803
FXUS63 KLSX 130733
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
233 AM CDT Tue Aug 13 2013
.SHORT TERM: (Through Late This Afternoon)
Issued at 228 AM CDT Tue Aug 13 2013
Cold front exited far southern sections of the CWA just before
midnight, and during the predawn hours lower dewpoint air has been
advecting south into the area, albeit at a very slow pace. While
saturation of low levels from yesterday's rain has allowed some fog
to form, it appears that advection of the lower dewpoints into the
region has prevented the fog from becoming widespread, and reports
of dense fog from regional ASOS/AWOS sites has been very spotty and
short-lived. So, any fog that does remain early this morning should
burn off quickly.
Otherwise, today will begin a string of days where the weather over
the mid-Mississippi Valley will be under the influence of the
well-advertised Canadian high pressure system, with increasing N-NE
surface winds accelerating the advection of lower dewpoints into the
area during the day. Cloud cover should be a combination of
diurnally driven cu as well as some high clouds overspreading the
region from upstream disturbances over the plains, but think that
most areas will get at least some filtered sunshine.
Progged 850mb temps, as well as conditional climatology based off
the above cloud trends, suggests afternoon highs will range from the
upper 70s to lower 80s.
Truett
.LONG TERM: (Tonight through Sunday)
Issued at 228 AM CDT Tue Aug 13 2013
NW flow pattern continues aloft with a Canadian high pressure center
settling across the region at the surface, leading to unseasonably
cool conditions for the next few days. Daily max/min temperatures will
be around 8-12 degrees below average through the end of the week.
In addition to the cooler temperatures, dew points will also be
much lower than what is typically seen at this time of year. Since
river waters are still fairly warm, there may be some highly
localized river fog over the next few days during the early
morning hours if skies are clear and winds are light. A shortwave
moving down the east side of the persistent western CONUS ridge
should produce increased cloudiness on Fri/Sat however no
precipitation is expected at this time. Temperatures will
gradually warm due to air mass modification.
Kanofsky
&&
.AVIATION: (For the 06z TAFs through 06z Tuesday Night)
Issued at 1148 PM CDT Mon Aug 12 2013
Cold front south of taf sites, but still have some lingering cu
and mid clouds. Otherwise, main issue to deal with will be
possibility of fog through daybreak. Several locations have
already seen conditions bounce around from VFR to IFR, even LIFR.
With drier air slowly filtering in as ridge builds in, feel that
KUIN will just see some MVFR visibilities through 12z, so kept
tempo mention there. As for sites along I-70 corridor, closer to
frontal boundary, plenty of low level moisture remains as well as
light and variable winds. So could see visibilities drop to IFR at
times, especially at KCOU, KSUS and KCPS, so have tempo mention
there between 08z and 12z. Just kept MVFR mention at KSTL. As for
winds through the forecast period, light north winds to veer to
the northeast by mid morning on Tuesday, then east by Tuesday
evening.
Specifics for KSTL:
Cold front south of metro sites, but still have some lingering cu
and mid clouds. Otherwise, main issue to deal with will be
possibility of fog through daybreak. Several locations across the
region have already seen conditions bounce around from VFR to
IFR, even LIFR, back to VFR. So even though drier air to slowly
filter in, there is plenty of low level moisture as well as light
and variable winds over the metro area. So could see visibilities
drop to IFR at times, especially in river valleys at KSUS and
KCPS, so have tempo mention there between 08z and 12z. However,
just kept MVFR mention at Lambert. As for winds through the forecast
period, light north winds to veer to the northeast and pickup to
near 10 kts by 15z Tuesday, then east by 01z Wednesday.
Byrd
&&
.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&
$$
WFO LSX