Area Forecast Discussion
Issued by NWS St. Louis, MO

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378
FXUS63 KLSX 140240
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
840 PM CST Fri Jan 13 2017

.UPDATE...
Issued at 840 PM CST Fri Jan 13 2017

The first round of FZRA has wrapped up for most areas and the
second is taking shape to our southwest.

Models have come in great agreement on how this will come about,
notably the NAM/HRRR/RAP. First, a northern slug of pcpn just
south of KC is expected to continue to track E-NE while expanding
in coverage overnight--tracking thru our CWA from around midnight
when it enters central MO and take until mid-Saturday morning
before exiting west-central and southwest IL and will largely
track thru the I-70 corridor regions and many areas further to the
north towards Quincy. It is with this that the forecast needed its
largest adjustment, where many PoPs were boosted to likely for
northeast MO and west-central IL and ice accretions overnight and
early Sat mrng boosted to between .1 and .2".

The second, more southerly slug of pcpn, is currently over
northeast OK and far northwest AR and this was handled already
well by the previous forecast and made few if any changes on its
handling. This will take the form of mostly freezing rain as it
tracks thru southeast MO late tonight and early Sat mrng, but some
areas in our far southern CWA where temps have edged to 33, where
this may fall as plain rain. Additional ice accretions here will
also be .1 to .2" for most areas, although some support for
convective processes may locally enhance this in spots.

Temps should largely remain steady overnight from where they are
now, with the best chance for temp drops being in northeast MO and
west-central IL where the largest dewpoint depressions exist and
should narrow considerably with pcpn moving in.

TES

&&

.SYNOPSIS...
Issued at 340 PM CST Fri Jan 13 2017

No major changes to the forecast. Headlines and ice storm warnings
continue into Sunday. Thus far highest ice accumulations reported
across parts of southeast MO and far southwest IL nearing .20
inches. Overall storm total ice accumulations of 0.25 to 0.50 and
locally higher still appear on track.

Glass

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 340 PM CST Fri Jan 13 2017

Focus thru this period continues to be the impending icing event.

Sfc ridge currently over the Great Lakes region will continue to
build ewd this evening. In the wake, another reinforcing sfc ridge
develops over the nrn Plains due to the snow pack over this region
as temps drop tonight.

Do not expect temps to move much tonight other than dropping a
couple of degrees after sunset. Also do not expect temps to change
much on Sat. Some locations may see a max temp on Sat of a degree or
two above freezing, but any temps above freezing shud be short
lived.

As for precip chances, ongoing FZRA will continue to move ewd this
evening. Expect more of a lull in precip activity tonight. This
creates a message issue with more shower activity expected tonight,
but it appears to be a high probability of it occurring. Have
therefore kept PoPs in the likely range thru much of the night.
Precip shud remain light thru much of the night with an increase in
intensity possible late tonight thru early Sat morning. There
appears to be two main areas of precip expected late tonight into
the morning, one across the mid to srn portions of the CWA and the
other across the nrn portions. initially, this more nrn area of
precip was expected to be light shower activity. However, the 13/18z
NAM, which is just arriving, wud suggest this area of precip may be
heavier than prev anticipated.

After this first wave of precip late tonight thru early Sat morning,
expect precip coverage to increase mid to late Sat morning, mainly
across srn portions of the CWA. With these warmer temps, albeit only
slightly warmer, do not anticipate much additional ice accumulation
across this area.

Overall, not a lot of change in the going forecast for ice
accumulation amounts at this time.

Tilly

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 340 PM CST Fri Jan 13 2017

A wave of more significant precipitation should spread into the
area from the west/southwest on Saturday night. This will be in
response to another impulse in the flow aloft and associated
forcing, as well as increasing isentropic ascent due to backing of
the low level/H85 flow in response to this impulse. Waves of
precipitation will continue on Sunday in response to yet another
impulse aloft and continued low-level WAA/isentropic ascent. The
overall precipitation shield should shift northward with time, due
to backing flow aloft, with the precipitation/freezing rain centered
thru northern MO and west central IL by the later part of the
afternoon.

Temperature tends continue to be the big question mark and in
general the models appear to be warming the surface temperatures
too quickly, especially the GFS. This forecast has trended a bit
colder for Saturday night into Sunday morning with clouds and
northeast surface winds/CAA at least maintaining the temps and
delaying the rise. The warm-up however should proceed on Sunday,
and primarily Sunday afternoon with only far northern MO and west
central IL remaining at or slightly below freezing at 00Z Monday.
On Sunday night the main focus of precipitation should shift to
primarily northern MO into the upper MS Valley in response to
continue backing flow aloft and a more decide southerly LLJ.
Increasing southeast surface winds should finally drive surface
temps above freezing across the remainder of the CWA by the
overnight hours.

Models diverge on Monday with the overall upper air pattern. The GFS
suggests a dominate northern stream upper low tracking across the
Midwest as a key player, while the ECMWF maintains the upper low
lifting northeast. The ramifications on this are the speed/timing/location
of the lifting associated surface low and cold front progression
across the area late Monday into Tuesday. The ECMWF has been most
consistent and has been followed most closely. Mild air will
overspread the area on Monday in advance of the cold front with
rain and isolated thunderstorms continuing along/ahead of the cold
front through Tuesday. Thereafter, an unseasonably mild pattern
appears on track the remainder of the week with temps some 20-25
degrees above normal.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 523 PM CST Fri Jan 13 2017

MVFR CIGs across COU and the STL metro sites currently prevail
with IFR just to the south--notably just to the south of the band
of pcpn which is going thru STL metro and SW IL at the moment.
Indications are once this band completes moving thru and heads off
to the east, that IFR CIGs will build a bit more to the north to
envelope the STL metro sites, and should be around 02z. UIN will
see somewhat better conditions but its CIGs should still drop into
MVFR category soon.

Of more importance is timing the main slugs of FZRA that are
expected to move thru. The first one is in the process of moving
out of STL metro now and should be gone by 01-02z. A break will
ensue, but of course cannot fully rule out a stray brief
occurrence of pcpn. The next batch of pcpn, taking form over SE
KS, is due in towards midnight in central MO and a few hours later
for UIN and STL metro sites. Once in, the next round of FZRA
should last about 5 hours. Again, another break from persistent
pcpn should ensue until the next round works in late Saturday
night.

An extended period of low MVFR or IFR CIGs are expected once they
eventually drop this evening, with the worst CIGs expected around
STL metro.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Ice Storm Warning until 6 AM CST Sunday for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

     Ice Storm Warning until 3 PM CST Sunday for Audrain MO-Lincoln
     MO-Monroe MO-Pike MO-Ralls MO.

     Ice Storm Warning until noon CST Sunday for Boone MO-Callaway MO-
     Cole MO-Franklin MO-Gasconade MO-Jefferson MO-Moniteau MO-
     Montgomery MO-Osage MO-Saint Charles MO-Saint Louis City MO-
     Saint Louis MO-Warren MO.

     Ice Storm Warning until 6 PM CST Sunday for Knox MO-Lewis MO-
     Marion MO-Shelby MO.

IL...Ice Storm Warning until 6 AM CST Sunday for Randolph IL.

     Ice Storm Warning until 3 PM CST Sunday for Calhoun IL-Greene IL-
     Jersey IL-Macoupin IL-Montgomery IL-Pike IL.

     Ice Storm Warning until noon CST Sunday for Bond IL-Clinton IL-
     Fayette IL-Madison IL-Marion IL-Monroe IL-Saint Clair IL-
     Washington IL.

     Ice Storm Warning until 6 PM CST Sunday for Adams IL-Brown IL.

&&

$$

WFO LSX



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