Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 221756
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1156 AM CST Mon Dec 22 2014

...Update to aviation forecast discussion...

.SHORT TERM...(Today and Tonight)
Issued at 240 AM CST MON DEC 22 2014

Water vapor imagery continues to show a broad upper level trough
over the central U.S. at 09Z. Profiler and airplane reports show a
150+ KT jet streak on the back side of the upper trough from the
Pacific northwest into the central Rockies. Meanwhile at the
surface, an area of low pressure was forming over the northern
plains with a trough axis extending south through western and
central KS.

For today the surface trough axis/front is expected to gradually
move east across the area as energy on the back side of the upper
trough causes a closed upper level low to develop over the central
plains. As it does, models prog a decent shortwave to swing through
northern KS providing forcing for showers along the trough axis. All
of the models seem to be in agreement on this, and the only real
difference in the solutions is the GFS seems to be the deepest with
the upper low and the surface reflection. This may explain why the
GFS is a little more aggressive with the surface winds this
afternoon on account of having the strongest pressure gradient. In
any case, it looks like the strongest vertical motion from the
shortwave moves overhead during the late morning and early
afternoon. With this in mind, have the highest pops along and east
of the expected trough axis location through the early afternoon.
Considered inserting a mention of thunder to the forecast as the
shortwave rotates through the area. The NAM and GFS show a small
window where mid level lapse rates steepen to around 8 C/km. But
since instability remains very modest due to the cool and moist
nature of the airmass, have left the forecast all rain. However I
would not be surprised if there were a couple rumbles of thunder
early in the afternoon across east central KS. Cloud cover and
precip are expected to keep temps from warming much and have highs
in the mid 40s, or only several degrees warmer than current
readings.

Tonight should see surface winds become westerly as a surface low
propagates into the MS river valley. This should usher in some
relatively dry air to the forecast area. Unfortunately the closed
upper low is expected to remain over the MO river valley with the
potential for upper level deformation and moisture wrapping around
the back side of the upper low back into northern KS. The GFS seems
to be the more bullish solution with its QPF overnight and am a
little concerned this may be due to it developing a deeper system
then other solutions. Nevertheless have held onto POPs tonight
showing the potential for precip. Models show mostly subsidence
developing this evening across east central KS, so think any chance
for precip overnight will be across northern KS and along the NEB
boarder. There looks to be enough cold air moving in that the precip
could be in the form of snow after midnight as well. At this point
with temps forecast to be in the lower and mid 30s overnight and
models showing the better mid level frontogenesis setting up over
southeast NEB, think snow may only amount to a dusting before 12Z.

.LONG TERM...(Tuesday through Sunday)
Issued at 240 AM CST MON DEC 22 2014

A complicated forecast in the mid term as minor changes between
model runs create major changes in the location of optimal lift and
consequential precipitation, especially from Tuesday through
Wednesday morning. Nevertheless, it does appear model guidance in
showing better consistency with an upper jet streak within the
northwest flow diving southeast towards the southern plains. As this
occurs, embedded lift enhances behind the departing upper low.
Better confidence in this system lead to higher precip chances for
the CWA Tuesday as the wave and cold front move through. Still
remains some uncertainty in strength of this wave. Cross sections
and previous runs reveal the GFS to be an outlier with the amount of
forcing and generous amounts of QPF Tuesday afternoon. Leaned closer
to the ECMWF and NAM solutions as ensemble solutions shows similar
characteristics with the evolution. As precipitation develops along
and behind the boundary, temps will fall close to or just above the
freezing mark. A mix of rain and snow is possible, transitioning to
all snow by early evening, and gradually exiting east central KS
Wednesday morning. At this time do not see areas of strong,
persistent lift and instability to indicate banding precip. Snow
ratios averaged at 12:1 lend to total snowfall accumulations near or
less than an inch by Wednesday morning. Will continue to monitor
trends in the upcoming days as this system has the potential to
impact travel.

For Christmas Eve, southerly winds increase as a lee surface trough
builds, advecting warm air back into the CWA through Christmas Day.
Dry air advection slowly pushes east Wednesday afternoon, as north
central areas may break out into mostly sunny skies. Highs recover
into the upper 30s with overnight lows in the upper 20s. Southerly
winds increase Christmas Day as a slow moving upper trough enters
over the Rockies. Skies become mostly sunny for all of northeast
Kansas as increased WAA boosts highs in the upper 40s. Bulk of the
system and associated QPF passes to the north of the CWA Thursday
evening and Friday. Still could not rule out very light snow
impacting north central areas, otherwise no impacts are seen in
terms of precip. Another cold front cools highs back into the 30s
for the weekend as dry high pressure takes control of the region.

&&

.AVIATION...(For the 18Z TAFs through 18Z Tuesday Afternoon)
Issued at 1145 AM CST MON DEC 22 2014

Expect improving conditions gradually over the first 6hrs of the
TAF period. Confidence is high regarding the overall outcome, but
there may be some timing variations. The main uncertainty will be
in the last half of the TAF period with some wrap around cloud
cover and mist associated with the main low off to the north of
the area. KMHK does reflect this idea currently, but KTOP and KFOE
have been left out for now since this may happen near the end of
the TAF period or early into the next TAF period.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Wolters
LONG TERM...Bowen
AVIATION...Drake






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