Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 041133

633 AM CDT Mon May 4 2015

.SHORT TERM: (Today and Tonight)
Issued at 330 AM CDT Mon May 4 2015

There are 2 main areas with precipitation potential this morning.
The first is from southeast/east central MO eastward into IL where
low-mid level warm advection has resulted in spotty showers within
the last few hours. The GFS and recent runs of the HRRR suggest
this potential will persist until around mid morning. The other
area is across northeast MO/west central IL extending into parts
of central MO. The frontal convection has in general become less
organized with one area from eastern IA into northern IL and another
cluster in NW MO. Both regions have generated a pre-frontal cold
pool and outflow boundary, but its the western-most area in NW MO
that bears the most watching. General movement has been to the
southeast and some of the hi-res WRFs want to bring this cluster
along with some additional development into north-central/central MO
this morning. The current cluster is within a region of moisture
convergence along the northern terminus of a 40+ kts southwesterly
LLJ and weak elevated CAPE. It should encounter a less favorable
enviroment as it moves southeast this morning, primarily after 12z,
however forcing may be sufficient for scattered showers and
thunderstorms within the area highlighted by the hi-res guidance.
Otherwise clouds should thin across the entire area during the later
half of the morning and into the early afternoon and when combined
with the present WAA regime and temps aloft, this should result in
another warm day with well above normal highs in the 80s.

There are some discrepencies in the guidance on the southward
position of the front this afternoon and evening with the RAP and
ECMWF showing a bit more southward position into the northern part
of the CWA by 00z, while the remainder of the models keep it near
or just south of the MO/IA border. I think it will ooze south into
the far northern part of the CWA by late afternoon. Present
indications are that heating and continued low-level moistening
late morning into afternoon will result in moderate instability
along the frontal zone. Good low level convergence along the
boundary combined with lift associated with a migrating short wave
moving across southern IA/northern MO should result in extensive
convective development during the later half of the afternoon
spanning from KS across northern MO/southern IA into central IL. I
am still thinking that the broad and veering southwesterly LLJ will
maintain numberous thunderstorms/clusters during the evening with a
decided eastward motion. Although the models suggest the front will
shift slightly northward overnight I am not so sure this will happen
with rain cooled air maintaining its position. However increased
ridging aloft both in the low-mid levels suggest that the forecast
of a northward shift in the convection overnight is reasonable. The
only noticeable change to the pops tonight is to expand the area of
slight chances southward a bit to I-70. The GFS and ECMWF QPFs are a
bit far south and well into the warm sector and I would think the
majority of the convection will reside along the front and northward
into the cool air.


.LONG TERM: (Tuesday through Sunday)
Issued at 330 AM CDT Mon May 4 2015

Not much has changed in thinking from 24 hours ago with respect to
the Tuesday through weekend period. The front will make a decided
shift northward on Tuesday as ridging aloft continues to build from
the lower TN Valley into the upper MS Valley, while across southern
sections of the CWA we continue to see the models generate some
light QPF. I`m still not sold on it but will maintain low pops in
the slight range given the uncertainty. Tuesday still looks to be
another day of above normal temps with highs in the 80s.

Flow aloft becomes more south-southwest on Wednesday and a lead
impulse within this flow will traverse the western CWA prompting
an increased threat of showers and thunderstorms across western MO
into central MO. Flow aloft becomes southwesterly by Thursday and
this regime persists into the weekend ahead of a upper trof/low
migrating from the western U.S. and into the central U.S.. The threat
of showers and thunderstorms Thursday into the weekend gradually
ramps up and spreads west to east due to the active southwest flow
aloft with embedded disturbances, moist unstable warm sector, and
slowly encroaching cold front. Above normal temperatures should
persist through the extended period.


.AVIATION:  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 625 AM CDT Mon May 4 2015

VFR conditions to persist through forecast period. Main issue will
be scattered showers and thunderstorms, especially this morning
and again later in the day today. Coverage is scattered this
morning, so for now have vcnty shower mention at all sites, except
KUIN where have vcnty trw mention as they are closer to frontal
boundary and activity firing along outflow boundary. Another round
of storms possible late this afternoon and evening for KUIN with
frontal boundary in area. Otherwise, south to southwest winds to
persist today with gusts to near 20kts at times, before
diminishing towards sunset.

Specifics for KSTL:
VFR conditions to persist through forecast period. Main issue will
be scattered showers this morning. Coverage is scattered, so for
now have vcnty shower mention in metro area. Otherwise, south to
southwest winds to persist today with gusts to near 20kts at
times, before diminishing towards sunset.



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