Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281147
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
647 AM CDT SAT MAY 28 2016

.SHORT TERM...
306 AM CDT

Through tonight...

The main weather concerns are for the potential of some
strong thunderstorms over portions of the area...especially this
evening.

a few widely scattered showers and perhaps a thunderstorm will be
possible today. This may end up especially being the case east of I-
57 as some weak mid level disturbances shift northward over the
area. However...most areas look to be dry for much of the day...and
as a result...I have lowered POPS to just slight chances in most
areas. However...the chances of more storms will be gradually
increasing over north central and northwestern Illinois late this
afternoon...and especially this evening.

An upper level trough...currently over the Plains...will gradually
shift northeastward to the upper Mississippi Valley by this evening.
This system will likely spawn additional thunderstorms to our west
over eastern Iowa this afternoon...in close proximity to the
entrance region of an 80 KT upper level jet...and as diurnal
destabilization maximizes. This activity then looks to progress
eastward into northern Illinois into the early evening. The main
question with this activity is how far eastward will it progress
before weakening. Many of the Convective Allowing Models
(CAMs)suggest this activity will weaken quickly through the evening
as the storms shift towards northeastern Illinois. While this
certainly seems plausible given the poor diurnal timing...I still
have mentioned higher end chance POPS into the Chicago area later
this evening as I think some storms could still persist this far
east.

Some stronger storms will be possible early this
evening...especially over north central and northwestern Illinois.
Farther east...the expected weakening trend with the convection
should limit the severe threat. These storms could present a hail
risk...along with some locally strong wind gusts given the higher
DCAPE values. This activity looks to wane during the late evening
and overnight as it shifts eastward.

KJB

&&

.LONG TERM...
345 AM CDT

Sunday through Friday...

The upper trough mentioned above will shift over southern WI and
northern IL by midday Sunday. This should help drive some
scattered diurnal shower and storm development again on Sunday.
However...at this time it appears the best chance of storms may
remain to our north in WI...but we cannot rule out a few storms
getting as far south as northern Illinois. Otherwise...Sunday is
looking dry for most areas...with temperatures again getting into
the lower 80s.

Our next good chance of showers and storms looks to enter the
picture for Tuesday and Wednesday. Model guidance/ensembles are in
good agreement with the development of another storm system over the
Dakotas into the Upper Midwest by Tuesday. This storm system looks
to push a cold front across the area by Wednesday evening. A period
or two of thunderstorms will be possible over the area ahead of
this frontal boundary. However...once it passes Wednesday
night...the threat of precip will come to an end...with the rest
of the week looking to be on the dry and cooler side.

KJB

&&

.AVIATION...
For the 12Z TAFs...

Main aviation concerns continue to be timing the greatest threat
of thunderstorms across the terminals late today/this evening...in
addition to gusty south winds through the day.

Low pressure over the mid-Missouri valley will lift northeast
across the upper Midwest through tonight. Warm...moist and
conditionally unstable air will continue to feed into the region
on gusty south winds...20-25 kts through this afternoon. While
isolated to scattered shra/tsra can not be ruled out during the
day...the best organized large scale forcing for higher coverage
tsra is expected to be late this afternoon and this evening.
Trends from various convective-allowing model runs continue to
suggest that tsra will develop across parts of eastern Iowa...
southwest WI and western IL later this afternoon...spreading
eastward into north central IL by early evening and into the
Chicago area mid-late evening. Guidance trends also indicate that
storms will likely be diminishing/decaying as they move east away
from the stronger large scale forcing as well as with diminishing
instability after sunset. Thus have carried a tempo for tsra at
KRFD early in the evening...and maintained prob30 mention for
Chicago area taf sites given less confidence of storms directlyimpacting
the eastern terminals. Activity should be greatly diminished or
ending by the 06z time frame.

Otherwise...gusty southerly surface winds are expected through
the day in the region of tight surface pressure gradient east of
the low. Forecast soundings support gusts of 20-25 kt...perhaps a
bit higher at times if boundary layer mixes as deeply as some
guidance indicates.

Ratzer

&&

.MARINE...
214 AM CDT

Relatively quiet period for marine weather...with the only issues
being some lingering dense fog over northern parts of the lake and
breezy south winds today and tonight. No recent visibility
observations...though model forecasts indicate fog should become
less widespread with time this morning/today. Will maintain
current dense fog advisory into this afternoon... though would
expect it could be cancelled earlier.

A diffuse area of low pressure was over the central Plains early
this morning...with a warm front arcing across the upper Midwest
and about two thirds of the way up the lake. This low will lift
slowly to the northeast over the weekend...reaching the upper
Mississippi Valley by this evening and moving across far western
Lake Superior by early Sunday. As the low...and the warm front...
move north...winds will shift to the south across the lake. Winds
will gust into the 20-25 kt range today...and 15-20 kt later
tonight and Sunday. Winds across WI and IL will shift west-
southwest behind a cold front on Sunday with gusts 20-25 kt also
affecting the immediate western shore of the lake. Cool water and
warm air will make for a stable marine boundary layer over the
open waters resulting in lighter winds away from shore especially
on the southern part of the lake. The low will continue to pull
slowly away to the northeast of the Great Lakes early next week
with a weaker surface pressure gradient across the lakes bringing
lighter winds. Another low may lift across the upper Midwest mid-
week...with another frontal passage Wednesday or Wednesday night.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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