Area Forecast Discussion
Issued by NWS Topeka, KS
FXUS63 KTOP 191808
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1208 PM CST Thu Dec 19 2013
...Update to aviation forecast discussion...
.SHORT TERM...(Today and Tonight)
Issued at 310 AM CST THU DEC 19 2013
0Z upper air data indicated a pattern change underway with strong
500 mb height falls across the western half of the CONUS as multiple
waves impact the West. Cold front at 850 mb was near the
Nebraska/South Dakota border, though recent 3-hour surface pressure
trends along the Front Range were below wintertime early morning
normals. Southerly winds and a good amount of cirrus were keeping
temps rather warm across eastern Kansas, still near to well above
normal highs, with dewpoints reaching the lower 40s.
Recent short-range guidance showing a slightly slower trend to the
cooler air filtering in this afternoon then surging on south tonight
as the northern end of the main West Coast wave slides east across
the Northern Plains. Despite continued high clouds, should be enough
mixing for the central and southern portions of the area to reach
the 50s. Saturation on south to southwest winds will continue below
an elevated mixed layer, but the lack of upstream stratus and model
forecast soundings suggest any drizzle should hold off until at least
early afternoon. Conditions become more concerning as the night
progresses with the cold air undercutting the moisture for more a
better opportunity for drizzle. Still looks like southern and
eastern areas will see the most precip -- still quite light however
-- where the deeper saturation and 800-900mb frontogenesis coincide.
With the slower cold air advection, appears any freezing precip
would hold off until at least early evening, spreading south by the
late night hours. Will need to monitor forecast trends for how
widespread and heavy any precip will be, as well as impacts as the
previous 24-48 hours of temps above freezing for much of the area
will have warmed area road surfaces.
.LONG TERM...(Friday through Wednesday)
Issued at 310 AM CST THU DEC 19 2013
Friday into Friday evening continues to bring the cold air down
into the forecast area with highs middle 20s to near 30 at best
and lows in the teens to mid 20s. By 12z Saturday, longwave upper
trof extends as a split flow type pattern with broad trof over the
Rockies and a southern extension southward into Mexico.
Southwesterly winds ahead of the southern trof continue to stream
moisture and warm air in the mid levels to the north, while the
colder air remains in place over the central plains. All these
features and how they interact are quite complicated for the
weekend forecast period. NAM had been the warmer outlier at the
surface with southerly winds continuing into early Saturday
morning, while the EC and GFS kept the colder airmass in place and
east to northeast winds reinforcing the placement of the colder
air. Leaned toward a colder solution, and appears 06z NAM and 3z SREF
may be trending in a a colder direction.
Once the cold air is in place, amount of precipitation progged
becomes a larger player. Surface temperatures are hovering right
around the freezing mark, and with no ice in the column anyway and
a good warm nose across most of the area, will start off carrying
a slight chance for freezing rain early Saturday, with a slow
increase in amounts from the south through the noon hour, then
need to introduce the potential for a changeover back to rain in
the afternoon hours and will carry a rain/freezing rain chance
across the southeast half with freezing rain or sleet north. As
the surface temps fall back below freezing and warm nose slowly
cools, evening hours will carry freezing rain or sleet chances.
May see some accumulations of a tenth of an inch in the morning
across the southeast and again in the evening, but will have the
potential for melting in rain chances in the afternoon. If
temperatures stay just a few degrees colder, then ice
accumulations could become more of a concern.
Best chances for changeover to snow come after midnight as column
saturates in the snow growth layer. Uncertainty in just where the
snow band will set up continues, with EC the farthest north across
the forecast area, the GFS middle and the NAM farthest south.
Looking at frontogenesis in the GFS led for a compromise somewhere
right in the middle and have gone with a broad 3 inch band across
the eastern half of the CWA and diminishing as you go northwest.
This is also similar to the NAM BUFKIT numbers. Placement of the
stripe of heavier snow will likely change as the event draws near,
and will have to watch ice/sleet potential early if the system gets
colder. Still good continuity to get the system east of the area
by Sunday afternoon, with brisk highs only in the low to middle
20s. A secondary cold front dropping out of the northern Rockies
keeps cold highs near 20 in place into Monday, although flow aloft
quickly becomes northwesterly late in the day.
Latest EC keeps slight recovery back into the 30s for Tuesday but
would be impacted by any snowcover we get from the Sat/Sun system.
Some differences arise for Wed as EC and GFS depict different
evolution of the next trof with the EC currently more amplified.
Could bring a cool down and even a chance for snow.
.AVIATION...(For the 18Z TAFs through 18Z Friday Afternoon)
Issued at 1203 PM CST THU DEC 19 2013
Stratus with MVFR ceilings have moved into the terminals. Lat this
afternoon and into the early evening hours a cold front will push
southward across the terminals. Winds will switch to the north and
IFR or low MVFR ceilings will develop with patchy freezing drizzle
possible through the mid and late evening hours. by 12Z FRI, the
freezing drizzle will end and ceilings will become MVFR.