075 
FXUS63 KLSX 230947
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
447 AM 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 06z TAFs through 06z Friday Night)
Issued at 1042 PM CDT Thu Apr 22 2021

Showers are expected to increase overnight primarily for areas 
near I-70 and south, with prevailing showers for KCOU and the STL
metro sites, with a VCSH at KUIN. A general weakening trend will 
occur around sunrise with a break for much of the day before a 
more significant round of heavier showers, possibly with 
thunderstorms, moves in late Friday afternoon into the evening. 
Thunder chances look low, however, due to limited instability. 
Surface winds are expected to settle from a S-SE direction early 
this evening then continue for the remainder of the period, 
peaking and becoming gusty during the day on Friday. MVFR CIGs
will expand late tonight and early Friday morning for all TAF 
sites and prevail for the remainder of the valid period for KUIN
and KCOU while expected to lift to VFR category for the STL metro
sites Friday afternoon before returning to MVFR category Friday
night with the return of the showers. 

TES

&&

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

$$

WFO LSX