646
FXUS63 KLOT 310945
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
345 AM CST Fri Jan 31 2025

.KEY MESSAGES...

- Flood watch in effect through 3 PM today for parts of the area.

- A quick shot of wet snow is possible north of I-80 Saturday
  night.

- Record or near-record high temperatures expected on Sunday.

- A chance for a wintry mix in spots Tuesday night into
  Wednesday morning.


&&

.DISCUSSION...
Issued at 345 AM CST Fri Jan 31 2025

Through Saturday:

Areas of dense fog will continue to be our primarily weather
concern through the early to mid-morning period before rain
develops across the area. The current dense fog advisory remains
in effect through 9 AM this morning. The only change was to add
my northwestern IN counties due to the recent eastward
expansion of the fog.

Otherwise, the main concern for today continues to revolve
around the threat of flooding today as a result of periods of
moderate rainfall falling on still frozen ground cover across
northern IL into parts of northwestern IN. A rather impressive
deformation induced band of frontogenesis is already driving an
area of moderate rainfall west-southwest of the area across
parts of northeastern KS into northwest MO and southern IA. This
activity will be shifting right across northern IL into far
northwestern IN today as the parent closed mid/upper low
continues to slide eastward over the Mid-Mississippi Valley.

The highest rainfall amounts today are generally expected from
mid morning into this afternoon right across northern IL into
far northwestern IN. This as a developing west-southwest to
east- northeastward oriented 850-700 MB frontogenesis axis sets
up overhead beneath a region of enhanced diffluent upper-level
flow associated with the exit region of a stout 135+ kt
southwesterly upper jet. Steepening lapse rates could even
support a few rumbles of thunder. While much of the area looks
to get rain today, the greatest amounts of up around 1.5" (or
even locally higher) should generally fall within about a 30 to
50 mile wide corridor, with lighter amounts expected eighter
side of the main axis. Current model and ensemble trends, as
well as observation trends support this main axis falling
potentially right across the I-80 to I-88 corridors right into
the Chicago metro area.

Given the still frozen ground, very little of this rainfall
will soak into the soil. Therefore, extensive areas of ponding
of water and even some flooding of poor drainage and low lying
areas appears likely today. Accordingly, we have opted to issue
a flood watch for portions of the area through 3 pm today that
include all but the far northern Chicago suburbs.

Temperatures today with the rain should hold fairly steady in
the upper 30s to around 40. However, as northerly winds pick up
this afternoon temperatures will begin to drop through the 30s.
As a result, the rain could briefly mix with a some wet snow
before ending this afternoon, but no accumulation is
anticipated.

Northerly winds will remain blustery through the evening as
high pressure builds into the Upper Great Lakes. Winds should
begin to ease overnight, with temperatures expected to bottom
out in the mid to upper 20s. Quiet weather, with seasonably mild
temperatures in the upper 30s, is expected for Saturday.

KJB


Saturday Night through Thursday:

A surface low pressure center will track from North Dakota into
Manitoba Saturday night into early Sunday, dragging its
trailing warm front across our area by daybreak Sunday.
Meanwhile, a shortwave trough embedded in zonal flow aloft will
approach the western Great Lakes late Saturday into Saturday
night. In response, low-level warm and moist advection will ramp
up, causing a precipitation shield to blossom over the Upper
Midwest. Compared to a few model cycles ago ago, there was again
more support amongst EPS, GEFS, and CMCE ensemble members for a
quick hit of measurable precipitation occurring south of the
IL-WI state line in the warm advection "wing".

Our forecast PoPs reflect the aforementioned southward/upward
adjustment noted from ensemble and non-GFS
operational/deterministic guidance, with chance PoPs down to
I-80 (highest at ~40% near/north of I-88), and slight chance
PoPs a bit south of that. Air temperatures near 32F and cold
enough thermal profiles near 32F should support snow as a
precipitation type, perhaps mixed with rain with southward
extent where it precipitates. Warming temperatures at the
surface and aloft in the pre-dawn hours of Sunday should result
in whatever precipitation is still falling at that time to
transition over to rain. With the precip likely to focus into a
2-4 hour window, there is a play for mainly wet snow, tapering
off to drizzle or simply ending. There may be minor slushy
accums on colder surfaces, though time of day and near to above
freezing temps should serve to limit travel impacts.

Warm air advection off of southerly to southwesterly flow will
continue into Sunday and will likely lead to another
unseasonably mild day with temperature records potentially being
threatened (for more information on that, reference the Climate
section of the AFD below). The NBM`s latest high temperatures
for Sunday are in the upper 40s to lower 50s north of I-80 and
lower to mid 50s south of I-80, and this continues to appear
reasonable. The main wild-card will be the potential for stratus
to hang on and prevent temps from maxing out to their potential
(some bias-corrected guidance suggests that upper 50s/near 60F
temperatures appear to be attainable). If stratus does prove
tougher to erode, then some locales may end up only topping out
in the mid 40s or so, especially the farther north and northeast
you go.

A cold front is then projected to sweep through the region
Sunday night into Monday and will result in temperatures
returning to more seasonable readings for this time of year
early next week. The latest indications are that this frontal
passage will likely be a dry one as well, though ensemble
support for this isn`t unanimous. Forecast confidence then
starts to break down heading into the middle of next week owing
to uncertainties regarding the amplitude of an upper-level ridge
over the eastern half of the CONUS and the longitudinal
positioning of an associated baroclinic zone -- one that could
be quite sharp.

There continues a respectable signal in ensemble guidance for
precipitation in the region sometime mid-late next week as an
upper-level trough races eastward across the CONUS. Given the
large range of temperatures that could span the aforementioned
baroclinic zone, all types of precipitation (including a period
of freezing rain) are currently in play for our area Tuesday
night into Wednesday morning. However, confidence in exactly how
things will play out is low at this time given the continued
large spread in ensemble guidance. On Wednesday night, a warm
front may surge north depending on the surface pattern, which
would result in spring-like conditions, along with a chance for
showers and even embedded thunderstorms. A cold front will move
across the area late Wednesday night into Thursday, which
conceptually should dry things out, though timing uncertainty
resulted in some PoPs hanging on. Seasonably cold conditions
will return to close out the first work week of February, with
mixed signals into next weekend regarding extent of any wintry
precip chances.

Castro/Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1137 PM CST Thu Jan 30 2025

Forecast concerns include...

Dense fog/Vlifr overnight/early Friday morning.
Lifr/ifr cigs/vis through Friday afternoon.
Rain through Friday afternoon.
Gusty northeast winds Friday into Friday evening.

Cigs/vis lowered more quickly this evening and other than some
brief improvement for the next 1-2 hours with heavier rain
showers, expect the trend to be down overnight with dense fog
expected across northern IL and at all of the terminals, though
there is uncertainty for the duration of the dense fog. The best
timing appears to be from 08z to 13z though trends will need to
be monitored and prevailing dense fog is possible.

Expect light rain/showers and drizzle overnight with the fog/BR
and then a larger area of moderate to briefly heavy rain will
spread across the area by mid morning Friday. This rain is
expected to end the dense fog but visibilities may remain around
1sm through much of the rain. The rain will end from northwest
to southeast during the afternoon and visibilities should
quickly improve once the rain ends. Cigs will slowly lift to ifr
Friday afternoon, then into low mvfr late Friday afternoon or
early Friday evening though low confidence on specific cig
trends.

Winds are in the process of becoming easterly and will turn to
the east/northeast overnight and then to the northeast Friday
morning. Speeds will slowly increase after daybreak with gusts
into the mid 20kt range expected Friday afternoon into Friday
evening. The strongest gusts may hold off until after the rain
ends. cms

&&

.CLIMATE...
Issued at 320 PM CST Thu Jan 30 2025

With a temperature of 55 degrees being reached earlier this
afternoon, Rockford broke its high temperature record of 52 for
today`s date (January 30th), which was previously set in 1926,
1944, and 2012.

High temperatures for Sunday, February 2nd are similarly
forecasted be at or near record levels. The existing February
2nd high temperature records for Chicago and Rockford are both
52, set in 1987 in Rockford and 2020 in Chicago. Interestingly,
this 52 degree record is Chicago`s coldest existing record high
for any date and is one of Rockford`s coldest existing record
highs as well, so certainly some lower hanging fruit as far as
records go.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 9 AM CST this morning for ILZ003-
     ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-
     ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

     Flood Watch from 6 AM CST this morning through this afternoon
     for ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
     ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Dense Fog Advisory until 9 AM CST /10 AM EST/ this morning for
     INZ001-INZ002-INZ010-INZ011-INZ019.

     Flood Watch from 6 AM CST this morning through this afternoon
     for INZ001-INZ002.

LM...Dense Fog Advisory until 9 AM CST this morning for the IL and
     IN nearshore waters.

     Small Craft Advisory from noon today to 3 AM CST Sunday for
     Winthrop Harbor to Wilmette Harbor IL.

     Small Craft Advisory from noon today to 9 AM CST Saturday for
     Wilmette Harbor IL to Michigan City IN.

&&

$$

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