Flood Day

It was kind of surreal for me this morning....I biked to work absolutely dry and in warm conditions, while ten miles to the south there was moderate rain and all hell was breaking loose in the mountains. One of the most profound rain shadows I have experienced in a while (see the radar). On the radar this morning you also see how the focus of the heaviest rain has moved east and south of Seattle since last night. And NO rain in an east-west band from Seattle to Lynnwood stretching west towards the Olympics. You don't have to go too far east to pick up heavy rain--North Bend is being pummelled. The computer models have consistently showed the development of this rain shadow and is a prime example of how for the weather prediction enterprise has come in the last ten years. Forecasting rain shadows is FAR easier than predicting snow!

An interesting graphic based on the radar I have never shown you is the storm-total analysis. Basically, one can relate the radar signal scattering off the rain to precipitation rate, and then add that up over time. So weather radar can be used as a rain gauge...although an imperfect one.

I have attached the storm total rain for the past three days. Remember, there is major blockage of the beams by the Olympics and Cascades...so you are not seeing the whole picture. But what you can see is fairly daunting. The is a broad region of greater than 6 inches (yellows) surrounding the rain shadow and substantial areas of ten inches or more (reds) on the windward slopes of the Cascades. We cannot see the heavy rains on the SW side of the Olympics because there is no radar coverage there (another reason why our congressional delegation needs to secure the resources to acquire a coastal radar! With all the infrastructure spending being discussed this essential piece of warming and prediction infrastructure should be a high priority. If we can spend 7 million for solar panels on top of Qwest field, surely we can afford the radar which would greatly improve weather prediction and warnings!).


Both Stevens and Snoqualmie passes have gotten more than 7 inches so far. North Bend has received around 5 inches. And we are only half-way through this event.

The hydrological models are going for a very major event on many of our local rivers, with extensive flooding to be expected. As I noted in my last message, the NWS has an excellent web site with detailed river flow information. I should mention that Chapter 3 of my book describes the nature of NW floods and reviews major floods of the past.

Want good news?...the weather models are going for a dry day on Friday, some light showers over the weekend, and an extended dry period next week. You will think you are in California.

Read more

Heavy rains

The rain rate has really picked up around the area..take a look at the latest radar (9:33 PM)...lots of yellow (heavy rain). You can see the rainshadow over central Puget Sound--but it has really attenuated. Another rainshadow east is of Vancouver Is. There is heavy precipitation over the mountains...but the radar beam is blocked so you can't see it.

The infrared satellite image shows the pineapple express plume of moisture stretching far back into the Pacific. The next 24-h will bring heavy rains to the mountains..with the moisture plume slowly moving south.

A number of mountain sites have already gotten 2-4 inches. And this is just beginning.

The NWS has a nice hydrology site for those who want to see current river levels and what their models are predicting:

http://ahps2.wrh.noaa.gov/ahps2/index.php?wfo=sew

Or check out the UW Hydrology page:
http://hydromet.atmos.washington.edu/index.html

Both are going for major flooding....

Read more

Winds

The winds have really picked up forced by a strong north-south pressure gradient and the mixing of high momentum air from aloft. Take a look at my favorite plot...the seattle profiler (I know its not pretty...all I got!). Temperatures above us have warmed immensely....14C (57F) right above the surface. Its 52 at my building. And very strong winds from the SW are aloft....the profiler shows 50 knot (58 mph) at 400 m (roughly 1300ft) above the surface. Sustained winds of 35 knots have been observed over the water and 20-25 knots over land (knot is 1.15 mph). Gusts are higher of course. The NWS did forecast strong winds...although not quite this strong. I am sure there will be some power outages.


Legend: height in meters, time in GMT increasing from right to left. Red lines are temperature in C.

Read more

Heavy rain in mountains, but rainshadowed Puget Sound.

It is an interesting observation that the heaviest precipitation in the mountains and worst flooding in area rivers can be associated with relatively dry, balmy weather in Seattle and the central PS lowlands. To get heavy, unusual precipitation in the mountains you need a good upward component along the western mountain slopes. If the winds are mainly southerly the flow is mainly parallel to the mountains (except for the southern Olympics) and you don't get big-time orographic rainfall. But westerly winds are another story....strong westerly winds, particularly with a warm subtropical origin, can give massive amounts of precipitation...often rain...that can add up to 10-20 inches over a few days. And that is what is going to happen over the next 48h hrs. The irony of this, is that westerly winds produce a nice rain shadow east of the Olympics that can protect Seattle and neighboring regions. That is why Seattle will get wet, but not flooded.
To show you what I mean, take a look at a recent latest radar image (attached). Look how dry it is over central PSound. But if you head east you hit the rain by North Bend.
The precipitation is now mainly over the northern portion of the State, but the current of moist flow will settle south this afternoon. I have attached the model forecast 24-h precipitation for two periods...ending 4 AM tomorrow and Thursday morning. One is struck by two things...the tremendous rainfall in the mountains (reds are 5-10 inches). Clearly, some mountain locations will get 10-15 inches. The other feature is the rainshadow over Puget Sound...particularly today. (Note..the first figure is from a higher resolution simulation). You can also see a rainshadow to the lee of the mountains of Vancouver Island over Georgia Strait. The air coming into the region will be very warm, with freezing levels rising to 6-7 thousand feet. The snowpack can absorb some of the water, but plenty will go into the rivers. That plus the melting of the huge low-level snowpack has led the NWS to put up a flood watch for many of the local rivers. If you live in an area that often floods...you should prepare...now.

Read more

Snow, Global Warming and Flooding

Over the past few days I had several people ask me whether the cold, snowy weather of the past several weeks indicates that global warming is either a fallacy or being stopped by lack of sunspots or some other cause. The answer is our cold spell has absolutely nothing to do with global warming. The "Global" in global warming is the key. The earth is going to warm up due to increasing greenhouse gases like CO2 and methane (and indirectly from increases in water vapor). But not every location will warm up the same amount or at the same rate--with the polar regions warming up the most, the continents more than the oceans, and the eastern oceans less than than the western oceans. We are downstream of the Pacific (an eastern ocean), which controls our weather. So the Pacific Northwest may well be a location where global warming is delayed. Our computer models suggest we will get very significant warming..but not until the second half of the century.
It is also important to note the atmosphere has quite a bit of local and short-period natural variability, which can produce short periods of cooling in a region, even if global warming is occurring.

My second point is about the upcoming heavy rains in the mountain and major warming. The high resolution WRF atmospheric model we run in the department (which I show you quite frequently) indicates the potential for 10-20 inches of rain over the next two days on the windward mountain slopes. We also couple the high resolution weather prediction models and hydrological models of stream flow. The results of the latest simulations are scary, with major flooding on several rivers. In addition to the rains there could be substantial melting of the low-elevation snow pack. More on this tomorrow.

Read more

Very wet and then very dry

As predicted the snow turned to rain last night, although I little later than I and most of the weather community had thought. A Puget Sound convergence zone formed this morning, producing a band of showers across the central Sound...but that has weakened now...leaving showers only on the windward slopes of the Cascades and Olympics. Tonight we will begin a wetter, warmer period...with rain over the lowlands and the lower slopes of the Cascades. The central Sound from Seattle northward will be partially rain shadowed...so there probably won't be any urban flooding. The precipitation over the mountains will be sufficient to cause flooding on some rivers...but I will deal with that later...I have to get ready to teach the next generation of meteorologists!

PS: Later this week a big ridge of high pressure builds over the eastern Pacific and we get a major break...dry, warmer....


Snow Removal Notes

Some of you commented about the use of salt last night by Seattle. As someone in the pro-salt-when-appropriate camp, I can't see why these used it......with temperatures rapidly warming anyway and no ice, but slush. They seems to be spending a lot of time going up and down primary roads...which in this case were melted out rapidly, when the plows where really needed on steep secondary and tertiary roads...yes even the neighborhoods. A quick run of a plow on the slush could really have helped my street...I was trapped last night again..and narrowly missed my neighbor's car.

Read more

Transition is now occurring

Much of the region is transitioning to rain and strong winds from the south at 11 PM...raining in Olympia, Shelton, and Tacoma...and working its way northward. By 1 AM it will be over...as will the steady precipitation...

January 4th is the climatologically the coolest day of the year...its all up from here.....

Read more

Snow is here

As predicted, the snow is here. But right now it looks like it will hold in for several more hours..roughly 10 PM..which means several more inches. One of the most valuable tools in these situations is the profiler at Sand Point in Seattle. The latest graphic is attached. Temperatures have cooled aloft (this is virtual temperature in centigrade, subtract one degree approximately to give the regular temp). 1C--which mean freezing-- was at roughly 300 m and dropping...that is why snow is falling to sea level now. SW winds are increasing aloft in the figure...and that will bring warm air down eventually...but it is going to take several hours to do so.
Take a look at the latest obs (see attached)...cool temps everywhere..even at coastal stations. (Note if you click on the graphics they will expand for easier viewing) Finally, the latest computer forecasts suggest the snow holding to around 10 PM.
The bottom line is that snow could well range from 1-4 inches, perhaps more in favored higher locations. Kitsap will get more.
Very strong SE winds are over SE Washington and near Destruction Is on the coast. As described in my book, this pattern of strong winds is due to the influence of the Olympics.

Read more

Clear Mountains

Take a looks towards the Cascades..there are crystal clear...it seems they are only a few miles away. Why? As the low approaches there is strong offshore flow as air from eastern Washington accelerates westward over the mountains. As the air sinks it dries and moisture on particles evaporates.

Read more

Diabatic snow

Meteorologists have a term for it..."diabatic" effects. Two major examples of this are evaporation and melting...and both can cause cooling. And both may be important later today.
A strong Pacific system is coming onshore this afternoon and evening and during the afternoon precipitation will begin over the region (see satellite picture). You will see that this is NOT a pineapple express type event where the moisture extends into the tropics. We will get wet, but only for 6-12hr this evening and tomorrow am.
The air mass over us is cool and relatively dry, but not quite cool enough for snow as is. But that could and will change in some areas due to the "diabatics." Precipitation nearly always starts as snow aloft around here...even during rain events. Consider what happens when precipitation falls into above-freezing air below. If the air is not saturated then there is evaporation, which is a very powerful and rapid cooling agent. You know how powerful it is when you step out of the shower or get wet outside. But evaporation only lasts a few minutes to a half hour..until the air saturates. The temperature that air cools to when it falls into dry air is called the "wet bulb" temperature...you guessed it...we measure it by putting a wet cloth on the bulb of a thermometer and see what the reading is.
Now if that doesn't cool the air enough to reach the surface then melting can do the job. Snow falling into above-freezing air melts...and that takes energy out of the air...and thus cools it. Melting can be VERY effective when precipitation rates are large..and can drive the snow level to the ground. Melting is often associated with Puget Sound convergence zone snows--with snow where the precip is heavy and rain elsewhere.
Well, now you are diabatic, savvy what about today? Our air is marginal for snow, but is unsaturated. There will be moderate precipitation rates associated with the incoming system, which eventually will cause substantial warming aloft. Later tonight it WILL rain. But there could be 1-3 hr at the initial portion of the event when there could well be snow...and in fact the computer models are suggesting it. (see graphic). Greater threat will be the Kitsap area, SW interior and high elevations...but everyone will probably see at least some wet snow. Right now 4-7 PM is the best bet for the white fun. But no plows probably needed for this...warming and rain will follow (but be ready for a wintry mix for a few hours if you live on a hill)....cliff

Read more

The Wet Warm Up Late Tommorrow

Right now there are a few light showers to the north and on the western slopes of the Cascades, but little else going on (see radar) in the area. But when I took at the latest satellite picture, I was taken aback (see attached)--a huge plume of moisture over the Pacific headed for the West Coast and the Gulf of Alaska...and we will get a piece of that starting tomorrow evening.
The approaching system will bring much warmer temperatures and rain in the lowlands and over the lower slopes of the Cascades. But the initial few hours could very well bring snow to the normally cooler locations...and even wet snow to the lower elevations. Cold air will be in place and falling precipitation will cause cooling due to evaporation in the dry air near the surface and melting of snow into rain. I have attached 24-h snow maps ending 4 AM Monday and Tuesday so you can see this. By the second 24-h time period the lowlands are all rain..but the mountains will get hammered with snow above roughly 3500 ft. Perhaps the ski resorts can catch up on lost business...

PS: Thanks for all your help with the questionnaire...will let you know the results...

Read more

You Can Help Improve Weather Forecasting

WE HAVE ENOUGH NOW...there may be more of these later..thanks everyone!>.cliff


I am working with psychologists at the UW to understand how individuals interpret weather forecasts, and particularly how they deal with uncertainty. How is the best way to describe an uncertain forecast so users get the most valuable information? Below is a link to a short survey that I encourage all of you to try...it will really help us make progress! Thanks, cliff

http://www.atmos.washington.edu/weatherforecasting/usertrust/index.php

Please make sure pop-ups and Java-script are enabled...the software needs them.

Read more

Sun and Showers

Last night's system is through now and cold, relatively dry air has spread over the region. Some of you got some very light snow last night as the last precipitation bands came through and the temps had fallen sufficiently so snow could reach the surface. But the roads are generally fine.

Right now there are some instability showers (produced by cold air over warm water) and they should move into the region...mainly south of Tacoma (see satellite image). The mountains should have a breather after the snow pounding they have "enjoyed." Both the passes have about 70 inches of snow and the snowpack in the central Cascades is getting close to normal and being way below normal for much of the fall.

So a pretty benign day.

Tonight a weak system from off the Pacific will invigorate the snow showers...but they should be light--not more than an inch or so at most. Many of you will get nothing. Below the salt criterion.

Sunday morning will be dry and cold...but a much stronger Pacific frontal system will approach later in the day. For some locations, especially above 500 ft and SE of the Olympics (e.g., Kitsap) may see several inches of snow before warming aloft turns the lowland precipitation to rain all rain. Warmer, low-level locations near the water may turn very quickly and see no accumulations.

The biggest threat next week will be heavy precipitation in the mountains and the potential for river flooding. The NWS is already talking about it. My take is that this does not look like a major pineapple express event and that rainshadowing will spare the urban areas. We should watch the flood-prone rivers though.

Read more

No snowstorm tonight in Seattle

Several of you have asked about a potential snowstorms tonight...particularly since the NWS discussion seems to be threatening one due to an errant "deformation zone." Anyway, I just don't see it. There could be a snowstorm..but that will be over SW Washington and Portland--perhaps extending into the very SE suburbs of Seattle. The latest satellite picture shows the low offshore of Oregon (look for the tight swirl in the infrared satellite picture). This low appears to be somewhat more intense than forecast...but it does seem to be going in well south of us..as the models indicated. The precipitation shield is south of Seattle and should stay that way with the low's trajectory. With heavier precipitation to our south and northerly winds wrapping around the low bringing in cooler air behind...the Portland area and the SW WA corner could see snow (see 24 hr snow predictions ending 4 PM tomorrow afternoon) and perhaps a significant amount. That is certainly true above 500 ft.
The low will move westward tomorrow morning and cooler, drier air will move in behind. But I hate to say it, there is another snow threat tomorrow night as a weak disturbance moves in while we have cold air in place. Will check that out tomorrow...but the city should get the salt ready!

Read more

Snow in the mountains and no major storm here.

Happy New Year!

Today, a modest low center will cross the Olympic Peninsula as it moves eastward across the area (see plot of 6 hr pressure and temp forecast). No windstorm for us. Modest winds (15-30 mph sustained) will be experienced to our south where the large pressure differences are found. South of the low we will first experience a warm front passage this morning, followed by a cold front this afternoon.


Astoria has already experienced the warm front...take a look at the attached plots to see what happens. With the warm front temps increased rapidly (into the 50s!), and the dewpoint (a measure of humidity) jumped as well. Winds shifted from SE to SW and increased in speed.

As discussed yesterday, substantial snow has fallen in the mountains, greatly increasing the threats of avalanches, particularly since the underlying layers are weak. WSDOT has closed I90 for the time being and I imagine they will be completing serious avalanche control before letting people through (it is interesting how Snoqualmie closes so much more than SR-2, I assume because of the close-in steep slopes to I90).

The heaviest precipitation from today's system will be from the central Cascades southward (see the 24h precip plot) and I think the Olympics will be spared the worst. We are talking serious precipitation down there--5 to 10 inches in the mountains--and flooding is a possibility. Regarding lowland snow later tonight and tomorrow morning, the biggest threat is over SW Washington and northern Oregon--but it will be close for us (see latest prediction by the weather model--the WRF-GFS) The temperatures over central PS will be marginal for snow and with the heaviest precipitation to the south, the cooling effects of melting precip from aloft will be correspondingly strongest southward where the intensities will be there. So the salt probably won't be needed tonight in Seattle. But don't be surprised to see some wet snow mixed in...and if you are at higher elevations...there may be a white coating.


PS: Comment on the mayor's new salt plan. This is a positive step to be sure...but the city must not constrain the use of salt to only such large (4 inch) snowfalls. There has been very disruptive events with lesser amounts..where salt could have help tremendously. One is noted in my NW weather book...on Nov 27, 2006 (Pg 72). I remember that event well.. I walked home that night due to all the ice..and only about twoto three inches fell. And we need more snowplows with metal edges. I just got an email from Sea Tac Airport...they have 27 pieces of snow removal equipment..as much as the entire city!

Read more