Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 130537
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1137 PM CST Thu Jan 12 2017

.UPDATE...
Issued at 950 PM CST Thu Jan 12 2017

The 00z NAM continues to support the current timing of onset of
what will be predominantly a freezing rain event for our region.
There is some indication for support of the initial onset from
approximately I-70 northward could briefly be sleet or a freezing
rain and sleet mix. QPF has come in higher for Friday afternoon,
particularly for STL metro, possibly in response to an atmosphere
more convectively favorable. But the NAM on the last snow event
had QPF that was much too high and something to take into account
before a knee-jerk bumping up of this factor.

The 00z GFS has just arrived and has quickened its northward pace
a bit (it was slower than the NAM) and now matches the NAM almost
exactly for onset times. This also is reflected well with the
current forecast. The 00z GFS likewise has boosted its QPF for
late morning and afternoon Friday, particularly around the I-70
corridor: to 0.20 to 0.35" for STL metro and up to 0.2" for
central Missouri. Not as high as the NAM, but also not showing an
atmosphere as convectively favorable. This would tend to give a
support to the NAM trend but not quite on actual magnitude. The
GFS also supports the idea of an initial pcpn-type as sleet or a
sleet/freezing rain mix briefly before switching to all freezing
rain.

Surface temps continue to be advertised as sub-freezing thru the
day on Friday and for many areas into at least Friday evening.

The main change for the evening update will be to incorporate some
sleet into the onset pcpn-types for around I-70 northward,
otherwise the next shift will hammer out any QPF increases.

TES

&&

.SYNOPSIS...
Issued at 319 PM CST Thu Jan 12 2017

Storm highlights -

1) All areas converted to an ice storm warning due to forecast of
0.25 ice accumulations and greater. Warning timing starts late
tonight in the south, and Friday evening in the far north. Ending
in the far south by daybreak Sunday and ending in the north late
Sunday afternoon.

2) Overall looking at ice accumulations from around .25 to .50
inches, perhaps a little higher in central MO.

3) Impacts will be largely confined to elevated surfaces - icing
on bridges, trees, and power lines, cars... Untreated roads may
have some issues Friday-Friday night when the coldest surfaces
temps are expected. Given this amount of ice and light winds,
power outages are typically isolated to scattered.

Glass/Tilly

&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 319 PM CST Thu Jan 12 2017

Focus thru this period is the impending icing event.

A large, arctic sfc ridge, stretching nwd well into nrn Canada, will
continue to build into the area this evening. As it builds ewd into
the Great Lakes region, sfc winds across the area will continue to
veer to become ely. Just off the sfc thru low levels, WAA will
return and begin the moisture transport nwd tonight.

Still expect precip to arrive in srn portions of the CWA around 09z
tonight. However, have low PoPs beginning in this area at Midnight
to show some uncertainty. This area of precip will slowly move nwd
late tonight and thru Fri. Precip shud begin late tonight as more of
a shower activity and become more stratiform on Fri with better
forcing. Precip may begin briefly as sleet, but shud transition to
FZRA fairly quickly and remain FZRA thru this period.

There is still some uncertainty regarding ice accumulations. Precip
tonight shud be rather light QPF, and therefore ice amounts are
probably on the higher end. With temps in the upper 20s to around 30
where precip is occurring, wud not expect a lot of ice. With the
recent warm temps the past couple of days, the ground temps remain
rather warm according to MO Mesonet sites. This shud help prevent
problems with roads and much of the accumulation limited to elevated
sfcs. Even sites thru mid sections of the CWA show 2 inch soil temps
of 34 degrees. However, across nrn portions of the CWA, these temps
are already aob freezing, which shud continue to cool tonight with
temps in mid teens expected.

Temps on Fri are going to be tricky. Temps at onset of precip in the
morning will likely remain steady until the precip comes to an end.
Temps may warm slightly north of the area of precip, but shud cool
again as precip begins with wetbulb effects. Regardless, it appears
that temps will max out right at the freezing mark, or perhaps a
degree or perhaps two warmer. However, those warmer temps shud be
brief.

Fri still appears to be the worst time for this event overall. Given
the recent warm temps, light winds and precip falling during largely
marginal temps, do not currently anticipate a major ice event. The
going ice accumulations shud be considered a higher end forecast for
now.

Tilly

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 319 PM CST Thu Jan 12 2017

Freezing rain will continue to overspread the remainder of northeast
MO and west central IL early Friday evening. Like other areas
earlier in the day on Friday, there could be a brief period of sleet
at the onset, but this will be short-lived as strong low-level WAA
continues above the shallow cold dome. There are also suggestions
that the precipitation intensity/rate and hence overall ice
accumulations will diminish on Friday night into Saturday. Reasons
for this appear to be two-fold: 1) the initial impulse aloft scoots
by early Friday evening and the LLJ veers to more westerly
diminishing the isentropic lift; 2) we get some warming of surface
temperatures above freezing across southern portions of the CWA, and
this is where the majority of the precipitation may occur late
Friday through Saturday. Another wave of more significant
precipitation will impact the area late Saturday night into Sunday
with high precipitation rates as an impulse aloft moves east-
northeast across the area and backing mid-upper flow once again
results in increasing isentropic ascent.

The thermal characteristics of the atmosphere remain the biggest
challenge. The elevated warm layer is stout, well above freezing, as
is the depth of the warm layer aloft. So surface temperatures are
the main issue. A broad corridor centered along I-70 has the most
uncertainty. There will be impacts of latent heat release from the
freezing rain acting to warm surface temperatures, and then late
Saturday into Saturday night the surface high builds in a bit from
the north and winds veer to more n/nely bringing weak CAA. Thus
this region could see a change from ZR to R on Saturday and then a
change back from R to ZR on Saturday night, lasting into Sunday
morning. On Sunday and into Sunday night, low level warming should
progress from south to north as surface winds become more
southeasterly and winds above the boundary layer strenghen from
the south, driving all surface temps above 32F. This will occur
attendant with the main surface low moving from the TX Panhandle
into south- central KS.

The continued north-northeast movement of the surface low from the
central Plains into the Upper MS Valley Monday into early Tuesday
will bringing an expanding warm sector, mild temps, and strictly
liquid precipitation. In fact the models show enough instability
Monday afternoon-night to suggest some thunder potential along and
in advance of the progessing cold front. The cold front with the
northeast moving surface low should exit the area by Tuesday
evening, and present thoughts are pops may be too high the
remainder of the week. Temperatures Monday and beyond look to be
above normal thru the week.

Glass

&&

.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 intial onset.
Conditions should continue to deteriorate sufficiently to IFR as
we head into Friday night.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Ice Storm Warning from 3 AM Friday to 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 Friday to 3 PM CST Sunday for
     Audrain MO-Lincoln MO-Monroe MO-Pike MO-Ralls MO.

     Ice Storm Warning from 9 AM Friday 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 Friday to 6 PM CST Sunday for Knox
     MO-Lewis MO-Marion MO-Shelby MO.

IL...Ice Storm Warning from 3 AM Friday to 6 AM CST Sunday for
     Randolph IL.

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

     Ice Storm Warning from 9 AM Friday 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 Friday to 6 PM CST Sunday for Adams
     IL-Brown IL.

&&

$$

WFO LSX



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