Saturday, May 25, 2019

Memorial Day Weekend Forecast: Moist to Dry

This weekend is going to be a mixed bag over the Pacific Northwest, with Saturday being the coolest and wettest, improvement on Sunday in the west, and a glorious day on Memorial Day.

Let's start with today.  An upper level trough/low is moving through now--and headed for California where it promises declining weather during the next few days.  The forecast map for 5 PM today shows the low center sufficiently offshore that the most substantial precipitation will be along the coast.  Puget Sound will see only a few sprinkles today.  And the central and north Cascades will enjoy rain showers (which is good for keeping the wildfire threat down).


The  9 AM radar shows showers over NW WA and the northeast Cascades


Looking at the latest NOAA/NWS HRRR model, the accumulated precipitation through 11 AM is limited to those coastal/NE WA locations.

And through 5 PM, there is a substantial moistening of the Olympics, SW Washington and the northeast slopes of the Cascades.   Exactly where moisture is needed. Relatively dry over Seattle the the western slopes of the Cascades.  But there will be generally cloudy and breezy conditions.


The 24h precipitation ending 5 AM Sunday, reflects the pattern, with the dry conditions along the western slopes of the Cascades due to downslope (easterly) flow associated with the coastal trough.


Sunday will be an improving day in the west, but not in the eastern part of the state.  Moisture will wrap around from the southeast into eastern WA, bringing showers and thundershowers.  The forecast  three-hour precipitation ending 2 PM Sunday illustrates this ((below).   The precipitation extend over the Cascades before dying over the western slopes of the mountains.


Why is everything reversed, with eastern WA and eastern Cascade slopes so wet?  Because there is persistent easterly (from the east) flow on Saturday and Sunday.

The 24-h precipitation ending 5 AM Monday shows impressive amounts over eastern WA and the SE Cascades.   Extremely beneficial precipitation.


But then there is Monday, when major improvement will occur.  High pressure will build into the eastern Pacific,  dry conditions should prevail, the sun should be out, with temperatures zooming into the 70s in western WA and a perfect day for memorial day activities should be enjoyed by all.




Thursday, May 23, 2019

Does Washington State Really Have a Drought Emergency?

On May 20th, Governor Inslee announced a drought declaration for a large portion of the State.


Media outlets like the Seattle Times had banner front page headlines about the threat.


So you may ask.  Are the conditions we are experiencing really that usual?  It there really a significant threat of hardship over large areas of Washington State?   

This blog will examine the observations and latest model forecasts to help answer these questions. 

The bottom line:  the situation is far less dire or unusual than advertised. There will be plenty of water for nearly all users, and that the forecast is for a wetter than normal summer.

So let's look at the data.  We can start with the departure from normal in inches for the "water year" precipitation, which starts on October 1st.  East of the Cascade crest, the vulnerable areas for agriculture and wildfires, precipitation has been at or above normal.  But conditions were clearly drier than normal over our normally sodden coast.  Of course, the coastal region has limited population and little vulnerable agriculture.  The rest of western WA has been modestly below normal.

Plotting the precipitation over the entire state for October 1- April 30 since 1930, shows that overall, the state was wetter than normal this year.

And the Palmer Drought Index, which considers the impacts of both temperature and precipitation on soil moisture, shows that the drought index is on the wetter side.

The spatial distribution today of the Palmer Drought Index shows normal or better than normal condition east of the Cascade crest, with drier than normal conditions limited to portions of western WA.
 

The latest USDA soil moisture report shows above normal soil moisture for the state

Steamflow are normal for the eastern 2/3 of the state, with below normal values on the lower western slopes of the Cascades.

 Reservoir storage is quite good over the State.  The critically important Yakima River System is just below normal storage.

And all major urban water supply systems are in good shape.  For example, Seattle's reservoirs, located in the middle of the drier than normal zone on the western side of the Cascades, is now above normal in terms of water storage.


There has been a lot of scary talk about low snowpack, but much of it has been HIGHLY deceptive.  The snowpack situation at the end of the winter (April 15) was really not that bad over Washington state and downright good over Oregon (see below). The southern part of Washington was roughly at 100%, while the northern portion was approximately 70-75%.

This situation is not unusual in an El Nino year.  The snowpack in early April gives a good idea of the amount of water available from melting snow, which fills our reservoirs and rivers.  The April snowpack was not bad at all...nothing like the bad years like 2015.   We had a warm period in late April through early May that caused a lot of melt, so the snowpack numbers dropped rapidly in some areas, but much of that that water was not lost.

In some places, the snowpack melted out a week or two early, which gave crazy low numbers for percent of normal snowpack for a few weeks--but that does not really mean much. 

Take Blewett Pass.  The snowpack (blue line) was ABOVE NORMAL in early March compared to normal (red line), but warm weather resulted in  rapidly melt, with the loss of snow cover about a week early.

During that melt-0ut period, the observed snowpack was10-30% of normal!  But does that really mean anything?  Not really.  But certain groups and individuals are making a big deal about it.  Total precipitation at Blett since October 1, was modestly below normal.

The bottom line in all this is to show you that although some parts of the state have been a bit drier than normal, there is nothing exceptional going on.  Water supplies to cities should be fine. There is enough water for the fish and agriculture.   There is no emergency and even calling this situation a drought is a big stretch.

But there is more.  There is all kinds of talk of this being a dry summer.   The Seattle Times said that in the headlines, but it turns out they made a mistake, mis-quoting State Climatologist Nick Bond.   The last ensemble-based seasonal forecast (IMME) for precipitation over the summer (June, July, August) is for a WETTER than normal summer in our region.

And the latest European Center prediction forecasts precipitation over the Memorial Day Weekend (sorry), with the heaviest precipitation over northeast WA, exactly where we need it.

My take on all this, is that our state is going into the summer in relatively good shape water-wise and that there is no reason to expect drier than normal conditions and excessive wildfires.  Summers are typically dry here and there will be fires, but it is important not to exaggerate or hype the situation. 

If one cries Wolf too many times, one day folks may not listen.


Monday, May 20, 2019

Should California Be Renamed the Evergreen State?

Times change and recent meteorological trends suggest that Washington State should pass on its title of the Evergreen State to the more deserving State of California.   Washington State could become the Golden State, which makes sense with the vast sagelands and wheat fields of eastern Washington and the dry conditions over western Washington.

OK, let's make the case for the switch!  Here is the percent of normal precipitation since January 1st over Washington State. Much of the Cascades, western Washington, and northeast Washington received only 50-70% of normal.  Ouch.

But California, the supposedly Golden State, has been uniformly wet, with large areas enjoying more than 200% of normal precipitation.



Who had more precipitation since January 1st, San Francisco or Seattle?    San Francisco, of course, with 18.31 inches compared to Seattle's lowly 13.71 inches.

Snow in the mountains?  Skiing into the summer?   California peaks have been buried in the white stuff and CHAINS have been required over major mountain passes.  Some CA ski areas plan operations into July!


California's reservoirs are full of water.   Washington State reservoirs are ok, but well below capacity

This situation is not temporary--there is no end in sight.   The latest forecast from the European Center for accumulated precipitation over the next week (below) shows plenty of wet stuff over the Sierra Nevada and northern CA (1.5-4 inches), with some of that precipitation extended into normally arid eastern Oregon.

Washington State?  The arid leftovers, with most of the state getting a few tenths of an inch (with a bit more over the Cascades).

So what is going on here?  Why is Washington State and California exchanging places?  The key issue is the anomalous upper level flow pattern, with the jet stream heading into California, plus an usually persist trough of low pressure somewhere around CA.

A very revealing diagnostic was created by National Weather Service meteorologist Alex Tardy of the San Diego office.  It shows the difference from normal of the height of 500 hPa surface (thinking of it as the difference from normal of pressure at around 18,000 ft).  Unusual ridge of high pressure east of British Columbia, but a band of much lower than normal pressure heading into California.  Striking. 


This pattern pushes a strengthened jet stream southward into California.  And things are NOT going back to normal soon.  The upper level (500hPa) forecast map for 2 AM Tuesday shows the jet stream (where the lines are close together) heading into California, with a low pressure areas moving southward into northern CA to its north.



Fast forward to 8 AM on Sunday.   An area of low heights/pressure moving into California. Clouds and precipitation will extend over the southern part of the State.



The second half of May 2019 will be the wettest in CA history for many locations.

California is going to be green for a while.  And the fire season in California and Oregon is going to be seriously delayed. 







Saturday, May 18, 2019

Will 5G Undermine Weather Prediction?

There have been a number of media stories this week about a major threat to weather prediction:  the sale of electromagnetic spectrum for new 5G cellphone service.   The problem is that some of the wavelengths being auctioned off for 5G are critical for an important class of weather satellites, with 5G signals potentially undermining our ability to forecast the weather.



Currently, 4G cellphone technologies provide roughly 100 megabits per second (100 million bits per second) of communication speed, while the proposed 5G service could achieve 10 gigabits per second (10 billion bits per second).  Downloading movies and animations would be much quicker, with hardwired connections becoming less critical for most uses. 

But to achieve such service one needs a larger communications highway, which means the use of more of the electromagnetic spectrum.   Electromagnetic energy, such as radio, microwaves, and visible light, are characterized by ranges of wavelength and frequency.  The use of these wavelengths is controlled by our government, which can auction off specific frequency/wavelength bands.



  Among the spectrum recently auctioned off by the FCC for 5G is a band of frequencies near 24 GHz (GHz is gigahertz, or a billion cycles per second).   Unfortunately, this is close to 23.8 GHz, a frequency in which water vapor emits microwave radiation and which is used by weather satellites to determine the three-dimension properties of the atmosphere.  And that information is very important for providing the description of the atmosphere that is required for numerical weather prediction.

Why weather satellite information is important for numerical weather prediction

Numerical weather prediction, the foundation of all weather forecasts, depends on securing a comprehensive, three dimensional description of that atmosphere--known as the initialization.  The better this initialization, the better the forecast. 

One of the key reasons why modern numerical weather prediction has gotten so good is that weather satellites now provide 3D data over the entire planet.  Even over remote oceans and the polar regions.  Roughly 95% of the total volume of weather information now comes from weather satellites.
Before weather satellites, radiosondes were the main source of 
weather information above the surface

And the most important source of weather information is from a collection of satellites that contain microwave sounders.  These satellites observe the earth by sensing microwave radiation being emitted by water vapor, liquid water, ice, and the surface. 

The amount of radiation being emitted can be related to temperature.  And different wavelengths/frequencies reveal the conditions at different levels of the atmosphere.   To put it another way, by sensing emissions at various wavelengths, one can secure a profile of temperatures at various levels in the atmosphere.  Kind of like have radiosondes (balloon-launched weather observations) everywhere.  Very valuable information

The Microwave Sounder Unit on the AMSU-A satellite

What is the most valuable of all satellite observations? 

Satellites with microwave sounders like AMSU-A (see below).  That platform ALONE contributed to a 17% reduction in forecast error in the European Center global model (the world's best)

AMSU A looks at the atmosphere in 15 wavelength/frequency bands or channels,  including sensing the atmosphere at wavelengths that the atmospheric water vapor has peaks in emission (see below). 
Channel 1 is at 23.8 GHz.   The problem is that the FCC has sold off 24 GHz, which is very close to 23.6 GHz.   And if the 5G transmitters aren't very high quality, with little spread to neighboring frequencies, they could well interfere with the microwave weather satellites. 

Why?  Because the weather satellite have very, very sensitive receivers because they are trying to sense the weak microwave emissions of atmospheric water vapor.  These sensors could be overwhelmed by the active TRANSMISSION in nearby wavelengths by thousands of 5G cell tower transmissions or other sources.

And the problem is even worse than that.  The FCC is planning to auction off more wavelengths/frequencies, some of which are close to other wavelength/frequency bands used by the weather satellites.

The potential harm to U.S. and worldwide numerical weather prediction by interfering with the 23.8 GHz band is certainly real, but difficult to quantify exactly. 

First, it will depend on the characteristic of the 5G transmitters and to what degree they will contaminate the nearby weather observation bands.

Second, it depends on how many wavelength bands would be affected.

Third, cell phone coverage does not include the entire planet.  One analysis suggests that only 34% of the earth's surface has cell phone coverage, suggesting that roughly 90% of the planet would be clean of interference (71% of the earth's surface is covered by water).  But if plans to establish satellite-based 5G on commercial ships and aviation come to fruition, the problem would be much worse.

NOAA, NASA, and U.S. Navy are quite concerned about this issue, with the Navy writing a strong statement of the potential harm.  On Thursday, NOAA Administrator Neil Jacobs warned of a potential loss of  1/3rd of current forecast skill.  These warnings need to be taken seriously.

The key now is to have close coordination between the FCC and NOAA/NASA/DOD, as well as other international players, to ensure that spectra close to the weather observing frequencies are not used and, if there are, investments in high-quality transmitters, with effective filters, are required by law.  

Improved forecast skill derived from weather satellites has had huge positive impact on saving lives and property, and in fostering economic growth.  Reasonable actions must be taken to protect the value of weather observations from space.





Wednesday, May 15, 2019

Deluge in California

Some areas of California will experience the wettest second half of May in the historical record.  Inches of rain will fall in the lowlands and several feet of snow will pile up in the high Sierra Nevada.


The accumulated precipitation forecast for the next five days provided by the European Center is stunning, with roughly 5 inches in the high terrain and even two inches in San Francisco.   This is a time of the year that California is normally quite dry.

Looking forward ten days, the totals get even more impressive.  Moderate precipitation even gets down to LA, and some mountain locations will experience 6-9 inches.

Want to be impressed? Most of California will be hit by MORE THAN 500% of normal precipitation (see below)


And some of the high Sierra Nevada will see 2-3 feet of snow.


Let me reiterate, this is a period when California is normally quite dry.  To demonstrate this, here are the historical probabilities of getting 2 inches in 7 days in San Francisco. Has never happened in late May before.  It WILL this year.   Other sites show the same thing--this kind of heavy rain in late May is very, very unusual, if not unprecedented.



The implications of this liquid bounty is enormous.  California's reservoirs are not only way above normal, many are nearly up to capacity (see below)


Soil and fuel moistures are above normal and the heavy rain and massive snowfall will push off wildfire season by several weeks at at minimum and quite possibly more.    This is very good news for us in the Northwest, since we received substantial smoke aloft from California during the past two years.  Expect less of that unwanted California import this summer.

And although California will get for more than our region, plenty of rain will make its way up to us.  The forecast precipitation total for the next 48 hr is substantial, particularly in the central and northern WA Cascades, where snowpack totals were low (see below).   Expect soil moisture and streamflows to move upward substantially there. 

And more will fall after that.






Monday, May 13, 2019

A Wet Period Ahead for the U.S. West Coast

The first half of May has been generally dry and warm over the Pacific Northwest, but soon everything will change.

The second half of the month promises to be cool and wet in Washington and Oregon, with northern California swamped by highly unusual amounts of rain.   As we shall see, this wet period has highly positive implications for the upcoming wildfire/smoke season.

To give some perspective, below is the precipitation departure from normal for the past 60 days over the western U.S.   Much of the region has had above-normal or near-normal precipitation, with the big exception being western Washington, which has been drier than normal.  Importantly for wildfire concerns, southwestern Oregon and eastern Oregon have been moist and the area east of Cascade crest in Washington has been near normal. 

Much of coastal California has been on the dry side during the past two months, but that is following a very wet winter.  The concern for California is that abundant winter precipitation will lead to lots of grass growth, which will burn later in the summer as it inevitably dries out. 

The forecasts for the next two weeks for the West Coast are consistent and wet.  The UW WRF forecast for accumulated precipitation over the next week indicate amazingly wet conditions over northern CA, with 5-10 inches over the Sierra Nevada and northern CA mountains.  This is HUGE for late May.  Heavy precipitation extends northward across the Oregon Cascades and eastern Oregon, and western Washington gets 1-2 inches (normal total May rainfall at Seattle is a little under two inches).  Forget watering your lawn.


Turning to the vaunted European Center model totals for a similar period, one sees very similar amounts of accumulated precipitation.  Folks, this is really wet for Oregon and California.


Now, any good forecaster does not look at a single model prediction, but views an ensemble of many of them to understand the uncertainty of the forecast.    Here are the 51 forecasts of accumulated precipitation at Seattle from the European Model ensemble for the next ten days.  90% of the members are on the same page, with precipitation starting on late Tuesday and 1-3 inches by May 24th.
Even light rain in Yakima, on the eastern side of the Cascades.


So what are the implications of all this?   

First, the atmosphere is not locking up in a warm/dry pattern as it did in 2015.  Second, this is an unusually wet pattern for California/southern Oregon so late in the season, one that will top off their reservoirs and ensure that there will be plenty of water in the Golden State this year. 

It will also delay the wildfire season there.  Third, western Washington will get some needed precipitation, lessening the chances of any early grass fires and will help fill our reservoirs, which are actually not in bad shape anyway.  The wet period will also radically reduce local water usage, leaving more water in the reservoirs.

There is a lot of fear-inducing headlines in the media and among some politicians about a terrible fire season because of the recent dry period and the low snowpack at some sites in northern Washington.


These worries are not realistic.   The upcoming wet period, which has been forecast for days, should moisten up the whole area.  The eastern side of the state had plenty of precipitation during the past winter and streamflow/soil moisture is fine there. 

The snowpack in much of California, Oregon, and southern WA is fine.   And low percentages in parts of WA this time of the year can be very deceptive.  Let me demonstrate this.

Below are the values of snow water equivalent (SWE, amount of water in the snowpack) and precipitation accumulation of this year versus normal at Stevens Pass--a location with anomalous low snowpack (SWE) today.     Total precipitation (black--this year, gray--normal) is just a little below normal (that will change during the next week!). 

But what about snowpack (Red normal, blue this year)?  The snowpack peaked at about 75% of normal and will melt out around May 15th, rather than the normal June 1.   So the % of normal today is crazy low (maybe 5%), but clearly the situation is not that serious...just an early melt out from the warm temperatures of the past month.


The bottom line:  At this point, it is premature to predict an early or more severe fire season for our region.