Flash Flood Warning 11, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
221 PM CDT Mon Jun 22 2020

.SHORT TERM...
216 PM CDT

Through Tuesday night...

Primary focus during the short term is on the convective potential
this afternoon and evening, as well as an attendant threat for
some severe weather.

Skies have cleared out nicely in the wake of this morning's
weakening MCs with essentially mostly sunny conditions prevailing
south and east of a La Salle to Waukegan line. A few areas of
enhanced surface convergence are noted in both observations and
visible satellite imagery, with one in the vicinity of the
differential heating zone across our northwest, with at least two
additional areas elucidated by streets of lumpier cumulus in the
vicinity of the I-55 corridor. With temperatures having warmed
into the mid and eventually upper 80s to near 90 where the sun has
come out and dewpoints hovering around 70, the airmass has become
moderately unstable with little to no lingering MLCINH. Large
scale forcing for ascent is still in the nascent stages of
overspreading the area, but jet divergence is clearly increasing
overhead based on recent mid and upper level moisture channel
satellite loops. As a result, it's not all too surprising to see
storms taking off as early as they have. Main corridor to watch
here over the next several hours is mainly near I-55 and I-65. As
well, we'll be monitoring the differential heating area which
seems to have encouraged increased convergence towards Kane to
Lake counties where additional convection could tend to focus. Deep
layer shear has increased sufficiently to support some storm
organization and rotating mid-level updrafts, although this is
sort of towards the lower end of that spectrum space. Still, even
with the rather paltry mid-level lapse rates, storms will probably
be capable of producing both hail and damaging wind gusts as they
get going through the afternoon. Semi-training may also take
place initially which could result in sporadic instances of mainly
urban and small stream minor flooding.

By late this afternoon and into the evening, things get a little
unclear, especially given the potential for somewhat widespread
shower/storm development through the afternoon hours here...what
this does to the environment still remains a bit unclear. Either
way, signs point to additional storms potentially developing
closer to the actual cold front across central Iowa ahead of the
main shortwave, and this activity could attempt to merge with the
mass of storms drifting north out of the St. Louis area. Should
upscale growth take place, a loosely-defined MCS could make a run
at mainly the southern half of our area during the evening hours.
If convective coverage this afternoon gets high enough, then
would tend to think the severe wind risk with this next complex
might be muted to a good degree, although there does seem to be a
bit of a 700-500 mb wind response with this shortwave which could
act to bolster cold pool amalgamation and subsequent
acceleration--and this is corroborated by some 40 kt wind
retrievals on DVN's VWP. We'll continue to highlight mainly our
I-80 and south locales for this later evening threat.

Still don't think the flash flood threat looks too appreciable at
this point, but certainly will be a potential for localized
corridors to pick up 2-3" of rain before things clear out from
west to east overnight. Main corridor for this seems to be near
and south of I-55.

For Tuesday, things are looking much cooler and probably will see
quite a bit of cloud cover develop through the day. An additional
shortwave will pinwheel overhead and could squeeze out some
showers, some of which could be pretty gusty given increased winds
just off the deck. Think the potential for thunder may remain
somewhat muted given fairly robust mid-level subsidence/drying.
Things look to dry out--at least temporarily--Tuesday evening and
overnight.

Carlaw

&&

.LONG TERM...
220 PM CDT

Forecast thinking remains largely unchanged from the previous
discussion. Due to ongoing active weather, the long term
discussion will not be updated today.

Previous discussion issued at 153 AM CDT

Wednesday through Sunday...

The Great Lakes will remain beneath slow-moving upper-level
troughing in the middle of the week allowing for a continuation of
overall pleasant conditions. Highs and lows Wednesday should top
off right around 80 / 60, respectively, with light northwesterly
winds. A lake breeze with muted inland penetration should allow
for locally cooler temperatures along the shore line especially in
northwestern Indiana. Scattered afternoon showers and perhaps a
weak thunderstorm are possible during the afternoon hours as a
shortwave embedded in the broad troughing aloft swings through the
western Great Lakes.

Thursday and Friday, the upper-level pattern will transition into
a more summer-like regime with zonal to at times anticyclonic
flow over much of the United States. As such, low- level flow will
become south/southwesterly enabling the return of warm and moist
conditions. Ensemble model guidance suggests chances for showers
and thunderstorms will increase markedly Friday night onward.

Borchardt

&&

.AVIATION...
For the 18Z TAFs...

1243 PM...Primary forecast concerns include...

Scattered thunderstorms this afternoon into this evening.
Wind directions through late evening.
Low mvfr...possible ifr...cigs tonight/Tuesday morning.

Isolated thunderstorms are developing along a pnt/jot line early
this afternoon and scattered thunderstorms are expected across
much of the area though only medium confidence for both timing and
coverage. There could be additional thunderstorms developing from
outflow boundaries making for a challenging forecast. If current
activity continues to increase in coverage...some timing tweaks
will be needed. Convective activity should slowly diminish
through the mid evening hours with some showers still possible
into the early morning hours.

Prevailing wind directions are expected to settle on a south/
southwest direction this afternoon but some westerly winds are
possible across the northwest cwa. Some gusts into the mid/upper
teen kt range are possible as well. As thunderstorms increase in
coverage and outflows form...wind directions will become
problematic and some east/southeast wind directions are possible.
Winds may become light and variable for a time this evening ahead
of a cold front that will shift winds to the northwest late this
evening or early Tuesday morning. Wind directions will remain
northwest into Tuesday with speeds 10-15kts and possibly some
higher gusts Tuesday afternoon.

Much of the guidance is showing low mvfr cigs developing by late
this evening and continuing through sunrise Tuesday morning. Its
possible ifr cigs develop but confidence is low. These cigs will
lift through mvfr and into low vfr mid morning Tuesday. There will
be a chance for some showers Tuesday afternoon. cms

&&

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

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
304 PM CDT Mon Jun 22 2020

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-232015-
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-
304 PM CDT Mon Jun 22 2020 /404 PM EDT Mon Jun 22 2020/

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

.DAY ONE...Tonight.

Weather hazards expected...

  Significant Thunderstorm Risk...with an associated:
     Limited hail risk...up to half dollar size.
     Elevated damaging wind risk...up to 65 mph.
  Limited Flooding Risk.

DISCUSSION...

An area of showers and thunderstorms, initially focused mainly
south and east of a La Salle to Waukegan line, will continue to
develop this afternoon. Some storms could be severe with a threat
for damaging wind gusts and large hail, mainly with the storms
south of I-80.

An additional area of storms may develop and move into the region
very late this afternoon and into the evening hours. A threat for
locally strong to potentially damaging winds could exist with this
activity as well, mainly south of I-88. Finally, localized
instances of flooding will be possible through this evening due to
multiple rounds of thunderstorms.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Wednesday through Sunday...
  Limited Thunderstorm Risk.

DISCUSSION:

Isolated afternoon thunderstorms are possible Wednesday and
Thursday. Additional storms chances exist late Friday into
Saturday.

.SPOTTER INFORMATION STATEMENT...

 Spotter reports will be appreciated this afternoon and evening.

GENERAL STORM MOTION OF THE DAY:

 Moving toward the east northeast at 25 mph.

LMZ740>745-232015-
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-
304 PM CDT Mon Jun 22 2020

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...

  Significant Thunderstorm Risk...with an associated:
     Limited hail risk...up to nickel size.
     Limited thunderstorm wind gust risk...up to 50 kts.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Wednesday through Sunday...
  Limited Thunderstorm Risk.

Carlaw

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