U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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Storm Prediction Center ac 261253 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0753 am CDT Mon Jun 26 2017 


Valid 261300z - 271200z 


..there is a marginal risk of severe thunderstorms over parts of 
the interior northwest... 


..there is a marginal risk of severe thunderstorms from parts of 
the Central Plains across much of New Mexico and the southern High 
Plains... 


... 
Thunderstorms with marginally severe hail and strong wind gusts are 
possible late this afternoon into early evening across parts of the 
Pacific northwest and from the southern and central rockies eastward 
into the High Plains. 


... 
In mid/upper levels, strong ridging that has dominated the western 
U.S. Will begin to break down and, in the case of the northern 
branch of the ridge, shift eastward through the period. This will 
occur as two initially separate Pacific perturbations phase into a 
single synoptic-scale trough today: 
1. A large, intense, northern-stream shortwave trough moving over 
the southern Alaska Panhandle and just offshore southwestern BC, 
anchored by a low/vorticity Max now entering northwestern British Columbia. The 
low will proceed eastward across northern British Columbia to northwestern ab by 
12z, with troughing southward to the Idaho Panhandle. 
2. A less-intense but still well-defined perturbation in 
moisture-channel imagery, moving northeastward toward the northern 
California and or coasts. This trough will weaken but still link with the 
northern-stream feature as it reaches western Idaho and northern Nevada 
late in the period. 


Meanwhile, as the downstream northern branch of the ridge moves 
eastward across the northern/central rockies, difluent northwest 
flow aloft will persist over the central and southern Great Plains, 
but generally not as strong as yesterday due to a lessening of the 
height gradient aloft. The latter is related to the eastward shift 
of troughing and related cyclonic flow across the Great Lakes and 
Ohio Valley. Associated cold air aloft and residual low-level 
moisture, beneath strong mid/upper winds, may support small hail and 
strong gusts from a few thunderstorms over parts of eastern/northern 
New York and VT, but severe potential looks too low/conditional for a 
categorical outlook at this time. 


At the surface, the 11z analysis showed a cold front from offshore 
of the NC coast across southeastern GA, extreme southeastern la, and 
southeast TX, becoming quasistationary northwestward over 
central/northwest TX, before being overtaken by an expanding 
outflow/cold pol from a dissipating mesoscale convective system. This front will become 
quasistationary along most of its length and slowly weaken through 
the period. Another frontal zone, evident from a weak low near pln 
across eastern WI, northern IA, and northern NE, is forecast to move 
southward over the mid/upper Mississippi Valley region and Central 
Plains through the day, stalling then returning northward late 
tonight over the Central High plains. 


..interior northwest... 
Widely scattered to scattered thunderstorms are expected to develop 
this afternoon east of the Cascade crest, moving northeastward 
across the outlook area into early evening, with some increase in 
coverage possible due to clustering and cold-pool effects. The 
potential exists for isolated severe gusts and large hail. Strong 
diabatic surface heating and adequate low-level moisture will 
underlie strengthening deep-layer winds and large-scale ascent aloft 
in advance of the Pacific perturbation. Moisture should remain 
relatively maximized in the preconvective environment from northern 
Idaho southwestward across south-central Washington and central/south-central 
or, despite effects of strong vertical mixing as afternoon heating 
persists. This will support MLCAPE 500-1000 j/kg, locally higher. 
To the south and north of the outlook area, despite strong surface 
heating and steep low/middle-level lapse rates, moisture and 
therefore cape will decrease. 


Some of this activity also will pose a risk for dry-thunderstorm 
fire ignitions, especially over eastern or and southwestern id; 
refer to the Storm Prediction Center day-1 fire-weather outlook for more details. 


..Central Plains... 
Widely scattered thunderstorms should develop near the frontal zone 
this afternoon and move southward to southeastward across this lobe 
of the outlook area, offering isolated hail and gusts near severe 
levels. With augmentation from evapotranspiration helping to offset 
vertical mixing of the initially very shallow moist layer near 
surface, dew points now mostly in the 50s f should increase somewhat 
by mid afternoon, generally reaching the upper 50s to low 60s f. 
Shallow but well-mixed subcloud layers should develop with diurnal 
heating beneath 500-1000 j/kg MLCAPE in a generally east-west 
corridor, based on modified 12z raobs and forecast soundings from 
several models. 


Despite the eastward motion of the Great Lakes troughing, cyclonic 
flow aloft with favorable deep/speed shear will remain over the 
region, yielding 40-50 kt northwesterly effective-shear vectors, 
aligned with a substantial component orthogonal to the frontal zone. 
This will support a mix of discrete and clustered modes, and some 
supercellular characteristics may be noted in the more persistent 
cells. The lack of both more robust moisture/instability and 
stronger low-level lift precludes a better-organized or denser 
severe threat. Convection should diminish after dark while 
encountering a progressively more-stable boundary layer. 


..south-Central High plains, co, nm, West Texas... 
A relative gap may exist between this regime and the Central Plains 
one described above, across parts of southwestern Kansas/southeastern 
Colorado/northeastern nm/western OK Panhandle region. Intense convective 
overturning and stabilization has been performed by last night's 
MCS, the remnants of which now are moving south-southeastward and 
weakening across a large swath of west TX, and this potential gap 
area will be the slowest to recover. However, spatial precision of 
such a gap, along with the tendency of the Raton Mesa and Sangre de 
Cristos to initiate convection in even more marginal thermodynamic 
environments than evident today, still leaves too much uncertainty 
to carve those parts of the outlook area away at this time. 


Farther southwest over the bulk of nm, and west TX, recycled mesoscale convective system air 
in the north, transitioning to a mix of mesoscale convective system-processed and 
unprocessed trajectories in the south (but in weaker shear/flow 
aloft), will be available for afternoon development on the higher 
terrain. Widely scattered to locally scattered thunderstorms are 
expected to form this afternoon as diabatic heating and areas of 
weak upslope flow reduce mlcinh. MLCAPE 1000-2000 j/kg should 
develop atop well-mixed subcloud layers, amidst effective-shear 
magnitudes ranging from around 35 kt over southeastern Colorado to less 
than 20 kt in southern nm and far West Texas. An mesoscale convective vortex and 
outflow/differential-heating boundaries related to the prior mesoscale convective system 
also may focus storm development/propagation over parts of southwest 
and far West Texas. This environment will support primarily a patchy, 
slow-moving multicellular storm Mode during afternoon and early 
evening with isolated severe gusts and hail. 


.Edwards/picca.. 06/26/2017 


$$ 

Mesoscale Discussion


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sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 260454 
txz000-nmz000-260530- 


Mesoscale discussion 1151 
National Weather Service Storm Prediction Center Norman OK 
1154 PM CDT sun Jun 25 2017 


Areas affected...the Texas South Plains into parts of the Permian 
Basin 


Concerning...severe potential...watch unlikely 


Valid 260454z - 260530z 


Probability of watch issuance...20 percent 


Summary...the corridor for 50-60 kt gusts will likely remain 
confined to areas northwest/west of Lubbock and North/East of 
Plainview during the next hour with a southward-migrating squall 
line. 


Discussion...radar mosaic shows a mature squall line from the Texas 
South Plains southwestward into southeastern nm. Measured severe 
gusts at multiple West Texas mesonet stations have observed gusts 
ranging from 50-71 kt during the past hour ---peak gust of 71-kt at 
Muleshoe. Despite nocturnal cooling of the boundary layer and a 
low-level stable inversion, a mesoscale convective vortex and associated rear-inflow jet 
will likely aid at least a risk for isolated severe gusts for the 
next hour and perhaps longer. Yet, the additional need for a severe 
watch due to a severe risk persisting for 3 or more hours south of 
the ongoing squall line/severe watch 370 is in doubt. 


.Smith/Hart.. 06/26/2017 


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


Attention...WFO...lub...Ama...maf...abq... 


Latitude...Lon 32380408 34360252 34870099 34370089 33090171 32090317 
32060380 32380408