Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 282318

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
518 PM CST Sun Feb 28 2021

.SHORT TERM...  (Through Late Monday Night)
Issued at 248 PM CST Sun Feb 28 2021

Widely scattered convection, including a few thunderstorms,
continues to develop roughly along and south of the MO I-44 / IL I-
70 corridor early this afternoon. While this began as early as 10 am
across south central MO, there has been renewed vigor over the past
hour or two thanks to a marginal increase in instability (200-800
J/kg of MUCAPE) along and south of an advancing surface cold front.
While the environment is strongly sheared underneath the right
entrance region of a robust southwesterly jet streak, thunderstorms
have yet to become particularly well organized, and have only
displayed the capability of producing brief heavy downpours,
occasional lightning strikes, and perhaps some small hail. Given
the marginal instability present, this is likely about all that
can be expected of this activity. While odds of observing a strong
thunderstorm are rather limited everywhere across the St. Louis
forecast area, the most organized activity is likely to occur
across southeast Missouri where instability is greatest.

Over the course of the evening, the surface front will continue to
push southeastward, and convective activity will follow suit.
Thunderstorms should exit the region by late afternoon or early
evening, but light post-frontal stratiform rain may linger through
the evening across southeast MO and southwest IL. Meanwhile, gusty
northwest winds will develop behind the front, with gust speeds
ranging from 15 to as much as 25 mph at times through the night.
Skies will also gradually clear, although some lingering low
clouds may persist until Monday morning. Overnight temperatures
will be much lower as well, and most places are expected to fall
into the mid 20s to low 30s.

After tonight`s front, Monday will feature seasonally cool
temperatures in the upper 40s to low 50s thanks to persistent deep
northwesterly flow, with mostly clear skies and no additional rain
to speak of.


.LONG TERM...  (Tuesday through Next Sunday)
Issued at 248 PM CST Sun Feb 28 2021

By Monday night, the synoptic scale pattern will begin a gradual
transition from northwest flow aloft to quasi-zonal, as a longwave
ridge very gradually flattens out and slides eastward into the
plains and Mississippi Valley. Overall, this will likely lead to a
long-term warming and drying trend for the region, but not without a
couple of potential flies in the ointment.

The first of these flies will be a weakening shortwave emerging from
the southern plains early Tuesday. Model guidance continues to
struggle with the track of this feature, and likewise, there remains
substantial doubt that the region will see any precipitation at all
on Tuesday. The most recent trend in guidance has been a slight
shift toward a more southerly solution, and likewise a lower
probability of precipitation. We have maintained a mentionable
probability of very light precipitation along and south of I-70
during the day Tuesday, but have lowered probabilities slightly.
Temperatures will continue to be near to slightly above normal

A more noticeable warming trend is expected Wednesday with high
confidence as a shortwave ridge builds in the wake of the departing
system. Uncertainty in the temperature forecast increases Thursday
and beyond, as much hinges on the potential redevelopment of
northwest flow and arrival of a cooler airmass from the northeast.
Considerable ensemble spread exists Thursday and beyond, which does
not lend itself to a high level of predictability. In addition, the
potential remains for the arrival of a southern stream shortwave
late in the week, but the development of northwest flow would be
more likely to shunt this system to the south of the region, much
like Tuesday`s system. Likewise, uncertainty also remains high
regarding this late week system.



.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 448 PM CST Sun Feb 28 2021

The cold front, precipitation and associated MVFR cigs have pushed
southeast of all terminals, so attention now turns to a narrow
band of MVFR clouds pushing into northern MO. These clouds are
also moving to the southeast, and most guidance does show at least
a brief period of MVFR cigs as these clouds move through UIN and
the STL metro terminals. However, these clouds have been thinning
out over the last few hours and I`m not confident on whether they
will hold together long enough to make it to those terminals. UIN
will be the most likely to see MVFR cigs so I`ve kept them in
that TAF. Enough of the guidance still has MVFR cigs reaching the
St. Louis area as well, so I`ve kept them going in that TAF as
well, though for a shorter period than in previous forecasts.

After those clouds move through the area or scatter out, VFR
conditions will prevail.





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