Area Forecast Discussion
Issued by NWS Topeka, KS
FXUS63 KTOP 061128
AREA FORECAST DISCUSSION
National Weather Service Topeka KS
528 AM CST Tue Dec 6 2016
...Update to aviation forecast discussion...
.SHORT TERM...(Today and tonight)
Issued at 315 AM CST Tue Dec 6 2016
Cold front associated with upper low over the Dakotas was through
the area at 08Z with strong pressure rises and gusty winds behind.
Enough wind and dry air coming in to get rid earlier fog. IR imagery
and surface obs indicate mainly mid and high moving through, but
clearer conditions upstream.
Cold air advection dominates the daytime period with some wind to
the surface and mainly sunny skies. Have trended a bit warmer on
temps based on latest short-range guidance. Surface high pressure
ridge steadily builds into the Central Plains allowing winds to
weaken late in the day, with cloud cover increasing from south to
north through the night as the next system approaches. Mid-level
isentropic upglide ramps up overnight on a respectable baroclinic
zone, but low levels remain rather dry and appears only very light
if any snow will reach the ground in the late night. The light winds
and cloud trends make for a challenging low temperature forecast.
Increased the gradient a bit with southern areas likely seeing
broken to overcast cloud by mid-evening.
.LONG TERM...(Wednesday through Monday)
Issued at 315 AM CST Tue Dec 6 2016
Focus for the extended remains fixed on the potential for snow
midweek and the well-advertised intrusion of arctic air for the
remainder of the work week.
Starting with the midweek storm, medium range guidance is mostly in
good agreement in the development of snow out ahead of a fast moving
H500 shortwave during the day on Wednesday. There remains up to a +/-
6 hr timing discrepancy between the various solutions, with the GFS
being a notable outlier in not bringing any precip to the CWA.
Mesoscale conditions are quite favorable for the development of
banded snow, with a region of strong H700 Fg forcing within the DGZ
coupled with a region of CSI/CI further aloft. Simulated radar
reflectivity fields in the CAMs depict this banded snow, with the
NMM and ARW cores actually pivoting this feature over the CWA.
Even though we are only 24-36 hours away from the onset of snow,
there remains several key uncertainties in how the event will
unfold. The first pertains to the onset of snowfall. A persistent 10-
15 kts of dry north-northeasterly flow in the lowest 5kft of the
atmosphere will slow the saturation of the lower troposphere and
likely result in a sharp gradient in snowfall amounts along the
northern side of the storm. However, how long the dry air will win
out is in question and directly affect overall snowfall amounts. The
second uncertainty lies in the placement of any mesoscale snow
band(s) This is a common forecasting problem for these types of fast
moving systems, though the spread amongst the guidance would keep it
somewhere in the CWA. In either case, the transient nature of the
storm will limit snowfall amounts, though there may be brief periods
of moderate snowfall that could have an impact on traffic. This
morning`s forecast update fine-tuned/raised the POP grids to denote
the regions most likely to see snow and also tweaked the QPF grids
to highlight the potential for banded snow that was not resolved in
the blended solutions.
Otherwise, made minimal changes to the remainder of the extended
forecast with the properties of the incoming arctic airmass well-
resolved by medium range guidance. This will bring the coldest
temperatures of the season with lows in the teens and single digits
for Thursday and Friday as surface high pressure moves through.
Return flow and warmer weather return for the weekend, though
solutions diverge with the evolution of a southern plains low for
the end of the weekend that may impact Kansas.
.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 528 AM CST Tue Dec 6 2016
VFR conditions anticipated. North to northwest winds will become
light around 0Z. Ceilings will steadily lower after 03Z but should
remain VFR for this forecast period.