Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 042334
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 PM CDT Tue Aug 4 2020

.SHORT TERM...

Through Wednesday night...

220 PM...Primary forecast concern remains high waves with the
associated lakeshore flood/dangerous swimming conditions this
evening. Only other minor concern is low temps tonight/Wednesday
morning.

Northerly winds are slowly diminishing this afternoon and that
trend will continue into this evening as high pressure slowly
builds into the area tonight. Waves at the Wilmette buoy are down
to 4 feet and plan to let the lakeshore flood advisory for the IL
shore expire this afternoon. The lakeshore flood advisory
continues into this evening for the IN lakeshore and will let this
continue though the trend will be for slowly subsiding waves. The
beach hazard statement continues into early Wednesday morning.

Lake effect rain showers have been hanging on into far northwest
IN this afternoon and these should be ending in the next few
hours.

Dewpoints have dropped into the mid/upper 40s across much of the
area this afternoon and as the high builds in tonight...winds will
become light and variable or calm across much of the area allowing
low temps to drop into the lower 50s away from the immediate metro
area. Medium confidence if skies remain mostly clear...as expected
that some upper 40s will be possible in outlying/usually cooler
spots by sunrise. Temps will warm quickly Wednesday morning with
highs in the mid/upper 70s Wednesday. A lake breeze is expected
Wednesday afternoon but low confidence for how far inland it may
move. cms

&&

.LONG TERM...
248 PM CDT

Thursday through Tuesday...

The long range forecast is summarized by transitioning back to
more typical summer weather - that is 80s for highs and potential
for touching lower 90s, along with chances for thunderstorms
including possibly multiple rounds at some point this weekend
and/or the first half of next week. The threat for high impact
weather looks mainly tied to if any thunderstorms could become
organized. Not surprisingly in the long range there is some signal
of that Saturday onward, but at this distance in the forecast the
signal for higher impact is not statistically strong nor does the
pattern scream big trouble.

The 1020 mb Canadian surface high pressure responsible for the
pleasant and cool weather on Wednesday will continue to prevail
over Illinois and Indiana on Thursday. The broader upper long wave
trough will still be established from eastern Canada into the
Northeast with attendant northwest flow aloft over the Upper
Midwest into Great Lakes. The current compact closed circulation
over southern Saskatchewan is forecast to become a muted open wave
by Thursday as it moves southeastward in this upper flow toward
southern Wisconsin/northern Illinois. This is expected to dampen
enough along with running into a dry air mass to squelch rain
chances into the CWA though there could be some clouds from this.
Otherwise Thursday and Friday both look to have light southerly
winds and lake breezes with still low dew points for this time of
year. Highs should be near normal by Friday.

Looking ahead to the weekend, the low-level thermal ridge is
aligned in guidance solutions to build northeastward into the
Upper and Mid-Mississippi Valleys. An associated elevated mixed
layer will accompany this into Saturday, with steepening mid-level
lapse rates impinging over the forecast area. Moistening
isentropic ascent on the leading edge of this instability gradient
warrants thunderstorm chances in the forecast area late Friday
night into Saturday. Also for Saturday, both the GFS and EC have a
short wave passing just to the north of the area and this has
resulted in higher PoPs (30-50) in the Saturday P.M. period.
Despite that increase, mesoscale driving factors upstream remain
key in dictating if and when the best chance for storms
materializes. That is difficult to resolve at this distance. The
ECMWF is notably slower than the GFS in its boundary layer
moisture recovery throughout Saturday, and if that drier solution
were to pan out it may keep thunder chances, at least any
stronger chances, limited. Saturday looks to at least be more
humid than any days this week with temperatures dependent on if
there is convection and when. Sunday looks to be milder, apart
from any convective influences, and do have some 90-91 values
forecast for highs.

A strong upper low is forecast to move across the northern regions
of the Canadian prairie provinces with its associated upper jet
gradually digging into the northern Great Lakes into Monday.
Funneling this influence down, it should result in a more active
baroclinic zone in the general region which by that point will
also be coupled with higher theta-e air in the low-levels. Again
ingredients are there for some stronger storms and possibly
successive storms presenting heavy rain issues, but very low
confidence on where. We continue with a forecast message of
thunderstorm chances (20-50) throughout the Sunday-Tuesday
period. Highs will continue hinging on thunderstorm chances and
timing, but there will remain a decent possibility of being north
of 90 degrees on those days, given forecast 850 mb temperatures
of 19C-22C.

MTF

&&

.AVIATION...
For the 00Z TAFs...

Wind speed and direction will continue to be the focus of the
TAFs through Wednesday evening. The lake breeze boundary has made
it well inland this afternoon and is bisecting ORD at this time.
Based on latest radar trends the boundary will remain about over
the field if not shift just to the east before fading out in the
next hour or so. Speeds will likely hover in the 10-12 kt range
for the next hour or so. Otherwise, winds will diminish close to
sunset and remain fairly light overnight. Direction will generally
be variable but should favor a west or southwest direction at
ORD/MDW.

High pressure will shift overhead Wednesday keeping winds
generally under 8 or 10 kt. A lake breeze is expected to push
inland and latest thinking brings it across ORD/MDW during the
mid/late afternoon, similar to today, with an earlier shift to
east or northeast at GYY. Wind speeds will again diminish early
Wednesday evening with a south to southeast direction expected.

MDB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ103-ILZ104 until 7 AM
     Wednesday.

IN...Beach Hazards Statement...INZ001-INZ002 until 7 AM Wednesday.

     Lakeshore Flood Advisory...INZ001-INZ002 until 10 PM Tuesday.

LM...Small Craft Advisory...nearshore waters
     until 4 AM Wednesday.

&&

$$

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