wednesday
forecast is a Pacific shortwave that will carve out a fairly deep
trough to our west and amplify height rises to our east on
Wednesday. Pattern evolution will open up Gulf moisture
trajectories with strengthening southerly warm and moist
advection. Initial southeast 850 mb flow and east-northeasterly
low level flow emanating from the dry high pressure off to our
east and northeast Tuesday afternoon will slow the northward
progress of much higher column moisture and likely hold back much
if all appreciable rainfall through the daytime hours on Tuesday
as dew points are held back in the 40s and lower 50s. Some light
showers and sprinkles may push into areas near and west of I-39
toward evening.

With the deepening synoptic system off to our southwest and
aforementioned lingering dry air influence from high pressure to
the northeast, think that for most areas the main push of soaking
rain and some embedded thunderstorms will be after midnight and
into Wednesday morning as the deepening surface low tracks from
near Kansas City to the mid MS Valley. PWATs pushing to 1.5-1.75"
or a bit higher, nearing 200% of normal for late May, and strong
ascent should result in moderate to at times heavy rainfall rates.
With weak mid-level lapse rates, probably won`t realize much
MUCAPE, but envision enough for isolated to widely scattered
embedded t-storms that could result in localized torrential
downpours. Limiting factor for notable hydro impacts is likely to
be shorter duration of any heavy rainfall rates as the mid-level
dry slot punches northeastward toward mid day Wednesday.

Turning to convective threats Wednesday afternoon, the ECMWF and
GFS and a majority of their respective ensembles have trended
toward a much farther north surface low path than just a few model
cycles ago. This isn`t a high confidence element yet, but
importance to the forecast is that a conditional severe threat
becomes a realistic concern with the farther north low path. Mid-
level lapse rates will only steepen slightly under the mid-level
dry slot Wednesday afternoon, but temps in 70s and dew points in
the 60s near and south of the northward lifting warm frontal zone
may yield 500-1000 J/kg of minimally capped MLCAPE amidst strong
low and deep layer wind shear, and favorable shear vector
orientation to the warm front. Given strong height falls ahead of
the system cold front Wednesday PM, we`ll certainly need to watch
for a "high shear, low CAPE" severe set- up in this time- frame,
with lower instability pointing toward a tornado and wind damage
threat if above described scenario comes to fruition.

However Wednesday pans out, all current signs point toward a
breezy, cool, and mostly cloudy Thursday in the cold advection
regime in the wake of the strong surface low over Lake Superior.
Might be a few light showers if saturation depth of the strato-Cu
is enough. The pattern will quickly shift into the Memorial Day
holiday weekend, with a warmer (highs in 70s) and drier Friday
(with small exception of an isolated PM shower/t-storm threat).
Building ridging over the eastern third of the CONUS and Plains
and western troughing will result in summer-like warmth for the
unofficial summer kick- off over the weekend. With some threat for
warm frontal activity on Saturday and some continued threat on
Sunday, low, mainly slight chance Pops are warranted. If front
does get hung up on Saturday, that would keep temps with northward
extent and especially near the lake cooler.



For the 06Z TAFs...

Aviation weather concerns during this period consist of:

* Low potential for brief MVFR cigs INVOF ORD/MDW/GYY behind a
  cold front tonight. Probabilities too low to include in the

* Gusty northwesterly breezes Sunday.

* Lake breeze wind shift late Sunday afternoon.

Showers will continue on a spotty basis near and just to the east
of GYY for a few more hours, but drier air will eventually bring
precipitation chances to an end tonight. Latest surface
observations indicate a small area of IFR and even LIFR cigs and
MVFR vsbys near the lake in SE WI which seem to be right behind
an incoming cold front. This potential is too low for inclusion in
the TAFs with the thinking this will remain tied more to the
immediate lakefront 07-09z.

Deep mixing will promote the quick development of gusty
northwesterly winds after sunrise. During the afternoon, there is
high confidence in lake breeze development, but only moderate for
specific timing of a northeast wind shift arrival into the
Chicago-area terminals. The inherited 23z timing at ORD/MDW still
seems appropriate, but this will continue to be refined in future
TAF issuances. Winds may trend essentially light and variable
overnight into early Monday morning, but northeast winds are
expected to quickly re-develop towards 12z Monday.





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