Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181758

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1158 AM CST Sat Feb 18 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 358 AM CST Sat Feb 18 2017

Scattered sprinkles and light rain developed across the southern CWA
early this morning ahead of an approaching shortwave which was
located near the OK/AR border per early morning water vapor imagery
and RAP PV fields. Scattered sprinkles/light rain should continue
through the morning across the southern one-third to one-half of the
CWA as the shortwave lifts ENE. Precipitation should taper off
during the afternoon hours. An upper ridge then builds in
tonight behind the departing shortwave.

The 09z air temperature at St. Louis was identical to that of three
cities in Florida: Orlando, Tampa, and Melbourne. This is very
unusual for St. Louis in February. High temperatures will remain
well above normal today but should be a few degrees cooler than
yesterday due to increased cloud cover and scattered precipitation.
Overnight lows will also be unusually warm. In fact, tonight`s
forecast lows in the mid to upper 40s are right around the normal
highs for this time of year.


.LONG TERM...  (Sunday through Friday)
Issued at 358 AM CST Sat Feb 18 2017

Very warm temperatures are fcst to continue thru the week. It would
appears as if winter may be over but a strong late week storm
system will bring conditions back to reality for next wknd.

Sun - Sun night

High pressure at the SFC and aloft will combine for another very
warm day. SFC high will slowly move across the ern CONUS while the
upper level ridge moves overhead. The sthrly flow and 850 mb temps
of 10C-12C will help temps rise into the lower 70s just about
areawide. Remarkably...lows should hold in the 50s going into Mon

Mon - Tue night

The strong storm system that impacted CA on Fri will move into the
cntrl CONUS for early next week. The vort max will become sheared as
the system separates into nthrn and sthrn stream energy. The nthrn
stream short wave is expected to remain more open which means a
quicker mvmnt E when compared to the sthrn stream system that tries
to close off across the nthrn Gulf of Mexico by midweek. This
evolution is partly due to a strengthening downstream ridge that
will hamper eastward progression of the system and will cause the
vorticity to become more of a N-S narrow ribbon as it slides thru
the region. The trof...vorticity and assoc cold front move thru the
FA on Tue ending the precip threat from W-E as the short wave
progresses E. There may be just enough instability for a fee rumbles
of thunder. Weak high pressure tries to build in behind this system
but winds are already becoming sthrly again by Wed mrng. Temps will
once again be well above normal with most locations in the upper 60s
to lower 70s on Mon...though slightly cooler than on Sun for cntrl
and NE MO and W cntrl IL due to cloud cover and the potential for
rainfall late in the day. Most locales will remain in the 60s on Tue
due to even more cloud cover and an even better chance for precip.

Wed - Wed night

Weak SFC high moves SE of the FA drng the mrng with return flow well
established by aftn. This will allow for another very warm day with
highs in the upper 60s to lower 70s...which may not be quite warm
enough. A weak nthrn stream short wave tries to drop a cold front
into the region Wed night but it stalls out along the IA/MO border.
Meanwhile...another strong Pacific storm system will come onshore
along the W Coast Tue night into Wed. The short wave will induce lee
side cyclogenesis Wed/Wed night. The stalled cold front will
transition to a warm from by Thu mrng as the system in the cntrl
Plains begins to strengthen.

Thu - Fri

The short wave is fcst to strengthen as it approaches the region
driving a deepening SFC low from the cntrl Plains on Thu mrng to
the Midwest by Fri evng. Models have a general consensus on the
overall evolution of this system but of course vary in the
meaningful details. There should be some WAA precip along/N of the
warm front Thu. The bndry is expected to lift N of the FA by Thu the latest...with the cold FROPA Fri mrng. Fri will
likely not be a very nice day with a line of SHRAs and possibly
TSTMs accompanying FROPA and then falling temps behind the front
as strong CAA overspreads the CWA. The nthrn FA could get a little
light snow Fri night as wrap around moisture skirts nthrn MO and
cntrl IL.



.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1148 AM CST Sat Feb 18 2017

Upper low over southeast Missouri will continue to slowly slide to
the east. So with taf sites to the north and northwest of system,
VFR conditions to prevail. Main question is precipitation
development. HRRR model indicating some increased shower activity
over STL metro area, so added VCSH mention through mid to late
afternoon today. As for winds, west to southwest winds to prevail
before diminishing to light and variable early this evening.

Upper low over southeast Missouri will continue to slowly slide to
the east. So with metro area to the north of system, VFR
conditions to prevail. Main question is precipitation development.
HRRR model indicating some increased shower activity over STL
metro area, so added VCSH mention through 21z today. As for
winds, southwest winds to prevail through tonight before backing
to the southeast on Sunday. So that means a cross wind at KSTL
through at least early this evening.


Record Highs

 2/17  2/18  2/19    2/20
STL: 77/1911  74/1971  77/2016  78/2016
COU: 74/2011  71/1930  76/1930  76/2016
UIN: 72/2017  68/1971  71/1930  72/2016



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