U.S. Severe Weather Forecast (Convective Outlook)

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 

.Broyles/Mosier.. 06/27/2017 


Mesoscale Discussion

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. 

.Smith.. 06/27/2017 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 46331991 47771808 48181497 47701367 46401363 46021498 
46611655 45911929 46331991