6:00 PM Update: Cold Air Arriving

November 22, 2010

The good news?  Cold air is definitely moving in from the northwest.  The bad news?  Radar is definitely drying out.

Take a look at some of the “upstream” obs.  Snow at Astoria with a strong west wind…down to 34 degrees now.  Tidewater RAWS, just inland from Astoria at 2,000′?  Dropped from 31 to 27 the last couple of hours, and I assume it’s even colder now at 6pm.  That air advects right over us in the next two hours…so a 30 degree temp at the top of the West Hills is likely by 8pm.  Note the dramatic drop in dewpoints too.  All a good indication that the much advertised sudden cooling in the upper-atmosphere is now coming south out of Washington.  And it’s getting even colder up there.  I see Hurricane Ridge at 5,000′ is down to +1 degree!  That very cold upper-level air is dropping south.

At the surface, all of Puget Sound has gusty northerly wind now at temps well down into the 20s.  Arctic front passed Olympia and appears to be somewhere between there and Chehalis.

So will moisture stick around?  I still would be surprised if we don’t get a dusting this evening at some point.  But that one inch might be a stretch!  We’ll see.

Still expect freezing temps by the time we get to 10pm or so.  Watch out on roads that are still wet.

Chief Meteorologist Mark Nelsen


1:30pm Update: Wet Through 6pm

November 22, 2010

Here’s the latest:

1.  No freezing/snow issues in the lowlands from Portland south through at least 6pm.  After that time things could change quickly, but more likely 8-10pm.

2. Will it snow in the lowest elevations around the Metro Area?  Still a definite possiblity.  I would be VERY surprised if there is not at least a dusting most areas by 10pm.  I would also be surprised to see more than one inch.

Quite a bit more precipitation than expected today, that’s due to a stronger-than-expected area of low pressure developing right around Forks, WA this afternoon.  Pressures down around 1000 millibars instead of the 1005 models were showing.  Check out Tatoosh Island…northeast wind gust to 45 mph, while just 30 miles south a buoy is gusting from the west over 40 mph!  That’s some great meteorological stuff.

That low should race SE and be near Pendleton by late this evening…opening the door for something very unusual here:  a big surge of arctic air from the north/northwest WHILE a west wind is increasing through the Gorge.  Almost every arrival of arctic air here in the Metro area is accompanied by that gusty east wind blowing out of the Gorge.  That’s not happening this time around.

So the obvious question is…where is the cold air going to come from?  Regardless of the depth and position of the low…ALL models show the big cold surge from the north this evening.  Look at these numbers…2,000′ temp over PDX @ 4pm = 32 degrees (accounts for snow level a little over 1,000′ this afternoon).  But by 10pm it’s down to 20 degrees @ the same elevation!  So where is the cold air coming from again?  Check out temps up around the Olympic Peninsula.  The RAWS sites are that cold (or colder) at that elevation, they’ve been cooling all day.  So once the low passes to the east, the cold air should whip around the back side of it.  This is an incredibly cold airmass moving south in the upper-atmosphere, and a good chunk of that is moving in at the surface too.  Should be fun to watch….let’s just hope there is still moisture around to generate SOME snow.

A side note:  Don’t want to sound cranky, but I don’t have any tolerance for bad behavior here today.  Especially with 50,000+ hits to this blog.  I haven’t read comments yet, but I know some will get very upset and lash out when their big snowstorm doesn’t materialize, but you know what?  That’s what forecasting weather is about.  Sometimes it doesn’t turn out like you expect.  Move on…

Chief Meteorologist Mark Nelsen


10:00 am Update: No Snow During the Day

November 22, 2010

Apparently the NAM is correct so far…low pressure is developing off the Washington Coast, pulling back the dreaded south and southwest wind up the Valley and over the Coast Range.  The arctic boundary (front) appears to be in a line from around Hoquiam to Chehalis to Mt. Adams west of the Cascades.  Areas to the south of that are getting a mild south wind (including metro area), to the north it’s cold and snowy.  Until the low pressure passes by late this afternoon, we stay in the “warm” airmass.

All models show the arctic air surging south behind the low this evening, supported by the incredibly cold upper-level trough dropping south.  THAT is our last chance for snow…with the arctic front.  We’re talking after dark.  Now obviously models have trouble with the small-scale details…for example once again there is more precipitation than expected today (just too warm!).  So we should get at least a dusting as the precipitation ends, maybe an inch or so if we get REALLY lucky.  Unfortunately the UW-WRFGFS decided to fail it’s morning run today (too cold?), but our RPM shows a brief burst of snow right along the arctic boundary; supposedly around 7pm.  We’ll see.  I will say that is the one feature our model has been very consistent on, regardless of where the low presssure system was expected to be today.

Disappointing for the snow lovers (I mean people that love snow, not lovers hanging out in the snow of course).  But it IS weather and half the excitement is the anticipation.  Plus, we still have the coldest November airmass in a generation descending on us tonight!

So here’s my latest forecast:

Portland Metro:  Rain/snow showers today…streets remain wet until at least sunset.  Then a Trace to 1″ of snow sometime after 5pm.  Clearing and freezing up by 10pm.

Salem:  Mainly rain showers today, then a mix or all snow showers briefly sometime between 7-10pm.  Could get a dusting then…freezing up after midnight.

 


9pm Update: One More Snow Chance

November 21, 2010

Well tomorrow is the “big day” for us weather weenies.  I just finished perusing the 00z model runs…here are my thoughts:

1.  South wind kept us from getting a 1-2″ surprise in the Metro area today.  Models handled our little snow burst well, except the timing was off…it showed up 2-3 hours early.  That light onshore flow was expected.  Now it’s disappearing as a 2nd low is spinning down the coastline.  It’s near Astoria right now; models show it spinning overhead and into eastern Oregon by morning.

2. Behind that low, a colder atmosphere is pulled down from the north overnight.  That plus some clearing should make for an icy, but dry start Monday.

3. Between daybreak and around 4pm tomorrow, a stationary front (the leading edge of the arctic air) sits between Astoria-Portland-Condon.  A 3rd surface low develops along the front and moves right overhead midday or afternoon.  I’m depending on that to give us a possible 1-2″ of snowfall…a bit more moisture than models show (but that happened today too).   We start cool again in the morning tomorrow, plus our model (RPM) and WRF-GFS show the boundary right over us, instead of up north of Kelso.  That should mean no south wind and we shouldn’t have a problem getting snow to stick just about anywhere from Portland north.

4.  The arctic front then suddenly makes it’s move behind the low after 4pm, sagging south to Eugene by 10pm tomorrow night.  Most likely at least a trace of snow would accompany that front even down into Salem and Eugene.  As a result, we should freeze up quickly after dark tomorrow.

5.  If you have plans to be out and about after the noon hour tomorrow, make sure you have a plan to get un-stuck if we suddenly get a surprise 2″ dump of snow and traffic grinds to a halt!  That seems unlikely, but definitely possible.

Now this is the scenario that I’m basing my forecast on.  The NAM moves the low by a bit farther to the north, which puts us into a similar situation that we saw today (minus the eastern Clark County cold spot).  If so, we’d only get a trace or so as the arctic front finally slides south.

So here’s my forecast:

1-2″ snow Portland north to Longview midday-evening tomorrow.

Trace from Woodburn south into the Valley.

I lowered Tuesday’s high temp forecast based on at least 1″ of snow cover in the metro area.  If we don’t get snow on the ground, I’d guess 33-35 degree highs.

We can talk about that sudden appearance of moisture for Wednesday AFTER we get through tomorrow.  Actually, I suppose you can talk about whatever you want on here, but I’m busy focusing on tonight and tomorrow.

It’s still hard to believe it’s such a cold atmosphere plunging into our area tomorrow night.  -13 to -15 degrees C. at 850mb?  Imagine if it was January…then it would be quite a bit colder at the surface.  I’d probably go for highs in the low-mid 20s.

Chief Meteorologist Mark Nelsen


2 pm Snow Update: Snow Level Forecasts

November 21, 2010

This afternoon is a prime example of why you shouldn’t forecast snow levels in 200′, 300′, or 500′ increments.

At 2pm, we have a rain/snow mix and nothing on the ground here in Beaverton at about the 220′ elevation.  Yet in NE Vancouver I can see snow on everything except the roadway…AT THE SAME ELEVATION.

Precipitation intensity, leftover cold pockets from overnight cooling, and influence of milder southerly wind all plays a larger part of the snow/rain story than just a few hundred feet of elevation.

That’s why you’ll never see a forecast for snow to 300′ (or anything similar) on my forecasts.

It sure looks as if at least everyone has SEEN some snow this afternoon, if you didn’t get a dusting at your home, you have probably seen it in the air.  The best spot seems to be in central/eastern Clark county, and areas up around 1,000′ elsewhere.  A mild southerly wind is keeping temps in the mid-upper 30’s south of the Columbia River, but a leftover pocket of cooler temps (and no south wind) around/east of I-205 up in Clark County is giving you folks your first snow of the season.

The back edge of the heavy precipitation line is crossing the West Hills right now, so the low elevation snowfall will stop in the next hour, but then more showers between now at 8pm may give us an even better chance for sticking snow as sunset approaches!

Chief Meteorologist Mark Nelsen


Weather Page Fixed

November 21, 2010

That was REALLY annoying.  My main weather page with all the links I use (and I know a lot of you folks use) had been down since 7pm last night.  I felt meteorologically naked.  BUT, it turns out my provider (www.cascadeaccess.com) was somehow making any ftp’d files into zero bytes.  I use a program that inserts current conditions every 15 minutes, then sends the file from my computer to the server.  Don’t know what was wrong, but it’s fixed now.  Page is working.  In the future if it happens try markswxlinks2.html and that is a “static” version that doesn’t change.  Well, at least it will be there about 10 minutes from now.

Looks as if precip is going to move in a bit faster this afternoon.  Marginally cold enough for snow to stick in the lowlands right now, and with the new surface low (sliding along the arctic boundary) slightly to our north, we seem to get a light southerly breeze this afternoon…kiss of death.  But maybe not.  Let’s just hope for briefly heavy precip. intensity and a quick dusting for most of us.  Either way it’ll freeze up overnight, so watch out for ice once that happens.

Still looks good for tomorrow with moisture picking up midday and a colder atmosphere.  I’ll be surprised if everyone doesn’t at least get a dusting out of this from Salem north.

Mark


Snow Outlook

November 20, 2010

I happened to be scheduled to work this weekend…how convenient in this case.

So is it going to snow in the city of Portland?  I would be surprised if the ground doesn’t at least get coated white either tomorrow evening or Monday afternoon/evening.  Will we get 4″ or more?  That seems very unlikely, but 2″ sure is within the realm of possibilites.

It’s getting cold out there tonight with clearing skies and drier dewpoints working down from the north and east.  Tonight should be the coldest so far this season (finally a frost at PDX!) for most of us.

I’m amazed at how much information we have nowadays and how detailed it can be.  When I started in this business around 1991, a cold upper-level trough and associated arctic airmass would plunge south, you could generally just say we’ll see somewhere between a trace and 2″ of snow within 12 hours of the frontal passage, and then it would get cold.  But look at today’s (and tonight’s) mesoscale modeling.  Our RPM, the WRF-GFS, and to a lesser extent the NAM-MM5 from the UW, all show two separate precipitation espisodes the next 48 hours.

The first is a burst of showers around sunset tomorrow as a weak surface low slides overhead and east towards Pendleton.  Larger and coarser modeling wouldn’t even be able to see this in the past.  The 2nd is a slightly stronger low or elongated low that slides by Monday afternoon. 

I feel a bit better about snow compared to last night at this time for these reasons:

1. Tomorrow’s burst of showers around sunset is just that…around sunset and beyond…good timing with a marginally cold-enough atmosphere for lowland snow.  Dark = cooler = good.

2. Wind is calm or light northwest both tomorrow and Monday afternoon…that’s always good to allow snow to fall a bit lower.  No south wind to mess things up.

3. Models (the last two mesoscale runs) have been steadily moving the Monday afternoon precipitation a bit farther north.

So here’s my forecast I’ll use at 10pm:

Portland: 

-Trace to 1″ possible tomorrow evening around sunset and just beyond.  Best bet for that 1″ would be on the hills and eastside (Gresham, Camas, Sandy, Battleground, etc…).  If the precipitation is heavy enough, it could be everywhere.  If we only get light showers; forget about the lowest elevations.

-1-2″ possible Monday afternoon/evening, even better chance south into Salem and Eugene. This is the one that could really make a mess.  If indeed snow DOES start to fall at mid-afternoon, roads are wet/slushy and then begin freezing up???  Well…I just keep thinking of December 29th, 2009.

Forget about the 300′, 500′, 750′ elevation distinctions…especially with showery precipitation!  That drives me nuts.  One place could get sticking snow to sea level (under a heavier shower), and another spot at 1,000′ might just get a mix under a light shower for no accumulation.

Chief Meteorologist Mark Nelsen