Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161120

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
620 AM CDT Wed Aug 16 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 315 AM CDT Wed Aug 16 2017

The synoptic pattern early this morning features southwest flow
aloft over the Midwest in advance of a fairly strong PV anomaly
ejecting out of the Central Plains. This feature will swing
through the region tonight, bringing several chances of showers
and thunderstorms ahead of it today into tonight.

Have high confidence in the showers/storms today, but the
convective evolution remains a bit murky at this early hour.
Currently, a disorganized complex of thunderstorms over southern
MO continues to lift northeast. This activity is tied to a weak
vort max traversing northeast, along with moisture convergence at
925-850mb. This area of moisture convergence associated with the
approaching vort max is forecast to lift northeast later this
morning while maintaining intensity, thus expect showers and a few
thunderstorms to continue to develop and lift northeast along the
I-44 corridor.

Once this first system passes by, confidence in the forecast
begins to decrease. Typically, would expect a lull in the activity
given some weak subsidence in the wake of a passing wave.
However, it appears the low-level jet will continue to focus
moisture convergence into the area as yet another weak wave slides
into the area from the southwest. Forecast soundings suggest a
fairly uncapped atmosphere today given the expected high amounts
of low-level moisture (although weak mid-level lapse rates should
limit overall instability values), thus it is hard to rule out
convection really at any point given the forcing mechanisms
passing through. Given this, have broad-brushed 30-40 pops in for
much of the late morning/early afternoon hours.

It does appear the most likely time for a relative lull in the
convection will be from about 5-9PM, as shortwave ridging
overspreads the region ahead of the approaching trough and cold
front. Convection should blossom along this front (likely becoming
severe) over eastern KS and western MO late this afternoon into
this evening. By the time it approaches central and northeastern
MO, it is likely to be organized as a linear MCS. As it pushes
east, however, it will encounter a less favorable thermodynamic
environment. Therefore, expect a general weakening trend in the
line as it moves eastward toward the Mississippi River. But, given
the fairly strong dynamics (30-40 knots of deep-layer shear) for
mid-August, can`t rule out a few severe storms mainly in the
9-midnight timeframe over central and northeast MO. The primary
threat would be locally damaging wind gusts. However, given 925mb
winds of 30-35 knots aiding in low-level helicity values, cannot
completely rule out an isolated tornado mainly over northeast MO.
The main limiting factor will be the tendency for convection to
become elevated with eastward extent as it outruns the main
instability gradient and encounters nocturnal stability effects.


.LONG TERM...  (Thursday through Tuesday)
Issued at 315 AM CDT Wed Aug 16 2017

Precipitation associated with the aforementioned PV anomaly will
taper off on Thursday. After a brief lull on Thursday night and
Friday, another approaching PV anomaly will bring a chance of
SHRA/TSRA to the area on Friday night and Saturday. A ridge will
then begin building into the region on Saturday night behind the
departing upper disturbance. Dry weather is expected on Sunday.

Please see the discussion section below for Monday.

Precipitation chances will begin increasing again on Monday night
ahead of an approaching trough axis and surface frontal system.

Temperatures will be seasonably warm (mid/upper 80s to lower 90s)
through the extended.


(Monday August 21st)

To have high confidence in good viewing conditions for the total
solar eclipse, one would ideally want to see a decent cold front
sweep through the area a day or two prior to the eclipse (to dry
out the column) followed by a strong high pressure system overhead
on the day of the eclipse (the resulting large-scale subsident
motion would limit any residual cloud cover). The current model
forecasts for August 21st do not depict anything close to this
ideal scenario, but they do offer some nuggets of hope for people
hoping to watch the eclipse within the path of totality across
central and eastern MO into southern IL.

The primary concern for Monday is still mid/high clouds
associated with either a nearby frontal boundary and/or upstream
convection. As noted in a previous discussion, diurnal cumulus
clouds may dissipate during the partial phase of the eclipse due
to reduced diurnal heating within the moon`s shadow.

Aloft, the latest model runs are similar to 24hrs ago and depict
a broad, flat ridge across the central and eastern CONUS at 12z
Monday. Several weak H5 disturbances are also noted on the ECMWF,
GFS, and GEM. Although some of these features appear to be noise
and/or the result of convective contamination, the ECMWF remains
consistent in its depiction of a shortwave over MN, which is
farther north compared to 24hrs ago. Both the overall upper
pattern and the ECMWF`s northward trend with the weak shortwave
are positive signs for this area.

At the surface, there are significant differences compared to
24hrs ago and between models. These types of shifts are to be
expected since it is still a day-6 forecast. One item of concern
is that it appears a secondary warm front may be developing and
lifting northeastward between 12-18z, which is poor timing because
of the additional cloud cover. However, timing details with
frontal boundaries and the placement of surface features are
likely to change several times over the next few days. The GFS
and ECMWF condensation pressure deficit plots are also more
pessimistic compared to 24hrs ago.

Temperatures will be seasonably warm on Monday (low 90s), although
local temperatures may be reduced by as much as 20 degrees during
the brief period of totality. NWS forecast grids at hourly
resolution will not capture this full temperature drop.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 620 AM CDT Wed Aug 16 2017

Main TAF concerns will be on convective timing/coverage and MVFR
cigs. Showers with an occasional rumble of thunder will continue
to affect sites this morning, with the most concentrated area
likely to shift northeast from COU to UIN. Otherwise, generally
VFR cigs are likely to prevail this morning into the afternoon
hours, but a few patches of broken MVFR will be possible at times
mainly this morning (currently a patch over the St. Louis metro

A second round of showers and thunderstorms is expected early to
mid afternoon. Confidence in timing is low and amendments will
likely be needed once trends are better ascertained. Finally, a
third and more widespread round of showers and thunderstorms is
expected this evening into the early overnight hours. Have highest
confidence in this round affecting UIN/COU, with it likely to
weaken as it approaches the St. Louis metro sites. Any
thunderstorm today/tonight will be capable of locally gusty winds
and IFR vsbys.

A few showers will continue to plague STL this morning, along with
occasional MVFR cigs. A second round of showers/storms is expected
this afternoon, however confidence in timing/coverage is rather
low with this round. A third and final round is expected late this
evening into the early overnight hours as a cold front moves
through the region. Any thunderstorm will be capable of IFR vsbys
and locally gusty winds.





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