000
FXUS63 KTOP 051150
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
550 AM CST Thu Dec 5 2013

...Update to aviation forecast discussion...

.SHORT TERM...(Today and Tonight)
Issued at 351 AM CST THU DEC 5 2013

Shortwave trough and speed max has/is exiting the CWA at 09z with
decreasing radar returns...and clouds in the north central.
Snowfall amounts have been light...primarily trace to a quarter of
an inch...except the far southeast corner near Garnett which
likely received no snow. The arctic air will continue to advect
into the area. The mid and high clouds will linger much of the day
helping to limit the highs to the low 20s north to mid 20s south...
however windchill indices this morning will commonly be in the
5 below to 5 above range. Although a few flakes of snow are
possible through sunrise...expect the day to be dry. The only
exception will be in the afternoon over the far southern CWA along
and south of a Herington...Emporia to Garnett line. This is where
increasing mid level lift ahead of approaching shortwave may be
just enough to overcome the dry levels for some snow to reach the
ground...although confidence remains low and will limit pops to
slight chance. A better chance for light snow will occur overnight
generally south and east of I 35 where a quarter to half inch of
snow will be possible...although the stronger lift will remain
well south of the area and a reinforcing surge of dry air in the
levels will limit the northern extent of the snow. The very cold
air will continue pour into the CWA through the period with lows
from near 8 above in the northwest corner to 15 with the clouds
and precip potential far southeast counties. Windchill readings
will commonly range from around zero far southern counties to
approaching 10 below across the northern counties.

.LONG TERM...(Friday through Wednesday)
Issued at 351 AM CST THU DEC 5 2013

By Friday morning, the better forcing for precip should be exiting
to the east as a shortwave passes over the state. Dry air is
expected continue its push south and any precip across east
central KS should come to an end. Friday could be one of the
coldest days as the arctic ridge exerts its greatest influence
over the forecast area. Persistent cold air advection through the
day with 925MB temps falling into the -13 to -15C range may keep
surface temps in the teens through the day. Think that Friday
night could clear out setting up decent radiational cooling.
However am not sure I buy into the sub zero temps from MOS since
snow cover is not likely to be wide spread and there remains some
wind in the boundary layer. Therefore have kept lows above zero
and in the single digits.

Models have shown good consistency from run to run and are in
fairly good agreement with a better defined shortwave affecting
the area by Saturday night and into the day Sunday. This leads to
good confidence in the forecast and have gone with likely pops for
Sunday. There should be good PVA and QG forcing for a broad area
of lift and precip ahead of the wave. Since there isn`t much of a
surface reflection or cyclogenesis progged with this wave,
mesoscale forcing appears to be minimized with the expected
precip. About the only question is the suggestion from the NAM and
to a lesser extent the GFS for some mid level dry air to move
into east central KS from the south. This could remove the
saturation from the dendritic and ice from the cloud causing
precip to switch from snow to freezing drizzle. Initially think
that the broad forcing ahead of the wave will probably maintain
some saturation for ice. By the afternoon when the dry air starts
to move in, the low level vertical motion starts to wane as
isentropic upglide switches to down glide. So am not sure the mid
level drying is a symptom of advection or just the loss of deep
vertical motion. At this point, think the mid level drying is more
a function of the loss of forcing and will just keep a mention of
snow in the forecast. However this will need to be monitored as
the system gets closer. The NAM and GFS continue to show snow to
water ratios between 15:1 and 20:1. Using a middle of the road
ration of 18:1 shows the potential for 2 to 4 inches of snow by
Sunday evening.

The snowfall should be tapering off by Sunday evening as forcing
from the upper wave moves northeast of the forecast area. Cold dry
air should once again dominate the forecast as another
continental polar airmass builds into the central plains Monday
and is reinforced by a clipper system moving over the northern
plains and upper Midwest on Tuesday. So have no precip mentions
with temps remaining below freezing for the first half of next
week.

&&

.AVIATION...(For the 12Z TAFs through 12Z Friday Morning)
Issued at 540 AM CST THU DEC 5 2013

Will keep the TAF forecasts vfr and dry through the fcst period
as -sn expected to remain south of the terminals. Scattered ac
clouds will thicken and lower to around FL050 by 00Z/06 as
northwest winds 10 to 15 kts veer to the north by 00z at speeds
around 10kts.

&&

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

$$

SHORT TERM...63
LONG TERM...Wolters
AVIATION...63