Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 190420
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1120 PM CDT THU AUG 18 2016

.UPDATE...
Issued at 925 PM CDT Thu Aug 18 2016

Made a few changes this evening to account for the latest guidance
and observational trends. Continue to watch a complex of
thunderstorms to the northeast over east-central IL. These have
really weakened over the past hour with warming cloud tops and
lowering lightning counts, thus think they will struggle to get
into our region as they continue to dive SSW along the MLCAPE
gradient.

Another concern tonight will be the potential for fog development
once again over west-central and south-central IL. Passing high
clouds and the potential for the outflow from the storms to the
northeast getting into this region and keeping low-levels mixed
are the two limiting factors to fog at this time. However, would
not be surprised to see some locally dense fog develop toward
sunrise once again so have included patchy dense fog in the grids.

Finally, have slowed the arrival of pops from the south tonight. A
weak PV anomaly will push through northern MO early Friday
morning. In response, a weak low-level jet and associated
isentropic ascent will develop and lead to a few showers/isolated
storms across portions of AR and far southern MO early Friday
morning. Latest CAMs have slowed the arrival of this precip which
makes sense given observational trends, thus have slowed down the
introduction of pops early Friday morning in this area.

KD

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 341 PM CDT Thu Aug 18 2016

Isolated to scattered showers and thunderstorms are possible
across the southern CWA overnight due to increasing moisture and
ascent, but widespread coverage is not anticipated attm.

Kanofsky

.LONG TERM...  (Friday through Next Thursday)
Issued at 341 PM CDT Thu Aug 18 2016

There are three primary forecast concerns over the next 7 days:
thunderstorm chances for Friday through Saturday night, cool
overnight lows on late Sunday/early Monday, and thunderstorm
chances for the middle part of next week.

A strong cold front will approach the region tomorrow and
tomorrow night before moving through the CWA on Sat/Sat night.
Large-scale lift ahead of an upper vort max will support
widespread coverage of showers and thunderstorms, especially on
Friday night and Saturday. Models depict 20-30 kts of 0-6km shear
across parts of southeastern MO and southwestern IL on Sat
aftn/eve, which may be sufficient for a few strong to severe
thunderstorms depending on instability.

A surface high builds in behind the front and ensures quiet wx
conditions for Sun/Mon. Highs on Sunday may struggle to reach 80
degrees F within the cooler post-frontal air mass. The current
forecast highs for Sunday (upper 70s) are 5-10 degrees below
average. The center of the surface high should be located nearly
overhead by 12z Mon, and the combination of nearly clear skies and
light winds should produce favorable radiational cooling
conditions on Sunday night/early Monday morning. The combination
of good radiational cooling conditions along with a cooler
post-frontal air mass should bring overnight lows down into the
mid to upper 50s across most of the region, which is roughly 5-10
degrees below average.

The surface high moves slowly eastward early next week and
southerly return flow commences over the plains, sharpening a N-S
boundary which eventually lifts northeastward as a warm front on
Wed. Models also depict a stalled cold front or quasistationary
front somewhere over the region during the middle and late part of
next week. A few weak shortwaves will move across the area during
the middle and late part of next week and may interact with the
sfc boundaries to produce periods of showers and thunderstorms.

Kanofsky

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1050 PM CDT Thu Aug 18 2016

Mainly just some high level cloudiness for the St Louis metro
area late tonight as persistent high level clouds continue to
stream northeastward through southeast MO and southwest IL. Light
south surface winds will continue late tonight. There may be some
fog and patchy stratus clouds late tonight/early Friday morning,
particularly at CPS and SUS, although the MOS guidance appears a
little overdone with its forecast of stratus and fog. Could not
rule out some stratus clouds advecting northward from southern MO
and getting into COU and the St Louis metro area Friday morning.
Scattered diurnal cumulus clouds is expected again late Friday
morning and afternoon. There may also be isolated showers/storms
Friday afternoon and evening, but for now may keep the tafs dry.
Surface winds will be southerly on Friday with the surface ridge
southeast of the region and a cold front approaching from the
northern Plains.

SPECIFICS FOR KSTL: Mainly just some high level cloudiness late
tonight as persistent high level clouds continue to stream
northeastward through southeast MO and southwest IL. Light south
surface winds will continue late tonight. There may be some fog
and patchy stratus clouds late tonight/early Friday morning,
although the MOS guidance appears a little overdone with its
forecast of stratus and fog. Could not rule out some stratus
clouds advecting northward from southern MO and getting into STL
Friday morning. Scattered diurnal cumulus clouds is expected
again late Friday morning and afternoon. There may also be
isolated showers/storms Friday afternoon night, but for now may
keep the STL taf dry. Surface winds will be southerly on Friday
with the surface ridge southeast of the region and a cold front
approaching from the northern Plains.

GKS

&&

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

$$

WFO LSX



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