SPC AC 301259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Sat May 30 2020

   Valid 301300Z - 311200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF CENTRAL/NORTHERN OREGON AND CENTRAL/EASTERN WASHINGTON...

   ...SUMMARY...
   Scattered severe thunderstorms, offering destructive winds and hail,
   are expected today over parts of the interior Pacific Northwest,
   east of the Cascades.

   ...Synopsis...
   A high-amplitude upper-air pattern will influence the CONUS this
   period, with a deep mean trough persisting over the northeastern
   Pacific, and another major trough extending from a cyclone over
   Baffin Island southward to the central/southern Appalachians.  In
   between, a sharp ridge will move slowly eastward from the Rockies
   across the adjoining High Plains.  A formerly cutoff cyclone -- its
   center now apparent in moisture-channel imagery west of the central
   CA coastline -- will accelerate north-northeastward while devolving
   into a strong shortwave trough.  By 00Z, the vorticity max
   representing the core region of the cyclone should reach
   northernmost CA.  By 12Z, the negatively tilted shortwave trough
   should reach southern WA, eastern OR and western ID.

   At the surface, 11Z analysis showed a cold front from the St.
   Lawrence Valley of southern QC southwestward across southeastern NY,
   central VA, northern GA, southern AL, to the TX shelf waters of the
   Gulf.  This front will move slowly eastward/southeastward over the
   Atlantic Seaboard and offshore through the period, while stalling
   then moving northward across south TX.  In the Northwest, an
   elongated area of low pressure was evident along a quasistationary
   frontal zone over central parts of WA/OR.  A more-coherent low
   should consolidate across southern OR by around 18Z as the strong
   upper support approaches, then move north-northeastward along the
   front over eastern portions of WA/OR.

   ...Interior Northwest...
   Scattered thunderstorms are forecast to develop this afternoon in a
   zone of strong deep lift over extreme northern CA/southern OR just
   ahead of the ejecting wave.  In addition, isolated to widely
   scattered thunderstorms may form farther north in the frontal zone,
   and in the warm sector (especially where local orographic lift is
   favorable).  This convection should sweep roughly northward across
   the outlook area during the rest of the afternoon and into the
   evening, producing severe winds and sporadic large hail.

   An anomalous combination of favorably timed deep forcing and rich
   low-level theta-e will support the severe threat.  The approaching
   cyclone/trough will be preceded by strongly difluent flow and a belt
   of relatively intense large-scale ascent/DCVA.  As this and the
   aforementioned surface cyclogenesis occurs, mass response will
   strengthen in low levels, increasing vertical shear area-wide.

   Meanwhile, steep midlevel lapse rates will be present on either side
   of the front, while the warm sector experiences strong heating and
   deep mixing that will yield favorable DCAPE, while maintaining
   favorable inflow-layer moisture content.  Lower-elevation surface
   dewpoints this morning are commonly in the 50s and 60s F, with
   moisture expected to remain favorable despite some mixing-related
   decrease in preconvective dewpoint values this afternoon.

   Surface-based effective-inflow parcels will exist on the west side
   of the front, where low-level winds will be veering with height,
   hodographs strongly curved, and deep shear relatively maximized.  As
   such, and with low-level vorticity maximized in the frontal zone,
   supercells are possible.  Though the environment generally favors
   high-based convection, boundary interaction and storm-scale/
   orographic effects may render a nonzero localized/isolated threat
   for a tornado, in addition to the more-prevalent wind/hail
   potential.  Forecast soundings reasonably depict peak preconvective
   MLCAPE in lower elevations in the 1500-2000 J/kg range (locally
   larger), amidst strengthening deep-layer flow and shear with time.

   Organized convection also may develop in the evening and overnight
   periods across parts of northern ID and western MT as deep ascent
   increases (atop favorable moisture), offering an anomalously
   late-hour threat for damaging wind and at least isolated large hail.

   ...Northern Rockies to central High Plains...
   Isolated to widely scattered thunderstorms are forecast to develop 
   this afternoon near the large-scale ridge, primarily over higher
   terrain at first.  Convection should spread off the mountains in a
   loosely fan-shaped pattern across the outlook corridor:  northward
   to northeastward in the ID/MT region, and eastward to southeastward
   across the central High Plains.  Isolated severe is possible as the
   convection moves across a marginally moist, but strongly heated/
   mixed boundary layer supporting maintenance of hail/gusts to the
   surface.

   Favorable buoyancy, with 500-1500 J/kg MLCAPE, should develop in a
   plume from northern CO across eastern WY to southern MT.  Strong
   directional shear (veering of winds with height) is expected over WY
   and southern MT, and isolated supercell formation cannot be ruled
   out there, despite the lack of substantial upper support and
   ridging-related weaknesses in low/middle-level wind speeds.  Weak
   bulk shear will limit organization with southward extent over CO. 
   Convection in this corridor should be tied strongly to diurnal
   thermodynamic processes, weakening after dark as it moves across
   lower terrain and a stabilizing subcloud layer.

   ...Carolinas, Tidewater VA...
   Widely scattered to scattered thunderstorms are forecast to form
   this afternoon over portions of the Piedmont and coastal plain,
   moving eastward to southeastward across the outlook area.  Sporadic
   damaging gusts and isolated severe downdrafts are possible.  A weak,
   elongated vorticity banner in mid/upper levels -- currently
   occupying the base of the cyclonic flow from eastern VA across
   central NC to central GA -- will move slowly eastward across the
   outlook area today, atop a diurnally destabilizing and richly moist
   warm sector.  Convection will be supported by diabatic heating in
   the prefrontal air mass, combined with lift near the front, thermal
   lift in the free warm sector amidst weak MLCINH, and sea-breeze
   boundaries.  Surface dew points generally in the upper 60s to near
   70 F will support MLCAPE of 1500-2500 J/kg.  Weak winds through most
   of the troposphere, and related lack of vertical shear, will limit
   organization of the severe threat.

   ..Edwards/Gleason.. 05/30/2020

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