Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 131805
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1205 PM CST Fri Jan 13 2017

.UPDATE...
Issued at 959 AM CST Fri Jan 13 2017

So far, onset of precip has occurred a little quicker than
expected 24 hours ago. Main changes for this update are to cool
temps in areas of precip a bit. May still see temps reaching to
around the freezing mark today where there is no precip falling at
the time. Temps where precip is falling are generally in the upper
20s to just below freezing. This is allowing ice to accumulate on
elevated sfcs, but coupled with warm ground temps, shud help keep
impacts on treated roads down.

Have added a slight chance for TS for the rest of today across srn
counties. CAPE is quite limited, but may see a isod strike. Have
also increased QPF amounts based on latest trends. Actual precip
amounts may still be on the low side, but will continue to
monitor.

Looking ahead to tonight, still analyzing latest data as it
arrives, but tonight appears to be a lull in activity. May be able
to change wording from a likely occurrence, which still looks
like it is, to a isod to scattered coverage wording. Will continue
to monitor and update ASAP.

Tilly

&&

.SYNOPSIS...
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 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
ice.

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.

42

.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.

42

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1144 AM CST Fri Jan 13 2017

Ongoing FZRA will continue thru this afternoon and is expected to
move nwd slightly. Expect precip to diminish late this afternoon
into the evening. Believe much of the precip tonight will be more
shower activity. However, as temps cool overnight, expect cigs to
lower as well as a threat of fog development and perhaps FZDZ. To
keep the TAF simple, just left as FZRA. Visbys may be lower than
current TAF due to fog developing, however, do not currently
expect it to cause a lower category than what the cigs will cause.
Precip shud move south of all terminals by mid morning or so.

Tilly

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     31  28  34  30 / 100  80  40  50
Quincy          28  25  33  25 /  30  30  20  30
Columbia        28  26  33  28 / 100  70  50  60
Jefferson City  29  27  33  30 / 100  70  60  60
Salem           32  30  35  30 / 100  90  60  40
Farmington      31  28  36  31 /  90  70  80  60

&&

.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 from 3 PM this afternoon to 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 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 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 from 6 PM this evening to 6 PM CST Sunday for
     Adams IL-Brown IL.

&&

$$

WFO LSX



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