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000
FXUS63 KLSX 220759
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
259 AM CDT Mon Apr 22 2024

.KEY MESSAGES...

- Temperatures rebound back toward normal today. With the
  combination of dry air and moderate winds, Elevated Fire Danger
  conditions will exist across much of the region today.

- Seasonable temperatures last most of this week. The next chance
  for rain is on Tuesday when mainly light showers are expected.

- A shift toward a warmer and more unstable period is expected
  this weekend. Multiple rounds of thunderstorms, some
  potentially strong to severe, are possible. The timing and
  magnitude of the threat with each round of storms is still
  uncertain, but the threat is greatest Friday and Sunday.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 252 AM CDT Mon Apr 22 2024

Surface high pressure over the Mid South this morning will gradually
expand eastward to the East Coast today. The immediate practical
effect for us will be winds turning southerly and opening us up to
warmer temperatures than we`ve seen in the last few days of a cool,
dry air mass. Moisture return begins across Texas on the back side
of the high, but we don`t see much of that today. In fact, the first
day of return southwesterly flow is a classic set up for
temperatures to over-perform and dewpoints to under-perform
especially downwind of the Ozarks. For this reason, we leaned
toward the upper end of guidance for high temperatures today, with
St Louis likely to see the warmest temperatures. Humidity will
fall below 30 percent for much of the area this afternoon with
deep mixing. This combined with moderate southwest winds will lead
to Elevated Fire Danger conditions today as any fires which do
start could spread quickly.

With more wind tonight we won`t be as cold as the last few nights.
Clouds will be increasing as well as the limited moisture return
across the Southern Plains gets pulled northeast by a trough
tracking along the Canadian border into the Great Lakes. A cold
front will push southward behind this trough with an increasing
chance of showers in the moist advection ahead of this front. There
could be a few such showers across northern Missouri into central
Illinois by morning, but the best chance of rain for most of us will
be during the day on Tuesday. Instability looks pretty limited at
this point, so while we couldn`t rule out a rumble of thunder we do
not expect a threat for severe weather or heavy rainfall. In fact,
ensemble guidance suggests most areas will receive only between 0.05
and 0.25 inches of rain which makes sense considering most of this
will be forced in the mid levels and fall into a pre-existing dry
air mass. Greater rainfall amounts will be further to the east
across central Illinois and Indiana where better forcing exists with
the trough. Temperatures on Tuesday will again be seasonable, but
perhaps a bit cooler in parts of the area due to the showers and
passage of the cold front. Southern areas have the best chance of
reaching 70 degrees again on Tuesday.

Kimble

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 252 AM CDT Mon Apr 22 2024

Tuesday`s trough moves east into New England on Wednesday with
surface high pressure moving through the Great Lakes. The surface
cold front pushes south of our area into the Mid South, but it
doesn`t clear the Gulf Coast. So while the limited moisture return
gets pushed out of our area on Wednesday in the cooler, drier air
mass behind the front, moisture return continues across Texas
helping to set the stage for a richer moisture feed later this week
into the weekend.

Looking at the overall pattern perspective, we see ridging advancing
eastward across the Rockies and into the central US late this week
as a trough sets up over the Southwest. This will have the effect of
shifting warmer air back into our area later this week into this
weekend when temperatures rise back into the 70s and 80s. It will
also set the stage for multiple days of strong to severe
thunderstorms across the Plains and occasionally extending into the
Mississippi Valley until the trough in the Southwest finally ejects.

Back to the daily perspective, Wednesday will be the coolest day of
the week in the post-frontal air mass. It`ll still be within a few
degrees of seasonal normals, though, with plenty of sunshine. While
the front to the south clears the moisture out of our area, westerly
flow over the Rockies keeps a lee trough over the High Plains and
maintains the low level southerly flow over the Southern Plains.
Like a heartbeat, this pumps more and more moisture northward out of
the Gulf of Mexico each day. The thunderstorm threat on Wednesday is
focused more toward Oklahoma where the front stalls and the moist
southerly flow begins to push it back north.

Thursday into Thursday night the low level jet ramps up in response
to the strengthening lee trough and an initial shortwave trough
ejecting into the Central Plains. This will push the moisture much
further northward through the Plains and a bit further eastward. The
focus in this period will remain to our west, but we could see some
showers and thunderstorms Thursday night as the moisture pushes back
into our area from the southwest. The greater chance for showers and
thunderstorms will be during the day on Friday into Friday evening
as the initial shortwave trough moves northeast toward the Great
Lakes. This is when we`ll see the best mid level lift over the top
of the increasingly moist and unstable air mass. With low level flow
out of the SSE and a 500MB southwesterly jet max of 50+ KT moving
through we will see enough shear for the development of more
organized strong to severe thunderstorms on Friday. The greatest
uncertainty on the severe weather threat at this time period is
actually the degree of instability and the location of where
thunderstorms will develop. There may be a fair amount of
synoptically forced showers through the day on Friday which would
limit the degree to which daytime heating acts on the moist air mass
to produce more intense instability. This could also set up locally
more favorable environments for thunderstorms to develop later in
the day.

As this first shortwave moves northeast, a cold front slides
southeast behind it but doesn`t make a lot of progress southeast.
Shortwave ridging in the wake of the shortwave trough in combination
with another strengthening lee trough reinforcing the southerly low
level flow in the Southern Plains will help to stall this front.
There are some differences in the guidance regarding how far south
the front makes it as well as the overall timing, however it is
pretty clear most if not all of our area will remain in the warm
sector on Saturday. Under shortwave ridging we will likely see the
development of a stronger cap and the local reduction in
thunderstorm chances across our area. That said, there is
uncertainty on how strong this ridging is. With the main trough
moving into the Southern Rockies on Saturday the main focus for
convective development will be across the Southern Plains. However,
additional development northeast along the stalled front is also
possible with the front likely in the vicinity of northern Kansas
into southeast Iowa, mainly northwest of our forecast area. Due to
the ridging and lower chances for showers and thunderstorms,
Saturday is the warmest day of the forecast with highs reaching the
mid 80s. NBM actually has some low probabilities of 90+ degree
temperatures on Saturday. For that to happen we will need to be on
the sunnier and drier end of the spectrum.

As the primary trough moves out into the Plains on Sunday it will
set up a broader threat for organized thunderstorms in the warm
sector ahead of it. Deep layer shear increases and capping weakens
with the arrival of the trough. It is likely at this point that we
will see the best moisture of this air mass and potentially the
greatest instability as well. All of this suggests another threat
for strong to severe thunderstorms near our area Sunday into Sunday
night. While the uncertainty on when the trough moves out has
decreased relative to prior model runs, there is still considerable
variability on that point. With a slightly slower trough, the threat
Sunday shifts to our west, with the best timing for thunderstorms in
our area Sunday night or even potentially into Monday. The trough
and best thunderstorm chances moving through Sunday night is a best
case scenario in terms of limiting the severe weather threat in our
area as instability will be nearing the diurnal minimum. If the
timing is earlier on Sunday we may see a greater local threat.

The cold front with this trough will at least temporarily squash the
moisture and instability in our area and give us at least a brief
break from the thunderstorms. Temperatures behind it are not that
cold, though, so we may end up staying near or above normal even
behind the front.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1035 PM CDT Sun Apr 21 2024

Dry and VFR flight conditions will continue through the TAF period.
Surface high pressure is shifting into the Mid-Mississippi River
Valley and is allowing for winds to be light and variable through
the overnight hours. By Monday morning, the surface high will be
east of the region, favoring southwesterly flow with increasing
winds and gusts around 20 knots.

MMG/Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory until 8 AM CDT this morning for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis MO-Sainte Genevieve MO-Shelby MO-Warren MO-
     Washington MO.

IL...Frost Advisory until 8 AM CDT this morning for Adams IL-Bond IL-
     Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey
     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Pike IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX