078
FXUS63 KLOT 190328
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1028 PM CDT Tue Mar 18 2025

.KEY MESSAGES...

- A storm system will move across northern Illinois bringing
  rain, high winds, and possibly strong thunderstorms to the
  area Wednesday.

- Chances for slushy snow to mix in Wednesday night into
  Thursday morning

- Seasonal conditions this weekend into early next week with
  several chances for precipitation, highest on Sunday

- Seasonably cool temperatures early next week to start
  astronomical Spring.

&&

.UPDATE...
Issued at 1028 PM CDT Tue Mar 18 2025

Main Forecast Changes This Evening:
- Cooler temperatures and northeasterly winds north of a sagging
  front tonight into Wednesday morning
- Wind Advisory for Livingston, Ford, Iroquois, and Benton
  Counties 10am-7pm CDT Wednesday
- Farther south low pressure track on Wednesday resulting in a
  sharp temperature gradient roughly near/north of I-90 corridor
- Afternoon severe threat will in all likelihood be confined to
  primarily near/south of afternoon warm front position, which
  is uncertain, but likely south of IL/WI stateline counties

A very complex and challenging forecast period is in store
through Thursday morning. First order of business was to adjust
for the previously stationary front sagging farther south into
northern Illinois this evening, resulting in cooler temperatures
and north-northeasterly winds. The farther south frontal
position will play a role in Wednesday`s key forecast elements
(winds, temps, convective trends and severe threat). East-
northeasterly winds north of the front will be slower to return
to southerly with northward extent, and as noted above, locales
particularly near the IL/WI stateline are unlikely to see
southerly winds and the expected mild (60s) temps. In fact,
can`t rule out some fog near the lake in far northeast Illinois.
The exact track of the strong upper 980s mb surface low will
dictate the precise location of the warm front in the afternoon
as an arcing dryline approaches.

An area of warm advection driven showers and perhaps some
isolated thunderstorms (due to steep mid-level lapse rates)
should develop over the northeastern half or so of the area in
the morning and then slide east through midday/early afternoon.
South of these showers (well south of the warm front), a very
strong low-level jet with 40+ kt flow down to around 1kft AGL or
less, and temps jumping into the 60s supporting mixing into the
base of the LLJ should result in 45-50 mph gusts across the
southern tier of the CWA toward midday and through the afternoon
(aside from convective influences). These stronger advisory
criteria (45 mph) gusts may spread to near the Kankakee River
Valley or so, which the midnight shift will assess, with respect
to possibly expanding the Wind Advisory northward.

A plume of fairly shallow 50s dew points, possibly as high as
mid-upper 50s, will be drawn northward by the strong southerly
flow in the warm sector, up to the warm front terminus. The main
period of interest is in the mid afternoon through early evening
as the aforementioned arcing dryline lifts northeast and
increases convergence in the weakly to moderately unstable warm
sector. While instability will be fairly minimal (generally less
than 500-1000 J/kg), cold temps aloft and strong low-level
turning/veering are a supportive pattern for the development of
low-topped mini supercells. If the more aggressive dew point
guidance verifies and the moisture is sufficiently deep, could
see 100+ J/kg of 0-3km CAPE.

This is important because there could be a corridor within the
CWA where low-level hodographs are favorable for more than the
current 2% tornado probs in the day 2 SPC outlook. If the
meridional flow aloft leads to more of a messy storm mode, then
small linear clusters could result in a wind damage threat given
the very strong winds aloft. Finally, low-topped mini
supercells aren`t typically associated with a significant hail
threat. The cold temps aloft and low wet bulb zero heights could
compensate somewhat for isolated marginally severe (~1"
diameter) hail in the most intense cores. Needless to say,
mesoscale trends will need to be closely monitored for the
~2pm-8pm CDT severe threat window.

Finally, given the farther south low pressure path, winds
should shift to strong northerly a bit quicker Wednesday
evening, with rapidly falling temperatures. A developing
backside deformation precip axis should produce a short window
of wind whipped accumulating wet snow, with the exact favored
corridor still uncertain. We may even need to consider another
wind advisory for the north-northwesterly winds Wednesday
night.

Castro

&&

.DISCUSSION...
Issued at 330 PM CDT Tue Mar 18 2025

Through Thursday:

Winds are starting to increase this afternoon as better daytime
mixing is occurring. A 30-40 knot low level jet is expected to
increase this afternoon, particularly south of I-80. Southerly winds
are expected to gust between 25 to 30 mph. Winds will slowly
diminish overnight, through gusts around 20 mph are possible
through the night.

Currently, there is an upper level trough passing over the
Rocky Mountains. Recent mesoanalysis shows a center of low
pressure is already developing over the Colorado front range as
surface cyclogenesis is underway. This low will move east
through the overnight and Wednesday bringing showers, stronger
storms and potentially snow (as it exits) to the area on
Wednesday into Thursday morning.

The upper level trough is expected to have a 100+ knot jet
embedded in it, with the left exit region over northern Illinois
late Wednesday morning providing additional synoptic lift.
Steep lapse rates will slowly sweep over the area through the
day. Showers and potentially some thunder are possible in the
morning and early afternoon, mainly in far northern Illinois.
Southerly winds will increase through the day on Wednesday.
While the forecasted gusts were bumped up, most of the area
should remain in sub-advisory criteria (below 45 mph gusts) for
any significant length of time. If winds over perform, a wind
advisory may be needed, especially for areas south of I-80.

There are no major changes from the previous forecast with the
severe threat in the afternoon. There is the potential for a
brief break in the showers between the initial morning boundary
and the dry line moving through in the afternoon. However, that
dry line is expected to be the main triggering mechanism for
storms to fire in the afternoon. With good synoptic forcing,
steep mid level lapse rates and decent MUCAPE, strong scattered
thunderstorms are possible tomorrow afternoon. The main limiting
factor is the amount of moisture available. Even leaning on the
HRRR for creating the forecast only netted dew points in the
low 50s. Storm mode will be mini supercells. The main concern
will be damaging wind, but hail is possible as equilibrium
levels will remain colder than minus 20C for a brief time. Model
hodographs depict strong low level shear, so a brief spin-up
tornado is possible, but LCLs on model soundings look a little
higher which is a limiting factor. All this being said, the
forecast area remains in the SPCs slight risk (level 2 out of 5)
for severe weather.

As the low moves east, winds will flip over to the northwest
ushering in cold air advection. Snow is expected to mix in with
the rain through the overnight and into Thursday morning. With
lower snow ratios, wet slushy accums are possible creating
slippery travel on Thursday morning. While the risk for slushy
accums is possible area wide, higher confidence is closer to
the Rockford Metro area and for areas along the Wisconsin
border. The northwest winds will gust around 40 mph, There is a
chance a wind advisory may be needed during this window as
well, however current models are showing the system weakening
as it moves through the area.

There is lower confidence on the exact timing the system exits
on Thursday morning. While there may be some lingering snow and
showers, it should dissipate in the afternoon. Otherwise drier
conditions are expected with temperatures returning to seasonal
normal values.

DK

Thursday Night through Tuesday:

The reprieve from the active storm track will be short lived on
Friday, but most areas will make it through the daylight hours
precipitation free. Transitory upper level ridging will pass
overhead Thursday night before it shifts eastward quickly Friday
morning. Warm advection ahead of Pacific origin upper trough
will hopefully keep low temperatures in check Thursday night to
prevent too much frost development, but will also allow
temperatures to recover back to the mid to upper 50s. It again
appears like Friday will be fairly breezy again. With the
increase in moisture and cloud cover, that should keep RH values
from falling too much during the day during peak mixing to
limit the fire weather threat, though wind gusts to 35 mph are
certainly plausible, even up to 40 mph south of I-80. We could
see some isolated showers during the afternoon Friday, most
favored west along the I-39 corridor, more so across NE IA and
NW IL.

The neutrally tilted trough will move through the area late
Friday and Friday night and send a weak cold front with it.
PWATs and low level moisture only modestly increase, scattered
showers with decent coverage (though not everywhere) is the
appropriate forecast at this distance.

This system will quickly exit east Saturday morning and no
precipitation is expected during the day. Temperatures on
Saturday will only modestly recover, as highs look to be
generally in the 40s around the area, maybe tagging 50 in the
west. Onshore winds will develop sometime during the day or
early evening which will also keep nearshore areas cooler.

Another more interesting northern stream Pacific origin trough
will pass through the Great Lakes region on Sunday. This system
packs a bit more punch than Friday night` system, and will
likely have a bit better moisture feed and thus precipitation
chances are correspondingly in the likely category. There are
still some timing discrepancies to resolve. Confidence is high
that temperatures Sunday night into early Monday will support a
mix or even a changeover to snow (across north central IL),
though it is not clear how much of the storm`s deformation axis
will affect northern IL.

Northwest flow will remain in place behind this system early
next week, and this will keep temperatures cooler than normal.
Subtle weak waves may bring low end precipitation chances at
times.

KMD

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 627 PM CDT Tue Mar 18 2025

Aviation Forecast Concerns:

- Lake-enhanced cold front expected to sag across the terminals
  this evening, producing a wind shift to the northeast. Front
  then lifts back north as a warm front with gusty south winds
  early Wednesday afternoon.

- Period of LLWS possible for Chicago metro terminals late this
  evening.

- MVFR conditions develop Wednesday morning with a period of
  SHRA.

- Period of scattered strong TSRA Wednesday afternoon.

- Winds become gusty from the north at 30+ kts Wednesday
  evening.

Complex forecast through the next 30 hours, as a significant low
pressure system moves across the area through Wednesday. In the
near term, gusty south winds will ease with sunset and are
expected to allow a cold front to sag across the terminals
(except perhaps KGYY) mid-late evening. Exact timing at ORD/MDW
is a little uncertain, though it appears by mid-late evening a
wind shift to the northeast at both airfields is likely. During
the late evening/early overnight hours, a southwesterly 40-50 kt
low level jet may support a brief period of LLWS conditions.
Latest high-res model forecasts suggest the front will then
remain south of the terminals through midday Wednesday, before
lifting back north ahead of the approaching low pressure center.
Winds would then shift south and become gusty Wednesday
afternoon (lowest confidence at KRFD, depending on the exact
surface low track), before eventually shifting north later
Wednesday evening as the low center passes east of the area.

As the low approaches Wednesday morning, and area of showers is
expected to develop across WI and northern IL, which will likely
result in the development of MVFR ceilings and occasional MVFR
vis. Scattered showers and MVFR conditions will persist through
midday, before improving to VFR as the warm front lifts north
of the terminals early in the afternoon. Within the warm sector,
a modestly unstable air mass will likely support scattered
thunderstorms ahead of a surface trough by mid-late afternoon,
and some of these could be strong to severe with gusty winds.
Some guidance is a little slower with the timing of these
storms, but given the larger scale forcing and diurnal timing,
the 20-24Z time frame appears quite reasonable at this time.

As the surface low moves east of the terminals Wednesday
evening, winds will shift to the north, and become quite strong
with gusts above 30 kts developing. A period of rain showers is
also expected, which will likely change over to wet snow later
Wednesday night, just beyond the end of this forecast period.
MVFR and potentially IFR conditions are possible within this
area of precipitation.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory from 10 AM to 7 PM CDT Wednesday for ILZ032-
     ILZ033-ILZ039.

IN...Wind Advisory from 11 AM to 8 PM EDT Wednesday for INZ019.

LM...Gale Warning from 9 PM Wednesday to 1 PM CDT Thursday for
     Winthrop Harbor to Wilmette Harbor IL.

     Gale Warning from 3 PM Wednesday to 1 PM CDT Thursday for
     Wilmette Harbor IL to Michigan City IN.

     Small Craft Advisory from 10 AM to 3 PM CDT Wednesday for the
     IN nearshore waters.

&&

$$

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