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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

                            
000
FXUS63 KTOP 210525
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1125 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 06Z TAFS through 06Z Thursday night)
Issued at 1122 PM CST Wed Feb 20 2019

Patchy fog is likely through mid-morning at all sites. Expect vsby
to remain MVFR to VFR through the night. Easterly winds will 
increase to 7-10 kt after sunrise and persist through the end of 
the TAF period. 

&&

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

$$

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