Joliet, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

695
FXUS63 KLOT 200019
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
719 PM CDT Mon May 19 2025

.KEY MESSAGES...

- Rain and some thunderstorms develop tonight into Tuesday;
  locally heavy rainfall likely at times.

- Monitoring a low severe potential south of the Kankakee River
  vicinity Tuesday afternoon.

- Cloudy, cool, with showers/drizzle on Wednesday.

&&

.DISCUSSION...
Issued at 227 PM CDT Mon May 19 2025

Through Tuesday night:

A broad area of sub-1000 mb surface low pressure was analyzed
across the Central Plains this afternoon, with a surface warm
front extending from southeast Nebraska through the lower
Missouri Valley and into Tennessee. Breezy, cool northeast low-
level flow was in place across the forecast area well north of
the front, and along the southern periphery of 1030+ mb Canadian
high pressure centered over Hudson Bay. Surface low pressure is
forecast to consolidate tonight into the Iowa/Missouri border
area, in response to strengthening ascent ahead of a negatively
tilted mid-level short wave rotating northeastward around the
deep upper trough and closed upper low currently across the
northern High Plains. A combination of strengthening
southwesterly flow and isentropic upglide north of the surface
warm front, differential cyclonic vorticity advection and strong
upper level diffluence ahead of the wave will result in a
north-northeastward spread of rain/showers across the forecast
area overnight and early Tuesday. Mid-level lapse rates are not
particularly impressive into Tuesday morning, approaching 6-6.5
C/km, but does look to support some embedded elevated thunder
across especially the southwest third of the cwa.

A decent soaking rainfall appears in store for the area into
Tuesday, as deep moisture continues to spread across the area in
association with the deep negative-tilt wave. Forecast
precipitable water values in the 1.50-1.75 inch range are
depicted, which is approaching 200 percent of normal climo
values, though hourly QPF rates are generally in the 0.25" per
hour range given the relatively weak instability. Locally
heavier rain rates will be possible where embedded convection
occurs, but for the most part it appears that our dry antecedent
conditions and low area river levels/flows should be able to
handle these rates with ensemble QPF totals generally between
1-2" expected.

The steadiest rainfall associated with the elevated warm
frontal zone is expected to begin to shift north of the area
through Tuesday afternoon, though showers and drizzle will
continue especially north of the I-80 corridor. Guidance
continues to show some spread in depicting how far north the
warm front lifts into the area Tuesday afternoon and early
evening, as the surface low elongates/tracks across Illinois.
Breezy easterly low-level flow and rain-cooled stable air mass
across northern IL/IN should tend to favor a warm frontal
position more in line with farther south model solutions which
depict the boundary south of the I-80 corridor, though we`ll
need to continue to monitor this as the vicinity of the boundary
and the warm sector to its south would be a focus for
surface/near- surface based thunderstorm development and a
potential severe weather threat. As previously indicated,
forecast shear profiles aren`t particularly impressive as
stronger low-level south- southwesterly flow shifts off to the
east of the area into IN/OH later in the day/early evening.
Nevertheless, the potential for some sunshine and increasing
low-level instability in the higher Theta-E air mass across our
southern cwa will need to be watched for convective trends later
Tuesday. SPC Day 2 marginal (level 1/5) mainly south of I-80
appears continues to appear reasonable at this time. Thunder
threat should end after the early evening hours, though
scattered showers/drizzle will likely persist at times through
the night as the mid-level trough remains overhead.

Otherwise, strong, blustery easterly winds are expected to
develop tonight and persist through Tuesday, gusting to 35 to 45
mph at times. Combined with the rain/clouds, this will make for
a rather raw feel compared to our recent warmer weather,
especially near Lake Michigan. Temperatures north of I-80,
downwind of the lake, will only be in the 50s for the most part.
Farther south, near and south of the warm front, afternoon
highs should reach 70/low 70s Tuesday afternoon.

Wednesday through Monday:

Weakening surface low pressure gradually drifts east of the
area Wednesday, as another mid-level short wave tracks through
the southern periphery of the elongating west-east oriented
upper level trough across the upper Midwest/Great Lakes region.
Cool northerly low-level flow persists with temperatures only in
the 50s to lower 60s, and lingering scattered showers
especially north of the I-80 corridor closer to the trough axis.

Extended guidance is in generally good agreement in wrapping
another closed upper level low from eastern Ontario south-
southwestward across the central and western Great Lakes region
Wednesday night through Thursday, carving out a broad eastern
North American upper trough and maintaining cooler than average
conditions and scattered shower potential. Ensemble guidance
develops increasing spread with the location of the upper low
across the Lakes late in the week, with blended NBM guidance
maintaining mainly dry weather Friday into Saturday, though the
degree of model spread keeps this of somewhat lower confidence.
From an ensemble perspective, precip potential increases again
Sunday into Monday especially south and west of the forecast
area as low level moisture begins to return across the central
and southern Plains. Pattern continues to support the potential
for additional short wave energy to wrap southward across the
region to the west of the slowly departing eastern upper trough.
While this would support some additional periodic shower
chances, it doesn`t necessarily mean a wash-out for the holiday
weekend. Temperatures do look to remain on the cool side
however, gradually moderating to the 60s/low 70s by Memorial
Day.

Ratzer

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 719 PM CDT Mon May 19 2025

Key Messages:

- Rain arriving overnight tonight and continuing into tomorrow.

- Ceilings likely to deteriorate to IFR tomorrow morning.

- Easterly winds turning blustery tonight into tomorrow.

- Chance for lightning tomorrow afternoon, mainly at/near the
  Chicago metro terminals.


A steady band of rain will arrive at the terminals overnight
tonight as a low pressure system approaches from the west. It
will likely take a bit for conditions to deteriorate below VFR,
but eventually, ceilings will likely lower to IFR, and at least
MVFR visibilities will likely be observed at some point as well.
Once ceilings fall to IFR levels, they may remain IFR (and
possibly even fall to LIFR at some point) through the remainder
of the current TAF period. With time, the rain may turn more
showery in nature, and there may be a break in the precipitation
sometime during the late morning or afternoon, but did not have
high enough confidence in that occurring during a specific time
frame to highlight that in the TAFs at this time.

Thunderstorms also appear poised to develop along and south of a
warm front that will likely end up positioned somewhere in
central or northern Illinois during the afternoon. While this
warm front will likely remain south of the terminals, some
lightning strikes may still occur north of the warm front and
main thunderstorm corridor, so opted to introduce a PROB30 group
into the Chicago metro TAFs to highlight this possibility.

Easterly winds will also become increasingly blustery tonight
into tomorrow in response to the approaching low pressure
system. Gusts in excess of 30 kts may be observed at times
tomorrow, though the rainfall may temper the frequency of these
stronger gusts. Nevertheless, sustained winds near and in excess
of 20 kts appear likely regardless of how the steadier rainfall
negates the overall gustiness of the winds.

Lastly, confidence in the forecast for tomorrow evening at the
Chicago metro terminals is very low owing to uncertainties in
where the center of the aforementioned low pressure system will
track and how quickly it will track eastward. For now, opted to
take a "middle of the road" approach in the TAFs and maintain
easterly winds while highlighting the potential for continued
shower/drizzle activity in the area and continued sub-VFR
conditions, but will note that if the low pressure center tracks
over the Chicago metro, then winds may end up turning light and
variable and make conditions more favorable for denser fog
development and/or further lowering of the already low ceilings
in the wake of steadier rainfall.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 PM CDT Thursday for the IL
     nearshore waters.

     Small Craft Advisory from 11 PM this evening to 1 AM CDT
     Wednesday for the IN nearshore waters.

&&

$$

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