Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 010519

Area Forecast Discussion
National Weather Service Topeka KS
1119 PM CST Tue Feb 28 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening and Tonight)
Issued at 340 PM CST Tue Feb 28 2017

A somewhat active evening and overnight period is in store for
the area with the potential for both strong/severe thunderstorms
in east central Kansas and a rain/snow mixture over northeast
Kansas tonight.

Starting off with the severe threat: Afternoon water vapor
imagery depicts a broad shortwave trough axis centered over the
High Plains with a 140 kt jet streak wrapping northeast along east
flank of the trough. At the surface, a pre-frontal trough could
be found stretching from Seneca to McPherson with the main cold
front just beginning to enter the NW CWA. Short-term models have
been struggling to resolve the evolution of the warm-sector
surface dewpoint field in NE Kansas, with the mid to high clouds
deck helping to dampen the magnitude of the afternoon mixing.
However, as of 20Z, dewpoints in the SE CWA had fallen into the
40s with mid to upper 30s not far to the north and west.

The window of opportunity for any strong to marginally severe
weather will be from approximately 00-04Z across the far
southeastern CWA. This threat will be tied to the the magnitude of
the return moisture flow streaming out of Oklahoma, which will
help to moisten the BL and enable the realization of steepening
mid-level lapse rates. Any convection that does manage to develop
in the far southeastern CWA should be close to surface-based with
SBCAPE values expected to be between 1500-2500 J/kg and effective
shear values of 40-60 kts. 0-1km SRH values will be between
150-250 m2/s2, thus the threat of an isolated tornado is non-zero.
The threat of convection further north/west of Franklin and
Anderson counties decreases quickly and will likely be elevated in
nature. Storm motions will be to the east/northeast at 40-50 kts
and quickly track out of the CWA. Large hail and localized
damaging winds will be the biggest threat.

Increasing 925-700mb frontogenetical forcing behind the surface
trough will lead to the development of a narrow band of
precipitation that will push across the CWA between 00-09Z. The
NAM has remained a stalwart proponent of a narrow, intense, but
short- lived band of snow with this forcing with amounts of 1-3
inches. However, the low-level thermodynamic profiles in the vast
majority of the short-range CAM soundings do not contain near-
surface wet bulbs that support snow accretion and even the 18Z NAM
has backed off considerably with snow amounts. Have continued the
going forecast of a rain/snow mixture with minimal snow amounts.

.LONG TERM...(Wednesday through Tuesday)
Issued at 340 PM CST Tue Feb 28 2017

Skies should be rapidly clearing Wednesday morning with dry and
subsiding air moving in behind the upper trough. Strong cold air
advection from the overnight keeps highs in the lower 50s. Weak warm
air advection takes place late Wednesday night into early Thursday
as a weak upper wave rotates around an upper low in south central
Canada. Winds will again be gusty from the northwest with some high
cloud particularly in northern areas, but highs should still be a
few degrees warmer for most locations.

Southerly winds dominate the Friday through Sunday periods with lee
troughing organizing ahead of a large-scale trough moving into the
Pacific Northwest and Northern Plains. This brings steadily warming
conditions, with the most uncertainty through these periods
currently being on Friday as the main warm front passes. Have kept
temps close to going values for the weekend but could see even
warmer values depending on mixing depths. As previously mentioned,
low-level moisture also increases with time, but capping aloft along
with little forcing for ascent keeps precip chances very low.

Models showing a decent spread in the timing of this trough moving
east across the Northern Plains, but a modified cold front should
pass in the Sunday night to Monday night time frame, with at least a
small chance for precip in southeastern areas Monday afternoon.
Modified continental air coming behind this system should still keep
temps above normal for Tuesday.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1118 PM CST Tue Feb 28 2017

Generally VFR conditions through the period, although will see
rain or rain/snow mix at MHK and near the TOP/FOE terminals for a
few more hours. Cigs also look to remain VFR. Winds prevail out of
the NW and are gusty early but diminish through the forecast


Issued at 340 PM CST Tue Feb 28 2017

Elevated fire danger conditions seem likely for the next few days.
For Wednesday and Thursday, northwest winds keep RH values on the
low side with incoming air still on the modified/warm side for early
March. Mixing depths will be fairly deep, and have kept dewpoint
values on the low side of guidance. South winds return for Friday
with moisture values slowly increasing into the weekend. By Sunday,
dewpoints should rise enough to keep minimum RH values above 30%,
but most previous days bring values in the lower 20s to lower 30s.
Winds speeds for these periods look to be the most likely element to
keep Extreme/Red Flag values in check, but could see Very High
Rangeland Fire Danger for much of the area for Wednesday through




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