Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171755
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1155 AM CST Fri Nov 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 345 AM CST Fri Nov 17 2017

Areas of fog across parts of southeast MO and southwest IL will
dissipate later this morning as southeasterly surface winds
strengthen and temperatures rapidly warm.  Patchy light rain or
drizzle can be expected today, particularly across northeast and
central MO and west central IL as a weak warm front approaches with
rising surface dew points.  A strengthening southwesterly low level
jet will also bring increasing temperature and moisture advection to
our area.  Despite the cloud cover the highs today will be much
warmer than yesterday and as much as 5 to 10 degrees above normal. A
deepening upper level trough over the northern Plains tonight will
cause a surface low to deepen along a cold front which will drop
southeastward through northeast and central MO and west central IL
late tonight, and be located just northwest of STL around 12Z
Saturday.  Showers and a few thunderstorms can be expected tonight
along and just ahead of the cold front. Temperatures will be quite
mild tonight ahead of this front, but temperatures will drop quickly
towards morning across northeast and central MO and west central IL
as strong and gusty northwesterly winds bring colder air into this
region.

GKS

.LONG TERM...  (Saturday through Thursday)
Issued at 402 AM CST Fri Nov 17 2017

A strong upper level disturbance will move thru our region on
Saturday, also driving a strong surface cold front.  Precipitation
ahead of the front will be driven by WAA from a 50kt H850 jet and
will be convective in nature with support for thunder.  The surface
cold front should exit the far southern and eastern sections of the
CWA by late Saturday morning.  While certain parameters will be very
supportive of severe thunderstorms, such as high shear both at 0-6km
and 0-1km and 0-1km helicity, and rather high mid level (H700-500)
lapse rates of 8C/km, much of the convection that develops will be
elevated in nature, and should limit much of the potential threats
for tornadoes and damaging winds from what could be.  However,
examination of model soundings also shows an increasingly finer line
between elevated and convection becoming more surface based, even
during the early to mid morning hours for areas S and E of STL
metro.  This will be helped greatly by what should be a region of
60+ dewpoints ahead of the front.  In light of this tendency and
factoring in margins of error in the models, there should be a
severe threat for storms Saturday morning, mainly for areas S and E
of STL metro, with the SPC Marginal Risk possibly being upgraded
within the next 24 hours.

Temps ahead of the front will begin the day on Saturday quite mild,
with readings easily in the 60s.  There are some model disagreements
on precise timing and how cold the air behind the front will be,
with most models dropping temps into the upper 30s to mid 40s during
the afternoon , with the NAM the most aggressive into the mid 30s
for many areas.  There should also be precipitation behind the cold
front, generated by deep lift from a left exit region of an upper
jet to frontogenesis towards H850.  It will be in this area of more
stratiform pcpn Saturday afternoon stretching from near STL metro
northeastward into central IL that a brief appearance of a flake of
snow might be possible if the more aggressive NAM verifies, but
currently is more of an outlier in the spectrum of possible
solutions.

One final item:  behind the front, several models depict not just a
very tight pressure gradient, but also a rather strong jet at h925
of 45kts.  If this depiction is maintained on subsequent model runs,
should support sustained winds of 20-30mph and gusts to 45-50mph,
which would warrant a Wind Advisory for much of the area.

The rest of the forecast period will be dominated by persistent NW
flow aloft and a persistently dry column.  So despite at least a
couple of shortwave disturbances to pass thru on Monday and again
late Tuesday (accompanied by a surface cold front), they should pass
thru uneventfully dry.  After a cool day on Sunday in the wake of
the front, temps should rebound well on Monday to close to 60 thanks
to gusty southerly winds and sunshine.  The front on Tuesday should
result in another brief bout of below average temps on Wednesday,
but any temp variations the next several days will be brief and
should not be any extreme departures

TES

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1142 AM CST Fri Nov 17 2017

Rather impactful TAF period in store. MVFR cigs to prevail
through much of the upcoming TAF period, with IFR cigs likely
tonight into Saturday morning at UIN. A low-level jet of around
40-50 knots will continue to develop this afternoon into tonight
especially over the STL metro sites. However, think most sites
should remain mixed enough tonight (gusts around 20 knots) to
leave out any LLWS mention for now.

While a light shower/drizzle will be possible at all sites this
afternoon, think the bulk of any concentrated rainfall activity
will hold off until tonight mainly after 06Z. Then, showers and
even a few thunderstorms will be possible along a cold front as it
sweeps southeast through all terminals Saturday morning. Behind
the front, winds will turn to the WNW and become very gusty with
gusts of 30-35 knots possible. MVFR conditions will persist even
behind the front with light showers lagging the front and
persisting into the early afternoon hours.

SPECIFICS FOR KSTL:
Rather ipactful TAF period in store. MVFR cigs to prevail through
much of the upcoming TAF period. A low-level jet of around 40-50
knots will continue to develop this afternoon into tonight.
However, think STL should remain mixed enough tonight (gusts
around 20 knots) to leave out any LLWS mention for now.

While a light shower/drizzle will be possible this afternoon,
think the bulk of any concentrated rainfall activity will hold off
until tonight mainly after 11Z. Then, showers and even a few
thunderstorms will be possible along a cold front as it sweeps
southeast Saturday morning. Behind the front, winds will turn to
the WNW and become very gusty with gusts of 30-35 knots possible.
MVFR conditions will persist even behind the front with light
showers lagging the front and persisting into the early afternoon
hours.

KD

&&

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

$$

WFO LSX



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