Wednesday, December 7, 2016

It Will Snow On Thursday Night, But Don't Expect Snow Drifts

Snow Update will be posted at 4 PM Thursday,  Snow has spread over Portland into SW WA at 1:30 PM.  
______________________________________

It is going to snow Thursday evening at most locations in western Washington--even near sea level.

And if you want an impressive accumulation of snow you will be disappointed, unless you are in a few favored locations.  Many will only see 1-2 inches. Some will only have a dusting.

And there will be lots of wind for those near the western Cascade foothills.

And yes, the forecast timing has shifted a bit:  a few hours later.

The origin of the snow is clear:  a warm front is approaching from the south and we have cold, dry air over us--air of continental origin.  Dry is important, because that allows evaporative cooling when precipitation from the warm front falls into it..

The forecast map for 1 AM Thursday shows the front  and associated trough off northern CA (this map shows sea level pressure in solid lines and lower atmosphere temperatures--actually at 925 hPa-- in color).  Note the large change in pressure over the Washington Cascades:  that will produce strong easterly winds.

 By 10 PM Thursday, the front is off the SW WA coast, with cold air still over western WA.

Run after model run has progressively weakened the warm front, with less precipitation.  That reduces the potential snowfall.   Furthermore, it has slowed down as it weakened.   The latest models hold off the snow until right after the evening commute--so less worry about driving tomorrow.   If the current models are right, snow will begin in Olympia around 7 PM and Seattle around 9 PM.
But a weaker front will be slower to scour out the cold air and will force less snow-eating easterly flow.

OK...you want to look at the snow forecasts....so let me not delay you anymore.  Here is the 24-hour total snow ending 4 AM Friday.    A big east-west snow gradient across the Sound.  Virtually nothing over the western Cascade foothills where easterly, downslope flow will be strong, and 3-4 inches over the western Kitsap Peninsula.    Seattle gets 1-1.5 inches, with more on the northern side of the city.  Half a foot in the mountains.    This is not a very wet system.


By 4 AM, the precipitation will turn to rain over the lower elevations.  So the early Friday commute could be a bit slushy, but by later in the morning driving should be ok.

Is there some uncertainty in this forecast?   Of course.  The latest National Weather Service short-range ensemble forecast (SREF) snow prediction for Seattle Tac shows an ensemble mean (the average of all the forecasts) of 1.5 inches at the airport, with quite a range of forecasts (from nearly nothing to 6 inches).


The European Center ensemble system has 1-2 inches at Sea Tac for most members for the late Thursday event, with more over the weekend.  But perhaps 30% of the members have nothing.


Based on these and other ensemble systems, there is a good 30-35% chance that the city will only get a dusting.

And winds?   The easterly winds have already started to rev up...here are the max gusts for the past 24 hr around the region (mph, locations with 31 mph and more).  Mountain stations are already getting above 50 mph and foothills reports have jumped to 40-45 mph.   They will get stronger during the next few hours.


It will be nice to see snow tomorrow evening and equally nice to know it won't stick around long in the urban areas.

____________
Help Weather Prediction Research and Enjoy a Great Free Weather App for Your Smartphone:  uWx

At the UW, we have developed a wonderful FREE weather app for Android smartphones that also collects pressure for use in weather forecasting. If you want to try it, please go to the Google PlayStore and download it.



11 comments:

Robert Magee said...

What about eastern Washington? Wenatchee area? Same system or different?

Alex said...

Thanks for the update, Cliff.

Any comment on the recent Euro ensemble? Historically, have ensemble means or the deterministic model been more accurate in predicting snow here in the NW. In which do you put more of your personal faith?

https://mobile.twitter.com/ScottSKOMO/status/806677056692043776/photo/1

JJM said...

Over hyping things again I see, Cliff.

A few days ago... "SNOW SNOW SNOW for everyone on Thursday. Inches of snow for everyone!"

Now... Well its not near as much as we thought, it's starting later, and wont stick around long at all. KING is forecasting .10 in the Seattle area this morning.

You seriously need to get over sensationalizing every weather event this region has. Its overkill.

Too bad readers of your blog will never see comments like these because you censor them. Only letting people post that hang on your every word.

sandy knoller said...

A question about HRRR presentations. This is specifically for the 11z rapidrefresh NOAA runs for 2pm PST today, but earlier runs have been similar. The “precip type” is clear that measurable snow would not arrive until much much later, presumably more in line with your timing. For the most part, the other presentations are consistent with the late snow hypothesis, which is also advanced by the NWS.

However, the “composite reflectivity” suggests a possibility of flurries during the afternoon! My guess is this doesn't contradict the late snow hypothesis, but some explanation would be instructive. Evaporation before it hits the ground? Too little to accumulate? Model quirkiness? All of the above?

Jacques White said...

Hi Cliff. The Washington Post is calling for a return of the "Polar Vortex" of displaced Arctic air over the upper plains states and the northeast next week.

http://www.msn.com/en-us/weather/topstories/polar-vortex-unleashed-severe-cold-snap-likely-in-us-next-week/ar-AAlgDmx?li=BBmkt5R&ocid=spartanntp

My question for you, is there any chance that these Arctic displacements could start to occur with any regularity over the North Pacific and the Pacific Northwest, or is there some atmospheric steering mechanism that "protects" us from recurrent dips into very cold temperatures? I notice from the maps in the Post article that anomalously cold air is extending into the North Atlantic, but historically winter temperatures at the same latitudes are much colder on eastern seaboard of North America compared with the western seaboard.

Shelley said...

Hi Cliff,

Some of us in Bellingham pay a lot of attention to your posts. I work at a native plant nursery and we are digging our bare root woody plants now, or rather we were digging and are now shut down because of the freeze. Any information you can give us in the Whatcom county area is very much appreciated. Thanks.

Shelley

Scott K. said...

Someone mentioned in the comment section of a previous blog about how predictions of snow and wind generally are updated (as the event gets closer) as less potent with each update.

I'm curious if there's statistical data to back this up, but I feel that it's true based on personal experience. This would refer to the greater puget sound area, Seattle, eastside, everett down to about Tacoma.

Basically, has there ever been a wind or snow event that included updated predictions of being more potent than it's original/early predictions as the even grew closer?

I'd really like to see data on this. It could help shine a light on those who are upset with missed forecasts (myself included) for these type of events. Do weather reporters/meteorologists over-predict these events more commonly then underpredict?

Mark said...

Scott,
During the first year I worked in Seattle, December 1990, the forecast called for a mix of rain and snow. Seattle got about 6 inches of thunder-snow in about 4 hours. The freeways turned into parking lots. The evening commute was a nightmare. The snow was followed by single digit temperatures. Seattle roads were impassable and water pipes froze all over town.

Several times in 1990s the roads were impassable due to snow and ice. My house lost power. We were trapped in a frozen world. Trees and large limbs collapsed under the weight of ice and snow. My daughters and I made snowmen and threw snowballs on those snow days. It was fun!

I can't recall a single snowstorm in the 2000s that matches my experiences from the 1990s. It's never been as cold as it was in 1990 and I've never seen as much snow as I did in the 90s.

Friends who lived in Seattle in 1960s recount a snowstorm where drifts piled up a couple feet deep.

We still get windstorms. The last big one back around 2006 or 8 blew down a big stand of Doug fir that I'm still harvesting for firewood. The warm, dry summers of 2014 and 15 killed a few medium sized Doug fir that I take down for firewood too.

I think if you look at the 30 year temperature averages for the 1960s, you will find Seattle used to be a little colder than it is now. A couple degrees makes a big difference in accumulating snow.

This last snow measured 0.5 inches on my wooden deck. Melted it measured 0.26 inches of water. That's a 2 to 1 ratio. Hard to even call that snow. Had it been a couple degrees colder the ratio might have been closer to normal about 8 to 1 or 10 to 1. It would have been a couple inches of snow. Enough to roll a couple balls of snow and stack them into a little snow man.

I'm still hoping for a decent fall of snow so my grand children can experience making a snow man. My experience is Lowland snow has been pretty sparse since 2000. So any forecast calling for heavy snow has failed to materialize.





Mary and Jim said...

You are being very generous calling it snow in Portland- driving through town now and it is just slushy rain.

larchitech said...

Check the WSDOT I-5 cameras and you can see the snow moving north.

Anisa Redmond said...

Scott, you took the question and thoughts right out of my mind. I want these weather events to happen so bad that I have considered refraining from reading forecast and following closely in an effort to just let things be a surprise. And to let Murphy's law play out. :)