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Day 1 Outlook Convective Tornado
Hail Wind
Categorical Day1 1300Z Outlook
  
Images courtesy of the NWS Storm Prediction Center
 Forecast Discussion


172 
ACUS01 KWNS 271259
SWODY1
SPC AC 271257

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0757 AM CDT Sun May 27 2018

Valid 271300Z - 281200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
CENTRAL AND SOUTHERN HIGH PLAINS AND VICINITY...AS WELL AS MUCH OF
FLORIDA...

...SUMMARY...
Severe storms are possible later today into this evening primarily
from the northern through southern High Plains. A modest threat for
a few tornadoes will also exist across the Florida Peninsula.

...Synopsis...
In mid/upper levels, a generally high-amplitude and slowly
progressive pattern will persist over much of the contiguous U.S.  A
large cyclone -- evident in moisture-channel imagery over the Great
Basin -- is expected to move slowly eastward through the period.  To
its east, a synoptic ridge will persist over the Plains States, with
some eastward shift and breakdown on the northern end.  That will
occur as a series of mostly low-amplitude shortwaves and vorticity
lobes eject northeastward through the difluent-flow regime in the
cyclone's peripheral northeast semicircle.  Meanwhile, a broad area
of weak heights across the eastern U.S. will incorporate -- and
become increasingly dominated by -- the circulation associated with
Subtropical Storm Alberto, as the latter moves poleward across the
northeastern Gulf.  See latest NHC advisories for all track/
intensity guidance and tropical watches/warnings related to Alberto.

At the surface, 11Z analysis showed a wavy/quasistationary frontal
zone from the lower Great Lakes to the Michiana area and
west-northwestward to southeastern ND.  A cold front was drawn
across southern WY and western SD, becoming ill-defined amidst an
expansive outflow pool over ND.  The cold front should move slowly
southward/southeastward today over WY and SD before stalling.  A
dryline will mix eastward across the central/southern High Plains. 

...WY to Black Hills, central/southern High Plains...
Widely scattered to scattered thunderstorms are expected to develop
in multiple, overlapping or interconnected regimes over this region,
primarily related to upslope lift, low-level convergence near the
frontal zone in the WY and Black Hills regions, dryline-related lift
farther south, and forced ascent on cold pools from mature
convective aggregations this evening and overnight.  Damaging gusts
and large hail will be the main concerns, though a tornado or two
cannot be ruled out.  

Surface dew points generally in the 50s F over the central/northern
Plains, and 50s to low 60s near the central/southern Plains dryline,
are evidence of the moisture needed to support the convective
potential this afternoon.  Strong heating/mixing is forecast along
the dryline, contributing to 2000-3000 J/kg MLCAPE.  Modest
low/middle-level winds near the ridge aloft will lead to weak
vertical shear for dryline activity, and generally outflow-dominant
multicellular modes.  Expect nodes or clusters of development by
mid/late afternoon with a few hours of severe potential into early
evening.  

Somewhat more concentrated/longer-lived convective potential is
expected to develop off the WY/SD near- and post-frontal convective
regime, taking advantage of moist inflow and favorable storm-
relative winds, though buoyancy will be somewhat lower than ahead of
the dryline.  The related easterly boundary-layer wind component
will combine with closer proximity to the cyclonic-flow field around
the Great Basin low to yield deep shear favorable for a blend of
multicell and supercell modes early, transitioning to clusters in
the evening.  

...FL/southern GA...
Given its still-broad wind structure (characteristic of cyclones
developing as subtropical), and the presence of the most favorable
low-level wind profiles for supercells east through north of center,
a fairly expansive area will remain under marginal to slight (2-5%)
tornado risks cumulatively for the period.  A pronounced dry/clear
slot aloft was evident over the Gulf that should remain offshore,
but another, messier one was evident from the lower Keys northward
over the southwest FL coast.  Although convective coverage should be
less under that eastern slot, related enhancement of surface heating
is expected to boost buoyancy available to any sustained, relatively
discrete convection that can form.  The most favorable juxtaposition
of backed low-level flow, enlarged hodographs, and low LCL
supporting supercell/tornado potential should remain over the east
coast of the peninsula northwestward across central FL, though
diurnal destabilization will be slow/gradual under relatively thick
cloud cover.  Favorable shear and at least marginal buoyancy should
spread/shift northward over northern FL and portions of southern GA
overnight, with more marginal/conditional potential. 

With the cyclone center pulling northward to northwestward away from
the southwest Florida/lower Keys region, per NHC track forecasts,
isallobaric forcing is expected to weaken.  This effect already has
led to veering of low-level flow and shrinking of hodographs at EYW,
per last few hours' time trends of VAD winds and comparison of
06z-12Z RAOB hodographs.  Increasingly unidirectional flow also will
be more favorable for quasi-linear modes (such as presently observed
west of the lower Keys) instead of discrete supercells.  Meanwhile,
the western slot may impinge on parts of southwestern and
west-central FL midday into afternoon, based on satellite trends and
forecast RH fields in midlevels.  Given these factors, the tornado
potential appears to have passed peak over those parts of the area
that are reverted to 2% unconditional probabilities.

..Edwards/Peters.. 05/27/2018

$$