August 31, 2019

Hurricane Dorian Will Spare Florida: Why So Much Uncertainty with This Storm?

It is becoming increasingly clear that Hurricane Dorian is going to make a sharp right turn as it approaches the eastern coast of Florida, resulting in only modest impacts to the Sunshine state.  Hurricane force winds (sustained at more than 64 knots) and heavy rains will probably remain offshore.

Early forecasts had predicted landfall of a category 5 hurricane on the central Florida coast,  with potentially devastating impacts, but now the real risk has shifted northward into the Carolinas.

Some critics have already started complaining, suggesting that there were too many cancellations and closures before the storm track was clear.  Folks along the Florida coast have stripped the food stores, gas stations are out of fuel, and even the Orlando Airport was closed.

 Hurricane Dorian at 2:40 PM PDT on 31 August

Was there a failure of weather prediction technology for this event?  Did the European Center Forecast outdo the American FV-3?    Did local governments over-react?

My conclusion:  this has been an inherently difficult storm to forecast, with more uncertainty than many recent storms, some which were predicted well over a week in advance (e.g., Irma).

The latest forecasts of all major centers are converging on a solution in which the storm approaches the Florida coast, but swings northward just offshore.  Let me show you the usual cream of the crop model, the European Center forecast.

The forecast for 5 AM Monday PDT, shows the storm approaching the Bahamas.

 A day later, it moved to the northwest...quite slowly.

 5 AM Wednesday it is crawling northward.

And Thursday morning it is just off the southeast tip of North Carolina

The strongest winds of a hurricane are in the right front quadrant--to the right of the direction of motion.   And thus the strongest winds are generally kept offshore.  Same thing with precipitation.

The accumulated precipitation totals through Friday morning show the heaviest precipitation offshore.  No problem at all for Florida and Georgia.   An issue for coastal North Carolina.

As I shall describe in a second, this is an inherently difficult forecast situation with high levels of uncertainty, particularly once the storm gets near Florida.  A measure of the uncertainty is apparent in the ensembles--in which multiple forecasts starts slightly differently and are run by both the European Center and the NWS.

To illustrate, here is the latest ensemble prediction of the position of the hurricane center during the next five days..  No much variation (spread) initially as the storm heads west. And then nearly all members make an abrupt right turn....but there is a lot of spread in the exact time when the turns start, resulting in a big difference in the low center positions in five days.  The U.S. ensemble is similar, but closer to the coast.

Why is there so much uncertainly?  It has to do with steering currents.  Hurricanes tracks--or trajectories in time---are controlled by the large scale flow around them....which is called a steering current.
For strong storms like Dorian, the steering flow can be represented by the flow speed and direction averaged from around 5000 ft to 30,000 ft (or roughly 850 hPa to 300 hPa in pressure).  If the flow becomes weak or if there is a large change in steering flow in distance, hurricanes can stagnate or move erratically.  
The current (2 PM Saturday)steering flow analysis by NOAA CIMMS is consistent with westward movement of the storm (the arrows show the flow direction and the hurricane is indicated by the red marker).

But this changes during the next few days, with the flow weakening over the region and the subtropical high moving eastward (steering flow prediction by HWRF model for Monday at 5 PM PDT shown).  The result is a northerly component to the steering flow and the shift to the right.

But let's be clear...this is all on the edge, and it would not take a large prediction error to allow the storm to drift further to the west...and thus onshore.   

The weak steering flows make this inherently a difficult forecast, even for the best of the current models.   A verification of position errors by Professor Brian Tang of U. of Albany suggests the U.S. FV-3 model (dark blue color) and been similar in skill to the European Model (purple color) for 12-96 hr forecasts, with the Euro being significantly better at 120 hr.  The position errors are quite large compared to recent hurricanes. But these are early times.

August 29, 2019

Unstable Sky

When I looked out this morning around 6:15 AM at the beautiful sunrise, I could see it.  The potential for showers and thunderstorms was there. 

Below are images from the Space Needle Pano cam at 6:10 and 6:20 AM.  Not only was the sunset beautiful, but the colors changed radically in ten minutes from red to yellow.

But look at the clouds.  There is a deck of mid-level clouds that are broken into lots of puffy, individual elements.   This is sign of instability--the tendency to convect--in the middle atmosphere.  And often precedes convection (e.g., thunderstorms) in our area.

And in fact, the 6:18 AM weather radar shows a line of showers moving northward over southern Washington (see below).    Olympia is feeling it as I write this.

There is convection--thunderstorms--embedded in this line, as shown by the lighting detection network.  For example, here are the lightning strikes (red and yellow crosses) during the 30 minutes ending 5 AM from the UW WWLLN network run by my colleague, Professor Robert Holzworth.  The background is the infrared satellite image at the same time.

This convection is associated with the lifting forced by an approaching upper level trough, which is invading the high pressure ridge that gave us the warm temperatures the past few days (see upper level map at 5AM below).  You see the (subtle) trough just off the NW coast?

Most of the showers should stay south of Seattle.  But today (Thursday) will be a major step down (from the upper 80s into the mid to upper 70s).   This weekend?  You may not want to ask.

The forecast of the upper level situation (500 hPa heights...roughly 18,000 ft)  for 11 AM Saturday shows a strong trough approaching our region. A trough that brings upward motion, clouds, and rain

In fact, the forecast 24h precipitation ending 5 AM Sunday, shows.....well....some rain. Saturday will be mostly cloudy with highs in the lower 70s.  Plan accordingly.  

This weather pattern is typical of what we have seen all summer:  normal variability and precipitation, going between warm and cloud/wet spells.   And what has prevented the anomalous wildfires/smoke of the past two summers.

August 27, 2019

Trump, Nuclear Bombs and Hurricanes

Today, I received calls from two media outlets asking about Trump's supposed suggestion that the U.S. might weaken hurricanes by exploding nuclear weapons inside of them.   

Now there is some debate on whether he said they in the first place:  the story was released by Axios, some kind of media outlet, with President Trump denying he even said it.

So putting the question of whether President Trump said it or not, does nuking hurricanes make sense?  If not, are there other ways to alter their strength or paths?    Hurricanes have been an interest of mine for a long time--in fact, my Ph.D. thesis was on the disturbances that lead to their development.

Would nuking hurricanes work?    This is not a new concept; in fact, a meteorologist Jack Reed of the Sandia Corporation wrote a paper on the subject and a more recent note has been distributed by two experienced hurricane scientists at NOAA's AOML lab.  The idea behind nuking was that hurricanes depend on having a warm inner core (which causes the low pressure).  Exploding lots of nuclear weapon in the hurricane eye would blast the hot air away, allowing cooler air in the periphery to surge inward, weakening the storm.

Let me save you some effort and give you the bottom line of these analyses:  it wouldn't work. 

Even a middle-of-the-road hurricane releases HUGE amounts of energy (mainly by condensing water vapor and releasing latent heat).  So much energy that human capabilities seem puny, even with nuclear weapons at hand.  An estimate by NOAA scientists is that a fully developed hurricane releases energy at a rate equivalent to detonating  a 10 megaton bomb every 20 minutes.  And there is no reason to expect that the warm air would be "blown away."    And even it worked, there is the little issue of a massive release of radioactivity.

So let's cross nuking hurricanes off the list.

Back in the 1960's, another approach to weakening hurricanes was suggested:  cloud seeding outside of the eye wall.  The idea is that cloud seeding would result in the conversion of liquid water droplets to ice, releasing lots of latent heat outside the inner eyewalls, causing the eye to expand or a new outer eyewall to form---and either change would weaken the winds.

This suggestion was taken seriously enough that the approach was tried in a NOAA/DOD  field project called STORMFURY, with aircraft flying into hurricanes, seeding with silver iodide or dry ice.  The results were mixed and no compelling evidence for the efficacy of the Stormfury approach was compiled.  The project ended during the 70s, both due to lack of evidence of impact and the threat of lawsuits.

More recently, another approach has been suggested:  cloud seeing storm systems in the environment of hurricanes, subtly changing the "steering currents" controlling storm direction and speed so that hurricanes miss major metro areas.   No serious attempts have been made of such an environmental steering approach.

Perhaps the best solution for dealing with huricances is to discourage the massive development that has been occurring on our southeast coastline, development that is putting millions of folks at risk.    Reducing CO2 emissions is also a good idea, since increasing greenhouse gases may well enhance the destructiveness of the most powerful hurricanes.

August 25, 2019

Near 100F in Portland But Low to Mid 80s in Seattle. Why the Difference?

The contrast in weather between Portland and Seattle, only 145 miles apart, will be substantial the next few days, with Portland approaching the century mark, while Seattle only warms into the lower 80sF.

Why does this happen?   To understand, one has to know about a crucial local weather feature, the thermal trough, and the impacts of terrain and land/water contrasts.  And it will also explain why it will get quite breezy in Seattle on Tuesday, with strong winds from the north.

Let's start with the latest forecast from the excellent system:

On Tuesday, Seattle is predicted to get to 80F, while Portland hits 97F.   Quite a contrast.  Byt why?

Let's start by looking at the latest surface air temperature forecast from the UW high-resolution WRF simulations.   At 5 PM Monday, it is warmer in the Willamette Valley  (80s), then Puget Sound (70s)

But then the heater is turned on within the Willamette Valley with temperatures rising into the upper 90s on Tuesday, even above 100F around Medford.   But way cooler over Puget Sound and almost chilly in NW Washington.

The key large scale feature that develops on Tuesday is an upper level ridge building over the region from the Southwest (see 500hPa--about 18,000 ft-- map at 5 PM Tuesday).

This feature is associated with increasing sea level pressure to the east of the Oregon/Washington Cascades and easterly (and thus offshore) flow across the Cascades. 

The sea level pressure map for 5 PM Monday below shows high pressure offshore that is canting into eastern Washington.  Temperature at 2 meters is indicated by the colors, with brown and red being the warmest air. You can see an area of low pressure over northern California that is associated with warm temperatures...this is the thermal trough and is associated with easterly flow descending down he Sierra Nevada and Cascade ranges.  Notice how this feature produces a large north-south pressure difference over the Willamette Valley and southwest Washington, with higher pressure to the north.  This pressure difference will accelerates the air towards the south (northerly flow).

 By Tuesday afternoon, the easterly flow across the Cascades will strength the thermal trough in the Willamette Valley causing the northerly flow to accelerate over western Washington and warming the Puget Sound region. This northerly flow will bring in cooler air from off the Georgia Strait and Strait of Juan de Fuca into western Washington, preventing Puget Sound from heating up like the Willamette Valley.

Expect the winds to get gusty in Puget Sound on Tuesday.  The wind gust forecast for 2 PM on Tuesday shows wind gusts hitting 25-30 knots from Seattle to Tacoma.   Get your sailboat ready!

So expect blustery/warm (low 80s) condition around Puget Sound on should wonderful to experience--while Portland and the Willamette Valley will be toasty. 

This heat wave won't last.  Wednesday will remain warm, but then marine air will push in on Thursday and seasonal temperatures will return.  And there is no expectation of lightning during the next few days, so I would not expect any high-terrain fires to accompany this short heat wave.  But human caused fires could occur in western Oregon if folks are not careful.

Announcement:  I will be giving a talk "The Great Storms of the Pacific Coast" in Ocean Shores at 6:30 PM on September 7th at the Shilo Inn as part of the Coastal Interpretative Center's summer lecture series.  More information is found here:   Shilo Inn is offering special room rates for those wishing to stay overnight, as well as a special buffet.   

August 23, 2019

Is Western Washington Really in Severe Drought?

For society to make good decisions on environmental matters, decision makers inside and outside of government require accurate and realistic guidance.  For that reason, I am a bit worried that there is a problem with a frequently cited source of information about drought across the U.S.:  the U.S. Drought Monitor website.

The latest release of this information suggests that Washington State is one of the most drought-affected locations of the U.S., with much of western Washington in severe drought.

Below is an expanded version of the graphic for just Washington State.  Severe drought along the coast, much of SW Washington and the western slopes of the northern WA Cascades.  Moderate drought over more than half the state.

Now is it reasonable to suggest that much of our state is in moderate to severe drought?   Severe drought sounds quite scary to me and one would expect extremely serious problems as a result.

Let us begin by looking at the difference from normal of precipitation for the summer so far. Most of the Northwest  has been near normal, including the supposed "severe drought" area.

Current streamflow?  Most sites are near normal (green colors).  A large number are above normal (cyan and dark blue), a similar number are below normal (orange and red).

Wildfires are below normal across the State this year, as is smoke. 

What about water resources?  All the major urban areas have plenty of water, without any drought restrictions.  Seattle's reservoirs are slightly below normal, with no water issues (see below).

The only water resource issues are on the Olympic Peninsula, where Port Angeles and few small water districts are asking for voluntary reductions in water usage.  I could find no impacts on agriculture or any other industry due to the "drought."

The origin of the drought declaration is from the relatively dry winter we had last year, something illustrated by the figure below, which shows the difference from normal of the total precipitation since October 1, 2018. Southeast Washington was wetter than normal, but the coastal zone and the western Cascade slopes were drier than normal.   But having a location that normally gets 100 inches a year, now receive 80 inches, is a far cry from a severe drought and the impacts are very modest at best.   And a dry winter was supplanted by a normal summer even in those parts.

Another way to appreciate this year's precipitation situation is to view the cumulative precipitation over the past year   For Seattle, we ended up with about 80% of normal precipitation (see below, normal is cyan, observed is purple)
And the situation is similar at Quillayute, on the northern Olympic Peninsula coast.
80% of normal precipitation does not represent an extreme drought and has occurred many times during the past 50 years.

My own opinion is that the term severe drought is not appropriate for the kind of situation observed in western Washington this summer--- one with few impacts and not that unusual.  Using such strong terminology makes it harder to get folks to react when a real severe drought occurs and doesn't accurately communicate the actual threat level.   And some politicians use such overly exuberant language to push their agendas. 

This is the kind of year that "abnormally dry" might be appropriate and the "D word" might not be the best choice.  Extreme drought seems over the top.

Severe drought

August 21, 2019

Why you can't trust your eyes

You watch a nice sunset.  You see the sun crossing the horizon and believe your eyes.

But strangely enough, at the moment you thought the sun had set, it had already set several minutes. ago.

What you experienced was an atmospheric mirage, and there are a lot more like this one.

The trouble is that we are trained to believe our eyes.  And we assume that light moves in a straight line.  That an object is where it appears to be.

But that is often not true, particularly when the atmosphere acts as a lens and causes light to bend.

Light is influenced by our atmosphere.  Where the atmosphere is dense, light slows down.  When it the atmosphere is less dense, the speed of light is faster.  And when light moves between layers of different air density, the direction of the light can change or bend.  You have seen such effect with pencils in water (see below)--now consider what happens at sunset.

The atmosphere is denser near the surface and less dense aloft.  The causes a lens effect that results in objects looking higher than they actually are--something known as a superior mirage.   The figure below shows the situation.  The apparent position of the sun...what it looks like to above the horizon.  The real position is below the horizon...the sun has already set.

The sun is not the only object that isn't where you think it is.  The stars, particularly near the horizon, are also shifted upwards by the lens effects of the atmosphere.  And this superior mirage effect can be seen locally as well when warm air passes over cool Northwest waters.   For example, cliffs on the opposite side of the sound can be elevated into sky as impressive walls of land (see example below).

So the moral of the story is don't always believe your eyes, reality can be different.    Same thing for what you hear.....but that topic belongs in another blog. 😃

August 19, 2019

Snow in northern BC, Rain in the Northwest

The fire season is over in northern BC.     Done.  Finished. Currently, it is SNOWING  up there, with all kinds of warnings and dangerous roads (see below).  And this is after quite a wet period recently.   What a difference a year makes!

The latest infrared satellite imagery is scary for this time of the year (see below).   A range of potent weather systems are lined up offshore, ready to move in on us.  Of particular note is the tightly wound cyclone offshore of southern Oregon.   Doesn't look like a mid-August satellite pic.

Tuesday is the last dry day for our region for a few days. The 24h total precipitation ending 5 AM Wednesday show the effects of a potent frontal system, with lots of rain on our coast and Vancouver Island.  Forget the catastrophic fire/drought warnings for that region.  Way too moist now.

During the next 24 h, the rain moves inland, with western WA getting a good wetting.  Even rain in eastern WA.  The last wildfire (Williams Flats on the Colville reservation is 87% contained and will further decline with this rain.

The action on Wednesday will be associated with a strong upper level trough moving through (see map for 8 PM Tuesday below).

And then as advertised, the flow configuration changes and a strong upper level jet moves right across the Pacific into our region (see winds speeds at 300 hPa....about 30,000 ft) for 5 PM Friday, shown below.  Some flights will use this wind field to head directly across the Pacific.

Enjoy the rain.   Our gardens will be happy.

August 17, 2019

The First Autumn Storm Makes Landfall on Wednesday

It is very typical for first "autumn-like" storm to reach our region during the third week of August....and this year will not be an exception.  In fact, we have two systems that will be making landfall this week, with the first one being the strongest.

You can see the impact of this "first autumn storm" in the precipitation climatology Seattle.  Below is the  daily  climatological probability of enjoying at least .01 inches at Sea-Tac.  At the end of August the probability jumps to roughly 25%! 

Rain comes in on Wednesday, associated with a robust Pacific front, with the 24 hr total ending 5 PM Wednesday being substantial over the Olympic Peninsula and north Cascades.  On the coast, some locations could get several inches.

The Wednesday system is associated with a front connected with a very intense low center moving into southeast Alaska (see surface weather map at 2 AM Wednesday). Let's say that some of the Alaska cruises will be enjoying some rock and roll that has nothing to do with music.

Another system comes in late Friday and Saturday, with substantial amounts shown in the 24h total ending 5 AM Saturday.

All this rain will be associated with a massive reconfiguration of the atmosphere to a more fall-like pattern.  To illustrate, here is the upper level map for around 30,000 ft ASL, showing the jet stream winds in colors.   At 2 PM today, there is a ridge (higher pressure and heights) stretching from the Aleutians to California, and the jet stream (yellows and light green colors) is to our north.

But by 11 PM next Saturday everything is different, with a fairly strong jet stream heading right into our region.   You won't have to worry about smoke in the Northwest.  You will have to worry about finding your umbrella.

This pattern change will have a huge impact on wildfire potential, greatly reducing the changes of new fire initiation or the growth of old, smoldering burns.

Thunderstorms Return to the Northwest

 Thunderstorms have been relatively rare this summer, but today will see some boomers over the Cascades and eastern Washington. In fact, the...