Area Forecast Discussion
Issued by NWS St. Louis, MO

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838
FXUS63 KLSX 200424
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1024 PM CST Sun Feb 19 2017

.UPDATE...
Issued at 1016 PM CST Sun Feb 19 2017

Fog has begun to develop already across parts of southwest IL
this evening and should continue to expand overnight, becoming
dense across parts of southeast MO and southwest IL late tonight.
Issued a dense fog advisory for late tonight and early Monday
morning across parts of southeast MO and southwest IL where
visibilities will likely drop to one quarter of a mile or less.
Also looking at the latest model guidance it now appears that
any showers will not make it into parts of northeast and central
MO until after 12Z Monday and probably not until after 15Z, so
delayed the pops a little early Monday morning.

GKS

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 329 PM CST Sun Feb 19 2017

Upper level ridge to continue sliding off to the east tonight as
next system approaches region. So will see increasing clouds ahead
of it. This along with southerly winds will keep temps from dropping
too much tonight, so expect mild temps with lows in the low to mid
50s. Otherwise, main issue tonight is possible redevelopment of
patchy fog/stratus, especially along and east of Mississippi River.
Because stratus lingered a bit longer in this area today, it may not
have mixed out enough to dampen fog chances tonight.

As for precipitation chances, models continue to keep both northern
and southern systems phased together with enough low level moisture
and instability tomorrow with best chances of showers and a few
storms over central/northeast MO and west central IL by Monday
afternoon. Highs will depend on cloud cover, fog and precipitation.
For now went close to guidance, in the upper 60s to mid 70s.

Byrd

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 329 PM CST Sun Feb 19 2017

An elongated upper trof will stretch from the Upper MS Valley into
the southern Plains early Monday evening. Weak large scale ascent
with the upper system along with low level forcing and moisture
transport via a southwesterly LLJ are expected to be supporting a
wide broken band of showers and some embedded thunderstorms centered
thru mid MO. This band of showers and few thunderstorms will spread
east on Monday night ahead of the advancing cold front and in
association with migration of the upper trof and shifting and
veering of the LLJ. The band will become more NE-SW with time as
the advancing trof becomes more positively-tilted, owing to
faster flow aloft across the northern tier of the CONUS. Some
residual showers may be onoging on Tuesday morning from southeast
MO into southwest IL, ahead of the cold front which at 12Z is
expected to be located near St. Louis. The upper trof will then
continue to shear quickly eastward on Tuesday with the
accompanying cold front moving to the east out of the CWA. The
front itself will gradually lose identity with time as the air
mass quickly modifies in its wake. Tuesday promises to be another
unseasonably warm day, but just how warm it gets will be dependent
on clearing. The most questionable area is southwest IL where the
models show abundant low level RH/clouds persisting into the early
afternoon. Further west, some locations from central MO into west
central IL will flirt with record high temperatures.

The flow aloft will veer to more west-southwesterly Tuesday night
into Wednesday associated with a digging upstream upper trof, and
this will be accompanied by the return of a pronounced low level WAA
regime. Present indications are that Wednesday will again be an
unseasonably warm day with more record highs possible. The only fly
in the ointment is that a good deal of high clouds will return with
the veering flow aloft.

The weather pattern gets interesting during the later part of the
week. First a west-east oriented front settles southward into the
region late Wednesday night into Thursday, eventually becoming quasi-
stationary north of I-70. This front will demarcate above average
temps to its north from above average and unseasonably warm
temperatures to its south. Steep mid level lapse rates along with
increasing low level moisture via a strengthening/backing LLJ will
result in elevated instability to the north of the front, and
support the potential for cool sector thunderstorms Thursday
afternoon and evening. The warm sector during this time however
appears to be quite capped owing to limited boundary layer
moisture and a stout EML.

Thursday night into Friday an upper trof will eject negatively
tilted across the Plains and MS Valley. A deepening surface low and
attendant cold front will sweep eastward from KS into the MS Valley
by 12Z Friday in association with the upper trof, with the surface
low moving into the Great Lakes by early Friday evening and the cold
front advancing into the western OH/TN Valley region. This
progression on Thursday night will prompt the west-east boundary
north of the CWA. The warm sector ahead of the advancing cold front
looks to remain capped until the front progress into western IL
early Friday afternoon. At that time, strong height falls and
cooling aloft will weaken the cap, which may allow for isolated
to scattered thunderstorms across eastern portions of the CWA.
Gusty northwest winds and good CAA in the wake of the front will
bring falling temps during the afternoon. The structure of the
upper trof and vorticity maxima not longer support any "wrap
around" precipitation in the cold air on Friday night.

Seasonable temperatures are then expected Saturday as surface high
pressure dominates, with some moderation on Sunday as the high
retreats.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 545 PM CST Sun Feb 19 2017

Surface ridge over the OH and TN Valley regions will move
eastward with southeast surface winds tonight on the backside of
this ridge and stronger south-southeast winds on Monday. Stratus
and fog will redevelop across southeast MO and southwest IL
tonight where the surface winds will be lighter and there is
plenty of boundary layer moisture. This will likely lead to IFR
conditions in UIN and much of the St Louis metro area late
tonight and early Monday morning. The fog will dissipate by late
Monday morning and the stratus will lift as the surface/low level
winds increase. Showers will move into UIN and COU by late Monday
morning or early afternoon ahead of an approaching cold front, and
eventually into the St Louis metro area by late Monday afternoon
or early evening.

SPECIFICS FOR KSTL: Surface ridge over the OH and TN Valley regions
will move eastward with southeast surface winds tonight on the
backside of this ridge and stronger south-southeast winds on
Monday. Stratus and fog will redevelop across southeast MO and
southwest IL tonight where the surface winds will be lighter and
there is plenty of boundary layer moisture. This will likely lead
to IFR conditions in the STL area late tonight and early Monday
morning. The fog will dissipate by late Monday morning and the
stratus will lift as the surface/low level winds increase. Showers
will move into the STL area ahead of an approaching cold front by
late Monday afternoon or early evening.

GKS

&&

.CLIMATE...
Record Highs

      2/19    2/20    2/21    2/22    2/23

STL: 77/2016 78/2016 79/1935 78/1995 77/1996
COU: 76/1930 76/2016 76/1935 76/1995 73/1933
UIN: 71/1930 72/2016 69/1930 70/1922 70/1922


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CST Monday for Iron MO-Madison MO-
     Reynolds MO-Saint Francois MO-Sainte Genevieve MO.

IL...Dense Fog Advisory until 9 AM CST Monday for Bond IL-Clinton IL-
     Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-
     Marion IL-Monroe IL-Montgomery IL-Randolph IL-Saint Clair
     IL-Washington IL.

&&

$$

WFO LSX



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