Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 271235

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
735 AM CDT Mon Mar 27 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 400 AM CDT Mon Mar 27 2017

A low pressure system will move through southern MO/IL today,
bringing periods of SHRA/TSRA to the area. LLJ interactions with the
system`s lifting warm front had already produced a couple of MCSs
early this morning across OK/KS/MO. Precipitation chances will shift
eastward this morning as the surface low approaches the region. The
system`s warm front continues lifting today and eventually bisects
MO/IL from SW to NE, putting the southeastern half of the LSX CWA
within the warm sector by afternoon. SHRA/TSRA are expected to
redevelop or increase in coverage this afternoon, and a few strong
to severe thunderstorms will be possible based on instability and
shear profiles. Neither instability (~1000 J/kg) nor shear (~30kts)
looks unusually strong, and with H7-H5 lapse rates of ~6.5-7 deg
C/km, the primary severe wx threat appears to be large hail and
damaging wind gusts. The severe weather threat is greater for areas
farther southeast of the LSX CWA where both instability and shear
values are greater. Precipitation should be ending by 03-06z. The
warm front will sink southward as a cold front tonight in the wake
of the departing low pressure system.


.LONG TERM...  (Tuesday through Sunday)
Issued at 400 AM CDT Mon Mar 27 2017

The synoptic pattern at the start of the long term period will
continue to feature an active flow regime due to an active Pacific
jet.  This active pattern will bring two systems of note into the
region through the period, one Wed/Thur and another one perhaps
Saturday night into Sunday.

Shortwave ridging will dominate on Tuesday as the Midwest lies
between a trough departing to the east and the next one approaching
from the southwest.  Despite the ridging aloft, enough low-level
moisture trapped beneath an inversion will lead to mostly cloudy
skies and temperatures in the upper 50s to low 60s.

The main focus of the period will be on the system which will begin
to impact the region as early as Tuesday night and linger into
Thursday night.  As the upper-level low ejects out into the Central
Plains, a low-level jet of 30-40 knots will focus into portions of
central and eastern MO.  The associated isentropic ascent will be
enough for a band of showers Tuesday night into Wednesday.

Things become more interesting Wednesday night into Thursday as the
upper-level low (now becoming more vertically stacked with the
surface reflection) ejects closer to the Midwest.  A stronger PV
anomaly will round the base of the upper-level low, which will help
to enhance large-scale ascent and once again lead to the development
of a low-level jet, this time on the order of 40-50+ knots.  This
low-level flow will have an open GofMex to work with leading to
efficient moisture transport into MO and western IL.  Given the
increasing dynamical forcing, it is likely a line of showers/storms
will develop over eastern OK and quickly race eastward Wednesday
night.  The southern part of the LSX CWA will likely mark the
northern extent of the better surface-based instability ahead of
this convective line.  SPC has placed areas south of I-70 in a
Slight Risk where it appears storms could remain surface-based into
early Thursday morning.  There is some uncertainty with the northern
extent of this severe threat, which will likely be driven by low-
level moisture advection (i.e. how far north the richer surface
dewpoints can get) ahead of the line given fairly poor lapse rates
aloft. Stay tuned.

On Thursday, the stacked low pressure system will move through the
heart of Missouri.  The latest guidance is in much better agreement
with this scenario than past nights, thus confidence is increasing
in additional showers/storms Thursday afternoon into Thursday
evening ahead of the surface low beneath the cold core of this
system.  In fact, this synoptic setup is fairly similar to what we
experienced this past Saturday.  Will once again have to keep an eye
on thermodynamical trends, as sufficient instability developing in
the heat of the day ahead of the surface low could support another
severe threat during the day on Thursday.

The upper-level low will finally push east Thursday night, leaving
much of Friday and the first part of the upcoming weekend dry.  The
second system will then approach Sunday into Monday, although
uncertainty is high given fairly large model spread.  Will continue
with an ensemble approach for now until trends become more clear.



.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 631 AM CDT Mon Mar 27 2017

Periods of showers and isolated thunderstorms will continue
today as a low pressure system moves across the region. Ceilings
will fall rapidly to MVFR or IFR after the onset of rain due to
saturation of the lowest levels. A separate round of SHRA/TSRA is
expected to develop this afternoon to the east of KCOU and to the
south of KUIN. Winds will become northwesterly to northerly after
00z behind the departing surface low.

SPECIFICS FOR KSTL, KSUS, KCPS: Periods of showers and perhaps an
isolated thunderstorm will continue this morning. Ceilings will
fall rapidly to MVFR or IFR after the onset of rain due to
saturation of the lowest levels. There may be a break between the
ongoing precipitation at TAF issuance and a separate round of
SHRA/TSRA which is expected to develop during the afternoon, but
it`s not clear whether the afternoon precipitation will occur far
enough north and west to affect the St. Louis metro area. Winds
will become northwesterly to northerly after 00z behind the
departing surface low.





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