Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 280915
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
415 AM 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 06z TAFs through 06z Friday Night)
Issued at 1155 PM CDT Thu Apr 27 2017

VFR conditions currently prevailing across the TAF sites should
continue thru the overnight hours, even as some scattered SHRA
develop into our region after midnight. There is a reasonable
consensus that lower CIGs will develop at some point just after
daybreak and overspread our region, and looks initially to be
MVFR and this should hold into Friday evening before the surface
boundary becomes more established and allow for a better chance of
IFR. Tried to target the rain opporunities in two main periods:
one for very late tonight thru early Friday afternoon, and the
other building from the south--for most TAF sites beyond the 24
hour valid time--during Friday night. Easterly surface winds will
continue thru Friday but as the front builds in over the region,
it looks to set up near STL metro and remain nearly stationary
thru Friday night, winds should lighten.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flash Flood Watch from 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 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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