Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 030529

National Weather Service Topeka KS
1129 PM CST Fri Dec 2 2016

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Saturday)
Issued at 326 PM CST Fri Dec 2 2016

Water vapor imagery showing upper pattern becoming more amplified
with an upper low digging south into the Baja area and high cloud
spilling northeast into the Central Plains. Weak, nebulous high
pressure at the surface keeping winds light.

Main challenge through Saturday is precipitation chances and
types. Models are in good agreement in generalities with moisture
column slowly saturating from aloft tonight into early Saturday as
moisture pushes north ahead of the southern wave and isentropic
lift increases over the weak high in response to a faster-moving
wave progressing east across into the Northern Plains. Best
isentropic lift takes place late tonight into early Saturday, with
upper forcing with the wave moving in in the afternoon. Models
tend to keep the night dry, but model soundings showing moisture
rather deep in decent forcing by morning, so have sided on the
caution side with small precip chances in the south and west late,
then increasing chances in the late morning and afternoon as the
western wave enters. Surface to lower level temperature values and
trends will be important in precip types, with evening cloud
persistence a prime factor. Latest satellite loops showing some
breaks upstream, so have a faster cool-down this evening, but may
be rather steady overnight. Expect the bulk of the column to be
below freezing for mainly snow potential in the northwest, but
temps could be close enough to freezing for some brief freezing
rain in southern areas, but with still warm surface temps, any
precip should struggle to make any significant impact. Amounts
continue to look light overall, with continued southeast winds
bringing warmer and wetter low levels Saturday for precip to turn
to rain.

.LONG TERM...(Saturday night through Friday)
Issued at 326 PM CST Fri Dec 2 2016

The long term will be dominated by a longwave trough pattern
across the western and central CONUS and will subsequently feature
a more active pattern than we have seen over the last week.

A sharp meridional 500 mb trough will be moving into the Great
Plains at the onset of the period Saturday night with increasing
kinematic forcing and modest moisture transport below 700 mb
helping to fuel precipitation through the overnight. The
difference between rain and snow will hinge on the near-surface
thermodynamic profile and a wet-bulb decrease of only a degree or
two could result in substantial changes in p-type. Given the
clouds and warm near- surface layer, did nudge up MinT values by a
degree or so above the blends in the NE CWA and lowered snow
ratios, but left up to a tenth of an inch of snow in the grids for
the 00-12Z timeframe. Given the strong 800-500mb ascent evident
in the STJ and FNB bufr soundings (right through the DGZ), am
leery about removing accumulations altogether. Either way, the
increasing WAA and short duration of the precipitation should
result in the snow melting very quickly an impacts should be

A more zonal flow pattern sets up for the latter part of Sunday
and early Monday before a strong cold front sweeps through Monday
night. This will bring the coldest air of the season for the
middle to latter part of the week with 850 mb temps plummeting to
-14 to -17 C by Wednesday night. Lows in the teens and maybe the
upper single digits in the NW will be likely Wednesday and
Thursday nights. A band of waning elevated Fg forcing behind the
departing front may bring some light snow to the area, but the
majority of the medium range guidance fails to produce any
measurable QPF. Confidence in the next possible storm on Wednesday
is much lower with little in the way run-to-run continuity amongst
the EC/GFS. The 12Z runs of both models de-amplify the culprit
trough and developing a more progressive system, resulting in
lower snowfall amounts (in fact, the 12Z GFS is completely dry for
NE KS). If this trend continues, POP grids for this time period
will need to be adjusted down significantly.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1119 PM CST Fri Dec 2 2016

VFR conditions are likely through 18Z at which time showers and
lower cigs may approach MHK but are likely to stay west. By 22Z-
01Z ceilings and visibility should decrease quickly into MVFR as
widespread showers impact all TAF sites. Expect cig/vis to
continue decreasing to IFR by 03Z or so.




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