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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

                            
000
FXUS63 KTOP 192340
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
540 PM CST Tue Feb 19 2019

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Wednesday)
Issued at 246 PM CST Tue Feb 19 2019

Little overall change has been made to the ongoing forecast. 
Isentropic ascent in the lower/middle troposphere amid a long fetch 
of southwesterly flow aloft continues to support snow development 
from northern Oklahoma into central and southeast Kansas -- in 
advance of a high-amplitude western states trough. This activity 
will continue to develop northward/northeastward through tonight.

A boost in potential instability, resulting from the interception
of a trough-accompanying dry slot with the preceding warm 
conveyor, has supported a ribbon of convectively enhanced snowfall
rates from northwest Oklahoma into central KS. This activity has 
consolidated in a continuous band of higher snowfall rates that 
will continue to develop north-northeastward into north-central 
KS this evening. Corresponding snowfall totals have been increased
in Clay and Ottawa Counties, which have been added to the Winter 
Storm Warning. Otherwise, no changes have been made to the winter-
weather highlight configuration, with 6-7 inches of snow expected
across the updated Winter Storm Warning area, and 2-5 inches of 
snow expected across the remainder of the forecast area warranting
the continuation of the Winter Weather Advisory.

Precipitation will taper off from southwest to northeast late 
tonight into Wednesday morning as the saturated layer becomes 
increasingly shallow, and stronger ascent moves northeast of the 
region. Precipitation may still be ongoing as cloud ice becomes 
negligible, resulting in freezing rain/drizzle mixing with the 
precipitation late tonight and Wednesday morning prior to its 
cessation. A glaze to around one-tenth inch of ice accumulation 
will be possible -- greatest along and southeast of the Kansas 
Turnpike. Winds will veer to the west behind surface low pressure 
departing the region on Wednesday, which will support weak cold 
advection through Wednesday afternoon -- with only partial 
clearing possible across north-central Kansas late in the day. 
These factors will mute diurnal heating, with highs expected to be
in the lower/middle 30s.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 246 PM CST Tue Feb 19 2019

In the long-range forecast period, concerns are increasing about 
potential impacts from the next system set to bring precipitation to 
the area on Saturday.

On Thursday, the upper atmospheric pattern over the country will 
resemble a similar look to a pattern that has become common this 
season. An upper level trough will be digging over the western CONUS 
with ridging southeast of the US, placing our region under southwest 
flow aloft. In the meantime, a surface ridge axis just to our 
northeast will begin shifting farther northeast deeper into the 
Midwest while a surface low develops near the Four Corners region. 
1000-500 mb thicknesses will be increasing over the area and 925 mb 
temps are expected to increase to between +2 and +4C. This WAA 
pattern could allow highs on Thursday to reach the low 40s in east-
central KS. 

Low level moisture will begin increasing on Friday ahead of the 
approaching deep upper level trough and associated surface cyclone 
that will be strengthening in the Front Range of the Rockies. There 
could be enough isentropic upglide on Friday for some light rain to 
develop in the warmer portion of the flow which will be in place in 
central and eastern KS. However, much better forcing for ascent will 
arrive on Saturday as a 100+ kt H5 jet rounds the base of the 
approaching upper low, placing the CWA under the left exit region of 
the jet. Additionally, medium range models depict negative tilting 
of the trough axis as it moves from near the TX panhandle into 
eastern KS. The negative tilt could introduce more cool air aloft 
and increase lapse rates and associated instability. Guidance shows 
boundary layer 300+ J/kg of CAPE developing in southern KS midday 
Saturday with little to no convective inhibition. As a result, 
thunderstorms are not out of the question on the warm side of the 
system Saturday, especially across southeastern and eastern KS. 
Models have come into better consensus on the placement of the 
surface low being located very close to Kansas City by Saturday 
evening. At that time, isobars could be packed very tightly together 
with a strong baroclinic zone on the backside of the low. This would 
indicate rapid CAA with very windy conditions. Given this setup, 
rain could quickly transition to snow as the cold air wraps into the 
area late Saturday. It is too early to forecast accumulation 
amounts, but snow combined with windy conditions has the potential 
to create many hazards in a very short period of time following 
precipitation onset. It should be stressed that this is a system 
that will be monitored very closely due to the potential of 
considerable impacts from both possible thunderstorms and snow this 
weekend.

Surface high pressure will slide into the area for Sunday and Monday 
with a switch to more of a zonal flow aloft on Sunday. Conditions 
will be cool with the Canadian based surface high in place to finish 
up the weekend. A subtle upper level shortwave trough traversing the 
flow could bring another chance for precipitation early next week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 533 PM CST Tue Feb 19 2019

Snow will continue to overspread the terminals within the next 1-2
hours. Cigs and vsby will rapidly decrease to LIFR and possibly
VLIFR through 06Z. A gradual improvement in vsby is anticipated after
06Z with IFR conditions returning through sunrise Wednesday. 
Snowfall is likely to mix with freezing rain after 06Z at the 
Topeka terminals. All precipitation is expected to come to an end 
by 14Z at all sites. Cigs are expected to reach MVFR near 14Z, 
with VFR conditions returning between 18-21Z. West winds will 
increase up to 10-15 kt during the daylight hours Wednesday. 

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 9 AM CST Wednesday for KSZ022>024-
026-035>040-054>056-058-059.

Winter Storm Warning until 9 AM CST Wednesday for KSZ008>012-020-
021-034.

&&

$$

SHORT TERM...Cohen
LONG TERM...Teefey
AVIATION...Baerg