Area Forecast Discussion Issued by NWS Chicago, IL
000
FXUS63 KLOT 151931
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
231 PM CDT Mon Apr 15 2024
.KEY MESSAGES...
- Periods of showers and storms Tuesday and Wednesday (90-100%
at times) with a potential for some severe thunderstorms,
especially Tuesday evening
- Gusty winds to 25 to 35+ mph Tuesday and Wednesday
- Another chance for showers (50-60%) and a slight chance for
thunderstorms Thursday into Friday
- Drier, but cooler conditions expected over the weekend
&&
.DISCUSSION...
Issued at 217 PM CDT Mon Apr 15 2024
Through Wednesday:
A surface cyclone is beginning to spin up this afternoon on the
lee of the Rockies as an upper low tracks across the
Intermountain West. The low-mid level wind field will really
ramp up around the system tonight into tomorrow and we`ll find
the surface low deepening quickly as it works across the Plains.
The storm`s warm front will push northward into the CWA
tomorrow morning into the afternoon with instability building
atop the region behind the front. Models like the idea of an
initial push of showers and a few thunderstorms beginning as
early as the morning, but chances become notably better after
noon. It looks as though we should remained capped to surface-
based convection through most of the afternoon and this elevated
activity does not appear to pose a big severe threat. If
anything elevated does go severe with this initial wave, it will
likely be closer to mid afternoon when we`ll have more elevated
instability to work with. The primary threat with any earlier
severe convection would be large hail.
The severe potential will increase approaching the evening.
Continued warm and moist advection will erode the cap and allow
for surface-based storms to go up in our west as early as the
late afternoon. Roughly 1,000-1,500 J/kg of MUCAPE will have
built over the area by then. Perhaps the most eye-catching
component of this environment is an awfully impressive low level
shear profile featuring as much as 40 kt of veering 0-1 km
shear. This will support the potential for organized, severe
convection with any surface-based storms and carry with it a
threat for some tornadoes later in the day. High res guidance is
honing in on a cluster of thunderstorms forming ahead of a low-
mid level speed max and closing in on the western CWA early in
the evening. This may be our key feature to watch for severe
weather tomorrow. The tornado potential as well as the overall
severe threat will be greatest in the western CWA out near I-39
where we`ll find the better instability and kinematics.
Nonetheless, the threat will certainly remain present across the
CWA through the remainder of the evening.
The brunt of the severe potential will wrap up close to midnight
after this primary cluster of storms moves off to our east.
While we will remain in the storm`s warm sector tomorrow
night, a lack of forcing and a revitalized capping inversion
should put us in a lull overnight. There is a chance for a
second round of strong to marginally severe storms Wednesday
morning. The surface cold front will charge across the CWA
through the morning which could help parcels tap into the still-
present instability aloft. Guidance is inconsistent on resolving
precip ahead of this front, but can mostly agree that the
greatest chances of seeing additional storms Wednesday morning
will be east of I-55 where we`re seemingly most likely to break
through the stable air. A few scattered showers may then develop
around the center of the low as it passes to our north and fall
on parts of the area through the rest of the day, especially
north of I-80.
Meanwhile, there is quite a bit of spread in temperature trends
for tomorrow. It will be heavily reliant on how the warm front
behaves during the day and the magnitude of the warm advection
behind it. In the forecast, went with the favorite and most
consistent signal which is lower 70s in our north and upper 70s
south of I-80. A potent low level jet will move atop the area
during the day with upwards of 45 kt expected at 925 mb and
surface winds will respond accordingly. The better part of the
day should see gusts out of the southeast between 25 and 35 mph
with a few gusts to 40 mph expected. Winds may want to stay
rather gusty overnight tomorrow. While we`re expected to mostly
decouple and restabilize the boundary layer overnight, such a
tight vertical wind gradient means any even shallow mixing will
result in some gusty winds. Relatively deeper mixing on
Wednesday will cause strong winds to return during the day,
similar in magnitude to what we`re expecting for Tuesday.
Doom
Wednesday Night through Monday:
As the previous low drifts eastward, drier conditions will
slowly move in overnight into Thursday. Winds will become more
west-northwesterly providing a bit of cooler air advection
helping overnight temperatures back down into the 40s, with
Thursday afternoon high temperatures in the upper 50s and lower
60s.
As an upper level low deepens over Manitoba, an associated mid-
level trough will swing over the northern Plains and Great
Lakes pulling in additional moisture from the south with
stronger vorticity. Models are currently projecting stronger
instability to stay a little farther to the south, but kept the
slight chance for thunderstorms to develop for areas south of
I-88 as a line of showers moves through Thursday afternoon into
Friday.
The previously mentioned upper level low will gradually move
east toward Quebec and allow steering flow to become zonal
aloft. While subsidence aloft keeps the forecast dry through the
weekend, cold air advection will help drive temperatures
cooler. Most guidance is suggesting that lows could drop back
down into the 30s over the weekend. However, there is some
disagreement in high temperatures. The Euro has slightly lower
cloud cover so it suggests that highs could be back in the low
50s, while the GEFS and its greater cloud cover caps high
temperatures in the upper 40s. A lot will have to depend on how
quickly that low can move east and take some of the cloud cover
with it.
DK
&&
.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1230 PM CDT Mon Apr 15 2024
Key Aviation Impacts for the current TAF period:
- Easterly wind gusts increase to around 25 knots after 15Z
Tuesday
- A strong storm system approaches the Chicago Metro Area
Tuesday night
There are no short term impacts to area terminals. Conditions
through 12Z tomorrow will be VFR with easterly winds around 10 knots
with occasional gusts up to 15 knots.
A strong low pressure system will move northeastward from the Great
Plains toward southern Wisconsin tomorrow. There is a chance for
some elevated showers to approach the area tomorrow morning.
Instability is not overly robust and forcing would have to be strong
enough to tap into it, but there is enough to keep a PROB30 group
for the potential for thunder to develop. Winds will remain out of
the east and increase with gusts to 25 knots (isolated up to 30
knots) by Tuesday afternoon.
The main wave of convection is expected after 21Z. Showers are
expected to move over the area from the southwest to northeast with
the potential for thunderstorms to develop with them. While timing
may be adjusted in later forecast packages, there was high enough
confidence to add it to the TAF as a TEMPO for ORD and MDW. Vis and
cigs may reduced down to MVFR levels during the strongest cells as
they pass over. Lastly, towards 00Z, a south southeasterly low
level jet will begin to ramp up at 925 mb. If winds continue to
mix down through the night, that will help inhibit any LLWS
threat, but a couple models have winds decoupling at the
surface. For now, it was left out of the 30 hour TAF due to
lower confidence at the end of the forecast period, but will be
monitored.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 PM Tuesday to 7 AM CDT Wednesday
for the IL nearshore waters.
Small Craft Advisory from 4 PM Tuesday to 4 AM CDT Wednesday
for the IN nearshore waters.
&&
$$
Visit us at weather.gov/chicago