Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 201733
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1133 AM CST Sat Dec 20 2014

...Update to aviation forecast discussion...

.SHORT TERM...(Today and Tonight)
Issued at 209 AM CST SAT DEC 20 2014

08Z water vapor imagery shows one shortwave lifting east through the
OH river valley while a second wave swings through the mid MO river
valley. Further west a weak mid level ridge was noted over the
Rockies while another trough moves towards the Pacific northeast
coast. Surface obs show the ridge axis has moved to the southern
plains with high pressure centered near IN and OH. A lee trough of
low pressure has gradually been deepening over the last several
hours.

For today and tonight, the models do not have a strong signal for
large scale forcing. However there is expected to be gradual height
falls spread east as the system along the northwest coast propagates
east. Thinking is with relatively dry mid levels as noted on the 00Z
RAOB, any weak forcing will not amount to much chance for measurable
precip. Again much of the challenges come from the low level
moisture trapped within the boundary layer. Recent satellite imagery
shows clearing slowly but steadily moving east and think this may
be due to the subsidence provided by the shortwave moving through
the mid MO river valley. Models have done poorly with the low level
clouds and do not have much confidence in the NAM`s prog of surface
RH since it always seems to keep temps to cool from the snow cover.
However it is one of the solutions that seems to have an idea of the
stratus clearing in it`s 925MB prog of RH. So think there is
actually a decent chance of seeing some sunshine today as there
appear to even be some holes in the stratus over northern OK.
Because of this, have bumped up highs for western and southern
counties with skies becoming at least partly sunny. Kept highs in
the upper 30s for northeast KS where there is a little more snow
cover and longer fetch of south and southwesterly winds over the
snow. Tonight the surface trough of low pressure is progged to
gradually deepen favoring the southerly low level flow to persist.
With this in mind think low level moisture is probably going to make
a run at the forecast area with stratus returning. Have kept lows in
the lower and mid 30s thinking cloud cover would prevent much
radiational cooling.

.LONG TERM...(Sunday through Friday)
Issued at 209 AM CST SAT DEC 20 2014

Extended period is characterized by an active upper air pattern with
multiple waves of energy impacting the CWA through the week. First
upper shortwave trough tracks across the Pacific Northwest while a
lead wave ushers east over southern Ontario. Isentropic ascent
increases in advance of the wave as the low levels remain saturated
with the stratus. Periods of light drizzle are possible Sunday
afternoon before the main wave impacts the CWA Sunday night and
Monday. The latest GFS still appears to be the outlier compared to
the GEM, NAM, and ECMWF with the more southern track of the upper
wave. Sided closer to consensus which gives lighter amounts of
rainfall across the area, focused primarily on Monday morning.
Increased southerly winds throughout the profile will usher warmer
air northward, bringing highs to the middle 40s Sunday and upper
40s, perhaps 50 on Monday. Uncertainty greatly increases after
Monday as the system slowly lifts northward with ridging building
over the eastern CONUS. Weak embedded areas of lift rotate on the
backside of the low, warranting slight chances for precip Monday
evening into Tuesday. Light rain showers are possible for east
central areas Monday evening and a rain snow mix over far northeast
areas Tuesday. No accumulation is expected. Certainty in wind speeds
is high as guidance is consistent on strong northerly speeds between
15 and 20 mph sustained Tuesday afternoon. Behind the cold front,
temps Monday night cool below freezing while highs Tuesday are 10
degrees cooler near 40. Some possibility we could see sunshine on
Tuesday, depending on where the upper low and subsequent moisture
resides.

Best chances for mostly sunny skies return in time for Christmas Eve
as dry northwest flow ensues. Great uncertainty lies in the next
trough progged to dig southeast over the Rockies on Christmas Day.
The track of the departing upper trough will greatly influence the
timing and moisture availability for the incoming system. At this
time, I added a slight chance for precip Thursday evening and Friday
as guidance is split between a decent storm system and a weak/dry
system. Also tweaked highs Friday down a few degrees as it appears
the cold air arrives faster than previous runs. Precip type at this
time is difficult to discern but may be a snow or mix of rain and
snow.

&&

.AVIATION...(For the 18Z TAFs through 18Z Sunday Afternoon)
Issued at 1126 AM CST SAT DEC 20 2014

The challenge for this TAF period will be CIGs/VIS restrictions.
Currently, the moisture is well overdone by guidance. Based on
satellite and current obs upstream, wouldn`t expect enough
moisture to build back into the region until roughly the 3Z time
frame near KMHK and 6Z time frame over the KTOP/KFOE region.
Continued mixing should allow CIGs to remain mostly above IFR
restrictions at this time, but confidence is lower for the last
6hrs of the TAF period.

&&

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

$$

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





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