SPC AC 081622

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1122 AM CDT Tue May 08 2018

   Valid 081630Z - 091200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS UPPER MIDWEST TO
   LOWER MO VALLEY...

   ...SUMMARY...
   Isolated severe storms are possible from the Upper Minnesota Valley
   south to the Lower Missouri Valley, and across parts of Oregon
   during the late afternoon and evening.

   ..Upper MN Valley to Lower MO Valley...
   A cluster of warm advection-driven convection is ongoing across
   parts of the Siouxland. This activity should diminish as warm
   advection decreases into this afternoon. However, convection should
   redevelop late afternoon into this evening along the trailing
   outflow/warm front intersection expected to lie across the northwest
   IA portion of the MO Valley. Additional storms may form farther
   north near the surface cyclone that will drift from northeast SD
   into the MN Valley, and farther south across parts of MO near the
   warm front.

   With a lack of a connection to richer boundary-layer moisture over
   the southwest Gulf, mixed surface dew points should remain in the
   50s across the warm sector. This should result in relatively large
   surface temperature/dew point spreads south of the ongoing
   convective cluster. Deep-layer shear of 20-30 kts will also limit
   sustained supercells across much of the Mid/Lower MO Valley owing to
   the low-amplitude nature of the SD shortwave trough and modest
   mid/upper-level winds. The presence of a southerly low-level jet
   that will restrengthen this evening will support enlarged hodographs
   near the surface warm front. Convective mode should quickly evolve
   into multicell clusters with potential for a few embedded transient
   low-level mesocyclones. Such a setup will tend to favor isolated
   damaging winds as the primary overall hazard, with isolated severe
   hail favored in the initial development stage, along with a risk for
   a brief tornado or two near the warm front. While convection will
   likely persist into the overnight and become increasingly elevated
   east of the surface warm front, an increasingly marginal CAPE/shear
   combination should result in the severe-storm threat waning tonight.

   ...OR...
   A shortwave trough off the Pacific Northwest Coast will brush the WA
   Coast tonight, aiding in the breakdown of the upper ridge. Meager
   buoyancy atop a deep mixed layer should be supportive of a few
   strong, high-based storms east of the Cascades during the late
   afternoon and evening. While mid/upper-level speed shear will
   strengthen with approach of the trough, effective shear will likely
   remain on the margins for organized storms given the lack of larger
   buoyancy. Isolated strong to severe wind gusts should be the primary
   hazard, enhanced by evaporative effects within the dry sub-cloud
   boundary layer. Risk for severe weather should wane after sunset as
   inhibition rapidly increases.

   ..Grams/Bentley.. 05/08/2018

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