Drought in Illinois, September 1 to 30, 2023

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1146 PM CDT Thu Aug 31 2023

.SHORT TERM...
Issued at 149 PM CDT Thu Aug 31 2023

Through Friday night...

High pressure centered over the Lower Michigan early this
afternoon is producing a downright pleasant weather day across
the region. Temperatures will top out in the middle to upper 70s
this afternoon (slightly cooler near the lake) under full
sunshine. Clear and cool conditions are expected tonight, as
temperatures drop back into the upper 40s to low 50s outside of
Chicago.

Temperatures will begin a warming trend on Friday as the flow
turns southerly again following the eastward departure of the
Great Lakes surface high. Expect highs to climb into the lower 80s
under sunny skies Friday afternoon. A weak lake breeze is likely
Friday afternoon, but it appears unlikely that it will make much
inland progress through the day. For this reason, any slightly
cooler conditions should be confined to areas right along the
lakeshore.

KJB

&&

.LONG TERM...
Issued at 258 PM CDT Thu Aug 31 2023

Saturday through Thursday...

Key Forecast Messages:

* Very warm to hot through Labor Day Weekend but low-moderate
  humidity with fairly comfortable conditions outside peak heating

* Hot, dry, and breezy conditions on Sunday may create an elevated
  risk of more rapid brush and grass fire spread

* Very warm to hot and more humid Tuesday-Wednesday, and possibly
  Thursday with periodic (mainly low) shower and t-storm chances

Despite the meteorological calendar switching to fall, the weather
will behave like the astronomical season we're still in (summer)
through the period. Warmest temperatures peaking in the mid 90s in
spots will likely occur on Sunday-Monday. Strong 500 mb ridging
of 594+ DM over the central Plains will spread across the Mid-MS
Valley on Saturday, then the ridge center will slowly migrate
eastward through Labor Day. A weak embedded mid-upper low may
gradually meander from the Arklatex to the southwest Ohio Valley
Tuesday-Wednesday. Timing of a cold front expected to move across
the area sometime in the Wednesday-Thursday period will likely be
modulated by mid and upper level pattern evolution, including
trajectory of aforementioned mid-upper low.

Following a seasonably cool Friday night, potent thermal ridging
featuring 850 mb temps of +21 to +24C will advect eastward on
Saturday. This will set up a sharp inversion, likely capping
mixing below 900 mb. Since the corn (and soybean) farming areas
are mostly out of evapotranspiration season, blocked off moisture
trajectories and antecedent dryness should support 50s dew points
for most areas during peak heating (and possibly lower than official
forecast). Given the above and plenty of sunshine except for some
mid and high clouds from weakening wave moving in from NW, highs
should jump to the upper 80s to around 90F (officially 86-91F
range), but with heat indices lower than air temps. Steady to at
times breezy southwesterly winds will diminish Saturday evening,
so conditions will become comfortable toward after sunset, with
forecast lows in the low-mid 60s outside of Chicago and upper 60s
to around 70F in the city.

Sunday will be similar to Saturday, though warmer starting point,
850 mb temps a degree or two warmer, 925 mb temps climbing to the
mid-upper 20s C, and near cloudless skies will set the stage for
highs in the lower to mid 90s. There's some uncertainty on exact
mixing depths and how much dew points mix out, but forecast is for
upper 50s Td to be common during peak heating, keeping heat
indices slightly below air temps. As noted in intro points,
antecedent dryness and hot and dry conditions may yield an
elevated fire weather risk as southwesterly winds are forecast to
gust up to 30 mph. Should again see fairly comfortable conditions
once near and past sunset Sunday evening. Slightly warmer still
forecast lows in the mid-upper 60s outside of Chicago and low-mid
70s in the city should set the stage for similar highs to those
of Sunday on Labor Day (Monday) despite lower 850 mb temps. Dew
points climbing into the 60s will make it feel more humid (heat
indices slightly above air temps), but still not close a week ago.

Should open up moisture trajectories more on Tuesday, and weak
forcing from mid South upper low mentioned in first paragraph may
touch off a few isolated showers or thunderstorms near and north
of I-80 in the evening and overnight. The timing and passage of an
approaching cold front during mid week is rather uncertain, owing
to a faster breakdown of the 500 mb pattern by some of the guidance.
On the other hand, slower guidance (ie. 12z ECMWF) keeps higher
500 mb heights nearby into Wednesday and delays the cold front
approach and passage into Thursday-Thursday evening. For continuity
sake, maintained low shower and thunderstorm chances Wednesday
afternoon and evening for parts of northern Illinois, and slight
chance PoPs on Thursday. Note that Thursday would likely be warmer
and more humid than official forecast if the slower front scenario
verifies.

Castro

&&

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

There are no aviation concerns through that TAF period. Light
southeasterly winds overnight should veer south-southwesterly
(180 to 190 degrees) during the daylight hours. A lake breeze
should develop during the afternoon and approach ORD/MDW, but
925mb southwesterly winds of 10-15 kt (and increasing in the
evening) should keep the boundary just to the east of the
terminals.

Borchardt

&&

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

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Hazardous Weather Outlook...CORRECTED
National Weather Service Chicago/Romeoville IL
359 PM CDT Thu Aug 31 2023

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-012045-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
359 PM CDT Thu Aug 31 2023 /459 PM EDT Thu Aug 31 2023/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

  No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Saturday and Sunday...
  Limited to Elevated Fire Weather Risk for brush and grass fires.
Monday and Tuesday...
  Limited Excessive Heat Risk during the late morning and afternoon.
Tuesday Night and Wednesday...
  Limited Thunderstorm Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotter activation will not be necessary through tonight.

LMZ740>745-012045-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
359 PM CDT Thu Aug 31 2023

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

  No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Sunday and Tuesday...
  Hazardous conditions for small craft are possible.
Tuesday Night and Wednesday...
  Limited Thunderstorm Risk.

Castro

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