Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240745

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
245 AM CDT Tue Apr 24 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



241 AM CDT

Wednesday through Monday...

Fluctuating temperatures are in store throughout the extended period
as a series of cold fronts move across the region through the latter
half week and into the weekend. A big warmup may be in store by
early next week, though, as a strong upper ridge builds across the
Great Lakes.

Cold front that sags through the region Tuesday night will usher
in cooler temperatures on Wednesday. A modest north/northeast
gradient will be in place early in the day ahead of an expansive
ridge that extends from the High Plains into the Great Lakes.
Ridge axis will gradually build south across northern Illinois
and northwest Indiana by Wednesday evening/overnight allowing the
gradient to gradually relax through the day. Temperatures will be
cool on Wednesday, topping out in the mid 50s inland and 40s near
the lake with persistent onshore flow throughout the day. Weather
looks to be otherwise quiet Wednesday and Thursday under the
influence of the building ridge. Temperatures should moderate
behind the ridge axis on Thursday with afternoon highs near
seasonal norms in the low to mid 60s. There probably will be a
lake breeze on Thursday, but boundary layer flow increasing to
around 10 kt out of the southwest may result in minimal inland
progress, or a late day surge that won`t have much impact on high

Thursday night, another cold front is poised to sweep across the
region. Guidance has been in good agreement indicating a line of
showers and thunderstorms developing within a narrow corridor of
instability near the front Thursday afternoon across portions of
Wisconsin into Iowa. There has also been good agreement on frontal
passage across the local CWA overnight with a rapidly stabilizing
boundary layer which will limit the intensity of convection as it
reaches northern Illinois. Would expect a decaying line of showers
and not terribly confident that it will maintain through the
overnight hours across the entire CWA. Behind the front on
Friday, afternoon high temperatures drop back into the 50s with
40s near the lake.

Another ridge axis builds across the region over the weekend
bringing a dry spell and a warming trend back to seasonal norms
by Sunday. Meanwhile, strong upper ridge will be translating east
across the Great Plains over the weekend then across the Midwest
early next week. A strong thermal ridge will move across the
region with 850mb temps warming to 11-14C and 925mb temps of
16-20C Monday and Tuesday. Late April/early May climo has median
surface high temps in the upper 70s given those temperatures
aloft. Cloud cover could be a complicating factor, especially on
Tuesday as another cold front approaches the region but both days
look favorable to make a run at 80F.



For the 06Z TAFs...

MVFR to briefly IFR ceilings with -DZ/-RA continue to filter
northwestward early this morning on the NW extent of an upper-
level low. MVFR ceilings have reached MDW and GYY already, and
should arrive at ORD and DPA over the next hour or two. Some
enhancement in the coverage of -RA across northeast IL toward
daybreak will allow for a period of IFR ceilings at GYY and
possibly MDW, but confidence was too low to include IFR ceilings
at MDW. A solid deck of low-end MVFR stratus should then persist
at all sites except RFD into the early afternoon before scattering
out by late afternoon as the upper-low shifts east. A cold front
will usher in gusty N to NNE winds beginning mid-evening.



241 AM CDT

Lake Michigan lies within a weak flow regime early this morning,
between systems, but a cold front will move down Lake Michigan
this evening and overnight with northerly flow strengthening in
its wake. Wind gusts are expected to peak around 30 kt late this
evening and overnight, then should very gradually come down
through the day Wednesday as a ridge of high pressure across the
region. A small craft advisory will be hoisted for the Illinois
and Indiana nearshore waters for this evening through Wednesday
evening. Winds flip to southwest behind the ridge axis on Thursday
and ahead of another approaching cold front. This next front is
expected to sweep across the Lake Thursday night bringing another
period of north winds gusting to around 30 kt.






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