548
FXUS63 KLSX 242346
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
646 PM CDT Sat May 24 2025

.KEY MESSAGES...

- A cool and cloudy end to the holiday weekend is forecast.
Occasional periods of light rain may occur, mainly across central
and southeast Missouri.

- While a gradual warmup is expected next week, the overall cooler
  pattern is expected to continue through at least Friday.


&&

.SHORT TERM... (Through Memorial Day)
Issued at 222 PM CDT Sat May 24 2025

Current surface analysis shows two west-east boundaries to our
south/southwest. One is near the Red River into southern Arkansas
and is an effective warm front that was augmented by earlier
convection this morning. Another is closer to the KS/OK border ESE
toward the bootheel. Increasing low-level moisture convergence this
evening is expected to develop widespread showers and thunderstorms
near this boundary. This activity should spread east/southeast
overnight, staying almost completely southwest of our CWA. Parts of
central and southeast Missouri may get clipped by some showers and
perhaps a few rumbles of thunder after midnight. Further northeast,
mostly dry weather is expected though there may be some scattered
light rain showers given weak, broad low-level moisture convergence,
increasing lift downstream of a midlevel impulse, and even some weak
low/mid level frontogenesis.

Precipitation trends are pretty ambiguous through the day on Sunday.
Any organized convection should be exiting southwest Missouri by mid
morning, with continued weak low-level moisture convergence and
low/mid level frontogenesis further to the north. The forcing for
ascent is fairly weak, not very deep, and transient in nature.
Therefore, it is difficult to pin down when/where the highest PoPs
should be on Sunday. I have a lot of "scattered" wording, which is
indicative of PoPs between 30-50%. Most locations I think will see
some brief light rain showers at some point during the day, but key
words being light and brief.

A near-repeat of the prior two nights is forecast on Sunday night as
more organized convection along/north of the quasi-stationary front
moves across southwest Missouri. This should not threaten our area,
but there are some signs that a more impressive midlevel impulse
will move toward the Mississippi River by Memorial Day morning. This
should help spread some more widespread showers northeast into parts
of central and southeast Missouri by daybreak. Widespread rain
showers then should continue to move northeast through a majority of
the area on Memorial Day. The possible exception may be parts of
northeast Missouri as well as west-central Illinois where persistent
dry easterly low-level flow may prevent the rain from making it that
far to the northeast. While the coverage of rain showers should be
higher to end the holiday weekend, rain amounts still look light.
Probabilities on the LREF for 24-hour amounts ending 7PM Monday are
below 50% everywhere except for a small portion of central and
southeast Missouri. Even in those locations though chances for even
a half inch of rain drop below 50%.

Temperature wise, there is reasonably good confidence that each day
this holiday weekend will be slightly cooler than the next. This is
because of gradually higher chances of light rain further to the
northeast with time, but more importantly, thick low stratus
expanding toward the Mississippi River by midday on Memorial Day.
These two factors along with the persistent easterly surface winds
will help lead to little in the way of a warmup from morning lows.
Those easterly winds really help keep advecting lower dewpoints
(upper 40s to low 50s) into the area. That means when it does rain
that wetbulbing will occur, helping to cool the air temperature, or
at the very least halt any further warming temporarily. This weekend
is still overall on track for the coolest Memorial Day holiday
weekend for many since 1984. In terms of record low maximum
temperatures, odds of tying or breaking any of the daily records
appears unlikely. This is in part due to the fact that the expansion
of lower clouds is a bit more delayed than it looked 24-48 hours
ago, as well as the fact that overnight lows have trended a bit
warmer as well.


Gosselin

&&

.LONG TERM...  (Monday Night through Next Saturday)
Issued at 222 PM CDT Sat May 24 2025

(Monday Night - Tuesday Night)

Confidence has continued to increase in the unseasonably cool
pattern lasting through Tuesday night along with chances of light
rain. Guidance is coming into better agreement that a surface low
will pass across southeast Missouri late Tuesday into eastern
Illinois Tuesday night. To the north of this low, an inverted trough
is forecast to move from west to east across the area as well,
helping to focus the better rain chances. Similar to this weekend,
any rain looks light for the most part. The one possible exception
is in parts of southeast Missouri where some heavier downpours may
be possible in any thunderstorms.  North of the track of the surface
low, there is a strong signal for thick, low stratus. Similar to
late this weekend, anywhere that experiences the persistent low
stratus will really struggle to warm much from morning low
temperatures. Widespread mid to upper 60s are forecast in central
and northeast Missouri as well as west-central Illinois. Further
southeast, low to mid 70s are expected where the low clouds are most
likely to advect out of parts of southeast Missouri and southwest
Illinois by Tuesday afternoon.


(Wednesday - Next Saturday)

Wednesday appears to be the best chance for a completely dry day as
surface ridging moves into the mid-Mississippi Valley. Weak low-
level cold air advection and lingering stratus should lead to
another cool day however. Highs in the low to mid 70s are forecast,
and wouldn`t be surprised to see these values come down a bit more
if the signal for low stratus lingering longer increases. With a
weak low-level ridge moving slowly across the area on Wednesday, the
low stratus tends to get "trapped" beneath the inversion. Very
weak/negligible advection within the low-level ridge also means the
stratus will not simply move out of the area but must lift/scatter
out. The late May sun is very strong, but this stratus likely will
be thick and may have the aid of some mid/upper level clouds above
as well. Long story short, the cloudy/cool conditions are likely to
continue through the day on Wednesday.

At least a bit of a warming trend will take place ending the work
week heading into next weekend, but confidence remains high that
below normal temperatures will continue through at least the end of
the work week. Clusters of the 500-hPa height pattern remain in
pretty good agreement through Friday, showing anomalous mid/upper
level troughing across the Upper Midwest/Great Lakes. Temperatures
at 850 hPa are expected to remain near +10C, or about 2 to 5C below
normal on both the GEFS/EPS. High temperatures should remain in the
70s, with lows in the 50s. These temperatures would be closer to
5 degrees below normal though vs. the 10-15 degrees below normal
this holiday weekend.

In terms of rain chances, they appear to be trending downward for
the rest of the period as the surface front should be well to our
south. There should be a couple of midlevel disturbances moving
overhead, but just not sure that will be enough to yield any shower
activity. Probabilities for measurable precipitation on the LREF
have come down compared to yesterday, and PoPs now are generally in
the slight chance to low chance category. For perspective, those are
near to slightly below normal for late May. If confidence in the
pattern increases, I would not be surprised to see PoPs lower in the
coming days.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 645 PM CDT Sat May 24 2025

Dry and VFR flight conditions are now expected to persist into
tonight across the region with most precipitation and lower ceilings
remaining to the southwest. Guidance has had a prominent low bias on
ceilings over the last 24 hours, limiting higher confidence in MVFR
ceilings to KCOU and KJEF on Sunday, decreasing to the northeast.
There is a 40 to 60 percent chance of showers at times across the
area Sunday morning into early afternoon as well, but they should be
largely light enough to prevent direct impacts.

Pfahler

&&

.CLIMATE...
Issued at 223 PM CDT Sat May 24 2025

Record cool daily high temperatures for the end of the Memorial Day
holiday weekend:

SUN 5/26MON 5/27
St. Louis (KSTL)59F (2001)56F (1961)
Columbia (KCOU)58F (2001)57F (1992)
Quincy (KUIN)54F (1992)56F (1997)


&&

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

$$

WFO LSX