Area Forecast Discussion
Issued by NWS St. Louis, MO

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683
FXUS63 KLSX 170339
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
939 PM CST Mon Jan 16 2017

.UPDATE...
Issued at 926 PM CST Mon Jan 16 2017

A band of rain was moving through this evening from UIN southwest
to VIH. This precipitation was associated with a surface low near
UIN and a trailing cold front. This precipitation along with
stronger surface winds behind the cold front was leading to
improvement in the visibilities, and may be able to cancel the
dense fog advisory across parts of northeast MO and west central
IL early. Looking at the latest HRRR model runs it appears that
the rain should be east of the Mississippi River by midnight, then
east of our forecast area by 3 AM. Although the rain will come to
an end the cloud cover will continue to plague our area with
plenty of wrap-around type low level moisture and cloudiness
behind the surface low.

GKS

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 230 PM CST Mon Jan 16 2017

Surface low centered over northeastern KS will continue to lift
northeast tonight into the Great Lakes region by 12z Tuesday.
Associated cold front to slide through forecast area, exiting by
daybreak on Tuesday. Scattered showers will continue to develop
along and ahead of boundary, tapering off from west to east as front
moves through tonight. Decent MU CAPES between 250 and 500 J/kg, as
well as lift along and ahead of boundary, so will see some isolated
thunderstorms. Kept slight chance mention in grids for tonight.

Otherwise, cloudy skies to prevail with lows in the mid 30s to mid
40s. With such strong cyclonic flow around system, clouds to linger
over region on back side of system Tuesday, so highs will only be in
the upper 30s to around 50, which is still about 5 degrees above
normal for this time of year.

Byrd

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 230 PM CST Mon Jan 16 2017

Weak surface high pressure will dominate the area on Tuesday
night as the short wave trof over the Great Lakes area departs to
the east, and this will probably be the coldest night of the week.
In the wake of this trof, heights aloft will be on the rise and the
surface high will be retreating eastward. A WAA regime gets underway
on Wednesday resulting from the combination of rising heights aloft
and the return of low level south/southwest flow. This will
bring a nice boost to the already above average temperatures.

After Wednesday, an increasingly energetic lower latitude storm
track emnanting from the Pacific will bring us several weather
systems thanks to a stout 150+ kt ULJ. Suprisingly, the models
are in reasonably good agreement with the large scale mass fields
depicting these systems during the longer range period. The first
of these will impact the area in the late Thursday-Friday time
frame as the positively tilted southwest U.S. upper trof and
embedded upper low eject east/northeastward and become negatively
tilted. The GFS is a tad slower with this sytem and hence the flow
aloft is more backed in the low levels and veered in the mid/upper
levels, which results in some variation in the depiction of the
warm conveyor belt, moisture transport, and resultant precipitation
threat. Basically we have followed a consensus approach at this
time. Moisture transport will begin on Wednesday night with the
main thrust from the lower MS Valley into the TN Valley region
into Thursday. Southeast MO and Southern IL will be the western
periphery with the greatest pcpn chances. The probability of
precipitation will increase and translate further west on Thursday
night with the approach of the actual upper system and negatively
rotating short wave.

Another upper low and trof will lift north/northeastward from the
southern/central Rockies and through the central U.S. on Friday
night and Saturday. Despite decent height falls and weak large
scale forcing for ascent with this system, moisture will be
negligible in the wake of the previous system. Friday night and
Saturday look largely dry and continued mild for later January.

Finally a potent upper low will evolve as it moves from the
southwest U.S. and impacts the area Saturday night into Sunday
night, bringing a good slug of rain. This would be a rather interesting
system and we would likely be talking winter storm IF temps were
much colder, however present progs suggest all rain and continued
above average temps.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 548 PM CST Mon Jan 16 2017

Surface low over west central MO with a trailing cold front
extending south through western AR. The low will move northeastward
through UIN later this evening dragging the cold front through the
rest of the taf sites later this evening. Band of showers and a
few storms moving through the St Louis metro area will shift east
of this area later this evening, while another area of rain and
embedded showers over the northwestern quarter of MO moves
northeastward through COU and UIN this evening. This second area
of rain may cause the very low visibilities from dense fog to
improve slightly in UIN later this evening. The stratus cloud deck
at 500 feet or less in height over the taf sites will continue
tonight and Tuesday morning, but should improve Tuesday
afternoon. The low visibilities due to fog should gradually
improve through the night into Tuesday. Southeasterly surface
winds will veer around to a westerly direction later tonight after
fropa.

SPECIFICS FOR KSTL: Surface low over west central MO with a trailing
cold front extending south through western AR. The low will move
northeastward through UIN later this evening dragging the cold
front through STL later this evening. Band of showers and a few
storms moving through the St Louis metro area will shift east of
this area later this evening.  The stratus cloud deck at 500 feet
or less in height over STL will continue tonight and Tuesday
morning, but should improve Tuesday afternoon. The low visibilities
due to fog should gradually improve through the night into
Tuesday.  Southeasterly surface winds will veer around to a
westerly direction late tonight after fropa.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until midnight CST tonight for Knox MO-Lewis
     MO-Marion MO-Monroe MO-Ralls MO-Shelby MO.

IL...Dense Fog Advisory until midnight CST tonight for Adams IL-Brown
     IL-Pike IL.

&&

$$

WFO LSX



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