Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210817

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
317 AM CDT Mon May 21 2018

249 AM CDT

Through tonight...

Early morning water vapor imagery reveals a number of features that
will bring active weather to portions of the region today. An
upper low is spinning across portions of Nebraska and South Dakota
early this morning while several lower amplitude waves are
lifting from the southern Great Plains into the Midwest this
morning. Lead wave lifting across the mid Mississippi Valley is
already resulting in showers and thunderstorms across Central
Illinois. Precipitation will continue to lift northeast today, but
should outrun the better instability to our south which will
greatly limit any thunder threat through midday today. Remnant
shower activity will lift across northern Illinois and northwest
Indiana this morning and early afternoon.

The aforementioned upper low is expected to shift east into the
Upper Midwest midday while a weak surface reflection develops and
moves into northern Illinois this afternoon. GFS/GEM/ECM in general
agreement with a roughly I-80 track of the surface low and suggest
that there will be modest diurnal recovery of instability this
afternoon within the warm sector. Temperatures are expected to warm
into the low to mid 70s south of the low track while areas north are
held in the 50s and 60s. For now the NAM is a southerly outlier with
the low track. Expect a second window of showers and thunderstorms
to develop mid to late this afternoon and continue into the
evening as the main upper low traverses the region. If we are
able to get some breaks in the cloud cover and the low tracks as
expected this evening, this would support modest instability of
500-1000 J/kg MLCAPE across southern counties in the CWA within
the warm sector which coupled with deep layer shear values right
around 30 kt would support a low end severe threat. For now it
appears the severe threat will mainly stay confined to portions of
east central Illinois and northwest Indiana south of the Iroquois
River Valley. Precip chances wind down around sunset this evening
and expect primarily dry conditions overnight.



249 AM CDT

Tuesday through Sunday...

Should have a break from precipitation Tuesday, Wednesday, and
maybe Thursday, then more active weather returns Friday through
the weekend. Also expect a gradual warming trend through midweek.

Upper low is expected to move over the Intermountain West early in
the week while upper ridging amplifies over the mid section of
the country. Mid level height rises and subsidence will begin to
overspread the local area resulting in primarily dry weather
through at least early Thursday when the 500mb ridge axis begins
to move overhead. Meanwhile, surface ridge will build into the
Upper Midwest Tuesday resulting in a light north breeze. This will
continue to keep temperatures cool near the lake with onshore
flow, but temperatures should rebound nicely inland with late May
sunshine helping to warm temps back into the 70s. Surface high
builds overhead on Wednesday which will allow an afternoon lake
breeze to form with upper 70s and 80s inland. Southerly flow on
Thursday and upper ridge axis passing overhead should allow more
widespread 80s Thursday with again an afternoon lake breeze
suppressing temps near the lake. Models indicate that convection
will be ongoing along the Mississippi Valley Thursday, and cannot
rule out some of that activity reaching the CWA, though most of
the day should be dry.

Chances for showers and thunderstorms increase more markedly by
Friday as buckle in the upper flow develops and a strong wave digs
from the Upper Midwest across the Great Lakes over the weekend.
Showers and thunderstorms will increase Friday in advance of this
wave then continue into Saturday as a trailing cold front sweeps
across the region.



For the 06Z TAFs...

Low ceilings and precip chances will continue this forecast
period, while an east northeast wind persists. In the near term,
ceilings range from VFR to MVFR, to IFR/LIFR across the TAF sites.
Do think a downward trend will be observed for all sites over the
next 1-2 hours, with similar trends that MDW has just observed of
going from MVFR ceilings straight to IFR. These lower ceilings
will then remain for much of the morning, and possibly longer into
the day than currently forecast. Some improvement may occur later
today, but do think another night of low ceilings will once again
be possible. Dry conditions in the near term, but think upstream
precip development across west central IL will track northeast
across the terminals early this morning and last through midday.
Have not included thunder for early this morning, but will
continue to monitor this possibility of thunder moving across the



316 AM CDT

Lighter winds in place across the lake this morning, as high
pressure is situated across the Great Lakes. As this high shifts
to the east and low pressure tracks towards the lake today, will
see the gradient over mainly the southern part of the lake
increase. With this occurring, will see the current light winds
increase today to the 10 to 20 kt range across the south half and
nearshore waters. This low will move across the southern end of
the lake and into southern Michigan late tonight into Tuesday,
with winds then becoming northerly. However, speeds won`t become
overly strong especially as new high pressure builds across the
Great Lakes region.






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