It doesn’t take brain surgery (sorry neurologists) to figure out that we have a very cool air mass for this time of year sitting overhead. You don’t need a meteorologist to tell you that with temps in the low 50s across the metro area at 5pm. Typically we’d be well into the 60s on an early May afternoon.
Showers will end overnight tonight, and models punch some drier air into our area. This is a typical spring setup for a chilly night. The areas that clear out away from the big cities will see temperatures plummet. So who clears out and might see frost? Here’s a still image from our RPM model showing forecast cloud cover at 6am:
With air flowing west to east, clouds tend to bank up against the Cascades, so eastern suburbs and outlying areas will be relatively warm compared to the west side. Out there I expect mainly clear skies tonight, allowing maximum radiational cooling (objects radiate heat out to space). Whew! I live way out eastside to I think all my blooming fruit trees (apple, pears, plums & cherries) are probably safe. But if you live in Vernonia, Hillsboro, Gaston, Forest Grove, Scappoose, or McMinnville, you might get a light frost tonight. That does fit the average last frost dates for our area…earliest in the city, latest out to the west and northwest of the metro area.
It’s not on this map, but Downtown Portland’s average last frost is February 24th! Portland Airport and most of the lowest elevations within 10 miles of downtown Portland (south, east, & north) is the last week of March or first week of April.
Saturday will be a cool day with increasing sunshine, but then we have another very cool night tomorrow night. No clouds or fog around to hold temps up tomorrow night, so some of us (my trees!) might see a light frost in the outlying spots north and east of Portland…those spots that escape it tonight. But…
I don’t expect frost either night in the main part of the Metro area either of the next two nights. It’s only possible in outlying areas.
Starting Sunday it’s on to Summery weather for a few days. Light offshore (easterly) flow Sunday continues with a warmer atmosphere Monday. Check out the WRF-GFS cross-section:
Easterly flow from Sunday morning to Monday evening from the surface up to around 4,000′. By the way, time goes from right to left. 07/12 is Monday morning at 5am, 04/12 was this morning at 5am. That, along with solid sunshine, will give us maximum heating…lower 70s Sunday and right around 80 Monday. We didn’t get too crazy with temps Monday since it appears 850mb temps top out around +11 to +12.
After a strong onshore push later Tuesday and Wednesday, weak ridging seems to come back later next week. Hmmm, looks like the marine air see-saw is starting up for the season. Both the ECMWF and GFS imply we are heading into a period of very mild May weather with temps more above average than below. If I lived in the lowest elevations here in the western valleys, I’d consider planting the warm weather veggies (corn, beans, tomatoes) Sunday or beyond. It’s worth a shot, the 2nd week of May isn’t too early IF we stay in a generally mild pattern the rest of the month. If you live in a cooler spot like I do…wait for another couple of weeks to make sure.
Here are the two 12z ensemble charts. The GFS operational would say well into the 80s next weekend, ECMWF not as warm but still mild:
Interesting to note the GFS is warmer than its ensemble members for the Mother’s Day weekend, but the ECMWF is cooler than it’s ensemble:
Chief Meteorologist Mark Nelsen