Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 291132
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
632 AM CDT Wed Mar 29 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 335 AM CDT Wed Mar 29 2017

...A Few Strong to Severe Thunderstorms Possible Tonight...

The synoptic pattern early this morning features a strong upper-
level low across the Southern Plains. This feature will be the
main focus of the period as it will bring a couple rounds of
showers/storms to the region through tonight.

The first round of showers/isolated thunderstorms is currently
pushing across northeast OK/southeast KS and southwest MO. This
convection is tied to strong isentropic ascent on the nose of a
40-50 knot low-level jet as sampled on the Tulsa, OK radar. In
addition, a MCV was present across eastern KS, which will lift
northeast across northern MO this morning. Despite the low-level
jet weakening with time this morning, the approaching MCV will
likely be enough to force showers and a few isolated thunderstorms
across portions of central and northeast MO this morning into
early this afternoon. There is some question as to how far east
these showers will be able to get given less forcing and drier
low-level air with eastward extent, thus have lowered pops a bit
across eastern/southeast MO and southwest IL.

Despite a few spotty showers this afternoon, think this afternoon
will largely be dry as we sit in weak mid-level subsidence behind
the first departing wave. However, attention will turn to the
west/southwest late this afternoon and into this evening. The
latest guidance suggests that portions of southeast KS/northeast
OK/and southwest MO will destabilize sufficiently this afternoon
with SBCAPEs rising to 1000-1500 J/kg just to the south of a warm
front sharpening from west central MO eastward into southern MO.
Severe thunderstorms, possibly initiating as supercells, are
expected to develop over the western SGF CWA and areas to the
south where this instability will be co-located with very
favorable shear parameters. However, as the evening wears on, deep
layer shear vectors will back and become more parallel to the
initiating boundary, thus supporting a transition to more of a
linear/cluster storm mode. The latest hi-res guidance is in fairly
reasonable agreement that the surface warm front will struggle to
lift northward this evening into tonight (possibly slowed by the
terrain of the Ozarks), which will limit any appreciable surface-
based instability ahead of these storms to south-central and
southeast MO. Therefore, think any damaging wind potential or a
possible isolated tornado (given locally backed winds along the
warm front) will be limited to our far southwestern/southern
counties (Moniteau, MO to Madison, MO). The large hail potential
may extend a few counties north of that, as storms cross the
boundary and slowly weaken as they move northeast.

Despite the expected progressive nature of these storms, some
locally heavy rainfall will be possible as a 50+ low-level jet
focuses moisture convergence into portions of southeast MO and
southwest IL overnight. Could see one to locally two inches of
rainfall in this region late tonight into Thursday morning (with
additional rainfall expected on Thursday, see discussion below).

KD

.LONG TERM...  (Thursday through Tuesday)
Issued at 335 AM CDT Wed Mar 29 2017

Recent model cycles have shown a much slower progression for the
aforementioned low pressure system which will have been
responsible for Wednesday night`s SHRA/TSRA. The upper low will
be centered near the KS/MO border at 12z Thursday, which means
that a larger portion of the LSX CWA will now be in a favorable
location for severe weather on Thursday afternoon compared to
forecasts from 24hrs ago. Instability will depend to some extent
on how much clearing occurs ahead of the cold front, but
decreasing mid-level temperatures with the approach of the
mid/upper low will still contribute to overall instability. H7-H5
lapse rates are forecast to be around 6-6.5 deg C/km, and model
forecasts of 0-6km shear of around 50kts will be sufficient for
rotating updrafts. Forecast values of 0-1km shear are fairly low
(10-15kts at best) across the LSX CWA until after 00z when they
increase to around 20kts across the eastern and southeastern part
of the CWA. Taken together, this suggests that the primary severe
weather threats will be hail and wind, but the environment could
become slightly more favorable for isolated tornadoes with time,
especially if more clearing occurs and diurnal heating causes
instability to be greater than currently forecast. Any lingering
precipitation behind the departing low pressure system should move
out of the area on Friday morning.

The cold front associated with this low pressure system will move
across the area on Thursday night, and temperatures on Friday will
be much cooler compared to the previous few days. Highs on Friday
may struggle to rise above 50 degrees across the northern CWA,
and most areas will see highs in the low to mid 50s, which is 5-10
degrees below normal for this time of year. Overnight lows on
Friday night will dip into the upper 30s across the northern CWA,
but no sub-freezing temperatures are currently expected.

Shortwave ridging will keep dry weather in place through the first
half of Saturday evening before another low pressure system begins
to influence the region. The 29/00z model runs show better agreement
between the GFS and ECMWF regarding the track of this feature
compared to 24hrs ago, but there is still poor agreement regarding
the evolution of other upstream disturbances.

Kanofsky

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 622 AM CDT Wed Mar 29 2017

Main focus of the TAF period will be on cigs and shower/storm
chances this morning and again tonight. MVFR cigs continue to
plague mainly COU/UIN this morning, with drier air having worked
into SUS/STL/CPS allowing for VFR cigs. Expect mainly VFR cigs to
persist in the St. Louis metro sites through much of the day
today, although brief forays into high-end MVFR will be possible
at times. A band of showers and a few thunderstorms will affect
COU/UIN this morning, and a few of these showers could also
briefly affect the St. Louis metro sites as well, before dry
conditions move in for much of the afternoon.

By this evening and into tonight, a line of showers/storms will
move from west to east through the region. There are still some
uncertainties on timing/strength of this line, but a period of
lowered cigs/vsbys will be possible as the storms move through.
IFR cigs then look likely towards the end of the period even after
the showers/storms have tapered off.

SPECIFICS FOR KSTL:
VFR conditions should persist through much of the day today,
although some high-end MVFR cigs will be possible at times. After
a brief shower chance late this morning, much of the afternoon and
early evening will be dry with easterly winds of 10-15 knots with
gusts to around 20 knots. A line of showers/storms will then
approach late in the period bringing reduced cigs/vsbys. Specific
timing/strength of this line is still uncertain so will just
introduce VCTS for now.

KD

&&

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

$$

WFO LSX


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