Area Forecast Discussion
Issued by NWS St. Louis, MO

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749
FXUS63 KLSX 172125
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
325 PM CST Fri Nov 17 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 208 PM CST Fri Nov 17 2017

An active short term period is in store as a fairly impressive
mid-latitude cyclone affects the region. Quasi-zonal flow is in
place now ahead of a strong PV anomaly approaching from the west.
This passing PV anomaly will help carve out a trough over the
Midwest by the end of the period, allowing for much cooler
temperatures to enter the region by Saturday afternoon.

The approaching PV anomaly has already caused a surface low to
deepen across portions of KS this afternoon. This low will
continue to slowly deepen tonight as it pushes northeast into
northeast MO by 12Z Saturday morning. Ahead of it, broad warm air
advection will continue tonight as an impressive low-level jet
develops over the area. Moisture convergence will focus mainly
across western IL and northeast MO this evening into the early
overnight hours, leading to the development of showers. As the
trough and surface low approach and forcing for ascent increases
along the synoptic cold front, expect showers and a few elevated
thunderstorms to break out initially just to the north of the
front early Saturday morning. However, with time surface-based
instability should increase to near 750-1000 J/kg with weakening
surface-based CIN. This will occur mainly after 11-12Z as the cold
front begins to surge to the southeast. While most CAMs aren`t
overly aggressive with the strength of a convective line before it
leaves the LSX CWA, given the impressive kinematics in place and
the presence of surface-based instability with little CIN, can`t
rule out a few severe thunderstorms mainly in the 12-17Z
timeframe. Main threat would be locally damaging wind gusts given
the low-level kinematic fields, but can`t totally rule out an
isolated tornado if instability and low-level lapse rates can
become greater than progged.

The cold front will sweep east of the area by late Saturday
morning, allowing for a rather chilly airmass to come crashing in.
In fact, after temperatures in the low/mid 60s ahead of the
front, they will crash into the upper 30s/low 40s by Saturday
afternoon. The NAM continues to be the most aggressive with this
cold air intrusion and actually would suggest a few snowflakes
possibly mixing in with the departing post-frontal deformation
band. However, it remains a cool outlier and think boundary layer
temperatures should remain just warm enough to keep much of the
precipitation liquid.

Last, but certainly not least, will be the wind gust potential
tomorrow behind the front. Guidance suggests 925mb winds around
40-45 knots tomorrow in the post-frontal airmass. Given the
strong cold air-advection and steepening low-level lapse rates,
think much of this wind will have the potential to mix down to the
surface. Therefore, gusts of 35 to 40 mph will be common behind
the front, with many areas approaching 45 mph. In coordination
with SGF/PAH/ILX, will go ahead and hoist a Wind Advisory for
tomorrow to cover the wind gusts to around 45 mph.

KD

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 208 PM CST Fri Nov 17 2017

Wind speeds will diminish on Sat night and the column will
quickly dry out after the aforementioned low pressure system has
moved through the region and a surface high pressure center has
settled across the southern plains. High temperatures on Sunday
will be much cooler within the post-frontal air mass, and most
locations will probably remain in the 40s. The surface high will
then shift eastward on Sunday night and allow southwesterly return
flow to bring a warmer air mass back into MO/IL for Mon/Tue.
Another cold front could bring a brief period of slightly cooler
weather back into the region during the middle of the week.

Although a couple of PV anomalies will move across the region
during the extended forecast period, the dry air mass will make it
difficult to generate precipitation. Frontogenetical circulations
and upper support behind a surface cold front almost bring the
column to saturation on Tue/Tue night, but there is considerable
model disagreement regarding the location and timing of the upper
PV anomaly. It is not yet clear whether the impact of this system
will be limited to an increase in mid/high clouds or whether
there could be some light precipitation.

Kanofsky

&&

.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 impactful 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...Wind Advisory from 6 AM to 6 PM CST Saturday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO-Washington MO.

IL...Wind Advisory from 6 AM to 6 PM CST Saturday for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-Washington
     IL.

&&

$$

WFO LSX



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