U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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acus02 kwns 210446 
swody2 
Storm Prediction Center ac 210445 


Day 2 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1145 PM CDT Fri Oct 20 2017 


Valid 221200z - 231200z 


..no severe thunderstorm areas forecast... 


... 
at this time, the risk for severe storms appears negligible across 
the U.S., Sunday through Sunday night. 


... 
Models continue to indicate that the mid-latitude westerlies will 
undergo considerable amplification through this period across the 
Pacific into western North America. This appears likely to include 
building upper ridging near the Pacific northwest/British Columbia 
coast, to the north of increasingly prominent subtropical ridging 
across the lower latitude eastern Pacific and much of southwestern 
North America. 


Downstream of the building ridge, larger-scale troughing appears 
likely to continue to develop eastward across central Canada and 
adjacent portions of the north central United States. Within this 
regime, one significant short wave perturbation may dig through the 
international border area and northern U.S. Plains, while another 
pivots through the Hudson/James Bay region, accompanied by a deep 
surface cyclone. 


Much more uncertainty persists with another initially digging short 
wave impulse, which is generally forecast to split away from the 
northern cyclonic regime, before turning eastward toward the lower 
Mississippi Valley. Rather large spread remains readily evident 
among the models and within the ensemble output of the various 
models concerning this latter feature. 


..south central u... 
One low-level southward surge of cooler air in the wake of the 
Hudson/James Bay cyclone is expected to reach the Ozark Plateau and 
Southern Plains by 12z Sunday, before continuing southeastward 
through the lower Mississippi Valley and Texas Gulf Coast region. 
Of particularly note, seasonably high moisture content air seems 
likely to continue to advect inland off the Gulf of Mexico in a 
corridor along/ahead the front. This may provide potential for weak 
to moderate boundary layer destabilization by Sunday afternoon, 
despite the presence of generally weak mid-level lapse rates ahead 
of the mid-level cold core, which is expected to lag to the west of 
the front. Coupled with the potential for modest strengthening of 
lower/mid tropospheric wind fields, an environment becoming 
conducive to localized pockets of strong/severe storm development 
may not be completely out of the question, mainly across parts of 
southeast Texas, Louisiana, and perhaps Mississippi. However, at 
least some guidance is suggestive that the front, or pre-frontal 
outflow associated with weakening convection, may surge through much 
of this region fairly early in the day. Given this complication, in 
conjunction with the previously noted sizable spread within the 
model output, the risk for severe storms still appears negligible 
through this period (i.E. Less than 5 percent). 


.Kerr.. 10/21/2017 


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