Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 271844
AFDLSX

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

.SHORT TERM...  (Through Late Tonight)
Issued at 407 AM CDT Thu Apr 27 2017

Showers are in the process of moving out of CWA early this morning.
This is happening as the negatively tilted upper trough lifts
northeast into the Great Lakes this morning.  GFS/NAM are showing a
shortwave ridge building into the area later today which will help
bring in some clearing temporarily.  But then the next shortwave
trough will already be advancing into the Central Plains tonight.
Showers will develop by late this evening and overnight as moisture
convergence develops on the nose of the low level jet.

There will be cold air advection this morning before it ends as the
surface ridge axis passes to the east this afternoon. Highs will be
5-15 degrees cooler today than yesterday.  There will not be much of
a diurnal change tonight with increasing clouds and winds
increasing from the south.

Britt

.LONG TERM...  (Friday through Wednesday)
Issued at 407 AM CDT Thu Apr 27 2017

Fri - Sun night

This is shaping up to be a very active prd with chances for strong
to SVR TSTMs and a prolonged prd of heavy rainfall that will likely
worsen existing flooding and cause additional flooding through the
weekend and into next week as a strong phased storm system impacts
the cntrl CONUS.

Upper level system that will eventually drive the SFC low is just
now coming onshore across the Pacific NW. Meanwhile, a short wave
rounding the base of the long wave trough will lift a weak SFC low
NE thru the FA. Both SFC and upper level systems will weaken thru
the day but will push a weak cold front thru the CWA. This bndry
will move thru the sthrn CWA drng the aftn with strong to SVR TSTMs
possible. The bndry/outflow will sink S of the FA Fri night...but
not too far. The upper level system will dig S into nthrn NM Fri/Fri
night. After midnight...the LLJ begins ramping up with elevated
precip bcmng widespread across the sthrn CWA overnight. The ECMWF is
further N than the GFS with the bndry and assoc instability so can`t
rule out elevated hailers. As the upper level low emerges into the
sthrn Plains on Sat...the SFC low will lift NE out of TX. The
ongoing precip may act to suppress the northward advancement of the
warm front but this just means that wherever that sets up will
receive an extended prd of heavy rainfall. The upper level low lifts
NE into the cntrl Plains on Sun with the SFC tracking from NE TX to
NE MO by Sun evng. The warm front will be slow to move until the SFC
low moves N on Sun. Expect persistent convection along the warm
front as it moves N thru the wknd. A continuous LLJ will provide a
constant supply of theta-e rich air from the Caribbean. Dps in the
mid/upper 60s will help contribute to PWATs in excess of 1.5" with
some indications of values approaching 2.0". These values are at
least 2-3 SDs above normal. If PWATs reach above 1.75"...it would
set a record for the season. Also, warm cloud depths are between 3.5-
4 km. These values would be impressive during the summer but are on
the far high side of the spectrum for this time of the year. The
combination of the strength and dynamics of the system and the heavy
rain parameters indicate that this could be a significant hydrologic
event. Therefore, will issue a flash flood watch from Fri evng thru
Sun evng for widespread 2-5 inches with higher amounts possible
across SE MO and on into sthrn IL. The heavy rain threat will come
to an end as the SFC cold front moves thru the CWA Sun aftn/evng and
the dry slot overspreads the region. Although SPC does not have the
FA outlooked on Sun...would expect a thin line of convection to form
along the cold front with wind being the primary threat. There will
also be plenty of gradient winds assoc with this system.

Mon - Mon night

CAA will continue with wrap around precip possible across NE MO and W
cntrl IL Mon. The system becomes stacked early next week as the low
moves from the Midwest to the Grt Lks. Temps will be 10-15 degrees
below normal Mon.

Tue - Wed

A short wave embedded in NW flow will deepen as it approaches the
region producing light precip. There are still significant
differences in model solutions drng this prd...but any addtnl precip
would not help any ongoing flooding.

2%

&&

.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

&&

.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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