Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200255
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
955 PM CDT Wed Apr 19 2017

.UPDATE...
Issued at 944 PM CDT Wed Apr 19 2017

There has always been some question on the coverage of storms
overnight and there is even more so now. The upstream coverage of
storms from southwest IA into northeast KS along the advancing
cold is scattered at best, and these are occurring within the best
surface-based instability. Further east and central and northeast
MO the instability has diminished significantly with the loss of
heating. Current thinking is that there remains a threat of
scattered showers or thunderstorms across parts of central and
northeast MO overnight thru daybreak in association with the
advancing cold front, weak instability, and a veering
southwesterly LLJ. I added some slight chance POPS on an earlier
update into east central MO and parts of southwest IL after around
09Z thinking WAA and moisture convergence attendant with the
veering LLJ could initiate some spotty activity. I think the best
threat of showers and thunderstorms will probably be after 12Z in
a corridor from central MO east and southeastward. This potentially
could be associated with the residual activity now in central KS as
it progresses eastward, as well as new development in the post-
dawn hours along and ahead of the cold front.

Glass

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 347 PM CDT Wed Apr 19 2017

Unseasonably warm weather this afternoon with gusty southerly winds
and a diurnal cumulus field, more typical of an early summertime
pattern. We are getting isolated showers across parts of east
central and southeast MO and southwest IL.  The diurnal cumulus
clouds along with any isolated showers will dissipate by early this
evening as the sun goes down.  More significant showers and storms
will move into parts of northeast and central MO and west central IL
late tonight and early Thursday morning as an upper level trough
moves eastward through the northern Plains and sends a cold front
southeastward into our forecast area.  May have some severe storms
across northwest MO tonight, but these will likely weaken in
intensity as they get into our forecast area late tonight.
Unseasonably warm temperatures will continue tonight with a
southwesterly low level jet translating eastward into our region
along with southerly surface winds.  The convection will redevelop
late Thursday morning and afternoon across southeast MO and
southwest IL as the cold front continues moving southeastward
through our forecast area. Temperatures may be nearly steady or fall
slightly Thursday afternoon across parts of our area due to low
level cold air advection behind the cold front along with low level
cloud cover and precipitation.

GKS

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 347 PM CDT Wed Apr 19 2017

A potentially active west-southwest flow aloft will continue for
much of this period thru the middle of next week.

The main wx-maker will initially be a surface cold front that will
be ongoing thru areas south and east of STL metro on Thursday
evening and should exit just to the south of the CWA later that
night.  However, the baroclinic zone aloft will be maintained thru
southern MO and far southern IL thru the night and into Friday
morning to justify some mentionable PoP in those areas.

The main wx-maker of this period will be in the form of an upper LO
that will slowly traverse thru our region Friday night thru Saturday
night.  It will overspread rain by late Friday night, continuing
thru Saturday morning before beginning a measured retreat during the
afternoon from northwest to southeast, finally exiting Saturday
night.  The situation is such that with a surface LO track expected
to the south of our region thru northern AR, a stout NE flow at the
surface, extensive thick cloudiness and high probs for rain, diurnal
temp swings will be minimal.  In fact, temps do not look to exit the
40s for many areas on Saturday during the daytime.

The storm system should be sufficiently far away on Sunday to result
in a dry day, although some model guidance is suggesting a slower
exit and could see another cool day if occurs with low cloud likely
lingering as well.  Either way, it will not be as cool, with temps
likely to make a recovery back into the 60s.

A surface RIDGE will dominate Monday with a frontal boundary
slipping southward into our area on Tuesday, but general model
consensus is on a very limited moisture return and have kept this
period dry with this package.  Max temps will continue to recover
back into the 70s by early next week, especially with flow from the
south.

The next system looks slated for late Wednesday and by this time
moisture looks sufficient to justify at least a climo PoP for many
areas, and something more further to the west.

TES

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 640 PM CDT Wed Apr 19 2017

VFR flight conditions are expected this evening as diurnal cu
dissipates and high clouds begin moving into the area. During the
overnight hours through Thursday morning, the residual showers
and thunderstorms occurring across the Plains this evening will
move into the area ahead of an advancing cold front. The main
uncertainty at this time is the coverage of this residual
activity and any potential additional development, and therefore
the exact terminal impacts. The new TAFS reflect the best estimate
of when showers and possibly thunder will impact the terminals,
however flight conditions could certainly be lower. Winds will
shift to the west-northwest with the passage of the cold front and
there will also be a period of MVFR CIGS.

SPECIFICS FOR KSTL:

VFR flight conditions are expected this evening as diurnal cu
dissipates and high clouds begin moving into the area. Clouds will
increase overnight with scattered showers possible as early as
09-10z. Later on Thursday morning, residual showers and
thunderstorms will move into the area ahead of an advancing cold
front. The main uncertainty at this time is the coverage of this
residual activity and any potential additional development, and
therefore the exact terminal impact. The new TAF reflects the
best estimate of when showers and possibly thunder will impact
KLSX, however flight conditions could certainly be lower. The cold
front will bring a wind shift to the west-northwest along with a
period of MVFR CIGS.

Glass

&&

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

$$

WFO LSX



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