Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 150417
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1117 PM CDT Mon Aug 14 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 323 PM CDT Mon Aug 14 2017

A warm front will continue to develop north of the Iowa and Missouri
border. Some scattered showers are possible overnight in north
central Missouri as a couple of mid level impulses ride along the
boundary. A cold front will approach from the west on Tuesday.
Scattered showers are possible tomorrow afternoon in central and
north central Missouri. Temperatures will be at or just above
seasonal values through Tuesday afternoon with light southwest
winds.


Kelly

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 323 PM CDT Mon Aug 14 2017

(Tuesday Night - Thursday)

A warm front is expected to slowly move northward late Tuesday night
and early Wednesday, providing a possible focusing mechanism for the
development of showers and thunderstorms. These storms are expected
to be aided by low-level moisture advection as well as forcing for
ascent aloft as a subtle midlevel shortwave moves northeastward
across the area. Still some uncertainty with how widespread
convection will (or won`t) be late Tuesday night into Wednesday
morning due to model differences in strength of low-level jet and
strength/track of aforementioned shortwave trough. There may be a
lull in convective activity (albeit possibly brief) between late
Wednesday morning and early/mid Wednesday afternoon. This lull in
activity should occur while the CWA is within the warm sector after
the passage of the warm frontal boundary.

Attention beginning late Wednesday afternoon through Thursday will
be on an approaching cold front from the northwest. This front is
still expected to bring the best chance of showers and thunderstorms
to much of the area. Best guess on frontal timing still appears to
be overnight Wednesday night into late Thursday morning, which
agrees with morning NAM/CMC/ECMWF models. GFS is about 3-6 hours
slower with the actual cold front, but does appear to focus
convection along a prefrontal trough, which appears to become the
effective front by Thursday morning anyways.

More typical summer-like weather will continue Tuesday night through
Wednesday ahead of the cold front. Dewpoints are expected to be in
the mid 70s Wednesday and Wednesday night, so it certainly will feel
very humid. Expect low temperatures Tuesday night and Wednesday
night to be mainly in the low to mid 70s, which agrees well with 12Z
MOS numbers. Highs on Wednesday are forecast to be in the upper 80s
to near 90 degrees, though clouds/precipitation may hamper those
numbers down if the warm frontal passage is a bit slower than
currently forecast or if elevated convection late Tuesday night into
Wednesday morning helps to retard its poleward progression.


(Thursday Night - Sunday)

A cooler and drier airmass will filter into the bi-state region late
this week and last into this weekend. Look for near normal
temperatures for this period, along with mostly dry weather as well.
There are some low PoPs late Friday night and Saturday associated
with a northwest flow shortwave trough however. Some moderation in
temperatures is expected between Saturday and Sunday as mid/upper
level ridge amplifies across the south-central plains and 850-hPa
temperatures approach +20C.


(Next Monday)

As to be expected 7 days out, there is quite a bit of uncertainty
with forecast specifics for the total solar eclipse passing through
portions of the area early next Monday afternoon. One key factor
will be how quickly (and where exactly) the mid/upper level ridge
builds into the central U.S. There has been signs of a ridge-running
vorticity maxima crossing the mid-Mississippi Valley for a couple of
days now, which does show up again on this morning`s 1200 UTC runs
of the GFS/CMC. This type of solution would favor a chance of
showers and thunderstorms, along with more clouds. However, the
ECMWF is more aggressive with amplifying the upper-level ridge and
would yield a much better chance at a sunny or mostly sunny sky
(along with warmer high temperatures) for the eclipse. Given this
degree of uncertainty at 7 days out, stuck close to climatology for
cloud coverage and chances of thunderstorms, but did lean slightly
above normal for high temperatures.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1046 PM CDT Mon Aug 14 2017

Light surface winds late tonight as a weakening cold front sags
southward into portions of northern MO and west central IL late
tonight and Tuesday morning. Just patchy low level clouds around
3000-4000 feet late this evening along with some cirrus clouds. It
appears that fog along with patchy stratus clouds will develop
late tonight and early Tuesday morning due to the light wind and
high surface dew points. The lowest visibilities of less than 2
miles will likely occur at COU, SUS and CPS with patchy stratus
clouds below 1000 feet.  The fog will dissipate by late Tuesday
morning with the stratus clouds gradually rising in height and
becoming cumuliform in nature. The surface wind will remain weak
on Tuesday.

SPECIFICS FOR KSTL: Light surface winds late tonight as a weakening
cold front sags southward into portions of northern MO and west
central IL late tonight and Tuesday morning. Just patchy low level
clouds around 3000-4000 feet late this evening along with some
cirrus clouds. It appears that fog along with patchy stratus
clouds will develop late tonight and early Tuesday morning due to
the light wind and high surface dew points. The visibilities at
STL will likely drop down to around 2-3 miles with patchy stratus
clouds around 1000 to 2000 feet. The fog will dissipate by late
Tuesday morning with the stratus clouds gradually rising in height
and becoming cumuliform in nature. The surface wind will remain
weak on Tuesday.

GKS

&&

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

$$

WFO LSX



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