Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 151111

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
611 AM CDT Tue Aug 15 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 346 AM CDT Tue Aug 15 2017

The synoptic pattern early this morning features a ridge across
portions of the north-central CONUS, flanked by two troughs over
the northwest CONUS and Great Lakes region. This ridge will
amplify and shift east through the period, turning the upper-
level flow over the Midwest to more southwesterly today and
especially into tonight.

First concern of the period will be fog this morning. Latest
GOES16 imagery depicts some areas of fog, especially across
portions of southeast MO. However, some stratus has continued to
develop from time to time, which has helped keep some
visibilities up. Further north along the I-70 corridor, it
appears much of the fog is confined to the typical river valleys
and low- lying areas. Given these factors, will hold off on any
Dense Fog Advisory for now, but will continue to monitor trends
over the next few hours.

Any fog should quickly mix out this morning with skies becoming
partly cloudy by late this morning into early this afternoon.
Currently, a weak cold front/stationary front is located over
portions far southeast IA/northern MO. This front should move
little today, perhaps even lifting northward by this afternoon as
an area of low pressure deepens to the west. Will leave a brief
window of slight chance pops along this front this afternoon, but
think if anything develops it may remain just north of the area.
Further south, an area of convection currently over northern AR
may make a run at southeast MO this afternoon. This area of
showers/storms is likely to remain mostly south of the region,
but can`t rule out an isolated shower/storm getting into
southeast MO this afternoon. All in all, most of the region stays
dry today with temperatures climbing into the upper 80s and low
90s, with heat index values in the low to mid 90s.

By tonight, the upper-level flow will back to southwesterly. A PV
anomaly will eject into the Central Plains, which will help
induce a low-level jet response across portions of the Central
Plains. While much of the moisture convergence associated with
this feature should remain off to the west/southwest of the area
through 06Z, guidance suggests as the low-level jet veers and a
surface warm front lifts northeast into the area, scattered
showers/storm development will be possible. The hi-res guidance
differs with coverage/timing of the convection tonight, so some
adjustments to pops will likely be needed as trends become more
clear today.


.LONG TERM... (Wednesday through Monday)
Issued at 346 AM CDT Tue Aug 15 2017

Warm and humid conditions are expected on Wed and Thu ahead of an
approaching frontal system. There is a chance of widespread
SHRA/TSRA, especially during the frontal passage on Wed night and
Thu morning. A few strong storms are possible across the
northwestern half of the CWA given the expected 0-6 km shear
values of 20-25 kts.

The next opportunity for widespread precipitation will be on Fri-
Sat when a low pressure system moves southeastward from the
northern plains into the Ohio Valley.

A broad upper ridge then builds into the region for Sun-Mon. The
ridge is not particularly amplified and there are several
indications of weak disturbances moving through a nearly quasi-
zonal flow pattern across the north central the CONUS. One or more
of those features could bring a chance of SHRA/TSRA to the area on



Please consider the following discussion to have the same caveats
which would normally accompany any discussion of potentially
significant weather events at the extreme edge of the long-term
forecast period. This forecast, like most other 7-day forecasts,
may change several times over the next few days as the state of
the atmosphere changes and as the models continually incorporate
new observations.

The primary forecast challenge for Monday is determining whether
clouds and/or precipitation over MO/IL might obscure ground-based
observers` view of the total solar eclipse. One factor supporting
good viewing conditions is that the 00z runs of the ECMWF, GFS,
and GEM all depict a broad upper ridge in place across the area.
Subsident motion beneath an upper ridge would be favorable for
mostly clear skies. However, the ridge is not particularly
amplified, which does not lead to high confidence.

The GFS and ECMWF also show a weak disturbance over IA/MN, and
it`s not clear whether this feature is a shortwave or whether it
is convective contamination in the numerical models. With a trough
axis extending from Canada into the the northern plains on
Monday, there is a concern that cloud cover could be advected
downstream over our area.

The 18z forecasts of condensation pressure deficits at multiple
levels are around 80-200 mb, which is on the higher (drier) side
of the middle ranges. I`d prefer to see consistently higher values
(200+) over a more widespread area to have greater confidence in
mostly clear conditions. The good news is that these values do not
indicate widespread multilayered cloud cover, but rather suggest
that some scattered mid or high clouds could stream in from the
west. This idea is consistent with model depictions of a
developing surface warm front oriented approximately NNW-SSE along
the KS/MO border on Monday morning.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 604 AM CDT Tue Aug 15 2017

Main TAF impact will be fog this morning. Worst conditions
expected at COU/SUS/CPS, where LIFR vsbys will be possible over
the next couple of hours before fog quickly mixes out. Conditions
should then return to VFR late this morning into this afternoon,
with some scattered diurnal cu along with some passing mid/high
clouds. Clouds should thicken tonight as a warm front moves
northward into the area. This front may bring some scattered
showers/storms, but most of the activity looks to be south/west of
the terminals through 12Z, thus will not include any mention at
this time.

Could see a brief period of MVFR vsbys in BR still this morning,
but generally VFR conditions are expected through the period.
Scattered diurnal cu are expected this afternoon, but VFR clouds
will thicken tonight as a warm front approaches the terminal.
Can`t rule out a few showers/storms towards 12Z, but will leave
out for now due to uncertainties in coverage.



MO...Dense Fog Advisory until 9 AM CDT this morning for Callaway MO-
     Cole MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-
     Jefferson MO-Madison MO-Montgomery MO-Osage MO-Reynolds MO-
     Saint Francois MO-Sainte Genevieve MO-Warren MO-Washington



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