Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 272352

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
552 PM CST Sat Feb 27 2021

.SHORT TERM...  (Through Late Sunday Night)
Issued at 251 PM CST Sat Feb 27 2021

In the near-term, the primary item of interest will be the potential
for heavy rain and localized flooding tonight through Sunday morning
across primarily southeast Missouri and southern Illinois, in
addition to a few thunderstorms.

That may be somewhat surprising considering the pleasantly warm
and dry conditions currently in place, although the increasing
high cloud cover is a hint of what`s to come. As the afternoon
gives way to the evening, a strengthening southwesterly low level
jet will transport a plume of seasonally rich low level moisture
northward into southeastern Missouri and southern Illinois, to the
tune of 1 to 1.5" of precipitable water by late this evening.
This moisture content is anomalously high for this time of year,
reaching 2 to 2.5 standard deviations above normal along and south
of the MO Interstate 44 / IL Interstate 70 corridor. Moreover,
this moisture will overspread a remnant surface boundary, and the
combination of moisture convergence and isentropic lift will be
sufficient to generate moderate to heavy rainfall between this
evening and early Sunday morning.

Complicating matters even more will be the potential for convection
and even a few thunderstorms. While a low level stable layer will
limit the potential for surface based thunderstorms, forecast
soundings do indicate that between 200 and 600 J/kg of MUCAPE will
develop during the evening, despite only marginal mid level lapse
rates. Given the strongly sheared environment, this will likely be
sufficient to generate a few local thunderstorms featuring heavy
rain rates at a minimum, with potentially some small hail within the
strongest storms. The highest probabilities for thunderstorms will
exist across southeast Missouri and far southern Illinois.

Confidence remains high that the most likely region to see heavy
rain will be across southeast Missouri and southern Illinois, with
forecast amounts reaching between 0.5 and 1.5 inches along and south
of the MO I-44 / IL I-70 corridor. Locally higher amounts will be
possible wherever training thunderstorms occur, which may be enough
to generate localized flooding. As such, we have maintained our
existing Flood Watch for Iron, Madison, and Reynolds counties in MO
for this evening through tomorrow morning. North of the 44/70
corridor, scattered showers are expected with perhaps a stray
thunderstorm or two, with rainfall amounts of less than 0.5"

On Sunday, a cold front will push southward through the area,
forcing the corridor of heaviest rain and thunderstorms southward.
While initial round of heavy rain will gradually diminish, there
remains some uncertainty regarding just how quickly the front will
clear the area, as there is some recent Hi-Res guidance that suggest
a slower FROPA. At a minimum it seems likely that at least some
light post frontal stratiform rain will linger in the area through
Sunday afternoon. However, a slower FROPA would leave open the
potential for redeveloping thunderstorms along and south of the
boundary. To be clear, the latter scenario remains a low
probability, but it cannot be completely discounted at this time. In
addition to prolonging the potential for thunderstorms, this also
casts doubt on Sunday`s temperature forecast, as a slower FROPA
would likewise lead to warmer temperatures.


.LONG TERM...  (Monday through Next Saturday)
Issued at 251 PM CST Sat Feb 27 2021

By Monday, deep northwesterly flow will spread into the region
behind the departing cold front, bringing a more definitive end to
precipitation. This post-frontal airmass will also keep temperatures
a bit cooler, although not far from seasonal averages for the first
day of March. For those still reeling from mid-February`s bitter
cold, Monday`s "cooler" weather featuring upper 40s to mid 50s will
feel downright mild, despite the occasional 15 mph gusts.

Tuesday and beyond, the large scale upper flow pattern will
gradually transition from northwest flow to quasi-zonal, as a
longwave ridge very gradually de-amplifies and moves eastward into
the Mississippi Valley. While the most likely scenario will be for
the region to remain dry through mid week, a weakening shortwave
emerging from the southern plains does present a low-probability
opportunity for precipitation across the southern half of Missouri
Tuesday into Wednesday. Confidence continues to be low regarding the
track of this system, and currently the most likely scenario is that
this system will move too far south to produce meaningful
precipitation locally. However, a non-trivial chance remains for
light precipitation to occur during this timeframe along and south
of I-70.

Guidance continues to strongly suggest that a warmer, drier period
can be expected Wednesday and Thursday with high certainty of above
normal temperatures and no precipitation. Confidence is also
increasing that another southern-stream shortwave will impact the
region at some point late in the week, but a wide range of potential
outcomes remains possible with this system, and very few specifics
are known regarding how this will impact sensible weather locally.


.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 551 PM CST Sat Feb 27 2021

Showers and a few storms continue to develop along and south of
Missouri/Arkansas border as of 23z. This activity will spread
northward through the evening hours, though best chances of
thunderstorms will remain south of the TAF sites. However, as the
rain approaches all of the TAF sites, ceilings will lower to IFR
with visibility reductions for the St. Louis metro TAF sites.

Otherwise, strong low level jet is still expected to develop with
LLWS for TAF sites along I-70 corridor between 06z and 11z Sunday.
As for winds, southeast to south winds will veer to the west to
northwest behind the cold front as it moves through between 15z
and 17z Sunday with showers diminishing. Ceilings will lift and
scatter out by Sunday afternoon.


MO...Flood Watch through Sunday morning for Iron MO-Madison MO-
     Reynolds MO.


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