Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220718
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
218 AM CDT Thu Jun 22 2017

.UPDATE...
849 PM CDT

Main concern is potential for any thunderstorms overnight. A warm
front from central Iowa to central Illinois early this evening
will lift north overnight, with temperatures likely slowly rising
after dropping back a bit this evening. Current shower activity
over western and northwest Illinois may clip portions of north
central Illinois with isolated showers/sprinkles over the next
few hours before dissipating, with lightning unlikely with this.

00z RAOB analysis indicates a moderate 850 mb low level jet in
place with some strengthening overnight. In tandem with a short-
wave currently over Minnesota, some convection should form on the
nose of this low level jet, but at this time it appears
increasingly likely that this activity will form over Wisconsin,
possibly well north into Wisconsin. Kept isolated shower/thunder
mention overnight in far northern Illinois, but most areas should
remain dry. If any thunderstorms do occur, can`t completely rule
out a very isolated stronger thunderstorm capable of hail/gusty
winds. For the lionshare of the area, it will be a mild and
tranquil overnight period.

Looking ahead to Thursday, the main focus is summer heat. Progged
925 mb temperatures support highs in the low-mid 90s based off
local climatology, but the limiting factor could be the large
cirrus shield of Tropical Storm Cindy. Thus highs in the upper 80s
to lower 90s (around 90) appear more likely. It will still be a
hot day with higher humidity levels/dew points along with gusty
southwest winds.

Castro

&&

.SHORT TERM...
257 PM CDT

Through Thursday...

Forecast concerns the next 24 hours remain rain and thunderstorm
chances. Heat and humidity also return Thursday with gusty southwest
winds during the afternoon.

Decaying showers left over from clusters of showers and thunderstorms
across Iowa earlier today just working their way into the
forecast area from the west. Low dew point air combined with very
dry low levels per forecast sounding data continue to present a
very unfavorable environment for rain to move into... hence expect
downward trend in coverage and intensity to continue. Yet with
every wave of rain... the atmospheric column moistens more. Hence
expect scattered showers into the evening mainly fox river valley
and points west... with isolated showers or sprinkles possibly
surviving into northeast Illinois and northwest indiana as well.
Can`t rule out an isolated thunderstorm given modest lapse rates.

Expect another good round of convection to develop later this
evening and overnight associated with intensifying low level jet
in pattern of decent warm air advection/isentropic lift.  Best
forcing appears to remain to the north however.  Short range
ensemble and CAMs suggest best window for scattered rain/storms in
IL (mainly north of I-80) to come in the 03-05z timeframe north
central IL and 04z- 06z northeast IL with any activity that does
develop then shifting east and north.

Southerly flow increases Thursday ahead of a cold front to drop
southeast across the upper Great Lakes to central plains. Look for
this front to become progressive being pushed along by a vigorous
upper level short wave trof dropping into MN by late in the day
and a surface 1024 high pressure area building in to the Rockies
and high plains behind the wave. May see a stray storm pop
Thursday afternoon in the unstable airmass but if cap holds which
seems more probable showers and storms will hold off until along
and ahead of the cold front Thursday evening and overnight.

Ed F

&&

.LONG TERM...
240 PM CDT

Thursday night through Wednesday...

For the longer term forecast period, general trends remain
consistent with previous runs and the short term forecast period
with the cold front sagging south through srn WI/ern IA.  Expect an
area of showers and thunderstorms with the boundary.  Latest
guidance suggests pwats approaching 2 inches pooling along the
boundary which would signal the potential for heavy rainfall with
thunderstorms through Thursday night.  With flow generally parallel
to the front above the boundary layer, there will be the potential
for echo training as the general area of thunderstorms slowly sag to
the south.  With the high pwats, echo training and slow progression
of the boundary, there will be the potential for locally heavy
rainfall and possible flooding.  The models are in relatively good
agreement on pushing the boundary south and east of the CWA by
Friday morning, with the possibility for some lingering showers over
nwrn IN.  Otherwise, pcpn should end by late morning Friday and
conditions then drying out through Friday night.  High pressure will
quickly build across the middle Mississippi Valley and across the
upper midwest Friday afternoon and through Friday night.  Following
the passage of the cold front, max temperatures for Friday should be
at least 10 degrees lower than Thursday, topping out around 80 F.

The cooler, drier air will gradually filer in Friday night with
dewpoints dropping back into the lower 50s.  With the cooler/drier
air overspreading the region, highs for Saturday will drop to the
lower to middle 70s.  The cooling trend will continue through the
weekend and into early next week as broad long wave troughing
develops east of the Rockies and upper ridging builds over the west
coast.  A series of shortwaves will drop out of the upper
Mississippi Valley and across the midwest through the weekend and
into early next week, helping to amplify the upper trough.  With the
individual shortwaves dropping through the nwly flow aloft there will
be periodic chances for some showers or a few afternoon
thunderstorms, but given the typical timing and intensity
differences among the models for the weak ripples through the nwly
flow aloft, will limit any PoPs to lower chance to slight chance
levels.  By midweek, the longer range guidance is still suggesting
some progression to the longwave pattern, with the upper ridge
moving east across the central CONUS.  Also, the sfc high pressure
will push to the east with lee troughing developing over the nrn
plains.  Increasing heights aloft and strengthening slwy flow in the
lower levels will lead to a return flow of moisture and increasing
temperatures, with highs in the low to middle 80s and dewpoints arnd
60 F.

&&

.AVIATION...
For the 06Z TAFs...

A warm front is pushing north and has gone through Peoria and
Bloomington, IL. Winds will become southwest behind the front.
Thinking a stable layer will keep the terminals dry through this
afternoon. Southwest winds will gust to arnd 25 kt most of today.

Convection is expected to continue across Iowa and Wisconsin
through this afternoon. A line of showers and storms is expected
to shift southeast across northern IL and northwest IN late this
evening and tonight. I have high confidence in convection
impacting the terminals, but only medium confidence in exactly
when the line will reach the terminals this evening. Guidance
also suggests ceilings will drop to MVFR or IFR with the
late this evening. Decided to include MVFR cigs in the forecast
as I do not have enough confidence to go with IFR at this time.
Winds become northwest behind the rain and associated cold front
Friday morning.

JEE

&&

.MARINE...
217 AM CDT

Southwest winds will be gusting around 25 kt today so issued a small
craft advisory through this afternoon.

The low over western Manitoba will weaken as it reaches Ontario
tonight and then continues to Quebec. Its cold front will pass over
the lake tonight into early Friday morning with winds becoming west
to northwest behind the front. Westerly winds are then forecast
through early next week.

The remnants of Tropical Storm Cindy will move over the southern
Mississippi Valley today and then the low will fill as it shifts
east over the Ohio and Tennessee River valleys late this week.  High
pressure builds over the plains Sunday and then builds over the
western Great Lakes early next week.  Winds become southerly behind
the high Wednesday.

JEE

&&

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

$$

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