109
FXUS63 KLSX 091913
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
213 PM CDT Sun Mar 9 2025

.KEY MESSAGES...

- Elevated fire danger is expected across central and northeast
  Missouri as well as west-central Illinois on Monday afternoon.

- A strong storm system will bring windy conditions with a high
  chance (70-90%) of showers and thunderstorms between Friday
  afternoon and Friday night.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 209 PM CDT Sun Mar 9 2025

Clear conditions with mostly light winds are forecast again tonight
as a surface ridge axis gradually slides southward more into the mid
south and Ohio Valley. Some weak west/southwest return flow, mainly
in parts of central and northeast Missouri as well as west-central
Illinois, should help keep temperatures a bit more on the milder
side. Lows are forecast to be mostly in the mid to upper 30s across
the area. The one exception is in favored valleys across parts of
southeast Missouri where winds will be light/variable. This should
allow for the boundary layer to really decouple and at least
approach dewpoints (mid to upper 20s). Lows in these locations in
the low 30s are forecast, and given those dewpoints mentioned
immediately above, would not be surprised to see the coldest
locations bottoming out in the upper 20s.

The warmup will accelerate on Monday as strong low-level warm air
advection moves in from the west. The 850-hPa thermal ridge
approaches the Missouri-Iowa border by early evening, with
increasing southwesterly surface winds ahead of an approaching cold
front. Plentiful sunshine will also contribute to the warmup, with
most locations cresting the 70 degree mark. One negating factor
holding me back from going more aggressive with temperatures is the
lack of mixing. Model soundings show mixing to around 875 hPa, not
abnormally deep by any means for almost mid March. Even the
notoriously (too) well-mixed 12Z HRRR has the top of the boundary
layer at about 825 to 875 hPa.

Milder trends will continue Monday night due to increasing
southwesterly surface winds. Lows in the 40s are forecast, or about
15 degrees above normal for date.


Gosselin

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 209 PM CDT Sun Mar 9 2025

(Tuesday - Thursday Night)

A cold front will move through much of the bi-state area on Tuesday
before stalling out Tuesday night. Confidence in the timing of this
frontal passage has increased somewhat compared to 24 hours ago,
with the likely location of the boundary bisecting the CWA from
southwest to northeast around 1800 UTC. As a result, there will be a
moderately strong temperature gradient from northwest to southeast
across the area, with highs in the low 60s in parts of northeast
Missouri to mid 70s in southeast Missouri. Surface winds veering
ahead of the front more to the west/southwest also should help
warm temperatures in east central and southeast Missouri as air
downslopes off of the Ozark Plateau.

Little change is expected in terms of temperatures through Thursday.
The air mass behind Tuesday`s front is Pacific in nature, and as
alluded to above, the front should stall out in southeast
Missouri/southwest Illinois Tuesday night. The boundary may push
back slightly northward as a warm front on Wednesday before washing
out entirely. Therefore, look for highs each day to be mostly in the
70s, with lows mostly in the 50s.

Predominantly dry weather is forecast through mid week, with the
only chance of rain showers confined to Wednesday night. Trends
that have been observed over the past 24-48 hours have largely
continued, with the track of the midlevel disturbance more toward
the mid south. Chances of rain showers are now mostly in the
15-25%, and mostly confined to portions of southeast Missouri and
southwest Illinois. If the shortwave keeps shifting south, PoPs
may need to be removed altogether.


(Friday - Next Sunday)

A strong mid/upper level trough is expected to eject out of the
desert southwest early Friday, inducing rapid leeside cyclogenesis
across the western Plains. WPC clusters of the 500-hPa height
pattern are in pretty good agreement with the track, though there
are some notable differences in timing and strength of the overall
storm system. Regardless, it appears increasingly likely (70-90%)
for a round of showers and thunderstorms to impact the area between
Friday afternoon and Friday night.

The very strong dynamics with this system look like a given, with
extremely diffluent flow aloft to the east/southeast of the closed
mid/upper level low moving into the Upper Midwest. Convection likely
will initiate sometime Friday afternoon along a prefrontal trough.
Given the strong dynamics/synoptic scale forcing in play, any storms
that do form likely will quickly evolve into a line. The amount of
instability is still a source of uncertainty, with concerns on when
the storm occludes still an open question. A faster occlusion to our
west, as alluded to yesterday, could "pinch off" the richer moisture
(dewpoints near 60F) to the south. However, it may not take much in
the way of instability given the strong dynamics in play. Joint
probabilities from the LREF early Friday evening for at least 500
J/kg of CAPE and 50+ knots of deep-layer shear range from 40-60%.
These probabilities have also been climbing steadily over the past
couple of days now. This potential event is still 5 days away, but
trends are pointing toward at least the chance of organized strong-
severe convection within the CWA.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1213 PM CDT Sun Mar 9 2025

Quiet conditions will continue with light west/southwest winds and
a mostly clear sky through Monday morning.

Gosselin

&&

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

$$

WFO LSX