Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 272036
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
336 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 18z TAFs through 18z Friday Afternoon)
Issued at 105 PM CDT Thu Apr 27 2017

MVFR ceilings are rising and scattering from west to east this
afternoon. Expect the entire area will be VFR by 22-23Z or shortly
thereafter. A warm front will move up out of Arkansas and through
most of the area late tonight into Friday morning. A few showers
are possible this evening into central Missouri, but the best
chance for precip will be after 09Z. Showers should mostly be
pretty light...though ceilings look likely to lower into MVFR
range toward daybreak as moisture increases south of the warm
front. Intermittent light showers will likely continue through
18Z.

SPECIFICS FOR KSTL:

MVFR ceilings at Lambert look like they might scatter out a little
faster than previously thought...clearing observed on satellite
pics is moving pretty rapidly toward the terminal. Expect VFR
conditions to prevail after these clouds scatter out...probably
until early Friday morning. Scattered showers will precede a warm
front which will move north of the terminal Friday morning. Think
ceilings will lower to MVFR range in the morning as the front
moves north...though timing and heights are low confidence at
this time. Showers will likely decrease in coverage during the
late morning...and ceilings should rise to MVFR...though most
likely not much higher than 3500-4000 feet.

Carney
&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     53  71  59  70 /  50  60  80  80
Quincy          48  64  50  55 /  60  70  50  80
Columbia        51  71  53  63 /  60  50  70  80
Jefferson City  52  72  55  67 /  60  50  80  90
Salem           50  72  62  75 /  30  50  80  80
Farmington      52  72  60  74 /  30  50  80  80

&&

.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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