105
FXUS63 KLSX 242022
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
322 PM CDT Tue Sep 24 2024

.KEY MESSAGES...

- Wednesday and most of Thursday will have the nicest weather of
  the week with temperatures in the 70s to near 80 and some sun.

- Rain returns late this week and this weekend as tropical
  moisture gets pulled into the region. The most widespread rain
  comes on Friday but occasional showers and thunderstorms
  continue through the rest of the weekend.

- While there is still uncertainty on where the heaviest rain will
  fall, there is increasing confidence that an axis of 1 to 3
  inches of rain will occur in the region.

&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 321 PM CDT Tue Sep 24 2024

Surface low pressure is tracking northeast of Chicago this
afternoon while upper level low pressure is digging southward
through western Missouri. Expect a few more showers through the
afternoon, with the potential for a few thunderstorms as well. But
this activity should decrease around sunset as instability wanes.

As the upper low continues to dig southeast toward the Bootheel on
Wednesday, we`ll find our area on the relatively more subsident
side of the low between the center of the low and a ridge nudging
eastward over the Upper Midwest. This should provide some clearing
skies and a nicer weather day than we`ve seen in a while. In fact
we`re already seeing the clearing approaching from the northwest
on satellite this afternoon, so there is hope for more sun
tomorrow. While subsidence is the general rule tomorrow, promoting
the dry forecast, we will still be near the cold core of the upper
trough with strong warming at the surface. Thus I wouldn`t be
surprised to see a few afternoon showers or perhaps a thunderstorm
on Wednesday, although the chances at any one location are too low
to bring mentionable PoP into the forecast.

Kimble

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 321 PM CDT Tue Sep 24 2024

While we finally end our streak of cloudy, damp weather, we`re
actually setting up for another one. The upper trough digging
across the region today will stall over the Midsouth. Meanwhile
an incoming tropical system, recently named Helene, will track
northward through the Gulf of Mexico and into the southeast US.
These two lows interact with each other, with the end result being
a wobbling upper low that lingers near the lower to middle
Mississippi Valley right through the weekend. While there are
understandably model differences in how these two lows interact
(will Helene get fully absorbed in the upper low or will the two
circulations remain distinct?) which contributes to general
uncertainty in the forecast, there are some things that have
become clear.

First, after some drying the next few days a surge of tropical
moisture will get pulled in from the southeast leading to an
initial widespread rainfall. This comes in the late Thursday night
into Friday time period. While NBM PoP has risen to 80 percent in
the southeast part of the forecast area on Friday, our confidence
in this round of rainfall is actually higher than that. Ensemble
guidance shows measurable rainfall in nearly 100 percent of
members in a 24 hour time period ending Friday night in this area,
so some minor remaining timing uncertainty is holding the PoP for
each period a bit lower. PWAT values surge to between 1.7 and 2
inches which indicates this synoptically driven rainfall has the
potential to be a real soaker. Within the longer range ensemble
guidance, the spread in the 25th to 75th percentile is large,
ranging from 0.5 to 2.75. It`s the higher end of that where we
probably should focus in on a bit more as this shows the more
likely higher end potential with the uncertainty primarily on
precise location of where this will happen. With this kind of
moisture being advected in beneath the upper low there is strong
confidence that an axis of 1 to 3 inches of rain will fall
regionally.

At the moment the most likely target area for this rainfall is
southeast Missouri and southern Illinois. Much of this area
actually missed out on some of the heavier rain in recent days so
this would be a beneficial rain in those areas. If this instead
focuses a little further north then we could start to see some
concerns about another round of heavy rain onto soils that already
saw 3 to 4 inches in recent days. In that case we could see some
localized nuisance flooding or tributary creeks and streams rising
significantly.

The upper low lingers in the region through the rest of the
weekend. Just as we`ve seen with the past few cut off lows, as we
go forward in time and the low slowly fills, we transition from a
more synoptically focused rainfall to scattered showers and
thunderstorms driven by diurnal heating beneath the cool pocket of
air aloft. So we`ll see in our forecast that PoP gradually lowers
through the weekend as knowing where these daily showers will pop
up gets harder to determine as forcing becomes more nebulous.
With high moisture in place we may see additional significant
rainfall from these showers and storms Saturday and Sunday, but it
is likely to be more localized in nature each day.

There`s a growing consensus in the guidance that the upper low
finally pulls away to the northeast on Monday while the next
incoming northern stream trough drives a clearing cold front
through the region on Tuesday. This tells us that our daily
showers and storms have an end date of Monday, with the cold front
Tuesday shutting down this activity and bringing in a drier,
cooler air mass at least briefly.

Kimble

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1238 PM CDT Tue Sep 24 2024

Conditions are gradually trending toward VFR through the
afternoon, although occasional drizzle showers continue in the St
Louis metro. Brief visibility restrictions are possible for the
next few hours. Drier air moving in from the northwest will end
this activity this evening with a trend toward clearing
conditions. If conditions do clear out this evening, then fog
becomes more likely overnight as conditions cool and low level
moisture hangs on. This would be most likely in valley locations
like SUS, but could occur at any location.

Kimble

&&

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

$$

WFO LSX