Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261509
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1009 AM CDT Wed Apr 26 2017

.UPDATE...
Issued at 1001 AM CDT Wed Apr 26 2017

Have decided to hold off issuing a Watch product for the ongoing
cell over south central MO. This storm is expected to continue ENE
and impact the southern three counties or so of the CWA and
potentially remain severe thru that area.

Uncertain how far NE this storm will remain severe. However, the
airmass ahead of this cell continues to destabilize but to what
extent is unknown. Still believe another round of potentially
severe storms is possible this afternoon for far srn and sern
portions of the CWA, mainly along and east of a KFAM to KMTO line.

Tilly

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 422 AM CDT Wed Apr 26 2017

Main concern through today will be the potential for severe
thunderstorms and locally heavy rainfall.

Band of showers and thunderstorms is already moving across region
early this morning.  Latest RAP runs are showing some increase in
instability ahead of this line this morning, so there could be a few
strong to possibly severe storms that could produce gusty winds
and some hail as they move east early this morning across the
area. Of concern is how will this precipitation and debris clouds
affect the atmosphere`s ability to recover this afternoon.
NAM/GFS are still showing that there will be a limited amount of
CAPE developing ahead of the cold front by mid-late afternoon,
particularly across southeast Missouri into southwest Illinois.
This is where the latest runs of the CAMS have been the most
consistent in showing the development of a line of thunderstorms
this afternoon. Given the deep layer shear of 50+ kts, still think
that a few severe thunderstorms will be possible this afternoon.

Also concerned about locally heavy rainfall as shear vectors become
aligned with low level boundary.  NAM/GFS are showing the upper
trough currently over the High Plains becoming negatively tilted by
late this afternoon and this evening.  PWATS and warm cloud layer
depths will favor heavy rainfall rates, so could see locally heavy
rainfall rates.  Widespread 1-2 inches of rainfall seems likely
through tonight.

Showers and thunderstorms will end from west to east late tonight as
trough lifts off to the east.

Britt

.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 12z TAFs through 12z Thursday Morning)
Issued at 712 AM CDT Wed Apr 26 2017

IFR/MVFR ceilings are expected over central and northeast Missouri
as well as west central Illinois including KUIN and KCOU through
tonight. These low ceilings will spread into the St. Louis area
terminals by this afternoon. Showers and scattered thunderstorms
will move across the area during the late morning and afternoon
hours as a cold front moves across the area. The rain will end by
this evening as the front exits the area.

SPECIFICS FOR KSTL: Current VFR conditions will turn MVFR by this
afternoon as showers and thunderstorms move into the terminal. The
showers and thunderstorms will persist through late this afternoon
before ending by this evening.

Britt

&&

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

$$

WFO LSX



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