Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 130446
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1146 PM CDT Thu Oct 12 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Friday)
Issued at 314 PM CDT Thu Oct 12 2017

Not much change in the large scale pattern from yesterday with 19Z
water vapor showing a flat upper ridge along the gulf coast and an
upper trough over the northwestern U.S. At the surface, the high
pressure system has been replaced with a trough of low pressure
extending from the southern high plains through the northern plains.
This has caused south winds to strengthen and low level moisture to
return north.

For tonight and Friday, models show southwesterly flow aloft
persisting with not much in the way of large scale forcing. A
frontal boundary is expected to move into north central KS by mid
morning Friday. And with some higher dewpoints ahead of the front,
there could be some surface based instability by the afternoon.
Forecast soundings however show a capping inversion holding through
much of the day. So the only POPs in the forecast are some slight
chances over northeast KS along the boundary. Stronger mixing
overnight should minimize any potential for stratus or fog in spite
of the low level moisture advection. And with continued warm air
advection overnight lows should be warmer. Have lows in the mid 50s
north to around 60 south. There is expected to be an area of stratus
developing behind the frontal boundary keeping much of northern
parts of the forecast area in the clouds. Forecast soundings hint
that there could be some patchy drizzle behind the front as well
with a stable saturated layer with some weak vertical motion.
Meanwhile east central KS should remain in the warm sector with
southerly winds and more sunshine. So there is a tight gradient in
the high temp forecast with readings ranging from the mid 60s along
the NEB state line to the mid 80s along I-35.

.LONG TERM...(Friday night through Thursday)
Issued at 314 PM CDT Thu Oct 12 2017

By late Friday night and early Saturday morning, the surface
baroclinic zone attendant to a midlevel shortwave trough --
presently crossing the southern Canadian Prairies and adjacent
north-central U.S. -- and a trailing disturbance is forecast to
be stalling across the area. A weak frontal wave is expected to
develop northeastward along the boundary Friday evening in
association with a subtle midlevel speed maximum embedded within a
long fetch of southwesterly flow aloft, downstream of a trough
gaining amplitude over the central Great Basin. The low-level mass
response to the speed maximum will continue to encourage the
return of at least partially modified Gulf moisture toward the
frontal zone, with sufficient isentropic ascent perhaps supporting
isolated to widely scattered convection rooted atop the sloped
frontal surface. Strong high-level flow (around 80 kt at 250 mb)
with 25-35 kt of effective shear suggest that incipient, discrete
updrafts may conditionally support isolated hail to quarter size
Friday night. However, midlevel lapse rates are not forecast to be
particularly steep, and substantial uncertainty exists regarding
the spatial extent of overlap of speed-maximum-related ascent and
favorably moist elevated inflow. These factors cast considerable
uncertainty regarding any severe potential for Friday night.

For Saturday, the front is expected to gradually advance northward
across eastern parts of the central Plains, ahead of the
amplifying midlevel trough and an attendant cyclonically curved
midlevel speed maximum. This front will roughly delineate the
northern extent of surface-based effective inflow layers within
the warm sector of a surface cyclone developing northeastward
along the front. A cold front trailing southwest of the cyclone
will sharpen through the day and spread eastward/southeastward
across the forecast area through the late afternoon and evening.
Present indications are that low-level flow may veer to
southwesterly across the warm sector in response to the quick
progression of the surface cyclone beneath the exit region of the
cyclonically curved speed maximum. With stronger ascent
accompanying the amplified trough lagging well to the west of the
area through peak heating, these factors offer limited confidence
for convection to mature within the open warm sector. At this
time, the most likely scenario will be for initial convection
(likely sub-severe) displaced to the cool side of the boundary to
drift northeastward toward the lower/middle MO Valley through mid-
day, and then for quasi-linear convection to form along the
sharpening front and spread eastward through the evening (with
some severe potential). Modest midlevel lapse rates (around
6.5-7.0 C/km) and the early-phase nature of the return of more
substantial moisture (surface dewpoints into the middle 60s)
offer limited confidence in the overall severe risk. However, the
presence of 40-55 kt of effective shear -- with deep shear vectors
oriented with at least some component into the warm/moist
sector -- support some risk for surface-based inflow for bands of
convection evolving near or just ahead of the synoptic front.
With 40-50 kt of 850-700-mb flow, convective momentum transport
could facilitate isolated to widely scattered severe wind gusts as
the front overtakes the richer boundary-layer moisture. Limited
effective SRH associated with the veered pre-frontal low-level
flow may limit prospects for sustained line-embedded circulations,
while the synoptic-scale cold dome would have a tendency to
undercut frontal convection.

Cooler and drier air will overspread the region in the wake of
the front for the remainder of the weekend, with a surface ridge
moving across portions of the central/south-central U.S. beneath
northwesterly flow aloft -- into early next week. Surface lee
troughing is presently expected to gradually become established
over the northern and central High Plains into middle and latter
parts of next week, as midlevel flow turns more
westerly/southwesterly across the central states.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1140 PM CDT Thu Oct 12 2017

LLWS is expected to continue until 10Z at MHK and 13Z at TOP/FOE.
A front moves its way into the area in the morning shifting
southerly winds to northeasterly winds throughout the afternoon.
MVFR ceilings behind this front may make it to MHK by the end of
the period.

&&

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

$$

SHORT TERM...Wolters
LONG TERM...Cohen
AVIATION...Heller



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