Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251153
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
653 AM CDT Sat Mar 25 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 348 AM CDT Sat Mar 25 2017

A closed low pressure system will drift from eastern OK towards the
MO/AR border today before lifting northeastward and passing over the
St. Louis metropolitan area around 00z. The system eventually
rejoins the flow as an open wave very late tonight/early Sunday.

Periods of showers and thunderstorms are expected today with the
approach and passage of the closed low pressure system. From a
severe wx forecasting point of view, an interesting aspect of this
system is that the temporal trends of instability and shear are in
opposition from approximately 25/12z - 26/00z. Instability will be
increasing today due to increasing H7-H5 lapse rates (~6-7 deg C/km)
when the core of cooler air aloft moves over the region. Scattered
diurnal heating within any breaks in the clouds, including within
the dry slot, will also increase the instability. However, values of
0-6km shear will be decreasing with time between 25/15z - 26/00z.
Models show that the period with the highest values of 0-6km shear
(30-50 kts) will be approximately 15-18z or perhaps 15-21z. Values
of 0-1km shear also decrease during the day but then increase again
after 26/00z. It`s possible that severe weather could occur earlier
than expected for any storms which develop earlier in the day when
there is greater overlap between 0-6km shear and instability. The
most likely time frame remains the afternoon hours when instability
will be at its peak. There may also be a separate period of
increasing severe wx potential near and after 00z across the eastern
half of the CWA when 0-1km shear begins to increase again.
Precipitation should taper off this evening with the passage of the
surface cold front.

Conditions today should be warm and breezy across most of the area,
but neither as warm nor as breezy as yesterday. The difference is
mainly due to increased cloud cover, periods of precipitation,
poorer mixing, and weaker winds aloft. The LSX CWA remains in the
warm sector until the surface low passes overhead and the N-S
oriented cold front moves across the area tonight. Overnight lows
should be around 5-10 degrees cooler than last night.

Kanofsky

.LONG TERM...  (Sunday through Friday)
Issued at 348 AM CDT Sat Mar 25 2017

The synoptic pattern at the start of the long term period will
feature two upper troughs of note, one lifting northeast across
IL/IN and another ejecting into the Southern Plains.  This active
wavetrain of troughs moving through the region will continue through
the period due to the seasonally strong/active Pacific jet, keeping
several chances for showers/storms in the forecast through next week.

In between the two troughs, shortwave ridging will dominate for much
of Sunday, keeping much of the region dry after a few morning
showers to start the day.  It will be slightly cooler behind the
departing upper-level low and associated cold front, with highs in
the upper 50s to mid 60s.

However, the next in the series of troughs/PV anomalies will slide
into the region late Sunday night into Monday.  The associated
surface low will push through southeast MO.  Along the track of this
surface low, enough surface-based instability may build to bring the
threat for a few stronger storms to portions of southeast MO into
southwest IL, but it appears the better instability/upper-level
dynamics will be just to the southeast of the region.

Conditions will then briefly dry out for Tuesday as upper-level and
surface ridging take hold.  However, the pattern will continue to
remain progressive, allowing for the next in the series of upper-
level waves to impact the region Wednesday and especially into
Thursday.  Guidance differs with the strength/amplitude of this
wave, with the GFS weaker/flatter while the ECMWF is stronger/more
amplified.  Regardless of the exact solution, there is enough
agreement to warrant fairly high pops Wednesday night into Thursday.

The broken record forecast will continue, with a day of dry
weather expected on Friday, But yet another system looks to impact
the region next weekend, but again strength/timing differences
are fairly large in guidance thus uncertainty is high in the
details for next weekend.

KD

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 619 AM CDT Sat Mar 25 2017

Showers and thunderstorms are expected to redevelop today ahead of
an approaching low pressure system. Initially south to southeast
winds will become lgt/var invof the surface low center, then
become southwesterly to westerly towards the end of the valid TAF
period after a cold front has moved across the area. IFR to MVFR
conditions were occurring at TAF issuance at most of the terminals,
but conditions should improve this morning for several hours until
SHRA/TSRA redevelop later in the day. Towards the end of the valid
TAF period, MVFR to IFR ceilings are expected to move southward
towards the terminals around the back side of the departing low
pressure system.

Kanofsky

&&

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

$$

WFO LSX



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