SPC AC 050552

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1252 AM CDT Fri May 05 2017

   Valid 061200Z - 071200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE NORTHERN ROCKIES...

   ...SUMMARY...
   Isolated strong/severe thunderstorms may develop over portions of
   western/central Montana and vicinity Saturday afternoon and evening.
   A strong storm or two may also develop near the southern
   Appalachians during the afternoon and early evening.

   ...Synopsis...
   A persistent, amplified mid-level trough will remain established
   east of the Mississippi Valley Saturday, as multiple embedded
   impulses rotate around its periphery. Farther upstream, a ridge will
   slowly build eastward across the Plains states, while a closed low
   pushes ashore the southern California/northern Baja coasts. In turn,
   broad southerly mid-level flow is forecast to persist from the
   Desert Southwest northward to the northern Rockies.

   ...Portions of the northern Rockies...
   With the aforementioned flow regime over the region, the combination
   of subtle embedded impulses, diurnal heating, and orographic
   influences will likely yield convective development over higher
   terrain from the Great Basin northeastward to Montana. Near and
   ahead of a cold front pushing east across western Montana, a pocket
   of somewhat greater surface moisture (e.g., dew points in the
   40s/lower 50s) will combine with sufficiently cool mid-level
   temperatures to promote modest mixed-layer CAPE (around 500-1000
   J/kg). In turn, as storms move north/northeast off higher terrain,
   they may be capable of isolated strong/damaging gusts, enhanced by
   ample sub-cloud evaporation. Additionally, effective shear on the
   order of 35-40 kt may favor a supercell or two that propagates
   east/northeastward into greater buoyancy over north-central Montana.
   Any such cell would be capable of isolated large hail and damaging
   winds. Convective intensity is then expected to wane during the late
   evening and overnight hours.

   ...Southern Appalachians...
   A mid-level shortwave trough and related speed maximum will
   translate south/southeast from the upper Great Lakes towards the
   southern Appalachians during the day. A related surface low will
   advance southeast towards the region as well, with a cold front
   trailing to its west. While surface moisture will be quite limited
   along/ahead of the front, with dew points in the 40s to lower 50s,
   cold mid-level temperatures will likely encourage weak mixed-layer
   CAPE supportive of isolated thunderstorms. Organized severe weather
   is not expected, but strong west/northwesterly convective-layer flow
   and steep low-level lapse rates may enable a few stronger wind gusts
   (in addition to small hail).

   ..Picca.. 05/05/2017

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