Area Forecast Discussion
Issued by NWS Topeka, KS

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639
FXUS63 KTOP 132149
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
349 PM CST Sat Jan 13 2018

.SHORT TERM...(This evening through Sunday)
Issued at 355 PM CST Sat Jan 13 2018

Water vapor loops Saturday afternoon depict a mid/high-level jet
stream segment positioned from Alberta southeastward through the
central Plains, with the leading edge of an embedded speed maximum
emerging over the northern and central High Plains. The ascent lobe
peripheral to the cyclonic-vorticity banner flanking the speed
maximum is responsible for midlevel moisture enhancements elongating
southeastward from eastern MT/WY through north-central KS. With
time, the continued southeastward translation of the speed
maximum -- and gradual eastward progression of its surrounding
broader midlevel cyclonic-flow area -- will result in increasing
large-scale ascent across the forecast area through the overnight
hours. Meanwhile, the low-level mass response to the ageostrophic
circulation accompanying the jet streak`s exit region will result
in an area of lower-tropospheric warm advection and associated
isentropic ascent spreading across the region from this evening
into Sunday morning.

Related moistening of sub-freezing thermal profiles will result in
areas of snow spreading southeastward/eastward across the region from
this evening into Sunday morning. Only modest phasing of
dendritic growth zones aloft and ascent profiles -- albeit limited
in magnitude -- should mitigate snowfall accumulation rates.
Regardless, the substantive duration of snow, combined with snow-
to-liquid ratios of 10:1 to 15:1, should favor 1-2 inches of snow
accumulation along and northeast of a line from Belleville to
Topeka to Ottawa through Sunday morning -- consistent with the
latest multi-deterministic/ensemble model guidance. A Winter
Weather Advisory has been issued for these areas, where the
greatest snowfall totals are expected. Lesser accumulations are
expected elsewhere, though minor/mesoscale variations in the track
of the midlevel speed maximum and related zone of ascent may
eventually warrant adjustments to the areal delineation of the
Advisory. Moreover, antecedent deeper baroclinicity preceding the
strongest of the aforementioned cyclonic vorticity is relatively
modest, and curvature amid the broader midlevel flow is diffuse,
and thus prospects for robust frontogenetic-precipitation banding
are limited. Nevertheless, there appears to be sufficient signal
among model solutions for adequate frontogenetic ascent to boost
snowfall rates in north-central and northeast KS on a spotty
basis, where snowfall accumulations perhaps locally reaching 3
inches cannot be ruled out.

Snow should taper off during the late morning/mid-day hours of
Sunday, with a brief break in precipitation expected through late
Sunday afternoon -- before a strong Arctic front enters the
region from the north. Sufficient pre-frontal warm advection and
diurnal heating should allow temperatures to reach the lower to
middle 30s Sunday afternoon.

.LONG TERM...(Sunday night through Saturday)
Issued at 355 PM CST Sat Jan 13 2018

The aforementioned Arctic front will spread southward from Sunday
night through Monday morning, with an accompanying band of stronger
frontogenetic ascent supporting a southward-progressing area of
light to moderate snow. This will occur as the broader area of
midlevel cyclonic flow is reinforced over the north-central and
northeast states. Given a preceding period of pre-surface-frontal
warm advection aloft across the region, and subsequent localized
areas of drying aloft in the post-frontal regime, pockets of light
mixed precipitation (sleet and freezing rain) might occur during
this event. However, snow should be the predominant precipitation
type, and increasing snow-to-liquid ratios responding to increasing
deeper cold advection following frontal passage should support an
additional 1-2 inches of snow accumulation (perhaps locally
higher). With snowfall rates potentially reaching 0.50-0.75 inch
per hour Monday morning, and moderate northerly winds creating
localized blowing snow, hazardous travel conditions may occur
especially Monday morning. A separate Winter Weather Advisory may
eventually become required for this event.

Precipitation will end from north to south mid-day Monday, as an
Arctic air mass continues to spread across the region. Low
temperatures Monday night will likely fall to the single digits
below zero. Present indications are that the boundary layer will
remain sufficiently coupled to the deeper atmospheric column. This
should support wind chill readings of -15F to -25F, likely
warranting wind chill highlights in later forecasts.

The very cold air mass will persist into the middle of the week, as
a surface ridge moves across the region. Thereafter, the upper-air
pattern may begin to undergo a transition phase, as a series of
cyclonic perturbations potentially spread across the western/central
states, after the large-scale wave pattern loses amplitude over the
CONUS per medium-range model guidance. In response to these
developments, the boundary layer may be modifying/warming by late
week into next weekend while lee troughing becomes established
over portions of the High Plains. Present indications are that any
phasing of moisture and ascent accompanying the aforementioned
disturbances will remain displaced away from the forecast area,
precluding precipitation impacts across the region. However, prior
to more appreciable erosion of the Arctic air, enhanced surface-
layer winds amid the tightened lee-trough-flanking pressure
gradient could support periods of cold/very cold wind chills at
times through much of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1130 AM CST Sat Jan 13 2018

VFR conditions are expected through this evening, with light
winds. Areas of light snow will spread southeastward/eastward
across the region tonight into Sunday morning, accompanied by
MVFR to IFR ceilings/visibilities. The longest duration of snow
will affect areas from far north-central KS into east-central KS,
including the TOP/FOE. Snow will taper off Sunday morning. VFR
conditions are expected to become re-established at the TAF sites
around the 12Z-14Z time frame, with generally light southerly
winds expected thereafter.


&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 PM this evening to 10 AM CST
Sunday for KSZ008>012-023-024-026-039-040-056.

&&

$$

SHORT TERM...Cohen
LONG TERM...Cohen
AVIATION...Cohen



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