There are numerous possibilities inside the SPC slight risk area.

There are  4 potential targets for a late afternoon/early evening IOP.

1. Atlantic Seaboard LWX/PHI CWA:  SVR watch issued just before 19 UTC.  Main threat appears to be straight line winds given moisture pooling along weak frontal boundary and 30-35 flow around 500 mb.  Storms moving pretty fast and contain lightning, thus LMA should be able to show “something”.
2. S. Oregon MFR CWA:  This would represent the first time HWT had a potential W. US event.  SBCAPE approaching 2000 J/kg in and around the Rogue River Valley in S. Oregon.  30 kt 0-6 km bulk shear out of the southeast is available as a large (spatially, but not particularly intense) vorticity maximum approaches from the south-southwest.  Timing should be near max diurnal heating for the west coast, meaning around 00 UTC.  Storms should move out of the SE and primary threat would be marginally severe hail (1″ criteria) and some marginally severe winds given the potential for good downdrafts.
3. LUB CWA:  Extreme SBCAPE approaching 4000 J/kg exists across the S. TX panhandle.  Wind shear is borderline (30-35 kts 0-6 km).  Models break out precip around 00 UTC and move the storms to the east. SPC has a 5% tornado threat here, hatched hail box (30%), and 15% straight line winds.  If storms can become surface bases here, would be a good IOP for severe and the best chance for any tornadoes.
4. OUN CWA PAR/CASA:  Storms will form along and ahead of the cold front this evening and well into the night.  Models seem to be overpredicting the amount of CAPE ahead of the front. Wind shear is around 30 kts 0-6 km out of the west.  Most likely time is 00-03 UTC across the CASA network as storms fire in NW OKlahoma and propagate with the gustfront/synoptic front to the southeast across the CASA network.

2:00-3:00 PM LMA training
3:00-4:00 PM LMA IOP in the Sterling, VA CWA inside current SVR watch
4:00-??? PM TBD:  We’ll need to pick an IOP target early and monitor for any potential changes, especially for PAR/CASA operations.

Liz Quoetone and Paul Schlatter (EWP Weekly Coordinators, 1-5 June 2009)

SHORT:  Most likely IOP will be for multi-radar/multi-sensor product evaluation along frontal boundary in C. Iowa and E Nebraska during the 5-9 PM timeframe.  PAR and CASA operations are not anticipated today.

LONG:  The strong northerlies are still north of the border in Canada, but thankfully there is enough moisture around to at least get deep moist convection in the CONUS.  A weak vorticity max (weak even for early June standards) will move NE out of the 4-corners region, helping to increase low-level moisture across the central plains.  At 1600 UTC quasi-stationary front was draped across C Nebraska, C Iowa and S. Wisconsin/N. IL.  The warm sector south of the boundary should remain mostly clear and with dewpoints in the low 60s should be more than enough CAPE to get things going today in that area.

Depth of low level mosture is of some concern in terms of overall risk of severe weather and the threat of more than just gusty outflow winds.  There is a fair amount of moisture aloft, leading to PWATs around 1.5 inches across NE and IA, and just above the shallow moist layer there is a stout EML.  Along the surface boundary storms should form in the early evening hours right around peak heating and have a decent chance of becoming severe with good threat of severe hail (assume 1 inch criteria) and straight line winds.  Wind shear is marginal in the area near and south of the surface boundary, with ~30-35 kts 0-6 km bulk shear.  Low level shear and helicity are low everywhere, thus tornadic threat from supercells would be small.  Near the surface boundary would still need to be monitored however and locally enhanced helicity/low level shear could be present.  Primary storm mode would be quasi-linear with bowing segments and perhaps 1 or 2 embedded supercell-like structures.  Here are the SPC convective outlooks issued 1300 UTC:

The boundary is expected to lift a little north throughout the evening until/unless reinforced by convection.  Though very unlikely, there is at least a VERY small chance things might fire south into Western Oklahoma so will need to keep an eye on that area in case something gets going, but the threat of severe weather is pretty slim even if storms do form.

Next update will be during the live briefing in the NSSL dev. Lab, ~130 PM.

Paul Schlatter and Liz Quoetone (EWP Weekly Coordinators, 1-5 June 2009)