911
FXUS63 KLOT 211746
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 PM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Near record heat expected today, fittingly for the last day
  of astronomical summer

- Scattered showers/t-storms tonight likely to build to a
  steadier beneficial rain with some embedded thunderstorms
  Sunday

- Another bout or two of rain possible Sunday night through
  Tuesday, especially south of I-80

&&

.DISCUSSION...
Issued at 324 AM CDT Sat Sep 21 2024

Dense fog blankets northwest Indiana and extreme eastern IL near
the IN border early this morning. Dense fog should persist
until after sunrise with some potential for a bit more westward
expansion into IL. Will monitor trends for possible westward
expansion of the dense fog advisory.

Once the fog burns off this morning, expecting mostly sunny
skies with just some passing mid-high level cloudiness today.
Guidance is in good agreement that 925mb temps will peak in the
mid 20s Celsius this afternoon, which is about degree or so
warmer than yesterday. Guidance pretty unanimously exhibited a
low bias with high temps Friday and have trended highs today
close to the warmest guidance, with forecast highs potentially
threatening the record highs for both Rockford (94F in 1920) and
Chicago (94F in 2017).

Our CWA will lie east of the axis of deeper moisture which
should be creeping into western IL this afternoon. The shallow
low level moisture over moist of our CWA should mix out with
dewpoints falling at least into the 50s, if not locally into the
40s this afternoon. Scattered thunderstorms could develop this
afternoon over western Illinois as the richer low level theta-e
air mass advects eastward, with a chance that some of these
storms could creep into our western CWA very late this
afternoon. Expectation is that this activity would weaken as it
outruns the better moisture and likely meets its demise as it
chokes on the drier air mass over our CWA early this evening.

As a cold front begins to move gradually southeast toward
northern IL tonight, the axis of deeper moisture pooled ahead of
the boundary is progged to overspread the area later this
evening into the overnight hours. While details in timing and
strength exists among the various models, there is a pretty
consistent signal in guidance that there could be a lower
amplitude shortwave potentially enhancing coverage of scattered
showers and thunderstorms at times tonight across the CWA.

Somewhat complex pattern Sunday with strong northern stream
trough over Manitoba with weaker trough axis extending south
into the mid-Mississippi Valley potentially interacting
favorably with a lead impulse emanating from an upper low
emerging out into the central Plains. Confidence is increasing
that there could be a period or two of some beneficial rains
Sunday with potentially a couple of embedded thunderstorms as
well. Rain should have a tendency to move east and potentially
end from the west during the afternoon. Given the slow moving
front and unseasonably rich moisture pooling near the front,
there is growing confidence that many areas could see 24 hour
rainfall totals of a half inch to around an inch of rain tonight
into Sunday. Embedded thunderstorms could result in some totals
well over an inch to approaching 2 inches.

Front and better rain chances should push south and probably out
of the CWA Sunday night into Monday. The complexity of the
pattern with a northern stream trough interacting with the
southern stream upper low over the Plains lends itself to higher
than normal uncertainty in precip chances. Increasingly, it
looks like late Sunday night through Monday will be mostly rain
free, particularly north of I-80. NBM pops have trended
downward, but are probably still too high, but opted to let them
ride given the uncertainty in this pattern.

The central Plains upper low is progged to weaken to an open
wave as it moves across the Mississippi Valley Monday night
into Tuesday. This could result in another period of rain and
possibly even some thunder depending on how far north this wave
tracks. Still uncertainty in how that will play out and NBM pops
reflect this well with mostly chance pops, highest southern CWA.

Cooler, more seasonable temperatures are expected Sunday into
early next week behind tonight`s front. Medium range guidance is
advertising a very blocky pattern to develop over North America
later this week. Models often struggle with the evolution of
blocky patterns like what is progged later this week, so not a
lot of confidence in the details. For now, it looks like Rex
Block type pattern with northern stream ridging over the upper
Great Lakes and southern stream upper low somewhere in the
central Plains or lower Missouri Valley region. If this Rex
Block develops as ECMWF suggests with pretty good ensemble
agreement, then our weather will hinge on the exact placement of
these features. If upper low remains far enough to our south and
west, more dry and warmer than average weather is possible later
in the week into next weekend, but stay tuned, pattern
evolutions like this are prone to significant model swings from
run to run.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1245 PM CDT Sat Sep 21 2024

Key messages:

- A lake breeze is expected to to provide northeast winds at
  GYY, east winds at MDW, and southeast winds at ORD - though
  magnitudes should be 10 knots or less

- A front will move through from west to east providing chances
  for showers and thunderstorms around northern Illinois and
  northwestern Indiana Sunday

There is a line of showers and storms over the Mississippi River
gradually moving east, but are starting to fall apart as they
interact with the drier air in Illinois. The line will be
monitored, but given the lower confidence in impacts, TAFs were
kept dry through this evening. The main impact in the afternoon
will be a lake breeze that develops. Winds ware expected to turn
to the northeast at GYY, near east at MDW, and southeast at ORD.
However, wind speeds should be at or less than 10 knots.

A front is expected to move across the airspace early Sunday
morning. Confidence with the onset of rain is low at this time,
it was opted to maintain the going PROB30 mention prior to 12Z
Sunday morning. Confidence then increases with showers at the
terminals after 12Z. While there will be instability present,
latest guidance is keeping the threat of storms more isolated
and embedded in nature. Therefore, the VCTS mention was
maintained in the latest TAF. The chances for thunder are
expected to diminish after 18Z, though showers should remain
around the terminals.

Winds ahead of the boundary will be out of the southeast before
flipping to the southwest. Guidance is suggesting that by
midday, wind directions will slowly become more westerly and
even northwesterly after 21Z. A lower level jet aloft may
develop and with proper mixing, occasional stronger gusts up to
20 knots are possible Sunday afternoon.

DK

&&

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

&&

$$

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