- Day Three
acus01 kwns 270553
Storm Prediction Center ac 270551
Day 1 convective outlook
National Weather Service 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
..there is a marginal risk of severe thunderstorms across parts of
eastern New York and southern New England...
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 above ground level. 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
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
acus11 kwns 270026
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 270026
Mesoscale discussion 1159
National Weather Service Storm Prediction Center Norman OK
0726 PM CDT Mon Jun 26 2017
Areas affected...southeast Washington...northern Idaho...far western Montana
Concerning...severe potential...watch unlikely
Valid 270026z - 270130z
Probability of watch issuance...20 percent
Summary...scattered 45-60 kt gusts are likely this evening as a band
of strong to severe thunderstorms continues to move north-northeast
across southeastern WA, northern ID, and into far western Montana.
Discussion...radar imagery over the past few hours has shown a
gradual upscale growth of convection over northeastern or. This
region is immediately downstream of an approaching upper-level pv
anomaly moving across northern California/western or with a 75 kt 300-mb
speed Max nosing into eastern or. 9-km cappi imagery over the past
several hours has shown scattered intense thunderstorm cores
associated with earlier reports of large hail across north-central
or. A 56-kt gust was observed by the Pendleton, or ASOS at 2347z
and a 47-kt gusts by the Walla Walla, Washington ASOS at 2311z. Surface
analysis shows temperatures near 100 degrees f over southeastern Washington
and into the lower 90s across northern Idaho and western Montana. Surface
dewpoints are in the upper 40-lower 50s---resulting in
temperature-dewpoint spreads in the 40-50 f range.
The very steep lapse rates from the surface through 500 mb will be
very favorable for evaporative cooling and the propensity of
scattered strong to severe gusts through the early evening. There
was a 8.9 degrees/km 700-500 lapse rate on the 00z boi raob with
40-60 kt southwesterly mid- to high-level flow. Likewise, the otx
00z raob exhibited steep lapse rates and was substantially moist
with a precipitable water of 1 inch. The combination of factors listed above
coupled with a fairly well organized cold pool/outflow suggests the
severe threat will persist into western Montana and perhaps the Flathead
Valley later this evening.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 46331991 47771808 48181497 47701367 46401363 46021498
46611655 45911929 46331991