Decatur, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

822
FXUS63 KILX 230959
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
359 AM CST Sat Nov 23 2024

.KEY MESSAGES...

- Low clouds will remain prevalent today. There is a small (about
  30%) chance that ceilings will continue building down toward
  the surface, resulting in visibility falling below 1 mile this
  morning.

- South winds will become breezy (20-25 mph) on Sunday, helping
  boost temperatures toward 60 degrees, albeit briefly. A cold
  front will bring rain chances (40-60%) to areas east of the IL
  River Valley by Monday, with temperatures turning sharply colder
  Monday night (wind chill values in the teens).

- There is a medium probability (40-50% chance) for wintry
  weather by the middle of next week, which will be followed by
  very cold, January-like temperatures by Friday and Saturday.

&&

.DISCUSSION...
Issued at 119 AM CST Sat Nov 23 2024

The main forecast concern for today will be timing the stratus
breakup, and how that might impact temperatures going into this
afternoon. Observational trends overnight have indicated a steady
build-down of stratus, and much of the recent guidance (RAP, HRRR,
and GLAMP) has trended even lower with the stratus build-down such
that surface visibility may fall below one mile in some areas.
With the residual moisture trapped beneath a strong subsidence
inversion, and with deep cyclonic flow still in place ahead of the
925-mb ridge axis, we have no reason to believe the low clouds
will make a quick exit today. It`s not until after 18z/12pm that
the ridge axis pushes across central Illinois, shifting low-level
winds southwesterly, do we see an opportunity for clouds to
gradually lift and break. If this is indeed how things evolve
today, afternoon highs will struggle to reach the upper 40s.

By Sunday, model guidance is in excellent agreement that a mid-
level shortwave will eject out of the Rockies and help bring deep
southwest flow across the Southern Plains and into the Mid-
Mississippi Valley. With increasing southerly winds and a strong
push of warm advection ahead of a sharpening baroclinic zone,
temperatures will surge into the low 60s across west central
Illinois Sunday afternoon. This sharp warm-up will be brief,
however, as a robust upper-level wave begins to dig toward the
Upper- Mississippi Valley on Monday, forcing a cold front into
central Illinois. Some rain is anticipated with frontal passage,
though model soundings exhibit a serious moisture discontinuity in
the mid-levels, which is taking a bite out of our QPE. Current
blended (NBM) and ensemble (LREF) guidance supports a 40-60%
chance for rain on Monday in areas east of the Illinois River
Valley, with QPE generally between a Tr-0.25".

With a colder, drier and breezy airmass filtering-in behind the
front late Monday evening, temperatures will bottom-out in the mid
20s with wind chill values in the teens. Below normal temperatures
will then keep its grip on the region through the remainder of
next week, with a reasonably strong signal for a January-like
chill by next weekend.

Of course, the thing overshadowing this forecast period is the
potential for wintry precipitation by the middle of next week,
which is accentuated by holiday travel. As we see it right now, if
the blended (NBM) and raw ensemble (LREF) guidance trends more
towards the `EPS Mean` in coming days, central Illinois will be
positioned for moderate winter weather impacts. In comparison to
the GEFS & GEPS, the EPS Mean exhibits a deeper/more-amplified
trough across the Southern Plains next Wednesday, which coincides
with a stronger, wetter and more northerly-oriented surface low
into Thursday.

That`s what we see right now. We do not favor one solution over
the other. Each solution seems equally plausible at this time. The
best thing to do is to not anchor toward any one solution, but
continue monitoring how the data trends throughout this weekend.

MJA

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1042 PM CST Fri Nov 22 2024

Ceilings have trended down near 1k feet AGL this evening, and we
expect IFR ceilings to dominate the terminals overnight and
through much of Saturday morning. Have delayed the clearing trend
to mid/late afternoon based on latest high resolution guidance,
as it appears that light winds under the subsidence inversion will
keep the low clouds later than previous forecasts. Light
northwest winds tonight to gradually back southwest on Sat.

25

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$