La Nina Digs In…

September 21, 2010

I briefly talked about La Nina last night on the blog (not so briefly on-air), and tonight’s 10pm weathercast details my thoughts on Cascade snow, and snow/cold here in the lowlands.  Let’s wrap it up…here’s what I see for this upcoming winter:

Moderate-Strong La Nina is here

Most likely it’ll be the lowest ONI (Oceanic Nino Index) since the strong La Nina of 1988-1989.  So I based all my graphics/research on a moderate-strong event.  All models keep it going through at least spring…great, another chilly spring???  But let’s not get too far ahead…

Likely Effects This Winter:

1.  Rainfall

Very unlikely that we’ll have a drought winter; far more likely precipitation will be above average.  La Nina winters in the Pacific Northwest are dominated by a strong jet bringing frequent disturbances across the region, interspersed with sudden ridging or northerly flow.   Then it’s back to the westerly flow.  For this reason they tend to be wet.

1a.  Flooding

This actually goes with the rainfall.  For obvious reasons we tend to have more flooding events in winter due to the wetter weather.

2.  Mountain Snow

Lots of precipitation and cool weather systems = tons of mountain snow.  This is probably the #2 most likely event.  11 of 12 moderate-strong La Nina events since 1954 have produced above normal snow at Government Camp.  Seems like a pretty good snow signal doesn’t it?  The last La Nina winter (2007-2008) was a massive snow year in the Cascades…definitely at the extreme upper end.  Get a season pass!!!  No, I haven’t accepted a free ski pass in years, so you can be assured I’m not biased.  I told folks last year that it would probably be a bad ski season with El Nino and that’s pretty much what happened.

3.  Foothill Snow

This happened in several of the past La Nina winters…significant snow to lower elevations (1,000′-2000′).  This MAY happen again if we get a succession of cold and wet systems coming in from the west and northwest.  Of course these are the same systems that give forecasters headaches because then snow it quite close to the Valley floor multiple times during the season.  News people get really excited about it too. 

4.  Wind Storm

We are overdue for a major windstorm here in the Portland Metro area.  The last BIG one was December 1995.  That’s fifteen years back!  14 years before that we had the major November 1981 storm.  Then if you take a look at previous moderate-strong La Ninas?  Whew!  A large majority had at least one big wind event.  It’s interesting that all those La Ninas from 1950 to the mid 70s had a wind gust of 60+ mph at PDX each time!  Not as frequent since that time though.  Of course during the last La Nina in 2007 the north Oregon Coast saw it’s strongest storm since the Columbus Day Storm…the Great Coastal Gale of 2007  But we missed out in the interior.  Of course I use the term “missed out” loosely.  By the way, when I say “major windstorm” here in the Portland Metro Area, I’d call it that when I see widespread 60-70+ mph gusts at the major airport weather sensors.

5.  Extreme Cold Snap

This is definitely not a guarantee, but we have a higher likelihood of a severe cold spell (arctic blast, like last December) during a La Nina winter.  That’s because we occasionally have the flow come down from the north.  BUT, two of the last three La Nina winters have NOT seen a blast (07-08, 99-00). 

6.  Portland Snow/Ice

This one is tough.  Anyone who says a La Nina winter means lots of snow in Portland is mistaken.  Average snowfall in moderate-strong La Ninas DOES go up a bit, but not a dramatic increase.  What I find interesting is that no La Nina in the last 30 years has produced a major snowfall here in Portland.  Of course you couldn’t say the same thing about 1989 down the Valley and up into Washington, but I’m just talking about right here.  As for ice (freezing rain), records for that are harder to come by, but it seems that we have a better likelihood for freezing rain in some La Nina winters.  This is totally anecdotal evidence from my experience.  I should point out that the “cool/wet” La Nina winters (such as the last one) often produce little freezing rain because we don’t get as many inversion episodes to our east, which means less east wind in the Gorge.  We need that for a good ice storm either in the Gorge OR in Portland.

7. Gorge Wind

For the hometown folks in the Gorge here…most La Nina winters tend to have less east wind through the Gorge during the winter season.  That’s because the strong easterly flow is mainly caused by stagnant high pressure sitting east of the Cascades during slow weather periods (upper-level highs overhead or split flow patterns).  During winters (like 07-08) we don’t get long periods of inversions due to frequent passage of cold fronts and low pressure centers.  I remember the winter of 98-99 (or maybe it was 99-00) was real quiet out in the Gorge too.  That said, when we DO get a big blast of cold air, we can get very strong easterly flow during the cold spell if a warm system approaches from the southwest.


I didn’t mention two other possible effects…PDO and Solar Minimum.  We ARE in the cold phase of the Pacific Decadal Oscillation, which some theorize that combined with a La Nina means a better chance for snow and cold.  That did not occur in the last 3 La Nina episodes (late 90s and 07-08).  So I’m not totally sold on that.  Very interesting and something to watch though for sure.  Then we have the unknown of the solar minimum.  This is well out of my area, but the sun has been VERY quiet and some theorize that we are entering a “grand minimum” or something similar to the Dalton/Maunder Minimum.  Both of these were associated with much cooler than normal global weather.  Also very interesting, but I didn’t factor that into the winter outlook either.

Now, let’s see how the winter turns out…my money is on “unusually wet” and “windstorm” and “Cascade snow”.

Chief Meteorologist Mark Nelsen

“Normal” September Ahead

September 20, 2010

What a soaker this weekend…record rain Saturday, then a few thunderstorms scattered around Sunday (although drier than I expected in general), just a bit gloomy today.  Things have returned to normal now with the moist subtropical airmass no longer in the Pacific Northwest.  As a result we’ve got cooler nights ahead and some sunshine the next couple of days.

The big picture over the next 7+ days is definitely not set in stone, but it appears that we PROBABLY have much more pleasant September weather in store for the last week of the month.  The main problem is: where will a warm ridge of high pressure in the upper atmosphere set up shop?  Will it be along the West Coast, or farther east, in the Rockies, next weekend and beyond?  I always like to watch for trends in the models;  the trend today and this evening is for the ridging to hang on at least through the first part of the weekend.  The 12z ECMWF and 00z GFS have very little rainfall Sunday, with ridging kicking in again Monday.  I don’t trust that for now, so we’ve left the rain in the 7 Day forecast Sunday and Monday.  It COULD be a very warm weekend (best in weeks!) if the cold front and rain holds off.

Late last week and today I pulled out the La Nina graphics, cleaned them up a bit from 2007.  and put them in the 10pm show.  Tonight I talked about the Fall/Winter rain possibilities.  Interesting to see how the winter of 2007-2008 lined up with all the graphics I made the Fall before.  Things turned out about as expected except no big windstorm in the Valley and no big arctic freeze.  Plenty of mountain snow, valley rain, and coastal windstorm otherwise that year.  Plus the hills around town really got pounded.  I had 39″ of snow at 1,000′, yet PDX only had a trace.

So what about rain the rest of the Fall?  Most likely above average; same for winter.  Very few La Nina Fall/Winters end up below average.   Temperature is a tossup.  As we just saw, lots of cloudy/rainy weather can be mild.

More La Nina tomorrow…

Chief Meteorologist Mark Nelsen

Very Heavy Rain Moving In

September 18, 2010

Check out the radar at 8:50pm:

Line of heavy rain moving north up the Valley…looks like all areas east of Hillsboro are going to get soaked soon.  This line was less intense when it passed through Salem and dumped about .50″!  No lightning yet, but I’ll keep the live radar on it here:


A Wet Night Ahead!

September 18, 2010

Rain arrived a few hours earlier than I expected, enough to ruin my bike ride.  That was one of the sweatier rides I’ve done…couldn’t hardly tell the difference between that and the rain pelting my face.  But at least a nice picture (on our FOX12Weather Facebook page).  Here’s a nice image showing the north-south oriented moisture plume over us right now: 

Plenty of rain to go as NESDIS Satellite Precipitation Estimates folks say in their midday discussion.

A warm, wet, and humid night ahead…slightly unstable too, so maybe a flash of lightning???  Most likely not but it’s possible.

Chief Meteorologist Mark Nelsen

Most Humid of the Summer

September 17, 2010

This 2-3 day stretch of warm and humid weather is unprecedented this summer.  Yes, I know, most of us would agree that summer ended a few weeks ago weatherwise…we’ve only had two days above 80 this whole month, with none above 75 in sight.  Here’s a nice graph (click on it for a good view) showing dewpoint at PDX for the last 3 months.  The last few hours are not plotted on it, but you can see only one other time we’ve reached a dewpoint above 63 degrees.  And that was only for a few hours.  We’ve had a dewpoint near or above 60 degrees for just about all of the last 48 hours.  And this evening it’s been up to 65 degrees at times.

Dewpoint is the direct measure of the amount of moisture in the air; technically it’s  “the temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur”.   Warmer air can hold more moisture than cooler air before it saturates.  Ever been to Florida, the USA Southeast, New Orleans, or Central America?  How about a Turkish Prison?  (that last one was a joke, no need to ask about gladiator movies either).  All those places are pretty disgustingly humid during the warm season.  And when it rains it REALLY rains!

The main band of rain has passed on to the east this evening, with almost constant lightning from storms in north-central Oregon lighting up the radar screen.    One particularly strong cell around 9:30pm has produced 150-200 lightning strikes in a 10×10 mile square in 15 minutes!  Right around Antelope/Shaniko.  It’s going to be a wild few hours across the wheat fields of north-central Oregon.

Humidity stays high tomorrow as another wave of rain moves in during the afternoon/evening.  Flow direction is from the south-southwest again.  Then drier air (more typical cooler showers) for Sunday means an end to the very high humidity.

A brief break the middle of next week and then we appear to get our first strong westerly jet of the season…very wet beyond Thursday!

Chief Meteorologist Mark Nelsen



Thunderstorms Popping Up

September 17, 2010

Some thunderstorms right around Multnomah Falls, east of Sandy and headed towards the Central Columbia River Gorge at this hour.  There are more storms down around Detroit Lake, also headed north.  Another warm and humid day out there with temps right around 70 even with the thick cloud cover.  A good soaking coming up the next few hours.  Thunderstorm chance not real great down here in the lowlands with no sunbreaks, but a moist southerly flow can always surprise us (in a nice way!).

12z WRF-GFS showed some very heavy rain this evening; we’ll see if that comes to fruition.  Our RPM model showed more of the scattered showers that we are seeing now.

Chief Meteorologist Mark Nelsen

Warm & Humid Weather Ahead

September 15, 2010

Now that’s a sunrise Tyler Mode!  He sent us this picture.  It’s the view from Women’s Forum State Park near Corbett down to Crown Point and all the way up the Gorge to Cascade Locks.  I live about 2 miles SE of that location, but I was asleep until 8am so apparently I missed out…

Moving on, looks like some interesting weather the next 48 hours.  First, the rain is definitely moving in too fast this afternoon.  As I write, heavy rain is pummeling the Willamette Valley from the south Metro area down to Corvallis.  No lightning strikes with any of this or the heavy showers along the Coast.    This is associated with a deep upper-level low pressure area out in the Eastern Pacific.  We are on the far eastern edge of it’s circulation; the warm southerly/southwesterly flow side.  We’ll be on the warm side all the way through Friday afternoon.  That means lots of moisture available for rain through the overnight hours tonight for starters.  I wouldn’t be surprised to see .50″ totals even inland by morning.

As I mentioned on last night’s broadcast, timing will be tricky for the following waves of rain, but it appears one just nicks the North Oregon Coast tomorrow afternoon/evening, largely leaving the rest of Western Oregon dry.  So we dry out tomorrow and get some sunbreaks too, even while 850mb temps rise up to around +12 to +13.  This adds up to a warm day.  Little to no wind and dewpoints around 60 mean it’ll feel quite humid too.  In general it’s going to be unusually humid (warm nights too) tonight through Friday.

Then Friday looks like it could be a fun day.  A cold front moves inland at some point in the late afternoon/evening.  But BEFORE that time, we get light easterly flow from the surface to around 3,000′, warm southerly flow above that with 850mb temps around +13, plus at least partly cloudy skies through early afternoon.  That says “warm” to me; similar to one of those spring days just ahead of a cold front where we see temps rise more than expected.  We could hit 80 Friday!  We should at least hit 75…it’s all dependent on cloud cover.  So we have a warm and humid atmosphere…precipitable water up around 1.30″ or so, lifted index dropping below zero, and some decent CAPE west of the Cascades.  Then a cold front sweeps in from the west or southwest during peak heating time; oh, and a surge of cooler marine air from the southwest too.  Can you say “afternoon thunderstorms” kids?  Sure seems like the ingredients are lining up well.  We don’t often get a nice combination like that.

The upper low offshore opens up and moves onto the West Coast over the weekend; more showers at times, but cooler temps.

Chief Meteorologist Mark Nelsen