Orographic Flow

November 11, 2008

Snapshot This evening's rain totals, up to 8pm, clearly show strong orographic flow in progress over us.  That means the strong westerly flow is running right into the mountains head on.  Over the next 24 hours wind around 4,000-5,000' (850mb) averages 60 mph.  Add a very moist and warm airmass to that and you get a ton of rain over elevated terrain.  The graphic here shows about 5 times as much rain at the crest of the Cascades compared to the driest part of the Valleys.  The areas just downwind of the Coast Range tend to be the driest, generally the west side of the Portland Metro Area.

Quite a break in the rain at midday today…actually the models did better than I did.  I saw the big rain-shadowed hole over us yesterday evening (for today), but just said the rain would be a bit lighter.  Models still show the same thing coming up for later tonight through midday tomorrow as the main jet and cold front are aimed at Western Washington.  So quite a break in the rain again the 1st half of tomorrow, then the heavier cold front rain moves down over us from Noon-6pm or so.  I still don't see much more than another 1" at most here in the valleys.  But maybe another 4" in the Cascades and Coast Range.

I notice the wind is picking up on the roof right now…due to the warm front moving in and tightening the southerly pressure gradients again.  The North Bend to Forks gradient made it up to around 13 millibars early today, dropped to less than 8 this afternoon, but now is back up close to 14 mb.  Wind seems to be responding…I see gusts to 47 the last hour at Yaquina Bay Bridge in Newport.  Gradient seems tightest later tonight through tomorrow morning.  The WRF-GFS from the UW has consistently shown a leeside-type low developing near Yakima overnight too.  It shows 15mb from Eugene to Yakima by daybreak.  This may bring strong west wind through the Gorge later tonight, but there's no sign of that for now.

Not much to talk about beyond tomorrow evening.  Some sort of longwave ridging sits over us for many days, through some time next week or even longer.  Looks like November will end up milder than average…unless we have a really cold 2nd half of the month that doesn't show up on the maps yet…Mark Nelsen