U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 171948 
Storm Prediction Center ac 171947 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0247 PM CDT Mon Jul 17 2017 

Valid 172000z - 181200z 

..there is a slight risk of severe thunderstorms over parts of Minnesota 
and vicinity... 

..there is a slight risk of severe thunderstorms from northeast PA 
into Vermont... 

..there is a marginal risk of severe thunderstorms over parts of 
the central and northern plains and upper Midwest... 

..there is a marginal risk of severe thunderstorms over parts of 
the northeast states... 

..there is a marginal risk of severe thunderstorms across portions 
of the lower Colorado River valley and vicinity... 

Isolated to widely scattered severe thunderstorms are expected into 
this evening across portions of the upper Mississippi Valley, and 
across portions of the northeast. 

A marginal risk area has been added for portions of the lower 
Colorado River valley and vicinity. Strong surface heating is 
ongoing along the northern zone of rich monsoon-related moisture 
across portions of the southwest states. Based on the Storm Prediction Center sounding 
climatology, the 12z Las Vegas sounding sampled an anomalously 
substantial magnitude of precipitable water this morning (around 1.4 
inches). With temperatures climbing well into the 90s to around 105f 
as dewpoints only slowly fall into the upper 50s to lower 60s, 
buoyancy is becoming moderate to strong amid steep low- to mid-level 
lapse rates. Convection will continue to deepen along the Mogollon 
Rim and westward across the higher terrain of southern Nevada and 
vicinity into the late afternoon hours. Intensifying cold pools 
should tend to congeal at the meso-Beta scale owing to the 
anticipated substantive convective coverage in the moist environment 
and along the northern periphery of an mesoscale convective vortex spreading westward across 
southern Arizona. Into the evening hours, there will be the potential for 
small convective clusters to spread southward/southwestward across 
the vicinity of the lower Colorado River valley. Given the 
substantial buoyancy, anticipated convective clustering, and dcape 
values in excess of 1500 j/kg, the risk for isolated severe wind 
gusts may exist -- warranting the addition of the marginal area. 

The western part of the slight risk area across portions of the 
northeast has been trimmed owing to antecedent convective 
overturning in association with loosely organized convective 
clusters crossing parts of New York and western New England. This activity 
will continue spreading across the updated slight risk area. Modest 
air-mass recovery in the wake of this activity could sustain some 
isolated severe potential into the early evening, as sufficient deep 
shear persists peripheral to a mid-level cyclonic perturbation 
crossing parts of southeast Canada. 

No other appreciable changes have been made to the ongoing forecast. 

.Cohen.. 07/17/2017 

Previous discussion... /issued 1109 am CDT Mon Jul 17 2017/ 

..Minnesota vicinity... 
Morning water vapor imagery shows a shortwave trough moving across 
ND toward Minnesota. Models remain consistent in the development of 
scattered afternoon thunderstorms over eastern ND/western Minnesota, with 
storms moving rapidly eastward across the slight risk area. Strong 
heating is occurring in this zone, while dewpoints climb into the 
mid/upper 60s. This will yield afternoon MLCAPE values over 2500 
j/kg with little cap. Forecast soundings also indicate favorable 
wind fields, with effective shear values over 35 knots and 
considerable low-level shear. These parameters should encourage the 
development of rotating/bowing storm structures capable of damaging 
winds and large hail. 12z cam solutions suggest the potential for a 
fast-moving bowing complex moving into The Arrowhead region this 

Current visible satellite imagery shows mostly clear skies with a 
growing cu field from central PA across southeast New York into central 
New England today. This corridor should become moderately unstable 
with MLCAPE values over 1500 j/kg. Thunderstorms are expected to 
form relatively early this afternoon over parts of PA/New York and move 
eastward across the slight risk area. Mid-level lapse rates are 
moderately steep, and winds in the upper-levels are favorable. 
However, relatively weak winds through much of the lower-half of the 
convective layer lend some uncertainty regarding the overall severe 
risk today. Nevertheless, the stronger cells and bowing lines this 
afternoon will pose a risk of gusty/damaging winds and hail. 


Mesoscale Discussion

acus11 kwns 171958 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 171957 

Mesoscale discussion 1331 
National Weather Service Storm Prediction Center Norman OK 
0257 PM CDT Mon Jul 17 2017 

Areas affected...portions of the northern plains and upper 
Mississippi Valley 

Concerning...severe potential...watch likely 

Valid 171957z - 172100z 

Probability of watch issuance...80 percent 

Summary...thunderstorms will continue to develop across the region 
through this evening, with an attendant threat of damaging winds, 
large hail, and perhaps a Tornado. Watch issuance is likely within 
the next hour. 

Discussion...a cumulus field continues to organize across portions 
of NE South Dakota, southeast North Dakota, and western Minnesota this 
afternoon, as a cold front and associated low pressure shift across 
the region. Surface heating (i.E., Temperatures in the 90s) has 
combined with sufficient boundary-layer moisture and instability 
aloft to promote MLCAPE values upwards of 1500-2500 j/kg this 
afternoon. While mid-level westerly flow remains modest, veering of 
winds with height has provided around 30-40 kt of effective shear. 
Combined with ample mid-level buoyancy, a mixture of supercells and 
multicell clusters will likely organize over the next few hours, 
with a resultant threat of damaging winds and large hail. 
Additionally, while low-level flow remains relatively weak, ample 
boundary-layer moisture and backed flow with eastward extent may 
favor a tornado or two as well this evening. As such, watch issuance 
is expected within the next hour. 

.Picca/Hart.. 07/17/2017 

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


Latitude...Lon 45199575 45189698 45509747 46309777 46659766 46819738 
47209646 47569451 47529304 47479248 47059216 46309220 
45619236 45269418 45199575