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000
FXUS63 KLSX 060840
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
340 AM CDT Thu May 6 2021

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 340 AM CDT Thu May 6 2021

Early this morning, an upper longwave TROF dominated eastern North
America with resultant northwest flow aloft over our region.  A pair
of upper level disturbances of note existed embedded within the
northwest flow aloft:  one crossing through western Missouri
kicking off scattered showers and a thunderstorm or two and
another located over western South Dakota.  At the surface, a
reflection of the western Missouri upper level disturbance existed
over southeastern Iowa in the form of a weak surface low pressure
with a cold front extending west southwest through northwest
Missouri into northeast Kansas.  Most of the scattered showers
currently over Missouri are courtesy of forcing from the upper level
system and not the front.  Temperatures had bottomed out in the
upper 40s and lower 50s in most locations, but thanks to low level
WAA ahead of the cold front and mostly cloudy skies, temperatures
have risen back into the mid to upper 50s for several locations.

The overall pattern will change little during this period, with
northwest flow aloft prevailing, guiding the two upper level
disturbances of note through our region:  the first through early
this morning and the second through this afternoon.

The scattered showers associated with the first upper level
disturbance are expected to exit or dissipate by late morning.
However, almost indistinguishable from this will be what gets
produced by the surface cold front and the second upper level
disturbance by midday and into the afternoon hours.  While coverage
will never progress much past isolated to scattered early this
morning, the precipitation later today will benefit from deeper and
stronger forcing during peak heating.  The only real limiting factor
will be moisture in the column.  The best location where this all
comes together continues to be for areas east and south of STL metro
during the midday through mid-afternoon hours.  It is here where the
highest PoPs will be along with the best chances for thunder.

Max temps today will be attained in most locations by midday with
falling temps in the afternoon due to a relatively early cold FROPA
and relative strength of the front itself.

Another cool night is on tap tonight as high pressure builds in
under mostly clear skies.  Min temps will be comparable to what we
saw a couple of nights ago with low-mid 40s for most locations but
upper 30s in a few spots cannot be ruled out.

Friday looked to be dry the last several forecast packages but model
guidance has come into better focus on the effects of another cold
front dropping south during the afternoon during peak heating.
There appears to be enough forcing with this front to now justify a
low PoP, mainly for areas near the I-70 corridor.  Most areas still
look to be dry this day, however.  Friday expects to be the warmest
day over the next 7 days, with max temps in the upper 60s and low
70s.

TES

.LONG TERM...  (Friday Night through Wednesday)
Issued at 340 AM CDT Thu May 6 2021

Heading into this weekend and early next week, the GEFS H500 mean
continues to advertise a pattern devolving through de-amplification
of the RIDGE and TROF as a storm system from the Pacific Northwest
becomes caught within the western RIDGE.  The result appears to be a
stuck primary storm system over the Rockies kicking off pieces of
energy eastward towards our region into the zonal flow downstream
for the rest of the CONUS.

Consequently, the GEFS mean MSLP develops a frontal boundary over
our region on Saturday and lingers it into Sunday before most
ensemble members drop this primary frontal boundary well south of
our region for Monday through Wednesday.

The end result continues to favor a wet weekend ahead, focused
particularly on Saturday night and Sunday morning, when ensemble
data continues to support a wave of low pressure tracking through a
stalled frontal boundary across our region. After Sunday morning,
ensembles then favor a steady southward push of this primary frontal
boundary.  We continue to favor a widespread 1-2" rain event this
weekend out of this.  If rainfall/thunderstorm potential focuses
enough there is a small chance that it could be significant enough
with respect to rainfall rates to result in the need for some kind
of Flood Watch.  Otherwise, rainfall of this magnitude spread out
over 24 hours will generally not cause any issues.

PoPs were trimmed back after Sunday night with increasing confidence
of the primary frontal boundary dropping well south, and also
supporting the continued below normal temperature regime.

TES

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1054 PM CDT Wed May 5 2021

Not much change in the forecast since previous TAF issuance. VFR
flight conditions are expected to prevail for the remainder of
the night. The cold front looks on track to move into northeast
Missouri and west central Illinois around 10-12Z Thursday morning.
A band of MVFR ceilings is likely in the vicinity of the front.
There will likely be some widely scattered showers in the morning
ahead of the front, but latest guidance is showing the showers
congealing into a band with some embedded thunderstorms ahead of
the front over central or east central Missouri around 15-16Z and
continuing southeast through the area. Some gusty winds and hail
are possible with the strongest storms. Precipitation should be
out of the area by 00Z.

Carney

&&

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

$$

WFO LSX