Area Forecast Discussion
Issued by NWS Topeka, KS

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790
FXUS63 KTOP 202331
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
531 PM CST Wed Feb 20 2019

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Thursday night)
Issued at 359 PM CST Wed Feb 20 2019

Southwest upper flow taking hold behind the recent upper trough with
large-scale subsidence in its wake. Low cloud slowly diminishing on
west to southwest low-level winds with a modest surface ridge axis
over southwestern Kansas at 20Z. Temperatures at most locations in
the lower to middle 30s in combination with overall good insolation
to allowing for some of the overnight snowfall to already melt.

A broad upper trough along the west coast digs south downstream
winds backing. Cirrus field to the south will likely work its way
north into at least east central Kansas overnight. Winds should
gradually weaken with the weak surface ridge moving in from the
southwest. Wind speeds near the top of the boundary layer remain
around 10 knots for much of the night, switching to the south to
southeast overnight. With these wind speeds and the increasing
cirrus, conditions aren`t prime for a widespread fog event, but
given the snow melt occurring, will go ahead with patchy fog at this
time.

A modest shortwave makes its way NNE out of the southwest states
early Thursday and there may be enough mid-level moisture and ascent
for some very light precip in light rain or light snow in portions
of north central Kansas in the afternoon. Increased cloud for the
entire area with an easterly wind component over the residual snow
field leads to a lower-confidence high temperature forecast. Leaned
toward the cooler guidance given the above factors.

Continued south to southeast low-level flow downstream of slowly-
deepening lee troughing brings some shallow saturation into western
and southern areas late Thursday night. Upglide remains weak as well
so drizzle concerns remain low at this point but will need to be
monitored.

.LONG TERM...(Friday through Wednesday)
Issued at 359 PM CST Wed Feb 20 2019

Embedded within a consolidated deep and highly amplified baroclinic
zone over the CONUS, a deep cyclonic perturbation will close off
along its northeastward track from the Southwest States to the
central Great Plains from Friday into Saturday. Rapid deepening of
the corresponding midlevel cyclone will correspond to an
intensifying, cyclonically-curved midlevel speed maximum within the
southern semicircle of the cyclone. This will especially be the case
Friday night into Saturday, once the exit region of the midlevel jet
streak emerges over the High Plains, resulting in surface
cyclogenesis owing to potential vorticity conservation.

Strengthening isentropic ascent well in advance of the strongest
deep forcing for ascent will support increasing light precipitation
chances through the day on Friday into Friday night, with
drizzle/light rain increasing in coverage. Pockets of freezing
drizzle will be possible across the northern half of the forecast
area, conditional upon subfreezing surface temperatures persisting
amid the cold side of sharpening surface baroclinicity. A light
glaze of ice -- at most -- is possible through Friday night.

An influx of partially modified Gulf moisture will continue within
the warm conveyor, and especially become enhanced within the eastern
half of the CWA, by late Friday night into Saturday morning. This
will contribute to increases in precipitation rates and slight
destabilization for parcels elevated atop the poleward-sloping
baroclinic zone. Isolated thunderstorms could occur as early as
Friday night, and then persist into Saturday. Precipitation should
become mostly liquid rain across the forecast area by Saturday
morning.

Then on Saturday, particularly intense deep ascent will overspread
the area. The small radius of curvature of streamlines accompanying
the compact midlevel cyclone will encourage extreme differential
cyclonic vorticity advection within its left exit region, fostering
rapid surface cyclogenesis northeastward across parts of Oklahoma
and Kansas. While models differ regarding the exact track of the
surface cyclone, the large-scale antecedent consolidation of
baroclinicity accompanying a more single-stream upper-air pattern
casts higher-than-normal confidence for a day-4 mass-field forecast.
This extends to growing confidence that a strong surface pressure
gradient will become established within the cold conveyor of the
cyclone, within which ample deep forcing for ascent will emanate
from a multitude of factors -- e.g., DCVA, frontogenesis, jet-streak-
related ageostrophic circulations, isentropic ascent. This will
support concurrent precipitation amid the strong winds in some
areas. Cold-air advection on the west and north side of the cyclone
track will support precipitation changing over to snow through the
day on Saturday, with heavy snow possible within the deformation
zone of the cyclone -- especially in north-central Kansas.
Elsewhere, the delayed onset of colder air may mitigate
spatiotemporal phasing of stronger ascent with adequate
thermodynamic profiles for snow. With strong winds overtaking the
region on Saturday within the western semicircle of the surface
cyclone, significant blowing and drifting snow will be possible, and
serious impacts may result.

The overall tendency of Numerical Weather Prediction model guidance
has been a slightly more southern track to the midlevel cyclone. As
a result, confidence in surface-based convective potential is
diminishing. Nevertheless, elevated buoyancy will foster a chance
for thunderstorms Friday night into Saturday, and the potential for
lightning to accompany the snow cannot be ruled out given the
intense forcing for ascent.

Ultimately, mesoscale details regarding the exact cyclone track and
accompanying heavier-precipitation bands cast some degree of
uncertainty regarding potential impacts. Present indications are
that the most dangerous winter-weather conditions will be northwest
of a line of a line from Marysville to Abilene, though high-
impact/dangerous winter-weather conditions will be possible across
the entire forecast area. Everyone is urged to monitor the latest
information from the National Weather Service in Topeka, as
consensus among model solutions probably improves in closer time
range to this event.

Also of note, the snow pack -- combined with some snow melt -- will
contribute to the potential for precipitation in the Friday-Saturday
time frame to run off quite efficiently. With upwards of 0.5 to 1.0
inch of liquid-equivalent precipitation expected to occur, rises on
area rivers and streams could support a non-zero potential for
localized flooding concerns during the weekend.

Drier air will overtake the region for Saturday night into Sunday as
the aforementioned strong system departs the region. Ample cold-air
advection behind the departing cyclone will reinforce a polar air
mass across the region for the remainder of this weekend into early
next week. Additional perturbations embedded within deep/broadly
cyclonic flow extending from southern Canada into the northern
States will correspond to additional polar air surges into the
forecast area through next week -- with wind chill readings
potentially falling below zero during the midweek time frame, with
chances for frontogenesis-enhanced snow bands at times crossing the
region.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 528 PM CST Wed Feb 20 2019

VFR conditions are expected through the bulk of the forecast
period. Patchy fog may, at times, reduce vsby between 07-15Z.
Current indications suggest vsby should remain MVFR to VFR through
the night, however.

&&

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

$$

SHORT TERM...65
LONG TERM...Cohen
AVIATION...Baerg



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