Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 142348
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
648 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 00z TAFs through 00z Tuesday Evening)
Issued at 531 PM CDT Mon Aug 14 2017

The surface winds will become light this evening as a weakening
cold front sags southward into northern MO and west central IL
late tonight and Tuesday morning. The low level cloudiness around
2500-3500 feet in height will continue to break up and shift east
of the taf sites this evening. The models were hinting at the
development of at least patchy fog and stratus clouds late tonight
and early Tuesday morning. Will include at least scattered low
level clouds along with visibilities down to around 3SM BR for
late tonight and early Tuesday morning at the taf sites. 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: The surface winds will become light this evening
as a weakening cold front sags southward into northern MO and
west central IL late tonight and Tuesday morning. The low level
cloudiness around 2500-3500 feet in height will continue to break
up and shift east of the taf sites this evening. The models were
hinting at the development of at least patchy fog and stratus
clouds late tonight and early Tuesday morning. Will include at
least scattered low level clouds along with visibilities down to
around 5SM BR for late tonight and early Tuesday morning at STL.
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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