Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 060542
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1142 PM CST Thu Dec 5 2013
Issued at 804 PM CST Thu Dec 5 2013
Grids and products have been updated to reflect higher PoPs slightly
farther north and west compared to the previous forecast. No
significant changes were made to the total snowfall forecast.
.SHORT TERM: (Through Tonight)
Issued at 356 PM CST Thu Dec 5 2013
Focus continues to be the ongoing winter wx system.
Latest mdl guidance is in fairly good agreement. However, seems to
be on the warm side regarding temps aloft based on latest reports of
sn/ip. Have therefore trended p-type turn over to sn a little faster
than based on progd soundings. Based on reports received within
the last half hour, precip is turning over to sleet, then sn
further S quicker than anticipated. Have therefore increased sn
amounts across sern portions of the CWA this evening. Some
uncertainty exists regarding how far E this heavier sn will make
it as some mdl guidance suggests the precip will weaken as it
Ongoing precip is expected gradually push s and sewd, before pulling
nwd again late tonight into Fri morning.
As for temps, have trended aob coolest MOS guidance.
No changes to headlines are expected based on tonight`s forecast,
but may be needed depending on precip rates this evening.
.LONG TERM: (Friday through Next Thursday)
Issued at 415 PM CST Thu Dec 5 2013
(Friday through Sunday)
Differences among mdls continue to plague the forecast process. Main
forecast problem is timing and placement of precip based on subtle
s/w within a mainly uniform flow. While the GFS/ECMWF are in good
agreement, the NAM/local WRF are suggesting a secondary band of
precip Fri morning across portions of central MO. This change
appears to be due to a stronger s/w in the NAM. While this s/w is
currently still across nrn portions of Mexico, there is some
legitimate concern this soln will verify.
Have therefore pulled POPS further NW and have also increased sn
amounts for Fri morning, esp across central and ern MO. With colder
air moving in, precip shield will struggle to move nw. However, due
to this cold air, some rather high SLRs may be realized over central
and into east central MO. If these trends continue in upcoming mdl
cycles, headlines may need to be upgraded and moved nwd this
SN is expected to pull ewd out of the area quickly Fri afternoon
with all precip expected to be E of the CWA by 00z Sat.
The next system to impact the area is expected to arrive late Sat
night into Sun. Latest mdl guidance has slowed the onset of this
precip. With uncertainty in timing, have lowered POPs, but have kept
basic message as the prev forecast.
With sn/ip expected across at least the srn half of the CWA, and
with strong CAA across the remainder of the area, have trended
colder again, esp with overnight lows over expected snowpack. Air
temps near zero will be possible Sat morning as clouds clear out of
the area. Since sfc winds are still expected to be around 8 to 10
kts, kept temps on the warmer side given how cold dewpoints are.
(Monday through Thursday)
Mean UA trof remains anchored over the CONUS into early next week,
with a very gradual transition to a more zonal flow regime by the
end of the work week. This should result in a very slow moderation
in temperatures during the extended period; however the overall
temperature trend during the Monday-Thursday time frame will remain
well below average for early December.
There are several shortwaves forecast to propagate through the trof
and across the nation`s midsection during this time. However,
surface ridging is forecast to dominate the mid-Mississippi Valley
during much of this time, and moisture is quite limited, so will
leave the forecast dry for now.
.AVIATION: (For the 06z TAFs through 06z Friday Night)
Issued at 1126 PM CST Thu Dec 5 2013
This first wave of snow is coming to an end across the St. Louis
metro with just flurries and VFR conditions are expected through
10z or so. The next wave of snow is then on track to spread
northeastward into the St. Louis metro between 10-12z and continue
into the afternoon. IFR flight conditions to dominate with this
second band, with accumulating snow of at least several inches at
the KSUS/KSTL/KCPS terminals. The northern extent of this second
wave remains problematic and at this time it appears to flirt with
KCOU and remain south of KUIN. The whole snow band should exit to
the east in the 21-00z time frame on Friday afternoon with rapidly
improving conditions in its wake.
Specifics for KSTL:
Well this first wave of snow moved a bit further north than
originally thought. It has now pushed east of KSTL and just
flurries and VFR conditions are expected through 10z or so. The
next wave of snow is then on track to spread northeastward into
the St. Louis metro between 10-12z and continue into the afternoon.
IFR flight conditions to dominate with this second band on Friday, with
accumulating snow of at least several inches at the terminal. The
whole snow band should exit to the east in the 21-00z time frame
on Friday afternoon with rapidly improving conditions in its wake.
MO...WINTER WEATHER ADVISORY until 6 PM CST Friday FOR Crawford MO-
Jefferson MO-Washington MO.
WINTER STORM WARNING until 6 PM CST Friday FOR Iron MO-Madison
MO-Reynolds MO-St. Francois MO-Ste. Genevieve MO.
IL...WINTER WEATHER ADVISORY until 6 PM CST Friday FOR Bond IL-
Clinton IL-Fayette IL-Monroe IL-St. Clair IL.
WINTER STORM WARNING until 6 PM CST Friday FOR Marion IL-
Randolph IL-Washington IL.