000
FXUS63 KTOP 130545
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1145 PM CST Thu Dec 12 2013

.UPDATE...
Issued at 840 PM CST THU DEC 12 2013

00Z NAM offers little in the way of additional confidence
regarding snowfall potential across the forecast area. The NAM
interpretation of this storm system has been a dry outlier of
sorts over the past several runs, and is continuing that trend
over the local area. So, while the GFS and especially the ECMWF
and GEM are on the aggressive side in terms of precipitation
amounts, the NAM is quite low. There is still indication of a
round of liquid precip in eastern KS between sunrise and noon, but
surface temperatures may be slightly above freezing as this occurs
and it will likely be difficult to know if there will be any
icing implications long before the precip actually moves in. By
late afternoon and into the evening, the NAM maintains the band of
weak ascent and saturation of the dendritic growth layer across
the area, but seems to indicate that the lift is too weak to
overcome drier air around 5000 feet and precip (and snow) amounts
are very low accordingly.

Where the 00Z NAM does lend some confidence is in the eventual
development of freezing drizzle across a large portion of the area
by mid evening and persisting overnight. This seems consistent
with previous runs of other models as well, and the main question
will be accumulation potential as well as whether it will freeze
on roadways given the relatively warm temperatures in the 36 hours
preceding the drizzle.

The potential still exists for accumulating snow in Eastern Kansas
in addition to the freezing drizzle, and the NAM remains at least
somewhat of a dry outlier compared to other model guidance. So
for now, will maintain the current forecast numbers as well as the
level of uncertainty regarding both precipitation type and
amounts. Do not anticipate any product issuance this evening due
to the significant uncertainty.

Barjenbruch

&&

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Friday)
Issued at 342 PM CST THU DEC 12 2013

The next system to affect the Central and Southern Plains was
spinning over southern California this afternoon at 20Z. An upstream
kicker was moving into the Pacific Northwest. The upper trough over
California will move east tonight into New Mexico and west Texas by
Friday morning. The upper trough axis will move east and extend from
central Kansas to near Dallas Texas by 00Z. With temperatures
falling below freezing tonight expect areas of fog, possibly dense,
to develop after midnight as low level moisture increases across
eastern Kansas into Friday. Lows tonight will range from the teens
along the Nebraska border to the upper 20s southeast of the Turnpike.

Friday, increasing chances of precipitation will occur across
eastern Kansas through the day as the upper wave approaches.
Increasing vertical motion along with a band of frontogenesis
extending from northeast Missouri southeast to near Emporia during
the afternoon hours. There exists discrepancies between the NAM and
the GFS, ECMWF and the GEM. The Nam was furthest south with light
QPF being generated while the others have been consistent with the
TROWAL and band of precipitation from north central Missouri into
east central Kansas. There exists potential for convection in the
afternoon hours across eastern Kansas and into Missouri. Temperature
profiles from the GFS and ECMWF suggest that precipitation should
begin as freezing rain or freezing drizzle. As temperatures warm to
above freezing by mid to late morning the precipitation is expected
to transition over the all rain. As the dynamic forcing cools the
column in the late afternoon expect the rain to change over to snow
by 4 PM. Accumulating snow is possible after 4 pm from northwest
Missouri to Topeka to Emporia. Amounts of around an inch are
expected by 6 PM in the above mentioned area. If convection does
develop then amounts would be locally higher. Currently plan to hold
off on an advisory for now through Friday afternoon and wait for the
next model runs to adjust upward or downward on expected wintry
weather on Friday.

.LONG TERM...(Friday Night through Thursday)
Issued at 342 PM CST THU DEC 12 2013

The period from 00Z to 06z Sat (Friday evening) will see some of the
strongest forcing within the dendritic layer over the eastern cwa as
trowel feature becomes better pronounced across the far eastern
counties. This will then coincide with the higher 6 hour
snowfall...generally from less than a half inch along and slightly
west of 99 highway to 1 to 2 inches additional along and east of 75
highway. This should give total snowfall amounts from less than an
inch w3est of an Emporia to Seneca line to 3 to 4 inches across the
far eastern county warning area in the Oskaloosa...
Eudora...Wellsville areas areas. Drier air aloft should overspread
the eastern counties through/beyond midnight with decreasing
snowfall as the system pulls away...but increasing chances for
freezing drizzle with little ice available. All precipitation should
end by 12z Saturday as colder air overspreads the areas again. After
lows in the teens and lower 20s...highs Saturday will range from the
middle 20s far northeast to lower 30s south. This will set the stage
of another cold night with the new snow and light winds beneath sfc
ridge axis. Have lows from near 10 above northeast corner to the
middle teens east central and north central.
As another trough digs into the western CONUS next week...the
northwest flow aloft will transition to a more zonal flow by mid
week with leeside troughing bringing sfc winds around to the south
and southwest and aiding a moderation in temperatures into the
middle and upper 40s all areas Monday through Thursday of next week.
Should also see little in the way of precipitation chances as the
stronger flow aloft will remain to the north of the cwa.

&&

.AVIATION...(For the 06Z TAFs through 06Z Friday Night)
Issued at 1145 PM CST THU DEC 12 2013

Confidence is increasing in the timing of limitations. Onset now
on track for much later MVFR issues in low cloud and showers.
Temperatures could be close to freezing initially but continued
south winds should keep the bulk non-freezing. IFR conditions in
SN and FZDZ still expected near 0Z through the end of the
forecast.

&&

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

$$

UPDATE...Barjenbruch
SHORT TERM...53
LONG TERM...63
AVIATION...65