Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181200

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
700 AM CDT Thu May 18 2017

354 AM CDT

Today and Tonight...

Gusty west-southwest winds especially this morning, cold frontal
passage with potential sharp temp drop along Lake Michigan late
this afternoon, and potential for a few thunderstorms across the
far southeastern cwa late this afternoon are the main near term
forecast concerns.

Surface low pressure system was to our north across Wisconsin
early this morning, with a cold front trailing southwest through
northern IL and into the Plains near the Kansas/Oklahoma border.
The front will lay out from southwest to northeast as it drifts
slowly southeast across the cwa today, as the low pulls away in
association with the vigorous mid-level short wave propagating
across the upper Lakes. Relatively tight gradient will persist
through the morning and midday hours, with forecast soundings
indicative of gusts to around 35 mph from the west-southwest,
eventually easing as the frontal trough elongates across the cwa.
Presence of mid-level dry slot and an inversion based around 850
mb suggest precipitation potential is pretty low across most of
the area despite the presence of the front and the southern
periphery of the mid-level wave, though some destabilization is
depicted particularly in WRF soundings across the far
southeastern tier of counties by late afternoon near the boundary.
Bulk shear in 0-6 km layer in the 30-35 kt range, with WRF
soundings generating 1500-2000 J/kg far southeast, and SPC does
have that area in the near day 1 outlook marginal risk. 925 mb
temps ranging from +13/16C in the northwest to +20/22C in the
southeast would support afternoon highs from the low-mid 70`s
north to the low-mid 80`s far south. Potential exists for
temperatures to drop sharply along the IL lake shore very late
this afternoon or early this evening, as winds shift northerly and
then northeasterly behind the cold front and with high pressure
building across the Upper Midwest to our north.

Front moves south of the cwa tonight as cool north-northeast flow
spreads across the forecast area. Mostly cloudy skies are expected
to develop or persist especially across the south/southwestern
parts of the cwa, as warm-advection isentropic ascent develops
atop the stalled frontal boundary from the Plains into the mid-
Mississippi Valley. Guidance is in pretty good agreement in
holding precip off overnight however.



419 AM CDT

Friday through Wednesday...

Rain potential ramps up again heading into the weekend, as another
low pressure system develops over the southern Plains and lifts a
warm front across the area. Pattern looks to remain fairly amplified
and generally progressive next week, with another trough arriving
mid-week. Guidance does develop some spread in handling of the
large scale features by days 6-7, with lower confidence in details
by the later portion of the period.

Closed upper low was evident over the western CONUS in water vapor
imagery this morning. This feature is progged to slowly lift
north-northeastward across the Plains and into the upper Midwest
over the weekend, with moist return flow developing downstream
atop the low level frontal boundary currently sagging through the
region. Isentropic warm-advection ascent atop the baroclinic zone
will result in periods of showers and thunderstorms beginning as
early as Friday afternoon, though focused primarily Friday night
through Saturday night for the local area as the front develops
north as a warm front. Rain chances look to linger into early
Sunday as the surface low lifts across the upper Mississippi
Valley, and the occluding cold front pushes east across the
forecast area. Locally heavy rainfall will be possible with pwats
increasing into the 1-1.50 inch range, and a conditional severe
weather threat will exist especially across the southern 2/3 of
the cwa Friday night into Saturday near the surface warm frontal
zone. Temperatures will start out much cooler Friday with north-
northeast flow off the Lakes holding temps in the 50`s in many
areas, though the northward push of the warm front will likely
bring 70`s back to much of the area Saturday except far
northeastern IL where east low level flow will likely persist.
While a bit cooler overall Sunday behind the cold front, dry
westerly winds will prevent lake cooling and should allow mid to
upper 60`s across the board.

High pressure spreads in Sunday night into Monday behind the
departing system with dry weather persisting along with slightly
cooler than average temps. Guidance then is in decent agreement in
bringing the next upper trough and surface low across the region
mid-week with the next chance of rain Tuesday into Wednesday.



For the 12Z TAFs...

Main concern is with winds, with additional concern regarding CIG
trends. Gusty southwest to west southwest winds ahead of a cold
front will provide a window for gust speeds around 30 kt this
morning through the early afternoon. Behind the cold front
passage, winds will become westerly and then northwesterly by mid
to late afternoon with some reduction in speeds/gusts. As strong
high pressure builds in from the northwest, winds will then
quickly shift to gusty north-northeast this evening. Have decent
confidence in this shift arriving at ORD/MDW/GYY around the time
indicated in the TAF of 01z. However, this type of setup often
features a lake enhancement to the wind shift progression, so
cannot rule out an earlier timing. Northeast winds with gusts
roughly in the 20-25 kt range will continue through Friday.

Lower clouds with patchy MVFR over IA are advancing toward RFD are
this morning. It appears that CIG heights are more likely to stay
low VFR there, with only a glancing blow of scattered to possibly
broken VFR clouds at the other TAF sites. Some guidance continues
to indicate lower clouds coming down Lake Michigan this evening
behind the north-northeast wind shift, but do not have enough
confidence for a prevailing MVFR CIG yet.



448 AM CDT

There are several marine concerns through Saturday. Low pressure
over Wisconsin early this morning will move to near or just north
of Lake Huron by mid day, with its trailing cold front sweeping
across the northern portion of the lake. Ahead of the cold front,
steady southwest winds with hazardous speeds in the Illinois and
Indiana nearshore waters will continue. Behind the cold front
passage, winds will shift to northwest on the northern portion of
the lake with speeds increasing to gale force by around mid day.
The gales will continue through early evening and then diminish as
winds shift north and northeast. To cover this, have issued a Gale
Warning from 10 am CDT through 9pm CDT.

For the southern portion of the lake and nearshore, winds will
veer to west and then northwest behind the cold front this
afternoon and then quickly north to north-northeast to 30 kt by
the early evening. Have some concern for a few hour window for
occasional gale force gusts through mid evening, but not enough
confidence to issue a Gale Warning. Strong 1030 mb high pressure
just north of the lakes will keep brisk northeast flow continuing
through Friday. As low pressure lifts north Friday night, reaching
Iowa by Saturday morning, winds will shift to brisk easterly, and
then southeast over the south half during the day on Saturday.
The low will move to near Lake Superior on Sunday and to near
James Bay on Monday with the cold front trailing from it moving
across the lake Sunday. Hazardous winds and waves will continue
into Saturday for the Illinois and northwest Indiana nearshore
waters, so have extended the Small Craft Advisory until mid
Saturday afternoon.



LM...Small Craft Advisory...nearshore waters
     until 3 PM Saturday.




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