Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 281804
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
104 PM CDT Fri Apr 28 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 414 AM CDT Fri Apr 28 2017

A baroclinic zone will sharpen and lift into MO/IL today before
stalling as a WSW-ENE boundary tonight. This boundary will provide a
focus for a prolonged period of SHRA/TSRA, moderate to heavy
rainfall at times, and the potential for severe weather.

Forecast values of 0-6km shear near 50-70kts are more than
sufficient for severe thunderstorms today and tonight. Severe
thunderstorms may occur this afternoon across the southern CWA
within the warm sector, but the northward extent of the affected
area will depend on how quickly the warm front lifts northward
today. There is also a separate concern that elevated convection may
produce large hail tonight/overnight due to favorable shear profiles
for rotating updrafts in the presence of H7-H5 lapse rates of 6-7
deg C/km.

Widespread precipitation should begin to overspread the area tonight
during the evening hours. Please see the discussion below for
information about moderate to heavy rainfall.

Highs today will be around 10 degrees warmer than yesterday.
Overnight lows will be 5-10 degrees warmer than last night for
locations south of the warm front.

Kanofsky

.LONG TERM...  (Saturday through Thursday)
Issued at 414 AM CDT Fri Apr 28 2017

Active, wet, stormy weather pattern this weekend.  NAM model
initially appears too far north with its QPF on Saturday and prefer
the GFS and ECMWF model solutions which are similar.  A warm front
will lift gradually lift northward into our area on Saturday with
numerous showers and storms along and north of it.  An upper level
low will move into the southern Plains Saturday night with a
developing surface low ahead of it moving into southwest MO around
12Z Sunday.  A strong southerly low level jet will bring abundant
low level moisture into our area ahead of the approaching 850 mb
low.  The upper level low will move northeastward through the
central Plains on Sunday with the surface low moving northeastward
into northeast MO by 00Z Monday,  The surface low will drag a
trailing cold front eastward through our forecast area Sunday
afternoon and evening.  There may be some severe storms along and
just ahead of this front.  Will continue the Flash Flood Watch for
much of our forecast area, except parts of northeast MO and west
central IL, through Sunday evening.  There is still some uncertainty
as to where the heaviest rain axis will occur, but with several
rounds of showers/storms the total QPF for tonight through Sunday
evening across the Flash Flood Watch area should be around 3 to 5
inches with locally higher amounts possible.  A dry slot will move
northeastward into our forecast area Sunday night with the
convection shifting east of the forecast area by late Sunday night.
There will be some wrap-around/deformation zone light rain mainly
across northeast MO and west central IL on Monday.  The rain will
shift northeast of the forecast area by Monday evening as the upper
level and associated surface low shift well northeast of the region.
Cooler conditions can be expected Sunday night and Monday due to low
level cold air advection behind the storm system, plus low level
cloudiness on Monday will hinder solar insolation. Low-mid level
warm air advection ahead of an approaching and deepening positively
tilted upper level trough along with increasing upper level
divergence ahead of the upper level trough will lead to showers,
mainly across southeast MO and southwest IL Tuesday night through
Wednesday night.

GKS

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1255 PM CDT Fri Apr 28 2017

Showers and occasional thunderstorms will impact the metro St.
Louis region through 20-21Z. FLight conditions will range from VFR
to MVFR although a burst of heavy rain could produce a short
period of IFR. Further north, showers/rain will be impacting
KUIN into the early-mid afternoon with occasionally MVFR flight
conditions. From late afternoon into the evening, I believe the
terminals will be predominately dry however cig heights will be
gradually lowering. During the overnight hours and into Saturday
morning additional waves of showers/rain and thunderstorms will
overspread the region. Flight conditions will be predominately
MVFR although periods of IFR with heavier rain are certainly quite
possible.

SPECIFICS FOR KSTL:

Showers and occasional thunderstorms will impact KSTL through
20-21Z. FLight conditions will range from VFR to MVFR although a
burst of heavy rain could produce a short period of IFR. I believe
the terminal will be predominately dry from late afternoon into
the evening, however cig heights will be gradually lowering.
During the overnight hours and into Saturday morning additional
waves of showers/rain and thunderstorms will impact the region.
Flight conditions will be predominately MVFR although periods of
IFR with heavier rain are certainly quite possible.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flash Flood Watch from 7 PM CDT this evening through late Sunday
     night for Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford
     MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Lincoln MO-
     Madison MO-Moniteau MO-Montgomery MO-Osage MO-Pike MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

IL...Flash Flood Watch from 7 PM CDT this evening through late Sunday
     night for Bond IL-Calhoun IL-Clinton IL-Fayette IL-Greene
     IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX


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