Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 300951
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
451 AM CDT TUE AUG 30 2016

.SHORT TERM... (Through Wednesday Night)
Issued at 251 AM CDT Tue Aug 30 2016

A pattn change will begin to unfold today. 06z satellite imagery
depicted a vort max moving into northwestern Ontario, and this
feature was well-handled by most models. As this feature
continues sliding southeastward, its upper trough axis will merge
with a separate disturbance noted over CO/NEB early this morning,
and this upper support eventually drives a cold front through the
LSX CWA.

The sfc front will arrive in two stages: first the wind shift
line today/tonight, then the moisture gradient tomorrow/tomorrow
night. Both stages will be accompanied by increased coverage of
SHRA/TSRA, partly due to sfc convergence and partly due to
increased upper support. After the moisture gradient has moved
through the area, a cooler and drier air mass will then settle
across the region for the end of the week.

Convergence along the wind shift line was partially responsible
for the clusters of SHRA/TSRA which developed early this morning
from KS through MO into IA. Scattered SHRA/TSRA will accompany the
wind shift line as it continues sagging southward today/tonight.
As mentioned above, an elongated PV anomaly was noted across
CO/NEB, and a slight eastward trend to this feature means that
increased upper support will be available today over part of the
CWA (mainly northern MO) compared to previous days. PoPs have been
adjusted to reflect higher chances farther north.

The moisture gradient is fcst to approach the MO/IA border by 12z
Wed. Although limited upper support combined with daytime heating
ahead of the bdry may initially slow its southward progress, the
arrival of the aforementioned upper trough after 18z will finally
push the bdry southward through the CWA. The combination of a
moist air mass, a sfc bdry to focus the convection, and upper
support should produce at least scattered SHRA/TSRA near and ahead
of the bdry. Locally heavy rainfall will be possible given that PW
values are forecast to be near 2", which is roughly +2 SD for
this time of year.

Kanofsky

.LONG TERM... (Thursday through Monday)
Issued at 251 AM CDT Tue Aug 30 2016

Cooler and drier conditions will settle across the region for
Thu/Fri. Friday in particular looks spectacular attm. Thursday
will be the first full day for the CWA within the post-frontal air
mass, and a sfc high pressure center building into the Great Lakes
will ensure dry easterly/northeasterly flow continues for Fri. The
drier air mass and limited cloud cover will promote good
radiational cooling conditions for both Thu and Fri nights, and
lows in the 50s to 60s still look reasonable. The surface high
begins moving off to the east over the weekend, leading to a
slight warm-up for Saturday and then a more noticeable warmup for
Sunday once return flow begins in earnest. Precip chances also
begin to increase late this weekend when the upper pattn shifts to
southwest flow aloft.

Kanofsky

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1100 PM CDT Mon Aug 29 2016

Surface ridge extended from MI southwest into northern MO late
this evening. Mainly just some high level cloudiness overnight,
although could not rule out some convection in the UIN area late
tonight. With a light surface wind along with high surface dew
points there may be some fog late tonight/early Tuesday morning,
especially in SUS and CPS. Scattered diurnal cumulus clouds are
expected again for late Tuesday morning and afternoon. The best
chance for diurnal convection will be at UIN and COU Tuesday
afternoon as a cold front approaches from the northern Plains.
Surface winds will be weak and variable on Tuesday.

SPECIFICS FOR KSTL: Surface ridge extended from MI southwest into
northern MO late this evening. Mainly just some high level
cloudiness overnight. With a light surface wind along with high
surface dew points there may be some fog late tonight/early
Tuesday morning. Scattered diurnal cumulus clouds are expected
again for late Tuesday morning and afternoon. There may be isolated
to scattered convection Tuesday afternoon and evening, although
the better coverage will likely be northwest of STL as a cold
front approaches from the northern Plains. Surface winds will be
weak and variable on Tuesday.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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