656
FXUS63 KLSX 171721
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1221 PM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Warm and dry conditions continue through at least Thursday.

- A few opportunities for rain begin late this week into this
  weekend.


&&

.SHORT TERM... (Through Thursday)
Issued at 315 AM CDT Tue Sep 17 2024

A fairly stagnant flow pattern continues across the country today.
A deep trough continues across the western US with downstream
ridging over Texas and the Southern Plains and continuing
into the Northeast US. A cut off low is swirling westward into
the southern Appalachians, undercutting the persistent ridge over
the Northeast.

The relative ridging in between the two troughs promotes
subsidence across our area today and for the next several days,
suppressing any attempts at convection. Meanwhile the surface high
associated with the Northeast ridge is rather expansive, extending
westward through the Great Lakes. It`s this surface high which has
brought in the drier air with dewpoints falling into the 50s on a
light easterly breeze becoming southeasterly with time. With dry
air and a mostly clear sky today, we`ll see temperatures warming
back up into the mid to upper 80s to near 90 degrees, perhaps a
degree or two warmer than yesterday. Nighttime lows will be
cooler, dropping into the 50s in the cooler valley locations
thanks to that drier air.

Temperatures warm ever so slightly Wednesday into Thursday, with
more and more 90 degree readings likely. This is in part due to
the expanding influence of the Southern Plains ridge and more
southerly low level flow.

Kimble

&&

.LONG TERM... (Thursday Night through Sunday)
Issued at 315 AM CDT Tue Sep 17 2024

As we get toward the end of the week and the weekend, our eyes
turn more to the west for our potential weather makers. Guidance
is coming into better agreement that the Southern Plains ridge
becomes firmly established this week while a deep trough digs over
the Southwest. This puts our area along the northern periphery of
the ridge. Any waves triggering thunderstorms to our west would
have a tendency to move eastward toward our area.

The first wave to bring rain chances to our region is the trough
currently over the Great Basin. While the core of this trough
tracks northeast through the Northern Rockies and into Canada, a
portion of this wave pushes eastward around the top of the ridge
with a surface cold front pushing eastward as well. As moisture
gets pulled northward ahead of this wave, thunderstorms are
expected to form late Thursday into Thursday evening across Iowa
and northwest Missouri. These may track into our region Thursday
night into Friday morning before dissipating. With the better
forcing to the north, the best rain chances will be in the
northern half of our forecast area, mainly northeast MO and
western IL. Convection is likely to at least be elevated by the
time it reaches our area, although it will also likely be
outrunning the instability as well, leading to a dissipating
trend. This idea is consistent with the new Day3 Outlook from SPC
which focuses the severe weather threat to the north (where wind
shear is stronger) and west (where surface based instability is
accessible). The main impact for us will be the chance for much
needed rainfall.

The surface cold front likely stalls or washes out near the area
as ridging takes back over behind the departing trough. The front
will serve as a focus for additional thunderstorms Friday into
Friday night, although this is more likely to our west across
Kansas or Nebraska where better moisture will be pushing northward
ahead of the deep trough over the Southwest. What this trough
looks like when it eventually heads out into the Plains and where
it tracks varies considerably among the guidance. If it gets
absorbed into the northern stream and takes a northeast path, much
of the energy will be steered around our area. If at least a
portion of the trough takes a slower eastward trek over the top of
the ridge it would represent a better opportunity for rainfall
for our region in the Sunday/Monday time period. The uncertainty
in the flow pattern causes the rain chances in our forecast to be
more broadly distributed through the weekend. The temperature
forecast is also more uncertain as warmer temperatures (near 90)
would be likely if the ridge holds on longer while clouds and rain
could prevent those warmer temperatures from occurring.

While there is uncertainty in how the trough ejects and the ridge
breaks down this weekend, guidance does agree that we will
transition to at least a period of troughing early next week. A
cold front brings an end to the warm temperatures and shifts us
back toward more typical temperatures for this time of year.

Kimble

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1218 PM CDT Tue Sep 17 2024

Confidence is high in dry and VFR flight conditions at all local
terminals through the forecast period. There is a low chance for
fog tonight, but it is currently expected to develop south of the
local terminals.

Elmore

&&

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

$$

WFO LSX