207
FXUS63 KLOT 131357
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
857 AM CDT Mon May 13 2024

.KEY MESSAGES...

- Showers and thunderstorms will move in this afternoon and
  persist through the day on Tuesday. These storms may produce
  locally strong wind gusts in excess of 40 mph this afternoon
  and early evening.

- A localized area of heavy rainfall likely this afternoon
  through tonight, but uncertainty remains in exact area.

- Chances for showers and storms appear poised to return by the
  end of the week.

&&

.UPDATE...
Issued at 855 AM CDT Mon May 13 2024

Broader cloud cover has been generally non-existent across much
of the forecast area this morning, allowing temps to increase
to around 70F prior to 9am. Recent CAM guidance that is inline
with current temp and cloud cover trends suggest max temps
should reach or surpass 80F nearly areawide, especially into the
Chicago metro. Dew points may also struggle to rise as quick as
previously expected given robust low-level mixing this morning.
Moisture advection from the southwest is expected to slowly
increase dew points into this afternoon, albeit a bit more
subdued given under-performing dew points upstream.

As for forcing, a mid-level wave with ongoing convection
near/over the AR/KS/MO/OK common borders is expected to be the
main driver of increasing coverage of convection this afternoon.
This feature is observationally trending slightly slower than
with previous guidance, so arrival of convection into the
forecast area may be delayed into mid-afternoon.

With the additional mixing this morning, a currently growing
DCAPE reservoir up to 1000 J/kg should begin to diminish with
time this afternoon with increasing dew points. However, any
delay with an increase in cloud cover will result in a slightly
warmer and more deeply mixed PBL ahead of any upstream
convection in central Illinois. Expectations are therefore that
outflow driven single cells in central Illinois may eventually
congeal into loosely organized multicell clusters capable of
producing pockets of sub- severe wind gusts in excess of 40 mph
across northern Illinois late this afternoon into early evening.

Kluber

&&

.DISCUSSION...
Issued at 343 AM CDT Mon May 13 2024

Through Tuesday:

The broad upper trough and associated upper low over the
central Plains will continue to advance into the mid-Mississippi
Valley today increasing low-level warm-moist advection as it
does so. Temperatures aloft will also cool as the trough
approaches with surface temperatures and dew points climbing
into the upper 70s and lower 60s, respectively. These
conditions will allow for ample destabilization and give way to
widespread shower and thunderstorms this afternoon. At the same
time, a cold front, currently analyzed across central Wisconsin,
will advance southward into northern IL during the later
afternoon and evening hours. As the front arrives the low-level
moisture is expected to pool along the boundary, which in
combination with relatively slow storm speeds (around 10 to 20
mph) and front parallel motions, is expected to establish a
focus for training storms and locally heavy rainfall.

While guidance remains in great agreement on this evolution,
the convective nature of this event continues to provide
uncertainties as to where exactly the axis of heaviest rainfall
will occur. The latest 00z guidance suite has started to hone in
a couple of potential areas, with global models have continued
to favor areas along the I-88 corridor while the higher
resolution CAMs favor areas along and just south of I-80.
Unfortunately, the aforementioned convective nature will likely
lead to notable changes in these favored areas until storms
develop and we can see how their respective outflows are
effecting the movement of the cold front. In the meantime, our
official forecast has decided to take a more middle of the road
approach and highlight the areas between I-88 and I-80 with
locally higher POPs and rainfall totals to reflect the 00z
guidance consensus. Additionally, our official forecast also
aligns well with the slight risk (level 2 of 4) area for
excessive rainfall from WPC. Regardless of where the axis of
heavier rain set ups, those within this axis should expect
rainfall totals between 1.0 and 2.0 inches with amounts
generally in the 0.5 to 1.0 inch range elsewhere.

Given the aforementioned uncertainty and the fact forecast rain
values are under Flash Flood Guidance thresholds, confidence is
still too low for the issuance of any flood outlook products at
this time. However, if these amounts occur over urban and/or
suburban areas, some instances of ponding or minor flooding may
occur.

Heading into Tuesday, the cold front is expected to push into
central IL and IN which will move the axis of heavier rainfall
out of our forecast area for Tuesday. However, the upper trough
will still be pivoting overhead as it tracks into the Ohio River
Valley which means showers and storms will persist through
Tuesday afternoon. Though the greatest coverage of showers and
storms is expected to be south of I-80 so expect gradually
improving conditions for those to the north. Aside from the
precipitation, breezy northeast winds are also expected on
Tuesday behind the front with gusts of 20 to 25 mph likely. The
northeast winds will also advect in notably cooler air which
will keep highs in the low to mid-60s for most of the area with
those near the lake expected to remain in the 50s due to
onshore flow.

Yack

Tuesday Night through Sunday:

On Wednesday, a surface ridge will move through the Great Lakes
allowing for plenty of sunshine and a rebound in highs back into
the upper 60s to lower 70s. With an onshore component to the
wind, temperatures along the Lake Michigan shoreline will remain
in the upper 50s to lower 60s. Outside a few lingering showers
in the morning in northwestern Indiana, Wednesday looks dry.

The forecast from Thursday onward carries much lower forecast
confidence than is typical due to the apparent differences in
how a myriad of upper-level shortwaves riding through the jet
stream in the northern Pacific Ocean will interact with a soon-
to-be eastward-drifting cut-off low emanating from southern
California. Will the cut-off lows phase with the northern-jet
stream leading to aggregate troughing across the Great Lakes, or
will they have a missed connection leading to pronounced ridging
across the northern US? Timing will be everything, and right
now, the ensemble envelope is as vast as can be. (Not to
belabor the point, the range in EPS 500mb height and 850mb
temperature spans nearly 30 dam and 10C, respectively, by
Saturday afternoon). Perhaps the only item of confidence is that
there should be a wave (or waves?) of precipitation sometime in
the Thursday to Sunday timeframe, warranting more or less
continuous low- to mid-range chance PoPs (20-50%). Anything
beyond that is pretty much a throw of a dart at this point.

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 608 AM CDT Mon May 13 2024

Key Messages:

- Scattered showers and thunderstorms are expected this
  afternoon and evening, primarily between 19-01Z.

- While southwesterly winds are preferred during daylight hours,
  erratic wind directions and magnitudes are possible in and
  around thunderstorms.

- Winds will turn northeasterly after sunset as ceilings build
  downward into MVFR and possibly IFR. Showers will prevail
  overnight and tomorrow morning.

Discussion:

An elongated low pressure system with embedded mesoscale
convective vortices continues to lift northeastward toward the
Mississippi River and will set the stage for a busy TAF period.
Heating of the moistening boundary layer will lead to minimal
convective inhibition as early as 17-18Z, supporting blossoming
showers and thunderstorms on a scattered basis. Coverage of
showers and storms is expected to maximize sometime in the 21 to
01Z timeframe, before the loss of sunlight leads to primarily
showers prevailing after dark. When it will be all said and
done, there may be an unusually long period of thunder at least
in the vicinity if not directly overhead some or all of the
terminals this afternoon and evening. For this reason, the
outgoing TAFs will feature prolonged periods of VCTS with
targeted TEMPOs. On-demand adjustments to the TAF appear
unavoidable this afternoon as the first convective cells develop
and overall trends become established.

Confidence remains lower than average on the forecast for wind
direction from this afternoon onward due to chaotic and merging
thunderstorm outflows. While a general southwesterly direction
is still preferred this afternoon, the wind direction and
magnitude may vary considerably from time to time in and around
thunderstorms. As the coverage of thunderstorms relaxes after
sunset, winds should slowly adopt a northeast to easterly
component before a cold front turns them decidedly northeasterly
between 06-07Z.

Primarily VFR ceilings and visbys are expected today, though
pockets of MVFR ceilings IFR visibility may occur beneath
thunderstorms. After sunset, ceilings should start to build
downward especially behind the aforementioned cold front, with
high confidence in MVFR developing and medium confidence in
pockets of IFR developing. Cigs may only gradually improve
toward the end of the TAF period as showers prevail.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 4 AM Tuesday to 7 AM CDT Wednesday
     for the IL nearshore waters.

     Small Craft Advisory from 7 AM Tuesday to 7 AM CDT Wednesday
     for the IN nearshore waters.

&&

$$

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