Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212351

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
551 PM CST Sat Jan 21 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 330 PM CST Sat Jan 21 2017

Low pressure centered over northeast New Mexico will begin moving
eastward tonight as the shortwave aloft ejects into the Great
Plains.  Short range guidance has slowed the timing of the onset of
precip into the late evening and overnight hours for most locations;
this is especially true for the convection allowing models such as
the HRRRs and WRFs.  This looks reasonable given the weak forcing
over the area for most of the evening.  That changes as the
shortwave shifts into eastern Oklahoma after 06Z, pushing the low
level circulation with it. RAP/GFS/NAM all show strong low level
moisture convergence wrapping around the circulation up over the
Ozarks by 06Z.  The moisture convergence weakens somewhat after 09-
12Z as the low level flow swings around to the east and then
northeast with the low moving through northern Arkansas.  Expect
widespread showers...and a few thunderstorms more or less along and
south of the I-70 corridor with this system, but think precip
further north will be more spotty since most of the moisture
convergence will be focused south of I-70.  Should see rain ending
slowly from northwest to southeast Sunday with most of the area dry
before 00Z.  Thunderstorm potential tonight should be limited to
areas along and south of I-44.  Forecast MUCAPE values drop off
quickly from around 1000 J/Kg this afternoon to 300-500 J/Kg this
evening.  Can`t rule out a few rumbles of thunder with values like
that, but don`t think there`s enough instability for more than
"isolated" thunderstorms.  Very mild temperatures for mid-late
January will continue tonight and Sunday...although with rain and
cloud cover it`ll be cooler than today.  Guidance lows ranging from
the mid 30s in northeast Missouri to near 50 in southeast Missouri,
and highs of 45 to near 60 look reasonable.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 330 PM CST Sat Jan 21 2017

The strong storm system center that will have passed to our south
will be pulling away on Sunday evening and some leftover rain can be
expected, mainly for areas to the south and east of STL metro.

After a brief interlude of a weak RIDGE thru our region late Monday
into early Tuesday, another strong Pacific storm system will
approach late Tuesday.  The models continue to show that the bulk of
the lift with this system will pass to our north on Tuesday night,
but have trended just a tad more southerly with the track over the
past 24hrs.  PoPs across our northern CWA have been edged higher as
a result.  Unlike with the last system on Sunday, enough cold air
will try to seep in during the late night and early morning hours to
justify some mention of snow for pcpn-types as well as rain.

A strong lobe of sheared vorticity aloft will couple with a surface
cold front as they drop thru on Tuesday night and carryover into
Wednesday.  Some limited PoP chances continue, but mainly north of
Interstate 70 with a mostly rain but some possibility of snow mixing
in during the cooler times of day.

Heading into Thursday and into next weekend, models continue to show
additional weak upper level disturbances sliding down into our
region from the resultant northwest flow aloft as a longwave TROF
carves into the eastern CONUS.  However, the column gets
progressively drier decreasing any confidence on the ability of any
one event to successfully precipitate given the lack of strength
currently depicted with any of them.  Have favored a dry forecast
for now until this changes.

Otherwise, temperatures are expected to remain above average until
the middle of next week when the longwave TROF develops, with
temperatures more appropriate for late January heading into next



.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 539 PM CST Sat Jan 21 2017

Low system continues to develop over the Plains and will swing
into the region on Sun. Elevated TSRA have developed across AR
heading into srn MO. Much of the guidance has slowed onset of
precip at terminals and raised doubt that precip will reach KCOU,
esp KUIN.

A lot of question on cigs and visbys tonight. There is a fair amnt
of guidance that wud suggest much of tonight remains VFR, but is
also some guidance that suggests IFR. Trended visbys and cigs more
twd the VFR/MVFR, but did not see too much evidence to suggest
drastic changes from the pref TAFs. Will continue to monitor.





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