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FXUS63 KLSX 171730
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1230 PM CDT Fri May 17 2024

.KEY MESSAGES...

- Patchy dense fog is expected very early this morning, mainly in
  parts of central and northeast Missouri as well as west-central
  Illinois river valleys. Any fog should quickly diminish after
  sunrise.

- Isolated to widely scattered showers and thunderstorms are
  expected today, mainly during the afternoon hours in southeast
  Missouri and southwest Illinois.

- A dry weekend is expected with temperatures warming up to around
  10 degrees above normal.

- A return to a more active pattern is expected for early/mid next
  week with multiple rounds of showers and thunderstorms possible.
  There is the potential for strong to severe thunderstorms with a
  cold frontal passage late Tuesday/Tuesday night.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 327 AM CDT Fri May 17 2024

Widespread rain has been impacting parts of southeast Missouri and
far southwest Illinois overnight, but has begun to move off to
the northeast. This rain has been persistent but light, and
generally falling in abnormally dry (D0) to moderate drought (D1)
locations. This widespread light rain should move out of the area
over the next few hours as a midlevel shortwave and stronger low-
level moisture convergence pushes off into the Ohio Valley. In its
wake, look for the development of isolated to widely scattered
showers and a few thunderstorms by this afternoon. The best
chances (>30%) this afternoon are in parts of southeast Missouri
and southwest Illinois closer to the stalled surface frontal
boundary. Any shower/thunderstorm activity should wane gradually
this evening as daytime instability lessens. Dry weather is then
forecast for the remainder of tonight, and for the most part,
through the day on Saturday.

Meanwhile, there has been some on/off patchy dense fog around in
areas that have had partial clearing (particularly river valleys).
We are keeping an eye on things, and if dense fog becomes more
widespread, we may issue a dense fog advisory. Any fog will be quick
to diminish this morning after sunrise.

Temperatures today will be generally near seasonal normals, though
there will be some variation from southeast (cooler) to northwest
(warmer). Highs in southeast Missouri and southwest Illinois are
forecast to be in the low to mid 70s due to thicker, more copious
cloud cover whereas parts of northeast Missouri and west-central
Illinois are expected to top out closer to 80 degrees under
plentiful sunshine. Lows tonight are expected to be in the upper 50s
to low 60s across the region, or a few degrees above normal for the
date. Coolest locations are likely to be in favored valleys in east
central and southeast Missouri.

Warmer conditions are expected on Saturday as mid/upper level
heights rise, low-level warm air advection strengthens, and most of
the area lies beneath a mostly sunny sky. Highs are forecast to
reach into the low to mid 80s, close to 7-10 degrees above
climatological normal values.


Gosselin

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 327 AM CDT Fri May 17 2024

(Saturday Night - Sunday)

Dry weather is forecast through the remainder of the weekend with
continued moderating temperatures. Widespread mid to upper 80s are
expected on Sunday afternoon, with some locations flirting with the
90 degree mark.


(Sunday Night - Thursday)

As we head into early next week, the mid/upper level ridge overhead
shifts off to the east with southwest flow aloft settling into the
region. Multiple midlevel shortwave troughs are expected to
transverse the Plains/Upper Midwest. The first two (Sunday night and
Monday night) likely will stay far enough to our northwest to keep
most of the region dry, though chances of showers and thunderstorms
look to return to northwest sections of the area. Better chances (60-
70%) of showers and thunderstorms comes late Tuesday/Tuesday night
associated with a cold frontal passage. There is still quite a bit
of spread with the timing of this cold front, with the ECMWF nearly
24 hours faster than the GFS. This difference in timing is due to
high uncertainty with the strength/track of a midlevel shortwave
trough. Clusters of the 500-hPa height pattern show a lot of spread
in the pattern on Wednesday, with zonal/northwest flow in 2 clusters
to strong southwest flow in 1 cluster. This latter cluster is a
minority (13% of total membership from the grand ensemble) and is
mostly comprised of CMCE members. This indicates a low likelihood in
this pattern, but if it were to come to fruition, a much slower front
(Wednesday-Thursday) would be likely. This not only would have
precipitation chance implications but also lead to a more prolonged
period of warmer than normal temperatures. The most likely scenario
is for a frontal passage as alluded to above sometime in the late
Tuesday/Tuesday night time period. Strong instability (>2000 J/kg)
is likely to be in place (>50% probabilities from the LREF) Tuesday
afternoon. This amount of instability combined with 30-40 knots of
deep-layer shear means the potential for strong to severe
convection. Both the SLU CIPS and CSU machine-learning guidance is
highlighting the potential for severe weather associated with this
cold front. In addition, the CAPE-Shear EFI values are over 0.7,
indicating high consistency within the EPS suite of anomalous
CAPE/shear in the region.

Well-above normal temperatures are highly likely through at least
Tuesday. Highs should reach well into the 80s each day, or generally
about 10-12 degrees above normal for mid/late May. Some type of cool
down is likely by Wednesday, either due to the front clearing the
area or due to increased rain chances/clouds associated with the
frontal boundary.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1230 PM CDT Fri May 17 2024

Isolated to scattered showers and thunderstorms will continue to
develop through the afternoon, with the greatest coverage across
southeastern MO and southwestern IL. Confidence in direct impacts at
any of the terminals is low, warranting only a mention of VCSH for
now. Showers and thunderstorms will dissipate as the sun sets this
evening. Otherwise, VFR flight conditions are anticipated the
rest of today.

Conditions will be favorable for areas of fog to develop again
overnight into Saturday morning, especially in river valleys and
including at KJEF, KSUS, and KCPS with MVFR to IFR flight
conditions. Lower visibilities than currently indicated are
possible. Fog will lift an hour or two after sunrise on Saturday
with area-wide VFR flight conditions returning.

Pfahler

&&

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

$$

WFO LSX