Area Forecast Discussion
Issued by NWS St. Louis, MO

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118
FXUS63 KLSX 191757
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1257 PM CDT Fri May 19 2017

.UPDATE...
Issued at 1152 AM CDT Fri May 19 2017

Main concern continues to be the potential for thunderstorm
redevelopment this afternoon and tonight. Front is now aligned
west southwest-east northeast across across the area from central
Missouri into central Illinois. There is quite a bit of
cloudiness to the north of the front which is holding down
temperatures compared to the south, where temperatures are
climbing through the 70s where there is more sunshine. MUCAPES are
already in the 500-1000 J/kg range along and south of the front
and this will likely increase into the 1000-2000 J/kg range by
later this afternoon. CAMS indicates are showing some scattered
development along the front during the mid-late afternoon hours.
Best chance for storms will occur during the late afternoon and
early evening hours when the RAP shows a more organized cluster of
thunderstorms associated with a vort max currently over Oklahoma
moving east along the boundary. There will also be another chance
for an MCS overnight. Lowered temperatures in the north for clouds
and placement of the front.

Britt

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 351 AM CDT Fri May 19 2017

Severe potential remains the main forecast issue for today and
tonight. As of 08z, storms along warm front continue to lift
northward into northeast MO/west central IL while MCS over western
MO has moved into central MO. Complex to track eastward, reaching
metro area between 5 am and 8 am, then out of forecast area by mid
morning. Still feel that there will be a bit of a break in the
activity from mid morning through early afternoon. Only concern is
will the atmosphere recover from this activity. Models still giving
mixed signals with some indicating MU CAPES well over 2500 J/kg,
while others now show little or no atmospheric recovery. For now
will go with low end chance pops for areas along and north of I-70,
higher pops south.

By mid afternoon, storms to refire to our west and southwest,
eventually increasing in coverage and tracking northeast through
forecast area through tonight. A few strong to severe storms are
still possible during this period with the main focus along the warm
front which will remain stalled over I-70 corridor.

As for temperatures today, will be dependent on cloud cover,
precipitation and location of warm front. For now they will range
from the mid 60s far north to the low 80s along and south of I-70.
Lows tonight will be mild once again, in the upper 50s to upper 60s.

Byrd

.LONG TERM...  (Saturday through Thursday)
Issued at 351 AM CDT Fri May 19 2017

(Saturday through Sunday)

Period begins with convection most likely exiting, or soon to exit,
the CWA as the leading s/w lifts out of the area. At the sfc, a low
shud be somewhere over nwrn MO or nern KS. Aloft, a slightly
negatively tilted trof is expected from Saskatchewan swd into the
Great Plains region. The sfc low is projected to lift nnewd into WI
by the evening with the cdfnt pushing ewd thru the area as the
system begins to occlude.

A number of questions still exist regarding evolution of convection
during the day. Would expect at least a short lull period after the
morning convection with another round later in the day. However,
with stronger forcing further west, this lull may be very short
lived. Any severe potential still appears to remain east of the
Mississippi River where the effective fnt shud reside. Regardless,
the cdfnt shud push all precip east of the CWA by Midnight Sunday.

Behind the fnt, a much cooler sfc ridge will build into the region
for Sun. This ridge will keep the area dry with temps struggling to
reach the 70s. Have still trended twd the warmer guidance for Sun
given ample insolation and deep mixing.

(Monday through Thursday)

Mdls begin the period in fairly good agreement. By the middle of
next week, differences appear, tho mainly timing issues. After the
sfc ridge pushes east out of the area during the day Mon, a s/w
rounding the base of the upper trof is expected to bring a chance of
rain ahead of a fast moving clipper system.

With the region under persistent NW flow and a strong sfc ridge
likely settling south of the region, expect temps to be well below
seasonal avg for much of next week.

Tilly

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1236 PM CDT Fri May 19 2017

Scattered thunderstorms are expected to redevelop along and south
of a stationary front this afternoon that currently extends from
south of KCOU to north of KSTL. MVFR and some IFR ceilings will
slowly lift during the afternoon hours. There will be a better
chance of thunderstorms toward 00Z when a more organized band of
thunderstorms moves across the area. A second area of
thunderstorms is expect to move across the area between 08-12Z
affecting all of the terminals. Any thunderstorms will have the
potential to produce MVFR/IFR conditionals and brief gusty winds.
MVFR/some IFR conditions are expect late tonight and early on
Saturday morning before they improve on during the day on
Saturday.

SPECIFICS FOR KSTL: There will be the threat for thunderstorms to
redevelop late this afternoon and early this evening. The best
chance for thunderstorms will be early this evening and again
overnight. Any thunderstorms will have the potential to produce
MVFR/IFR conditionals and brief gusty winds. MVFR/some IFR
conditions are expect late tonight and early on Saturday morning
before they improve on during the day on Saturday.  Any
thunderstorms will have the potential to produce MVFR/IFR
conditionals and brief gusty winds. MVFR/some IFR conditions are
expect late tonight and early on Saturday morning before they
improve on during the day on Saturday. Current southwest winds is
expected to turn to the north this afternoon.



Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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