Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201757

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1257 PM CDT FRI MAY 20 2016

Issued at 1257 PM CDT Fri May 20 2016

Strong vort lobe over northeast MO, representing the old center
of the storm system that was over the southern Plains yesterday,
is very slow to move and is expected to continue to be very slow
to exit this afternoon. Have maintained high PoPs in this area as
a result of the near-steady light rain that has occurred there.

A lobe of vorticity that extends further south from this old storm
center into southwest MO, fulfilling what is the upper level TROF
axis, will also slowly swing thru our region this afternoon and
into early this evening, and interacting with diurnal enhancements
from the heating of the day, should be able to maintain the threat
for isolated to scattered showers during that time. Loss of
heating into the evening and eventual exit of old storm center up
north should eventually result in loss of pcpn chances by mid-
evening for the entire area, save the far eastern sections around
Salem and Vandalia, IL, where a deformation zone region will form
and take much of the night to exit. These areas are close enough
to this region for a low PoP at this time.

Temps today will be close to persistence, with slightly warmer
airmass offset by pcpn chances, clouds, for maxes near 70 most
locations, and 60s in northeast MO.

Min temps in the upper 40s and lower 50s look on target for much
of our region tonight.



.SHORT TERM...  (Through Late This Afternoon)
Issued at 308 AM CDT Fri May 20 2016

An upper low was located over northeast KS early this morning with
the attendant upper trof extending southward into the southern
Plains. Several vort maxes were rotating northeast through the
trof and contributing to a sizeable area of rain/showers stretching
across southeast and south central MO into central MO. The vast
majority of the guidance, with emphasis on the CAMS, have this
precipitation lifting to the north/northeast this morning
gradually diminishing in areal coverage as the upper low/trof
lifts east/northeast. The greatest coverage this afternoon, which
should be scattered in nature, is expected to arc from northeast
MO into south central IL, with a more isolated shower threat
elsewhere. Present indications are the air mass will remain too
stable to support any thunder. Extensive clouds will limit heating
with highs in the 60s, still below average.


.LONG TERM...  (Tonight through Thursday)
Issued at 308 AM CDT Fri May 20 2016

The upper low will continue to move to the east northeast tonight
and actually will gradually deepen as the large scale pattern
amplifies. The upper trof axis will be located over the far
eastern CWA early this evening supporting at least a slight chance
for showers in SC IL. Otherwise surface high pressure will nose
back into the area tonight. There will be potential for fog
tonight into early Saturday morning across parts of eastern and
central MO given the extensive clouds today, late decrease in
clouds this afternoon/evening and light winds. The SREF
probabilities hit this region rather hard.

The large scale amplification will result in a prominent omega
block over the CONUS and the eastern lobe/trof will maintain
surface high pressure Saturday into Saturday Night. Slow progression
of the block will allow the upper ridge to shift from the Plains
into the MS River Valley on Sunday, and slow eastward retreat of
the surface high pressure system. This will result in continued
moderation of temperatures that will begin on Saturday.

By early next week a positively tilted long-wave trof evolves over
the western U.S. and the upper ridge weakens and shifts east into
the TN/OH Valley region. On Monday/Monday Night this will lead to
continued retreat of the surface high, improving low level WAA and
moisture return, and a chance of showers and thunderstorms. By
Tuesday a broad and unstable to very unstable warm sector will be
in place. Weak impulses within the southwest flow aloft and a
southwesterly LLJ will result in continued thunderstorm chances
through midweek and seasonably warm temperatures.



.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1257 PM CDT Fri May 20 2016

VFR conditions and a light north to northeast surface wind are
expected to prevail at the TAF sites thru the valid period. The
shower threat will continue thru at least this afternoon as upper
level energy rotates thru and combines with the heating of the
day. We expect the showers to increase by 19-20z for much of the
area, and diminish toward sunset. Otherwise, VSBY reductions in
mist were added to most sites with late clearing and light north
winds expected with no real change in airmass. May need to be
reduced further on subsequent updates.





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