Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 190829
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
329 AM CDT Tue Mar 19 2024

.KEY MESSAGES...

- Gusty winds, low relative humidity, and dry fine fuels will lead
  to heightened fire danger across much of the area today, though
  particularly in southeastern Missouri.

- The next chance for rain (up to roughly 40%) comes late Thursday
  into Friday.


&&

.SHORT TERM... (Through Wednesday Night)
Issued at 325 AM CDT Tue Mar 19 2024

Current water vapor imagery shows northwesterly flow aloft over the
CWA on the backside of a trough moving toward the east coast. At the
surface, observations show an area of high pressure centered over
the Mid South and edging eastward. The track of this high has had
winds across the CWA slowly turning southerly, which has begun to
advect warmer air into the CWA and slow to stall temperatures from
cooling further.

This slowing/stalling of temperatures signals the start of a brief
warm up for today. Winds will strengthen this morning as mixing
begins and the pressure gradient tightens due to a surface low
moving through the Great Lakes. Initially, winds will be
southwesterly, giving an extra boost to our temperatures due to
downsloping off the Ozarks. In turn, temperatures will climb into
the mid to upper 60s for most locations - about 10 degrees above
normal. This warmth will couple with the dry air in place over the
CWA and gusty winds to lead to Elevated Fire Danger this afternoon
across much of the area, though particularly over southeastern
Missouri where relative humidity will be lowest.

Late this afternoon into this evening, a cold front associated with
the aforementioned surface low will sweep through the area, causing
winds to shift to back out of the northwest overnight. Thanks to the
lack of upper-level forcing and moisture return, this will be a dry
FROPA. The northwesterly winds will advect cooler air back into the
region, with overnight lows bottoming out in the mid to upper 30s
for most locations.

Deep northwesterly flow will remain over the CWA on Wednesday as the
upper-level trough broadens over the eastern CONUS and an area of
high pressure edges into the region. Guidance varies on how far into
the CWA this high progresses, and therefore, temperature spread for
highs Wednesday afternoon vary by about 5 degrees within the IQR of
ensemble guidance. Regardless of the exact values, temperatures for
much of the region will be right around to just below climatological
normals.

Elmore

&&

.LONG TERM... (Thursday through Monday)
Issued at 325 AM CDT Tue Mar 19 2024

Starting Thursday into Friday, two different systems are set to
impact the region. A shortwave will pass through the Mid South as
another passes through the Midwest. The amplitude and speed of the
former varies among ensemble clusters and deterministic guidance,
and the longevity of the latter is in question, as most guidance
weakens it as it moves through the Midwest. The result will be
bifurcated rain chances for the CWA, with greater probabilities for
rain remaining north and south of the region late Thursday into
Friday. If the southern system is able to track further north, we
will see rain chances increase, though it seems that the trend has
been more southerly with only a low chance (20%) for rain from this
system over portions of southeastern Missouri. The better chance
(currently 30-40%) for rain comes with the northern system and its
associated cold front. However, these probabilities are dependent on
the strength of an already weakening shortwave and how much moisture
is able to return ahead of the front given reduced access to Gulf
moisture given the southern system. Given these factors, some of the
area may see light rain late Thursday into Friday, but my confidence
is low that any area will see notable rainfall during this portion
of the period.

As we get into the weekend and early next week, confidence in the
evolution of the upper-level pattern and the resulting surface
conditions drops. In the wake of Friday`s front, guidance consensus
is that a large area of high pressure will move into the central
CONUS. However, how far south this high progresses differs
drastically among deterministic and ensemble guidance. Additionally,
guidance shows varying degrees of southwesterly flow ahead of an
upper-level trough Sunday into Monday. This impacts how quickly the
high will retreat and southerly flow will return to the CWA ahead of
a surface low moving through the region sometime early next week.
The result of these differences among guidance is anywhere from a 5-
20 degree spread in temperatures within the IQR this weekend into
early next week. It also impacts the timing of our next chance for
rainfall; though, guidance consensus is currently that we`ll see
chances (70-80%) ramp up during the latter half of Sunday into
Monday. This early week system does have the potential to bring much
needed widespread rainfall to the area, but the lead time and spread
among guidance prevent me from saying that confidently.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1020 PM CDT Mon Mar 18 2024

Dry, VFR conditions with a clear/mostly clear sky are forecast to
continue through the period. There may be a very brief (1-2 hours)
window for low-level wind shear at KCOU/KJEF/KUIN around dawn
Tuesday morning, but the expectation is for surface winds to
increase enough out of the southwest by that time to preclude
mention. Speaking of the southwest surface winds, they will
increase through the morning hours and become gusty. Gusts in the
25 to perhaps 30 knot range are forecast before they diminish
around or just before sunset. Winds are expected to lighten up
between the late afternoon and evening hours and veer more to the
northwest behind a cold frontal passage.

Gosselin

&&

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

$$

WFO LSX


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