The Subtleties of Warmth

For many in western Washington and Oregon, today was the warmest day of the past seven months... this was true at Sea-Tac Airport, where temperatures rose to 77F this afternoon (see plot of temperatures since 30 was the warmest). 
The plot of high temperatures today (Tuesday) shows some interesting contrasts:  mid-50s near the water.  70s over land around Puget Sound, and 80s near the coast and in the warm Willamette Valley. 

Why so warm along the coast?  Because there was strong easterly flow aloft.  This is shown by a plot of winds over time above Sea-Tac Airport (red are temperatures, blue are wind barbs, time is in UTC/GMT with 24/18 indicating 11 AM this morning, the y-axis is height in pressure with 850 being about 5000 ft)  Strong easterly flow developed in the lower atmosphere last night and this morning.
Easterly flow pushes the cool marine influence offshore and produces downslope warming on the west side of terrain (like the Cascades and coastal mountains).  The coast enjoys the downslope warming benefits of two mountain barriers and thus gets very warm.

Another interesting oddity is the minimum temperatures this morning.  72F near Black Diamond in the eastern foothills of the Cascades and 60s nearby.  Why so warm?  Because of downslope warming off of nearby terrain!

 Tomorrow will be warm, but not as warm as today.  Why?   Because the easterly flow has weakened and remain weaker on Wednesday.  The sea level pressure forecast for 2 PM tomorrow shows a thermal trough of low pressure extending into southwest Washington and less east-west pressure difference than today.  Such a pattern tends to give us more northerly than easterly flow over the Sound...thus, a cooler pattern.

Thursday at the same time, the trough has pushed northward to Bellingham and the flow is more easterly over the Cascades--thus I expect warmer conditions (upper 70s that afternoon around Seattle).  But the end of warmth will be upon us soon.   With the approach of a low offshore, a ridge of high pressure builds along the coast and a strong coastal pressure gradient is building.

The result will be an onshore push of marine air late Thursday/early Friday, with considerable cooling on Friday. 

So enjoy the next two days....precipitation and cooler temperatures are in store for the weekend.

Remarkably Good and Bad Snowpacks at Your Doorstep

We're now deep in April and near the time of maximum upper-elevation snowpack in the interior western United States.  The contrast in snowpack characteristics from north to south through our region could not be more striking.  If we look at SNOTEL stations at elevations at or above 8000 feet, there are many in Nevada, Utah, and southwest Colorado that have already lost their snowcover.  In contrast, to the north, there are some in the Greater Yellowstone area with more than 100 inches.  The maximum is at the Fisher Creek Snotel, which is just north of Cooke City, Montana. 

Snowpack water equivalent tells a pretty similar tale.  There's a total loss of snowcover at many sites in Nevada, Utah, and southwest Colorado.  The snowpack is phat to the north.   Sites above 50 inches of water are Fisher Creek noted previously and Grand Targhee in the Tetons.  Northern Colorado and south-central Wyoming have some decent numbers as well, including the Tower and Medicine Bow Snotels, both at 10,500 feet, with over 40 inches. 

We can also identify stations that are at their all-time lowest and highest snowpack water equivalents in the period of record.  This focuses on sites with at least 20 years of data.  Sites at record low snowpack can be found in the Ruby Mountains of Nevada, the southern Wasatch, the plateaus of southern and central Utah, the southern Wind River Range, and the mountains of southwest Colorado.  In contrast, sites at all-time highs can be found in the Dear Lodge Mountains of southwest Montana, the Absaroka Range of Montana and Wyoming, and the Big Horn Mountains of Wyoming. 

Finally, we can look at the percentile rank for these sites, with 50 indicating that the site is right in the middle of past years, values above 75 indicating that it is in the top 25% of past years, and values less than 25 indicating that it is in the bottom 25% of past years.  The contrast in snowpack as one moves northward couldn't be more striking, especially in the Wyoming and Wind River Ranges where one moves from a snowpack that is near or below median to solidly above median as one moves northward. 

It is not uncommon for the southwest to be dry when the northwest is wet (and vice versa), a pattern sometimes described as a precipitation dipole.  This dipole is in part related to ENSO, with La Nina favoring dry years and El Nino favoring west years in the southwest.  However, the contrast this year is especially abrupt and striking. 

If you are a skier, the dwindling Wasatch snowpack may be a bit depressing, especially as it becomes increasingly snirty this week.  On the other hand, you are not far from a remarkably deep snowpack.  Perhaps you should be thinking about a road trip. 


ACSPO SSTs constructed from AVHRR, MODIS and VIIRS data from various overpasses at Guam on 18 April 2018 (Click to enlarge)

Sea Surface Temperatures (SSTs) produced from the Advanced Clear-Sky Processor for Oceans (ACSPO) are now being created in real time at the National Weather Service Forecast Office on Guam (where the National Weather Service day begins). The algorithm is applied to data broadcast from polar orbiter satellites and received at the Direct Broadcast antenna sited at the forecast office.  Because there are so many polar orbiters broadcasting data — NOAA-18, NOAA-19, Metop-A, Metop-B, Suomi-NPP, Terra, Aqua — cloudy pixels on one pass are typically filled in with data from a subsequent pass.  When ACSPO software for NOAA-20 is available, data from that satellite will be incorporated as well.  The result is a very highly calibrated, accurate depiction of high spatial resolution tropical Pacific SSTs.  A composite created every 12 hours from the imagery is also available at the forecast office.


Interesting pattern of contrails off the coast of Southern California

As pointed out by NWS San Diego, an interesting pattern of contrails formed off the coast on 23 April 2018. A comparison of GOES-16 (GOES-East) “Red” Visible (0.64 µm), Near-Infrared “Cirrus” (1.37 µm) and “Clean” Infrared Window (10.3 µm) images (below) showed signatures during the daylight hours — with Visible images revealing contrail shadows cast upon the low-altitude cloud tops at 0142 and 0147 UTC — and an Infrared signature persisting after sunset. These contrails were likely caused by military aircraft performing training exercises, since chaff was seen with radar on the previous day.


GOES-16 “Red” Visible (0.64 µm, left), Near-Infrared “Cirrus” (1.37 µm, center) and “Clean” Infrared Window (10.3 µm, right) images [click to play animation | MP4]

A better post-sunset signature was seen on a NOAA-15 Infrared Window image at 0212 UTC (below). A comparison with the corresponding GOES-16 “Clean” Infrared Window image displayed a significant northwestward GOES-16 displacement due to parallax — and the 1.1 km spatial resolution of AVHRR data resulted in a clearer contrail signature.

NOAA-15 AVHRR Infrared Window (10.8 µm) and GOES-16 ABI

NOAA-15 AVHRR Infrared Window (10.8 µm) and GOES-16 ABI “Clean” Infrared Window (10.3 µm) images [click to enlarge]

The contrails could also be followed after sunset using GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) imagery (below).

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images [click to play animation | MP4]

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images [click to play animation | MP4]

The GOES-16 Water Vapor weighting function plots (below) displayed a bi-modal distribution for all 3 spectral bands, with peaks near 300 hPa and 500 hPa. The absence of a contrail signature on the 6.2 µm imagery suggests that these features were located closer to the 500 hPa pressure level.

GOES-16 Water Vapor weighting functions, calculated using rawinsonde data from San Diego CA [click to enlarge]

GOES-16 Water Vapor weighting functions, calculated using rawinsonde data from San Diego CA [click to enlarge]

The Upcoming Heat Wave in the Pacific Northwest

During most years there is what I call a mid-spring heat wave, typically between mid-April through mid-May.

And we are going enjoy such a heat wave this week.  But why a mid-spring heat wave?

By mid-April, our sun is strong and the days are long....keep in mind that the sun's strength today is the same as August 20th.

But to get the heat we have to turn off our natural air conditioning...onshore flow off the cool Pacific.  And to get real warmth, some nice downslope flow down the western slopes of the Cascades, allowing air to warm by compression.

And there is a way to do this...have high pressure build to the east of the Cascades....and that is exactly what will happen this week.

The fact we often get heat waves in April and May is obvious by looking at a climatological plot of extreme daily temperatures at Sea-Tac Airport (yellow line below).  Temperatures have zoomed up to 80 in April, 80s (even low 90s) during May.

At the UW we run a high-resolution ensemble of 15 forecasts to get a handle on the uncertainties in the forecast.    Here is prediction for surface air temperature from last night's run.  The black is the ensemble average...normally a very good forecast.   Nearly right-on for today (high got to 59F).  Tomorrow's model forecast bringa Sea-Tac to 70F...roughly a ten-degree hike.  And 75F on Tuesday.

Want more assurance of the warmth?  Here is the latest output of the National Weather Service Short-Range Ensemble System (SREF). 70F tomorrow, 75F on Tuesday and a bit warmer on Wednesday, although you will notice that there is increasing uncertainty (the forecasts vary more).

 You really want to enjoy the warmth before it happens?  Let me show you the surface temperature forecast maps from the UW system.

First, the forecast for 5 PM tomorrow (Monday).  68-72F in much of western Washington.  72-76F around Portland and over the western foothills of the Cascades.

Tuesday at 5 PM.  Amazing. Upper 70s from Seattle to Portland.  Even along the coast, thanks to the offshore flow.

Wednesday at 5 PM.  A bit cooler over Seattle, but much warmer around the Columbia Basin of eastern Washington.  This is a sign that the high pressure is moving eastward.

Enjoy the warmth...after a cool, wet few weeks it will be nice to experience the spring-time lushness of the Northwest under sunny skies and perfect temperatures. 

There is one problem, though....the pollen count is high right now and according to will get higher with all this warmth (see below).   Sorry...

Peak Snowpack, Emerging Snirt

Based on the forecasts I'm seeing right now, there's a pretty good chance we're at at the apex of the seasonal upper-elevation snowpack right now.

The latest from the Snowbird SNOTEL shows a snowpack water equivalent of 30.1 inches (green line), which is 71% of median for the date and 70% of the peak median.  Those are close because the average time of peak median snowpack is about right now. 

Forecasts for the next 10 days show loaded dice for warm, dry conditions.  You know, the stuff we can't seem to shake.  There's always hope that a weak system or two can generate something, or that the forecasts are simply wrong, but the 6-10 day outlook is pretty telling. 

Thanks to the dust storm earlier this week, snirt is emerging pretty quickly and was visible in many areas this morning.  The photo below is of Lake Peak in upper White Pine. 

The dust is co-mingled with the melt-freeze crust that developed prior to the Tax Day snowfall. 

Sadly, wind and warmth are quickly laying waste to that snowfall and dust is emerging on the top of the snowpack.  This is the basic problem with dust.  The storms are most prevalent in spring, they put dust near the surface of the snow, and it quickly emerges once the snow starts to melt.  In this instance, most of the dust is from a local source in the Cedar Valley west of Utah Lake, although there may have been some more remote sources, such as the Sevier lake bed, contributing later in the day on Monday. 

Turns?  I'm not sure what to call the conditions above 10,000 feet.  Settled powder if generous.  It didn't ski too bad above that level. 

Our future is quite clear in the photo below.  Look how dark that dust is!  It's going to be really really ugly in a few days. 

My advice: Ski this weekend if you can.  After that, it's going to be Snirty Dancing. 

The Connection Between Heavy Rainfall in Kauai and Washington State Last Weekend

Last weekend brought catastrophic rain to Kauai, resulting in floods and landslides.  And last Saturday, portions of Washington State were inundated with heavy rain that brought localized flooding.

The interesting thing:  they were directly connected.  Let me explain.

Kauai by any measure is a wet place. But it is also a place of great precipitation contrasts. These contrasts are directly related to the terrain, with the two highest peaks being Kawaikini (5243 ft) and Mt. Waialeale (about 5150 ft.).   Moist tropical air ascending this terrain results in huge amounts of precipitation.

The annual precipitation illustrates this (below), with much of the higher terrain soaked by over 150 inches a year. In fact, the heaviest average annual precipitation in the U.S. occurs at Mt. Waialeale with 460 inches a year-- much more than our Olympics, which enjoy the record for the highest total in the rest of the U.S. (about 150-180 inches a year on the windward slopes).  The northern/northeastern side of Hawaii tends to have heavy precipitation since it faces the incoming trade winds, while the southeastern side (e.g, Poipu) is relatively arid, with only 20-40 inches a year.  Most vacationers head to the south side of the island for obvious reasons.

Last weekend there were unimaginable amounts of precipitation hitting Kauai. Below are the 48-h totals for the 48-hour period ending at 6 PM HST on April 15, 2018 provided by the National Weather Service. All values are in inches.
Wainiha and Hanalei on the north side got huge amounts (28-32 inches). No wonder there was massive flooding. But Kalaheo on the southern side only had 1.55 inches (which is still a significant amount of rain)

Wainiha : 32.35 
Hanalei : 28.41 
Mount Waialeale : 22.34 
Princeville Airport : 14.60 
Kilohana : 13.19 
North Wailua Ditch : 10.62 
Kapahi : 10.12 
Wailua : 8.21 
Lihue Variety Station : 3.36 
Anahola : 3.20 
 Lihue Airport : 1.92 
 Kalaheo : 1.55 

A map of the 24h amount ending 9 AM HST April 15th shows the amazing contrasts.  Keep this in mind when you consider a vacation in Kauai.

Back to the connection with Washington precipitation on Saturday.   As noted in an earlier blog, an atmospheric river brought heavy precipitation to our area that day, with up to 2 inches in Seattle and 3-6 inches in the mountains.

It turns out that there was a direct connection between the plume of moisture hitting Washington State and the moisture inundating Kauai.   The UW model forecast for the vertical total of precipitation at 5 AM Saturday (PDT) shows this.

And a satellite-based measurement of moisture for the 12h ending 5 AM (PDT) Saturday, shows the connection, with Kauai getting hit by MUCH larger values (orange colors) than we endured.
 And the connection was quite clear in satellite-based water vapor imagery, with the lighter colors indicating more water vapor in the upper troposphere.

But what caused the connection in moisture?  An upper-level (500 hPa) map at 5 PM Friday (0000 UTC 14 April) shows an extensive upper level trough that extended back to Hawaii.  The strong southwesterly flow associated with the trough (where the lines are close together) moved moisture into our area, while the trough extension over Hawaii contributed to vertical motion and precipitation there..

A surface map 12-h later (5 AM our time) shows a front approaching our coast, while a trough (area of low pressure indicated by the dashed lines) extended back towards Hawaii,with moderate northeasterly flow that rose up the northeast slopes of Kauai.

Hawaii and the Northwest have a lot of weather connections..... atmospheric rivers (which we call pineapple expresses) are, of course, one.   But there is a lot more, including the ocean currents which, move water from off our coast to Hawaii.

And those currents will assist a brave UW student in rowing from the West Coast to Hawaii in June.

Announcement:  The Northwest Weather Workshop is on April 27-28

The NW Weather Workshop is the big annual meeting for those interested in Northwest meteorology.  This year we will have a major session on the meteorology of NW wildfires and others on other aspects of our regional weather.  The gathering takes place at the NOAA facility in Seattle.  To view the agenda and to register, go to the meeting website.  The workshop is open to everyone, but registration is required.

How I Became a Meteorologist

How (or why) did you become a meteorologist?  It's a question I ask my students on the first day of class each semester.  Many people who are meteorologists have wanted to be one for most of their lives.  Often they can point to a specific weather event, perhaps a tornado, hurricane, or winter storm, as the moment that they decided to go into meteorology.  In some cases, I know multiple people who were motivated to become a meteorologist by the same storm

I was infected early by the weather bug.  My path to meteorology was paved by youthful experiences in the Adirondack Mountains.  The weather in the Adirondacks is usually somewhere between partly and mostly crappy,  and in the late 1970s and 1980s the forecasts were truly terrible.  Whether hiking, backpacking, canoeing, nordic skiing, or alpine skiing, we had to be prepared for anything. 

There are, however, two backpack trips that were essential for sparking my interest in weather.  My father and I used camp or backpack in the Adirondacks every Columbus Day weekend, which is a long weekend in New York State.  In 1981, the trip was especially memorable because it had snowed at upper elevations.  We spent three days in the High Peaks Wilderness and hiked much of the Great Range, one of the most spectacular areas in the Adirondacks.  Our experiences greatly stoked my love of snow and my interest in mountain weather.

Dad and I on the trail.  Heavily clothed in cotton.  We had plastic ponchos too.  It was a different time!
The Great Range from Big Slide Mountain.  Gothics is just left of center, named by Adirondack Guide Orson Phelps for the slides that resemble Gothic architecture.   
Filling water bottles from a local stream.  I don't remember ever filtering or treating water.  It was a different time!

The Great Range and other high peaks from Armstrong Mountain

On the summit of Gothics.  This was my 9th "high" peak, one of 46 Adirondack Peaks thought to be at or above 4,000 feet in early 20th century surveys.

On "belay".  Well not really, but some Adirondack trails have cables and other contraptions to aid in the ascent and descent.
A second trip, which I usually cite as the moment I decided to become a meteorologist, occurred a couple of  years later.  We were on an overnight backpack in the High Peaks when we were caught in a torrential thunderstorm.  We hastily descended to a low pass and pitched our tent, eventually waking the next morning in a pond of water.  It was at that time that I decided that was never going to happen to me again and that I would become a meteorologist. 

A strong desire to understand and predict the weather I encounter in the mountains has been a great source of ideas and motivation for me through my career.  When in the mountains, I still feel like a kid in a candy shop.  So much to observe and understand!  The paths of others in my field may be somewhat different, but passion for weather and the natural world seems to flow through nearly all of our veins. 

Dense Fog: Unusual This Time of the Year

This morning brought fairly dense fog to portions of Seattle near Lake Washington and in some river valleys around the western side of the State.  The fog was beautifully illuminated this morning around 7 AM in the view looking north from the Space Needle PanoCam

And the view southeastward towards Mount Rainier was impressive as well.

The fog was apparent is the visible satellite image at 7 AM, with some of the river valleys of SW Washington in fog.

It  turns out the dense fog is relatively unusual this time of the year in Washington State.    I can show this using the following graph from my NW Weather book. Here in Seattle (red color) fog reaches the minimum number of days per year from April through June.  Spring is a low time for fog in most locations of our region.  Fall is the big fog season.

But why is Spring so bad for fog?  For several reasons.

First, most of our fog is radiation fog, which develops over long nights as the earth radiates infrared energy to space.    Our nights are getting MUCH shorter now, so much less time for the cooling to occur (remember our day-night periods today are the same as around August 23).

Second, fog likes stable conditions that does not produce a lot of vertical mixing.  Such mixing is bad for fog, since the cooling at the surface gets spread over too much air in the vertical. Stable conditions occur when temperatures don't decrease rapidly with height.  The opposite is true in Spring...the sun is fairly strong, causing the surface to warm, but the air is still cool aloft. Not stable and bad for fog.

Third, in spring around here we still have a lot of clouds and clouds slow up the radiational cooling from the surface. 

Fourth, rain is generally letting up in Spring and rain helps moisten the surface and lower atmosphere...which is good for fog.

Why fog today?  We had lots of rain recently so the lower atmosphere was moist.   And high pressure moved in late yesterday, which helped make the atmosphere more stable and lessened the amount of clouds aloft. 

Can our models forecast fog?   They can, but there skill is not great.  Fog plays into almost all our modeling weaknesses.   Here is the 14 hour forecast for low clouds valid at 7 AM from the UW WRF model.  Right idea, but somewhat too much fog/low clouds around Puget Sound.

And now the big news....a major warm up will occur early next week, with highs getting into the upper 60s on Monday and Tuesday.  And pretty decent the rest of the week.  More on the "big heat" in my next blog.

Passage of a Bowling Ball

Interesting forecast situation through Friday night as a closed upper-level low, which rumbles like a bowling ball across the American southwest, moves along the Utah–Arizona border.

If you look at the 3-h NAM precipitation forecasts above (color fill) you can see that northern Utah misses out on the action initially, but does get some after the low enters Colorado thanks to so-called wrap-around precipitation.  In the NAM forecast above, that precipitation primarily affects northeast Utah, especially the Uinta Mountains.  The Wasatch are at the edge of the action and in fact Alta gets no precipitation from the forecast above (but it is right on the edge of it). 

The strong contrast in precipitation probability from west to east across northern Utah is better illustrated by the probability of more than 0.10 inches of precipitation during the 12-hour period ending at 0600 UTC 21 April (0000 MDT Saturday) based on forecasts by the Short Range Ensemble Forecast System (SREF).  Note the sharp gradient from about Antelope Island to the western Uintas.

Indeed, if we downscale those SREF runs to account for local terrain effects, we see large spread at Alta, with members producing anywhere from 0 to 0.7 inches of water.  This would fall mostly as wet snow above 8000 feet, with snow levels possibly flirting with 8500-9000 feet Friday night should precipitation linger.
My take is that thisI is that we may see a few snow showers Friday and Friday night, but accumulations will likely be 1-4 inches.  A skunking is perhaps more likely than a surprise dump.  The latter would require the wrap around to extend further westward than indicated by most of the models I'm looking at this morning. 

After Friday night, the forecast for the weekend looks warm and pleasant.  If we do get a decent dump Friday or Friday night, it will turn to mank quickly on Saturday if it isn't already mank at sunrise.