Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261940

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
240 PM CDT THU MAY 26 2016

212 PM CDT

Through Friday...

Showers and thunderstorms from this morning have moved East...with
diffluent atmosphere left in the wake this afternoon with becoming
mostly sunny skies. Clouds have been slow to erode...but expect by
late afternoon mainly just a lingering thin cirrus shield
overhead. Upstream across Northern Missouri an area of
thunderstorms was developing...however guidance does not indicate
that this will lift towards Northern Illinois this evening. Have
trended towards a dry late afternoon persisting...and likely into
the evening hours as well. Some of the hi-res forecast guidance
continues to indicate a few showers/thunderstorms developing as a
weak lobe of vorticity lifts North after 4z...however with 500mb
heights likely to hold through this evening the potential for any
organized convection looks very low through at least 6z...then
have trimmed back on pops through the remainder of the overnight
with just a slight chance increasing towards chance by daybreak.

Winds should diminish with the lack of a gradient in the near
surface environment later this evening as well...and with dew
points in the middle 60s progged to advect North/Northeast pushing
PWAT values towards 1.5 to 2 is conceivable that some
patchy fog may develop. At this time have held it out of the
current forecast as there may be just enough wind at 1000-1500ft
to keep some mixing. With the high water content in the lowest
layers of the atmosphere...temps will be slow to cool tonight and
likely remain mild. Have adjusted temps to the mid/upr 60s...but
could easily see temps not dropping below 70 closer to downtown
Chicago and perhaps a few other locations. Cloud cover tonight
will also play a role in lows.

For Friday a deep trough remains anchored across the Central
Plains with the continued Southwest to Northeast feed of moisture
towards the Western Great Lakes/Ohio Valley. There is a weakly
defined surface low that will remain near the upper level
trough...but a weak mid-lvl vort wave will lift Northeast across
the forecast area later Friday morning through the afternoon. At
this time not anticipating any severe storms for Friday with the
continued lack of shear in the lowest 0-3km along with weak
forcing...but if any storms develop Fri aftn the greater concern
will be on brief heavy downpours given the anomalous PWAT that is
progged for the area.

With anticipated considerable cloud cover for Fri...temps will
likely hold in the lower 80s with some locations nearing the mid
80s mainly in the Southeast CWFA.



345 AM CDT

Friday through Wednesday...

The ejecting upper low as mentioned above will shift a high PWAT
air mass /AOA 1.5 inches/ over the Mississippi River valley by
midday Friday. The challenge is how quickly and how much rain will
spread east as similar upper lows over the past few weeks have
struggled to expand rain into the area within a similar semi-
blocked pattern. The difference with this is a wide open Gulf of
Mexico with a strong fetch of moisture transport. Almost all
guidance provides the western CWA with more rainfall on
Friday/Friday Night than the east and have tightened the gradient
on PoPs and rainfall amounts but feel it will probably need to be
done more so during the day Friday and possibly Friday evening.
Still expect eastern areas to get showers...just not sure how
quickly and it may not be until Friday night or even Saturday
morning. The synoptic setup gets less favorable for severe weather
and more so for heavy rainfall with PWATs approaching the 99th
percentile. The model ensemble spread for rainfall is quite large
over the area...with higher members in the two to three inch will need to monitor hydrology concerns...especially
with some localized amounts over 1.50 inches last night. The
chances for rain and some storms will continue into
Saturday...with more of an unstable air mass trying to push back
into the area...although forcing could be diminishing.

Upper ridging atop a weak surface high is expected to set up by
Sunday. As this draws near and our confidence has increased...we
are able to keep more of the area dry in the forecast for Sunday
and Memorial Day. Highs are forecast in the lower to mid 80s.




Earlier this morning a couple areas of showers and thunderstorms
were approaching the TAF sites...however one area of thunderstorms
lifted Northwest and dissipated and the other went South and has
since developed into a line of strong thunderstorms stretching
across Southeast Indiana stretching West through Southeast
Missouri. In the next several hours across Northern Il/Northwest
IN quiet weather looks to persist...with visible imagery
indicating a steady erosion to the lower clouds with continued VFR
conds. Most forecast guidance continues to lean towards a dry
afternoon and evening timeframe...with the next potential for
showers and thunderstorms nearing the TAF sites around 9Z Fri at
the earliest...and perhaps around daybreak across the ORD/MDW
sites. This continues to be a low confidence scenario though...and
may end up remaining dry through this period as well. Winds
throughout the TAF time will be generally from a 220 degree
direction trending to 190 with speeds between 7-12kt.

Thunderstorms will increase by late Friday morning through the
afternoon hours...and could easily see the VCSH trending towards
either a PROB30 or likely a TEMPO for thunderstorms Fri aftn at
ORD. This has increased confidence in impacting the TAF sites.




142 am...A warm front will continue lifting north across northern
portions of Lake Michigan this morning shifting winds southerly. A
large trough of low pressure is expected to remain across the
plains into Saturday maintaining moist southerly flow across the
lake. This will lead to continued fog development and likely dense
fog today through Friday and a dense fog advisory may be needed.

A weak cold front is expected to move across the lake Saturday
night shifting winds westerly with weak high pressure settling
over the lakes region early next week resulting in winds turning
northeast Monday night or Tuesday. cms


LM...Dense Fog Advisory...LMZ777-LMZ779 UNTIL 9 AM Friday.




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