Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 280033
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
733 PM CDT Thu Apr 27 2017

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 332 PM CDT Thu Apr 27 2017

Developing storm system over the Great Plains will begin spreading
clouds and precipitation over the area tonight.  The associated warm
front will move from Arkansas up through Missouri late tonight
and tomorrow morning and stall near the Missouri/Iowa border by
18Z Friday. Warm advection ahead of the front this evening and
tonight will spread isolated to widely scattered showers into our
area this evening...but best chances for precip are overnight into
Friday morning. Showalter index values do not drop below 0 until
Friday afternoon, so have kept mention of thunder out of the
forecast until after 18Z. Severe weather potential for our area
during the afternoon appears to be limited as the highest
instability looks to be down over far southeast missouri and
southern Illinois. However...if the warm front can shift further
north allowing the higher CAPE into our area 0-6km shear of
50-70kts will be more than sufficient for severe thunderstorms to
develop.

Carney

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 332 PM CDT Thu Apr 27 2017

Active period through the weekend as frontal boundary sinks back
south stalling out over southern portions of forecast area Friday
night through early Saturday. As the low level jet ramps up will
see increasing chances of showers and storms through the overnight
hours Friday, especially along and south of Interstate 70.

During the day on Saturday, boundary becomes a warm front once
again while upper level low, over the 4 corners region, begins to
lift northeastward towards region. Will see several rounds of
showers and storms Saturday through Sunday. Still some differences
on location of frontal boundary which will affect where the heavy
rain axis will be. NAM continues to be the outlier with a further
north solution. So kept a GFS/ECWMF blend and after coordination
will go with WPC qpf, which shifts the higher amounts a tad
further north than previous forecast. As for chances of severe
weather, strong to severe storms possible, especially over
southeast Missouri and southern Illinois through Sunday.

By Sunday as surface low moves north, warm front to slide north of
forecast area, while surface low drags cold front through region
during the afternoon and early evening hours. So dry slot to move in
behind cold front with precipitation tapering off towards Midnight
Sunday.

As for qpf amounts, dewpt values in the upper 50s to mid 60s through
the weekend will contribute to PW values in excess of 1.5 inches.
This combined with strength and dynamics of system will generate a
prolonged period of heavy rain. At this time expecting between 2 to
5 inches of rainfall with locally higher amounts possible,
especially over the eastern Ozarks. This could lead to a
significant hydrologic event. Flash flood watch to remain in
effect through 06z Monday.

Will see wrap around showers on backside of system late Sunday night
through Monday. Beyond that, another brief break in the
precipitation before next system moves Tuesday afternoon through
Thursday. Extended models having differences in timing and placement
with this system, so went with a blend for now. Otherwise, coolest
temps will be on Monday with highs in the 50 to 60 degree range,
then moderate back into the 60s for the remainder of the forecast
period.

Byrd

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 715 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, but how low is a point of
contention. For now will carry MVFR category but may need to drop
this into IFR if model guidance comes into any better agreement.
Tried to target the rain opporunities in two main periods: one for
late tonight and Friday morning and the other building from the
south--for most TAF sites beyond the 24 hour valid time--during
Friday night. Surface winds will become easterly later this
evening and thru Friday morning but then as the front builds in
over the region, it looks to set up near STL metro and remain
nearly stationary thru Friday night. For now have transitioned
winds to variable at those sites.

TES

&&

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