Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 131133

Area Forecast Discussion
National Weather Service Saint Louis MO
533 AM CST Fri Jan 13 2017

Issued at 414 AM CST Fri Jan 13 2017

Winter storm highlights:

1. No changes to existing headlines.

2. The headline "Ice Storm Warning" is due to the expected ice
accretion amounts of between one-quarter and one-half of an inch
of ice. There are several mitigating factors which will tend to
lessen the impacts of receiving this amount of ice. See
discussion below for further details.



.SHORT TERM... (Through Late Saturday Night)
Issued at 414 AM CST Fri Jan 13 2017

A stalled front located over AR/TN has brought a shallow Arctic
air mass into MO/IL. Above this layer, temperatures are fcst to be
sufficiently warm throughout sufficient depth of the atmosphere
such that thermal profiles support freezing rain as the dominant
ptype. Isentropic ascent over the bdry has led to the dvlpmt of
clouds and pcpn on the cold side of the bdry, and these processes
will continue today/tonight. There may be brief pds of sleet
mixed in with the rain at pcpn onset due to evaporational cooling
effects. The extreme northern CWA may experience longer and more
frequent pds of mixed precip including sleet and snow, but the
dominant ptype will still be FZRA. Although some of the models`
QPF seems high, PW values of 0.8-1.0" are at +2 SD for January.

Although the forecast still calls for between one-quarter and
one-half inch of ice accretion, there are several mitigating
factors which should lessen the impact of receiving this amount of

First, the Arctic air mass arrived yesterday, not several days
ago. The CWA has not experienced multiple days of sub-freezing
temperatures leading up to the onset of FZRA. In fact, it was so
warm over the last few days that new record highs were set at STL
and COU. Ice accretion will initially be less efficient on
residually warmer roads and ground surfaces which have not yet
fallen below freezing. Expect accretion on elevated surfaces and
untreated roads.

Second, the Arctic air mass will not persist for multiple days
after pcpn ends. The stalled front will lift northward as a warm
front late this weekend. All locations initially rise above
freezing on Sunday and then rise well above freezing on Monday. In
other words, ice accretion melts relatively quickly rather than
persisting for several days. A short duration lessens the impacts
of receiving this amount of ice.

Third, air temperatures near the surface will be near freezing for
most of the event, which in itself tends to limit ice accretion.
Latent heat released during the freezing process may also locally
nudge temperatures to near or above freezing at times.

Fourth, wind speeds are not particularly strong either during or
after the pcpn. This will tend to limit the extent of tree and
tree limb damage to isolated/scattered rather than the widespread
and extensive tree damage which occurred during ice storms in
December 2006 and January 2007. The iso/sct nature of tree damage
would then limit the spatial extent of power outages, damage to
structures, and/or tree-blocked roads.

Finally, the initial period of FZRA on Fri/Fri night is followed
by either a lull or a period of non-freezing rain on Saturday
across the southeastern half of the CWA when temperatures rise
slightly above freezing.


.LONG TERM... (Sunday through Thursday)
Issued at 414 AM CST Fri Jan 13 2017

A large cutoff low pressure system located off the coast of CA on
Friday will move eastward over the weekend. Another disturbance
begins to interact with this feature on Sunday night and helps to
eject the initial disturbance northeastward into the plains.
Models diverge quickly due to multiple shortwaves and a plethora
of interactions between them. There is general agreement between
models that a disturbance or three ought to be present over the
plains, and that the surface reflection of one of these features
will lift the stalled cold front northward into MO/IL as a warm
front. Temperatures should rise well above freezing on Monday
behind the warm front. The developing low pressure system`s
associated sfc cold front then moves across the CWA on Mon night
and Tue. There may be sufficient instability for thunderstorms
ahead of the cold front.



.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1137 PM CST Thu Jan 12 2017

Low clouds have moved off to the southeast--for now--with VFR
conditions prevailing at the TAF sites. Clouds will return Friday
morning with lowering CIGs into MVFR range by late morning or midday
Friday for COU and the STL metro sites with pcpn entering the
picture almost at the same time. Pcpn-types continue to look
overwhelmingly FZRA thru the valid period with increasing evidence
of PL mixing in for the first 2-3 hours of initial onset.
Conditions should continue to deteriorate sufficiently to IFR as
we head into Friday night.



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 from 3 PM this afternoon to 3 PM CST Sunday
     for Audrain MO-Lincoln MO-Monroe MO-Pike MO-Ralls MO.

     Ice Storm Warning from 9 AM this morning to 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 from 6 PM this evening to 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 from 3 PM this afternoon to 3 PM CST Sunday
     for Calhoun IL-Greene IL-Jersey IL-Macoupin IL-Montgomery
     IL-Pike IL.

     Ice Storm Warning from 9 AM this morning to noon CST Sunday for
     Bond IL-Clinton IL-Fayette IL-Madison IL-Marion IL-Monroe
     IL-Saint Clair IL-Washington IL.

     Ice Storm Warning from 6 PM this evening to 6 PM CST Sunday for
     Adams IL-Brown IL.



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