Snow Wrap-Up Christmas Evening

December 25, 2007

Whew!  I’m real happy with the forecast I put in the previous post this morning.  Models absolutely nailed the sudden drop in the snow level to very close to the surface, just as a heavy line of snow gave just about all of us in the metro area a (brief) dusting of snow that has since melted with the south wind that kicked in.  You can see below that I gave a 50/50 chance of 1/2" in the lowest part of the metro area.  Looks like some of you on the hills got that? 

At my parents home in west Gresham there was a dusting on the ground 1-2pm but it was pretty much gone when we left at 3:30pm.  Up at 1,000′ here at home I had maybe an inch or so…but it’s melting with a breezy south wind.

So what about the weather service office?  Was any snow officially recorded?   I see the PDX and TTD obs. never went to solid snowfall, but of course those are automated observations too.  Hopefully the NWS will put out a statement soon.

At 5:30pm I’d estimate the snow level to be around 1,500′ or so, it’s pretty obvious we are getting a milder onshore flow.  Of course with clearing inbetween showers temps at the surface can quickly drop to freezing, so watch out for spots of ice on roadways, mainly on the west side of town overnight.

The rest of my thoughts haven’t changed…see the previous post…Mark Nelsen

Christmas Morning Snow Update

December 25, 2007

No picture today…it’s Christmas Morning and I’m not working, so you only get the bare minimum.  Here are my latest thoughts.

CHRISTMAS DAY:  White Christmas above 1,000′!  A dusting possible anywhere early afternoon.  We’ve seen exactly the same thing several times this season, but this time it’s slightly colder.  Cold overnight airmass, then a front moves in early afternoon with precipitation.  I notice KPTV Tower temps are all at/below freezing, plus the air up above us is quite dry, which should give nice evaporational cooling up there.  Not really any Gorge effect today, so what we have right now is all we get cold-wise.  The MM5-GFS cross section shows the lowest freezing level in the next 72 hours between 1-4pm this afternoon as the precip arrives.  I would say the Gorge (east of Multnomah Falls or above 500′ west end), and areas above 1,000′ west of the Cascades will see a white Christmas today with 1-4".  But I’d put the chance of sticking snow (more than 1/2") in Portland metro area this afternoon at about 50%.  Notice this is higher percentage than what I thought two days ago.  It’s going to be close!  If we get sudden heavy precip with this front immediately as it starts falling, everyone could get a white Christmas!  This evening strong onshore flow develops behind the cold front, so the snow level lifts to around 1,500′ or higher, before dropping back to 1,000′ late tonight as the colder onshore flow works inland.

WEDNESDAY:  Possible below 1,000′ in heavy showers.  It’s ALL dependent on moisture availability.  More accumulation is likely in heavy showers at/above 1,000′.  With those heavy showers, we could get brief accumulation even down in the city.  Keep in mind though that temps in the afternoon between the showers will be up around 40 in the lowest elevations. I say maybe a 20-30% chance of sticking snow in the city

THURSDAY:  Very unlikely below 1,000′.  We do it all again with another system moving in over cooler air.  This one is slightly warmer than Christmas Day, so snow stays at/above 1,000′. 

FRIDAY-SATURDAY:  Warmer, snow level 1,500′ or higher both days…very wet.

So to sum things up…I still don’t see a GOOD chance for snow in the lowest elevations (I-5 Corridor below 1,000′) except briefly this afternoon.

Merry Christmas!  Mark Nelsen