Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 260836
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
336 AM CDT Sun Mar 26 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 334 AM CDT Sun Mar 26 2017

A stacked low pressure system will continue its slow northeastward
progress today before rejoining the prevailing flow as an open wave.
Isolated to scattered rain showers will continue in the vicinity of
the upper circulation today, and there may be some diurnal
enhancement of SHRA coverage this afternoon. The cold front
associated with this system moved through the region last night, but
the boundary will be lifted back northward as a warm front later
today in response to the flow induced by another low pressure system
located upstream. The combination of large scale ascent ahead of the
approaching low pressure system along with a southwesterly LLJ
impinging on the aforementioned boundary will produce a broad area
of precipitation tonight which will spread across the entire CWA
tomorrow (see discussion below).

Kanofsky

.LONG TERM...  (Monday through Saturday)
Issued at 334 AM CDT Sun Mar 26 2017

The active Pacific jet stream and thus the active pattern across the
central and southern CONUS will persist through the period.  The
synoptic pattern at the start of the period will feature one PV
anomaly/shortwave trough pushing into southeast MO.  This will be
the first in a series of two systems to impact the region through
next weekend.

The main focus of this period continues to be on strong/severe storm
potential on Monday afternoon into early Monday evening.  The
morning will start with showers and elevated thunderstorms forced by
isentropic ascent on the nose of a 40-45 knot low-level jet.  This
activity should become less widespread through the late morning
hours as it shifts off to the northeast.  The exact details of
potential convective evolution on Monday afternoon into Monday
evening remain a bit murky.  Thermodynamic profiles will become
increasingly favorable for severe weather across southeast MO and
southwest IL late Monday morning into Monday afternoon ahead of a
surface low and trailing cold front with MLCAPE increasing to 500-
1500 J/kg. Deep-layer shear, while not overly impressive for this
time of year at 25-35 knots, will also be supportive of some strong
to severe weather. However, the 12kNAM and some of the hi-res cams
suggest that activity could focus more along a pre-frontal trough
(or perhaps a convectively-induced cold pool from overnight Plains
convection) across southern IL and western KY (mainly east of the
LSX CWA). Others suggest that the more vigorous afternoon
development will remain along the surface front, putting more of
southeast/south-central MO and southwest IL in a severe threat.
Therefore, there is some uncertainty with how Monday will play out,
but as of now it looks like the best severe chances will be across
southeast MO and southwest IL in the LSX CWA (and locations
south/east of the LSX CWA) from the early afternoon into the early
evening. The main threats look to be large hail and damaging winds.

The cold front will push through the region Monday night, bringing
an end to the showers/thunderstorms.  Tranquil conditions will
continue into Tuesday night, before the second system arrives
Wednesday into Thursday.  Guidance continues to differ with this
system, as the GFS seems to be a southern outlier with the surface
low compared to the ECMWF/GEM solutions.  Therefore, have continued
to favor the northern solutions which places another vertically
stacked low-pressure system right over central/eastern MO Thursday
into Thursday night.  This will once again set the stage for showers
and low-topped thunderstorms.

That system will meander to the east by Friday afternoon, allowing
conditions to dry out for the remainder of the period.  Despite the
systems passing through, we will never really experience much of an
airmass change, so temperatures in the low to mid 60s are likely
through much of the upcoming week.

KD

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1053 PM CDT Sat Mar 25 2017

Surface and upper level low near or just northeast of COU will
move slowly northeastward late tonight and Sunday. Most of the
scattered showers will move north-northeast of the taf sites late
tonight. Prevailing cloud ceiling will drop into the IFR catagory
late tonight with light fog possible, then gradually rise into
the VFR catagory Sunday afternoon. The south-southeast surface
wind will veer around to a west-southwest direction late tonight
and Sunday morning.

SPECIFICS FOR KSTL: Surface and upper level low near or just
northeast of COU will move slowly northeastward late tonight and
Sunday. Most of the scattered showers will be north-northeast of
STL late tonight. Prevailing cloud ceiling will drop into the IFR
catagory late tonight with light fog possible, then gradually
rise into the VFR catagory Sunday afternoon. The south surface
wind will veer around to a west-southwest direction late tonight
and Sunday morning.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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