Severe Storm Outlook Narrative (AC)
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
ACUS01 KWNS 271258
SWODY1
SPC AC 271257

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Tue Jun 27 2017

Valid 271300Z - 281200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
CENTRAL/EASTERN NEBRASKA AND EXTREME SOUTHERN SOUTH DAKOTA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM THE
CENTRAL NORTH/SOUTH DAKOTA BORDER REGION TO NORTHWESTERN KANSAS AND
WESTERN IOWA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND EXTENDING NEAR THE WYOMING/MONTANA BORDER AND TO THE
NORTHERN TEXAS PANHANDLE...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
EASTERN NEW YORK AND NEW ENGLAND...

...SUMMARY...
Severe thunderstorms capable of large hail, damaging winds and a
tornado or two are expected over much of Nebraska and South Dakota
late this afternoon and evening.  A few gusts may exceed hurricane
force.  Isolated marginally severe storms are also possible across
parts of the High Plains, North Dakota, western Minnesota, eastern
New York and New England.

...Synopsis...
A more progressive mid/upper level pattern has evolved over roughly
the northern 2/3 of the conterminous U.S.  Heights are forecast to
rise across the upper Great Lakes as troughing and related cyclonic
flow shift eastward toward New England, driven largely by a strong
shortwave perturbation now located from southern ON to the OH/WV
border and south-central KY.  By 00Z, the primary associated 500-mb
vorticity max should reach the Adirondacks, with the shortwave
trough arching across Long Island, the Delmarva Peninsula and
north-central/northeastern NC.  An associated surface trough or weak
cold front should move eastward over eastern NY and New England
ahead of the mid/upper perturbation.

Farther west, as a mid/upper-level cyclone crosses north-central AB,
a basal shortwave trough -- now evident in moisture-channel imagery
over ID and eastern NV -- will move eastward faster.  By 00Z, the
trough should reach from eastern MT across the Black Hills region to
southeastern WY.  Related surface frontogenesis is expected across
eastern MT and northern WY today.  By 00Z, the resulting surface
cold front should extend from a warm-frontal triple point over
central ND south-southwestward across the NE Panhandle to
north-central CO.  Lee troughing will strengthen ahead of the front
through the day from the Black Hills to eastern CO, while being
frontally overtaken from north-south.

A surface warm front -- analyzed at 11Z from western KS across
eastern OK to the Arklatex region -- should move/re-develop
northeastward across the central and northern Plains today,
extending by 00Z from the central Dakotas southeastward across
east-central NE and eastern KS to southwestern MO.  The warm front
should continue northeastward overnight across much of IA and MO,
though its definition may be compromised by overtaking convective
complex(es).

...Central/northern Plains and vicinity...
Scattered thunderstorms are expected to develop mid/late this
afternoon near the lee trough and front.  Large hail and damaging
gusts will be possible for the first few hours, evolving to primary
a severe-wind threat with eastward extent especially over parts of
NE and SD.  Hurricane-force thunderstorm gusts are possible from the
most intense surges of convection.

Storms will initiate in a regime of very warm surface temps at
higher elevations, with a deeply well-mixed subcloud layer and large
DCAPE.  This will support production of strong to severe outflow
winds relatively early in the convective cycle, in addition to the
large/significant-hail threat implied by the favorable CAPE, steep
lapse rates aloft and supercell-favoring wind profiles.  Therefore,
despite substantial components of deep-shear and mean-wind vectors
oriented normal to the axis of convective-scale lift, discrete mode
should be rather short-lived, and aggregation into one or two at
least loosely aggregated, forward-propagational convective complexes
appears to be the most probable modal/morphological scenario.

This activity should outrun the shortwave-related large-scale ascent
that supported its initial growth, but move into a progressively
more moist boundary layer with lift aided by forced ascent atop a
strengthening cold pool.  Rear-inflow jets and foregoing bow echoes
may develop, offering a locally maximized threat for corridors of
significant wind, the potential for which is greatest in and near
the newly introduced 10% significant-wind area.  As the convection
moves eastward over the Missouri Valley and into progressively more
stable air tonight across IA and perhaps southern MN, severe wind
still will be possible, but the overall intensity/coverage should
diminish.

Large low-level hodographs and SRH values are possible ahead of this
activity, especially across portions of central NE near the warm
front.  Accordingly, a tornado or two cannot be ruled out in the
risk area, whether from embedded supercell(s), bookend vortices or
QLCS spinup processes.  However, that threat currently appears to be
limited by
1.  The presence of high LCLs where modes are likely to still be
somewhat discrete in the zone near and downshear from convection
initiation, then
2.  By the time activity gets farther east into lower LCLs, messy
and likely outflow-drive/bowing to quasi-linear storm modes.

...Northeastern States...
Widely scattered thunderstorms may develop this afternoon and move
eastward to east-northeastward across the outlook area, offering
isolated large hail and the potential for strong gusts.  The
convective potential here is ambiguously focused in low levels,
given the expected weak MLCINH, gradual wind shift progged through
the frontal zone and the related lack of strong boundary-layer
convergence.  Modest low-level theta-e will be in place, as well as
only gradual, muted surface diabatic heating related to cloud cover
now evident in satellite imagery there and upstream.  Still,
large-scale lift/cooling aloft ahead of the trough will offset those
factors enough to steepen low/middle-level lapse rates and yield
500-1000 J/kg preconvective MLCAPE.  Strong mid/upper-level flow
will contribute to 30-40 kt effective-shear magnitudes over the
southern and eastern parts of the outlook area, and somewhat weaker
farther northwest.  Convection should weaken as it moves eastward
over either land-based evening diabatic cooling or the Atlantic
marine layer.

..Edwards/Picca.. 06/27/2017

$$


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.