Area Forecast Discussion

707
FXUS63 KLSX 212343
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
543 PM CST Sat Dec 21 2024

.KEY MESSAGES...

- A warm-up will commence on Sunday with above normal
  temperatures expected for the duration of the upcoming week.

- The upcoming week will also be active featuring multiple rain
  chances Monday through Saturday, including on Christmas Eve and
  Christmas Day.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 140 PM CST Sat Dec 21 2024

Gradual large scale deamplification will begin tonight with weak
height rises aloft as the eastern longwave trof moves into the
western Atlantic. This progression will prompt the eastward
retreat of the surface high currently dominating the area and the
onset of east-southeast surface winds and weak WAA aloft. Lows
tonight will be coolest in SE MO and SRN IL in closest proximity
to the retreating high where winds are lightest and the most
efficient radiational cooling will occur.

We should see a noted warm-up on Sunday as a result of continued
deamplification across the CONUS and more progressive zonal flow,
and the establishment of a prominent WAA regime. We maintain a S-SE
wind component in the boundary layer (which is somewhat shallow with
mixed layer depths below 2.5 kft), with more preferential SW flow
atop it. While not an optimal set-up, the combo of WAA along with
ample sunshine should still result in highs in the 40s, which is
5+ degrees above normal from central/northeast MO into west-
central IL.

Minimum temperatures on Sunday night should be a good deal warmer
than the past few nights. Southerly winds should "stay up" and low-
level southwest flow is expected to bring a return of stratus
overnight into Monday morning.

Glass

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 140 PM CST Sat Dec 21 2024

All guidance continues to indicate an active week ahead with
multiple rain chances still on the plate.

While there could be some sprinkles early Monday morning associated
with the initial thrust of stratus return and WAA, the real initial
chance of measurable rain still remains centered on Monday afternoon
and night with the primary focus across the southern CWA centered
through southeast MO and southern IL. Little has changed in the
overall scenario with light rain/showers initiating Monday afternoon
and evening well ahead of an advancing cold front, near the tail
end of a progressive upper trof moving through the Great
Lakes/Ohio Valley and largely in association with low-level WAA
and low-level moisture convergence and transport. There has been
some signs that the initial rain across the south may be slower to
initiate owing to initially weaker moisture transport and
moistening, however probabilities and coverage should increase on
Monday night as the cold front continues to sag southward into the
southern CWA and both forcing and moisture improve.

The last guidance cycle continues trends from yesterday with
increasing rainfall chances and an increasing areal footprint of
rain Christmas Eve into Christmas morning as the first in a series
of upper trofs approaches and ejects across the Mid MS Valley. The
ejecting short-wave trof will be weakening but the accompanying
large scale ascent and attendant improved thermal/moisture
advections and forcing should lead to a period of more widespread
rain. Timing/position and strength differences are still present
with this lifting trof that ultimately impact the onset of rain and
the northern extent of the rain shield within the CWA, as well as
the longevity of rain into Christmas Day. Some solutions never
get rain into NC and NE MO and also have the rain departing by
midday, while others have the rain impacting the entire CWA and
not ending in the eastern counties until Christmas evening.
Confidence in rain is highest across the southeast 2/3rds of the
CWA where the latest LREF has exceedance probabilities of 90+
percent for measurable rainfall, largely centered on the 00-12z/25
window with around 50 percent chance from eastern MO into
southwest IL from 12-18z/25.

Another and much stronger upper low/trof is still on track to impact
the area late next week in the Thursday-Friday time frame. The
latest ensembles and LREF show the same issues as yesterday with
significant differences in the position/timing and depth of the
upper system as it lifts northeastward through the Mississippi
Valley. The trof position differences are large as well. The
cluster analysis has one cluster with a trof axis position at 12z
Friday well northeast of the CWA, while another cluster has the
trof axis from eastern KS to the ARKLATEX. Despite these
differences, the latest LREF suggests that late Thursday night-
Friday morning is the most-favored window for rain with 50-60% of
the membership producing measurable rainfall.

The NBM temperature guidance maintains it will be a mild week with
above to well above normal values. Confidence is high (>90%) with
the entire interquartile range (IQR) of the NBM max temp guidance
above normal Tue-Sat, and highs well into the 50s with some
locations maybe in the lower 60s Thursday and beyond.

Glass


Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 542 PM CST Sat Dec 21 2024

Dry and VFR flight conditions will persist through Sunday evening
across the entire region. Light southeasterly to variable winds will
become southerly Sunday morning and strengthen slightly with a few
sporadic gusts of 15 to 20 kt becoming likely.

Pfahler

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion