NWS: Regional Thunderstorms Forecasted This Memorial Day Weekend

The pattern for thunderstorms becomes more interesting on Sunday as the upper low continues south toward the Bay Area of Oregon . The flow around this low should move any thunderstorms that develop to the west or southwest. The favored area of thunderstorm development will be across Modoc and Siskiyou counties but isolated thunderstorms could extend into Lake and Klamath counties. The steering winds are weak so any storms that develop may not make it across the Oregon Cascades into the Rogue Valley but that’s the direction they will tend to move.

Overall, thunderstorm movement will vary from day to day but similar weak instability and moisture availability will keep the focus in the same region for thunderstorm development for at least the next few days.

A very different air mass (relative to thunderstorms) is impacting the coast this morning. A weak front and marine push is occurring. The stratus layer is still shallow early this morning but this front should stir the air and raise the stratus layer so that it makes it into the Umpqua Basin after sunrise. The low clouds will probably not make it into the Rogue Valley this morning but higher clouds will pass through and it will be a little cooler today compared with yesterday, about 5 degrees cooler in Medford.

The new low pressure system will fuel an area of shower and thunderstorm activity that is wider than today, with showers across Douglas County and along the Siskiyous in Jackson and Josephine County, and all other areas along the Cascades and east, and across NorCal. Chance for thunderstorms ranges from about 10% in the west side Cascades foothills around Steamboat and Butte Falls, to the Ashland Area, increasing to the southeast up to 35-40% in southeast Klamath County, eastern Siskiyou, and Modoc County.

There is strong model agreement that thunderstorm activity will occur in this region. There is also a more isolated bullseye for thunderstorms in the Trinity horn area of southwest Siskiyou County that ranges from 20 to 40% depending on the guidance source. NWS Medford is leaning towards the 40% side because it will be closer positioned to the upper level shortwave (the trigger) and is a bullseye for instablity.

Even with an influx of new moisture, the storms should not be very wet. High bases will keep accumulations to a few hundreths except under the cores of the most vigorous storms where 0.1″ to 0.25″ is possible. There is a 30% chance that rain rates lead to 1″ of rain accumulation, but this is still an inch shy of the expected flash flooding thresholds in this area. Storm motions should be from the west to east or east southeast at around 20mph. Hail will also be present, but it should be the typical pea size or smaller based on the lower lapse rates and poor shear environment.

An upper level low will traverse southwards by the start of the extended forecast period. The latest forecasting tools shows a 20 to 30 percent chance of rain showers east of the Cascades Monday evening. There appears to still be some upper level energy helping keep things unstable. The probability of thunder is roughly the same around 20 to 30 percent.

The flow pattern will allow some cooler air to push in from the north as higher pressure builds well to our west in the Pacific. The cooler more unstable air will push into southern Oregon for most of the week keeping daytime highs around their climatological normals for this time of year. In addition, the cooler air will allow afternoon instability to build every afternoon and evening. This will be most notable east of the Cascades since the energy to break the cap seems to be weaker. NWS is predicting a 25% chance of lightning within 25km of any point east of the Cascades this upcoming week. Those probabilities fall off the farther one travels to the west and is essentially zero over valleys west of the Cascades and along the coast.

Source: NWS Medford