Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171752

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1252 PM CDT Sat Jun 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 345 AM CDT Sat Jun 17 2017

Main concerns with this forecast is dangerous heat over central
Missouri this afternoon and severe thunderstorms late this afternoon
and this evening.  Have issued a heat advisory for parts of central
Missouri for this afternoon.

Line of showers and thunderstorms continues to move southeast across
the area early this morning.  This line will continue to move
southeast and weaken through 12-14Z as mid-level shortwave trough
moves east and low-level forcing weakens.  Then most of the day
today will be dry and there will be some clearing which will allow
temperatures to climb back into the low 90s, particularly over
central Missouri.  Dewpoints will also increase into the low to
middle 70s ahead of a cold front that will drop down into northern
Missouri.  The combination of these factors will allow for heat
index values to reach 105 in Boone, Moniteau, and Cole counties this
afternoon, so went ahead and issued a heat advisory for this

Aforementioned cold front will move south into the area late this
afternoon and this evening.  Instability will be extreme ahead of
the front with MLCAPES between 3000-6000 J/kg.  Deep layer shear
will be supportive of organized severe weather with values around
40kts.  With a mid level trough approaching the area tonight that
will be increasing mid level ascent, expect thunderstorms to become
likely ahead of the front.  CAMS are showing thunderstorms
developing over northwest MO by late afternoon that will grow
upscale into a line by early evening and then move southeast across
area late in the evening and overnight.  Expect there to be a risk
for damaging winds and large hail and possibly a tornado.


.LONG TERM...  (Sunday through Friday)
Issued at 345 AM CDT Sat Jun 17 2017

The upper TROF that had been quasi-stationary over the north-central
CONUS the past few days will deepen on Sunday, giving a fresh
southward impetus to a surface cold front that was moving thru the
area Saturday night and should allow it to exit the CWA Sunday
morning, resulting in rain ending from north to south and for most
areas.  The cold front will usher in relatively much cooler air
where it will push max temps to around average values for mid June
in the lower 80s.

A dry stretch of wx is then expected late Sunday thru Tuesday with
NW flow aloft and high pressure at the surface building in.  Look
for another day of near average max temps on Monday with temps
already returning to above average on Tuesday with the surface high
scooting off to the east and a west-southwest flow.

The forecast gets muddier from Tuesday night onward, with some
models already bringing back a surge of moisture into the area and
as flow aloft backs more zonal, could re-introduce an active period
of wx.  While there are model differences on how soon to bring back
the rich deep moisture and instability, there is good consensus on
the zonal flow with what should be a higher likelihood of upper
level disturbances zipping thru.  So with some confidence adding at
least mentionable PoPs for most areas for mid to late week for these
increasing rain chances is justified.  There are also indications of
further surface frontal intrusions, but timing and location is all
over the place for the latter half of next week, especially run to
run.  Above average temps also looks favored to continue for much of
the rest of next week, but first glance is that it may be more
seasonably hot versus necessitating a heat headline.



.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1240 PM CDT Sat Jun 17 2017

Swly winds are expected to prevail thru the afternoon with VFR
conditions. TSRA are expected to develop later this afternoon over
nwrn MO into srn IA and drop swd late this afternoon thru the
evening hours. These storms will be capable of producing large
hail and damaging winds. Winds are expected to become nwly behind
these storms and remain thru the rest of the TAF period.

SPECIFICS FOR KSTL/KSUS/KCPS: Current indications are that storms
mentioned above will impact terminals around 06z and for only a
couple of hours. These storms may also be severe with damaging
wind gusts as the primary threat.



MO...Heat Advisory until 7 PM CDT this evening for Boone MO-Cole MO-
     Moniteau MO.



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