Things seem to be coming together for some very active late summer/early fall weather from tonight through Friday afternoon.
Several “weather events” to talk about:
1. Another round of nocturnal thunderstorms is possible in the Willamette Valley, sometime after 10pm tonight.
2. A wet day westside tomorrow, a few scattered thunderstorms possible at times too.
3. Powerful severe (and supercell) thunderstorms with damaging hail, wind, and flooding rain are possible tomorrow afternoon across north-central and northeast Oregon (E. Washington too).
4. Steadier and heavy rain will show up in Western Washington and possible down into NW Oregon tomorrow night and Friday morning.
5. First dusting of snow above 10,000′ will be on the high volcanic peaks once skies clear Saturday.
What’s going on?
Another upper-level low is spinning away offshore. But it’ll make its move tonight and tomorrow; should be in the Central Oregon Cascades by sunset Thursday, ride north along Highway 97, and end up in north central Washington by Saturday morning. The areas to the north and east of the low center have the best lifting, thus the severe weather possible tomorrow afternoon east of the Cascades. But as it starts to make its move tonight, showers and thunderstorms will break out west of and over the Cascades during the overnight hours. Just like two nights ago, big issues with models resolving exactly where the main action will setup. Here’s our 18z RPM model for the middle of the night tonight showing thunderstorms in the Willamette Valley and north towards Puget Sound:
As the low heads north and east tomorrow, the juicy air mass, lifting, and warm afternoon should combine to create another severe weather event like we saw about 9 days ago. The Storm Prediction Center has this area in the SLIGHT risk for severe storms category during the afternoon:
Here is the text from their outlook, noting the likelihood of long-lived supercells:
INCREASING FLOW ALOFT WILL CREATE FAVORABLE SHEAR FOR SUPERCELLS...THOUGH INITIAL ACTIVITY MAY BE HIGH BASED ALONG THE NOSE OF DRY SURGE. BY 06/00Z AN ARCING BAND OF SCT SUPERCELLS MAY EVOLVE FROM SERN WA/NERN ORE/WRN ID WHICH SHOULD MOVE NNEWD AT 25-30KT. LARGE HAIL IS THE MOST LIKELY SEVERE THREAT...SOME OF IT COULD EXCEED GOLF BALL SIZE...AND DAMAGING WINDS ARE ALSO POSSIBLE WITH THIS ACTIVITY. GIVEN THE STRENGTH OF THE MID LEVEL SPEED MAX SUPERCELLS ARE EXPECTED TO SPREAD ACROSS NRN ID INTO NWRN MT DURING THE LATE EVENING HOURS.
Looks good for storm chasing (with a lot of driving) eastside tomorrow afternoon…also, a Flash Flood Watch is up for some areas eastside due to slower moving storms possibly heading over recently burned areas:
“Regular” showers and possibly a few thunderstorms continue west of the Cascades tomorrow through the day, so just a mostly cloudy, cool, and wet September day for the metro area. Then as the low heads north tomorrow night, models are all creating a “deformation zone” on the northwest side of the low. This is the area where moisture wraps around the low and turns into steady (and possibly heavy) rainfall. With such a juicy air mass, the Seattle NWS has issued a flood watch for most of Western Washington since most models have the main rain up there. Any southward shift of the deformation zone would bring steady and heavy rain to our area too.
Our RPM is far too heavy with rainfall totals, especially with the overnight storms…here’s the 3 day forecast total:
The WRF-GFS is a bit more reasonable keeping totals in the metro area around an inch or so:
They both show the heavier rain up in Western Washington tomorrow night and Friday.
So hold on, stay dry, and hope for a good soaking for our lawns and gardens the next 2 days!
Warm and summery weather definitely returns next week.
Chief Meteorologist Mark Nelsen