Drought in Illinois, August 1 to 31, 2021

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 AM CDT Sun Aug 1 2021

.SHORT TERM...
206 PM CDT

Through Sunday...

Skies are a bit hazy this afternoon as some of the wildfire smoke
aloft is advected our way ahead of a cold front. Otherwise, the near
term attention grabber is an area of thunderstorms that have been
increasing in coverage across north-central Wisconsin this
afternoon. The storms are along and ahead of a cold front that is
working southeastward toward our area tonight. With some warming
ahead of the front and colder temperatures aloft in the northwest
flow aloft pattern, mixed layer CAPE approaching 1000 J/KG and some
lower end shear values should allow storms to continue this
afternoon.

By the time the front approaches, we will be past the heating of the
day which has had some influence on convective coverage to this
point. To that end, much of the near term guidance continues to
suggest that these storms will likely decrease in coverage this
evening and weaken or dissipate. Forecast soundings do depict skinny
CAPE values aloft hanging on into the evening hours. Current
thinking is there will be a few weakening showers or storms,
especially since guidance is a bit slow, and we may not decouple
if they arrive a bit quicker. Current timing is about 9 pm into
the Chicago area if they hold together. Afterwards, the front
should help clear out some of the hazy skies for Sunday.

Sunday will feature more sunshine, and we will have a reinforcing
shot of northeasterly winds and cooler dewpoint air. These winds
will again lead to a period of high swim risk conditions at southern
Lake Michigan beaches through Sunday evening. Highs will top out in
the 70s with afternoon wind gusts near 20 mph inland, making for a
somewhat seasonally cooler day.

KMD

&&

.LONG TERM...
243 PM CDT

Sunday Night through Saturday...

After the dangerous swimming conditions at the beaches on Sunday
night, there are no significant weather impact concerns through
next week. There's just a few items of interest, starting off
with early season lake effect shower potential Sunday night into
Monday. Temperatures will start out below normal with comfortable
humidity levels and moderate back to around normal by mid week.
Signs are then pointing toward above normal warmth and higher
humidity returning next weekend.

850 mb temperatures over southern Lake Michigan will cool down to
9C or even slightly lower toward daybreak Monday. With lake
surface temperatures in the lower to mid 70s, this will yield a
respectable 850 to lake delta T of nearing 15C. Increasing lake
induced equilibrium levels (inversion heights) with several
hundred J/kg of CAPE and convergence should generate cloud cover
initially and then even some at least isolated to scattered shower
activity. Have some concern for isolated thunder, though well
above freezing level ELs are only brief and there's a good amount
of mid-level dry air when parameters are maximized. The
convergence should shift to the southwest portion of the lake
overnight, with low PoPs added downwind into parts of northeast IL
and extreme northwest Indiana through Monday morning. Aside from
the lake effect, forecasting coolest overnight lows. in quite some
time of lower-mid 50s inland of lake effect clouds.

A short-wave pivoting in on the western flank of longwave
troughing over northeast North America will move overhead Monday
afternoon. As this occurs, the convergence axis from the lake
will be shifting inland. Expecting a fairly healthy Cu field due
to seasonably cool air mass aloft steepening lower level lapse
rates. Moisture profiles are fairly meager and ELs generally top
out near or below the freezing level, so while some
isolated/spotty showers are possible, think they would be on the
lighter side. Highs will be 5-10F below normal in primarily mid
70s (locally upper 70s) on Monday, with lower 70s lakeside due to
onshore flow.

Tuesday through Friday have dry forecasts in the official grids.
Can't rule out some spotty shower/sprinkle activity at times.
However, think that splotchy afternoon-early evening QPF on some
of the guidance, especially from NCEP, is probably owing to
unrealistically high dew points (even when accounting for some
crop ET) and instability. Humidity levels will remain in check
likely through the work week as temperatures warm back to normal
low-mid 80s, so no dog days of August yet. Next week should
finally bring a return to very warm to hot and humid conditions,
with some variance on the magnitude of the warmth. Opening up of
moisture trajectories will allow for the aforementioned more humid
conditions and also potentially set the stage for a somewhat more
active stretch for the second half of next weekend into the
following work week. Low chances for showers/thunder return
Saturday-Saturday evening in the official forecast.

Castro

&&

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

Aviation Forecast Concerns:

* Surface cold front moves through during the pre-dawn hours, with
  wind shift to the north-northeast. Breezy north-northeast winds
  with gusts 15-20 kt this morning into the afternoon.

* Isolated lake effect -SHRA possible this afternoon and again
  late tonight over northeastern IL.

A cold front over southern WI early this morning will shift
southward across the area prior to daybreak, resulting in a
northerly wind shift at the terminals. Expect north-northeasterly
winds to then become a bit gusty (15 to 20 KT) off the lake
during the day. However, wind speeds are expected to quickly abate
across the area early this evening.

It appears the cold front will move through overnight precipitation
free. However, we will have to watch for the possibility of a few
isolated to widely scattered lake effect rain showers developing
off the lake this afternoon and possibly again late tonight. If
this occurs, a period of rain showers could impact the Chicago
area terminals after 18z today. However, the chances and coverage
of these potential lake effect showers look too low to include a
formal mention in the TAF at this time. Some lower end VFR CIGs
could accompany any developing showers.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ103-ILZ104...7 AM Sunday to
     7 AM Monday.

IN...Beach Hazards Statement...INZ001-INZ002...7 AM Sunday to 7 AM
     Monday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...7 AM Sunday to 7 AM Monday.

&&

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Hazardous Weather Outlook...UPDATED
National Weather Service Chicago/Romeoville IL
334 PM CDT Sat Jul 31 2021

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-
334 PM CDT Sat Jul 31 2021 /434 PM EDT Sat Jul 31 2021/

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

.DAY ONE...Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk...with an associated:
     Limited damaging wind risk...strong enough to break tree limbs.

DISCUSSION...

A few isolated thunderstorms are expected this evening into the
overnight as a cold front moves south, mainly near and north of
I-80. If any strong or severe thunderstorm occurs over far
northern Illinois in the state line counties, winds could be
strong enough could move light objects or break small to medium
sized tree limbs.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Sunday...
  High Swim Risk at Lake Michigan beaches.

DISCUSSION:

High wave action and strong currents from brisk northerly winds
over Lake Michigan will cause dangerous swimming conditions at the
beaches through Sunday night.

.SPOTTER INFORMATION STATEMENT...

  If any severe weather occurs this evening, spotter reports will
  be appreciated.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the southeast at 30 to 40 mph.

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-
334 PM CDT Sat Jul 31 2021

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.

Weather hazards expected...

  Limited Thunderstorm Risk...with an associated:
     Limited thunderstorm wind gust risk...up to 35 to 45 knots.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Sunday...
  Hazardous conditions for small craft are likely.

Castro

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