Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231200

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
700 AM CDT Thu Mar 23 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 332 AM CDT Thu Mar 23 2017

Low pressure will develop over the Great Plains today as a strong
high over the Great Lakes moves off the East Coast.  The developing
trof is producing increasing low level southwest flow, and this is
brining moisture up into the area.  Moisture convergence and
isentropic lift is already producing isolated showers of the
Mississippi.  Expect this activity will continue to develop, become
more widespread, and refocus further north after sunrise where the
low level jet and moisture convergence is strongest.  Could also
hear a few rumbles of thunder as the RAP is showing 200-600 J/Kg of
MUCAPE across most of the area through the morning.  Showers should
dissipate by early afternoon as a slug of drier air pushes up into
the area from eastern Oklahoma/Texas.  The drier air should clear
out the clouds this afternoon and temperatures should jump pretty
quickly into the 60s and low 70s west of the Mississippi.  Eastern
zones will stay cloudier longer and won`t have as much  time to warm
up so will likely stay in the 50s.

Tonight looks quiet and mild with southerly flow continuing.
Temperatures will likely drop off a few degrees in the evening, but
will probably steady off after midnight in the upper 50s.  There
could be a few showers over the eastern Ozarks before 12Z as the low
level jet increases to around 50kts.  Some moisture convergence
showing up down there in the models, but baroclinicity is lacking
(unlike this morning).  Have therefore opted to keep the forecast
dry through the night for now.


.LONG TERM...  (Friday through Wednesday)
Issued at 332 AM CDT Thu Mar 23 2017

Active period expected over the next 7 days with a pair of closed
lows moving across the Mississippi Valley.  One low will bring a
round of showers and thunderstorms to the area Friday night into
Saturday night with the next one moving through early next week.

NAM/GFS/ECMWF are showing decent agreement that the first upper low
currently over northern California will move steadily eastward into
the high Plains by Friday morning.  There may be some scattered
showers early in the day as moisture transport increases ahead of
the upper level low.  Better chances for showers and thunderstorms
will occur later on Friday afternoon and Friday night when both the
GFS/NAM shows a band of strong low level convergence moving out
ahead of the cold front.  This low level forcing combined with
strong mid-level ascent from the approaching trough warrants likely
or categorical PoPs on Friday afternoon into Friday night. It still
looks like the attendant cold front with upper low will not pass
through until Saturday.  With the upper low moving over the area,
there is still the potential for some modest instability to develop
near the low, and ahead of the cold front.  As mentioned in previous
discussion, will need to monitor the possibility for low-topped
supercells capable of producing all hazards near the path of the
upper low with more of a hail threat along the cold front during the
day on Saturday.  Latest SPC Day 3 outlook has a marginal risk over
parts of the area.

The upper low will begin moving east of the area on Saturday night
and a shortwave ridge will move across Missouri and Illinois on
Sunday.  This will cause rain to end from west to east Saturday
night and Sunday morning with a break in the precipitation Sunday
afternoon and Sunday evening.  Then the GFS and ECMWF are showing
the next upper low moving across southern Missouri Monday and Monday
night. Ahead of it, strong low level moisture convergence and
instability will cause showers and thunderstorms to develop across
the entire area late Sunday night and Monday.  This upper low will
open up and move out by Tuesday and an upper ridge will move in
leaving Tuesday and Tuesday night dry.

The ECMWF and Canadian are quicker bringing the next southern stream
system into Missouri and Illinois mid-late next week compared to the
GFS. Have added a chance of showers and thunderstorms on Wednesday
as this third system moves into the area.

Temperatures will be mainly above normal this weekend and next
week based on the GEFS mean MOS.



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 631 AM CDT Thu Mar 23 2017

A broad area of showers is moving east through central and
southern Illinois. Another smaller area of showers with embedded
thunderstorms is moving east through eastern Missouri. This
precipitation is expected to continue this morning. VFR conditions
are prevailing in this precipitation, but brief MVFR visibility is
possible with the thunderstorms in the second area of rain. An
area of MVFR ceilings with bases just below 3000 FT in northeast
Missouri will spread into west central Illinois this morning...but
should lift northeast quickly. Rain and thunderstorms this morning
will dissipate by early to mid afternoon. VFR flight conditions
are expected to prevail thereafter. Low level wind shear
conditions are expected to develop this evening with a southerly 40-50kt
low level jet at 1000-2000ft. There is a chance some MVFR ceilings
could develop before 12Z Friday across southern Illinois and the
eastern Ozarks.


An area of showers and thunderstorms will affect the terminal over
the next hour. Visibility could briefly be reduced to MVFR...but
this is a low probability. 6 mile or greater visibility is much
more likely as the heavier rain looks to pass south of the
terminal. VFR conditions are expected to prevail after the rain
moves east of the terminal and ceilings are expected to clear out
this afternoon. Low level wind shear conditions are expected to
develop this evening with a southerly 40-50kt low level jet at
1000-2000ft. Wind shear conditions will diminish after 14Z Friday.





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