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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

FXUS63 KTOP 210958

Area Forecast Discussion
National Weather Service Topeka KS
358 AM CST Thu Feb 21 2019

.SHORT TERM...(Today and tonight)
Issued at 347 AM CST Thu Feb 21 2019

This weekend's upper trough is currently deepening southward across 
the western conus, resulting in southwest flow aloft this morning as 
mid level moisture advects into the region. Sfc high pressure 
centered over southeast Kansas has allowed for patchy dense fog to 
develop over portions of the area. Visibilities may occasionally 
fall below a mile at times, however the high clouds should help 
mitigate widespread dense fog from forming and the need to issue an 
advisory at this point in time.

Dry and warming trends expected this afternoon as gusty southerly 
winds at 925 mb carry warmer temps into the region. EML around 925 
mb on forecast soundings mixes down to the sfc, resulting in highs 
potentially in the upper 40s. However, given the snow cover in place 
went closer to the latest RAP and HRRR runs which were slightly 
cooler in the lower to middle 40s. Still monitoring a lead impulse 
that lifts northeast through western Kansas in the afternoon. While 
medium range guidance stretches precip into our north central 
counties, most short term runs hold the optimal lift towards western 
Kansas. Decided to keep a slight chance mention with precipitation 
type being mostly rain perhaps mixing with light snow if it persists 
after peak heating.

Main upper trough begins to shift eastward this evening inducing lee 
troughing over eastern CO overnight. Moisture advection within the 
sfc to 850 mb layer spreads northward after midnight noted by 
increasing isentropic lift towards central Kansas by 12Z. Given 
temps below freezing at this time, light freezing drizzle is 
possible in the morning period for the western half of the CWA. 
Little to no ice accumulations are expected. Chances for drizzle 
increase and spread east across the entire area in the afternoon 
Friday as the aforementioned sfc trough nudges eastward. Precip type 
for much of the area should be drizzle as temps warm into the low 

.LONG TERM...(Friday through Wednesday)
Issued at 347 AM CST Thu Feb 21 2019

The biggest story for the extended period continues to be focused 
around a potentially impactful winter storm system intensifying over 
the region into the day Saturday before exiting the area by early 
Sunday morning.  

Into the morning on Saturday, the semipermanent Bermuda High and 
tropical ridge will play a role in the western CONUS trough 
undergoing further amplification before finally lifting into the 
central Plains early Saturday morning. Unfortunately, some spread 
in guidance remains at this time with colder and more southern 
track solutions of the NAM, ECMWF and Canadian suggesting that the
forecast area may see more impacts from the winter weather 
perspective than from a thunderstorm perspective. The GFS overall 
deepens and intensifies the entire system but maintains a more 
northerly track of the surface and upper level features across the
area into the midday and early afternoon time frame before 
developing an intense deformation zone and frontogenetical snow 
bands on the northern and northwestern side of the system. 

Regardless, a strong upper level closed low rapidly undergoes 
cyclogenesis on Saturday over the central Plains with strong surface 
and low level mass responses.  As a result, there is just a short 
period of time across the area mainly north I-70 and toward the Hwy 
36 corridor where temperatures profiles remain cold enough to 
support a short duration of freezing rain -- with only minor ice 
accumulation expected -- before WAA boosts temperatures and dewpoints 
more supportive of a transition to all rain for much of the morning 
and into the afternoon across the area.  Intense upper flow and a 
strong jet streak work into the area as the upper trough pivots into 
a negatively tilted aspect and enters its maturing phase.  As a 
result very strong ascent is expected to enhance a broad 
precipitation shield over the area. Right now PW amounts around 1 
inch look possible as the TROWAL feature and warm conveyor wrap into 
the area.  Instability should support thunderstorms for portions of 
the day. Deeper more surface based storms look to stay southeast of 
the area though at this point, especially with trends of a more 
southern shift in the overall system appears to be the more likely 
outcome. With several previous systems bringing moisture to the area 
and still cold ground temperatures, there could be some localized 
areas that see at least some concern for minor flooding impacts.  

The transition to the CAA component over the area starts by mid to 
late afternoon over north central areas.  As temperature profiles 
cool and remain deeply saturated with plenty of opportunity for ice 
introduction, do expect snow to take over as the dominate p-type. 
There could be a couple main bands of snow set up with intense 
frontogenetical forcing behind the system.  Mechanical mixing will 
likely help strong H85 winds mix to the surface.  This along with 
the snow could be potentially very impactful to areas over north 
central KS and again along Hwy 36 mainly.  However, with a large 
deformation zone across the area as the system continues to lift out 
of the area, the threat of high winds and snow could work as far 
south and east as the I-335 and I-70 Kansas turnpike vicinity. Right 
now, at least the opportunity for several inches of snow look 
possible over north central areas with generally less further 
southeast.  Blowing snow could be a concern even with small snow 
amounts.  Have held off issuing a winter storm watch for now as the 
system is just far enough out and high enough uncertainty remain 
that defining the most likely areas are still in question. 

As the storm system exits the region, the upper flow pattern 
transitions to a more progressive zonal pattern.  The next possible 
opportunity for mixed precip comes into the picture by mid week of 
next week.  Much variability on this set up, so have maintained only 
slight chance POPs brushing the area.  Temperatures remain slightly 
below normal with highs in the middle 30s and lows around 20.


.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.