Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 171745
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1145 AM CST Wed Dec 17 2014

...Update to aviation forecast discussion...

.SHORT TERM...(Today)
Issued at 331 AM CST WED DEC 17 2014

Main concern in the forecast package is the wintry precipitation
expected later today and tonight for northeast Kansas. The main
upper trough axis arriving this evening was currently rotating
across southern AZ as seen on water vapor imagery. Additional
areas of vorticity maxima were lifting north and east from western
TX while a much weaker wave was shifting east through southern NE.
Bands of weak reflectivity have since developed across the CWA and
are quickly evaporating within the dry airmass at the surface. Low
to middle 20s are common for lows with the patchy low level
stratus in place.

Surface high pressure this morning quickly lifts northward as the
storm system begins to lift northeast towards the CWA. Mid level
moisture increases through the day, as stratus returns by early
afternoon. Highs were lowered a few degrees to the lower 30s as
short term guidance hints at a thicker mid level cloud deck with
the lead trough arriving as early as late afternoon.

.LONG TERM...(Tonight through Tuesday)
Issued at 331 AM CST WED DEC 17 2014

The first stronger short wave trough energy is expected to work
into east central KS by mid afternoon with enhanced Q-Vector
convergence and pressure advection on the 1.5 PVU surface. Still
expect quite a bit of dry air in the mid levels at this time, but
several models including GFS/ECMWF/NMM/ARW/RAP/HRRR indicate that
partial to full saturation will occur between 3 and 6 PM in east
central KS. This seems reasonable given presence of instability
which can result in showery precipitation even in the presence of
dry air. This early precip will fall through a weak warm layer
before wet bulbing cools temps aloft, so have included sleet or
snow in the afternoon forecast. The focus of this early lift
remains south of I-70 so the forecast is initially slow in
bringing precip north.

Expect precipitation to become more widespread through the evening
hours with deep moisture advection expected and continuous weak to
moderate vertical motion throughout the troposphere. Broad lift
supported by jet divergence aloft, broad height falls, a
continuous train of weak embedded short wave energy, and broad
isentropic upslope flow in the lower troposphere. Expect mesoscale
or smaller enhancements to be provided by convective and slantwise
instability and multiple bands of frontogenesis expected to
develop from SW to NE across the forecast area. Any early sleet
potential should quickly translate to all snow as saturation cools
the initially warm layers. The peak of the event can be expected
between about 9 PM and 4 AM. Snow will then gradually decrease and
some drier air is likely to move in aloft in the snow growth
temperature zone. While vertical motion will be waning by sunrise,
still see enough to support some drizzle potential. Temperatures
will likely be below freezing during the morning so could have
very minor freezing drizzle accumulation. This potential looks to
decrease through the afternoon and vertical motion all but
disappears by the evening so only kept drizzle near/south of I-35
after 6 PM.

Accumulations: The convective and slantwise instability, along
with the initial dry air presence and sleet potential make this a
difficult accumulation forecast. Current expectations are for only
very light sleet accumulation at the onset of precip. Then, as
snow increases, expect widespread amounts of 1 inch or more but
with bands of snow approaching 4 inches where that enhanced lift
can persist for any decent duration. If frontogenesis bands can
interact with this instability as well, then could see a couple
locations flirt slightly higher amounts. But again, this is
uncertain and identifying the location of snow bands is impossible
at this time. They may also be quite narrow with several inch
variations across areas as small as a county.

The rest of the forecast is rather benign. See a gradual warm-up
by the weekend. A deepening short wave trough will move into the
region by late Sunday, but may not get organized enough or have
access to enough moisture to produce much precipitation until it
gets east of the local forecast area. Do expect cooler conditions
as the wave passes, and a second piece of upper energy early next
week could keep at least some potential for light precipitation in
the area.

&&

.AVIATION...(For the 18Z TAFs through 18Z Thursday Afternoon)
Issued at 1130 AM CST WED DEC 17 2014

Low level dry air will generally keep KTOP/KFOE vfr until aft
02z/18 when moisture and forcing will increase with lowering
mvfr ceilings and vsbys in light snow and fog. Confidence has
increased that lowest vsbys in snow should occur in the 05Z-09Z
timeframe and have lowered to 1 mile...but may briefly go lower
at times in moderate. Snow intensity decreases at all terminal
sites by 09z...but ceilings and vsbys will lifr/mvfr through the
remainder of the TAF forecast in very light snow/fog...although
brief fzdz cannot be ruled out as well.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY from 6 PM this evening to 9 AM CST
Thursday FOR KSZ020>024-026-034-036-038>040-055-056.

WINTER WEATHER ADVISORY from 9 PM this evening to 9 AM CST
Thursday FOR KSZ008>012.

WINTER WEATHER ADVISORY from 3 PM this afternoon to 9 AM CST
Thursday FOR KSZ035-037-054-058-059.

&&

$$

SHORT TERM...Bowen
LONG TERM...Barjenbruch
AVIATION...63





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