020
FXUS63 KLSX 221927
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
227 PM CDT Thu May 22 2025

.KEY MESSAGES...

- Cool and dry weather will continue tonight and Friday.

- This is forecast to be the coolest 3-day Memorial Day holiday
  weekend since at least 1984. In addition to the unseasonably
  cool temperatures, periods of rain showers along with a few
  rumbles of thunder are expected.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 219 PM CDT Thu May 22 2025

Current surface analysis shows a strong surface anticyclone (~1025
hPa; >90th percentile of climatology) across the northern Plains.
This area of surface high pressure is forecast to slide southeast
into the mid-Missouri Valley overnight tonight. A mostly clear sky
and calm or light/variable winds are forecast in the vicinity of
this feature. Abnormally low dewpoint values (<10th percentile of
climatology) will also help allow for plenty of radiational cooling
tonight. Lows in the in the low to mid 40s are forecast across the
area, with the exception of some upper 40s in the urban heat
island of metropolitan St. Louis. A couple of spots dropping into
the upper 30s in favored valleys would not be too surprising
either. For lows tonight, stayed near or just below the 25th
percentile of the NBM given the favorable conditions detailed
above.

Persistence is a pretty good forecast for the day tomorrow, with
seasonably cool conditions and dry weather. Winds will be quite a
bit lighter though as the surface high slowly moves off to the east.
Mid/upper level cloudiness will also begin to increase from west to
east within the northwest flow aloft. These clouds shouldn`t play
too big of a role on temperatures however as plenty of morning/early
afternoon sunshine is expected. Highs in the upper 60s to low 70s
are forecast, or about 5-10 degrees below normal for the date.


Gosselin

&&

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 219 PM CDT Thu May 22 2025

(Friday Night - Memorial Day)

Unseasonably cool daytime temperatures should be the biggest story
this upcoming Memorial Day holiday weekend from a climatological
standpoint. The CWA will remain well to the north of a quasi-
stationary front wobbling back and forth across the southern
Plains into the mid south. Multiple rounds of widespread rain
(mainly across central and southeast Missouri), along with a few
rumbles of thunder, are expected north of the boundary in our
region associated with midlevel impulses interacting with this
front along with low-level moisture convergence. The strongest
mid/upper level forcing actually may hold off until Memorial Day
as the trough moves overhead with increasing ascent downstream
aided by some upper-level jet support. Right now, Sunday and
Memorial Day seem to have the best chances of widespread
rainfall, so current thinking is that the coolest highs for many
areas will be either Sunday or Memorial Day. The setup really does
not get much better for unseasonably cool high temperatures in
the warm season than persistent east/northeasterly surface winds,
thick low clouds, and rainfall. This trifecta typically limits
diurnal warming to as little as 3 to 5 degrees from observed
morning lows. Widespread low to mid 60s are forecast for daytime
highs each of those afternoons, with some spots likely staying in
the upper 50s. These values may tie or break daily record low
high temperatures. Please see the CLIMO section for more details.

In terms of 3-day rainfall totals, they continue to have come down a
bit as the favored axis of heaviest rainfall has shifted about 50-75
miles to the south/southwest. This axis may shift a bit more to the
southwest with time, as it will be very difficult for the surface
front to make much if any progress northward with continued
convection along/north of it and a lack of a strong midlevel wave
moving overhead. This lack of strong mid/upper level forcing should
not allow for any real surface cyclogenesis along the boundary to
help push the front poleward. Probabilities on the LREF for at least
1" of rain remain pretty high in parts of southeast Missouri (70-
90%), with a sharp tail off as you head north and east. For at least
3" of rain, chances drop to 10-30% in southeast Missouri. These
probabilities are about 10% less than yesterday, which matches well
with the overall shift to the southwest for the highest rainfall
amounts. Any hydrological impacts look minor at this time, barring a
fairly significant shift to the northeast. The latest HEFS
probabilities for minor flooding have dropped on area rivers, with
only the Meramec River at Sullivan having at least a 50% chance.
This site has a lower flood stage than surrounding stations and
has some elevated flows from the previous event. There is no
change to the thinking in terms of flash flood threat. There
appears to be little/no threat given the long duration nature of
the event and lack of higher rainfall rates. Thunder probabilities
are only in the slight chance category, and mainly confined to
parts of central and southeast Missouri. In the absence of
organized deep convection, you just can`t get high enough rainfall
rates to cause flash flooding barring a connection to purely
tropical moisture (which won`t exist this weekend). Neither warm
cloud depths nor precipitable water values look impressive either,
which also argues against any really efficient rainfall
processes. Precipitable water values peak out close to 1.50" in
far southeast Missouri this weekend, but even that would be less
than the 90th percentile of climatology. Most heavy rain events
that cause flash flooding in this part of the country have at
least a brief period above the 95th if not 99th percentile.


(Tuesday - Next Thursday)

Clusters of the 500-hPa height pattern remain in good agreement even
through the middle of next week with the overall synoptic pattern,
with anomalous mid/upper level ridging across the intermountain west
and troughing over the Great Lakes/eastern US. We should tend to dry
out and warm up slightly with time. However, the cool pattern does
look to continue through next work week and potentially beyond as
both the GEFS and EPS have 850-hPa temperature anomalies of -3 to -
5C even through Thursday. High temperatures should climb above the
70 degree mark everywhere on Wednesday, with readings more in the
mid to possibly upper 70s next Thursday. Those values still would be
about 5 degrees below normal. With more clearing and light surface
winds, lows will actually decrease the middle of next week.
Widespread upper 40s to low 50s are expected both Tuesday and
Wednesday mornings.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1154 AM CDT Thu May 22 2025

Northwest winds will weaken this evening, with gusts ceasing
around 0000 UTC Friday. Winds will then become light/variable
before midnight as a surface ridge of high pressure approaches
from the west. Some river valley steam fog is expected, which
could impact KJEF/KSUS. Added some MVFR fog at each location late
tonight into very early Friday morning, but it is low confidence
in terms of intensity and longevity.

Gosselin

&&

.CLIMATE...
Issued at 219 PM CDT Thu May 22 2025

Record low daily high temperatures for the upcoming Memorial Day
holiday weekend:

SAT 5/25SUN 5/26MON 5/27
St. Louis (KSTL)58F (1924)59F (2001)56F (1961)
Columbia (KCOU)56F (1924)58F (2001)57F (1992)
Quincy (KUIN)55F (2001)54F (1992)56F (1997)

&&

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

$$

WFO LSX