Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261939
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
239 PM CDT Wed Apr 26 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 126 PM CDT Wed Apr 26 2017

Deep upper trof will swing thru the region this evening with the
cdfnt pushing sewd thru the area. Ongoing TSRA are expected to
continue along the sern CWA border for the next few hours with
mainly a wind threat.

Expect precip event to transition to a stratiform RA with sct to
isod TS spreading over much of the CWA. Some areas of heavy
rainfall will be possible this evening as the sfc low approaches
the area. Have held off any flood headlines for now, but will need
to monitor how the event unfolds.

Thurs is expected to remain dry with a weak sfc ridge building
into the region ahead of the next approaching system. Have trended
temps twd the warmer guidance with clouds expected to clear out
during the morning hours. Have not gone above the warmer guidance
due to at least some clouds expected to spill back into the region
during the afternoon.

Tilly

.LONG TERM...  (Thursday through Tuesday)
Issued at 422 AM CDT Wed Apr 26 2017

Thu - Fri

Storm system that is expected to bring SVR WX and decent rains will
be pulling away from the region Thu mrng. Neg tilted short wave will
be mvng thru the cntrl/ern portion of the FA at the beginning of the
prd with some post frontal SHRAs possible across the far ern portion
of the CWA early in the prd. A short wave ridge will build into the
region for later Thu/Thu night which should provide a brief break in
the active pattern. This break will be brief though as the weak high
at the SFC and aloft quickly slide E late Thu night. This will allow
for warm advection to commence again as a ripple in the wrn trof
lifts NE in the broad SW upper flow which will induce a weak SFC low
with precip breaking out late Thu night into Fri. The low is fcst to
move into sthrn Ontario by Fri evng and leave a weak bndry draped
thru the OH R. Valley into the middle MS R. Valley and on to the
sthrn Plains. It is along this bndry that the next round of rain
will focus.

Temps on Thu will be seasonally cool averaging about 5-10 degrees
below normal. There will be a decent temp gradient across the area
on Fri with warm front bisecting the region. Temps should range from
lower 60s in NE MO to the mid 70s across SE MO and sthrn IL.

Fri night - Sun night

The next short wave approaches the region Fri night with a SFC low
dvlpng across TX. Widespread precip is expected to break out along
and S of the residual bndry from the Fri system. The entire system
strengthens thru the day Sat. The upper level system is fcst to move
from the 4-Corners area to the sthrn High Plains while the SFC low
lifts NE to ern OK. Rainfall should be impressive with this system
as well aided by PWATS in excess of 1.5 inches. Widespread totals
of 2-5 inches are expected from NE MO to SE MO with higher totals
possible across the ern Ozarks from Fri night thru Sun night
from. This is on top of the 1-2 inches expected between now and
then.

There will also be plenty of dynamics assocd with this system along
with impressive frontogenesis and moisture convergence along a
tightening baroclinic zone with an impressive 20+ degree temp spread
across the CWA from NW to SE on Sat. The entire system lifts NE Sat
night and Sun with the SFC low tracking thru the CWA reaching ern
IA/NW IL by Sun evng. The warm front is expected to clear the NE FA
by early aftn Sun. Meanwhile the cold front is fcst to push E thru
the aftn with the dry slot overspreading the entire CWA by evng.
Although SPC removed our area from the outlooks for the wknd...I
still think there is the potential for strong to SVR TSTMs in the
warm sector along the northward advancing warm front Sat/Sat
night/Sun and the cold front Sun. Temps in the 70s and Dps in the
upper 50s/lower 60s will combine with lapse rates near 7.5 C/km thru
the wknd to produce SBCAPE/MUCAPE of 1000-1500 J/kg. There certainly
won`t be a lack of shear either. SE MO and sthrn IL have the best
chance of seeing the strongest storms thru the wknd. I would think
the threat along the warm front would be elevated supercells Fri
night and again Sat night with hail being the primary threat. SFC
based convection should dvlp Sat aftn/evng along the warm front. SFC
based convection should also dvlp along the cold front Sun aftn. One
mitigating factor will be ongoing convection along the warm front
thru the wknd. Nevertheless...the potential does exist. **Since
writing the AFD earlier in the shift...SPC has since put the sthrn
CWA in a Day 3 (Fri...for potential of elevated convection Fri
night) enhanced risk.**  The low will continue to lift N into sthrn
WI thru the night Sun with wrap around/def zone precip approaching
NE MO by late in the night. Winds in going fcst likely not high
enough thru the wknd.

Significant temp gradient will continue on Sun as warm front doesn`t
clear NE CWA until the aftn and CAA already underway across NW FA
behind SFC low.

Mon - Tue

The upper level low is fcst to drift into ern Canada by the middle
of the week. Light rain may brush far nthrn zones Mon as the low
lifts out of the the region. Another brief respite for later Mon/Mon
night into Tue as weak SFC ridge assoc with ridging aloft traverse
the region. The next upper trof approaches for mid week with WAA
precip dvlpng as early as Tue evng. Winds on Mon likely not high
enough either.

Mon will be a windy raw day with a stiff W/NW breeze and temps well
below normal with highs in the mid 50s N to mid 60s S.

2%

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 229 PM CDT Wed Apr 26 2017

Area of rain will persist across much of the area impacting
terminals thru this evening. Cigs will remain in the MVFR and IFR
range thru tonight with clouds clearing out Thurs morning. Mid
level clouds will spill back into the area Thurs afternoon.

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     48  64  54  72 /  90  10  40  50
Quincy          42  60  49  67 /  90  10  40  60
Columbia        42  62  51  70 /  80   5  60  40
Jefferson City  42  64  51  72 /  80   5  60  40
Salem           50  63  51  73 / 100  10  20  40
Farmington      47  66  52  74 /  90  10  40  40

&&

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

$$

WFO LSX


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