981
FXUS63 KLSX 180824
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
324 AM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Dry and warm conditions continue for at least a couple more
  days. Highs near 90 are expected.

- There will be multiple opportunities for rain through the
  weekend, although widespread drought relief is unlikely.

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 324 AM CDT Wed Sep 18 2024

A broad trough over the western US and a blocking ridge in the
east continues to be the story as it has been for quite some time
now. The ridge centered over Texas expands its influence in our
direction today and tomorrow, keeping conditions warm and dry.
Temperatures nudge slightly upward today and again tomorrow with
gradually more 90 degree readings being observed as the ridge
asserts more influence. Dry air in the low levels will also
promote efficient warming during the day and rapid cooling in the
evening.

Kimble

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 324 AM CDT Wed Sep 18 2024

While the ridge persists over the Southern Plains, the trough in
the west sees one potent wave lift out of the northern Rockies and
into Canada while another digs in across the West Coast. The first
wave is moving through Montana this morning, with a narrow
corridor of moisture pulled northward through the western High
Plains ahead of it. This has led to thunderstorms from the Texas
Panhandle northward into the Dakotas early this morning. While
the core of the trough continues northeast into Canada, the
surface cold front pushes slowly eastward across the Plains with
additional rounds of thunderstorms occurring within the band of
moist advection ahead of the front. By Thursday this axis of
instability will be just to our west from western Missouri north
to Minnesota where the Day 2 SPC outlook has highlighted a risk
for severe thunderstorms. While we are not expecting enough
surface based instability and shear in our area for severe
thunderstorms, we will see the low level moist advection move into
central and northeast Missouri into western Illinois overnight
Thursday night. Elevated instability within this low level jet
will produce and maintain thunderstorms late Thursday night into
Friday morning, although their intensity should be waning as they
reach our area. Rain chances have increased as confidence on the
positioning of the low level jet has increased, with now locally
up to 60 percent chance of rain Thursday night in the northern
portion of our forecast area. Thunderstorms should be relatively
progressive, so overall rainfall amounts will not be excessive.

The cold front enters our area from the northwest on Friday but
will be gradually lose its definition as the trough moves further
away and the ridge asserts dominance. The ridge will be
strengthening on Friday in response to the digging trough over the
Southwest US. Subsidence on the northeast side of the ridge is
likely to suppress convection on Friday even with the decaying
frontal boundary in our area and a moist, unstable air mass in
place. The focus for convection Friday is likely to be to our west
where another round of moist advection begins over the High
Plains in response to the approach of the Southwest trough.

Uncertainty grows as models differ in how to handle the
interaction between a northern stream trough and the low over the
Southwest US. There seems to be a growing consensus that the two
will not phase, but the northern trough will influence the
progression of the southern one as it moves out into the Plains
this weekend. Another cold front stemming from the northern stream
wave will move through the Northern Plains, with moist advection
ahead of the southern wave interacting with this front to produce
the most widespread shower and thunderstorm activity over the
Central Plains late Saturday into Sunday. While the best rain
chances will be to our west this weekend, the westerly flow around
the top of the ridge will have a tendency to push some of this
activity eastward toward our area. So we see rain chances becoming
more widespread Saturday into Sunday, with the uncertainty
deriving from where the cold front is positioned and thus where
the dominant forcing mechanism to trigger convection will be.

Eventually the cold front will make its way through our area and
clear out the moisture and instability and end the rain chances.
However models still vary widely on when that will be. On the
early side, some solutions have it pushing through quickly on
Sunday leaving Monday dry, while others are considerably slower
with it not moving through until Monday. As a result, we still
have some rain chances in the forecast on Monday if a slower
progression occurs.

Temperatures ahead of the front this weekend will be largely in
the 80s to near 90, although do depend on clouds and timing of any
thunderstorms. There is a solid cool down behind the front,
though, with daytime temperatures only in the 70s even with good
mixing. The greatest uncertainty is on Monday due to the timing of
the cold front, so Monday`s temperature forecast currently
represents something of a middle ground between the two extremes.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1108 PM CDT Tue Sep 17 2024

Predominantly dry and VFR flight conditions will prevail through the
TAF period along with light winds. Patchy river valley fog is
expected to develop again tonight/Wednesday morning, but with less
coverage and duration than last night/this morning. Impacts from fog
are high enough at KSUS to maintain a short period of MVFR
visibilities.

Pfahler

&&

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

$$

WFO LSX