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000
ACUS01 KWNS 151254
SWODY1
SPC AC 151252

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Tue May 15 2018

Valid 151300Z - 161200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
PENNSYLVANIA AND NORTHERN NEW JERSEY TO SOUTH-CENTRAL NEW ENGLAND...

...SUMMARY...
Severe thunderstorms, capable of significant damaging wind as well
as hail and a few tornadoes, should affect part of the Northeast
States and the Mid-Atlantic this afternoon into the evening.

...Synopsis...
Mid/upper-level split flow will be maintained over the western U.S.,
but with some pattern evolution underway.  The longstanding Great
Basin cyclone continues to fill and will begin ejecting northward
this evening and overnight.  That will occur as a strong shortwave
trough with embedded cyclone -- now evident in moisture-channel
imagery west of CA near 130W -- moves east-northeastward toward the
CA coast.  The latter feature should move inland early day-2.
Meanwhile, a belt of zonal to slightly cyclonic flow will persist
over the Great Lakes and Northeast, south of an intense cyclone
moving eastward from Hudson Bay across northern QC.  Embedded within
that flow belt is a low-amplitude shortwave trough and embedded MCV
-- now apparent in satellite and composited radar imagery near the
northern portion of the IN/OH state line.  This feature should
progress eastward or east-northeastward across western NY and
portions of western/central PA by 18Z, then eastward over New
England through this evening.

At the surface, 11Z analysis shows a wavy frontal zone from an
occlusion triple point over Lake Ontario eastward across south-
central New England, and westward to southwestward over southern
Lower MI, northern IL, eastern KS, and the TX Panhandle.  Initially
quasistationary to weakly cold frontal along most of its length, the
boundary generally will move southward/southeastward as a cold front
through this period, but should remain almost stationary in the
preconvective environment today across the Northeast.  A dryline
initially drawn over portions of west TX and southeastern NM will
mix eastward across the southern High Plains.

...Northeast...
Thunderstorms are expected to develop and grow upscale from midday
through afternoon, sweeping eastward across the region with the main
risk being severe wind.  The potential for a well-organized swath of
damaging wind -- including a few gusts around hurricane force --
appears greatest over the enhanced-risk area.  Large hail also is
possible, especially in the first few hours of the convective cycle
when storm modes can be more favorably discrete or semi-discrete.  A
few tornadoes also may occur, whether from supercells or
QLCS-embedded vortices.

A nonsevere area of convection now moving across western NY and
northern PA should reinforce the baroclinic zone across parts of
south-central and eastern NY, which should act as a fairly sharp
northern delimiter for substantial severe potential, given strongly
stable air to its north.  The UVV field related to the MCV appears
well-timed to encounter the destabilizing warm sector across
northern PA today, along and south of the frontal zone, in support
of initial development, which then should expand/intensify to severe
levels as it moves rapidly eastward toward parts of southern NY, NJ
and New England.  Whether the resulting convective wind event
qualifies as a derecho may be a semantic exercise; impacts could be
of that caliber in the area affected.

One source of uncertainty in this scenario is an area of outflow to
the south, across parts of VA/MD/Delmarva and eastern PA,
originating from yesterday`s MCS, and sampled peripherally by the
12Z IAD sounding.  Airmass recovery is expected from the southwest,
around the northwest rim of that outflow pool and south of the
morning convective/frontal baroclinic zone.  Expect midday to
afternoon preconvective destabilization arising from both theta-e
advection and diabatic surface heating.  68-70 F surface dew points,
such as forecast by the NAM, may be overdone considering the
available recovery trajectories, and the nearest dew points that
large are 300-400 nm away over NC, on the other side of the outflow
pool.

Regardless, a plume of EML air advecting over this region will
foster steep midlevel lapse rates, overlying strengthening
boundary-layer lapse rates and low/mid-60s F surface dew points.
That combination still supports peak MLCAPE in the 1500-2500 J/kg
range, amidst strong west-southwesterly mean-wind and deep-shear
vectors.  Forecast soundings suggest that, despite a nearly
unidirectional vertical wind profile, effective-shear magnitudes of
45-55 kt may be realized.  Downward momentum transfer from strong
flow above 700 mb, into a well-mixed preconvective boundary layer,
should offer favorable conditions for severe thunderstorm winds.
Given the strong westerly component of the near-surface flow,
more-unstable inland air may be shunted eastward to very near the
coast across much of the region, extending the severe threat
accordingly, before the MCS encounters too much stable marine-layer
air and weakens.

...Southern Plains to central High Plains/foothills...
Overall convective potential today looks lower in coverage, more
mesoscale-dependent, less well-focused, and more disorganized
compared to the last two.  Widely scattered thunderstorms, some in
clusters, are expected to develop near the dryline and along
segments of the frontal zone and outflow boundaries from yesterday`s
convection.  In the absence of more substantial deep shear, activity
should be multicellular in mode, with isolated damaging gusts and
large hail possible.

Surface analysis and RAOBs indicated a patchwork low-level moisture
field of inconsistent depth across the southern Plains, further
complicating the convective regime.  An area of relatively dry air
across west-central/northwest TX, with a dry axis southeastward over
the Edwards Plateau.  This air mass will mix more deeply/readily
today, decreasing but not eliminating favorable CAPE for convection,
though the mixed layer also should support maintenance to the
surface of any severe gusts/hail generated by thunderstorms in the
area.  Most of the convection should initiate in the afternoon to
early evening, diminishing in coverage and intensity overnight.

Thunderstorms also should form this afternoon in the southern Sangre
de Cristo foothills, Raton Mesa area, Front Range/foothills, and
Palmer Divide area, moving east-southeast to southeast across
portions of the adjacent High Plains.  Weak mid/upper winds will
limit overall organization, though strong directional shear and
MLCAPE reaching 800-1500 J/kg should support at least marginal/
isolated severe before the activity weakens with evening boundary-
layer stabilization of the postfrontal airmass at lower elevations.
Relatively maximized afternoon supercell potential may be over
northeastern NM where the CAPE/shear phase space looks most
favorable.

..Edwards/Goss.. 05/15/2018

$$

The Thunderstorm Outlook is created by the Storm Prediction Center in Norman, OK.
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