Text Products for AFD Issued by LSX

Area Forecast Discussion
Issued by NWS St. Louis, MO
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FXUS63 KLSX 212332
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
632 PM CDT Sun Apr 21 2024

.KEY MESSAGES...

- Another seasonably chilly night is on tap, with widespread frost across
  much of the area developing overnight. Please see the frost
  advisory for more details.

- The next chance of rain comes on Tuesday when mainly showers are
  expected.

- Warmer temperatures and chances for thunderstorms return later
  in the week, with the best chance for thunderstorms on Thursday
  night through Friday night.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 157 PM CDT Sun Apr 21 2024

(Tonight)

An area of surface high pressure was located near the Kansas-
Oklahoma border early this afternoon, with a ridge axis extending to
its east/northeast. This ridge will gradually progress through the
area tonight, shifting into southeast Missouri and southwest
Illinois. The combination of a clear sky and light/variable winds in
proximity to the aforementioned ridge will yield very favorable
conditions for radiational cooling. Most locations should drop back
into the mid 30s overnight, with frost development across much of
the area expected. The exception is in the urban corridor of
metropolitan St. Louis where lows closer to 40 degrees is forecast.
On the other end of the spectrum, favored valleys in east central
and southeast Missouri should be a bit colder, with lows around the
freezing mark. Widespread subfreezing lows are not forecast so will
not do a freeze warning, but would expect the heaviest/most
widespread frost in these locations.


(Monday)

Return flow around the ridge departing into the mid south will
increase across the area quickly on Monday, with southwest winds
strengthening to 10-15 mph. This increasing low-level warm air
advection and plenty of sunshine should allow for a rapid warmup
during the day, with highs reaching into the upper 60s to low 70s.
These readings are around the 90th percentile of available model
guidance and a degree or two warmer compared to the previous
forecast. This slight increase in high temperatures was due to the
favorable conditions outlined above for warming and the propensity
for model guidance to be too cool in these situations so far this
spring.


Gosselin

&&

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 157 PM CDT Sun Apr 21 2024

(Monday Night - Tuesday Night)

Chances of rain showers will begin to increase late Monday night as
low-level moisture advection moves into the area and a midlevel
shortwave trough approaches from the mid-Missouri Valley. These rain
chances will shift southeastward with time as a cold front moves
through the region. The best chance of rain continues to look like
for areas along/east of the Mississippi River (60-75%) where
mid/upper level forcing for ascent is a bit stronger closer to the
track of the midlevel shortwave trough. A couple of rumbles of
thunder are possible with this activity, but limited moisture return
and weak lapse rates really help limit instability. Deterministic
model guidance shows only up to 150-200 J/kg of MUCAPE across the bi-
state area.

The cold front should is expected to clear the CWA to the south and
east by Tuesday evening, with rain chances rapidly decreasing in its
wake. Total rainfall amounts continue to look on the light side,
with most areas expected to receive around 0.10". Probabilities for
>0.25" of rain on the LREF are below 25% across the entire area,
with odds of >0.10" highest (50-60%) on the Illinois side of the
Mississippi River.


(Wednesday - Wednesday Night)

A period of dry and slightly cooler temperatures is forecast for
midweek behind Tuesday`s cold front. High temperatures on Wednesday
are expected to be mainly in the 60s, or about 2-3 degrees below
normal for the date.

Wednesday night will be cool, but concerns for frost are a bit less
compared to yesterday. This is due to the the surface high looking
to be a bit further east/northeast (more toward Great Lakes),
yielding easterly winds that should stay up enough to help prevent
frost. In addition, model RH plots in the 700-850 hPa layer suggest
an increase in cloud cover, particularly in central and southeast
Missouri. Lows in the 40s are forecast, with coolest locations in
parts of south-central Illinois. This is where the easterly winds
should be lightest and clouds should stay off to the west overnight.


(Thursday - Next Sunday)

Ensemble guidance continues to show anomalous mid/upper level
troughing in the southwest ejecting into the central Plains late
Thursday into Friday. Differences though remain in the timing and
track of the initial shortwave, with the GFS/GEFS a bit faster and
further southeast. Even with this type of solution however, most of
Thursday at least is dry, with better chances of showers/elevated
convection further to the southwest closer to a stalled frontal
boundary. Better chances for showers and thunderstorms come late
Thursday night into Friday as low-level moisture advection increases
above the retreating warm front.

At some point late Friday/Friday night the entire CWA likely will
get into the warm sector, and this is when the threat for strong to
severe convection would be higher. The GFS/GEFS remains cooler and
more stable than the ECMWF/EPS due to the further southeast track of
the midlevel shortwave trough and associated surface low and more
antecedent showers/storms early Friday. Probabilities on the LREF
for at least 1000 J/kg of SBCAPE Friday evening have not changed
much at all compared to yesterday, generally ranging from 10-30%
from east to west (highest chance in central Missouri). Deep-layer
shear however will likely be quite supportive of organized
convection with at least 40 knots expected. That means it may not
take a lot of instability to have a threat of at least some isolated
strong to severe thunderstorms. Given the uncertainty though on the
storm track and resultant instability, we continue to keep an eye on
this timeframe but are not messaging the threat explicitly at this
still very early juncture.

The active pattern looks to continue through next weekend (and
likely beyond), but confidence in PoPs is low. Ensemble means from
both the GEFS/EPS suggest some mid/upper level ridging settling into
the area behind Friday`s system, but differ on the longitudinal
location. The ECMWF/EPS is a bit further west (more over the
Mississippi Valley) than the GFS/GEFS. The further west solution of
the ECMWF/EPS would suggest a better chance of dry (and hotter)
weather than the GFS/GEFS. Given the uncertainty, the NBM`s chance
PoPs Saturday/Saturday night looked like a good compromise. Better
chances of storms should come late Sunday as another midlevel
shortwave trough ejects into the south-central Plains. Given that
the CWA should be in the warm sector all weekend, instability should
not be a problem but there may be less deep-layer shear, and more
importantly, a bit more uncertainty as to what will initiate
convection (particularly on Saturday). In other words, at least a
conditional threat for a few strong to severe thunderstorms probably
will continue at least through next weekend.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 631 PM CDT Sun Apr 21 2024

Dry and VFR flight conditions are expected to persist through the
entire TAF period. Surface high pressure will trek across the area
tonight and allow winds to become light and variable. With the
surface high to the east by Monday, expect winds to increase from
the southwest.

MMG/Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory from 2 AM to 8 AM CDT Monday for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis MO-Sainte Genevieve MO-Shelby MO-Warren MO-
     Washington MO.

IL...Frost Advisory from 2 AM to 8 AM CDT Monday for Adams IL-Bond IL-
     Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey
     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Pike IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX
National Weather Service Regional Headquarters
