Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 161747
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1147 AM CST Tue Dec 16 2014

...Update to aviation forecast discussion...

.SHORT TERM...(Today and Tonight)
Issued at 306 AM CST TUE DEC 16 2014

Upper shortwave trough this morning continues to lift northeast and
away from the region as ridging builds over the inter-mountain west. The
next system to impact the region was noted entering the western
coast. On the backside of the departing trough, strong pressure
gradient and northwesterly winds at the surface continue to advect
colder air into the CWA. Gusts over 30 mph were observed at times as
weak low level forcing produced areas of patchy drizzle earlier this
AM. This precip has since dissipated.

Increasing subsidence this afternoon will attempt to clear out the
low level stratus. Current observation in conflict with short term
guidance show decent uncertainty on timing and extent for cloud
cover to break. Forecast soundings advect drier air southward in the
mid levels, attempting to mix out the low level moisture while some
high resolution models are holding on to a broken cloud deck. Forecast
highlights best chances for sunshine across west central KS while
the Eastern half indicates scattered to broken clouds lingering
through late afternoon. Gusty northerly winds prevail through the
morning period before waning below 10 kts in the afternoon. Highs
struggle to warm this afternoon, despite the sunshine, into the
lower and middle 30s. A few upper 30s are possible across west
central KS if the clouds clear faster than anticipated.

Winds become light and variable tonight as surface high axis centers
just south and east of the area. Mid level moisture begins to
increase in response to the incoming shortwave trough. Scattered
clouds should provide some insulation, but not enough for lows to
descend into the upper teens near the Nebraska border, to the low 20s
elsewhere.

.LONG TERM...(Wednesday through Monday)
Issued at 306 AM CST TUE DEC 16 2014

The primary forecast focus through this period was on snow,
freezing drizzle, and drizzle potential from Wednesday evening
through Friday morning.

On Wednesday, will have weak upper ridging overhead with a broad
trough from the Rockies west to the Pacific. A short wave moving
within the base of the broad trough will move from the Oklahoma
panhandle through eastern KS by sunrise on Thursday. This system
will provide an area of enhanced vertical motion in the mid
levels, and will provide steeper lapse rates and less stable air
to interact with persistent isentropic ascent in the low/mid
levels. Also expect a period of frontogenesis on Wednesday night
to perhaps enhance lift a bit more in one or more bands across the
area. Temperature profiles on Wednesday night into Thursday
morning are expected to support snow as the primary precipitation
type, but the main question lies in saturation of the ice-bearing
layer. Expect this saturation to occur during periods of peak
lift, but otherwise could see some drying out of the dendritic
growth zone and probably the whole ice crystal growth zone at
times. This would result in periods of drizzle in addition to the
snow...and at least some of this drizzle would likely fall on sub-
freezing surface temperatures. All told, do expect at least some
period of snow with a good chance for light accumulations
generally less than 2 inches. If a band of stronger frontogenesis
indeed develops, could have a band of a bit more snow. But at the
same time, if moisture aloft is less-than-forecast, some areas
could end up with no snow on the ground in favor of drizzle. It
will be a delicate balance.

Drizzle is increasingly likely from 12Z Thursday through 12Z
Friday as the isentropic ascent persists from near the surface
through 750 hPa but the mid level lift and moisture further
dissipates. Kept at least some mention of light snow through this
period as there could be periods of deeper saturation to allow
some snow to mix in, but think drizzle will dominate. With drizzle
expected, the question then turns to temperatures, which are
expected to be very close to freezing through the period. Northern
Kansas may have the best chance of prolonged freezing drizzle at
this time, but any part of the forecast area could experience some
light icing and hazardous travel through Friday morning.

Drizzle and light snow are expected to decrease and shift
southeast on Friday as the next storm system will take a track
farther to the south...keeping the vast majority of precip out of
the local forecast area. Do not see any great precip chances
through the rest of the forecast, with a gradual warming trend
through Sunday. A northern stream short wave trough is forecast to
cross the area for Monday, but it seems to lack moisture locally,
so have not included any precip in the forecast until a slight
chance Monday night.

&&

.AVIATION...(For the 18Z TAFs through 18Z Wednesday Afternoon)
Issued at 1147 AM CST TUE DEC 16 2014

MVFR Stratus becoming rather thin with some breaks via visual
inspection and satellite imagery, though obs in clearing area
struggling to report no ceiling. Have backed off on VFR once again
with confidence remaining somewhat low. Light winds and high cloud
should dominate the remainder of the forecast.

&&

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

$$

SHORT TERM...Bowen
LONG TERM...Barjenbruch
AVIATION...65





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