789
FXUS63 KLSX 171103
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
603 AM CDT Wed Jul 17 2024

.KEY MESSAGES...

- A few redeveloping showers and thunderstorms remain possible
  this morning south of I-44, with bursts of heavy rain the
  primary threat. While confidence is low that this will occur,
  any storms that develop in areas that are already waterlogged
  will have the potential to cause flooding.

- Isolated to scattered showers and thunderstorms will again be
  possible in similar areas this afternoon. Bursts of heavy rain
  will again be possible along with gusty winds. Again, if storms
  manage to develop in waterlogged areas, a brief flooding threat
  will exist.

- Below average temperatures begin today, and will persist along
  with dry conditions through at least Saturday.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 326 AM CDT Wed Jul 17 2024

After a day of extensive heavy rain and flash flooding, things have
quieted down considerably as convection has diminished as of 2:30
am. While a narrow, nearly stationary line of showers produced a
quick 1-2 inches of rain over a narrow corridor in St. Francois
and and St. Genevieve counties overnight, convection has otherwise
remained well to the south and new flooding has not been
reported.

Still, rich moisture remains in place across the Ozarks, although
the highest PWAT values have shifted slightly farther to the east. A
slow moving synoptic cold front also remains to our north, and with
a modest enhancement of the LLJ likely, there remains enough
potential for renewed convection across the Flood Watch area to
maintain the headline. Should additional showers and thunderstorms
develop, once again they will be capable of producing high rain
rates, and if they occur across the northern counties in the Watch
where significant rains have fallen over the last 24 hours, flooding
will not be difficult to achieve. A bit of morning fog will also be
possible in a few locations, although this will be highly variable
due to patchy low clouds, and will likely erode as the cold front
advances south.

Over the course of the morning and through the afternoon, the
previously mentioned synoptic boundary will continue to slide to the
south. Meanwhile, gradual destabilization is expected to occur along
and ahead of the boundary due to the residual boundary layer
moisture, although the persistence of cloud cover may inhibit this
somewhat. In any case, around 1000-2000 J/kg of SBCAPE will be
achievable by early afternoon, and forcing from the boundary may
provide enough lift to generate isolated to scattered
thunderstorms. Exactly where this occurs is a bit uncertain, but
it could be as far north as I-44, or may hold off until the
boundary is farther south into the Ozarks and southern Illinois.
The former, more northerly scenario will need to be monitored
closely considering that new storms would be developing over
waterlogged areas, and this could exacerbate existing flooding
issues for brief periods. At the present time this potential does
not warrant an extension of the Flood Watch as only one or two
time-lagged HREF members develop convection this far north, but it
wouldn`t be completely inconceivable that a stray thunderstorm or
two could pose a problem if it develops in the right place. The
good news is that PWAT values are expected to erode a bit (1.2 to
1.8 inches), which may cut into the efficiency of rainfall.

Meanwhile, winds in the low and mid levels (through roughly the 0-
6km layer), are expected to remain weak today and likewise 0-6km
shear values are only expected to be around 25 to 30 kt. While there
is some stronger anvil-level flow present in the upper half of the
storm layer, these profiles suggest that organization will be
difficult, favoring slow-moving pulse-type storms and clusters, and
not an ideal environment for severe storms aside from some
occasional gusty downburst winds. Again, this poses at least a
limited threat locally heavy rain, but the likelihood of storms
developing in the areas that are most vulnerable does not appear
to be high.

Otherwise, the primary implication of the cold front is exactly as
it sounds...a much cooler airmass will filter into the area, leading
to below average high temperatures today that will climb only into
the low to mid 80s.

BRC

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 326 AM CDT Wed Jul 17 2024

From tomorrow through the end of the week, a cool and dry pattern
will take hold of the region thanks to northwest flow aloft and
surface high pressure. Ensemble mean 850mb temperatures continue to
be forecast below the 3rd percentile of climatology in both the
NAEFS (GFS/Canadian) and the ECMWF ensemble suites, with the lowest
values forecast for tomorrow followed by a very gradual warming
trend through the end of the week. This correlates to surface
temperatures generally in the upper 70s to low 80s Thursday through
Saturday. Meanwhile, this is also accompanied by an anomalously dry
airmass and subsidence, which should keep precipitation to a minimum
during this timeframe.

By Sunday, a meandering, weak upper low is expected to dig into the
Mississippi Valley region and very slowly drift east, if it moves at
all. While the predictability of weakly forced upper lows like this
tends to be low, this does reintroduce the potential for showers
Sunday onward, particularly in our western areas, although this is
far from certain. While the signal for precipitation does increase
during this period, very few members produce significant
precipitation amounts even among outliers, and this makes sense
considering the persistence of neutral PWAT anomalies relative to
climatology. Following this week`s cold front it may take time for
significant moisture to return north, and considering that this
upper low is forecast to be weak, we will likely be dealing with a
modified airmass and a limited ceiling for significant rain heading
into early next week. Meanwhile, the position of the upper low to
the west also will likely maintain neutral to slightly negative
temperature anomalies, and likewise prolong our period of mild
surface temperatures.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 558 AM CDT Wed Jul 17 2024

VFR categories are likely to prevail for most of the 12Z TAF
period, with a few potential exceptions. While coverage is very
sporadic this morning, an hour or two of fog remains possible at a
few terminals, particularly SUS and JEF. There are also patches
of MVFR stratus lingering around I-70 corridor terminals, and this
may reduce categories briefly this morning as well. While a very
low probability remains for a stray shower or thunderstorm near
STL/SUS/CPS this afternoon, most of this activity is more likely
to remain well south of the terminals and this is not included in
the TAF. Light winds and clear skies may also allow for another
round of patchy fog overnight tonight through the end of the TAF
period as well, particularly at fog prone terminals like
SUS/JEF/CPS.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch until 7 AM CDT this morning for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

IL...Flood Watch until 7 AM CDT this morning for Randolph IL-
     Washington IL.

&&

$$

WFO LSX