Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171145

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
645 AM CDT Fri Mar 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 339 AM CDT Fri Mar 17 2017

Main concern today will be rain chances.

Latest surface analysis showed a surface low over western Iowa with
a warm front extending southeastward from the warm front into
eastern Missouri and southern Illinois.  A cold front also extended
southwest from the surface low into the central Plains. Temperatures
early this morning are running 5-10 degrees above normal.  Latest
radar mosaic shows some weak returns already across northeast
Missouri and west central Illinois.  These returns are occurring in
an area of low level moisture convergence under some modest mid
level ascent ahead of a weak shortwave trough.  Expect scattered
showers to be on the increase across the entire area through mid
morning as this forcing increases.  Eventually the low level
moisture convergence will shift eastward and and best chance for
showers will shift south and east of the CWA by late morning and
early this afternoon.  The cold front will enter northeast and
central Missouri by late this morning and move southeast across the
area, reaching southeast Missouri and south central Illinois by 00Z.
Isolated thunderstorms will be possible over parts over southeast
Missouri into far southwest Illinois where the RAP and GFS are
showing some modest values of MUCAPE.  The chance of rain will end
by early this evening as the cold front moves south of the CWA and
drier moves in.

Went close to agreeable MOS guidance for highs today ahead of the
cold front.   Lows tonight will fall back close to mid March normals
behind the front.


.LONG TERM...  (Saturday through Thursday)
Issued at 339 AM CDT Fri Mar 17 2017

High pressure over the central and northern Plains will build
southeast across the Midwest and into the Ohio Valley.  This should
provide quiet and seasonably cool weather for Saturday and warming
temperatures on Sunday as return flow develops over Missouri and
Illinois on the west side of the ridge.  Most short range guidance
is picking up on some low level moisture convergence on the nose of
a 20-30kt low level jet behind the retreating ridge.  Most also
print out a few hundredths of an inch of precip west of the
Mississippi so think slight chance/chance PoPs continue to be

Medium range guidance starts to diverge early next week with the
next cold front passage during the Monday-Tuesday time frame. The
week will start out warm Monday with temperatures running 15 to 20
degrees above normal in southwest flow.  The GFS is faster pushing
the next cold front all the way through the area by late Monday
afternoon while the ECMWF is slower as it develops a wave on the
front which delays the FROPA by 6+ hours. I`ve leaned a bit toward
the ECMWF in this case and am therefore keeping a chance of
thunderstorms in the forecast for Monday night. Northeast flow,
clouds, and some lingering post-frontal precip will keep Tuesday`s
temperatures down to near normal levels in the 50s.

High pressure will once again build southeast from the northern
Plains Wednesday with return flow developing Wednesday night or
Thursday.  Along with the return flow will come another slug of low
level moisture convergence and a chance for more precip Wednesday
night into Thursday.  Temperatures for Wednesday look to remain near
normal in the 50s until the return flow kicks in.  Should see a
pretty rapid warm up on Thursday in south-southeast flow ahead of a
rapidly developing low in the Great Plains.  Ensemble guidance has
highs rising into the 60s Thursday, but this may not be warm enough.
Didn`t bump up over guidance this morning since it`s so far out in
the forecast, but may have to go warmer if we maintain this



.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 630 AM CDT Fri Mar 17 2017

MVFR ceilings with scattered showers and isolated thunderstorms
are expected this morning ahead of a cold front that will move
southeast through the area. Drier air will move in behind the
front allowing for VFR and dry conditions by early afternoon.

SPECIFICS FOR KSTL: MVFR ceilings with scattered showers are
expected at the terminal through 19Z. A cold front will move
through the terminal shortly thereafter allowing drier air to move
in causing dry and VFR conditions the rest of the period.





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