Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 121130
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
530 AM CST Thu Jan 12 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 345 AM CST Thu Jan 12 2017

Upper flow becoming more southwesterly as low organizes along the
west coast. A smaller scale wave was exiting eastern Nebraska
early this morning with precip remaining well north of Kansas.
Cold front continues to push south, exiting the southeast corner
of the state near 06Z though pressure rises were weakening over
northern Nebraska. Some thin stratus remains over much of the area
with next batch of high cloud entering central Colorado.

Cold air deepens somewhat today with 850mb winds continuing to
veer, but southwest trajectories at this level continue over
eastern Oklahoma through tonight. Models are struggling with this
thin stratus, but much of the area should see some peaks of sun
around midday before the high cloud returns. Kept highs fairly
similar with only 10-20F rises through the day. Low level moisture
return begins overnight as the surface high center pushes
northeast. Depth of moisture remains rather small with limited
isentropic lift as well so only have very small chances in far
southeast sections late tonight with freezing drizzle or very
light freezing rain possible.

.LONG TERM...(Friday through Wednesday)
Issued at 345 AM CST Thu Jan 12 2017

Friday noon through Saturday sunrise - Light freezing rain /
freezing drizzle lifts into the east central counties late Friday
Morning and continues through the afternoon hours. Amount of
precip increases in the far southeast through 12Z Saturday, with a
few tenths of accumulation possible by sunrise Saturday. Although
light, any freezing precip will make for slick roads, walkways and
elevated exposed surfaces. Through this time period models
continue to agree that surface temps will remain below freezing
across the area. Likely to meet advisory criteria and will start a
winter storm watch for this period.

Saturday/Saturday night - Through the daytime hours most of the
precip is along and south of I70, with the GFS most bullish with
qpf amounts with a few tenths falling by 0z. The counties south of
I35 flirt with 32F in the NAM and GFS but stay well below in the
EC. Used a bit of a blend. With some guidance a little faster in
the onset of QPF return, will start next segment of winter storm
watch to cover the afternoon hours Saturday. Next heavier round
comes in closer to the 06z hour and continues overnight, with
another quarter to half inch coming in from the southwest with the
higher amounts along and south of I70.

While Sunday brings in even higher forecast QPF amounts, it also
brings in more uncertainty. I can`t stress enough how much this
becomes a game of inches as GFS and NAM have widespread
temperatures across the entire area very, very close to 32F toward
Sunday morning...while the EC to this point remains the colder
outlier. This will clearly have considerable impact on
accumulations of ice. It almost looks like as soon as the heavier
precip / qpf starts to move into our area, that perhaps the warm
nose just off the surface is being mixed into the column with the
rainfall enough to bring it near or just above freezing. Or
perhaps its latent heat released by the ice forming at the
surface. Worth noting that these drops will fall through a shallow
layer of temperatures a few C cooler than the surface on their way
down which seems plenty cold to refreeze...but this layer is very
shallow.

Current forecast ice totals are a few tenths from noon Friday
into sunrise Saturday, then this second round from sunrise
Saturday through the noon hour Sunday bringing around a quarter
inch far northwest to half to three quarters inch farther south.
Sunday afternoon brings more chance for ice accumulation, but its
complicated.

By 18z Sunday the GFS lifts the freezing line to roughly a
Minneapolis to Holton line, while the EC still has it near the I35
corridor. This is indeed the difference between rain and freezing
rain, and the colder EC has over an inch of rain in the 12-0z
timeframe. At this time, would have to say that the northwestern
counties by mid day Sunday are of higher confidence for
accumulating ice, while the southeast counties could stay in a
cold rain. The zones in between - with the higher total water -
are the ones of the most uncertain /along the I70 corridor/. It
remains prudent given the above solutions for the afternoon hours
on Sunday, that locations generally north of I35 should plan on
freezing rain to continue at least into part of the afternoon and
it could linger longer if the slower EC verifies. Currently have
generally a quarter inch of ice centered along the I70 corridor
out of this for Sunday afternoon.

These ice amounts could jump considerably higher if the colder EC
hangs on for the Sunday afternoon forecast.

By 0Z Monday, the EC lifts the freezing line near I70, while the
GFS has it into the northern counties. GFS would pivot that
freezing line out of our area through 12z Monday, with EC similar
just a bit slower on its progress. Will need to watch the far
north central counties as to whether or not they make it out of
freezing, but overall temperatures rise enough for forecast to
change over to rain late Sunday for most of the area.

GFS spits out another streak of nearly an inch and a quarter of
liquid precipitation overnight Sunday night into Monday, with
lesser amounts toward half an inch in the southeast counties, and
the EC also puts down another half an inch area- wide. There is
even a sliver of MuCAPE in the eastern counties by 06z Monday that
lifts northeast out ahead of the upper low as it lifts out of our
area on Monday, indicative of the potential convective potential
for the precip. This round of moderate to heavy rainfall should
be able to melt ice over much of the area that accumulated over
the weekend.

Things quiet down for the Tuesday-Thursday timeframe as
temperatures rise back into the 40s and 50s with lows in the 20s
and 30s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 530 AM CST Thu Jan 12 2017

Think MVFR stratus a bit harder to pick out on satellite imagery,
but should be clearing TOP and FOE in the next few hours based on
this and observation trends to the north and west. Conditions
will be VFR after this with some high cloud returning.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday afternoon through Sunday
afternoon for KSZ008>012-020>024-034>038.

Winter Storm Watch from Friday afternoon through Sunday
afternoon for KSZ026-039-040-054>056-058-059.

&&

$$

SHORT TERM...65
LONG TERM...67
AVIATION...65


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