SPC AC 251302

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0802 AM CDT Tue Aug 25 2020

   Valid 251300Z - 261200Z

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

   ...SUMMARY...
   The greatest severe-thunderstorm threat today appears to be for
   damaging thunderstorm winds over portions of the Mid-Atlantic.

   ...Synopsis...
   In mid/upper levels, a persistent anticyclone will remain anchored
   over the Four Corners vicinity through the period.  The northern
   part of a shortwave trough -- now evident in moisture-channel
   imagery over western/northern CA -- will eject northeastward to
   western MT by 00Z, reaching Lake Winnipeg near the end of the
   period.  A gradually deamplifying mean ridge will extend
   northeastward from the Four Corners high to the Dakotas and MB,
   suppressed temporarily by a shortwave trough now located over
   eastern MT.  This perturbation will move across MB/ND today,
   becoming elongated and diffuse tonight over northern ON and the
   upper Great Lakes.  Downstream, a strong synoptic cyclone will dig
   southeastward from far northern QC to the western Gulf of St.
   Lawrence through 12Z tomorrow.  Amidst related height falls, a
   shortwave trough now over northeastern ON and southwestern QC will
   dig southeastward over NY and New England between about 21-06Z. 
   Meanwhile, a weak, positively tilted, inverted trough will drift
   northwestward over the southern Plains.

   At the surface, 11Z analysis showed a cold front from southern QC
   essentially along the length of southern ON to southern lower MI and
   southern WI, becoming a stationary to warm front over southern MN
   and east-central SD, to a low between PIR-PHP.  The frontal zone
   arched from there along a moist axis over northwestern SD and
   western ND.  Another cold front was drawn from south-central SK
   through a low near GGW across south-central/southwestern MT.  The
   leading cold front is forecast to sweep southeastward across the
   northeastern CONUS through the period, clearing New England and most
   of NY and PA by 00Z.  The trailing frontal segment should stall
   today across the southern Upper Great Lakes region, and move
   northeastward slowly as a warm front tonight over the eastern
   Dakotas and southern/central MN.  The northern Plains cold front
   should move eastward/southeastward by 12Z to northwestern MN,
   central SD and eastern WY.

   ...Northeastern CONUS...
   A prefrontal area of clouds, precip and isolated embedded
   thunderstorms is crossing parts of southern/eastern NY and
   central/eastern PA this morning, which will delay diurnal
   destabilization for several hours.  While this is not expected to
   preclude enough destabilization to support near-frontal,
   surface-based convection initiation and movement over some of the
   same areas later today, along with some severe potential, it does
   cast enough uncertainties on the organization/intensity of the
   second round to preclude any upgraded severe probabilities for that
   region, on this outlook cycle. 

   More-sustained diurnal destabilization and greater theta-e are
   expected farther south in and near the 30%-wind/enhanced area.  The
   main uncertainty there remains convective coverage in an otherwise
   favorable environment; however, frontal-prefrontal lift impinging
   upon this air mass may support enough convection to yield scattered
   damaging-gust events with some severe wind possible.  Isolated to
   scattered thunderstorms, sometimes in clusters, are possible further
   west and northwest near the frontal segment extending across
   portions of WV and OH.  Diurnal heating and surface dew points in
   the 60s F (70s near the Delmarva and parts of NJ) should contribute
   to MLCAPE ranging from around 1000-1500 J/kg over the NY/New
   England/PA portions of the outlook, to around 2000 J/kg in parts of
   OH and over the Delmarva/NJ region. 

   ...Upper Midwest/Northern Plains...
   Widely scattered to scattered thunderstorms should develop mainly
   this evening and tonight, in discontinuous swaths, 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 should be present
   area-wide.  Vertical motion contributing to this development will
   include:
   1.  Post-cold-frontal large-scale ascent increasing over the western
   Dakotas ahead of the ridge-topping shortwave trough from the
   northern Rockies, atop residual low/middle-level moisture, and
   2.  Warm advection, moisture transport and frontal lift along and
   north of the surface warm front across the central/eastern Dakotas
   and MN.
   Isolated strong-severe downdrafts also may penetrate to the surface,
   accelerated by areas of subcloud drying above the diabatically
   cooled, near-surface stable layer. 

   ...Southwestern ID vicinity to northern Rockies...
   Non-severe thunderstorms already have been apparent the past few
   hours across parts of northern NV in advance of the northern segment
   of the CA shortwave trough.  The associated plume of UVV should move
   northeastward across southern ID and southwestern MT this morning
   into early afternoon, with a well-heated/mixed boundary layer
   potentially available ahead of the convection over the Rockies.
   Given the limited moisture/instability, however, severe potential
   with this round appears minimal.  

   A second episode of convection should develop 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 the remaining shortwave segment, along with a combination
   of at least marginal low-level moisture, strong surface heating in
   the wake of the morning activity, 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, 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
   hazard; 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 southwestward to
   westward.  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 proximal high, may limit
   coverage/duration/intensity for activity forming on the higher
   terrain.  Greater moisture/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.  Accordingly, NHC has issued its last advisory on this
   system.  Though low-level hodographs remain curved and somewhat
   enlarged 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.  A brief supercell cannot be ruled
   out, but tornado potential has become too small to continue even 2%
   unconditional probabilities.  

   Very late in the period (09-12Z), low-level shear may increase again
   near the LA coast as Hurricane Laura approaches, based on current
   NHC forecast track/intensity/wind radii.  Still, unless this system
   moves substantially faster, enlarges more, and/or gets much stronger
   than currently forecast, unconditional tornado probabilities appear
   too limited to introduce an outlook until day 2/Wed.  Refer to NHC
   advisories for latest forecast info and tropical watches/warnings
   related to Laura.

   ..Edwards/Leitman.. 08/25/2020

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