SPC MD 648

MD 0648 CONCERNING SEVERE POTENTIAL...WATCH POSSIBLE FOR PORTIONS OF WESTERN/NORTHERN OR...SOUTHERN/CENTRAL WA

Mesoscale Discussion 0648
NWS Storm Prediction Center Norman OK
0534 PM CDT Thu May 04 2017

Areas affected...Portions of western/northern OR...southern/central
WA

Concerning...Severe potential...Watch possible 

Valid 042234Z - 050000Z

Probability of Watch Issuance...60 percent

SUMMARY...Isolated strong thunderstorms already have developed
between west-central WA and the southern Willamette Valley. 
Thunderstorms should continue to develop over higher terrain of
western OR and southwestern WA initially, before moving/spreading
north-northeastward across the discussion area.  A gradual increase
in storm coverage is expected, especially along and east of the
Cascades, with the risk increasing through the remainder of the
afternoon for severe hail and damaging gusts.  The threat should
persist into early evening, and a watch looks increasingly probable
for some part of this area.

DISCUSSION...Weak low-level flow will continue to limit hodograph
sizes, as evident in available VWP and modified soundings.  However,
as the Pacific shortwave trough approaches, the tightening height
gradient aloft will increase deep shear across the entire discussion
area, in support of severe potential with any persistent/sustained
convection.  The main factors in the total severe threat, therefore,
will be thermodynamic support and storm coverage. 

The boundary layer continues to destabilize across lower elevations
on both sides of the Cascades due to persistent surface diabatic
heating, helping to remove CINH and poor low-level lapse rates that
were evident in the 18Z MFR and SLE soundings.  Meanwhile, surface
dew points remain firmly in the 60s F across the Willamette Valley
and northward along the I-5 corridor toward CLS in southwestern WA,
with GPS PW near an inch and only minor vertical mixing anticipated.
 Modified soundings and model soundings in the valley suggest
1000-1500 J/kg MLCAPE.  CINH and the shallower nature of the lift
off the coastal range, however, may continue to restrict convective
coverage west of the Cascades, though isolated severe remains
possible.  Along and east of the Cascades -- and especially near the
Columbia River region where low-level moisture has accumulated
around the gorge, modified forecast soundings also show 1000-1500
J/kg MLCAPE where the northern part of the warmer/more strongly
mixed boundary layer overlaps the antecedent moisture max/axis with
surface dew points generally in the 50s F.  CAPE decreases sharply
southward into even drier air over the central/eastern OR deserts. 
Buoyancy also diminishes gradually northward into the Palouse Hills
with somewhat lower theta-e overall, although additional heating
will destabilize the boundary layer and reduce CINH enough to
support some severe threat over parts of southern WA.

..Edwards.. 05/04/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...OTX...PDT...MFR...SEW...PQR...

LAT...LON   42792345 44652320 46182347 46812311 47542185 47802133
            48222067 47801892 46741838 45651882 44932010 44092159
            42442314 42792345 

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