602
FXUS63 KILX 310925
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
325 AM CST Fri Jan 31 2025

.KEY MESSAGES...

- Dense fog should gradually improve from southwest to northeast
  mid to late morning. In the meantime, visibilities will range
  from around a mile to a quarter mile, requiring motorists to
  use extra caution.

- A band of 1 to 1.5 inches of rainfall is expected to set up
  somewhere near or north of a roughly Macomb to Pontiac line.
  This could lead to localized flooding of poor drainage and urban
  areas.

- Warmer than normal conditions will continue through at least
  Sunday, when there is a 50-80% chance high temperatures exceed
  55 degrees. Beginning Monday, forecast uncertainty increases
  but above normal temperatures are generally favored through mid
  week.

- The next batch of precipitation arrives mid next work week.
  Temperatures are forecast to be warm enough for all rain, but
  there is around a 20-30% chance that a wintry mix occurs at some
  point.


&&

.DISCUSSION...
Issued at 319 AM CST Fri Jan 31 2025

*** DAMP, COOL, AND CLOUDY START TO THE WEEKEND ***

A dreary morning continues across central and southeast IL with
temps in the upper 30s to low 40s and visibilities running between
1/4 and 1 mile from fog. Current water vapor satellite shows an
expansive area of moisture transport streaming into the region from
the Gulf of Mexico ahead a surface low spinning in the Great Plains.
It is this moisture, in combination with lift associated with the
approaching low, that will perpetuate the mist, widespread cloud
cover, and occasional showers throughout the day. The most
persistent rain is slated to be with the deformation axis on the
northwest side of the low which most CAMs put somewhere near/north
of a roughly Macomb to Pontiac line mid morning to early afternoon.
While uncertainty surrounds where specifically this narrow band of
heavier rain totals will set up, virtually every model has a
corridor of over an inch of rain with the more bullish models
showing nearly 2 inches in spots. Due to this model spread in the
band`s precise location, HREF yields a broad 20-50% chance for more
than an inch of rain, which could result in some localized ponding
of water across roadways given the frozen ground which is unable to
absorb the rain. For this reason, we`ve added a Flood Watch to Knox,
Stark, and Marshall Counties. Elsewhere, expect rainfall totals
generally between 0.1 and 0.5 inches (highest amounts northwest) by
the time the precip departs from northwest to southeast late
afternoon into early evening. Can`t rule out a brief period of
drizzle this evening as cyclonic flow maintains low level moisture
and a surface ridge pokes briefly into the region to provide
subsidence and a drying of the column from top-down - eliminating
cloud ice. However, both the GFS and NAM suggest 0-1km moisture is
lost pretty quickly with the surface high dropping down from the
Upper Great Lakes tonight, so any drizzle should be short lived.
This evening, the transition between the departing low and that
surface high will be fast, and those sharp pressure rises will favor
a couple-hour period of breezy conditions with north winds gusting
to 30-35 mph - mixing being aided by evaporational cooling of the
precip`s back edge via dry air entrainment.

The stratus may finally break up tomorrow, but mid-high clouds will
be pouring in from the northwest ahead of the next shortwave trough.
As its attendant elongated and ill-defined surface low moves toward
us from the northern/central Plains tomorrow evening, the surface
pressure gradient will tighten once again to make for blustery south-
southeast winds, but thinking is the warm advection regime will not
be conducive to deep mixing so gusts should stay generally 25-30 mph
or lower despite the strengthening LLJ overhead. Tomorrow evening-
night there appears to be potential for a period of advection fog or
even drizzle, with the GFS and RAP suggesting 0-1km RH > 85% and low
(<30%) probabilities for cloud ice present, with upward omega and
diminished surface visibilities. The NAM, HRRR, and ARW, however,
are less aggressive with low level moisture and do not support such
a set up. Thankfully, given the warm advection 2m temps are likely
(80% chance) to be above freezing during this time, which should
prevent drizzle from freezing; we`ll keep one eye on areas across
our north where temps could be closer to that 32 degF freezing
point, as evaporational cooling on elevated surfaces could result in
localized riming, but potential for impacts appears low at this
point.

*** UNSEASONABLY WARM SUNDAY, UNCERTAINTY INCREASES BEYOND ***

Part two of the weekend should turn nicer as forcing for precip
shifts north and southwesterly warm advection brings 850mb temps
into the 6-8 degC range. Some uncertainty surrounds how warm we`ll
become at the surface given potential for cloud cover, but with any
breaks we could end up verifying at the higher end of guidance. NBM
suggests a 50-80% chance high temps surpass 55 degF area-wide (about
20 degrees above normal for early February), with the highest
chances across our south.

Thereafter, uncertainty increases in whether a cold front enters our
area or remains to the north, and it seems that, once again, a split
is evident between ensemble systems - the GEFS being the coolest,
the EPS warmer, and the GEPS being somewhere between the two. The
mean of NBM, which incorporates all of these models, splits the
difference and lands our deterministic forecast in the middle of the
extremes. However, it`s important to know the forecast for early to
mid next week is characterized by abnormally vast uncertainty given
how critical the aforementioned stationary/cold front`s position
will be to our weather, and given how different each model (and
successive iterations of the same model) output is in resolving that
feature. NBM`s probabilistic distribution for high temps gives us a
little insight into this uncertainty, with its 25th to 75th
percentiles at 40 and 64, respectively, by next Wednesday in Lincoln
(roughly the mid-point of our CWA). This means that there is a
50% chance the high temperature eventually observed on Wednesday
will fall within that (24 degF!) range, and a 50% chance it will
fall outside of it; or put another way, there is a 25% chance the
high temperature will be lower than 40 and a 25% chance it will be
higher than 64. In addition, the proximity of that boundary and
likelihood of a surface low developing along it at some point next
week suggests that precip is a good bet; in fact, nearly all
ensemble members give us precip at some point next week. Cooler
ensemble members would give us wintry precip (20-30% chance
Tuesday and Wednesday mornings, and again late Thursday), while
warmer solutions suggest potential for thunder and even severe
weather. The ECMWF EFI, for example, has a Shift of Tails (SOT) of
1 to 2 across much of our CWA for CAPE and shear next Wednesday
into Thursday, and the CSU MLP paints 5% probabilities for severe
weather across our area. Of course, the model forecasts which lie
nearer the middle of the unusually large ensemble envelope next
week would give us temps 5 to 15 degF above normal and regular old
rain. However given the large ensemble spread, we`ll have to keep
a close eye on model trends for next week as guidance continues
to hint at an active pattern conducive to some type of weather
impacts. Keep up to date with our latest forecast thoughts here.

Bumgardner

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1037 PM CST Thu Jan 30 2025

Widespread LIFR ceilings blanket the central IL terminals
tonight. This is forecast to persist into Friday afternoon, with
some rises to IFR from the south. LIFR/IFR visibilities will also
dominate the region before some improvement by late morning.
Models indicate a band of scattered showers shifting north over
the area late tonight, and for a few hours this could cause
temporary visibility improvement. Southeast winds tonight will
veer southwest Friday morning, then northwest by late afternoon as
low pressure departs the region. 20-25 kt gusts are expected
Friday evening.

25

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 10 AM CST this morning for ILZ027>031-
036>038-040>057-061>063-066>068-071>073.

Flood Watch until 3 PM CST this afternoon for ILZ027-028-030.

&&

$$