Wetter Days Ahead, A Little Gorge Snow, & Lots of Mountain Powder

9pm Tuesday…

Christmas vacation so far has given us a mix of weather; from warm/mild rains to cool easterly wind. Some fog and freezing fog in spots too. In the central/eastern Gorge, you experienced the much desired “White Christmas”. 1-6″ fell from Cascade Locks to The Dalles and down into north-central Oregon. Now we’ve seen a few dry days. Portland is running near to a little below normal for rain this month. A good chunk of the region has been drier than average over the last two months; somewhat unexpected in a La Niña winter.

Most of the USA has been warmer than average this month; including the Pacific Northwest. This is our 3rd consecutive warm December.

We are approaching the “halfway point” of the Pacific Northwest storm season. That’s November-February. It’s been a disappointment so far if you’re looking for lots of active weather from this “La Niña” winter. No lowland snow, no significant windstorms, flooding, tornadoes etc… We are in the coldest time of the year (by the averages) right now. It’s always interesting to note the western USA is coolest when you would think it would be; lowest sun angle and shortest days. That’s right now. Yet the coldest of winter is more like February 1st once you get into the upper Midwest and Great Lakes regions. That’s due to the continuing cold Canadian airmasses moving south and thick snow cover reflecting much of that increasing solar insolation.

It appears weather action will be picking up again over the next week; these 12 Days of Christmas will be turning quite wet! A mid-winter Pacific jet stream will be ushering a series of wet weather systems into the PACNW starting tomorrow. We still have a little cool air stuck in the Gorge. The layer of cold air isn’t very thick and easterly gradient through the Gorge is weak. This may be just enough to bring very light snow (less than Christmas) tonight through midday tomorrow. A real “marginal” event though.

I see at least 6 separate cold/occluded fronts coming through the region tomorrow through Wednesday the 6th. You can see 4-5 of them moving quickly west to east in this loop of sea level pressure and precipitation type. That’s now through next Monday.

In general we seem to be headed toward slightly colder systems after Sunday as upper-level heights lower next week. You see the cool/lower height anomaly max out the middle of next week, before the ECMWF ensemble average pushes heights up again about two weeks from now.

At this point I don’t see any especially strong storms, although models have been waffling around each run with exact low placement and precipitation intensity. It’s quite the message here…no significant gaps in the 24 hour precipitation from any of the ECMWF ensembles

I see quite an increase in mountain snowpack the first week of January. The ECMWF model thinks 20-40″ is likely on Mt. Hood over the next week. Since snowpack is running a bit below average this is good news!

With almost constant onshore flow it’ll be tough to get snow down to sea level over the next 10+ days. Notice almost no members of the ECMWF ensembles produce significant snow over Portland the next two weeks.

That’s not to say we couldn’t have something happen like what Puget Sound saw last week. A low pressure area and dynamic cold front brought heavy post-frontal precipitation overhead for a few hours, dropping sticking snow all the way to sea level. It didn’t last long, and it warmed up in the hours following, but something quick/crazy like this could show up in the models just a few days ahead of time.

To wrap it up, my main points…

• There’s NO sign of a widespread lowland snow/ice event in the next 10+ days…January begins mild
• It’s also unlikely we’ll see an “arctic blast” in the first 15 days of January. That refers to a setup with cold arctic air drops south on north or east wind, keeping high temps in the 20s west of the Cascades and lows down in single digits or teens. I don’t see that in the cards.
• Expect rain almost every day during the next week; some days will be wetter than others, but I can’t find a totally dry day.
• Keep a close eye on the forecast during this time; a wind storm or river flooding could show up in the forecast maps just 3-4 days ahead of time.

Chief Meteorologist Mark Nelsen

A White Christmas In The Gorge, & Some Christmas Evening Wind Westside

11am Christmas Eve (Thursday)

Merry Christmas! It’s almost here, although a much changed version from any other year…very quiet for obvious pandemic reasons. It’s a dark time across the USA. That said, it HAS been nice to see bright sunshine the past two days.

I’ve been off the last 6 days, but will be back at work tomorrow evening through the middle of next week. We had quite a well-forecast soaking, warm temps, and gusty southerly wind. Now it’s quiet except for dense fog in the Willamette Valley and strong easterly wind blowing in the Gorge.

A wet weather system moves inland Christmas Day. With cold air stuck in the Gorge, that means some of you will have a White Christmas! In this case I’ll define that as seeing at least a dusting on the ground.

HIGHLIGHTS

1. Nothing interesting happens through tomorrow morning, except lingering areas of dense fog south of Portland metro and easterly wind gusts 50-70 mph at the west end of the Gorge. Crown Point has gusted to 75mph this morning.
2. Rain moves inland by late morning tomorrow; it’ll be a gray, wet, & cool Christmas west of the Cascades
3. As that moisture rides over cold air in place, snow begins falling early afternoon from Bonneville Dam eastward to The Dalles and down into north-central Oregon (Dufur, Maupin). Expect 1-5″ Christmas afternoon through early Saturday morning in those areas. A White Christmas for Hood River, Stevenson, White Salmon, Lyle, & The Dalles. Least would be at freeway level and at The Dalles, most up around 1,000′ and above. Expect there may be some snow on I-84 late tomorrow afternoon possibly through Saturday morning.
4. Depending on how cold the airmass is, there could be the usual spots of freezing rain/sleet west of Bonneville Dam to hills above Corbett on Oregon side, and hills above Washougal on Washington side of the Gorge. Likely only up above 500-1,000′. East wind will increase further; gusting 60-80 mph midday/afternoon Christmas before suddenly turning light around midnight tomorrow night.
5. A sudden surge of southerly wind should push up the coastline and Willamette Valley in the late afternoon & evening hours Christmas Day. Most models are relatively weak with the wind, but WRF-GFS suggest gusts to 40 in the valley and Portland metro area are possible around sunset and beyond Christmas evening. We will see…ECMWF isn’t on board

A quick look at the “supporting documents”. Sea level pressure forecast for 10am Christmas day shows a developing low offshore…

Then at 4pm the low has strengthened a bit. Pressure gradient through the Gorge is up to 8 millibars = cold & windy there.

Then at 10pm you see the surge of south wind and tight pressure gradient in the valley. It’s important to note that this model is strong than others. Much weaker low would mean just light southerly breezes.

This WRF-GFS model brings 70kt wind down to around 2,000′. Again, the strongest I’ve seen of all this morning’s runs

As for snow, ECMWF has been looking like this for the past 4-5 days! Several inches in Hood River and an inch or so at The Dalles.

That’s because it’s holding in a cold pool at 925mb tomorrow afternoon and evening. It also thinks Hood River and The Dalles only top out in the upper 30s today. If it gets well into the 40s (today), then it’ll be obvious modeling has been a bit too cold.

The brand new IBM GRAF model (along with soon to be extinct RPM) thinks there will be no pool of cold air east of the Cascades and no snow anywhere near the Gorge. I think the GRAF missed the last snow event out there too.

It’s interesting that the 2.5km (very high resolution) HDRPS model (Canadian) brings plenty of snow to the central/eastern Gorge. Strange little no-snow zone around Lost Lake to Parkdale, apparently punching in a little warmer air aloft.

But just for fun, you can see what it’s doing. Slightly warmer air overhead = sleet. Accumulation graphic here “fills in” those no-snow holes.

The main message when looking at differing models/maps? You should never take any one image and say “that’s going to happen”. Far smarter to stick with ensembles, not just of one model but combine several of them together to get a general picture. In this case, “there will be some white tomorrow afternoon/evening and a little silver in spots east of Portland in the Gorge”.

That’s it for today. Enjoy your Christmas!

Chief Meteorologist Mark Nelsen

A Soaker On The Way Sunday & Monday

9:45pm Thursday…

December has been very mild and a bit drier than average. The mild weather is here to stay, likely through Christmas. Sorry kids, hard to find cold & snow in this weather pattern! But I think we’ll be making up quite a bit of ground with respect to rain. A strong westerly jet and atmospheric river will arrive in the Pacific Northwest Saturday night. Expect about 48 hours of moderate to heavy rain (depending on location) across NW Oregon and SW Washington from Saturday night through Monday evening. Notice the west/southwest flow at 500mb midday Sunday

Then by Tuesday a cold trough is pushing the whole system well east and south of us

Look at the precipitable water loop from Saturday through Monday; you can see the moisture plume moving into the region

Integrated Vapor Transport (IVT) shows the situation very well. Strong wind overhead and copious moisture supply combines to create an “atmospheric river” aimed at the mouth of the Columbia River Sunday morning

How much rain? Each model is slightly different of course, but you get the idea…somewhere between 2-4″ in the western valleys and 5-8″ in the mountains!

This might be enough to produce minor river flooding on the usual suspect north coastal rivers: Wilson, Nehalem, Trask, & Willapa. Since we’ve been relatively dry with no recent flooding, my gut feeling is we won’t see anything widespread in the western valleys. But we’ll keep a close eye on it of course.

Beyond Monday, we’re headed into a least a brief period of drier weather, chilly east wind, and even some sunshine Tuesday-Christmas Eve. You can see a bit of that precipitation gap in the ECMWF ensemble forecast

Notice temperatures cool off a bit on the ensembles too

But if you want snow? Very unlikely west of the Cascades. Possibly some sort of ice/snow transition around Christmas or beyond IN THE GORGE if that cold east wind is still blowing…maybe.

That 15 day ECMWF ensemble snow forecast now takes us to New Year’s Eve. Wow, just no sign of snow/cold on any of the 51 members.

That’s it for now, I’m out of time this evening. We’ve been busy the past few days (and this evening) implementing a graphics change. Nothing too dramatic, but changing all those fonts, banners, backgrounds, colors takes awhile…

Chief Meteorologist Mark Nelsen

Wet Weather Ahead; But No Sign of Lowland Snow/Ice/Cold

9pm Sunday…

It’s been 9 days since I last posted. Part of the reason (other than keeping busy at work) is the lack of stormy weather. As mentioned in the previous post this first half of December was looking drier than average. Sure enough…the 14 day anomaly across the western USA shows a very dry month SO FAR…

Temperatures have been running near average over the Pacific Northwest. Although wintertime inversions in some valleys/basins have given cooler than average (Pendleton, Tri-Cities)

Again, nothing too exciting. We had a pretty good easterly wind episode the first week of the month; Troutdale saw 7 consecutive days with gusts 35 mph or higher. That said, it is December and that sure isn’t unusual. We’ve seen snow in the central/eastern Gorge twice now, although real marginal both times… the 2nd was last night. I had a wonderful day of light rain showers with the temperature between 33-36 degrees all day long (at home). Snowpack IS running near average in the Cascades which is great news. As I write this I see Hwy 26 through Government Camp has turned snowy again

A couple of notes…nowadays I spend time putting out shorter thoughts/maps etc… on Facebook, Twitter. It’s much easier on a busy night to quickly post a model chart or other weather information there as opposed to a long blog posting. You can find me on Facebook as @marknelsenweather or Twitter as @MarkNelsenKPTV Also, we have a Northwest Weather Podcast. Lots of fun! Just this past week the four of us (Brian MacMillan, Jeff Forgeron, Anne Campolongo, & I) discussed La Niña so far, Vista House wind, Solar Eclipses, & holiday plans for the weather center. Several weather folks having a jovial conversation…what could be better? You can find it here: https://www.kptv.com/podcasts/weather/

What’s Ahead?

1- We are turning wetter now and should see more regular rainfall over the next 10-14 days. I don’t see a significant dry spell (2+ days) between now and at least the middle of next week.

2- There’s NO sign of real cold weather or lowland snow west of the Cascades for the next 10+ days. That bumps us up right against Christmas of course. My gut feeling is that we won’t be seeing snow anywhere near the lowlands for Christmas Week (next Sunday through Christmas Day).

3- I don’t see flooding or an especially stormy pattern for the next 7 days. Most of the weather systems moving inland are relatively week; I’m not seeing big deep low pressure areas in the eastern Pacific. Of course that could change, but not this week.

I can explain both #1 and #2 by showing you the 500 millibar forecast from the GEFS (GFS ensembles) for the next two weeks. 2 weeks in about 4 seconds.

It’s a continuing train of weather systems moving west to east in a “zonal” flow. That’s as opposed to a “meridional” flow which would lead to more north-south movement. Also notice there are no prolonged ridging episodes (dry weather). In this pattern it never gets cold because the chilly arctic air is bottled up to the north of the jet. You aren’t going to get lowland snow in this setup. Look at the 850mb ensemble chart from the ECMWF model. I’m amazed that through the entire 2nd half of December there are almost no members (each line is one of 51 ensemble members) that drop below -7. That’s about what we need for lowland snow in onshore flow. Very consistent westerly flow = mild.

Confirmation comes from the GFS ensemble forecast of Portland snowfall the next two weeks (through Sunday the 28th). Hardly a single member tries for 2″ or more snow.

There are hints that Christmas Week might be a bit drier than this week and coming weekend. You see quite a few more gaps in 24 hour precipitation totals during the 2nd week.

What About La Niña? Shouldn’t Weather Be Wilder at This Point?

No, not necessarily. Remember that La Nina doesn’t guarantee lowland snow, and it sure doesn’t guarantee lowland snow and stormy weather between November 1st and January 1st. For all we know, we’ve got crazy wild weather action coming in January or February! For fun I looked back through the last 10 La Nina winters to see what happened. Some start just like this…mild and dry-ish. My brief notes on each.

2017– A boring November, dry ridging 1st half of December. No decent skiing until Christmas Vacation

2016– Crazy and wild ride. At this point we had just gone through an ice storm and were preparing for a 2pm arrival (December 14th) of a snow storm. Everyone hit the road at the same time and you remember what happened. I always remember the date 12-14-16.

2011– Stormy November, Dry December. Then a big atmospheric river wipes out quite a bit of snow on Mt. Hood right after Christmas. January 2012 following was stormy

2010– Stormy November, arctic air at Thanksgiving

2008– Boring November, dry December start. Arctic blast arrives right about now (Dec. 14th). Two weeks of snowy mayhem followed…

2007– Boring/dry/mild November, huge coastal storm and widespread flooding just north/west of metro early December. A series of colder storms began in mid December bringing feet of snow to the foothills and many feet the rest of winter to the Cascades

2000– Nothing all that interesting, cold/dry November, in fact entire winter was dry-ish. Didn’t fit La Nina pattern at all

1999– Warm & wet November

1998– Very wet November and December, arctic blast around December 17th

1995– Warm and wet November, stormy December included the last major regional windstorm (12-12-95)

You can see this current year isn’t a very good fit for any the last 10 La Nina winters. Maybe similar to the last one 2017-2018? Interesting…

That’s it for now, enjoy the rainy weather. Best chance for some dry is tomorrow and maybe sometime between Thursday showers and renewed rain on Saturday…possibly Friday.

Chief Meteorologist Mark Nelsen

Slow 1st Half of December; Plus Lots of East Wind

10pm Friday…

A bit of a schedule change which means I’m working a Friday night (rare). What a beautiful first four days of December! Solid sunshine for most today; check out the upper 50s and lower 60s along the coastline.

Low 50s again in Portland; this has to be one of the warmest east wind episodes I’ve ever seen in December. Typically we stay in the 40s, or colder. An approaching low pressure area is tightening the pressure gradient across the Cascades as a cool area of high pressure sits in the Columbia Basin. It’s been running around 8 to 10 millibars this evening.

That tight pressure gradient leads to strong wind in the “outflow” area of the Columbia River Gorge. Gusty east wind from Cascade Locks all the way into East Portland. Don’t expect the wind to drop off until Saturday night as a weak cold front passes by; it’s going to be a windy Saturday out there. Peak gusts have been in the 40-50 mph range around the mouth of the Gorge (Troutdale/Camas/Gresham) and 60-80 mph in the western Gorge itself. I don’t see a long period of strong wind NEXT week, it’ll likely come and go. By the way, Vista House wind sensor is working fine, but data isn’t making it all the way to NOAA yet. That should fix itself next week. A peak gust of 80 mph occurred late this afternoon.

Let’s look ahead…

THE BIG PICTURE

1. In general, benign weather will be the rule for the next 1-2 weeks.
2. I don’t see a weather setup that would bring lowland snow/ice west of the Cascades in the next 10+ days. That takes us through mid December. There won’t be an “early winter” this year. By that I mean some years we get a cold spell and/or lowland snow in late November or the first half of December. Not the case this year…hmmm. Surprising, but not unprecedented in a La Niña winter.
3. There’s no sign of a stormy weather pattern through the next 10 days either. Most weather systems passing by appear to be weak.

So what’s going on? It all boils down to the fact that upper-level heights will remain above normal over the eastern Pacific and western USA much of the time over the next 10 days. Any significant change is more than 10 days away. Models are in pretty good agreement about this. Take a look at the ECMWF ensemble average 500mb height map for North America for tomorrow evening; a weak upper-level wave is pushing through the strong ridge over us

Notice the ridging is back by Monday. Just some light showers tomorrow evening through Sunday morning.

Other than another stronger upper-level trough with cooler air passes through about next Thursday, followed by more ridging. This is one week from today (Friday the 11th)

We don’t get all that much rain out of these weak systems. 7 day precipitation forecast

The ridging flattens a bit, but notice heights are still above average (warm colors) by Tuesday the 15th.

We’re halfway through December at this point. Ah, but then both ECMWF and GEM models are trying to push the ridge west about two weeks from now. A cooler/wetter pattern as we head toward Christmas Week. We’ll see…

One tool we use is the “850mb ensemble chart”. Each line represents one of the 51 ensemble members, showing temperature in Celsius around 4,000′ or so. To get snow down to sea level we generally need it below -6. Notice almost no lines go down that low THROUGH THE NEXT TWO WEEKS!

Confirmation of that comes in the 24 hour snowfall forecast for each of those 51 ensemble members for the middle of the north Willamette Valley. It’s rare December-February to see so few members producing at least some snow at some point in the two week forecast.

To summarize: weather is unlikely to significantly alter your life or plans during the next 10+ days.

Chief Meteorologist Mark Nelsen