Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 241935
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
235 PM CDT Thu Apr 24 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 234 PM CDT Thu Apr 24 2014

(Tonight)

Main focus through tonight will be cold front and associated risk for
showers and thunderstorms. Degree of instability is still a concern and
a source of uncertainty. Dewpoints have really struggled to climb so
far today due to persistent southeasterly surface flow. Winds are expected
to veer to the south/southwest as a subtle warm front lifts north to near
the I-70 corridor by 0000 UTC this evening. This should allow sfc dewpoints
to at least climb into the 50s across the southern half of the area. That
being said...instability is likely to be on the marginal side for severe
thunderstorms...on the order of several hundred J/kg. Wind shear will be
supportive however of organized convection over the southern CWA with
40+ knots of 0-6 km shear just downstream of approaching shortwave trough.
Regardless of the severity of storms tonight...believe widespread showers
and thunderstorms will affect the entire CWA due to very strong forcing for
ascent aloft via DCPVA ahead of vort max and jet coupling. This strong
upper-level support may help overcome the lack of instability and may
lead to more strong/severe storms with damaging straight-line winds being
the primary threat. Frontal timing and associated ending to precipitation
is even more delayed than this time yesterday...with frontal timing just
after 0000 UTC likely along the Mississippi River including the St. Louis
metropolitan area and around 0400 UTC for KSLO/K3LF.

Skies will quickly clear overnight tonight with northwesterly winds gradually
lightening up a bit. Look for lows in the 40s to near 50 degrees across the
area and went with a consensus of guidance for mins tonight.


Gosselin

.LONG TERM: (Saturday through Wednesday)
Issued at 403 AM CDT Thu Apr 24 2014

Guidance continues to come into better agreement for the weekend.
The secondary front which was forecast to sweep all the way
through Missouri into Arkansas earlier in the week is now only
going to get to southern Iowa according to the model collective.
Still showing a good low level baroclinic zone stretching from
southwest Missouri up into Iowa Saturday through Sunday with a
southwest low level jet ramping up to 40-50kts flowing
perpendicular to the thermal gradient. All models print out some
light precip in response to this on Saturday, primarily over
central and western Missouri but I wouldn`t be surprised to see it
a bit further north/northeast as well if this verifies.
Temperatures will be highly dependent on how much sunshine we end
up getting, but I leaned heavily on the warmer MAV numbers.

Strong warm advection continues ahead of the ginormous trof which
will be moving over the Rockies Saturday night, and it looks like
there could be some nocturnal convection as a a result...mostly
across northern sections of the CWFA.  The trof moves into the Great
Plains on Sunday and closes off while also becoming negatively
tilted.  The resulting wound up, slow moving storm system will draw
abundant moisture into the Mid-Mississippi Valley from the Gulf
producing at least a couple of rounds of showers and thunderstorms
Sunday into Monday night before the associated cold front comes
through and drier air works into the area.  Temperatures should stay
seasonably mild through the Monday with highs near normal.

Tuesday and Wednesday the storm system moves slowly overhead and
then on into the Ohio Valley.  Models print out widespread precip
over the area as cold air aloft moves in with the cut off low.
Heaviest amounts look to be across northern Missouri and southern
Iowa associated with the deformation zone.  While this makes sense
conceptually, still not sure I totally believe the medium range
models as this is a highly anomalous pattern for late April/early.
Stuck relatively close to guidance though as the mass fields on the
GFS and ECMWF are in very good agreement considering strength of
this system and its anomalous nature.

Carney

&&

.AVIATION:  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1216 PM CDT Thu Apr 24 2014

Round of pre-frontal precpitation beginning to affect KCOU and
should also affect KUIN with shra/embedded thunder possible.
Ceilings/visibilities may dip down into MVFR category early this
afternoon. Frontal timing a bit slower than earlier
forecast...around 2100 UTC at KCOU...2300 UTC at KUIN...and
0000-0200 UTC at metro TAF sites. More instability as you head
south...but round of thunderstorms expected at terminals with
brief periods of IFR visbys in heavy rain possible. Showers are
expected to linger behind front for a few hours. Winds will shift
to the northwest behind front with brief gusts along/just behind
front from 25-30 knots. Skies will quickly clear out overnight
tonight with winds backing to the west/southwest Friday.

Specifics for KSTL:

Round of pre-frontal is expected to weaken as it heads east toward
terminal so uncertain whether or not some light rain showers will
affect the site. Frontal timing a bit slower than earlier
forecast...around 0100 UTC. More instability as you head
south...but round of thunderstorms expected at terminal with
brief periods of IFR visbys in heavy rain possible. Showers are
expected to linger behind front for a few hours. Winds will shift
to the northwest behind front with brief gusts along/just behind
front from 25-30 knots. Skies will quickly clear out overnight
tonight with winds backing to the west/southwest Friday afternoon.


Gosselin

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     51  78  56  83 / 100   0   5  10
Quincy          46  76  52  78 / 100   0  10  10
Columbia        45  78  53  81 /  80   0  10  30
Jefferson City  47  79  53  82 /  90   0  10  30
Salem           50  76  52  80 /  80   0   5   5
Farmington      49  77  53  81 /  90   0   5  10

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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