Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 191141
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
641 AM CDT Sat May 19 2018

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 302 AM CDT Sat May 19 2018

Severe thunderstorms are likely across portions of the forecast area
today. However, the exact convective evolution is rather unclear at
this time, and confidence regarding the timing and location of
greatest severe potential is lower-than-average for a day-1 severe
forecast. The combination of widespread early-day convection and
convective debris, and an unconsolidated upper-air pattern, greatly
complicate the predictability of the details for this potential
severe-weather episode. Regardless, there will likely be one or more
corridors of robust convective threat with severe hail and winds,
and perhaps a few tornadoes, all possible.

Early-morning surface observations indicate a baroclinic zone
oriented northeast-southwest from the middle Missouri Valley to the
TX Panhandle vicinity, which has been locally fragmented by patchy
theta-e deficits accompanying the decaying remnants of nocturnal
convection across KS. The disorganized area of convection and
related zones of stratiform precipitation are rather widespread
across KS early today, with lift preceding a pair of mesoscale
convective vortices facilitating arcs of elevated convection from
southern NE into east-central KS, while isentropic ascent and
favorable moisture transport along a low-level jet continue
supporting slightly more intense convection across southern KS. The
low-level jet precedes a compact midlevel cyclonic circulation
centered over northeast CO vicinity, which is somewhat disconnected
from a series of speed maxima embedded within a belt of enhanced
southwesterly midlevel flow.

Convection-parameterized and convection-permitting model solutions
vary substantially regarding some aspects of the aforementioned
features and attendant convective signal. However, there is general
agreement that the midlevel cyclone will advance east-northeast and
then minor out within confluent flow aloft. The corresponding de-
amplification of this perturbation and its small spatial extent,
coupled with antecedent static stability deposited by
nocturnal/early-morning convection, should collectively dampen the
low-level mass response. Differential cyclonic vorticity
advection/height falls preceding the midlevel cyclone will be
focused within a narrow zone from north-central to northeast KS
through the diurnal heating cycle. As a result, there should be a
tendency for surface low pressure over the TX/OK Panhandles to
develop northeastward along the deeper baroclinic zone through
central KS into north-central KS and southeast NE in association
with the migratory zone of midlevel height falls. However,
substantial uncertainty regarding the evolution of early-day
convection extends to the eventual locations of the effective
boundary and any outflow boundaries/differential-heating zones that
would influence subsequent diurnal convection, along with the
overall low-level mass response.

Moreover, the reinforced elevated mixed layer will not be
particularly warm -- despite being accompanied by 7-8 C/km midlevel
lapse rates -- compared to the sustained influx of underlying
boundary-layer moisture on the warm side of the synoptic boundary
characterized by middle 60s surface dewpoints. As a result, multiple
zones of convergence from meso-beta-scale to augmented-synoptic-
scale origins will all be foci for potential diurnal convective
initiation and/or intensification through the day -- for which most
are associated with substantial spatial uncertainty. While deep-
layer upward motion will be strongest from north-central to
northeast KS, the potential for early-day, upshear-regenerating
convection to shunt an effective boundary farther south beneath a
regime of midlevel height neutrality may displace the focus for more
robust diurnal convective development farther south into
central/east-central KS later in the day.

Needless to say, the aforementioned thermodynamic profile will be
capable of supporting robust convection where cloud-thinning/cloud
breaks evolve on the warm side of the synoptic boundary. A parameter
space characterized by 2500-3500 J/kg of MLCAPE and 30-45 kt of deep
shear -- where diabatic surface-layer heating is most prominent --
should foster clusters of intense deep moist convection. Uncertainty
regarding boundary orientations extends to convective mode, though
at least some component of boundary-relative storm motion should be
orthogonal to initiating boundaries to support initially separated
rotating updrafts. However, multiple hodograph inflections within
the mid/upper convective-cloud layer, reflecting the
fragmented/loosely phased upper-air pattern, suggest storm
splits/mergers may tend to facilitate localized corridors of upscale
convective growth, especially provided 1000-1300 J/kg of DCAPE
enhancing cold-pool strength. Substantial MLCAPE through the hail-
growth zone suggests that initial supercell structures will be
capable of producing very large hail -- perhaps as large as golf-
ball to tennis-ball size. However, the storm mergers/upscale
convective growth, and instances of convective-scale dynamic
subsidence accompanying the wavy hodograph structure, may abbreviate
the significant severe hail risk. Corridors of severe winds will
also be of substantial concern as cold pools elongate along 35-40 kt
of southwesterly 700-mb flow, and wind-driven hail may also be of
concern.

Any tornado potential will be largely influenced by heterogeneity in
the boundary-layer thermal/moisture fields for which uncertainty is
substantial. Consistent elongated clockwise curvature in the low-
level hodograph and accompanying 100-250 m2/s2 of effective SRH
could support low-level mesocyclones, QLCS-embedded mesovortices,
and accompanying tornado potential. The strongest environmental
streamwise vorticity will be confined to north-central/northeast KS
where the low-level isallobaric response accompanying the
progression of the midlevel perturbation will encourage the greatest
backing of surface winds. However, substantial uncertainty regarding
boundary-layer recovery extends to the tornado risk in this region.
Regardless, low-level shear profiles will be sufficient across the
entire area for some tornado risk, which will tend to be maximized
where convection interacts with pre-existing vertical vorticity
focused around remnant boundaries and/or in the vicinity of the
synoptic/effective front.

Ultimately, there still remains substantial uncertainty regarding
the severe risk today. The key points are as follows: (1) The
potential for very large hail, severe winds, and possibly tornadoes
will exist across the entire forecast area; (2) Not all areas will
experience severe storms, but the potential for severe weather will
exist across the entire area; (3) The location of greatest severe
risk is very uncertain; (4) The timing of greatest severe potential
is very uncertain, though the period from 1 PM to 10 PM will likely
contain the major of severe-weather risk; (5) We encourage everyone
to pay attention to the weather today/this evening for the potential
for rapidly changing weather conditions and monitor subsequent
forecast refinements; (6) Storms today/this evening will be fast-
moving.

The cold front trailing southwest of the surface low will move
across the area during the mid/late overnight hours, with drier air
being advected into the region by early Sunday morning bringing an
end to the risk for deep moist convection.

.LONG TERM...(Sunday through Friday)
Issued at 302 AM CDT Sat May 19 2018

By Sunday morning, the bulk of precipitation is expected to be east
of the area although some wrap-around showers behind the surface low
may linger through the late morning.  Winds from the northwest will
keep high temperatures much cooler than previous days, ranging from
the upper 60s to upper 70s.  On Monday, a mid-level cut off low
moves over southern California which will be the main feature
attributing to the shower and thunderstorm chances through the
work week and weekend. Models are in a bit of disagreement with
PoPs for next week, so confidence with these chances remains low
at this time. A cooler day is expected Monday as well with
temperatures in the upper 70s and low 80s, before winds once again
gain a southerly component contributing to a another hot week.
High temperatures in the upper 80s are seen from Tuesday through
the end of the period.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 611 AM CDT Sat May 19 2018

Showers will move across the region this morning, with a few
thunderstorms possible south of Interstate 70. Localized MVFR
visibilities may accompany this activity generally to the south
of the TAF sites. This afternoon, thunderstorms are expected to
develop and quickly spread to the northeast/east across the
region into the evening. Some storms may be severe. Localized
MVFR ceilings and visibilities may accompany this activity, and
the greatest likelihood for this activity to affect areas around
the TAF sites will be between 21Z and 04Z. A front will move
eastward across the region during the overnight hours, with
showers and thunderstorms tapering off behind the front and light
winds becoming westerly to northwesterly. Areas of low clouds
behind the front are expected to yield MVFR ceilings.

&&

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

$$

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



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