Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232340

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 PM CDT Mon Apr 23 2018

259 PM CDT

Through Tuesday...

Main question for the near term will involve the progression of
rainfall north, or actually northwest, into the area as a center of
low pressure spins through the Tennessee Valley over the next 24
hours.  Guidance both from deterministic as well as ensemble sources
suggests the forecast area could see periods of light scattered
showers beginning in our southeast corner this evening and spreading
through the Chicago metro area overnight.  The limiting factor would
be relatively dry air at lower levels, especially compared to
existing areas of Indiana and eastern Illinois where surface
dewpoints are in the mid 50s.  Across much of the local area the
dewpoints are not expected to climb out of the 40s through tomorrow,
especially in locations downwind of the northeasterly flow off the
lake.  The exception would be in our southeast which could see
values in the lower 50s.  In this sense the models are probably on
track with the idea of mid to upper level moisture and forcing being
sufficient for random periods of light showers through mid day
Tuesday, and with low level dry air limiting the extent of this



310 PM CDT

Tuesday night through Monday...

There are currently no significant forecast concerns through Day
7/next Monday. The best chance for rain is with a period of rain
showers associated with a cold front passage Thursday night into
Friday. After a bit of a typical spring temperature roller coaster
through the rest of the work week, signs are pointing toward above
normal warmth arriving likely sometime over the weekend and
continuing into at least early next week.

On Tuesday evening, isolated/spotty rain showers associated with
upper low exiting off to the southeast, if any within CWA, will
be confined to northwest Indiana. The first cold front passage of
note of the week will occur Tuesday night as a fairly strong
trough swings across the western Lakes and brings a surge of cool
advection. The main sensible weather effect initially will be a
strengthening of prevailing north-northeast winds, particularly
lakeside where gusts could approach or exceed 30 mph at times into
Wednesday morning. High temperatures Wednesday will be knocked
back into the 50s inland for most spots despite mostly sunny
skies. Can`t rule out tagging 60 in some areas, especially north
central and central Illinois. The gusty onshore flow will keep
areas near Lake Michigan in the mid to upper 40s.

Wednesday night will be clear and cool in the 30s under surface
high pressure. As the surface high slides southeast on Thursday,
dry westerly warm advection aloft and southwest surface winds
ahead of the next cold front will set the stage for a nice
recovery in temperatures, with 60s likely area wide. Possible
exception would be if winds aloft are a bit lighter near the
lakeshore and enable a lake breeze to push inland a bit.
Unfavorable diurnal timing of the cold front Thursday night, very
minimal moisture return and thus little/no instability will likely
preclude any thunder potential. Models vary on the strength of
the forcing with the front (GFS most robust), which will affect
shower coverage, thus kept PoPs in blended low-mid chance range.

Clouds should clear fairly quickly Friday behind the front, but
breezy northerly flow and cool advection through mid day will keep
most inland areas in 50s to possibly near 60 well away from Lake
Michigan, while the lakeshore will top out in the upper 40s. Over
the weekend, most recent operational guidance varies on timing and
strength of another possible cold front passage, which would
mainly affect high temperatures on Saturday with it otherwise
appearing most likely to be dry. Building mid-upper ridging and
steady southerly winds on northwest periphery of surface high will
set the stage for at least a few warm days to start next week.
Well into the 60s if not tagging 70 are probable on Sunday, with
possibly well into the 70s next Monday to close an exceptionally
cold April on a thankfully warm note.



For the 00Z TAFs...

Primary aviation concerns are the potential for periods of light
rain overnight and Tuesday morning, and development of MVFR and
potentially IFR cigs.

Early evening satellite imagery depicts mid-level low pressure
circulation drifting slowly east across the lower Ohio and
Tennessee river valleys. A series of smaller-scale short wave
disturbances will rotate counter-clockwise around the main
circulation through Tuesday, pulling moisture north-northwestward
across northwest IN and portions of northeast IL. Greatest
potential for rain will be across Chicago terminal, while RFD
likely remains dry with higher ceilings through the period. GYY,
farthest south, has the greatest potential for rain and lower
ceilings. Northeast low level flow will maintain a feed of
relatively dry low level air into the region, which may tend to
erode rain as it spreads north-northwest, and thus does add lower
confidence to extent of lower cigs. At least patchy MVFR cigs
appear likely into ORD/MDW area, with a lower potential for IFR
conditions mainly Tuesday morning. The system will pull away to
the east later in the day, which should allow for a gradual
increase in north winds, and improving ceiling conditions. In the
end of the 30 hour forecast period for ORD/MDW, a strong cold
front will push south across the area mid Tuesday evening, with
winds becoming gusty near 25 kts.



259 PM CDT

High pressure over the lake through mid-day Tuesday will allow for
generally light and somewhat variable flow. Tuesday afternoon and
evening the passage of a frontal boundary down the lake will turn
winds northerly and increase the pressure gradient, ramping up
wind speeds Tuesday evening and supporting rapid wave growth
across the south half. This long northerly fetch will persist into
Wednesday evening before winds again relax. Presently it appears
that wind speeds and gusts will remain just below gale strength
Tuesday evening and overnight into Wednesday.






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