Thursday, December 8, 2016

Thursday Snow Nowcast at 3 PM: Snow Moving in Faster Than Expected

Well, our models may have had the right idea of the threat of snow, but the recent runs clearly have a serious timing error.    The snow will arrive 4-5 hours earlier than expected.

The latest radar image  (2:27 PM) shows the story--the precipitation shield from the warm front has reached the south Sound.  Once precipitation starts aloft, it takes about an hour to reach the surface (need to humidify the layer below)

The latest visible satellite image shows an impressive cloud band with the warm front.

Let me show you some traffic cams.

It has been snowing in Portland for hours.


Here is the Chehalis Airport

Dupont, north of Olympia at 3:30

It should starting snowing at the surface in Olympia by 3PM.  At the current rate of progress, the snow will get to Tacoma around 4 PM and Seattle by 5-5:30 PM.

There appears to be multiple bands...the first band is producing the above snow.  It appears to be weakening over southern Puget Sound...and slowed down.  So it looks like Seattle commute will be ok.   The second band, which should arrive later (if the models are right), well be more of a threat for Seattle and points north...later in the evening.

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. want to look at the snow 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 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.

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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.

Tuesday, December 6, 2016

Increasing Probability of Lowland Snow Late Thursday Afternoon Through Early Friday

Update Wed AM.   The latest runs are shifting the precipitation several hours later, with a start on the early evening on Thursday.  So the Thursday PM Seattle commute may be OK.  Each model run is shifting the low farther offshore and delaying the precipitation onset.  All runs suggest a large east-west snow variation, from little along the western slopes of the Cascades to serious amounts (roughly a half foot) over the area west and southwest of the Olympics (e.g., Hood Canal area).

It is becoming increasingly likely that we will see some significant snowfall over the lowlands late Thursday afternoon into early Friday morning, with amounts of up to 2 to 5 inches before the precipitation turns to rain.  But there will be a large variation of snow across the lowlands, with less near the western Cascade foothills.

My work tonight is made more difficult by major systems failures at the National Weather Service, slowing of Comcast internet (surprise), and the lack of availability of the UW WRF runs (probably due to the NWS problems).

Thursday afternoon, cold and dry air will be in position over western Washington and a strong warm front will be moving up the coast (see graphic for 4 PM showing low-level temperature, sea level pressure, and winds).  Where you see blue colors the air will be cold enough for snow to reach the surface.

Moisture associated with the warm front will be able to fall through the cold air as snow.   By the next  (Friday) morning, as the warm front and associated low moves northward, rain will spread into western Washington (the situation at 4 AM Friday is shown).

The latest run of the UW WRF forecast system shows snow over the region of various amounts.  For example, the 24-h snowfall ending 4 AM Friday indicates 2-6 inches over over the western Kitsap Peninsula and  more over the high terrain, and several inches inches over NW King County.  There is less over the eastern Seattle suburbs due to easterly flow down the western Cascade slopes (downslope flow causes drying).

The European Center snow depth forecasts for 4 AM Friday (below) shows 2-3 inches around Seattle, with less right next to the Sound and over the eastern suburbs.

Talking over easterly flow, there should be strong winds in the western Cascade foothills (see graphic of max gusts at 10 am on Thursday).   45 knots in some locations SE of Seattle.   Very strong winds over the eastern Strait and offshore water.

This situation is more threatening than Monday, since we will have cooler and drier air in place and the amount of precipitation is greater.  Furthermore, with low pressure approaching cold air will be pulled into the region through the Fraser River Valley.

The timing right now suggests that snow will reach the Seattle Metro area between 4 and 6 PM Thursday.

The snow forecasts are critically dependent on the amount of precipitation and the exact temperature structures over us... and the models have been shifting in their solutions somewhat.  Thus, there is still considerable uncertainty in the forecasts, which I will quantify using ensemble predictions tomorrow.   But the nearly all of the ensembles I have seen so far start this event off as snow.  All warm up the area by Friday morning.  The question now is really about the amount of snow and the exact timing.

Monday, December 5, 2016

Puget Sound Snow Threat is Done, But More Exciting Weather is Yet to Come

Serious Snow Threat on Thursday: Update Tonight (Tuesday) at 9 PM

Our minor lowland snow event is nearly over, with only some wet snow near sea level and modest snow above 500-700 ft.  As noted in my previous blog, the problem was temperature:  too warm at low levels.  But plenty of snow in the mountains.

 The only real snow during the next few hours will be around Port Angeles and the NE Olympics, where Fraser Valley flow will be forced to rise (see forecast for 3-h snow ending 4 AM).

But there is a lot of weather fun that will occur over the next few days over the Northwest, including:

  • The coldest low temperatures over the western lowlands since last January (down to the mid-20s!)
  • Strong Fraser River outflow winds during the next day or so.
  • Strong easterly winds along the eastern slopes of the Cascades
  • Strong easterly winds in the Columbia Gorge
  • Lowland snow on Thursday
  • Strong winds along the NW WA coast on Thursday.

More in future blogs.

Sunday, December 4, 2016

Lowland Snow: It's All About Temperature

Forecasting snow over western Washington is made hugely more difficult because our temperatures are often marginal for snow, with a few degrees making all the difference.

And the snow forecast for Monday is no different.  

Whenever air is coming off the relatively mild (roughly 50F) Pacific, it is difficult to get lowland snow unless the air is very, very cold aloft.

As scheduled, a strong cold front moved through this morning with cool onshore flow in its wake.  This transition was clearly shown in the winds and temperatures over Sea-Tac Airport (see time-height cross section below, red temperatures, blue are winds, time goes from right to left in GMT/Z/UTC, and heights are in pressure --850 is roughly 5000 ft).  The wind shifted around 0412 (Dec 4 and 12 UTC-4 AM) and the temperatures dropped.
It cooled enough aloft that a few areas above 500 ft or in intense showers (which brings the snow level down momentarily) that snow reached the surface at some Puget Sound locations.  Here is an example from Snoqualmie Ridge

Picture taken by Rob Nelson

Or Ken's Korner on Whidbey Island:
Picture courtesy of Susan Kieffer

The current snow threat is associated with cold unstable air that is being lifted by an upper level trough (or disturbance).  The latest infrared satellite image Sunday night show the convective showers associated  with the unstable air (the white mottled look).  An area of enhanced showers is off our coast and headed our way overnight--that is the immediate threat.

But although the air has cooled considerably, it is still marginal for snow near sea level and near the water.  In fact, the freezing level rose a bit during the day and is now (7:30 PM) roughly 2200 ft.  Keep in mind that the snow level (the level below which the precipitation is all rain) is about 1000 ft below the freezing level (the altitude of 0C).  Thus, the snow level is now around 1200 ft.
The approaching precipitation will bring the freezing and snow levels down (due to evaporation and melting of precipitation), but it will be difficult to bring snow all the way down to sea level, particularly locations near relatively warm Puget Sound.   Let's look at the latest model runs.

The NOAA/NWS HRRR (High Resolution Rapid Refresh) snow total through noon shows only .1-1 inch over most of Seattle, but much higher amounts south of Olympia and the Olympics.

The even higher resolution UW model run  for the 24-h snow ending 4 PM Monday is similar, but with more snow over Snohomish County and north Seattle (few inches).

The marginal (and thus uncertain) nature of this event is highlighted by the latest National Weather Service SREF ensembles for snow at Sea Tac (452 ft).  The mean total snow is around 3/4 of an inch, but there is a lot of variability.

The bottom line is that if you are near Puget Sound and close to sea level, you will see mainly rain showers, perhaps accompanied by less than an inch of wet snow.    But away from the water and a few hundred feet up, an inch or two is possible.  The roads should be ok other than steep hills since the soil is warm.  Higher up (400-800 ft), several inches are quite possible.  The main area of showers/snow should get to us roughly 5-7AM--so if you are starting your commute on a hill or inland, leave extra time.

The Seattle media has already started its favorite past-time, with "live storm coverage" and it is starting to have an impact on local supermarkets (see below).

And if you think the snow will be over tomorrow...think again.  Much colder arctic air will surge southward Tuesday morning, with potential snow in Sequim/Port Angeles and even northern Puget Sound.

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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.

Saturday, December 3, 2016

Snow Update with a Probabilistic View

I will have an update at 9 PM Sunday.... snow has already hit some higher-elevation locations and expect more tonight...

Let me update you on the snow situation, using all the technology at my disposal, both using ensembles for a probabilistic viewpoint and high-resolution forecasts.

Let's begin by noting an important point:  after the warmest November in Seattle history and mild temperatures the last few days, the road surfaces are quite warm, which will help melt any light snow falling on them. To illustrate, here is the latest air and road temperatures (road surface temperatures in rectangles) from the Seattle SnowWatch website.  Most roads are in the mid to upper 40s.   This will be a powerful weapon against icing during any initial onslaught of snow.

As in any good forecast we should start from a probabilistic viewpoint with a clear understanding of forecast uncertainties, and as I have discussed many times in the past, ensemble forecasts (using many forecasts, each a little different) is a powerful tool to do so.

The National Weather Service 21-member GFS ensemble's cumulative snow forecast for Seattle (below) has all members (forecasts) going for some snow this week (remember this is amount of snow falling out of the sky, not depth on the ground).   Black is the average of the all the forecasts and blue is the operational high-resolution GUS run. As noted earlier, there are two possible events.  The first, on Monday, produces about 1-2 inches and the late Wednesday/early Thursday event has a little more (but more uncertain).
The more skillful European Center ensemble system, with more ensemble members, higher resolution, and better physics is also showing snow in two steps (see below).  About a quarter of its members suggest no snow, while the rest produce generally light amount (.5-1.5 inches).  The second event is much larger with 3-6 inches for several members.  

The National Weather Service also has a short-range ensemble system using two models (NMM and WRF) and higher resolution (about 16 km).   This system shows a mixture of solutions for the first potential snowstorm, with a lot of uncertainty and about a quarter of runs showing very little snow.

So what is my take away from all this?    There is uncertainty, with about 25% of the forecasts showing virtually no snow.  Roughly 75% of the model runs are predicting an inch or two at Seattle-Tacoma Airport (452 ft).    The temperatures on Monday are colder than we have had, but they are still on the margin near sea level and the water.   So expect a substantial gradient with elevation (more on the hills).  

To get a better spatial view of the snowfall, lets look at the UW WRF, which is driven by the NWS GFS operational model.   Since the GFS solution is close to the mean of the ensemble and seems to have a representative solution, the UW ultra-high resolution view should be useful.

Here are the 24h snow totals from the UW 1.3 km domain (very high resolution in the weather business).  First the 24h ending 4 AM Monday.  Scattered snow showers, with heaviest lowland values south and southeast of the Olympics, which is not unusual with relatively warm snow events and southerly/southeasterly flow.  Not much in Seattle--thus the Seahawks game should be fine.  Some snow over SW Washington.

But the next 24 h, ending 4AM Tuesday is another story. The Puget Sound area gets a half to a few inches, with north Seattle to Everett getting more than the south Sound.  Less near the water.

 The 9-km European Center snow total through Monday evening shows loads of snow over the mountains (12-15 inches) and zero to a few inches over the lowlands (with much less near the water).

The bottom line of all this?  Nothing is certain.  But it appears probable (say at the 75% level) that much of the lowlands away from the water will see some light snow on Monday, particularly above a few hundred feet and over north Seattle and southwest of the Olympics.  

We will leave the Wednesday event for another blog.

Thursday, December 1, 2016

Significant Cold/Snow Event Next Week

Will update about snow at 2 PM Saturday

As we approach the weekend, it is becoming increasingly clear that a major cold/snow event will occur next week, but with substantial uncertainty about lowland snow.  One thing is certain: it will be an extraordinary week for snow in the mountains, and everyone who enjoys mountain snow recreation should get ready for a wonderful period of light, bountiful snow.

Right now the snowpack is still a bit below normal over about half of the region, with near normal snowpack (green colors below) and crazy above-normal snowpack in a few areas (the Olympics are over 250% of normal!)  By the end of next week the entire mountain region should be above normal.

Early next week will be the coldest period since last January, with temperatures dropping into the 20s over western Washington..... so drain you hoses and protect vulnerable plants.

The models, both high-resolution and ensembles, are in agreement that we will have a much colder period early next week.  The NWS GEFS ensemble system (21 members in gray, ensemble average black, GFS high-resolution member-blue), show unanimity of solution towards colder temperatures in Seattle early next week (highs in mid-30s, lows in the mid-20s), with modest warming at the end of the week (see plot below)
The European Center ensemble has the same idea.

During the next week, the cold and snow will occur in roughly three acts.  The first act, as shown by the low-level temperatures (colors), winds, and sea level pressure at 4 AM Sunday,  will encompass a strong cold front that will move southward across our region in Sunday, bringing much colder air (see below).  By later on Sunday, the air above our region will be cold enough for snow to reach near sea level.

In the second act, which will take place on Monday, cold air will extend over Washington and Oregon with frigid conditions to the east of the crest of the Rockies.  A low center will form over western Washington (map at 4 PM Monday shown below).  How the low develops and positions itself will determine how much snow falls in the lowlands.

In the third, and perhaps the most dramatic act, a weak occluded system approaches the coast bringing warming temperatures aloft and precipitation.  With cold air in place over western Washington, the precipitation will start as snow at low levels, and perhaps a lot of snow.

Now lets look at the ensemble snow forecasts.  The NOAA/NWS GEFS ensemble show a lot of variability (and thus uncertainty) with the forecast, with the average of all the forecasts (black line) and the high-resolution forecast (blue line) looking very similar.  There are a few inches on Monday, a gap, and then a lot more snow later Wednesday and Thursday.  All the models show at least 3 inches by the end of the period.

The European Center ensemble shows a similar picture:  light snow Sunday/Monday, but heavier amounts Wednesday/Thursday.

Now, to see the spatial distribution better, lets turn to ONE forecast, the UW high-resolution prediction based on the NOAA/NWS global model.  For the 24-h period ending 4 PM Monday (below), there is plenty of snow over the region, some of which extends to ground level over Puget Sound an SW Washington.
But the 24h snowfall ending 4 AM Thursday really is impressive...a foot over the mountains and several inches over the lowlands.

So at this point, much colder temperatures is virtually guaranteed Monday through Wednesday, and there is consensus among the ensembles that we will get at least some light snow at low levels.  The warm road temperatures will help to mitigate the impacts on highways in contact with the soil.

Tuesday, November 29, 2016

Will Snow Hit the Puget Sound Lowlands in a Few Days?

Some of the media have already started to talk about it: the possibility for much colder temperatures and lowland snow during the Sunday through Tuesday period.

Let's analyze the possibilities, making use of the most powerful probabilistic forecasting tools at our disposal.  We will attempt to avoid the problems experienced during the October 15th storm by highlighting the forecast uncertainties and the use of ensembles.

Snow Forecasting is HARD

Let me begin by noting a key point:  it is difficult to get lowland snow around Puget Sound because the Pacific Ocean and Puget Sound are relatively warm (around 50F).  It is easy for us to be mild and wet and frequently we are cool and dry.  But to be cool and wet is very hard, demanding a rare configuration of pressure and winds.  As a result,  snow is the most difficult forecast problem in our area.

As I have mentioned frequently, all forecasts should be probabilistic and the most potent tool to create probabilities and to determine uncertainty is through the use of ensemble forecasts--running weather models many times with different initial states and model physics (e.g., moist processes).

So let's look at some ensembles!  We can start with the NOAA/NWS GEFs ensemble of 21 members (relatively coarse 35 km grid spacing) for snow over the next week (see below).  Dates/times are in UTC/Z, the average of all the ensemble members (the ensemble mean) is shown in dark black, the individual ensemble members are in light gray, and the blue line is single high-resolution (13 km)  GFS forecast.

Note that the ensemble mean shows about 1.5 inches by 4 AM Monday (12Z, 5 December) and there is considerable spread (from 6 inches to 0).   Most members produce 1-4 inches.  The high-resolution run is very different, with no snow until later in the week.

Next, lets examine the snow forecast of the best global ensemble system in the world (the European Center) using the wonderful WeatherBell web site. The European Center (EC) ensemble is larger (51 members) and higher resolution (25 km) than the US version.  The graphic below will take some getting used to.

The top panel shows the accumulated snowfall for each ensemble member for Seattle. Note that most members show snow starting on Sunday or Monday (Dec 4th or 5th) and several members show much more snow later in the week (8th or 9th).

The bottom panel shows the ensemble mean (green) and the high resolution snow forecast (deterministic, blue) for Seattle.   The ensemble average shows some very light snow on Sunday and Monday (maybe a half inch), but more more later in the week (with total accumulation of a few inches).  The much higher resolution deterministic run (single 9-km grid spacing run) shows more snow (3-4 inches).

What about temperature in Seattle?  Here are the ensemble forecasts for high and low temperatures from the EC model.  The single high resolution forecast is black, the ensemble mean is green, and the range of the forecasts are shown by the blue "whiskers".   The rectangles encompass the 50% of forecasts closest to the mean.

The big story....a major cool down, with daily highs dropping from 46-47F to around 40 F on Monday.  Even lower after that.  After around 200 h, the uncertainty becomes very, very large.

So based on the ensembles, I feel pretty confident to forecast much cooler temperatures next week.  And the ensembles suggest a good chance of getting at least a dusting early next week, with more later.

Finally, with all that I have said about uncertainty, lets look the latest UW high-resolution run, which is driven by the high-resolution NOAA/NWS GFS model.

On Sunday at 1 AM, a very strong Pacific cold front is pushing southward over NW Oregon, with cold temperatures behind.

By 4 AM Monday, cold air (blue/purple/white colors) had spread over our region.

By 1 PM Monday, a low-center had formed just off of the SW WA coast-- this is getting close to a typical snow pattern for Puget Sound.

The 24h total now forecasts ending 4 AM Sunday indicates LOTS of snow over the WA Cascades (skier will rejoice)
The subsequent 24 h brings lowland snow, mainly over SW WA and western Oregon.  But the uncertainty is very large, so be prepared for the details to change.

Bottom line of the above:  there is a high probability that we will see substantially colder temperatures over the region starting Sunday.   It is probable but not certain that there will be some light snow at lower elevation on Monday (with the potential for more in some locations).   There is a good chance of heavier snow (a few inches) later in the week.  At this point, the ensemble don't indicate a major regional low level snow.    The mountains will get large amounts of snow, including the lower passes.