636
FXUS63 KLSX 102345
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
645 PM CDT Wed Jul 10 2024

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are expected
  this afternoon. One or two of these thunderstorms become strong
  to severe with the primary threats being gusty winds and small
  hail, mainly across northeast MO.

- A warm up is expected to begin Saturday with temperatures by
  early next week being between 5-10 degrees above normal for mid-
  July.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 242 PM CDT Wed Jul 10 2024

Current water vapor imagery shows an upper-level low over the Great
Lakes region with an associated shortwave disturbance across the
Upper Mississippi River Valley and upper-level ridging over the
western CONUS. The shortwave will slowly round the upper low and
provide the focus for showers and thunderstorms this afternoon and
evening mainly along and north of the I-70 corridor. Under abundant
daytime heating and moist low-levels, the atmosphere will easily
become unstable with SBCAPE values between 1500-2000 J/kg and
support isolated to scattered showers and thunderstorms. An isolated
strong to severe thunderstorm cannot be ruled out this afternoon
through early evening. The primary threat in the strongest storms
will be gusty winds and small hail as current model guidance
soundings are showing DCAPE values between 800-1000 J/kg and weak
effective shear. Shower and thunderstorm coverage and intensity is
expected to diminish around sundown. However, there remains a 20%
chance for isolated showers and thunderstorms during the overnight
hours.

Winds overnight will become light and variable. These light winds
combined with the moist grounds there is potential for patchy fog,
mainly in the river valleys and low lying areas. The development of
fog will be dependent on how much remnant cloud cover from the
afternoon convection remains across the area as well as if there is
any shower activity. Should cloud cover diminish, more locations
could see patchy fog overnight.

The shortwave will continue to round the upper-low and provide weak
support aloft for isolated showers and thunderstorms on Thursday
into Thursday night. The greatest chances (20-30%) for thunderstorms
will be Thursday afternoon when instability will be greatest. Shower
and thunderstorm activity is expected to be isolated to scattered
and pulse by nature as there is a lack of low level forcing to focus
convection. Otherwise, expect temperatures on Thursday to be near to
slightly below normal.

MMG/Elmore

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 242 PM CDT Wed Jul 10 2024

There is consensus among ensemble guidance that ridging over the
Western CONUS will continue to expand eastward by late week, leaving
the Mid-Mississippi Valley in generally zonal flow through the
weekend. As this ridge expands east, there is signal that two mid-
level disturbances will round the ridge`s eastern periphery. These
disturbances will provide additional support for a 20-30% chance of
showers and thunderstorms with the existence of a surface warm
front. Current deterministic guidance is in agreement that the warm
front will drape across the southern portions of the CWA Friday into
Friday evening and provide focus for showers and thunderstorms.
As the ridge continues to expand east through Saturday, the weak
warm front will also trek northwards. This will also provide an
isolated chance for showers/thunderstorms Saturday. Otherwise, a
warm-up to near normal temperatures is forecast for the early
weekend in the southerly surface flow.

With regions proximity to the expanding upper-level ridge Sunday and
into next week, a warm up will be underway. Starting Monday,
ensemble soundings are showing 850-mb temperatures being in the low
20s C across the area and this will allow surface temperatures to
climb into the mid to upper 90s. To accompany these hot
temperatures, dew points in the 70s are forecast. Confidence
continues to increase in these higher dewpoints with over 70% of
ensemble members have dew point values in the 70s. Should these
dewpoints and temperatures be realized, hot conditions will be
possible. Apart from these potentially hot temperatures, the next
chance for showers and thunderstorms will be sometime late Tuesday
into Wednesday as a vorticity maximum rounds the upper-level
ridging. Deterministic guidance currently has an accompanying cold
front somewhere across the Upper Mississippi River Valley and trek
south into the region mid next week. There is uncertainty in the
timing of shower and thunderstorm activity along the front due to
differences in current guidances timing of the cold front.

MMG/Elmore

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 645 PM CDT Wed Jul 10 2024

As of 2330z, two lines of thunderstorms are moving southeast through
northeast Missouri and west-central Illinois. These thunderstorms
have produced 38kt winds at KMBY as recently as 2315z and may keep
producing elevated winds and wind shifts as they arrive at KCOU and
KJEF. MVFR/IFR visibilities are also expected with these
thunderstorms. Confidence decreases in whether the St. Louis
metropolitan terminals will see thunderstorms tonight, so maintained
a dry forecast. Tonight, patchy river fog is possible in areas that
see more clearing and the calmest winds. As of now I added MVFR fog
at KSUS and KCPS, the two terminals where low-level cloud cover may
hold off the longest. Will monitor and amend overnight as needed.
Tomorrow afternoon there is a chance of diurnal convection, but
confidence is low in any terminal seeing a direct impact at this
time.

Jaja

&&

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

$$

WFO LSX