SPC AC 251958

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0258 PM CDT Tue Aug 25 2020

   Valid 252000Z - 261200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE MID-ATLANTIC...

   ...SUMMARY...
   Isolated to scattered damaging wind gusts will remain the primary
   severe threat over portions of the Mid-Atlantic and Northeast this
   afternoon and evening.

   ...20Z Update...
   The only substantial change to the outlook was to remove severe
   probabilities along/behind a broken line of storms across the
   Mid-Atlantic and Northeast. Damaging wind gusts will likely remain
   the primary severe threat with these storms as they move
   east-southeastward through this evening. For more information on the
   near-term severe threat across these regions, see Mesoscale
   Discussion 1570.

   ..Gleason.. 08/25/2020

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Tue Aug 25 2020/

   ...Northeast/Mid-Atlantic region into portions of the Ohio Valley...

   Substantial cloud cover over northern portions of the Slight Risk
   area this morning has tempered diurnal heating, however sufficient
   clearing is anticipated to result in 1000 to 1500 J/kg MLCAPE ahead
   of the cold front. Farther south/west into the mid-Atlantic and OH
   Valley, on the order of 2000 to 2500 J/kg MLCAPE should develop by
   late afternoon. Strengthening low/mid-level wind fields will develop
   as an upper-level trough amplifies slightly while moving east, and
   an increase in large-scale ascent associated with the upper trough
   should aid in thunderstorm development or re-intensification this
   afternoon. Clusters or line segments capable of damaging winds will
   be the primary risk as storms move southeast through this evening.
   No changes were made to the Enhanced Risk area, with some minor
   adjustments to northwestern portions of the Slight risk areas based
   on current trends and latest high-res model guidance.  

   ...Northern Plains/upper Midwest...
   Widely scattered to scattered thunderstorms should develop mainly
   this evening and tonight across portions of the western/central
   Dakotas into northern MN. The most vigorous convection will be
   capable of isolated large hail and strong-severe gusts. Steep
   EML-related midlevel lapse rates will be present, and contribute to
   moderate-strong MLCAPE. Lift associated with a weak shortwave trough
   moving east across northern Rockies/Plains and low-level warm/moist
   advection should contribute to thunderstorm development.

   ...Southwestern ID vicinity to northern Rockies...
   Thunderstorms should develop/intensify across the northern Great
   Basin, and eastern OR into the Snake River Plain and vicinity this
   afternoon. This activity will be aided by large-scale lift ahead of
   a shortwave trough lifting north from CA, along with a combination
   of at least marginal low-level moisture, strong surface heating and
   local orographic enhancements of lift. Deep-layer speed shear will
   increase westward across the outlook area, with effective-shear
   magnitudes reaching the 35-45-kt range over parts of eastern OR into
   central ID. MLCAPE of 200-500 J/kg should be common. Deep and
   well-mixed subcloud layers will contribute to locally
   strong/isolated severe gusts. Much of this area also is at risk for
   isolated dry thunderstorms and related fire hazards; see the SPC
   fire-weather outlook for more details.

   ...Mogollon Rim to southern AZ...
   Widely scattered to scattered thunderstorms should form this
   afternoon into early evening, both over the Mogollon Rim area and
   deserts near and south of I-10, moving generally west or
   southwestward. Forecast soundings depict a reasonably deep but not
   very strong (5-10 kt over the Rim and southwestern AZ, somewhat
   stronger but not as deep over south-central/southeastern AZ) layer
   of easterly to northeasterly flow aloft that would assist convection
   in moving across adjoining lower deserts. However, weak
   mid/upper-level lapse rates related to the proximity to the
   upper-level high, may limit coverage and intensity for storms
   forming on the higher terrain. Greater buoyancy and strong heating
   over southern AZ, along with deep/well-mixed subcloud layers
   area-wide, may still support strong/isolated severe gusts with the
   best-organized convection.

   ...Central/western Gulf Coast...
   Low-level remnants of Marco continue to move westward just off the
   LA coastline, weakening and well-removed from any sustained deep
   convection.  Though low-level hodographs remain somewhat curved well
   northeast of the low-level vorticity center across parts of the
   central Gulf Coast, weakening of both residual wind fields and
   low/middle-level lift will proceed through most of the remainder of
   the period, as the remnant trough proceeds westward while further
   losing definition. 

   Meanwhile, the increase in tornado potential associated with
   Hurricane Laura is not expected to arrive until Wednesday as outer
   bands and strengthening low-level wind fields overspread the region.
   Please see the latest information from the National Hurricane
   Center.

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