Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 181720

Area Forecast Discussion
National Weather Service Topeka KS
1120 AM CST Thu Jan 18 2018

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 344 AM CST Thu Jan 18 2018

08Z water vapor imagery shows a closed low along the MN and Canadian
boarder moving towards the Great lakes while another weaker upper
low digs southeast towards the TX Big Bend. A closed low was noted
off the British Columbian coast with ridging from southern CA into
MT. At the surface, the arctic high pressure remained along the TX
gulf coast while a lee trough gradually deepened along the northern
and central high plains.

The forecast for today and tonight remains dry. There isn`t much if
any dynamics progged by the models within the sheared northwest flow
aloft. And if there were some, there isn`t any moisture to speak of
while the Gulf of Mexico remains closed off. So there may be a few
high clouds to interrupt what should be another sunny and dry day.
Highs are expected to be about 15 degrees warmer as models maintain
a southwesterly low level wind advecting warmer air north. However
snow cover is likely to inhibit the full potential of warming by
limiting the depth of mixing this afternoon. All the forecast
soundings struggle to mix the boundary layer past 950MB. So have
forecast highs in the lower and mid 40s thinking the raw model
output is to cold given good insolation and southwesterly winds, and
the MOS guidance is to warm due to the snow on the ground. There
should be a pressure gradient across the forecast area overnight
helping to keep a surface wind of 5 to 10 MPH. With some mixing and
continued warm air advection, lows are expected to be in the lower
and mid 20s.

.LONG TERM...(Friday through Wednesday)
Issued at 344 AM CST Thu Jan 18 2018

Midlevel ridging develops late week as a strong closed low
approaches the Pacific Northwest. Some of that energy will eject
from the parent low pressure towards the Southwest US. General west
to to southwest flow over the central Rockies will support deepening
of the lee side trough and eventually cyclogenesis once the dynamics
increase over the plains. On Saturday strong southerly flow advects
warmer temperatures and gulf moisture northward especially once the
low pressure tracks into western KS. A baroclinic zone should ensue
within the development of an inverted surface trough somewhere in
our area. This could potential cause sub-freezing temperatures on
the poleward side of the aforementioned surface boundary. With lower
tropospheric moisture in place any weak isentropic ascent within
this layer may support drizzle and or freezing drizzle depending on
the location relative to the boundary. Although the lost likely area
for ascent should be north of the boundary across portions of
southern NE, but any shift in the main system track could change
this location. There are still difference among the deterministic
models regarding the exact track, speed, and strength of the
midlevel closed low pressure. The potential for convection stills
exists with dew points in the 50s advecting into east central KS on
Sunday. MUCAPE is still on the order of a few hundred j/kg and the
deep layer shear is modest. There is not a strong connection with
the arctic air mass which by this time is well into Canada, but
there is enough cold air to support snow within the deformation
zone. Where exactly this band of snow sets up is still uncertain
with the better chances still across north central and far northeast


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1119 AM CST Thu Jan 18 2018

VFR conditions are expected through the TAF period with
southwesterly winds of 10 to 20 kts. Marginal LLWS is possible
again tonight but confidence in its occurrence was too low to
mention in the TAFs.




SHORT TERM...Wolters
LONG TERM...Sanders
AVIATION...Skow is the U.S. government's official web portal to all federal, state and local government web resources and services.