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000
FXUS63 KLSX 202257
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
557 PM CDT Sat Apr 20 2024

.KEY MESSAGES...

- Seasonably cool weather is forecast through Sunday night. A
  light freeze is forecast in parts of northeast Missouri tonight
  (Knox county), with frost expected further to the southeast.
  Please see the frost/freeze products for additional details.
  More frost is possible Sunday night across much of the area.

- Temperatures closer to seasonal normals are forecast for
  early/mid next week. Dry weather is also forecast for the most
  part other than a chance of showers and possibly a few
  thunderstorms on Tuesday associated with a cold front.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 225 PM CDT Sat Apr 20 2024

(Tonight)

A weak midlevel shortwave trough is forecast to move through the
Mississippi Valley/western Great Lakes early tonight. This feature is
expected to bring an area of mid/high level clouds which is expected
to move from northwest to southeast through the night. The timing of
the clearing behind these clouds is key, and will be the primary
factor in overnight low temperatures and frost potential. Model RH
plots show the clearing line reaching parts of northeast Missouri
and west-central Illinois shortly after midnight, then progressing
southeast into parts of southeast Missouri and southwest Illinois
between 0900 and 1200 UTC Sunday. This progression should yield the
coldest low temperatures in parts of northeast Missouri and west-
central Illinois which should have several hours of a clear sky.
This clear sky combined with light/variable winds on the eastern
periphery of a strong surface anticyclone (~1030 hPa in Kansas)
should yield very favorable conditions for radiational cooling. This
not only is expected to allow temperatures to drop into the low 30s,
but also lead to widespread frost development. Some spots near or
slightly below 30 degrees are also possible, particularly in/around
Knox county. This is where a freeze warning has been issued for
tonight, with a frost advisory further to the southeast. Pinpointing
the southeastern edge of the frost advisory was a challenge, but
elected to keep it in areas where there was relatively high
confidence in lows getting down into the mid 30s (central/northeast
Missouri, parts of east central Missouri, and west central
Illinois). There very well may be at least some patchy frost further
to the southeast, but it will depend on if the clearing can reach
those areas in time for temperatures to cool into the mid 30s and
allow for (more widespread) frost development.


(Sunday)

All signs point toward another beautiful day to conclude the weekend
with plenty of sunshine, light winds, and seasonably cool
temperatures. Slightly warmer temperatures are forecast across most
of the area, with less in the way of cloud cover and some slight
modification of the air mass in place. Most of the bi-state area is
forecast to have high temperatures near or just above the 60 degree
mark. Those readings would be about 5-10 degrees below normal for
the date.


(Sunday Night)

The attention on Sunday night will once again be on the seasonably
chilly temperatures and frost potential. The surface ridge slowly
moves across the area Sunday night, and by 1200 UTC Monday is
forecast to be located across parts of southeast Missouri into
southwest Illinois. Some weak return flow should begin to its
northwest, helping keep temperatures in central and northeast
Missouri as well as west-central Illinois from dropping into the low
to mid 30s. That being said, frost development (especially in
sheltered locations) still may occur as the southwesterly return
flow winds look weak (~5 mph). The frost may be at least patchier
however due to the more marginal low temperatures (generally 35-
38F). The best chance for more widespread frost development will be
in parts of southeast Missouri and adjacent southwest Illinois. Calm
or light/variable winds combined with what should be a crystal clear
sky should allow for temperatures to drop back into the low to mid
30s. Favored valley terrain in southeast Missouri may have a light
freeze (lows 30-32F) as well, but at minimum, would expect pretty
widespread frost development across these areas. An additional frost
advisory likely will be needed for parts of the area tomorrow/Sunday
night, but will leave that for tomorrow to take one last look at
things and to get past tonight`s headlines first.


Gosselin

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 225 PM CDT Sat Apr 20 2024

(Monday - Thursday Night)

After a chilly start to the day on Monday, a quick warmup is
expected by afternoon as southwesterly surface winds increase behind
the departing surface ridge. The warmup will also be aided by plenty
of sunshine, increasing low-level warm air advection, and mid/upper
level ridging moving overhead. High temperatures should reach back
into the upper 60s to near 70 degrees. These values are near the
75th percentile of available model guidance, and would not be
surprised if highs need to be raised another couple of degrees
with future forecast updates.

Deterministic and ensemble guidance is in good agreement that a
midlevel shortwave trough will dive into the Upper Midwest early
Tuesday. At the surface, an area of low pressure is expected to move
east/southeast across the eastern Great Lakes, with an attendant
cold front draped to its southwest. This frontal boundary is
expected to move through the bi-state area Tuesday afternoon/evening
from northwest to southeast. Showers and a few thunderstorms are
expected with this cold front, with the best chances (>60%) in parts
of northeast Missouri and west-central Illinois where mid/upper
level forcing for ascent will be strongest. These PoPs may tend to
climb upward too as confidence in the exact frontal timing
increases. The latest LREF probabilities for measurable rainfall are
70-90% over a 24-hour span in these same areas. Precipitation
amounts though do look light given the transient nature of the
forcing. Probabilities for at least 0.25" of rain are low, generally
below 25%.

Temperature wise, Tuesday should be a warm day ahead of the front
(low 70s), but the exact timing will be key to pin down high
temperatures. This will be particularly true for parts of northeast
Missouri and west-central Illinois given the expected frontal
passage in the early afternoon. A couple of hours earlier (later)
could yield high temperatures several degrees cooler (warmer) than
currently forecast. Behind the front, a return to more seasonable
conditions (highs in 60s, lows 40-45F) are expected. Cannot rule
out some frost potential Wednesday night/Thursday morning given
the air mass coming in, but that threat will depend on the timing
and location of the surface anticyclone.


(Friday - Next Saturday)

While model uncertainty increases late week into the following
weekend, the general pattern definitely points more toward active
weather with multiple chances of showers and thunderstorms.

The first in what should be multiple midlevel shortwave troughs
ejecting out of the southwestern CONUS is on Friday. Guidance varies
considerably with the timing, track, strength of this feature, but
generally agree that a surface low will move to our northwest.
Increasing low-level moisture and instability is expected on Friday,
with increasing chances of showers and thunderstorms. Some of the
storms may be strong to severe in the warm sector late Friday. Deep-
layer shear likely will be seasonably strong (>40 knots) but
antecedent clouds/elevated convection could limit instability. An
early look at probabilities for >1000 J/kg of CAPE early Friday
evening from the LREF show a 10-30% from east to west. Additional
showers and thunderstorms are possible next Saturday, but this will
depend on where a cold front resides and how strong mid/upper level
ridging settles in behind Friday`s departing wave.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 556 PM CDT Sat Apr 20 2024

VFR flight conditions are expected through the forecast period. A
mix of mid and high clouds will impact the region tonight with
clear skies on Sunday.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory from 2 AM to 8 AM CDT Sunday for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Lewis MO-Lincoln MO-Marion MO-Moniteau MO-Monroe MO-
     Montgomery MO-Osage MO-Pike MO-Ralls MO-Shelby MO-Warren MO.

     Freeze Warning from 2 AM to 8 AM CDT Sunday for Knox MO.

IL...Frost Advisory from 2 AM to 8 AM CDT Sunday for Adams IL-Brown
     IL-Calhoun IL-Greene IL-Jersey IL-Macoupin IL-Montgomery IL-
     Pike IL.

&&

$$

WFO LSX