Saturday will be the first day this “warm” season that we see strong thunderstorms develop east of the Cascades. A perfect mix of a very warm atmosphere, moist SE flow aloft, and low-level cool air moving in from the west Saturday afternoon and evening could lead to quite a thunderstorm outbreak there. West of the Cascades the low-level air mass will be much more stable, so I think it’s unlikely we’ll see thunderstorms. More likely lots of cloud cover and just a chance of a shower. This is the reason we haven’t had thunderstorms on our 7 Day forecast all week. Southeasterly upper-level flow ahead of a cold trough approaching is often a good setup for storms west of the Cascades, but not always. You may remember 2 years ago we were in similar pattern for a week or so in mid-June and saw almost nothing.
Meanwhile, we had a nice day today, if a bit chilly for the first half. Portland missed its record low for the date by only 1 degree! (38). Some isolated spots in the metro area even had frost. That’s totally inappropriate (in my mind) for mid-May. In fact with all that sunshine we were still 3 degrees short of reaching our AVERAGE high.
Lots of high clouds for tomorrow, so I have panicked and dropped the forecast high back to 68. Hopefully that won’t be a mistake, but we’ve been over-forecasting several “warm” days this season. When forecasting high and low temperatures, I do it a bit differently than I did the first 10-15 years of my career. With the report card each Monday, I need to be within 3 degrees to get the forecast correct. So while in the past I might have gone 70 or 71 for tomorrow. In the case of tonight, I figure it’s very unlikely it’ll go above 71 tomorrow, more likely somewhere between 65-70, so I end up with 68.
I looked at the long-range maps today and almost cried, or at least went to our vending area and bought some cookies for comfort food. Yuck…troughing or westerly flow for the 3rd week of May! No sign of significantly warmer weather through at least the end of next week…Now that was until the 00z GFS. It looks significantly better, at least very typical (and mild) May weather after next Wednesday.
Chief Meteorologist Mark Nelsen