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000
FXUS63 KLSX 120418
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1118 PM CDT Thu Apr 11 2024

.KEY MESSAGES...

- Gusty winds and low RH values will allow for conditions to be
  favorable for Elevated Fire Danger in northeast Missouri and
  west-central Illinois on Friday.

- Anomalous warmth will push temperatures 10-20 degrees above
  seasonal normals starting Saturday and continuing through the
  middle of next week.

- A more active pattern early next week will bring the potential
  for multiple rounds of showers and thunderstorms, some possibly
  strong to severe.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 225 PM CDT Thu Apr 11 2024

Current water vapor imagery shows an upper level trough extending
south through the upper- and mid- Mississippi River Valley. As this
upper trough`s surface reflection continues northeastward out of the
Ohio River Valley, the area is left in deep northwesterly flow. An
associated tight pressure gradient, as well as mixing from 850-mb,
is allowing for strong gusty winds in the 30-35 mph range area wide.
When daytime heating diminishes this evening, the gusts will also
diminish slightly.

As the upper level trough begins its shift east, a shortwave and an
associated vorticity maximum will traverse across Illinois. Surface
instability will be rather limited with ensemble guidance indicating
less than 300 J/kg across western Illinois. This will provide weak
support for scattered showers in its axis during the early evening
east of the Mississippi River. An embedded thunderstorm cannot be
ruled out.

By Friday, upper level and surface ridging will begin to build into
the Mid-Mississippi River Valley. As this ridging makes its way into
the area, the surface pressure gradient will continue to tighten and
allow for another day of strong, gusty winds. In addition to the
gusty winds, the deep northwesterly flow will advect dry air into
the region. The mix of gusty winds, low RH values, and relatively
dry fuels provides conditions for erratic fire weather to be
possible across northeast Missouri and west-central Illinois on
Friday. Temperatures on Friday are forecasted to be near normal for
mid-April in the 60s.

MMG/Elmore

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 225 PM CDT Thu Apr 11 2024

Guidance consensus is that at the start of the extended forecast
period on Saturday, the Middle Mississippi Valley will be beneath
northwesterly flow aloft as an upper-level trough moves eastward and
a ridge builds over the Intermountain West. Through the day
Saturday, this ridge will expand eastward, causing a surface ridge
oriented along the Mississippi River to shift eastward and return
southerly low level flow to the region. This will kick off a
prolonged period of warm air advection and boost temperatures
Saturday afternoon by at least 10 degrees compared to those on
Friday. This is supported by ensemble means approaching roughly 12
degrees C at 850 mb (at least 90th percentile of climatology) and
assuming deep mixing.

A weak shortwave will traverse the northern periphery of the ridge
through the Midwest Saturday night into Sunday morning per
deterministic guidance. A majority (80%+) of ensemble and
deterministic guidance have the CWA dry with the passage of this
shortwave, so most of the area is forecast to remain dry.

Despite the passage of the shortwave, the upper-level ridge only
continues to build into the Midwest Sunday into Monday as mid-level
temperatures warm into the mid teens degrees C per ensemble means,
corresponding with an increase in surface temperatures. This will be
the warmest portion of the period, as temperatures climb into the
mid to upper 80s - roughly 15-20 degrees above climatological
normals. While we are not forecasting record-breaking warmth,
current forecasted highs do come within a few degrees, and an even
warmer forecast can`t be ruled out on Sunday thanks to southwesterly
surface flow. See the climate section below for more information.

Monday into Tuesday, a trough over the western CONUS will eject out
into the Plains and move through the Midwest, dragging a surface low
through the Central Plains into the Upper-Midwest. Ensemble clusters
show different phasing of the trough, leading to uncertainty in the
exact track and timing of the surface low and its associated cold
front. However, the majority of guidance keeps the better forcing
and thermodynamics for convection west of the CWA on Monday. On
Tuesday, the low begins to occlude as the front swings through the
area. Ahead of the front, a majority of ensembles have 500 J/kg or
less of SBCAPE among roughly 40-60 kts of 0-6 km bulk shear. Even
with roughly 500 J/kg of SBCAPE, the amount of shear will lead to a
threat of strong to severe thunderstorms, highlighted by the SPC Day
6 15% Severe Weather Outlook that includes a portion of our CWA.

In the wake of this system, ensembles diverge on the phasing of the
upper level pattern, though the general consensus is that the trough
will reload as some degree of southwesterly flow remains over the
Middle Mississippi Valley and a stalled front. If this occurs, the
region may see at least another round of convection, some strong to
severe, during the mid-week period as highlighted by CSU and CIPS
probabilistic severe guidance.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1115 PM CDT Thu Apr 11 2024

Strong sustained winds currently plague all terminals, posing the
only aviation weather hazard in the valid TAF period. The wind
will persist overnight and tomorrow as a strong weather system
gradually exits the region tonight to the northeast. Winds on
Friday will gust as high as 30kts in the afternoon. Conditions
improve near 00Z with sunset and a loosening pressure gradient
weakening winds considerably into tomorrow evening.

MRB

&&

.CLIMATE...
Issued at 225 PM CDT Thu Apr 11 2024

  4/14  4/15
St. Louis92 (2006)89 (2002)
Columbia89 (2006)90 (1896)
Quincy86 (2006)88 (2002)


&&

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

$$

WFO LSX