Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161136
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
536 AM CST Mon Jan 16 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 315 AM CST Mon Jan 16 2017

The synoptic pattern early this morning features a fairly potent
PV anomaly embedded within a longwave trough across the Southern
Plains, which will eject into the Midwest through the period.
This will be the main focus of the period as a couple rounds of
showers and even a few thunderstorms will accompany this system
as it passes through today into tonight.

Latest radar imagery depicts a broad shield of precipitation
stretching from western Arkansas northwestward into southwest MO
and into eastern Kansas. This activity was located just to the
north of a surface warm front within the warm conveyor
belt/isentropic ascent region of the approaching mid-latitude
cyclone. This activity will all lift northward today as the
upper-level PV anomaly approaches.

Before more widespread precip arrives, we will have to deal with
dense fog on the cool side of the surface warm front. Given most
locations in the CWA reporting visibilities in the 1/2SM to 1/4SM
range, decided to expand the Dense Fog Advisory to cover all LSX
counties. 16Z expiration time still looks good, but may be able to
trim a few southern/western counties sooner than that, or may need
to extend some northern counties longer. Will make adjustments as
needed through the morning hours.

Focused warm conveyor belt of this cyclone associated with a low-
level jet of 40-50 knots will move into central MO by mid-morning,
and spread across the rest of the region through the early afternoon
hours.  In its wake, a very brief lull is expected in the action as
the region gets into a small dry slot, but this lull will only be a
couple hours at any given point, making it hard to get into the
grids. Nonetheless, additional showers and a few thunderstorms are
expected to develop in the wake of the warm-air advection band by
Monday afternoon into early Monday evening due to the combination of
the approaching negatively-tiled trough and surface cold front. It
is with this activity that thunderstorms will be the most likely,
with many hi-res guidance members suggesting around 300-600 J/kg
MUCAPE. It must be noted that this instability will be rooted
above a fairly stable boundary layer, thus think any severe threat
today is rather low despite the strong deep-layer shear.

For tonight, the cold front will pass through the region by early
Tuesday morning, bringing an end to the precipitation. Daytime
highs may actually be reached just ahead of the front this evening
(near 60 in southeast MO/southern IL and low 40s over northeast
MO), before temperatures fall through the 40s and upper 30s by
Tuesday morning.

KD

.LONG TERM...  (Tuesday through Sunday)
Issued at 315 AM CST Mon Jan 16 2017

By 12z Tuesday morning, the aforementioned potent PV anomaly will
have moved into NE IL and NW IN. However, three other disturbances
will remain across or near the region: a compact shortwave diving
southeastward from the US/Canadian border towards MN/IA, a weaker
shortwave or shear zone over OK/KS, and a nearly cutoff low over
northwestern Mexico. It`s unclear whether the weaker shortwave
will remain a distinct feature or whether it will merge with the
northern shortwave, but the best lift will be downstream of the
LSX CWA unless the track of the northern shortwave begins to trend
westward. At best, these two disturbances may bring increased
cloudiness to the area on Tuesday night and Wednesday.

Precipitation chances look minimal until Thursday/Thursday night
when low over northwestern Mexico will have lifted northeastward
into the plains, causing increased synoptic scale lift
downstream. Meanwhile, a much stronger disturbance is forecast to
reach the CA coast by 12z Thu followed by another shortwave which
reaches the CA coast by 00z Sat. Both of these features might
bring increased precipitation chances to the region towards the
end of the week and over the weekend.

Slightly cooler high temperatures are expected on Tuesday in the
wake of Monday night`s cold front, but it will still be 5-10
degrees above average for this time of year. The warming trend
continues through the end of the week; highs from Wednesday
through Saturday are forecast to be 10-20 degrees above average.
Despite these unseasonably warm temperatures, no record highs
appear to be in jeopardy attm. Record highs at STL, COU, and UIN
for Jan 18th-21st range from the mid-60s to mid-70s, but the
current forecast highs are in the upper 40s to lower 60s. There
is more variability in the record warm lows during that time
period, and it`s possible that one of the cooler record warm lows
at COU or UIN could be broken.

Kanofsky

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 536 AM CST Mon Jan 16 2017

High impact forecast early this morning as dense fog and low cigs
will plague sites through the day. All sites are currently
experiencing LIFR cigs, vsbys or both this morning. These
conditions will persist through the next few hours, although some
improvements are possible from time to time as light showers move
through the region. A warm front will approach and attempt to
lift north through the region this afternoon, which could bring a
slight improvement to IFR late this afternoon mainly at COU and
the St. Louis metro sites. A cold front will approach late this
afternoon into tonight, bringing a slight chance for thunderstorms
along with a continued chance of showers. Given expected low
coverage of storms, will continue to leave out mention for now.
Easterly surface wind will veer around to a southeast direction
through the day and strengthen as the surface ridge centered over
the Ohio Valley region shifts eastward.

SPECIFICS FOR KSTL:
LIFR conditions will persist for several hours this morning into
this afternoon, both with vsbys from time to time and with cigs.
An improvement to IFR is possible late this afternoon as a warm
front lifts into the region. Showers will be around through much
of the day, with even a slight chance of a storm this afternoon
into this evening. Winds around 5-10 knots will veer from
easterly this morning, to south/southeast by this evening ahead of
a cold front which will push through tonight.

KD

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 10 AM CST this morning 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...Dense Fog Advisory until 10 AM CST this morning 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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