SPC Jun 27, 2017 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1251 AM CDT Tue Jun 27 2017

Valid 271200Z - 281200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL AND NORTHERN PLAINS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
SLIGHT RISK AREA ACROSS PARTS OF THE GREAT PLAINS AND NORTHERN
ROCKIES...

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

...SUMMARY...
Severe thunderstorms capable of large hail, damaging winds and a
couple tornadoes are expected over much of Nebraska and South Dakota
late this afternoon and evening. Isolated marginally severe storms
are also possible across parts of the High Plains, North Dakota,
western Minnesota, eastern New York and southern New England.

...Central and Northern Plains...
A shortwave ridge will move eastward across the northern and central
Plains today as a shortwave trough moves across the northern
Rockies. At the surface, a front will move eastward into the western
Dakotas and western Nebraska as a corridor of maximized low-level
moisture sets up across central Nebraska and southern South Dakota.
Surface heating should result in moderate instability ahead of the
front by afternoon but warm air aloft will cap the airmass for much
of the day. Isolated convection is forecast to initiate along the
front during the late afternoon with thunderstorms developing and
moving eastward across Slight Risk area during the early to mid
evening. Several small clusters of thunderstorms may persist into
the late evening and overnight period as low-level flow increases
across the region.

Due to the warm air aloft and delayed timing of the shortwave
trough, convection may struggle until early to mid evening when
large-scale ascent increases markedly from the west. The strongest
instability is forecast across central Nebraska where NAM forecast
soundings at 03Z/Wednesday have MLCAPE around 3000 J/kg, 0-6 km
shear near 40 kt and substantial directional shear in the lowest 3
km AGL. This combined with steep mid-level lapse rates in excess of
8.0 C/km will be favorable for supercells with large hail.
Hailstones of greater than 2 inches in diameter will be possible
with the more dominant supercells. 0-3 km storm-relative helicities
in the 350 to 450 m2/s2 range will also support a tornado threat
especially as the low-level jet strengthens during the early to mid
evening.

As the shortwave trough moves across Montana and Wyoming this
afternoon, a few marginally severe thunderstorms will be possible in
the northern High Plains and northern Rockies. Moderate deep-layer
shear and sufficient instability should be enough for hail and
strong gusty winds. A marginal severe threat may also develop across
North Dakota and western Minnesota but the upper-level ridge will
limit storm coverage.

...Eastern New York/Southern New England...
An upper-level trough will move across the lower Great Lakes and
Mid-Atlantic today. At the surface, a cold front will advance
eastward across the Northeast. Ahead of the front, a narrow corridor
of maximized low-level moisture is forecast to set up across western
parts of southern New England this afternoon where surface dewpoints
should be in the 50s F. This will allow a corridor of instability to
develop around midday with thunderstorms initiating along the front.
This convection is forecast to move eastward across southern New
England this afternoon. Forecast soundings at 21Z in southern New
England show 0-6 km shear around 45 kt with steep low to mid-level
lapse rates. This may be enough for storm rotation within the
stronger cells. Hail and strong wind gusts will be the primary
threats.

..Broyles/Mosier.. 06/27/2017

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