Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201806

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
106 PM CDT Tue Mar 20 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 326 AM CDT Tue Mar 20 2018

The synoptic pattern early this morning features a trough and
associated PV anomaly sliding through portions of the central
CONUS. This trough will deepen across the east central CONUS
through the period, leaving the region with a cool northwesterly
flow regime through the period.

Showers persist early this morning mainly across areas
along/south of I-70. The last few radar scans depict some of this
shower coverage may be expanding a bit, especially across
portions of southwestern MO. This is likely due to enhanced large
scale ascent ahead of an approaching secondary PV anomaly dipping
through the northwest flow aloft. Therefore, think we will see
showers expand in coverage once again this morning over mainly
southeast MO and southwest IL, before all precipitation tapers off
across the area around 18Z or so. Can`t completely rule out a
couple of snowflakes mixing in this morning in the heavier shower
activity, but think the low-level thermal profile will continue to
support mostly rain with this activity.

Further north, an intrusion of drier air in the low-levels which
has caused some dissipation of the low-level cloudiness. However,
guidance suggests that RH in the mid and upper levels will remain
high today, thus expect mostly cloudy skies to persist across much
of the area. Given the expected cloudiness and the brisk
northerly winds, have lowered temps a bit from the previous
forecast which puts them in the low to mid 40s.

By tonight, the clouds should slowly taper off from west to east. It
will be a cool night as northerly winds continue to advect in a
chilly airmass, with lows expected in the upper 20s/low 30s.


.LONG TERM...  (Wednesday through Monday)
Issued at 326 AM CDT Tue Mar 20 2018

A weak PV anomaly is forecast to move across the area on Wed, but
only increased cloudiness and perhaps a few sprinkles are expected
at this time. Upper ridging across the western CONUS will
initially amplify as it shifts eastward before it is affected by a
series of strong disturbances upstream.

Although model disagreement remains high and temporal continuity
remains low for Wed through Mon, there are some general
similarities including a lead shortwave or separate PV anomaly
which reaches the central CONUS late this week and early this
weekend, the anomaly`s resulting surface reflection and frontal
boundaries, and a large and complex upper low over the PacNW.
Precipitation chances on Thu/Fri are based on H85 moisture
convergence along an NW-SE baroclinic zone, but the models have
not been very consistent with the placement of frontal boundaries
with this system.

Temperatures will gradually warm this week, and some areas in
central and south-central MO might even reach the 70s on
Saturday. Because of the upper ridge in place when the initial
disturbance arrives, models show that the resulting low pressure
system initially takes a typical track to the northeast before
transitioning to more of a clipper-like track to the southeast on
the downstream side of the ridge. This would keep warm
temperatures in place over most of the area this weekend while the
baroclinic zone pivots in place as the surface low rides along
it, but as mentioned above, the models have not been consistent
with the placement of frontal boundaries, and there is
considerable room for changes in this forecast over the next few

Similar to 24hrs ago, the ridge still looks too strong and the
upper PV anomaly still looks too weak/disorganized to support the
breakdown of the upper ridge. There has also been poor
run-to-run model continuity with both surface and upper air
features. These factors produce low confidence in the timing details
of the forecast for the upcoming weekend.



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1241 PM CDT Tue Mar 20 2018

VFR flight conditions are expected to prevail across most of the
area through Wednesday morning. MVFR ceilings will hang over parts
of the eastern Ozarks and southwest Illinois for a couple of more
hours this afternoon, but expect it should be VFR down there by
20-21Z. Another upper level disturbance will move across Missouri
tonight which may bring some light showers and possibly some MVFR
ceilings to parts of central Missouri, but current thinking is
that the lower ceilings will stay a bit further west near Sedaila
or Whiteman AFB.


VFR flight conditions are expected to prevail at Lambert through
Wednesday afternoon. Another disturbance will move through
Missouri tonight, but current thinking is that any lowered
ceilings/precip from this system will stay well west of the





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