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FXUS63 KLSX 181037
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
537 AM CDT Sat May 18 2024

.KEY MESSAGES...

-  Patchy dense fog in river valleys is expected across parts of
   southeast and east central Missouri as well as southwest
   Illinois through early this morning.

-  Seasonably warm temperatures are expected this weekend with
   highs well into the 80s each afternoon. Some spots may hit 90
   degrees for the first time this year on Sunday.

- The weekend is also mostly dry, though some isolated showers and thunderstorms
  are possible Sunday afternoon, mainly in parts of northeast
  Missouri and west-central Illinois.

- Confidence continues to increase that an active weather pattern
  will be in place early/mid next week with multiple rounds of
  showers and thunderstorms possible. The best chance is
  along/ahead of a cold front late Tuesday/Tuesday night where the
  best potential for organized severe thunderstorms exists.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 326 AM CDT Sat May 18 2024

Latest satellite imagery shows river valley fog in parts of
southeast and east central Missouri as well as southwest Illinois.
Most observations have been in the 1-5SM range for visibilities,
though KFAM and KMDH have had brief periods of 1/4SM. Some patchy
dense fog in river valleys will remain possible through about
7-8AM, but the current thought is that it will not be widespread
enough to warrant an advisory.

The persistent midlevel shear axis that has been draped across the
mid south/Ohio Valley the past couple of days will finally move out
to the east today, with mid/upper level ridging moving across the
Mississippi Valley. While a stray shower/weak thunderstorm cannot be
ruled out in parts of southeast Missouri through early this
afternoon in proximity to the aforementioned shear axis, chances
look much lower than yesterday and believe any activity will be
south and east of the forecast area. Early this evening, a cold
front will approach parts of northeast Missouri. By the time the
boundary reaches our area, it will become more diffuse with
weakening surface convergence (and instability). As a result, we are
not expecting any thunderstorm activity to leak into the area this
evening. Therefore, dry weather is expected today/tonight with
warming temperatures. High temperatures are forecast to reach into
the mid 80s and leaned toward the 90th percentile of available
model guidance due to sunshine expected today and increasing low-
level warm air advection.

The warmer than normal conditions will continue for the remainder of
the weekend with lows tonight only dropping back into the mid 60s
for most locations (10-15 degrees above normal for the date). Highs
on Sunday should be at least a couple of degrees warmer due to the
warmer start to the day and higher afternoon 850-hPa temperatures.
Highs in the upper 80s to near 90 degrees are expected, which should
rival or exceed the warmest day so far this year for most locations.

A weak warm front is expected to push northward gradually during the
day on Sunday, and by mid-late afternoon be located from west to
east across parts of northeast Missouri and west-central Illinois.
Convergence along this weak front is not terribly strong, mid/upper
level heights continue to rise a bit during the day, and the
stronger forcing for ascent resides well to our northwest. These
factors lead a lot of uncertainty to whether any storms will be able
to fire along the front. In addition, soundings show some weak
capping that will need to be overcome. Some CAMs do have initiation
in the vicinity of the front and we are carrying slight chance
PoPs (~20%) for the afternoon. IF any storms are able to develop,
there is plenty of instability (SBCAPE>2000 J/kg) for strong to
severe storms. However, deep-layer shear is poleward of the front,
with only ~20 knots along the boundary. This points more toward
pulse convection and/or multicellular clusters. Marginally severe
wind/hail would be possible in any storms that develop. Given the
uncertainty in convective initiation though, we are not explicitly
mentioning any severe threat at this time.

Gosselin

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 326 AM CDT Sat May 18 2024

(Sunday Night - Monday Night)

Sunday evening is expected to be dry as the weak warm front moves
north of the area and becomes more diffuse with time. A midlevel
shortwave trough is forecast to move through the mid-Missouri Valley
overnight Sunday into Monday morning. Widespread thunderstorms will
be accompanied by this shortwave and increasing low-level moisture
convergence. Some of these storms may move into parts of northeast
Missouri and west-central Illinois after midnight. How far east and
how long the storms continue however into the day on Monday is a
question mark as the low-level jet abates and the midlevel shortwave
trough shifts off into the Great Lakes. These factors would seem to
point to a lull in activity (if not outright dry weather) Monday
afternoon. In addition, the synoptic cold front is well to the
northwest of the area so there shouldn`t be a surface boundary
with associated convergence to help initiate convection. An
outflow boundary from prior early morning storms is a possibility,
but unsure if this alone would be enough to re-generate any
afternoon storms.


(Tuesday - Tuesday Night)

Speaking of the synoptic cold front, spread on the timing has
decreased in terms of its passage across our area. There is now
higher confidence that it will enter northeast Missouri by late
afternoon/early evening. Convective initiation should occur
along/ahead of this boundary as a strong midlevel shortwave trough
tracks through northwest Missouri. Convection likely will be
discrete initially with the deep-layer shear vector having a
strong orthogonal component to the cold front. There does not
appear to be a lot of inhibition and the synoptic forcing is
strong. Therefore, there may be a lot of updraft competition and
mergers. Hodographs through the afternoon are also pretty
straight, so there may be splitting supercells. Large hail will be
the primary threat during this period. By early evening, would
expect storms to congeal into a line/possible QLCS. The primary
threats would shift more to damaging straight line winds and
possible tornadoes as hodographs become a bit more
elongated/curved.

Temperatures Tuesday should be even warmer than Monday ahead of the
cold front with readings likely climbing into the upper 80s to low
90s across the region.


(Wednesday - Friday)

The cold front is expected to slow and eventually stall out by early
Wednesday, but exactly where is a question mark. Tuesday night`s
convection also may be able to push pretty far to the south and
east, which in turn likely would become the effective front. The best
chances of showers and thunderstorms on Wednesday are in parts of
southeast Missouri/southwest Illinois closer to the expected
position of the synoptic cold front. Cannot rule out the
possibility of strong/severe convection Wednesday
afternoon/evening, but antecedent convection seems likely to push
the axis of higher instability southeast of the forecast area. A
trailing midlevel shortwave trough may interact with a region of
increasing low/mid level frontogenesis to produce elevated
shower/thunderstorm activity further to the northwest. This is
uncertain however as deterministic guidance varies with the
strength of the aforementioned low/mid level frontogenetical
forcing.

A (relative/brief) dry period seems more likely across most of the
area Wednesday night/Thursday morning as an area of high pressure
moves into the region. Chances of showers and thunderstorms then
looks to increase once again late Thursday into Friday as another
midlevel shortwave trough moves out of the south-central Plains.
Model spread with this feature is quite high, so confidence in
where/when thunderstorms are favored for the remainder of the
forecast period is low.

Temperatures are expected to cool behind the cold front on
Wednesday. Exactly how much cooler though is a question. Not only is
the position of the front a bit uncertain on Wednesday, but also the
possibility of elevated shower/thunderstorm activity further to the
northwest. Temperatures may struggle to get out of the 60s if any
elevated activity actually does materialize. Meanwhile, there is a
least a low chance of temperatures near 90 degrees in parts of
southeast Missouri and southwest Illinois if the front is slow
enough. The NBM 75th percentile in these locations is near 90, and
the 90th percentile in the low 90s for reference.

Above normal temperatures are forecast to end the next work week,
but generally about 5-8 degrees above normal with highs mostly
ranging from the mid 70s to the low 80s.


Gosselin

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 532 AM CDT Sat May 18 2024

Some river valley fog may impact KSUS/KCPS very early this morning
but should evaporate quickly as the sun continues to rise.
Otherwise, dry/VFR conditions are forecast through the period with
light/variable winds and plenty of sunshine.


SPECIFICS FOR KSTL:

We are keeping an eye on some fog/stratus on satellite imagery
just to the south of the terminal. It has been advecting a bit to
the north over the past 1-2 hours. Cannot rule out a brief period
of IFR visibilities/ceilings, but not confident enough to add a
TEMPO at this point in time. Any fog/stratus threat is very short
lived this morning, with dry/VFR conditions and light/variable
winds expected through Sunday morning.


Gosselin

&&

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

$$

WFO LSX