Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 252313
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
613 PM CDT THU AUG 25 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 316 PM CDT Thu Aug 25 2016

The synoptic pattern this afternoon features a strong subtropical
ridge holding strong across the Southeast, with a trough amplifying
across the Rockies.  This has placed east-central MO and western IL
on the southern fringe of the better southwesterly flow aloft which
will persist through the short term period.

An MCV has weakened and pushed off to the northeast this morning
into early this afternoon.  In its wake, the cold front along which
it was traveling has slowly sagged into portions of west-central IL
and northern MO.  Latest visible satellite shows enhanced cu along
this frontal surface, but thus far anything has really struggled to
sustain itself.  This is likely due to some weak mid-level
subsidence behind the departing MCV/vort max which can be inferred
in some of the forecast/AMDAR soundings which suggest a weak capping
inversion still in place.  As the afternoon wears on and heating
continues, still expect isolated to scattered development along the
frontal surface, and perhaps some isolated thunderstorm development
in the more unstable airmass across east-central and southeast MO.
Given swift mid-level flow, deep-layer shear around 40-50 knots
is supportive of storm organization (mainly north of I-70), so a
conditional strong to severe thunderstorm is still on the table
through this evening. Main threats if these storms were to get
more organized would be hail to one inch and damaging winds to 60
mph.

Any convection that develops along the front this evening should
diurnally weaken. However, a low-level jet will refocus along the
boundary across the Central Plains overnight, which will cause
renewed convection. This convection will move/develop east
northeast along and north of the boundary early Friday morning as
the low-level jet veers and increases isentropic ascent across
central and eastern MO. Am not overly confident in timing/coverage
tonight into Friday morning, thus have left pops in the 40-50
percent range.

Deitsch

.LONG TERM...  (Friday through Next Thursday)
Issued at 316 PM CDT Thu Aug 25 2016

Stalled frontal boundary to begin lifting back north as a warm
front on Friday. So will see continuing chances of showers and
thunderstorms throughout the day. With decent instability and
shortwave sliding east through region, could see strong to severe
storms, especially during the afternoon and early evening hours. By
Friday night, front to shift just north of forecast area.

On Saturday, front begins to shift back to the south once again as
next in a series of upper level shortwaves lifts northeast towards
Great Lakes region, pulling surface low with it. Will see increasing
chances of storms with best chances over northeast Missouri and west
central Illinois. Then frontal boundary washes out and weak surface
ridge builds in.

This weak surface ridge to remain over region through the middle of
next week. Despite the ridging, will see diurnal chances of storms
each day. Temperatures through the period look to remain seasonably
warm with highs in the 80s and lows in the mid 60s to low 70s.

Byrd

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 540 PM CDT Thu Aug 25 2016

Convection developing across southwest MO, ahead of a weak upper
level disturbance, was moving into the COU area. More isolated
storms were also developing near UIN just north of a frontal
boundary which extended through the COU area to just north of STL.
This front will sag slowly southward tonight to just south of the
taf sites by Friday morning. If the current convection holds
together it may impact the St Louis metro area later this
evening. Most of the showers/storms should dissipate later
tonight due to loss of daytime heating, but more convection may
move into the UIN area late tonight/early Friday morning on the
nose of a modest southwesterly low level jet. Scattered diurnal
convection is expected again Friday afternoon as the weakening
front begins to lift back northward with moist and unstable
conditions. Surface winds will be light tonight and Friday
morning, then east-southeasterly Friday afternoon.

SPECIFICS FOR KSTL: Convection developing across southwest MO,
ahead of a weak upper level disturbance, was moving into the COU
area. More isolated storms were also developing near UIN just
north of a frontal boundary which extended through the COU area to
just north of STL. This front will sag slowly southward tonight to
just south of STL by Friday morning. If the current convection
holds together it may impact STL later this evening. Most of the
showers/storms should dissipate later tonight due to loss of
daytime heating. Scattered diurnal convection is expected again
Friday afternoon as the weakening front begins to lift back
northward with moist and unstable conditions. Surface winds will
be light tonight and Friday morning, then east-southeasterly
Friday afternoon.

GKS

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     73  86  72  87 /  30  30  40  50
Quincy          66  81  69  85 /  40  50  50  60
Columbia        68  83  68  85 /  50  40  50  50
Jefferson City  69  84  69  87 /  50  40  50  50
Salem           72  86  71  86 /  20  30  30  50
Farmington      71  86  70  85 /  30  40  30  50

&&

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

$$

WFO LSX



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