967
FXUS63 KLSX 300639
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
139 AM CDT Tue Apr 30 2024

.KEY MESSAGES...

- Patchy fog is expected across portions of southeast Missouri and
southwest Illinois, as well as in the main steam river bottoms
through tomorrow morning.

- Many river forecast points have exceeded minor flood stage, with a
few forecast to reach moderate flood stage this week.

- Temperatures are expected to be above normal through the period.
There are numerous chances for rain through next weekend with the
best chance (60-80%) Thursday into Friday.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 220 PM CDT Mon Apr 29 2024

Many of our forecast points along the main stem rivers and
associated tributaries have risen above flood stage thanks to the
widespread rainfall received over the weekend. Depending on the
basin and river, points may remain above flood stage for several
days. Additionally, some rivers like the Illinois, may be slow to
respond and may not go above flood stages for a few days.

A cold front is moving through the forecast area behind a stratus
deck. As of 2 PM the cold front is stretched from roughly Pittsfield
through the St. Louis metro to Farmington. Behind the cold front a
surface high is pushing into the region. Under the influence of the
high, a clear sky and near calm winds overnight will promote patchy
fog development. This is particularly the case across portions of
southeast Missouri and southwest Illinois where the sky will clear
much later in the day and subsequent near surface moisture will be
higher. Additional patchy fog development in the valleys of the main
stem rivers and in the hollows of southeast Missouri are also
possible.

The high will move eastward overnight and by tomorrow winds will be
from the south, increasing in speed as the surface pressure gradient
tightens ahead of another, weaker cold front. Warm air advection and
a clear sky will push high temperatures on Tuesday into the upper
70s to low 80s across the area. Cloud cover will begin to increase
Tuesday evening as a shortwave disturbance and weak surface low
slide across the upper Mississippi River Valley overnight. The
previously mentioned weak cold front will move through northern
Missouri bringing a chance (20-40%) of showers and thunderstorms to
the area. Coverage and intensity of the showers and thunderstorms is
expected to be light and weak given the weakness of the front as it
moves into our forecast area.

MRM

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 220 PM CDT Mon Apr 29 2024

A mid-level ridge will strengthen over the Ohio River Valley
Wednesday, and to the northwest of the forecast area, a mid-level
trough will slide along the US-Canada border in the upper Plains. At
the surface a low will form and strengthen over the southern Plains,
with a warm front developing to the north of the forecast area and a
cold front sagging into Texas. The forecast area will stay within
the warm sector ahead of this system on Wednesday and Thursday,
increasing temperatures and low-level moisture.

By Thursday the mid-level trough axis will swing into the mid-
Mississippi Valley and the surface reflection will progress through
the region during the evening and overnight. Showers and
thunderstorms are expected ahead of and along the attendant cold
front as it moves east to west through the forecast area overnight
Thursday into Friday. Ahead of the front, the warm sector will be
characterized by high instability (1000+ J/kg of MUCAPE via
deterministic members), minimal cap, and high precipitable water
(90th percentile via NAEFS situational awareness tables), and weak
effective shear. Conditions are favorable for widespread showers and
thunderstorms, particularly once the near surface dry layer can be
eroded. However, the weak shear limits the organization of strong to
severe thunderstorms. The cold front is expected to be progressive
enough to also limit slow moving thunderstorms and the associated
heavy rain potential. The cold front will clear the forecast area
Friday, bringing a relative and brief cool down to the region before
temperatures warm over the weekend.

The mid-level trough will continue to push east along the US-Canada
border into and through the Great Lakes region during the end of the
work week. With the system so far to the north, the region remains
open to mid-level disturbances through the weekend. Ensemble and
deterministic guidance varies on handling the mid-level pattern
beyond Friday, and we certainly cannot rule out disturbances moving
across the forecast area and bringing additional chances of rain.

MRM

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 139 AM CDT Tue Apr 30 2024

The main concern this TAF period will be the development of fog
overnight into Tuesday morning. Outside of more widespread fog in
southwestern IL, fog is anticipated to be largely limited to river
valleys, with confidence highest in at least periods of MVFR to IFR
flight conditions at KJEF, KSUS, and KCPS. Fog will dissipate and
visibilities will recover within an hour or two of sunrise Tuesday
morning. Other than fog, dry and VFR flight conditions are expected.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT this morning for Iron MO-
     Jefferson MO-Madison MO-Reynolds MO-Saint Francois MO-
     Sainte Genevieve MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CDT this morning for Bond IL-
     Clinton IL-Fayette IL-Marion IL-Monroe IL-Randolph IL-Saint
     Clair IL-Washington IL.

&&

$$

WFO LSX