Saturday, May 30, 2015

Thunderstorms and Convection over the Mountains

During the last week, there have been several days when cumulus clouds were towering over our local mountains, such as the Olympics and the Cascades, while it was sunny and dry over the lowlands.

This is not unusual and is a frequent feature of our summer weather.

Let's talk about it and explore why it happens.  To illustrate this, here are a sequence of visible satellite images from Thursday (May 28th). Starting at 8:40 AM, the mountains are clear, with the white  near their crest being the snow fields.  The extensive white clouds along the coast, the Strait, and southern Sound are stratus/stratocumulus (low clouds).

By 11 AM, you will notice increase white over the mountains...these are developing cumulus clouds.

 By 2 PM, they are much more prominent, with some of them precipitating and developing anvils of ice clouds (fuzzy looking look).

At 4 PM, the clouds are a bit more prominent

The radar image at 4 PM Thursday shows moderate to heavy rain associated with the thunderstorms  over the mountains.  Might have dampened the hikes of a few folks.

We know that some of the cumulus developed into lighting because the lightning detection network showed plenty over the Cascasdes, the Okanogan, the Rockies, and to some degree the Olympics (here is the 24h lightning strikes ending 1 AM Friday).  They are over and immediately downstream of the mountain crests.

We can view the processes from ground, and what better location than Hurricane Ridge in the Olympic range?  Here is a sequence of images showing the process.

So why do thunderstorms preferentially form over higher terrain?  

There are several reasons.   First, cumulus clouds and thunderstorms are often initiated by an area of upward motion.   The atmosphere is frequently what we call conditionally unstable, which means that without vertical motion the air stay put, but if lifted enough, the air becomes buoyant...rising into cumulus towers.

And vertical motion is greatly enhanced over the mountain crests (see figure).  Air tends to rise up heated slopes (something one often experiences when hiking).   With air rushing up the slopes and meeting near the crest, there is substantial upward motion over the crest.  Upward-moving air will tend to cool to saturation (air cools as it rises), producing clouds.  And if the upward motion is sufficient, instability results as the air become buoyant.

There are other reasons that mountain tops are favored around here.  For example, the lowlands are often flooded by cool, dense marine air (not good for thunderstorms), but the mountain crests are above such air.

If there is westerly (from the west) flow aloft, the convection (thunderstorms) are often blown to the eastern side of the mountains (as happened on Thursday).  The eastern slopes of the Cascades have received substantial amounts of precipitation from mountain thunderstorms the last week or so, something that is aiding the water situation in Yakima County and other eastern slope regions.


I will be giving a talk on the future of NW Weather on June 7th at 7 PM in Hansville on the Kitsap Peninsula:

Wednesday, May 27, 2015

A coastal eddy, Eastern WA rain, and early melt-out

Even coastal low clouds can have their subtleties and beauty.   Yesterday morning (Tuesday), visible satellite imagery showed an impressive swirl (or eddy) in the low stratus/stratocumulus clouds off the northern WA coast (see regional and close-up view).

There were several contributors to this beautiful swirl.  There was moderate northerly flow in the lower atmosphere (see sounding at Quillayute, on the north Washington coast), resulting in what is called lee troughing (low pressure) on the lee (downstream) side of the terrain.   There was also a large amount of horizontal wind shear (change of wind over some horizontal distance) in that region, with strong northerly flow offshore and weak winds near the coast (see weather map plot at 8 AM on Tuesday).
There is inherent rotation with wind shear.  If you want to prove this to yourself, imagine putting a pinwheel into a flow with northerly flow on one side and southerly flow on the would spin!  Such spin distorts the clouds into a swirl.

And if you like swirling low pressure, we have had persistent circulations over eastern Washington that have been associated with lots of thunderstorms there the last week.  Here is an infrared image for Tuesday afternoon at 4:15 PM--do you see the swirl of thunderstorms over eastern Washington?

Lots of rain over eastern Washington yesterday and today from this circulation.  Here are the totals for the last 24h.... substantial amounts, particularly along the fire-prone eastern slopes of the Cascades.

But the real wet story may be is the  forecast 72h precipitation total ending 4 AM next Wednesday....a very wet pattern, indeed, particularly west of the Cascade crest.  Stay tuned.

NEWS BULLETIN (courtesy of UW's Mark Albright)

The winter snowpack at Paradise Mt Rainier melted out today (27 May 2015), which is about 6 weeks earlier than normal.  This year experienced the 2nd earliest melt-out of the past 99 years.  The earliest melt-out date occurred 74 years ago on 24 May 1941.  Here are the 5 earliest melt-out dates at Paradise Mt Rainier over the past 99 years from 1917-2015:

1) 24 May  1941
2) 27 May  2015
3) 30 May  2005
4) 1 June   1926
4) 1 June   1934
Announcement:  On June 3, I will be on the Seattle Channel's Civic Cocktail with Governor Inslee.  If you would like to be in the audience, you can get tickets at:  They charge, but you get appetizers with their no-host bar.

Monday, May 25, 2015

June Gloom Starts in May

Sometimes the atmosphere doesn't follow our calendar.   June Gloom, one of the frustrations of life west of the Cascade crest, has arrived early and the results--incessant low clouds--have arrived.    Look outside or view a few of the local web cams to see it in action:



The source of June Gloom is the large pool of low clouds that develop over the eastern Pacific.  Here is the image this morning....the eastern Pacific is FULL of low clouds.  Strangely enough, such extensive clouds occur under high pressure conditions, which develop as the East Pacific high builds and extends northward.

Why does high pressure help low clouds?  One reason is that high pressure is associated with sinking and warming conditions aloft, which forms a low-level inversion that traps moisture at low levels. The vertical sounding (plot of temperature and dew point with height from a balloon-launched radiosonde) at Qullayute, on the WA coast, shows exactly this structure (see below).
Dew point is shown by the blue dashed line and red is temperature.  Height is in pressure (900 is about 3000 ft up, 700 about 10,000 ft).  In the lowest few thousand feet the atmosphere is saturated (low clouds), with inversion (temp increasing with height) above.   Primo conditions for the gloom.

Here is the pressure pattern for 11 AM this can see the high offshore (solid lines are isobars, lines of constant pressure)

In June, we generally have the high parked offshore, lower pressure over land, and an onshore pressure difference that pushes the low clouds to the Cascade crest.  June gloom.   As shown in the satellite image above, their is often a sharp end to the gloom near the crest, so sun is just short drive east on I90.    Yesterday, I did just that with a friend, mountain biking to the UW Manastash Ridge observatory near Ellensburg.  Sun and amazing wildflowers.

We can escape the gloom for a while when easterly flow develops, displacing marine air with dry, continental air.  That happened earlier this week when it warmed into the 70s.    But real relief often has to wait until July, when the high pressure builds northward and the winds aloft become more northerly.

And if it makes you feel any better, June Gloom is a phenomenon that hits the entire West Coast, including our brethren in southern California (see pic).  They always have their hot tubs.

June Gloom at Seal Beach, CA

Friday, May 22, 2015

Lightning Fest Starts the Memorial Day Weekend

Nature is supplying its own fireworks to signal the Memorial Day weekend.   For the past several days, large numbers of thunderstorms have moved westward across eastern Oregon and Washington, while others have developed over the Cascades and Olympics.

To illustrate, here is summary of the lightning strikes for Thursday.  Eastern Oregon, Idaho, and the Washington Cascades were covered by lightning.

A particularly strong thunderstorm cell hit Yakima Thursday evening, which received about an inch of rain within an hour.  The radar at 7:50 PM Thursday shows the heavy rain (red colors)

And localized flooding was observed around the area.

A lot more action occurred today (Friday), with a squall line of thunderstorms moving across eastern Washington, with heavy rain and gusty winds.   The radar at 7 PM this evening shows the action (red is the heaviest rain or hail):

And, of course, this heavy rain was associated with lots of lightning.   We are fortunate that all this is happening early in the summer, with the ground moist enough so that the lightning is not initiating fires.  The story will be different in two months.

The configuration of unusually persistent easterly flow and lots of thunderstorm east of the Cascade crest, which has held in place over the past week or two, may be the result of the strong El Nino that is developing.   My colleague, Nick Bond, who is also State Climatologist, examined the relationship between El Nino and regional precipitation (see the figure below, which shows the correlation between El Nino and precipitation for the month of May).  Eastern WA and Oregon are wetter than normal, while western WA is drier than normal--EXACTLY what has happened.

The million dollar question all of you are asking is what this weekend will be like.  Will the Folklife Festival be a rain out?  Will you get wet hiking in the Cascades?

Actually, it doesn't look that bad.  Here is the total precipitation for the next 72 hr starting 5 AM Saturday.  Relatively dry for most of western Oregon and Washington, with the major exception being light showers in the Cascades.  Much wetter over the Rockies and Nevada.

We do have marine air pushing in west of the Cascades, so no heat waves are ahead.  Highs in the mid-60s and considerable clouds over the lowlands.

Wednesday, May 20, 2015

Drought Misinformation

There have been a number of stories in the press and on the web about water issues this summer in the Northwest; some are reasonable but a disturbing number are exaggerating and distorting the situation, predicting a catastrophic wildfire season and billions of dollars in crop loses.  And some folks in the political arena are playing a bit lose with the facts as well.

You want some examples?  Crosscut calls out a SUPERDROUGHT.

This over-the-top piece does not stop with this year:

"The 2015 superdrought will likely be a harrowing experience for many state citizens, and could be followed by another, similarly terrible year.'

 Others talk about a TINDERBOMB due to low snowpack

Or in the Oregonian:

Will there be a superdrought this summer over the Pacific Northwest?   Will low snowpack bring huge wildfires?  Let's get beyond the hype and look at data and forecasts.

First, let's be clear.  There is no drought over the Pacific Northwest and by that I mean no precipitation drought.  For the last six months, here is the % of normal precipitation over the western U.S.  Most of Washington State and the eastern half of Oregon received 90-110% of normal.  The rest was at least 70%.   California and Nevada is a different story, with real drought.   You can see why William Shatner wants our water.

What about conditions on the ground?  One of the most considered measures of drought is the Palmer Drought Severity Index.   Here are the latest values.  Most of Washington State is near normal and eastern Oregon is wetter than normal!

What about the the NOAA crop moisture index?  Normal.
And the last two weeks have been unusually wet  (green and blue) over the eastern side of the Cascades.

And this evening's WRF precipitation forecast for the next 72h looks like something that would inspire Noah to start building his ark:

While the latest forecast of total rain over the next 9 days from the National Weather Service's GFS model would bring happiness to any Northwest duck.

And keep in mind that Columbia River flows, draining off the higher terrain of British Columbia, which had a better snowpack AND more precipitation, will only be slightly below normal.   Plenty of water for hydropower, irrigation from the Columbia, and fish in the river.  And because of substantial reservoirs and wise stewardship, nearly all of the populated areas of the region will have sufficient water.

So this talk about the Northwest being in a drought, super or not, is really problematic.    We have had near normal precipitation, our soil moisture is near normal, our reservoirs are full, the Columbia is near normal, and the drier parts of the state are being hit by wetter than normal conditions during late spring.    

We DO have a lack of snowpack because of the warm temperatures last winter that resulted in rain but little snow in the mountains.    So maybe it is ok to talk of a snow drought, but that is a bit confusing to some perhaps.  This snow drought will be problematic for fish on some rivers (e.g., draining the Olympics), but there is little we can do for them.  Nature is cruel sometime.

So how worried should we be about wild fires due to the low snowpack?   And what about other factors?

I have talked to my colleagues in the wildfire business and most of them feel the snowpack issue is overblown.   They tell me that the key determiner of whether there will be extensive fires in the summer are:
  1. Whether we are much warmer than normal in July and August
  2. Whether there are a lot of thunderstorms to initiate fires.
  3. Whether we are much drier than normal in July and August

Regarding the last bullet, we are normally dry during our summer, but rain can moisten the "fuels" and lessen the fire risk.

There is good reason to suspect snowpack has only a small impact on wildfire frequency and intensity.  First, the differences in snowpack between good and bad years is pretty minimal by the time of the main fire season, which is roughly from mid-summer to early fall.   Second, differences in snowpack early in the season between good and bad years is mainly at higher elevations, while most of the significant fires, and certainly the ones the affect most people, are at low elevations.   To show this, here are the areas of the major fires of the past 15 years.  Most are on the lower slopes of terrain east of the Cascade crest or in relatively low (and snowfree) coastal mountains.

There are a few papers correlating lack of snowpack with increased wildfire threat (e.g., Westerling et al. 2006), but I believe they have a fatal flaw.   Their low snowpack years were also precipitation drought years, which is NOT TRUE of this year.

So if snowpack won't be a major issue, what about temperature, precipitation, and thunderstorms?

Our best guidance for the seasonal predictions is the NOAA Climate Forecast System (CFS) model.   For temperature, its forecasts have been clear and robust.   Warmer than normal (see below) over our region.   That would enhance wildfire risk.
 Precipitation is a different story.  Nearly normal over Washington, but a bit wetter over eastern Oregon.
Predicted soil moisture?  Generally, near normal except for eastern Oregon, where it would be wetter than normal.
But then there is the thunderstorm issue...and folks, THAT is the wildcard.  The enhanced precipitation over the Rockies suggests more thunderstorms over that region and perhaps into eastern Oregon.  Might those thunderstorms get to up and start fires?   On the other hand, the CFS model predicts enhanced ridging over our region, which would lessen thunderstorms.

The bottom line in all this, is that a more nuanced view of the current "snow drought" situation is called for and the prediction of more wildfires is no slam dunk.  Talk of superdroughts is really nonsense.  

There is certainly no reason to expect fewer than normal wildfires and the warm temperatures will modest increase wildfires potential.  But the lack of snowpack is a minor issue.   Much is riding on the frequency of thunderstorms, and there is considerable uncertainty how that will play out.

As I have said many times, this year has many of the characteristics of 2070, when global warming will be significant.  That is why the work of Governor Inslee and the Department of Ecology to take all possible steps to adapt to a low-snowpack year is appropriate and welcome.  We need to figure out how to deal with a lack of snowpack, while preserving our agriculture, drinking water, and as much as possible of natural fauna/flora (e.g., salmon).  But hyping and exaggerating the situation is not helpful.

And for those in western prepared for thunderstorms later tomorrow and into the early morning hours of Friday.

Monday, May 18, 2015

Why last winter was so unusual


So unique that it should give forecasters pause.







So why are these unusual events happening?


Random, natural variability is more likely as the cause.




Saturday, May 16, 2015

Weather Role Reversal

It is always fun when the normal weather patterns reverse.

This week for instance, the normal precipitation pattern reversed over Washington State, with eastern Washington getting hit hard, while only light showers damped the west.  Here is the percentage of normal precipitation over the past week.  Pretty dramatic!  Some locations around Yakima and Tri-Cities had over 800% of normal rain.  We are talking about 25% of their ANNUAL precipitation in one day!  The Olympics, the home of the rain forests?  Bone dry.

This role reversal was associated with a reversal of the wind direction and storm locations, with a low center moving northward east of the Cascades and easterly flow causing upslope over the eastern slopes of the Cascades.

But role reversals have been occurring over a larger scale as well.  For much of the winter and early spring, the eastern U.S. has been anomalously cold, while the western U.S. has been unusually warm (the reason for the famous snow drought that has caused Governor Inslee to make a statewide drought declaration).  But what about last week?

The roles have reversed.  As shown by the graphic below (max temps, for the last week), it has been warmer than normal over the northeast and colder than normal in the West.

Why?  Because the large scale atmospheric circulation pattern has changed dramatically.  For much of the last year there has been enhanced ridging (high pressure) in the west and troughing (low pressure) in the east.  Like this.

But last week the pattern reversed, with a broad trough over the western U.S.  Big difference!

This change of circulations has also produced a role reversal along the West Coast.  For most of the winter and early spring, the Northwest has been relatively wet while California has been dry.  Oregon has been much drier than Washington.   But with the deep West Coast trough, the jet stream has been heading into California and they have been much wetter than normal.  Heavy rain hit San Diego and the folks freaked out there (see graphic)

And this atmospheric pattern will reverse the situation between us and Oregon, which is going to get hit VERY hard with moderate to heavy precipitation during the next week.  Here is the forecast precipitation for the next 72h.  Lost of rain from eastern Oregon into the Willamette, as well as northern California.  Oregon ducks will be happy.

The next 72h?   Lots of rain over eastern Oregon, but spreading into SW Washington.  California gets plenty.  We are talking about substantial drought relief for many, considering such sustained heavy precipitation is unusual for this time of the year in the West.

Why is this happening?  Some wags might smirk that it results from all the drought declarations, which are like red flags in front of the meteorological weather gods.  But there is another possibility: an extraordinarily resurgent El Nino that is revving up in the tropical Pacific in a way that is highly unusual for this time of the year.

Sunday Morning Update:  The eastern slopes of the north Cascades have been hit by substantial rain, with some locations getting over an inch of rain during the last day.  Here aer the 24h numbers ending 9 AM Sunday.  There are just the areas that often experience wildfires.
