000
FXUS63 KTOP 010543
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1143 PM CST Sat Nov 30 2013

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Sunday)
Issued at 337 PM CST SAT NOV 30 2013

An intense upper level trough across southeast AK will dig southeast
into the pacific northwest late Sunday afternoon. A strong upper
level jet will dig southeast across the central Rockies into the
northern and central high plains by 00z MON.

Tonight, a weak cold front across eastern NE will push southward
across the CWA. Temperatures are not really much colder behind the
front. Forecast soundings show a very shallow layer of saturation
developing near the surface south of the KS turnpike. There is very
dry air about 50M above the surface. Not really sure if moisture
will deep enough for any ground fog to develop...but there could be
some patchy ground fog in river valleys and around lakes near
sunrise on Sunday morning across east central KS.

Sunday, a weak surface ridge will build southwest across northeast
KS. Expect increasing high cloudiness with light northerly surface
winds. Highs may be a few degrees cooler across extreme northeast KS
with upper 40s but the southern and western counties should reach
the mid 50s once again.

.LONG TERM...(Sunday Night through Saturday)
Issued at 337 PM CST SAT NOV 30 2013

Sunday night, an embedded shortwave develops within the mid-level
zonal flow over the Dakotas and is expected to track just east of
the cwa by Monday morning. As this wave continues to dig
southeastward into the Ohio River valley, a broad mid-level trough
will set up across much of the eastern U.S. with the central CONUS
left under northwesterly flow aloft. Main focus, though, for the
extended period will be with the progression of the strong mid-level
low that models show moving into the Pacific Northwest on Monday.
Ahead of this system, surface high pressure will prevail over the
central U.S., but will quickly get pushed eastward as the expansive
surface low pressure system associated with the mid-level low
advances eastward into the northern Rockies on Monday and starts
nosing southeastward into the Central Plains by Monday afternoon and
evening. With the forecast area wedged between these two systems,
southerly winds will prevail across the region Monday and Tuesday,
resulting in decent waa and afternoon highs reaching nearly 10F
degrees above normal into the mid/upper 50s.

Tuesday into Wednesday, as the mid-level low progresses eastward
across the far northern U.S., a positively-tilted trough will
develop, significantly digging southwestward toward California. As
this trough deepens while advancing toward the central U.S., it will
help to push a cold front southward toward the region. However,
there is still a great deal of uncertainty with the timing of this
front as there continues to be both model-to-model and run-to-run
inconsistencies. The most significant model inconsistencies can be
seen in the GFS, in which it was become much more progressive than
its earlier runs, bringing the cold front into north central KS by
Tuesday morning -- nearly 24hrs faster than yesterday`s 12z run. The
tail-end of the NAM 12z run extends into the Tuesday night period and
seems to be trending more with the GEM in bringing the front through
Tuesday night. The ECMWF still remains the slowest, bringing the
front through Wednesday morning, but it`s worth noting that it has
been the most consistent from run-to-run amongst the models. As a result
of these model comparisons, have trended with a blend of the
ECMWF/GEM/NAM for mid-week and have ignored the very inconsistent
GFS. Even with this blend, there is still some uncertainty with
temperatures during the Tuesday night and Wednesday timeframe,
depending upon the exact timing of the front. In this case, have
trended a bit more toward the much more consistent ECMWF with the
frontal passage. With this in mind, have kept the cwa in the warm
sector for Tuesday night with southerly winds keeping temperatures
in the mid 30s to low 40s. For Wednesday, have the spread of mid-50s
over east central KS to upper 30s over north central KS to account
for the frontal passage. Again, will need to continue to monitor
model trends to gain better confidence in this frontal passage and
its affects on the temperature and wind forecast.

One aspect that is consistent amongst the models is that as winds
veer to the north behind the cold front, this northerly flow will
advect much colder air into the region, resulting in temperatures
plummeting well-below the seasonal normals for the latter part of
the week into the weekend. By Friday, models are in pretty good
agreement in having 850mb temperatures tank into the -8C to -15C
range. This translates into a significant drop in surface
temperatures with highs generally in the mid/upper 20s and lows in
the teens and possibly even single digits Thursday through Saturday.

As for precipitation chances, while models show some modest forcing
with the frontal passage mid-week, soundings show very little in the
way of available moisture over the region. As the surface low
advances into the Central Plains, the southerly winds on the south
side of the system will eventually be able to advect moisture from
the Gulf of Mexico northward, but models continue to trend with this
better surge of moisture remaining east of the area. As a result,
have removed PoPs for Tuesday night and continued to trim back PoPs
for Wednesday, keeping slight chances only over extreme eastern KS.
Another area of low pressure moves into the central Rockies mid-week
and will slide southeastward into the Southern Plains during the
latter part of the week. As this low pivots around the area, there
is a slight chance for some light precipitation Wednesday night,
which could be in the form of snow for most locations as
temperatures quickly plummet behind the cold front. As this system
progresses into the Southern Plains, models are in decent agreement
in there being a combination of both decent forcing as well as
significant available moisture being advected northward from the
Gulf of Mexico. The question lies with how far north this
precipitation will extend. GFS has surface high pressure surging
into the area, bringing in drier air. The ECMWF/GEM on the other
hand keep the surface high further northwest of the cwa and have a
heavier band of precipitation developing over the area Thursday
night into Friday before finally exiting to the east Friday night.
Will need to continue to monitor these model trends in future
updates, but at this time have slight-chance to chance PoPs in for
Thursday through Friday.

&&

.AVIATION...(For the 06Z TAFs through 06Z Sunday Night)
Issued at 1143 PM CST SAT NOV 30 2013

The forecast for the next several hours is not much clearer. KTOP
is already experiencing some reduced visibilities, but this appears
to be more of an FU issue than BR, though RH values are rather
high. Still believe BR/FG will become more consistent with little
if any mixing and crossover temps already surpassed. Have gone
lower at KTOP given trends, and put off the return to VFR a bit
longer.

&&

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

$$

SHORT TERM...Gargan
LONG TERM...Hennecke
AVIATION...65