I'm a long time weather watcher from British Columbia. For the last few years, I've been a follower of Scott Sistek over at KOMO-TV as well as Cliff Mass' blog. I actually prefer these soruces (along with the forecasts from the NWS Seattle office) over Environment Canada's forecast. Especially in summer, EC has gotten it wrong on convection countless times.
Anyway, from reading Cliff's blog, I am familiar with the weather patterns in the PNW in summer and how they limit convection. However, I want to know the dynamics that could trigger thunderstorm activity: west or east of the Cascades; in or out of summer, from Bellingham/the Lower Mainland down to Eugene...how do cold or warm-core convection events get triggered in this part of the continent when the dominant air mass is the North Pacific High, onshore flow dominates, the Cascades and Rockies block moisture, and dewpoints rarely surpass 60F/15C? As for east of the Cascades or the Willemette Valley, what makes warm-core convection so much more frequent than in, say, the Puget Sound or the Lower Mainland?
Essentially, what I want to know is something like this, but for thunderstorms.
There are a couple of things I know:
- Most convective events west of the Cascades occur in the spring, when the air is coldest aloft (source: Cliff Mass)
- Warm core convective events west of the Cascades are typically a dry lightning event. Thundervirga is also not uncommon. As such, the biggest danger is not wind, hail, or heavy rain, but actually wildfires. To my count, there has been one Tornado Warning west of the Cascades in my lifetime (the 5/31/97 tornado outbreak), one Severe Thunderstorm Watch north of Portland (on 8/12/14, in SW WA), and one Severe Thunderstorm Warning in Metro Vancouver/Fraser Valley (on 7/25/09, for the western Fraser Valley including Abbotsford). However, there are Red Flag Warnings every summer.
- Cloud tops aren't as high as east of the Rockies; very rare to see echo tops over 30,000 feet (east of the Cascades is a different story)
- At least here in the Lower Mainland, thunderstorms almost always come in a general south-north direction, or east-west, whereas fall/winter storms usually travel southwest to northeast.
- CAPE values max out at 800-1000 J/kg west of the Cascades*. East of the Cascades (example) and in the Willamette Valley, CAPE readings can hit 1500-2000 J/kg, but CAPE potential is more limited because the source of moisture is the North American Monsoon instead of the Gulf of Mexico, which basically acts like a heat pump/sauna for most of North America east of the Continental Divide. CAPE values in the Midwest and Mid-Atlantic can reach 5000-6000 J/kg.
This question has been bugging me for many years. I emailed both Cliff Mass and Scott Sistek for answers, but I never got a reply. I would also love to learn how to analyze the data on Twisteradata.com, like professional storm chasers do, and infer clues as to thunderstorm development. Like, "oh, the 850mb wind direction is different, and the 850mb temperature is 2C, and the CAPE is 850 J/kg with high voricity and water vapour, we will get thunderstorms".
* I'm making an educated guess because I haven't been able to find archived surface CAPE reading that are available to the public. UW's GRF-WRF product (which includes CAPE readings) only archives data for 12 days, and the NCDC doesn't have surface CAPE data archived. Though, I think there hasn't been a 1000 J/kg day west of the Cascades since 8/3/99 or maybe even 5/31/97.
Edit: 5/31/97 was tornado warning, not STW.