931
FXUS63 KLOT 041149
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
649 AM CDT Thu Jul 4 2024

.KEY MESSAGES...

- Showers and isolated thunderstorms mainly along and well
  south of I-80 mid morning into early afternoon with brief
  moderate rainfall

- Isolated thunderstorms late this afternoon into early
  evening, low confidence on coverage.

- Mostly dry conditions for the weekend, but there will be increasing
  rain chances late Sunday through Monday.

- Seasonable temperatures will continue through next week.

&&

.DISCUSSION...
Issued at 309 AM CDT Thu Jul 4 2024

Through Friday:

The first weather feature of interest for Independence Day is a
convectively induced vort/MCV across northern Kansas/western
Missouri. Satellite/radar depiction also nicely portrays some
warm advection induced showers and thunderstorms ahead of a
broad upper trough axis back across the northern Rockies. At the
surface, a cold/stationary front extends from southern Missouri
northeastward to the southern/eastern Great Lakes, with a high
PWAT airmass along and south of the front feeding into the
southwest flow.

The lead upper jet core will drive through northern IL this morning
and with the continued southwest flow this morning, the frontal
boundary will lift a bit farther north as well and try to bring some
precipitation with it. Upper air analysis depicts a relatively dry
airmass below 500 mb, and there is some ridging closer to the
surface, analyzed in the 850-925 mb and surface charts. A bit deeper
saturation is forecasted to be focused south of I-80 in response.
There may be a decent amount of echoes on the radar later this
morning initially with the warm advection, however with the dry air
coupled with mid level lapse rates being quite poor and not very
strong low level winds, whatever reaches the ground will be more
showery/sprinkely in nature. The main convectively induced wave
may try to drive some of this farther north in the late morning
to early afternoon. Whatever does make it through could have
some brief moderate rainfall given PWATs around 1.75",
especially along and south of Highway 24. With the passing of
the main wave, nebulous forcing would then be limited to the
front and the lake breeze boundary. Thus we would expect
precipitation coverage would still be somewhat limited, and
thunder coverage would not be high either - isolated at best.

For tonight, the mid/upper level trough axis will encroach on the
mid Mississippi Valley. Associated height falls and upper jet
segment look to lead to an expansion of showers and potentially a
few thunderstorms along the frontal boundary. There is some
uncertainty as to how quickly this will expand into northern
Illinois, plus the better warm advection  Chances look to be
around 15-30% by mid evening (9 PM) along the I-39 corridor,
with higher chances later. Coverage would expand more so late
in the evening and overnight toward the Chicago area.

The main low will pass to our north and the stronger warm
advection will be across central Illinois in Indiana, thus we
get somewhat dry slotted, but will need to maintain some widely
scattered shower chances into Friday morning mainly along and
east of I-55. Scattered showers would then spread in from the
northwest to areas north of I-80/88 in the afternoon as the
trough axis will be passing over area. The core of colder
temperatures aloft do slide by across Wisconsin, thus the
thunderstorm probability is low (20% or less). It will also be
breezy behind the system as the surface low continues to deepen
across lower Michigan.

KMD


Friday Night through Wednesday:

The upper-level trough that brought us the rain chances on
Friday will begin to pivot into Lower Michigan and the eastern
Great Lakes Friday night. As this occurs, modest upper-level
height rises are expected to develop overhead by Saturday
morning which will diminish any lingering showers Friday evening
and promote rain-free conditions through the day on Saturday.
Given that the area will also be behind a cold front during this
time, dew points should fall into the lower 60s generating
modest humidity levels. Though temperatures are still expected
to be in the seasonable category with readings forecast to be in
the upper 70s and lower 80s Saturday afternoon and low to
mid-60s both Friday and Saturday nights.

The modest ridging is forecast to drift eastward Saturday night
into Sunday as an upper-level shortwave trough ejects into the
upper Midwest. While initially it looked as if this trough would
struggle to have much rain with it, guidance has started to
show a complex of showers and thunderstorms developing over the
northern Plains Saturday night and moving eastward towards IL by
Sunday morning. However, the dry low-levels and resultant
sparse instability forecast to be over northern IL Sunday
morning should weaken (and possibly erode) any showers/storms as
they cross the Mississippi River. Thus, suspect that any rain
chances Sunday morning will remain low (<15%) for areas east of
I-39, but a stray shower in our far western CWA (west of I-39)
cannot be ruled out.

Heading into Sunday afternoon, the aforementioned shortwave is
forecast to pivot into the northern Great Lakes as a secondary
trough pivots into southern WI. This second wave is what
previous forecasts have been noting could bring us some
scattered showers and/or thunderstorms Sunday afternoon. While
guidance does continue to produce some slight chance to chance
POPs (20-30%) across northern IL and northwest IN, the limited
low-level moisture and dry mid-levels may prove sufficient to
hold any rain off until better moisture arrives Sunday evening.
Therefore, expectation for many dry hours on Sunday remains
valid with temperatures forecast to be slightly warmer than
Saturday as highs warm into the low to mid-80s.

Rain chances are expected to increase Sunday night as the
aforementioned trough becomes more situated overhead and better
moisture filters in at the surface. This trough looks to be
notably slower moving with guidance continuing to fester showers
and storms overhead through much of the day on Monday and
possibly into Tuesday. Therefore, have maintained the 40-50%
POPs offered by the NBM for this period which looks reasonable.
While rain chances should diminish on Tuesday as the core of the
trough pushes east, guidance does show at least broad troughing
hanging around the Great Lakes through much of next week. So
additional periods of showers and/or thunderstorms may linger
throughout the week depending on available moisture and
instability. Otherwise, expect temperatures to remain near
typical early July readings in the low to mid-80s.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 648 AM CDT Thu Jul 4 2024

Forecast concerns for the terminals...

- Lake breeze to turn winds easterly at Chicago terminals later
  this morning

- Low end chance for showers and storms along the lake breeze
  this afternoon with better chance for showers late tonight
  into early Friday morning

- Lowering ceilings with showers overnight through Friday
  morning

- Breezy westerly winds Friday afternoon


Generally quiet weather conditions are expected at the terminals
this morning as light north-northwest and VFR cirrus prevail.
Though, winds will gradually become easterly at ORD, MDW, and
GYY late this morning as a lake breeze develops and surges
inland. While the more pronounced winds with the lake breeze may
hold off until 18z-19z this afternoon, the wind shift looks to
occur between 14z and 15z. Elsewhere winds will become more
southeasterly this morning before turning southerly this
afternoon and evening.

Dry conditions should prevail through early evening at the
terminals, but the potential for isolated showers and
thunderstorms to develop along the lake breeze remains. Given
that instability this afternoon looks modest and the lack of
upper-level forcing, confidence in any showers/storms
developing this afternoon remains too low for a formal mention
at this time. However, better shower and thunderstorm chances
are expected late this evening and overnight as a cold front
moves through the area. While confidence is higher on rain
occurring overnight, the timing and coverage of thunder tonight at
the terminals remains uncertain. Therefore, have decided to
maintain the PROB30s for -SHRA at the Chicago area terminals but
did introduce a PROB30 for TSRA at RFD where potential looks to
be a bit higher. Regardless, showers/storms should come to an
end around daybreak Friday morning leaving mostly dry
conditions for the rest of the TAF period.

Additionally, ceilings are also expected to lower with the
showers overnight and persist into Friday morning. Guidance
remains in decent agreement that MVFR ceilings will be the most
common, but pockets of IFR are possible especially near RFD.
While ceilings should lift back to VFR by mid-morning on Friday,
there is a signal that the lingering upper trough overhead may
keep lower ceilings around through the rest of the period. Since
winds are also expected to increase Friday morning I have
decided to lean more towards the quicker improvement.

Finally, winds will become westerly behind the front Friday
morning with speeds increasing into the afternoon. Gusts are
expected to top out in the 20 to 25 kt range Friday afternoon
but there is the potential for gusts to near 30 kts at times.

Yack

&&

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

&&

$$

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