Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 061748
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1248 PM CDT Sun Jul 6 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 352 AM CDT Sun Jul 6 2014

Weaker convection across southwest IL, east and southeast of STL
associated with a weakening MCV shifting east of the area should
weaken and shift east southeast of the forecast area by 12z Sunday.
Stronger convection, developing MCS across northwest, north central
and west central MO associated with low level warm air advection and
moisture convergence on nose of w-swly low level jet should
gradually shift into portions of northeast and central MO later this
morning as the low level jet weakens and veers.  This convection
should weaken later this morning as it moves eastward into more
stable air.  Some uncertainty as to the location of new convective
development this afternoon, but it appears that the most likely
location for more organized and possibly widespread convection may
be across west central IL ahead of an MCV from the current MCS and
ahead of a weak warm front currently between IRK and COU.  High
temperatures should be warmer than yesterday due to gradually rising
850 mb temperatures and southwesterly surface winds.  Humidities
will also be rising as well.

GKS

.LONG TERM:  (Tonight through Saturday)
Issued at 352 AM CDT Sun Jul 6 2014

Several rounds of showers and thunderstorms should threaten the
region Tonight-Tuesday. Given the somewhat complex nature of the
pattern over the region, there is fairly decent consensus in the
synoptic scale models WRT the general evolution of the storm threat;
this evolution also dovetails pretty well with going forecast trends
(with some, and generally minor, adjustments).

While there may be a few lingering storms over the CWA very early
this evening, the primary threat tonight in our CWA should come
from storms that develop late this afternoon along cold front over
IA. These storms...or additional development in the vicinity of ne
MO/se IA from low level jet...should affect northern sections of
the CWA late in the evening/overnight. By the time the storms
reach our FA, would think that a linear storm mode will be the
most likely scenario, with the threat of damaging winds as well as
some hail. These storms should track southeast Monday morning,
with some redevelopment Monday afternoon along the first weak cold
front pushing into and then stalling over the area, especially
over southeast sections of the CWA where the instability should be
the greatest.

Monday night into Tuesday will likely be the most active time period
in the short range.  Approach of next shortwave, along with
convergence and overrunning of very unstable AMS (MUCAPES forecast
aoa 4000 j/kg) into the stalled frontal boundary, should cause a
large thunderstorm complex to form Monday night over northern
sections of the CWA. Storm propagation vectors indicate that this
large complex should charge southeast across the CWA during the
morning, with additional storms forming along the primary cold
front (that will be ushered into the area by the aforementioned
shortwave) during the afternoon.

Heading into midweek, the latest 00z guidance is following trends of
the previous medium range solutions in being more aggressive with
the southward push of the cold front, which in turn should cause the
shower and thunderstorms to wind down Tuesday evening over southern
sections of the CWA.   The stronger southward push of the Canadian
airmass should mean dry weather on Wednesday and Wednesday night.
However, slight chance/low chance PoPs will slowly return to the
region Thursday and Friday, and continue into Saturday, as Canadian
airmass retreats to the east, allowing warmer and more humid air to
push back into the mid-Mississippi Valley.

Truett

&&

.AVIATION:  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1235 PM CDT Sun Jul 6 2014

Another tricky forecast when it comes to convective trends this
forecast period. Activity from this morning has all but diminished
across the area, however, a disturbance that will move through the
region this afternoon could initiate some diurnal
showers/thunderstorms that could impact metro area TAF sites.
Given uncertainties in timing and the exact location of any storms
that develop, have maintained VCTS mention for KSTL, KSUS, and
KCPS this afternoon. A front to the north of the area will sag
southward overnight tonight, with showers and thunderstorms
expected to impact KUIN by 06Z. Uncertainty remains regarding how
far south any nocturnal activity will make it, though models
indicate at least some light precipitation across central and
eastern Missouri by daybreak. Thus, have mentioned VCSH for KCOU
and metro TAF sites in the 09-13Z time frame, though this trend will
likely need revisited in future TAF issuances. Otherwise, VFR
conditions will prevail outside of any storms, with winds
gradually veering more southwesterly through the period.

Specifics for KSTL:
Currently expect VFR conditions to prevail through the forecast
period. There is quite a bit of uncertainty regarding convective
trends, with the possibility of diurnal shower/thunderstorm
development later this afternoon. Have maintained the VCTS mention
from 21-00Z, as the timing and exact location of any storms cannot
be pinned down at this time. The other concern is that of
precipitation that could impact the terminal after roughly 09Z
tonight, associated with a cold front that will slowly sag into
northern Missouri overnight. While it appears a bulk of the
precipitation would remain north of KSTL, models are indicating
at least some light precipitation by daybreak, hence have included
VCSH mention from 09-13Z. This will likely need refined in future
TAF issuances. Otherwise, southwesterly winds will continue to
slowly veer, becoming west-southwesterly by the end of the period.

JP

&&

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

$$

WFO LSX





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