Area Forecast Discussion
Issued by NWS Topeka, KS
FXUS63 KTOP 241734
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1134 AM CST Wed Dec 24 2014
...Update to aviation forecast discussion...
.SHORT TERM...(Today and Tonight)
Issued at 321 AM CST WED DEC 24 2014
Water vapor imagery at 0830Z shows the continued phasing and
resultant eastward translation of the northern and southern stream
shortwave troughs from the upper Midwest to the southern plains.
As the trough axis shifts eastward through mid morning the
precipitation...primarily in the form of light snow...will
steadily diminish and come to an end this morning. Will not carry
any precipitation chances west of a Council Grove to Seneca line.
The overcast conditions will erode west to east across the CWA
through the day...although at a much slower rate than
precipitation. The sun should begin to break out by late morning
in the north central before reaching the northeast and east
central counties after 3 pm. This will help to limit high temps
across the area as gusty northwest winds decrease in the
afternoon. Will keep highs in the middle to upper 30s. Mostly
clear skies and much less mixing will allow temps to fall off into
the middle 20s most areas.
.LONG TERM...(Thursday through Tuesday)
Issued at 321 AM CST WED DEC 24 2014
There is below normal confidence in the forecast as models
continue to show poor continuity from run to run. The
ECMWF/GEM/NAM have trended slower with the next trough developing
over the Rockies. Meanwhile the GFS appears to be a fast outlyer
in swinging the trough through the plains on Saturday and building
a surface ridge of high pressure into the area. Therefore have
followed the WPC`s preference of a slower trend and relied more
on the ECMWF and a model blend for the forecast.
Christmas Day and Thursday night are expected to remain dry and
mild due to a relatively dry airmass in place and low level warm
air advection. Southerly winds continue to look gusty and have
bumped highs up a degree or two thinking the model forecast
soundings may be under doing the mixing.
The models continue to show some moisture advection into western
MO along and ahead of the front on Friday and Friday night. While
there is not an obvious shortwave moving overhead at the time,
there should be some reasonable frontogenesis along the front with
possible deformation within the axis of speed shear aloft.
Therefore have kept a chance of precip for precip beginning Friday
afternoon through the day Saturday. Since the GFS seems to be to
fast with the surface ridge and dry air, it did not make much
since to use its forecast soundings to figure out precip type.
Therefore tended to draw transitions of liquid to frozen precip
based more on the thicknesses of the ECMWF. There will likely be
some fine tuning to the forecast in this respect.
Think that by Saturday afternoon, things should be shifting east
of the forecast area. But since trends have been to slow these
systems down, have not removed POPs completely by Saturday
afternoon in case models continue to show a slower progression.
The forecast is dry for Saturday night through Tuesday, and this
may be optimistic. But with there being little agreement on the
synoptic pattern among the medium range models, its hard to pick
out these transient chances for precip when models change them in
the next run. The one period I`m a little concerned about is
Monday and Monday night when the ECMWF brings a wave within
the mean trough across the plains generating some light QPF. If
there starts to be some consistency with this idea, can have
better confidence in POPs for this period. For now, will opt to
keep a dry forecast rather than have slight chances spread out
over several days.
With a longwave trough developing somewhere over North America
next week, arctic air should be able to move south bringing an end
to the above normal temps we`ve had over the last week or so. The
forecast reflects this chance with highs on Tuesday in the 20s.
.AVIATION...(For the 18Z TAFs through 18Z Thursday Afternoon)
Issued at 1130 AM CST WED DEC 24 2014
Previous forecast on track. Back edge of stratus just west of a
CNK to SLN line and slowly eroding east. Expect a very gradual
lifting of stratus this afternoon, but still within MVFR
category. MHK should break out in the 21-23Z time frame while
expect TOP and FOE break out between 00 and 03Z. After that, VFR
expected with winds switching to west and then south at less than
10 kts overnight. Pressure gradient strengthening later tonight
and southerly winds of 12 to 20 kts should be in store for