621
FXUS63 KLSX 231124
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
524 AM CST Sat Nov 23 2024

.KEY MESSAGES...

- Temperatures will rise above normal by Sunday. A cold front
  arrives Monday, marking the start of a cold stretch that will
  persist through the Thanksgiving holiday.

- There is a chance (30-50%) of rain over parts of the area Monday
  along a cold front. Additional chances of precipitation exist
  from mid to late week but there is high degree of uncertainty
  with specifics regarding the timing, track and precipitation
  type.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 319 AM CST Sat Nov 23 2024

A few minor adjustments were made to the short term forecast early
this morning. For one, temperatures were pulled down slightly over
portions of central Missouri, where earlier clearing and light/calm
surface winds allowed for more efficient cooling. Locations under
patchy clearing have fallen into the mid-20s and lower 30s. Another
change was to delay and reduce the broad spread of patchy fog
over central Missouri as high clouds streamed over central and
eastern sections of Missouri overnight into early this morning.
Few observations have shown minor reductions to visibility in the
range of 5-8 miles. While some patchy fog is still possible, it
may be short-lived and confined to locations where skies remained
clear for a bit little longer.

As for today, the main challenge will be how quickly stratus clears
out. This should occur as flow turns out of the southwest but
initially, it remains weak during the morning before increasing
through the afternoon. Guidance has been consistently holding a
shallow veil of low level moisture over northeastern sections of
the forecast area, where high temperatures were trimmed a result.
Considering the behavior of stratus yesterday, neighboring
offices collaborated to lower highs over the northeast half of the
forecast area by 4-5 degrees (mid to upper 40s under clouds) with
low to mid-50s where southwest winds resulted in earlier clearing
(central Missouri).

Warm air advection will strengthen heading into Sunday, which will
feel a great deal warmer than today. Consistency and very little
spread place high confidence in high temperatures in the low to mid-
60s.


Maples

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 319 AM CST Sat Nov 23 2024

A mid-level shortwave trough will be approaching from the west late
Sunday night into early Monday morning as a weak surface low moves
northeast through the Mid-Mississippi Valley along a strong, but
slow-moving cold front. Outside of minor timing differences between
deterministic guidance, rough estimates track the front from
northern Missouri around 00z Monday (6 p.m. Sunday evening) to
southern Missouri/Illinois by 18z Monday. This period now falls into
the later portion of the HREF, which is placing probabilities
(approaching 60%) for measurable rainfall across sections of
northeast Missouri and west-central Illinois as early as Sunday
evening. This may be a bit quick to latch onto, but could be a trend
to watch in the event PoPs need to be expanded further northwest
Sunday night. What falls will likely be just a couple hundredths of
an inch as LREF probabilities for 0.05" or more fall around 10
percent or less. The bigger story will be the colder air as non-
diurnal trends have temperatures falling through the 50s and 40s,
then sticking around through the remainder of the week.

Surface high pressure traverses the Midwest late Monday into
Tuesday, keeping conditions dry and cool. Just how far the surface
high makes it into the area Monday night could determine how cold
things get. While much of the area if forecast to be in the 20s, the
calm winds and clear skies will invites lows in the teens Tuesday
morning over central Missouri. Any further east with the surface
high will just expand the teens further east.

Overnight temperatures won`t be quite as cold Tuesday night and
Wednesday as clouds begin to be drawn northeastward into the region.
The pattern remains complicated from Wednesday into the Thanksgiving
holiday. The most difficult part of the forecast is the timing and
track of shortwave energy in the quasi-zonal upper level flow.
Guidance has flip-flopped on the north/south extent of the system
with a wide spread between ECMWF and GFS ensemble members. While
deterministic GFS soundings show temperature profiles supporting
snow down through the mid-level, surface temperatures are marginal
or too warm for accumulation along and south of I-70. LREF
probabilities for 1" or greater snowfall run along and north of I-70
and generally range from 15-30%. This will be a period to continue
to watch as it comes into better view of medium range solutions.

There is high confidence that a very cold airmass will spill
southward out of Canada later in the week. CPC outlooks show a 80-
90%. LREF temperatures probabilities (50% plus) areas north of I-70
not getting out of the 30s Thursday. This cold air only sags
southward into Friday begin a reinforcing cold front with widespread
highs in the 30s Friday.


Maples

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 523 AM CST Sat Nov 23 2024

MVFR stratus has hugged the KUIN terminal all night, remaining
just to the east. Meanwhile, the back edge has pulled east of KSUS
and will put east of KSTL over the next few hours, if not sooner.
KCPS will be the last to improve to VFR by mid-morning. To the
west, the primary concern around KCOU and KJEF have been the
potential for patchy fog development. However, high clouds have
increased overhead, likely mitigating this potential. All fog has
be pulled from KCOU and KJEF with this in mind.

Wind will turn out of the southwest and eventually south this
afternoon. VFR conditions and light southerly flow is expected
through the remainder of the period once MVFR clouds break/clear
to the east.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX