June 05, 2024
A Very Wavy Atmosphere
The high-resolution MODIS satellite image taken mid-day yesterday was remarkable, with extensive wave-like cloud features east of the Cascade crest in Washington and Oregon (see below). More waves over Montana as well.
Here is a closer look at the waves over eastern Oregon. Very regular features with north-south cloud lines and clear zones between them.
Over southern Washington, there were still waves of clouds but they were distorted east of Mt. Rainier into more of a "v" shape.
What is going on here?
You are looking at mountain lee-wave clouds or, more precisely, trapped lee-wave clouds. You will learn about the "trapped" business in a second.
When air approaches a mountain barrier with sufficient wind speed and under stable conditions, air is pushed up by the mountains and then oscillates up and down downwind of the terrain (on the lee side). Think of a swing--you give a push and the swing goes back and forth for a while.
When the air goes up it cools (moving to lower pressure causes air to expand and cool), which can result in air becoming saturated (cool air can hold less water vapor than warm air). leading to withcloud formation. So one can get a series of clouds with clear zones in between (where air is sinking) downwind of a mountain barrier.
The mountain waves are stronger in the lower atmosphere and weaken with height. Thus, they are "trapped" in the lower atmosphere downwind of the mountain barrier.
If the mountain barrier is an elongated linear feature that is oriented north-south (like the Cascades), one ends up with north-south lines of clouds and clear zones downstream of the barrier. Exactly, what we see downstream of the north-south Cascades.
If the barrier is more isolated, the mountain wave clouds appear more like the wake of a ship (see below). The mountain wave clouds downstream of Mt. Rainier looked like the "ship wake" clouds.
Can high-resolution weather prediction models correctly simulate and forecast such waves?
I am proud to say that the answer is definitively yes. Below is a nine-hour prediction of the cloud field from the UW ultra-high resolution WRF prediction model (the kind of thing I work on in my real job) for 2 PM yesterday.
Wow. Near perfection.
Trapped mountain lee wave clouds are popular destinations for glider pilots, who try to stay in the upward motion associated with clouds. Free vertical lift courtesy of Mother Nature!
Posted byCliff Mass Weather BlogatJune 05, 2024No comments:
June 03, 2024
A Winter Storm in June
A combination of heavy rain and wind, more reminiscent of November than June, came through yesterday and this morning, and the effects were substantial.
Winds gusting to 35-50 mph, acting on fully leafed out trees, led to massive loss of branches and trees. My bicycle commute to the UW along the tree-lines Burke-Gilman trail was interrupted by several major treefalls (see picture below).
Tens of thousands of Seattle City Light customers lost power this morning. Same for Puget Sound Energy.
Below is a plot of maximum winds overnight. Some mountain locations gusted over 60 mph, with over 45 mph near Puget Sound, the Strait, and the Washington coast.
Why were the winds so strong?
The figure below shows the sea level pressure (solid lines) at 8 AM this morning. A Pacific low-pressure system passed north of western Washington, creating a large north-south pressure difference that accelerated the winds northward.
Precipitation totals? Over the past 36 h (see below), the western slopes of the Cascades and Coastal Mountain received 2-4.5 inches.... a lot for June.
Puget Sound was rainshadowed by the Olympics and Seattle "only" received about 1 inch.
If you really want world-class mountain rainshadowing, head to a station just south of Port Townsend, which only received 0.11 inches over the same period (see red arrow). No wonder folks retire there.
Western Washington and Oregon rivers are running very high now, some at record levels (see below, blue and black dots are much, much higher than normal).
I hope the Drought Monitor folks will consider dropping the "moderate drought" designation for the western Cascades slopes.
More showers are occurring today across the region, as shown by the latest radar image (see below)
And the precipitation is not over! Another plume of water vapor (a.k.a. an atmospheric river) is rapidly approaching (see water vapor satellite image below)
Rain will rev up tomorrow morning. By the time precipitation ends Wednesday AM, substantial rainfall will have occurred, particularly over the north Cascades and mountain of SW British Columbia (see 48 h total ending 5 AM Wednesday below).
Enjoy this wet interlude. Drier conditions and a slow warm-up are expected Wednesday through Saturday, as our highs climb into the mid-70s.
Posted byCliff Mass Weather BlogatJune 03, 202410 comments:
June 01, 2024
New Podcast: The Heavy Rain Event, Heat Wave, and Perhaps Thunderstorms!
Mother Nature is keeping the meteorological plate full for us.
My new podcast describes the latest on heavy rain events occurring on Sunday through Tuesday. But we are now close enough in time to see what's next: a minor heatwave peaking over next weekend, with the potential for thunderstorms from the Cascade crest eastward.
Friday afternoon, the European Center model has thunderstorms over eastern Washington
The simulated radar imagery from 11 PM next Saturday from the UW WRF model shows an area of thunderstorms moving into western Washington.
You can listen to the podcast below or through your favorite podcast server.
Posted byCliff Mass Weather BlogatJune 01, 20242 comments:
May 31, 2024
Flooding of the Snoqualmie River and Record Rainfall
We are about to begin a historic rain event, with the one or two-day rainfall being in the top five of the historical record over Puget Sound. Some other locations we do even better.
Impressively, the Snoqualmie River is predicted to flood with record-breaking flows for any day in late May or the first half of June.
The rain will start mid-day on Sunday and it will go on for several days.
Let's start with the predicted total precipitation through Tuesday morning (below). Just wow...much of our mountain areas will receive over 3 inches, with limited areas getting over 5 inches.
An extraordinarily intense atmospheric river, one quite rare for early June, will slam into the region. The water vapor transport map for Sunday afternoon illustrates this (see below). This would be impressive in November. In June, startling.
Shockingly, this plume of moisture can be traced all the way back to southeast Asia!! (see map below for the same time). Another Chinese import (😃).
The sustained heavy rain is predicted to take the Snoqualmie River to flood stage--and the highest level ever observed this time of the year (see plot below from the NOAA/NWS Portland River Forecast Center). The orange line shows flood level and the upturned red arrows show the highest historical levels for each day.
How unusual is the event?
Below are the top ten precipitation values for SeaTac for the first half of June. The current predicted two-day total at that location is about 1.5 inches, so could enjoy the third or fourth wettest totals since 1945,
Keep dry!
_____________________________________
Announcements
I will be doing a podcast about the event later today.
I will be teaching ATMS 101 this fall--the introductory atmospheric sciences class.
The class will be BOTH in person at the UW and available over Zoom. Thus, folks can take it remotely.
If you are over 60, you can take the class through theACCESS programfor a very nominal charge (something like $20). During the past few years, several hundred have done so.
So if you are a UW student looking to learn about weather or a non-student interested in the topic, I welcome you to join me this fall.
Posted byCliff Mass Weather BlogatMay 31, 20245 comments:
May 29, 2024
Heavy Precipitation Event with Record River Levels
An extraordinarily heavy precipitation event for early June will occur next week, with daily rainfall records falling and many regional rivers hitting record levels for June.
Rain so heavy that the wildfire threat will be greatly diminished for an extended period and regional reservoirs will get a mighty "top off" before our normally dry summer season.
Let me show you the latest forecasts, and be prepared to be amazed by the soggy bounty ahead.
Consider the latest European Center accumulated precipitation forecast for their model run starting 5 AM this morning (below).
The total through 5 PM today (Wednesday) is wet enough, particularly on the western side of Cascades where as much as an inch should fall.
With more showers on Saturday, the totals become more substantial over Northwest Washington.
But you haven't seen anything yet. A very moist atmospheric river comes in on Sunday/early Monday and precipitation totals go crazy, with 2-4 inches on the western slopes of the regional mountains.
The totals by Wednesday morning are staggering, with some favored locations getting to 4-6 inches of rain. This is unusual for June, to put in mildly.
Even in rain-shadowed Seattle, the ensemble forecast of many simulations suggests we will receive about 2.5 inches (blue and black lines are the ones to look at).
Much more than the average TOTAL rainfall for the ENTIRE MONTH OF JUNE here in Seattle (1.42 inches) Such heavy precipitation will saturate the regional soils and cause rivers to rise rapidly, with the NOAA/NWS River Forecasting Center (RFC) predicting a rapid rise to action levels and the exceedance of the previous daily records (red up arrows).
Most impressive of all is that this heavy precipitation period is associated with multiple strong atmospheric rivers. To illustrate, below are plots of water vapor transport, the best measure of atmospheric river activity. Blue colors are very high.
Sunday night at 8 PM an amazingly strong river heads across the Pacific and then slams into our region.
On Tuesday morning, ANOTHER river plows into the Northwest. Really unusual for June.
Expect many precipitation records to be broken.
Posted byCliff Mass Weather BlogatMay 29, 202423 comments:
May 27, 2024
Is Global Warming Causing Aircraft Turbulence to Increase?
After the turbulence encounter by a Singapore Airlines aircraft, there has been a slew of articles claiming that severe turbulence incidents are rapidly increasing due to global warming. The articles below in the Seattle Times and on the BBC are just two of hundreds of such stories
Several stories have been quite specific stating that turbulence has increased 55% in 50 years, based on a study by investigators at the University of Reading.
Is any of this true? As I will describe below, there are substantial problems with these apocalyptic claims of radically more bumpy skies.
But before we discuss these turbulence claims, keep in mind that aircraft turbulence can be caused by several different mechanisms, and not all of them are related to changes in climate.
Much of the turbulence is due to convection associated with big cumulus clouds, like cumulonimbus, which produce thunderstorms. The turbulence on the Singapore Airlines flight was associated with tropical convection.
Other turbulence is associated with high-amplitude atmospheric waves associated with mountains (see figure). Such turbulence is often observed downstream of major mountain barriers such as the Rockies and the Sierra Nevada.
Source: Whiteman (2000)
There is also turbulence associated with vertical wind shear: if wind speed changes rapidly with height, the atmosphere can break down into turbulence eddies. This kind of turbulence often occurs without clouds and thus is often called clear air turbulenceand is generally the least severe of the major turbulence mechanisms. Finally, it is possible to get turbulence near the surface, called mechanical turbulence, as air passes around objects.
How frequent are the different sources of turbulence? The best records and observations are found in the United States and the contributions of various types of turbulence by month are shown below (from an FAA document) for 2009-2018.
Convective turbulence is number one and dominates in spring and summer. Clear air or shear turbulence is number two and is most dominant during the cool season.
FAA records don't seem to suggest this. Below is a graph showing the percentage of aircraft incidents/accidents caused by turbulence (dark line). No increasing trend since the late 1980s!
So where are all these dramatic claims for increasing turbulence with global warming coming from?
Answer: from a handful of papers from one group at the University of Reading. The key paper is here.
In this paper, they DON'T USE ANY TURBULENCE OBSERVATIONS. Rather, they use analysis of weather data on a grid (called a reanalysis) and use a theoretical model that they suggest should indicate shear-forced turbulence. But they really don't have the data to back this up or to demonstrate their approach is reliable.
Even more problematic, the 55% increase in turbulence they cite is NOT for the entire world but just for one region in the North Atlantic. If you read the actual paper (I have), there are all kinds of qualifiers that never made it into media stories.
To gain some perspective, their 55% increase in this one location represents an increase from 17.7 to 27.4h PER YEAR. Ten hours more a year is 0.0011 of a year. For the least severe source of aircraft turbulence (vertical wind shear).
The media did not tell you that the increase was so minor. Still worried? And that paper said nothing about the other sources of strong turbulence, which in total are more important than the shear-induced, clear-air turbulence they considered.
And there are other technical problems with the above paper that could be a real problem. For example, the weather observational system has been greatly expanded and enhanced over the past 40 years, allowing observations to far better define atmospheric structures that can produce turbulence. Massive additions of satellite data and aircraft observations have occurred during the period in which the Reading group claimed the threat was increasing the most. Was the threat really increasing or was our ability to define strong wind shear layers getting better. They didn't say.
In summary, I believe there are no reliable studies that indicate a substantial increase in aircraft turbulence due to changing climate conditions. The media is needlessly causing folks to worry about flying.
Posted byCliff Mass Weather BlogatMay 27, 202411 comments:
May 25, 2024
New Podcast: Memorial Day Weekend Forecast and More on Western Washington Wildfires
This weekend will be a mixed bag west of the Cascade Crest but warm over eastern Washington.
The predicted high temperatures for the next few days are shown below. Today (Saturday) will be cool in the west, frigid in the mountains, and comfortable in the Columbia Basin.
Sunday says cool in the west, while eastern Washington warms considerably.
Monday is the best day. Decent over the western lowlands, warm enough for hiking in the Cascades, and downright warm in eastern Washington.
I talk about the forecast in some detail in my podcast (see below), and discuss the apocryphal forecasts of enhanced western Washington wildfire threats this summer.
You can listen to the podcast below or through your favorite podcast server.
Posted byCliff Mass Weather BlogatMay 25, 20244 comments:
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A Very Wavy Atmosphere
The high-resolution MODIS satellite image taken mid-day yesterday was remarkable, with extensive wave-like cloud features east of the Cascad...