248 
FXUS63 KLSX 231752
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1252 PM CDT Fri Apr 23 2021

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 343 AM CDT Fri Apr 23 2021

Showers were moving through parts of central and southeast MO 
and into a small portion of southwest IL early this morning, 
mainly west and south of STL. This activity was ahead of a 
vorticity maximum in southwest MO, embedded within a shortwave 
trough extending from SD south-southeast into western MO. A 
southwesterly low level jet was also bringing increasing low level
moisture into our area as well. Most of this precipitation will 
shift east of our forecast area later this morning. We will 
continue to have below normal high temperatures today with plenty 
of cloud cover over the area. Another round of showers and a few
embedded and elevated storms will initially move into parts of 
central and northeast MO and west central IL, north and west of 
STL late this afternoon and early this evening as upper level 
divergence increases ahead of a southern stream upper level trough
moving through the Southern Plains. This precipitation will be 
along a weak inverted surface trough/warm front extending 
northeast from a surface low in the Southern Plains. The showers 
will become heavier this evening into the overnight hours, 
particularly in a region of low level moisture convergence on the 
nose of a south-southwesterly low level jet across central MO 
between COU and STL. Showers will continue Saturday morning 
mainly across southeast MO and southwest IL, from STL south and 
east in the deformation type precipitation just ahead of the upper
level trough and north and northwest of the surface low moving 
from northern AR into western KY. The heaviest rain totals will 
occur Friday night and Saturday morning across parts of central 
and southeast MO and southwest IL where 1 to 1.5 inches is 
possible. This could produce isolated and minor nuisance type 
flooding and ponding of water or roads. The rain will taper off 
Saturday afternoon in this area as the upper level trough and 
associated surface low shift east of the region.

GKS

.LONG TERM...  (Saturday Night through Thursday)
Issued at 343 AM CDT Fri Apr 23 2021

Colder low temperatures can be expected Saturday night as a 
surface ridge moves eastward through our forecast area with a 
clearing sky and diminishing winds. A warming trend will begin 
Sunday afternoon as an upper level ridge builds over the central 
US, and the surface/low level wind becomes southeasterly/southerly.
It appears that any rain Sunday through Monday will be north of 
our forecast area, along and north of a warm front. The warming 
trend will continue through at least Tuesday night with 
temperatures becoming well above normal. The threat for 
convection will increase by Tuesday afternoon and Tuesday night 
as low level moisture and instability increases well ahead of a 
positively tilted upper level trough and associated cold front 
approaching our area. The models have been trending slower and 
deeper with the upper level trough, and are now develop a surface
low along this front. This will result in the threat for showers 
and storms continuing into at least Thursday. Cooler temperatures 
can be expected Wednesday night and Thursday due to lower upper 
level heights in the vicinity of the upper level trough/low, and 
also because of low level cold air advection after passage of the
surface low and trailing cold front.

GKS

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1252 PM CDT Fri Apr 23 2021

Predominantly dry and VFR flight conditions are anticipated at all 
terminals this afternoon. However, a brief period of MVFR ceiling 
cannot be ruled out at KUIN. Southerly to southeasterly winds this 
afternoon will become light by this evening as well. Flight 
conditions will begin to deteriorate to MVFR through the evening as 
showers gradually develop and spread into the region. These showers 
are expected to become widespread overnight. A thunderstorm or two 
are also possible from this evening into overnight in the vicinity 
of KCOU and St. Louis metro terminals, but low probability precluded 
inclusion in this TAF suite. MVFR ceilings will likely continue to 
descend to IFR at KCOU and KUIN late tonight; however, it is 
uncertain if IFR ceilings will occur over St. Louis metro terminals. 
Showers will begin to exit the region from west to east on Saturday 
near the end of the TAF period.

Pfahler

&&

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

$$

WFO LSX