Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230842
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
332 AM CDT Sat Mar 23 2024

.KEY MESSAGES...

- Low relative humidity, strengthening winds, and dry vegetation
  will result in elevated fire danger across Missouri tomorrow
  afternoon and early evening.

- Rain chances will extend from late Sunday through early Tuesday
  with the highest potential (90-100%) progressing west to east
  Monday afternoon through Monday night. Rainfall totals of 1-1.5
  inches could fall over much of the area with exception to
  central and northeast Missouri.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 328 AM CDT Sat Mar 23 2024

Cooler air continues to work into the region behind a cold front
that pushed through the area yesterday. Surface observations show
temperatures ranging through the 30s sub-freezing temperatures being
curbed by the overcast sky conditions. Regional IR satellite shows a
few breaks appearing over section of southwest Illinois with the
northern edge of the stratus deck running along the Missouri border.
Where clouds have cleared north of the border, surface temperatures
fall into the mid-20s. It is expected that this clearing line will
continue to work southward through the morning with northerly flow
rotating around the surface high building in from the northwest.
While temperatures are largely above freezing this morning, areas of
northeast Missouri and west-central Illinois could fall several
degrees before sunrise as clearing occurs from north to south.

The center of the surface high will slide west to east across the
Upper Midwest and into the Great Lakes tonight. A weak mid-level
shortwave track along the MO/IA border tonight, as weak warm air
advection begins to draw slightly warmer air into the area. Low
chances (20-30%) for rain/snow exist over northeast Missouri and
into west-central Illinois, but will have a difficult time
overcoming dry air in the lowest 5k feet. What does fall is expected
to be light and short-lived, if it does materialize. Above freezing
surface temperatures and the light nature of precipitation should
prevent any impact if snow happens to mix in.

As easterly surface flow back out of the southeast tonight into
Sunday, milder air returns temperatures to near normal. Though upper
level clouds will begin to stream in ahead of the next system
Sunday, dry weather is favored. An inverted surface ridge extends
southwest through the Appalachians with east/southeasterly surface
flow feeding into the mid-Mississippi Valley. Surface dewpoints in
the 20s are only one indication of how dry it will be ahead of the
approaching system. Model soundings show the dry layer extending
from the surface through approximately 700mb with dewpoint
depressions as high as 30 degrees within the layer. The tightening
pressure gradient will also result in increasing winds, especially
as mixing heights deepen Sunday afternoon. Southeast sustained winds
of 20-25 mph could gust 30-35 mph at times. Coupled with the dry
surface, winds and warmer temperatures will lead to elevated fire
danger Sunday afternoon into Sunday evening west of the Mississippi
River, where RH values will dip below 30%.

Maples

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 328 AM CDT Sat Mar 23 2024

Deterministic guidance is in generally good agreement with a late
weekend/early week system that could bring widespread, beneficial
rain to the region. The NAM/GFS/ECM each show surface low pressure
over the CO/KS state line Sunday evening (00z Monday). The surface
low begins to round the base of a broad upper before ejecting
northeastward into the upper Midwest by 12z Tuesday morning.

Dry air preceding the system will introduce challenges to timing the
onset of steadier, soaking rainfall. Southerly flow running ahead of
the system will pull moisture northward out of the Gulf of Mexico
into the central Plains, where dewpoints in the mid-40s to low-50s
will originate before advecting eastward Sunday evening into Sunday
night as strong mid-level WAA pushes southwest to northeast over the
region. The past couple of forecast cycles have continued to delay
likely (60+ percent) to the west until near/after 06z Sunday. Much
of the onset Sunday night into early Monday will be in the form of
scattered showers with a pool of MUCAPE remaining over the eastern
Plains. As the system near late Sunday, the strengthening LLJ will
continue to introduce the potential for winds inching toward
advisory criteria. Depending on how well we mix, gusts could reach
45 mph late Sunday night with better potential heading into Monday
as mixing deepens.

Warm air surges northward into the region by Monday with corridor of
moisture lining up with the Mississippi River Monday afternoon.
While precipitation chances will increase from Sunday night into the
Monday, the highest probabilities (90-100%) arrive Monday afternoon
through early Tuesday. Though the surface low drive northeast into
IA/MN, a secondary upper level impulse pushed northeastward ahead of
the upper trough and over top of what looks to be a north-south
oriented pre-frontal trough. This feature is highlighted in surface
to 700mb layer over central MO by 18z Monday, moving east into
Illinois by 06z Tuesday. While a rumble of thunder cannot be
entirely ruled out, CAPE progs are not great with deterministic
guidance showing 100-200 J/kg of MUCAPE that wanes with eastward
progress of the trough. Fortunately, ample ascent and deeper
moisture late Monday into early Tuesday will be enough to provide a
soaking rainfall over most locations with highest totals still
favoring southeast Missouri/southwest Illinois. GEFS/ENS/GEPS each
show mean rainfall totals around an inch over most of the CWA with
exception to sections of the northeast Missouri. Grand ensemble
spread is about 0.50 at any given location, placing the low end of
the amounts near one-quarter of an inch with the high end just over
an inch over southeast Missouri and southwest Illinois.

The remainder of the week is largely quiet with cooler than normal
conditions following behind the system through midweek. Temperatures
begin to warm late in the week in advance of another system that
ejects out of the Plains. However, diverging solutions result in
greater uncertainty heading into next week with regard to overall
rainfall potential.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1029 PM CDT Fri Mar 22 2024

MVFR stratus now envelops the terminals, with brief instances of
IFR CIGs being observed right along the cold front aloft. Light
showers are pushing southeast of the TAF sites, and will exit the
region in the next hour or two. Conditions will remain MVFR
overnight, but dry air from the north will erode the stratus from
north to south. By around sunrise, the stratus will scatter out
and VFR conditions will prevail. Winds will gradually shift from
out of the north to from the east by Saturday evening as high
pressure centered to our northeast establishes across the region.

MRB

&&

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

$$

WFO LSX


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