683
FXUS63 KLSX 280829
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
329 AM CDT Thu Aug 28 2025

.KEY MESSAGES...

- Dry conditions are favored over much of the area through early
  next week. Slight chances of rain creep in through midweek, but
  forecast confidence is low toward the end of the forecast period.

- Temperatures remain comfortable, wavering from below to near
  normal over the next several days.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 328 AM CDT Thu Aug 28 2025

(This Morning)

The high amplitude upper level pattern persists with a longwave
trough over the eastern CONUS and the axis to a ridge extending
northward through the Intermountain West and Plains. Cloud cover
parallels the northern fringes of the ridge from the Pacific
Northwest through the Mid-Mississippi Valley. It then becomes
sandwiched between the eastern side of the ridge and the southwest
periphery of the eastern trough with cloud cover extending into the
Tennessee Valley. Surface high pressure shifted eastward over the
Mid-Atlantic seaboard, but maintains some influence locally as
surface ridging extends westward into Missouri and Illinois.

Dewpoints have inched up in the last couple of days with the latest
observations in the 50s. Temperatures are also a little milder with
mid-50s to low-60s with exception to localized warmth near the STL
metro (mid/upper 60s) due to urban heat retention. The only impact
is localized to river valleys with the potential for another round
of fog. However, the potential is lower than past mornings with
cloud cover and the separation from the surface high. The potential
is likely to be short-lived and confined to areas that are shielded
by from winds (albeit light). This mainly includes the usual
suspects - KSUS, KJEF and KSET. Anything that develops is likely to
be shallow, relatively light and will erode quickly around 12z.

(Today through Friday Afternoon)

Clouds mentioned above aren`t necessarily a result of a steady
stream of moisture, but rather a regional prevalence associated
with impulses navigating around the western ridge. One of these
waves is progressing southeastward through the central Plains,
along with a corresponding surface low. Trends have proven to be
fruitful with the southwest shift in the axis of rainfall over the
last 48 hours, which now excludes all of the CWA from measurable
rainfall. HREF and LPMM are telling with no guidance showing
measurable rainfall through today. The only consideration then is
the impact on temperatures. Despite the removal of PoPs, the 10+
degree disparity between Reynold Co. and STL is questionable,
even with cloud cover. The lack of cold air advection and
southwest shift in the narrowing corridor of HREF temperature
spreads gives higher confidence to lean on the warmer side (NBM
75th percentile) of ensemble guidance (upper 70s).

The westward arm of the Mid-Atlantic surface ridge will maintain
light and variable (locally calm) winds tonight under mostly clear
skies. Efficient cooling will allow temperatures to cool back
into the 50s areawide.

By Friday, a separate surface high builds southward from the Great
Lakes into the Ohio Valley and expands westward into the Mid-
Mississippi Valley. This sends a backdoor cold front northeast to
southwest, which slows or stalls as it parallels the Mississippi
River late Friday. Additionally, a weak upper level shortwave tracks
overhead from northwest to southeast. There are weak convergence
signatures along this boundary, coinciding with a narrow corridor of
mid-level moisture (DewPt: low-60s/PWAT around 1") and SBCAPE that
is on the low end (500-800 J/kg). The HRRR/WRFarw/RAP show some
semblance of stray showers, but timing of these features will be
key. The shallow layer of saturation in the 850-750mb layer, while
increasing cloud cover, will need to tap into diurnal instability
and upper ascent to produce anything appreciable. Dry air beneath
the layer and the overall pattern isn`t favorable for anything more
than a very short-lived, isolated shower, if anything at all. I
maintained a dry forecast, but incorporated ~10% PoPs from KUIN to
KSTL into southwest Illinois, mostly for situational awareness to
monitor future trends.


Maples

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 328 AM CDT Thu Aug 28 2025

(Friday Night-Sunday)

Dry and seasonably cool to near normal temperatures are favored
through the upcoming weekend. The amplified longwave pattern remains
intact through at least Saturday before focus begins to shift toward
an upper low over the northern Plains Sunday. A dry forecast is
favored through Sunday thanks to surface/mid-level ridging and
soundings that show a deep dry layer from the surface through about
700mb. However, as the upper low approaches, mid/upper level
moisture converges at the western periphery of the ridge late
Saturday night into Sunday, resulting in increasing mid/high clouds
that expand from north to south. Highs will largely be in the low to
mid-80s with lows in the upper 50s to low-60s. The coolest
conditions will be on Sunday over northeast Missouri and west-
central Illinois. These locations may not get out of the 70s with
the earlier arrival of thicker cloud cover and dry, easterly surface
flow rounding the Great Lakes surface ridge.

(Labor Day/Monday)

Labor Day, while largely dry, does present some question with
relation to the approaching upper level shortwave. Global guidance
begins to diverge on the evolution of this feature. The GFS is a
little more complicated as the longwave trough to the east expands
westward as the shortwave shears out over the Midwest. It then shows
a second shortwave trailing close behind the lead wave. On the other
hand, the ECM not only shows the longwave trough less pronounced in
its westward extent, but tracks the shortwave almost due south
through the Plains. The eastern solution in the GFS would result in
more cloud cover and late day precipitation chances over central
Missouri, while the ECM would favor dry conditions with an eastward
limit to thicker cloud cover. NBM data is tightly clustered, which
would suggest that forecast confidence remains high through Monday.
However, the upper pattern might bear watching, as LREF 24-hour
precipitation probabilities show 20% of the members bringing
measurable rain to central Missouri late Monday. At this time, I
think a dry forecast is still heavily favored over much of the area
with dry air locked below the mid/upper cloud layer (however
extensive it may be). If trends support precipitation, it`s more
likely to come later in the day or Monday night, well after most
Labor Day celebrations are wrapped up. Nonetheless, this would also
impact temperatures across northeast Missouri and west-central
Illinois if clouds are delayed from current forecasts.

(Tuesday-Wednesday)

Considering the questions that exist Monday, it`s not surprising
that spreads increase as the dominoes fall through the midweek
period. It become more of a smattering of guess work as long range
guidance differs vastly between the surface ridge and the evolution
of the upper level pattern. This explains the NBM`s broad spread in
low chance (<30%) PoPs Tuesday and Wednesday. While there are
chances for rain, nothing is very well organized, along with a
surface ridge that differs in breadth and placement. Therefore,
forecast confidence drops quickly beyond Monday.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1009 PM CDT Wed Aug 27 2025

VFR conditions and light winds are expected throughout the 06Z
TAF period. While some shallow, patchy fog can`t be ruled out at
KSUS early in the morning, increased cloud cover and slightly
warmer temperatures should limit this development and impacts are
not expected at any local terminals. Precipitation is also
expected to remain well to the southwest of local terminals.

BRC

&&

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

$$

WFO LSX