Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191817

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
117 PM CDT Sat Aug 19 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 331 AM CDT Sat Aug 19 2017

Earlier convection which affected portions of south-central Missouri
last night has weakened and dissipated very early this morning.
Upstream, a shortwave trough was nearing the Mississippi River with
an associated diffuse frontal boundary across the northern CWA.
Scattered showers and thunderstorms across portions of central and
southeastern Iowa are expected to rapidly weaken before
dawn with only some remnant mid/high level cloudiness reaching the
bi-state area.

Aforementioned shortwave trough will rapidly push southeastward away
from the CWA this morning with rising heights and subsidence in its
wake. Decreasing cloudiness and near normal conditions for the
middle of August are expected across the area today and tonight.
Look for high temperatures near 90 degrees this afternoon, with lows
tonight in the mid 60s to low 70s.


.LONG TERM...  (Sunday through Friday)
Issued at 331 AM CDT Sat Aug 19 2017

(Sunday - Sunday Night)

The main band of westerlies will be located across the northern tier
of the CONUS at the beginning of the period, with an elongated upper
ridge in place from the southwest Atlantic across the lower MS
Valley and into the southern Plains. Within the southern portion of
the westerlies will be a favored region for weak migratory short
waves and the models have one such feature moving from eastern
NE/northeast KS into the western Great Lakes region on Sunday. Model
continuity and agreement with this feature and any associated QPF is
less than desired. Yesterday it looked like the best potential for
any showers and thunderstorms would be further west across parts of
western IA and western MO in the vicinity of an ill-defined lifting
boundary and where deeper moisture and better instability are
forecast. I still like that area better and the forecast will remain
dry on Sunday, however ultimately some POPS may need to be
introduced into parts of northeast and central MO for Sunday
afternoon. Increasing south-southwest low level flow in the wake of
the retreating anticyclone will result in good WAA and increasing
lower trop temps, and support a seasonably hot day with highs in the

The lack of model agreement and run to run model continuity continues
on Sunday night and into Monday, and this leads to lessing confidence
in forecast details. The NAM and GFS are particularily bad with
run-to-run continuity, with the ECMWF showing the greatest consistency.
In light of these trends, the ECMWF has been given higher weighting
with the forecast. The primary zone of showers and thunderstorms
still looks to located north of the area from the central Plains
into the upper MS Valley in the vicinity of an east-west
baroclinic zone, the short wave track, and terminus of a well
defined and veering southwesterly LLJ. There is now less support
for a late night/early Monday morning north-south zone of low
level WAA generated showers and thunderstorms extending into
central MO. Only the GFS shows much of anything in the LSX CWA and
its confined to northeast MO and west central IL through 12Z

(Monday - ECLIPSE DAY)

The models continue to show the upper ridge building/strengthening
westward on Monday resulting in a bit more west and southwesterly
component to the mid-upper level flow. This flow regime continues to
make clouds a concern, both from anything well to the west in the
eastern Plains/western MO or perhaps in central MO. South-southwest
low level flow is more pronounced than Sunday resulting in deeper
moisture return and potential for greater diurnal cu. As far as
precipitation potential, I think the main threat in the morning will
be in the northern CWA in the region of strongest low level WAA
accounting for both development and anything meandering southeast
from IA. I still like the northern CWA in the afternoon, however
there are also some indications that spotty diurnal activity may
impact parts of central and southeast MO. This would probably be in
the mid-late afternoon in anticipation of the brief early afternoon
temp cool down accompanying the eclipse, followed by a quick temp
recovery and highs in the low-mid 90s. As mentioned in the previous
discussion, the thought at this time is the best eclipse viewing
still appears to be in southern IL which should be farthest away
from any clouds and storms.

(Monday Night - Friday)

The latest model runs continue to support an increasing threat of
showers and thunderstorms across northern MO and west central IL
late on Monday night along and ahead of the advancing cold front.
This threat will continue to expand with numerous showers and
thunderstorms accompanying the southward moving cold front Tuesday
and Tuesday night.

An impressive upper trof will evolve across eastern NOAM mid-late
week as part of large scale amplification. This will result in
northwest flow aloft and maintain a large high pressure system as
the controlling weather feature and pleasant below average temps and
humidity levels Wednesday into Saturday.



.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1249 PM CDT Sat Aug 19 2017

Expect VFR flight conditions to prevail across the area through
18Z Sunday. An isolated shower or thunderstorm is
possible...primarily over the eastern Ozarks. Localized river fog
is also possible late tonight.


VFR flight conditions and light winds will prevail at Lambert
through Sunday. Some isolated showers or storms are possible well
south of the terminal this afternoon...and there is a possibility
that storms will develop on Sunday afternoon northwest of the
terminal...but current indications are that the storms will not
directly impact Lambert through 00Z Monday.





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