Damaging Windstorm Possible This Tuesday

Sunday, November 15, 2015
1:00 pm

Wow, we have a lot to talk about. It’s been a very long time since the weather has been this exciting!

Credit: WSDOT Flickr Page

First off, the rain has died down around Western Washington and most rivers have receded below flood stage. The Snohomish River at Snohomish, The Skokomish River near Potlatch, and the Chehalis River at Grand Mound are still above flood stage, with minor flooding occurring. Although the rivers have already crested at Snohomish and Potlatch, the Chehalis at Grand Mound is actually still cresting. This is because the Chehalis is a big, slow river, so floodwaters take a while to reach areas downstream. The Chehalis will crest this afternoon and fall below flood stage midday tomorrow, the Snohomish will fall below flood stage any minute now, and the Skokomish will fall below flood stage this evening. Minor flooding is occurring on all these rivers.

We also saw some snow this morning in the South Sound and foothills. Even places near sea level on the northeastern Olympic Peninsula saw snow. When was the last time we had lowland snow and river flooding at the same time? I don’t know, but I can’t remember any. It’s still pretty early in the season, but not too early to get snow; in fact, mid-November 1955 was one of the coldest stretches on record for Seattle. Today’s record low was 6 degrees at Sea-Tac airport, set back in that fateful year, which was also the snowiest on record for Snoqualmie Pass. They got 828 inches from 1955-1956. That sounds like a snow total you’d expect at Mt. Baker during a strong La Niña year! Check out this video of snow this morning in Auburn!

While it’s cool to talk about snow and rain, we have a much more important issue to talk about: the potential for a damaging, region-wide windstorm on Tuesday.

This is a pretty difficult forecast, particularly because there is a large spread in the models and they just started showing serious wind within the past couple runs. However, more and more are jumping on board with the idea of having a major windstorm on Tuesday. Additionally, there seems to be a lot of spread in the forecasters themselves, with the guys at the Seattle National Weather Service office mentioning the possibility for a serious windstorm while those at the Portland NWS office don’t make any mention of it at all.

First, let’s take a look at some excerpts from the NWS Seattle forecast discussions. You can really see how they are getting more and more concerned about the threat of a windstorm. I’ve put some of the highlights in bold.

Last night, 10:08 pm

A STRONG JET WILL REMAIN AIMED AT THE PACIFIC NORTHWEST MON NIGHT 
THROUGH WED. MOST MODELS SHOWED A DEEPENING LOW MOVING NORTH
ACROSS SOUTHERN BRITISH COLUMBIA ON TUESDAY. THIS COULD BRING
STRONGER WINDS TO THE PUGET SOUND REGION...THOUGH LIKELY BELOW
WARNING CRITERIA
. SUBSEQUENT MODEL RUNS WILL NEED TO BE EVALUATED
IN CASE A HIGH WIND WATCH MAY BE NEEDED FOR PARTS OF THE CWA FOR
TUE. MERCER/05

Early this morning, 4:02 am.

I FIND TUESDAY TO BE THE MOST ALARMING PART OF THE 
FORECAST...MAINLY DUE TO WIND CONCERNS.
THE 00Z GFS AND ECMWF ARE
IN GOOD AGREEMENT WITH BRINGING A DEEPENING SURFACE LOW QUICKLY
EAST ACROSS SOUTHERN B.C. ON TUE. THE 00Z NAM WAS WEAK AND
UNIMPRESSIVE WITH THIS FEATURE...BUT THE 06Z NAM IS TRENDING IN
TOWARD THIS SOLUTION TOO. THE 06Z GFS IS EVEN STRONGER AND MORE
PRONOUNCED WITH THIS FEATURE. A SHARP COLD FRONT WILL FOLLOW THE
LOW...DROPPING IN FROM THE N-NW ON TUE AFTN. THE FRONT WILL BE
PRECEDED BY A FEW HOURS OF INTENSE PRECIP...BUT THE SHORT DURATION
OF JUST A FEW HOURS WILL LIMIT THE HYDROLOGIC EFFECTS. HOWEVER
REGARDING WIND...AS THE SURFACE LOW OVER B.C. DEEPENS INTO THE
980-990 MB RANGE ON TUE AFTN...AND PRECEDING THE COLD FRONTAL
PASSAGE...S-SW PRESSURE GRADIENTS REALLY PICK UP...AND WINDS ALOFT
INCREASE TO IMPRESSIVE LEVELS
. THE MOST BULLISH OF THE
MODELS...THE 06Z GFS...BRINGS THE OLM-BLI PRESSURE GRADIENT TO
+12.8 MB AT 21Z TUE...WITH THE UIL-BLI GRADIENT PEAKING NEAR +5.3
MB AT 00Z TUE AFTN. IN ADDITION...THE RELATIVELY BEARISH 06Z NAM
BRINGS LARGE SWATH OF SWLY 925 MB WINDS OF 50-60 KT THROUGH THE
STRAIT AND THROUGH SOUTH AND CENTRAL PUGET SOUND. MEANWHILE...THE
MORE BULLISH 06Z GFS BRINGS 925 MB WINDS OF 60-70 KT...JUST AS THE
COLD ADVECTION IS ABOUT TO SET IN. COLD ADVECTION IS
NOTEWORTHY...BECAUSE IT HELPS TO MIX THE ATMOSPHERE THROUGH A
GREATER DEPTH AND PULL DOWN STRONGER WINDS ALOFT.
STATISTICAL
GUIDANCE IS ALSO IMPRESSIVE. THE 06Z GFS-BASED MOS GUIDANCE FOR
SEA-TAC PEAKS THE SUSTAINED WIND AT 37 KNOTS...WHILE THE GFS-BASED
MARINE MOS GUIDANCE SHOWS AN IMPRESSIVE AND ALARMING 48 KT
SUSTAINED AT WESTPOINT
...WHICH IS AT THE TIP OF SEATTLE`S
DISCOVERY PARK. BOTTOM LINE IS TO CHECK BACK TO SEE IF LATER MODEL
RUNS ARE STILL STRAYING THIS WAY. IF THERE IS GOOD NEWS...IT IS
THAT THE TRACK OF THE LOW COULD SUPPORT A LULL OR SUCKER HOLE IN
THE WIND DOWNWIND OF THE OLYMPICS. STRONGEST WINDS IN THIS PATTERN
WOULD BE IN LOCATIONS OPEN TO THE STRAIT OF JUAN DE FUCA...AND
LOCATIONS SOUTH OF DOWNTOWN SEATTLE.
DUE TO THIS EVENT STILL BEING
60 HOURS OUT...WILL SAVE DECISIONS ON A HIGH WIND WATCH FOR THE
DAY SHIFT.HANER

Later this morning, 10:30 am:

THE 12Z NAM12 AND GFS20 SOLUTIONS CONTINUE TO SHOW THE DEVELOPMENT 
OF A WAVE ON THE JET THAT RIDES EASTWARD ACROSS ABOUT THE NORTHERN
TIP OF VANCOUVER ISLAND AND INTO ALBERTA. THE FAST MOVING WAVE WILL
BE DEVELOPING AS IT MOVES EAST. A QUICK LOOK AT THE 12Z ECMWF SHOWS
THE WAVE DEVELOPMENT A BIT FARTHER SOUTH SUGGESTING THAT LOW
PRESSURE MAY RIDE EAST ALONG THE CANADIAN BORDER ON TUESDAY INSTEAD
OF ABOUT 100-200 MILES (to the north)
. IN ADDITION TO PRODUCING A PERIOD OF INTENSE
RAINFALL ON TUESDAY WITH IMPRESSIVE AMOUNTS SHOWN IN THE MODELS FOR
THE WEST FACING SLOPES OF THE CASCADES AND OLYMPICS...WIND IS
BECOMING AN INCREASING CONCERN. 925 MB WINDS TO THE SOUTH OF THE
FRONT RISE TO AROUND 70 KT AS THE WAVE MOVES BY TO THE NORTH AND MOS
VALUES FOR SEATTLE SHOW IMPRESSIVE 34 KT SUSTAINED WIND FOR 2 3 HOUR
PERIODS. GRADIENTS AND WINDS ALOFT SUPPORT RATHER WIDESPREAD SOUTH
TO SOUTHWEST WIND AHEAD OF THE FRONT OF 40 MPH WITH GUSTS TO 70 MPH.

THEN...WESTERLY WINDS THROUGH THE STRAIT OF JUAN DE FUCA COULD
IMPACT AREAS ON THE EAST SIDE OF THE STRAIT LIGHT WHIDBEY ISLAND AND
POSSIBLY THE NORTHERN HALF OF SNOHOMISH COUNTY. WILL LIKELY NEED TO
ISSUE A HIGH WIND WATCH FOR TUESDAY INTO TUESDAY EVENING FOR MUCH IF
NOT ALL OF THE FORECAST AREA WITH THE AFTERNOON PACKAGE.

So, in the space of 12 hours, the forecasters went from ‘could bring stronger winds to the Puget Sound but likely below warning criteria’ to ‘sustained winds of 40 mph with gusts to 70’ and saying we will probably need a high wind watch for everywhere in Western Washington. Big change, huh?

So that’s the National Weather Service. Here’s my forecast.

The most recent 18z NAM and GFS runs came out, and they are weaker than the previous runs. The GFS could still give us a high-wind-warning, but we wouldn’t see gusts to 70 with sustained winds of 40. Still, I expect that the models will wobble around a lot before finally settling on a solution. And remember, the 12z European model put the low into central/southern Vancouver Island, which would likely give us even larger wind. I have limited access to the ‘Euro,’ so I don’t know the specifics, but it looks very windy. Also, keep in mind that the 06z and 18z runs are generally not as reliable as the 00z and 12z runs, and this is because the 00z and 12z runs have more upper air observations.

The reason that these models are having such a tough handle on this system is because of the extremely strong jet stream driving it. Currently, there is an area of 170 knot winds up at the 300 hPa level (around 30,000 feet) driving west, and these winds will help power the storm and will reach our area by Tuesday.

Valid 01:00 pm PST, Tue 17 Nov 2015 – 57hr Fcst
Credit: UW Atmospheric Sciences

The chart above shows the 500 hPa geopotential heights (contours) and absolute vorticity (colors). Don’t worry about the vorticity for now… even after all these years, I still don’t fully grasp it. The geopotential heights represent the elevation of the atmosphere at which the given pressure level exists. For example, that high pressure system well of the California coast has a height of 5909 meters, while the low pressure to its northwest has a height of 5498 meters.

The important thing about this chart, however, is how close together the height lines are in our neck of the woods. The closer together the height lines, the faster the upper-level winds are.

Diagram of geostrophic flow in the Northern Hemisphere. Credit: Hong Kong Observatory

In the upper atmosphere, there are two main forces acting on the velocity of these winds: the pressure gradient force (PGF) and the Coriolis force. The PGF points from high to low pressure, while the Coriolis force is always to the right of the flow in the Northern Hemisphere (and the left of the flow in the Southern Hemisphere). Eventually, the two forces reach a balance, and the winds blow completely parallel to the height/pressure lines (it doesn’t matter what coordinate you use in the vertical, as you get the same result). The resulting flow is said to be geostrophic.

We know that this jet stream will be in our neck of the woods. However, we don’t have a good handle on whether it will result in a major windstorm for us.

Now, let’s take a look at some other maps closer to the surface.

Here is a map of the forecast winds at 925 hPa level on Tuesday afternoon. This was taken from the 06z GFS, with some annotations added by the NWS.

Credit:  NWS Seattle Graphical Area Forecast Discussion

These are extremely strong 925 hPa winds. The last time I remember seeing something this strong was during the Great Coastal Gale of 2007, and while we’ve probably had instances since then of equal or higher 925 hPa winds, we haven’t had many. The reason this is important is because these winds can easily get mixed down to the surface when the atmosphere is unstable. As these winds come in, cold advection (cold air moving into the region) will occur, mixing the atmosphere and helping bring some of this energy down to the surface. The mixing of winds from the upper atmosphere to the surface is what causes our wind gusts, so the NWS forecasters are worried about high wind gusts – and rightly so.

Strong winds aloft are correlated with strong pressure gradients, but those pressure gradients are often associated with temperature gradients. This storm looks to have one of the most well-defined warm fronts I’ve seen on any storm system, not just here in the Pacific Northwest. And that’s saying something, because due to the moderating effects of the Pacific Ocean, our fronts (both warm and cold) are usually pretty weak.

Valid 01:00 am PST, Tue 17 Nov 2015 – 45hr Fcst
Credit: UW Atmospheric Sciences

However, the warm front (east of the main low) above is incredibly strong. Note how abruptly the winds shift from weak northerly to strong westerly. The cold front looks just as strong. Warm front passages are generally pretty nonchalant. It will be interesting to look at some of the buoys off the coast and see what they measure with this front.

Here’s another picture of the front, showing sea level pressure (SLP), winds at the surface, and temperature at the surface.

Valid 01:00 am PST, Tue 17 Nov 2015 – 45hr Fcst
Credit: UW Atmospheric Sciences

Finally, although wind will be the main story of this storm, it will bring heavy rain as well, particularly to the mountains. The Seattle-Everett corridor will be shadowed, with the greatest shadowing occurring on the Northern Kitsap Peninsula. Places like Poulsbo and Kingston could see less than a tenth of an inch of rain, while exceptionally orographically favorable places on the Olympics and Cascades could see over 10 inches.

Valid 04:00 pm PST, Tue 17 Nov 2015 – 60hr Fcst
Credit: UW Atmospheric Sciences

Freezing levels will be high – around 7-8000 feet, and this will lead to additional flooding concerns on rivers. Pretty much all the rivers in Western Washington are still running very high from the heavy rains this past weekend, and as I previously mentioned, some are still flooding. Because of that, this flooding could be worse than the flooding we saw this past weekend.

By the way, the National Weather Service Seattle office just issued a high wind watch for the entire Western Washington lowlands from Tuesday through Tuesday evening, calling for gusts of up to 75 mph and the threat of large tree failures due to the recent heavy rainfall. The highest gusts will likely be in the most exposed places (Naselle Ridge, Destruction Island, etc.) but I would not be surprised if Sea-Tac hit 60 from this event.

There is considerable uncertainty with this event, so please keep that in mind. But one thing’s for sure: when the Seattle NWS homepage looks like this, it’s a fun time to be a weather nerd!

Credit: Seattle National Weather Service Office

I’ll keep you posted.
Charlie

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