Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 162036

Area Forecast Discussion
National Weather Service Saint Louis MO
236 PM CST Mon Jan 16 2017

.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.


.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.



.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.

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.





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