Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240755

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
255 AM CDT Tue Apr 24 2018

253 AM CDT

Through tonight...

A mid/upper-level low continues to meander across the western KY/TN
state line, with a notable trough axis and weak low to mid-level
frontogenesis extending NW from the main low over central IL early
this morning. This has resulted in an active region of light rain
showers and drizzle arcing from near St. Louis to southeast
Wisconsin. The region should remain active through mid-morning as it
interacts with another mid-level trough axis shifting northward
across Indiana. Short-term guidance supports this trend, placing the
main focus for light rain across NE Illinois, including the Chicago
metro, around daybreak. Patches of light rain should then continue
through the morning before slowly ending from west to east as the
upper low continues to shift east.

Plenty of low-level clouds will inhibit some daytime heating, but
with temps starting out the day around 50F, max temps just a few
degrees cooler than Monday are attainable.

A cold front will surge southward across the area this evening,
reaching the WI state line around 9pm, and clearing the southern CWA
during the early morning hours. N to NNE winds are expected to gust
25 to 30 mph through much of the night behind the front, with
gusts up to 35 mph along the Lake Michigan shore. The main trough
axis associated with the front will remain well north of the
area, so precip is not expected through the night.



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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