Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200438

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1138 PM CDT THU MAY 19 2016

Issued at 933 PM CDT Thu May 19 2016

Forecast appears to be on track as showers have moved into western
MO this evening ahead of the upper low currently over the Central
Plains. Radar trends and HRRR reflectivity still suggest that
showers will spread into central MO later this evening and into
eastern MO as well as west central and southwest IL by late
tonight. Temperatures will likely fall into the low 50s as the
showers move into the area.



.SHORT TERM...  (Through Late Tonight)
Issued at 326 PM CDT Thu May 19 2016

Small but potent upper level storm system located over western OK
and southwest KS has resulted in a wet day for many areas in
southern KS with cloud cover associated with it as far east as our
region.  Over our way, it has been continued cool for late May with
temperatures hovering in the upper 60s and lower 70s with a light
easterly wind.

The clouds from the system in southern KS are expected to increase
and thicken tonight with rain chance onset timing looking faster
than it did 12-24hrs ago, with the upper level storm system tracking
to the MO-KS line by 12z/Fri and should yield decent rain chances
over the western half of MO with somewhat less towards the MS river.
Instability looks quite limited so kept pcpn types to just showers.
Min temps will be higher than persistence (in the lower to mid
50s) due to increased cloud cover.

The upper level storm system itself crosses thru on Friday,
especially during the afternoon hours, but in a weaker state.
Its timing will nonetheless be good though with it happening during
peak heating and should yield a new round of scattered SHRA over
most areas. Temps should stay close to persistence, with a slightly
warmer airmass being offset by more clouds and better rain chances.


.LONG TERM...  (Friday through Next Thursday)
Issued at 326 PM CDT Thu May 19 2016

Rain chances should be coming to an end Friday evening as
the midlevel shortwave moves southeast and the low level circulation
moves away from the area. Model guidance continues to spit out a few
hundredths of QPF over southern portions of the CWFA Friday
evening...though would think east-northeast flow out of the surface
high would tend to dry out the lower atmosphere.  Will keep PoPs in
the unmentionable category at this time, but am tempted to add in
some sprinkles to cover the possibility of some very light precip.

The high pressure system over the Great Lakes will build back to the
southwest on Saturday as the mid and upper level longwave pattern
amplifies sharply over the CONUS.  Models are showing another
shortwave moving southeast out of the Upper Midwest into the Mid
Mississippi Valley Saturday afternoon and Evening ahead of the upper
level ridge.  There`s some fairly impressive lift in the 600-300mb
layer associated with the wave but low level ridging is showing very
strong subsidence.  Think it`s very unlikely that any precipitation
will reach the ground due to the strong subsidence and dry lower
levels.  The upper ridge builds overhead Sunday and with ridging
through the depth of the tropopause it should be a sunny and dry day.

Medium range models seem very consistent with moving the upper level
ridge east while weakening it early next week.  This will allow
southerly flow to redevelop over the Mississippi Valley.  This will
allow low level moisture to flow back up into the region from the
Gulf of Mexico.  The GFS is faster today than it was yesterday...
bringing mid 60s dewpoints up into central Missouri by the end of
the day Sunday.  This looks too fast given the strength and breadth
of the surface ridge which should still be pushing dry air southwest
as far as Louisiana and eastern Texas.  The ECMWF looks much more
realistic keeping the 60s dewpoints out of the area until Monday.
I`m still concerned that all guidance might be too fast breaking the
omega block over the area...but as I mentioned above, they`ve been
pretty consistent.  PoPs for Monday are low confidence but possible
if the ridge clears out as fast as guidance is currently
advertising.  Still looks like another wet period for Tuesday
through Thursday with the CWFA stuck in low level southwest flow.
Tough to pick one period that might be wetter than the next as
forcing is rather ambiguous.  It might come down to how much
convection can develop during peak heating and then what carries
over into the night time when the nocturnal low level jet develops.

Expect temperatures will remain near or a few degrees below normal
through Sunday. The southerly flow which develops early next week
will push temperatures up above normal into the low to mid 80s.



.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1136 PM CDT Thu May 19 2016

Scattered showers are now moving into central MO which will affect
KCOU. These showers will spread eastward the rest of the night
into tomorrow morning affecting KUIN and the St. Louis area
terminals between 09-13Z. The showers will last into Friday
afternoon before exiting the area to the east. While conditions
are expected to be mainly VFR in the showers, some MVFR ceilings
and visibilities will be possible. Winds will remain out of the
east at less than 10kts.

Specifics for KSTL: Scattered light showers currently over south
central MO will move into the terminal after 09Z. While conditions
are expected to be mainly VFR in the showers, some MVFR ceilings
and visibilities will be possible. Winds will remain out of the
east at less than 10kts. The rain will move out of the area by 00Z
Friday evening.





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