516
FXUS63 KLOT 141924
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
224 PM CDT Tue May 14 2024

.KEY MESSAGES...

- Showers and embedded thunderstorms will continue through
  around sunset roughly south of a Pontiac to Valparaiso line.

- A narrow axis of heavy rainfall (20-30% chance of >1" of rain)
  resulting in localized flooding remains possible through this
  evening over locations that experienced heavy rainfall across
  Livingston, Ford and Iroquois counties on Monday.

- Chances for showers and thunderstorms will return on Thursday,
  followed by warming temperatures into the weekend.

&&

.DISCUSSION...
Issued at 224 PM CDT Tue May 14 2024

Through Tonight:

We remain under the influence of an upper-level low over
central Missouri this afternoon, with the associated surface low
over far western Kentucky. An ongoing light rain band near the
IL/WI state line will gradually dissipate over the next few
hours as mid/upper forcing begins to wane north of the exiting
low. Focus then settles on an active mid-level deformation axis
stretching from near Pontiac ENE toward South Bend this
afternoon. This axis has roughly aligned with an exiting (and
stalling) surface boundary in an area with locally higher low-
level instability after some morning sun. While overall surface-
based instability is modest at best, marginal lapse rates and
low LCLs will support a general axis of broken showers and
embedded storms that should persist through sunset.

The main concern is on localized flooding for areas that
experienced heavy rain across east-central Illinois on Monday.
Had considered a targeted Flash Flood Watch for Livingston, Ford
and Iroquois counties through early evening, but the expected
small footprint of higher convective rates supported more of a
tactical gridded/graphical forecast approach. Would not be
surprised if an areal Flood Advisory is needed at some point
through early evening, but the likelihood of a Flash Flood
Warning remains under 30%. If a Flash Flood Warning were needed,
the most favored area would be from southeast Livingston into
northwest Iroquois counties where the highest axis of rain on
Monday has resulted in a Flash Flood potential with rates as
little as 1"/3hr this afternoon.

Overall activity will only slowly shift southeast through the
night as the upper-level drifts east, with some showers
lingering into Wednesday morning south of a Watseka to
Rensselaer line.

Kluber


Wednesday Night through Monday:
Issued at 342 AM CDT Tue May 14 2024

The forecast for the long-term period continues to suffer from
unusually low forecast confidence owing to a myriad of forecast
scenarios originating from how the northern jet stream may or
may not interact with a cut-off low pressure system drifting
eastward across the southern US.

Wednesday night into Thursday, a baggy upper-level trough is
expected to pass over the Rocky Mountains just to the north of an
eastward-drifting cut-off low pressure system drifting away
from southern California. The amount of phasing that will take
place between the two features remains unresolved in available
model guidance. However, ensemble guidance appears to favor each
maintaining some sort of individual integrity as they progress
eastward Wednesday night. Somewhat broad upper-level diffluence
across the Southern Plains between both features atop a
northeastward-moving moisture axis should allow for clusters of
thunderstorms to develop and instigate their own MCVs/vort
maxes Wednesday night, which would then be advected
northeastward toward the Mississippi River Valley on Thursday.
Even if one such MCV/vort max were to then move over our area
Thursday afternoon, chances for showers and thunderstorms would
depend entirely on how fast low-level flow can flip from
northeasterly to southwesterly during the day to facilitate the
rapid northeastward progression of any meaningful instability.
While NBM guidance paints likely (55+%) PoPs across our entire
area Thursday afternoon into Friday, there are hence forecast
scenarios where parts (much?) of our area actually ends up dry.

Friday through the weekend, ensemble model guidance appears to be
trending toward the southern-stream cut-off low pressure system
remaining, well, cut-off from the northern jet stream along the
US/Canadian border, causing it to remain well to our south and
along the Gulf Coast. The tandem effects of mid-level warm-air
advection on the north side of the cut-off low into the middle
Mississippi River Valley and the digging of a secondary trough
into the Pacific Northwest would then favor the development of
long-wave ridging across the northeastern US, altogether
supporting a pleasant slide into the weekend. Highs Saturday
and Sunday would be poised to surge into the upper 70s to lower
80s (mid 80s on Sunday?), and chances for showers and
thunderstorms would be relegated to Sunday evening onward as the
pattern "opens up" to upper-level shortwaves emanating from the
southwest.

However, there remain forecast scenarios where the cut-off low
phases, even partially, to the northern jet-stream, leading to
either a quasi-zonal jet or even pronounced troughing across the
Great Lakes by this weekend. The net effect of even a "flatter"
upper-level pattern will be more seasonable temperatures and
waves of rain for at least parts of our area, particularly
Friday into Saturday. It`s tough to say which forecast outcome
is more likely by this point, so our gridded database will
follow a true "middle of the road" approach with just above
average temperatures and low-end chances (20-30%) for rain from
Friday onward.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1226 PM CDT Tue May 14 2024

As an upper-level low drifts south of the area today, MVFR
ceilings will persist through the afternoon before becoming SCT
this evening. A few -SHRA are expected to develop as far north
as northwest Indiana mid-afternoon through early evening and may
brush the MDW vicinity. Otherwise, NE/NNE winds of 10-15 knots
are expected though the period.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 AM CDT Wednesday for the IL and
     IN nearshore waters.

&&

$$

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