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000
FXUS63 KTOP 032344
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
544 PM CST Tue Dec 3 2013

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Wednesday)
Issued at 333 PM CST TUE DEC 3 2013

The short-term focuses on the transitioning conditions that are
expected across the area from the advancing storm system. As of this
afternoon, the large mid-level low was situated over the northern
Rockies with a trough beginning to develop, stretching southwestward
into northern California. Tonight into Wednesday this mid-level low
is expected to progress eastward into the Northern Plains with the
deepening positively-tilted trough advancing into the Rockies. As
this low progresses into the Northern Plains, it will finally help
to push the area of surface low pressure currently stretching from
the Texas/Oklahoma panhandles northeastward into Iowa south of the
region tonight into Wednesday. The associated cold front had nosed
into north central Kansas earlier this morning but has stalled out
over far northern Kansas. But the advancing surface and mid-level
lows will help to push this cold front slowly southeastward across
the area tonight. By Wednesday morning, models show the front with a
southwest-to-northeast orientation over extreme eastern Kansas near
the Kansas/Missouri state border before progressing into Missouri
through the day.

As this front tracks southeastward through the forecast area tonight
into Wednesday morning, models show modest frontogenetical forcing
from the surface to around 850mb along with some isentropic lift in
the 300k to 315k layer. However, there is very little in the way of
available moisture. But model soundings do show a shallow layer of
saturation up to about 800mb developing after 06z across much of
eastern Kansas and remaining in place through much of Wednesday. Due
to the combination of this shallow moisture and frontogenetical
forcing, a handful of short-range and synoptic-scale models have
highlighted the potential for areas of drizzle to develop. In
addition to the potential for drizzle, models are once again
suggesting the development of some patchy to dense fog over much of
east central Kansas overnight into Wednesday morning as this stratus
deck develops. With the front and associated forcing progressing
into Missouri by Wednesday afternoon, expect the potential for
drizzle to diminish and the stratus deck to slowly mix out through
the afternoon hours.

As for temperatures, winds tonight will shift to the north-northwest
behind the front, resulting in significant caa as much colder air
comes surging southward into the area on Wednesday. Low temperatures
tonight are a bit tricky with the combination of caa into the region
and a developing stratus deck over east central Kansas. At this
time, have a spread of low 40s over east central Kansas and middle
20s over north central Kansas for tonight, but where exactly the
tightest temperature gradient sets up will be dependent on how
expansive the stratus deck will be over the area. The cold surge will
continue to progress southward through the day on Wednesday with
850mb temperatures dropping into the -8C to -12C range over north
central Kansas. Temperatures will once again be a bit tricky for
Wednesday as the stratus deck over east central Kansas may not start
to dissipate until the afternoon hours. Regardless, do not
anticipate temperatures to warm up more than a few degrees from the
overnight lows with highs ranging from the mid 40s over east central
Kansas to around 30 degrees over north central Kansas.

.LONG TERM...(Wednesday Night through Tuesday)
Issued at 333 PM CST TUE DEC 3 2013

Temperatures:
The recurring theme throughout the long term forecast is a period of
very cold temperatures. Highs from Thursday through Tuesday look to
stay in the 20s...almost 20 degrees below normal. Overnight lows
will be more dependent upon cloud cover, of which there will be
plenty, but single digits seem likely to occur more than once in the
coming week, and if conditions come together a few negative readings
would not be out of the question.

Precipitation - Late Thursday into Early Friday:
A short wave trough, rather evident in model PV analysis, will
impact the region during this period. Some of the best PV advection
will occur over the forecast area, and is quite likely to coincide
with a period of favorable upper jet dynamics and bands of moderate
frontogenesis in the mid levels. Those factors all suggest a period
of moderate forcing for ascent in the mid and upper levels during
this period. However, the continental polar airmass will have
overtaken the entire area by this time, and while plenty cold for
snow it will also be characterized by very dry air along with
persistent dry air advection. It seems that there will be a battle
between the strength of the ascent aloft and the strength of the dry
air in the low levels. The uncertainty in this balancing act has
been present in recent model runs as the 12Z NAM indicated stronger
lift would overcome the dry air and produce accumulating snow across
much of the forecast area while the 12Z GFS/ECMWF/GEM were weaker
with the lift and drier with the airmass...resulting in little to no
precipitation. At this time, all solutions seem plausible, although
a drier solution is favored. The best chance for precipitation will
be in far southern parts of the area where the dry air is not as
deep nor as strong, and have gone with higher POPs and some minor
accumulations possible south of I-35. As for the remainder of the
forecast area, it seems that slight chance to chance POPs for snow
are quite appropriate. If the lift is stronger and the snow bands
develop, then minor accumulations could occur, but there seems to be
a better chance for almost no snow at all.

Precipitation - Saturday night through Sunday night:
Another short wave trough is forecast to drop into the four corners
region by Saturday night and then eject northeast across the Plains
on Sunday. Expect a prolonged period of weak to moderate ascent with
this system beginning on Saturday night as a broad area of moisture
advection and persistent isentropic ascent overspread the region.
The ascent will then continue as the upper trough crosses the area
on Sunday. Moisture appears to be adequate, and forcing for ascent
adequate as well, to support widespread light snowfall through this
period. At the current time, it appears that accumulation would be
possible across the entire area, but amounts would very likely be
light for everyone as well.

&&

.AVIATION...(For the 00Z TAFs through 00Z Wednesday Evening)
Issued at 544 PM CST TUE DEC 3 2013

Another night of a complicated forecast. Boundary layer near to
upstream of KTOP and KFOE remains near saturation with patchy IFR
stratus remaining. Light winds and only some high cloud should
bring visbys down in the next several hours at these locations,
possibly going to LIFR before a cold front arrives. Should see
some improvements behind the front but still IFR in stratus and
possible DZ. KMHK is already behind the front with IFR stratus coming in
by 06Z. All sites should reach MVFR by 18Z. Uncertainty on the
specifics is again low.

&&

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

$$

SHORT TERM...Hennecke
LONG TERM...Barjenbruch
AVIATION...65