Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270901
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
401 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.

Kanofsky

.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.

KD

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1045 PM CDT Sun Mar 26 2017

Showers and thunderstorms across eastern portions of KS and OK
and west central and southwest MO, ahead of an approaching upper
level and surface low will spread into the taf sites late tonight
and early Monday morning. The cloud ceiling will drop down into
the IFR catagory as the low levels of the atmosphere saturate. The
surface wind will become easterly as the surface low approaches
southwest MO late tonight, then back around to a northwest
direction Monday afternoon as the surface low moves eastward
through southern MO. The showers and storms should shift southeast
of the taf sites by late afternoon or early Monday evening with
improvement of the cloud ceiling into the MVFR catagory.

SPECIFICS FOR KSTL: Showers and thunderstorms across eastern portions
of KS and OK and west central and southwest MO, ahead of an
approaching upper level and surface low will spread into the STL
area early Monday morning. The cloud ceiling will drop down into
the IFR catagory as the low levels of the atmosphere saturate. The
surface wind will become easterly as the surface low approaches
southwest MO late tonight, then back around to a northwest
direction Monday afternoon as the surface low moves eastward
through southern MO. The showers and storms should shift southeast
of the STL area by early Monday evening with improvement of the
cloud ceiling into the MVFR catagory.

GKS

&&

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

$$

WFO LSX


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