607
FXUS63 KLSX 261921
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
221 PM CDT Wed Jun 26 2024

.KEY MESSAGES...

- Multiple rounds of showers and thunderstorms are expected Friday
  and Saturday. Some of the storms Friday night will be strong to
  severe, with damaging wind gusts and large hail being the main
  threat.

- After a period of below normal to near normal temperatures
  through this weekend, heat returns early to mid next week.


&&

.SHORT TERM... (Through Thursday Night)
Issued at 220 PM CDT Wed Jun 26 2024

Water vapor imagery currently shows a shortwave passing over the
CWA, which is continuing to aid in forcing mostly stratiform rain
over southeastern Missouri and southwestern Illinois along and ahead
of a cold front that is roughly bisecting the CWA from the northeast
to the southwest. Given the cloud cover and rain along and south of
the front, the effective boundary has temperatures actually cooler
south of the front than north of the front. All this translates to
that temperatures are running at to below normal this afternoon,
with maximum temperatures reaching the mid 70s to around 80 north of
I-70 and in the low to mid 70s south of I-70. As the shortwave
progresses eastward through the remainder of the afternoon, rain
chances will diminish west to east, with rain exiting the CWA to the
southeast by late this afternoon/early this evening.

Thanks to the cool and cloudy weather today and temperatures cooling
into the low to mid 60s tonight, patchy fog can`t be ruled out
tonight. This will especially be the case across southeastern
Missouri and southwestern Illinois where rain has been falling
through much of today.

Tomorrow, a trough will deepen over the eastern U.S., keeping
northwesterly flow aloft over the region and high pressure in the
Midwest. The result is temperatures remaining at or below normal,
though a couple degrees warmer than today due to clearer skies and
ample sunshine.

Elmore

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 220 PM CDT Wed Jun 26 2024

A potentially complicated convective scenario is expected across the
region on Friday, with multiple rounds of convection possible.
During the early morning hours, a subtle shortwave will move through
the Central Plains and into the Midwest ahead of a trough digging
into the Northern Plains coincident with warm front lifting through
the Middle Mississippi Valley. Warm air advection associated with
the front will lead to a cluster of convection that will move out of
the Central Plains in the morning and into the Middle Mississippi
Valley through mid-day. Both deterministic and ensemble guidance
have the bulk of the convection passing just north of the CWA, with
more scattered coverage further south across much of the CWA. In the
wake of the warm front and early convection Friday evening and
overnight, guidance consensus is that the atmosphere will
destabilize across the Mid to Lower Missouri Valley among 35-40 kts
of 0-6 km bulk shear. During the evening, a surface low and
associated cold front will move into this airmass and force
convection that will likely be supercellular in nature at least
initially due to the shear magnitude and profile. Given the current
expected track of the surface low and placement of the more potent
airmass, the greatest chance for scattered severe storms is across
portions of central Missouri Friday night. Initially storms will be
capable of all severe hazards: large hail, damaging wind gusts, and
tornadoes. However, convection will likely grow upscale into
clusters and lines that will pose mainly a damaging wind threat.

Given the trajectory of the surface low and quasi-zonal surface flow
aloft, there is much uncertainty regarding the progression of the
cold front through the CWA Friday night into Saturday. Faster
solutions have the front clearing much of the CWA if not the entire
CWA prior to peak heating on Saturday, while slower solutions have
the front draped west to east somewhere across the CWA during peak
heating and among an unstable airmass. A majority of guidance favors
some flavor of the latter solution, which is reflected in the
current forecast. This solution may support at least a low chance
for severe storms, but this threat is highly conditional in what
happens Friday and Friday night and is uncertain at this lead time.

On Sunday, an upper-level trough will amplify over the eastern U.S.
with northwesterly flow developing over the Middle Mississippi
Valley, ushering high pressure into the region and pushing the front
well south of the CWA. This will bring in an unseasonably cool
airmass, with ensembles clustering around 80 degrees area-wide. How
long this cool down lasts is uncertain, as guidance diverges on the
eastward progression of the upper-level trough and how quickly
ridging builds into the area early next week. A quicker solution
favors temperatures warming on Monday relative to Sunday, while a
slower solution favors similar temperatures Monday compared to
Sunday. A majority of guidance has been trending toward the latter
solution, which is reflected in the current forecast. Regardless,
ensemble consensus is that the heat will return by mid-week as
ridging builds across the Southeast.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1213 PM CDT Wed Jun 26 2024

A weather system is slowly departing the area, and is currenlty
impacting KSUS, KSTL, and KCPS with light rain. This light rain is
expected to depart these terminals early this afternoon. Low
stratus is also accompanying this weather system mainly south of
the local terminals, but is currently impacting KSUS. This low
stratus is expected to be limited in both timing and spacial
extent this afternoon. Once the stratus and light rain clear the
area, VFR flight conditions are expected through the forecast
period at all local terminals.

Elmore

&&

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

$$

WFO LSX