Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 252017

Area Forecast Discussion
National Weather Service Saint Louis MO
317 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 40-5o 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.


.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.


.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1249 PM CDT Thu Aug 25 2016

An MCV continues to push across northern MO along a cold front
located from UIN southwest to near MCI. This system will be the
focus for renewed thunderstorm development this afternoon/evening.
Have highest confidence in TSRA at UIN, with less confidence due
to lower expected coverage further to the south at the other
sites. Therefore, will continue with just VCTS at these sites and
amend as necessary once convection begins to develop. Outside of
any storm, expect MVFR cigs to continue for the next hour or two
at UIN before lifting to VFR cigs as the thunderstorms approach.
VFR cigs expected elsewhere with WSW winds around 10 knots.

Convection should develop to the northwest this afternoon and
slowly spread east/southeast through the early evening. A bit
uncertain with thunderstorm coverage along the I-70 corridor, so
will introduce just VCTS for now. Winds will continue out of the
SSW at around 10 knots, slackening tonight. Additional convection
will be possible overnight into Friday morning, but uncertainty
too high to include at this time.


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



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