Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 240910
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
410 AM CDT Fri Mar 24 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 409 AM CDT Fri Mar 24 2017

A PV anomaly was noted near the OK/TX panhandles at 08z as per water
vapor imagery and hourly RAP analysis of the 1.5 PVU surface. The
closed low pressure system associated with this feature will
continue moving slowly eastward over the next 24-36 hours before
drifting northeastward towards the Great Lakes. This feature will be
the primary driver of sensible wx trends for the LSX CWA today and
tonight.

Little precipitation is expected during the day due to the slow
eastward motion of the low pressure system. However, models do
depict mixing to near or slightly above 850 hPa, and the stronger
winds aloft ahead of the system should mix down to the surface and
produce breezy conditions today within the broad warm sector. The
combination of a warm start to the morning, good diurnal mixing, and
persistent low-level WAA due to southerly winds will produce high
temperatures in the mid to upper 70s across the area. It would not
be surprising to see some spots reach 80 degrees today, but the
presence of mid/high clouds streaming into the region suggests that
seeing areawide temperatures in the upper 70s is the more likely
outcome.

By late afternoon, the surface cold front should be approaching the
western edge of the LSX CWA, and a band of SHRA/TSRA is expected to
move across the area overnight. The presence of 40-50 kts of 0-6 km
shear suggests that isolated strong to severe thunderstorms are
possible. Areas of precipitation will continue into Saturday morning
as the upper low moves into the region.

Kanofsky

.LONG TERM...  (Saturday through Thursday)
Issued at 409 AM CDT Fri Mar 24 2017

The synoptic pattern at the start of the long term period will
feature an impressive upper-level low near the KS/OK border, cutoff
from the main westerlies displaced well to the north across southern
Canada.  The passage of this upper-level feature will be the first
of many through the long term period, as a very active Pacific jet
continuously brings systems onto the West Coast, across the Plains
and into the Midwest.  These multiple systems will make for an
active and stormy long term period.

The main focus of the period continues to revolve around the
potential for severe convection on Saturday afternoon into early
Saturday evening.  The latest guidance continues to suggest that the
mid-level dry slot of the mature, vertically-stacked cyclone will
push into the region Saturday morning.  This descending dry air may
be enough to allow for some breaks in the cloud cover Saturday
morning, which should help develop some surface-based instability as
the central, cold core of the upper-level low (500mb temps around
-20C) moves overhead.  Just how much instability is able to develop
is where the uncertainty in the forecast lies, with more solar
insolation resulting in a higher severe threat, and less sun/more
clouds resulting in a lowered threat.  The latest guidance does
suggest we will see anywhere from 250-1000 J/kg of SBCAPE by early
afternoon. Favorable deep-layer shear profiles will support the
development of low-topped supercells (equilibrium levels likely only
around 25k feet) or perhaps small bowing segments capable of large
hail and perhaps some locally damaging winds. While the main core of
the strong low-level wind fields (850mb jet) will have pushed into
central/eastern IL by Saturday afternoon, the proximity of the
surface low and its associated boundaries/backed winds will allow
for a conditional tornado threat in storms that are able to form to
the northeast of the surface low.  The latest SPC Day2 Outlook keeps
the region in a Marginal risk, but this could be upgraded in later
forecasts as instability trends become more clear.  Stay tuned.

Any thunderstorm activity will come to an end overnight Saturday,
with perhaps a few lingering showers into the first half of Sunday.
However, the bulk of Sunday will be dry as shortwave ridging moves
across the region.  However, by Sunday night into Monday, the next
system will be quickly approaching from the Plains.  This will
spread renewed showers and thunderstorms across the region Sunday
night through Monday, before activity tapers off Monday night.

After another period of shortwave ridging and dry weather Tuesday
into Tuesday night, the next in the series of upper-level systems
will approach Wednesday into Thursday.  Guidance is a bit displaced
with regards to timing and surface low location with this system,
but enough agreement is there to warrant likely pops Wednesday night
through Thursday.  While we certainly don`t need any severe weather,
this active pattern will be highly beneficial to many areas that
have been very dry over the past few months. Over the next 5 to 7
days, the area could see anywhere from 2 to 4 inches of much needed
rainfall.

KD

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1155 PM CDT Thu Mar 23 2017

VFR conditions will prevail for the TAF sites for much of the
first half of the valid period. LLWS will be the main issue
overnight with a 45+ kt low level jet overhead with a base at
about 1600ft AGL. Low stratus cloud potential to form towards
daybreak Friday morning looks to be more to the east now but
should still see it expand as a low-VFR deck during Friday
morning. The main band of TSRA will develop over western MO by
midday Friday and propagate eastward, reaching COU by mid-late
afternoon and STL metro and UIN by early evening Friday. Big
question mark will be to what extent low clouds develops Friday
night and have played it conservatively for now with very
inconsistent guidance currently.

TES

&&

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

$$

WFO LSX



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