074
FXUS63 KLOT 141729
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1229 PM CDT Tue May 14 2024

.KEY MESSAGES...

- Showers and embedded thunderstorms will redevelop this
  afternoon (well south of I-80) and taper after sunset.

- A narrow axis of heavy rainfall (20-30% chance of >1" of
  rain) may materialize along and south of a Morocco, IN to
  Pontiac, IL this afternoon and early evening.

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

&&

.DISCUSSION...
Issued at 342 AM CDT Tue May 14 2024

Through Wednesday:

A broad upper trough continues to slowly pivot its way across
the mid-Mississippi Valley this morning which has allowed for
the development of a baggy surface low across the Missouri
Bootheel and adjacent areas of Illinois. To the north of the
low, a cold front has stalled across northern IL (currently
analyzed near the I-88 corridor) which has trapped a modestly
warm and moisture rich airmass across the majority of our area.
As the trough and surface low gradually move into the Ohio River
Valley this afternoon, the cold front is expected to begin to
drift south which should allow for the redevelopment of showers,
and perhaps even a few thunderstorms, mainly south of I-80.
Precipitation will gradually come to an end from north to south
as the aforementioned front progresses across the area this
evening.

Given that the better synoptic forcing is expected to remain
more focused into central IL and IN, and destabilization today
will be slow due to prevailing cloud cover, shower and storm
coverage today should be lower than yesterday especially with
northward extent. However, PWATs still around 1.3 inches will
once again provide an environment conducive for instances of
locally heavy rainfall. In fact, the latest CAM guidance (and
even some of the 00z global guidance suite) has been in decent
agreement on another narrow axis of locally heavy rainfall
occurring during the 1 PM to 7 PM CDT timeframe. The convective
nature of this set up once again makes pin pointing the exact
location of the heavier rainfall axis difficult due to
uncertainties as to how quick the cold front will surge south
and where the greatest coverage of storms ahead of the front.
While the general consensus is for this axis to form along and
south of a Valparaiso, IN to Pontiac, IL line, this axis could
establish as far north as I-80 or as far south as US-24.

In addition to the uncertainty in location, there is also the
uncertainty in amounts that needs to be discussed. Most guidance
(roughly 50-60%) is keeping rainfall totals within this heavier
rainfall band around 0.5 inches, but there is a notable 20%
potential for amounts to be near if not exceed an inch. If the
higher totals where to verify and occur over the same areas that
saw heavy rainfall yesterday, then we could be looking at a
threat for impactful flash flooding as opposed to nuisance
ponding in ditches and low-lying areas. Given uncertainties in
location and amount, confidence remains too low for the
introduction of a formal Flash Flood Watch at this time.
Therefore, trends will need to be closely monitored this morning
to see if confidence grows enough for such a product.

Aside from the precipitation, expect winds to once again be
breezy this afternoon with gusts around 20 mph out of the
northeast. These northeast winds will also usher in cooler air
off Lake Michigan and keep highs generally in the low to
mid-60s with the exception of those near the lake which should
remain in the 50s. On the bright side, this cool spell is
expected to be short lived as mid-level ridging will build into
the area on Wednesday allowing temperatures to rebound back into
the 70s away from Lake Michigan with ample sunshine as well.

Yack

Wednesday Night through Monday:

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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