550
FXUS63 KLOT 091819 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
119 PM CDT Tue Jul 9 2024

.KEY MESSAGES...

- The remnants of Hurricane Beryl will move through the general
  region later today into early Wednesday with a threat for
  gusty winds and flooding, particularly along and east-
  southeast of the I-55 corridor.

- Isolated to scattered showers and storms may continue Thursday
  and Friday, followed by increasing temperatures and humidity
  levels this weekend.

&&

.UPDATE...
Issued at 1030 AM CDT Tue Jul 9 2024

All eyes are on the remnants of tropical system Beryl with the
center of low pressure now pinwheeling steadily northeastward
across Arkansas. Ahead of it, numerous diffuse boundaries are
analyzed across our region, including a wavy quasistationary
front which stretches from near Valparaiso to Pontiac to Quincy,
and another one across SE Wisconsin and far northwest Illinois.
Aloft, additional confluence axes are noted in the 850-700 mb
layer which are forecast to drift northward towards the I-80
corridor through the afternoon. Finally, a lake breeze will also
develop across northeast Illinois.

A very moist airmass has pushed into the area, with PWATs
near 2 inches on latest objective analyses and the 12z Lincoln,
IL sounding, with a classic "tropical look" with deep saturation
and a relatively warm column characterized by near moist-
adiabatic lapse rates.

Arcs of slow-moving (nearly stationary) showers and the occasional
thunderstorm have developed along one of the aforementioned
surface boundaries in the vicinity of the Kankakee River. These
exhibit classic low-echo centroids, with pretty much all of the
45+ dBZ returns confined below the freezing level. We`ve noted
some localized rates nearing 1-2 inches per hour with these
cells on several personal weather stations.

While this set-up may not fit all of the definitions of a
Predecessor Rain Event, the dynamics/driving factors are the
same, with augmentations to the anticyclonically-arcing upper
jet from the incoming post-tropical low leading to an enhanced
reservoir of divergence aloft. As this enhanced lift interacts
with low-level boundaries, slow-moving showers/storms result.

Going forward, the main concern today will be on highly
localized corridors of heavy/torrential rainfall leading to
instances of flooding which will be tied to the aforementioned
boundaries. As such, we`re not concerned with a widespread
flood/flash flood threat through the balance of the day today,
but isolated rainfall amounts over 3-4 inches are certainly
plausible. While an occasional lightning strike is possible
today, widespread electrification appears unlikely given the
moist adiabatic profiles.

We`ll continue to assess incoming guidance as it relates to the
heavy rain and more widespread flood threat tonight with the
afternoon forecast package. There are also some hints in hires
output that winds may need to be nudged upwards, as well.

Carlaw

&&

.DISCUSSION...
Issued at 332 AM CDT Tue Jul 9 2024

Through Wednesday night:

The main focus continues to revolve around the increasing threat
for periods of very heavy rainfall and strong winds later today
into Wednesday morning as the remnants of Beryl move into the
Lower Great Lakes.

As of this writing, the center of Tropical Depression Beryl was
located over southeastern AR, and it`s movement is north-northeast
just under 20 mph. Model and ensemble guidance have been coming
into better agreement with the track of Beryl later today into
Wednesday as it begins its post tropical transition. While this
is the case, modest ensemble spread is still noted tonight into
Wednesday morning as the system tracks northeast from just
southeast of St Louis to near Fort Wane, IN. While these
differences could still play a role in the exact placement of the
heaviest rainfall axis, confidence continues to increase that this
axis will fall roughly along and east-southeast of the I-55
corridor. For this reason, we have opted to issue a Flood Watch
from late this afternoon through Wednesday morning for this
general area. We also opted to add Cook county into this watch,
though it is possible that the far southern portion of the county
(from the city south) experiences the highest rainfall amounts
(and hence the highest threat for flooding.

As Beryl approaches into the area through the day, it will
began to lose it`s tropical characteristics while phasing with
an upper-level trough approaching the Great Lakes from the
northwest. This will set the stage for a notable increase in
synoptic-scale forcing as a coupled upper-level jet structure
sets up overtop of a strengthening band of low to mid-level
frontogenesis oriented roughly along the I-55 corridor. Strong
and deep forced ascent within this region will act upon
tropical-like moisture (PWATs approaching 2.25"), resulting in
very efficient warm-rain processes. The net result of this will
be a band of very heavy rainfall, which should peak in
intensity tonight in and even near the watch area. Rain rates of
1"+ per hour will be likely within this band, and this could
result in total rainfall amounts in excess of 5 inches across
part of the watch area by late Wednesday morning. Rainfall
amounts of this magnitude would support significant rises and
flooding of small streams and rivers, and possibly even some
instances of flash flooding.

In addition to the heavy rainfall, a period of strong gusty
northeasterly to northerly winds are expected across eastern IL
into northwestern IN tonight into early Wednesday morning as the
center of circulation shifts into northeastern IN. The strong
gradient along the northwestern periphery of the system will
support gusts up to 40 mph. This will result in building waves
on southern Lake Michigan, and the onset of dangerous to even
life threatening swimming conditions along the southern shores
of the lake tonight through Wednesday. A small craft advisory
and beach hazard statement will be needed as a result.

The remnants of Beryl will begin to exit our area into the
afternoon on Wednesday. While this will end the the heavy rain
threat, the threat for a few afternoon showers, and possibly even
a few thunderstorms will continue as a second mid-level impulse
begins to shift into the Midwest late in the day.

KJB


Thursday through Monday:

The thinking during the extend period has not changed. Reference
the most recent discussion below.

Thursday and Friday, upper-level troughing will remain overhead and
provide the means for isolated to scattered diurnal showers and
thunderstorms. Looking toward the weekend and beyond, ensemble model
guidance supports the development of an upper-level ridge across the
central United States with the "downward" branch oriented into the
Great Lakes. Such a pattern will support rising temperatures and
humidity levels, as well as chances for episodic thunderstorm
clusters.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 116 PM CDT Tue Jul 9 2024

The primary aviation weather concerns for the 18Z TAF period include:

* A prolonged period of rain today into Wednesday with bouts of
  heavy downpours expected.

* Periods of MVFR to IFR cigs and vsbys associated with the rainfall.

* Particularly at GYY, a potential for strong northerly winds late
  tonight and Wednesday morning.

A large plume of light to moderate rain is in the process of filling
in across northeastern IL early this afternoon. Mostly non-impactful
rain can be expected for just about the rest of the afternoon and
evening. Overnight into early Wednesday is when we expect the
heavier widespread showers and the greatest potential for seeing
MVFR conditions, although the signal for MVFR has been lessening
with time. Nonetheless, several hours of MVFR to even IFR conditions
remain very much in play. Lighter rain is expected through the
morning tomorrow before the rain looks to move off to our east by
early afternoon. The lightning potential throughout the event is
very low, although a couple of stray strikes can`t be disregarded.

Additionally, SE winds under 10 kt will back to NE this evening.
Winds will build overnight while backing to near-northerly with
gusts getting to up to around 20 kt. NNE looks to be favored through
most of the night, though we could teeter on either side of
northerly. NNW becomes favored by early Wednesday morning with gusts
expected to step down through the morning.

Attn RFD: The brunt of the precip will remain just to east of the
RFD airfield and coverage remains somewhat uncertain. Best guess is
they too will see rain for much of this afternoon and evening,
though coverage looks far more scattered and intensity lighter. Rain
will remain possible overnight, although model guidance has been
trending toward a drier forecast at RFD tonight. VFR is favored
through the period barring any isolated downpour.

Attn GYY: Confidence is growing in a period of gusty northerly winds
late tonight into tomorrow morning at GYY. Gusts to 30 kt are now
expected but could very well climb even higher possibly up to around
40 kt. Winds should gradually but steadily ease during the late
morning and afternoon.

Doom

&&

.MARINE...
Issued at 356 AM CDT Tue Jul 9 2024

Tonight into Wednesday, the remnants of Tropical Depression
Beryl are expected to lift northeastward into the Lower Great
Lakes region. As Beryl lifts into northeastern IN Wednesday
morning, it may strengthen a bit (minimum surface pressure near
29.5 inches). This is likely to set up a period of strong
northerly winds on southern Lake Michigan late tonight into
Wednesday morning. Winds to 30 kt are currently forecast, but a
few gale force gusts will be possible over southern Lake
Michigan, centered around daybreak Wednesday. At this point,
confidence in gale- force or stronger winds is not high enough
to warrant the issuance of a Gale Watch. However, a small craft
advisory will be needed. Mariners are encouraged to stay up to
date on the forecast.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through Wednesday afternoon for ILZ013-ILZ021-
     ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...Flood Watch through Wednesday afternoon for INZ001-INZ002-
     INZ010-INZ011-INZ019.

LM...None.

&&

$$

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