Hurricane Sandy's Transformation

A lot has been said of the severe damage that may accompany Hurricane Sandy---10-20 inches of precipitation west and southwest of the storm, coastal wind gusts exceeding 80 mph, severe damage to barrier beaches, flooding over western L.I. sound and other vulnerable locations.  The threat is great and imminent.

But what has not been discussed in depth is the extraordinary transformation that will occur as Sandy makes landfall and moves inland....from a tropical to an extratropical storm, the first with a warm core and the latter with a cold core.  I will talk about that here.

Right now Hurricane Sandy is almost exactly one day from landfall on New Jersey.  It possesses sustained winds of 75 mph and a central pressure of 950 hPa, and is very large storm and will expand even more.  Here is a recent infrared image, showing the band of moisture feeding into the center of the storm.

As I have mentioned before, hurricanes and tropical storms derive their energy from the warmth and moisture of warm oceans, and Sandy is no exception.  Such tropical storms are warm-core systems, with the warmest air in their centers.  In general, tropical storms are not found in regions with large horizontal changes in temperature.

 Recently, there has been a substantial warm anomaly of the sea surface temperature (SST) of the western Atlantic (SST much warmer than normal), which helps tropical systems like Sandy move northward and stay strong longer than normal.  Here is a recent map showing the SST anomaly:  much warmer than normal along the path of Sandy, by over 1.5C!  So a very attractive welcome mat had been laid for tropical storms over the western Atlantic.

Now midlatitude or extratropical storms get their energy not from warm oceans, but rather horizontal temperature changes---generally warmer to the south, cooler to the north.  It turns out that the temperature changes on our planet are concentrated in the midlatitudes...and this temperature gradient fuels nearly all of the storms we experience in the winter.  Midlatitude storms tend to be cold-core, with the coolest temperatures in their center.

Sometimes a tropical storm moves northward and undergoes an amazing transformation, from a warm core tropical system to a cold-core extratropical system, and switches its energy supply from the warmth and moisture of a tropical ocean to the temperature gradients of the midlatitudes.  This is called Extratropical Transition (ET) in the business.

Sandy is about to go through the transformation.

Let me show you.  The figures below show you the temperatures and pressure pattern (actually heights of the 850 hPa pressure surface) at around 5000 ft above sea level for three times (Monday morning, Monday evening, and Tuesday morning--Seattle Time).  At the initial time there is warm air at the center of the storm (the dark orange color).  Fairly symmetric structure (which is typical of tropical systems).

By tomorrow evening, just as the storm is making landfall,  the warm core is fading, and temperature structure is becoming much more asymmetric.

 By Tuesday morning, the transformation is complete....cold air has reached the center of the storm and the warmest air is found far to the east of the center

Many tropical storms weaken when they go through this transition, but for a small subset the opposite occurs.   The two energy sources work synergistically for a while, resulting in an strengthening and expanding system....such is the forecast fate of Sandy.

In fact, during this transition stage, Sandy will continue to intensity until it makes landfall.  Here is the latest forecast from the NWS NAM model:

956 hPa low center at 2 AM Monday morning (PDT)

12 hours later 948 hPa pressure (2 PM Seattle Time).

My next blog will take a look at the scorecard for this storm....and return to talking about Northwest weather.  And there is a lot to talk about, including an atmospheric river situation setting up and 5-10 inches forecast over the next 72 hr over NW mountains.

A More Perfect Storm Will Strike the Midlatlantic Coast

There is now a high probability that an historic storm will strike the mid-Atlantic states next week.  

A storm far more intense than the Perfect Storm, of book and movie fame.

One of the most intense landfalling cyclones in several years and perhaps the equal of the great Columbus Day Storm of 1962 that brought 100 mph plus winds to large swaths of the Pacific Northwest.

The storm that will strike the East Coast early next week is eerily like the Columbus Day Storm and the famous Perfect Storm in several ways.  Each of these started as a typhoon or hurricane and  transitioned into a powerful midlatitude cyclone, undergoing what we call extratropical transition.

As I will describe below, although there is still some uncertainty, the major modeling systems, American and European, have now locked on to a very similar solution.

Hurricane Sandy is now a Category 1 storm with sustained winds about 75 mph and a central pressure of 971 hPa.  Now located just north of the Bahamas and still over warm water.

At the same time, an upper level trough is approaching from the west and the interactions between the two will cause Sandy to morph into a hybrid storm, expand,  and intensify rapidly.  The National Weather Service is doing something very rare:  adding more radionsonde (weather balloon) launches (four times a day instead of twice day)...a very wise investment.

This figure shows the situation on Saturday, showing the upper trough over land (red line) and an upper low over Sandy (purple circle) (the figure is for 500 hPa--about 18,000 ft above sea level).  These will join up during the next few days.

Sandy will not only grow larger, but the pressure will fall, and the winds will increase.  Perhaps the most dramatic description of this process (but not technically quite accurate) was given by a supposed meteorologists in the Perfect Storm movie, click on the picture to view it:

Click to see video

Until recently, two of our most important models (the European Center model and the U.S. GFS) were substantially at odds, with the European model taking the storm into Maryland/Delaware and the U.S. model taking the storm out to sea.   In more recent runs, the U.S. high resolution model began recurving the storm back to the East Coast, first over New England and now across Long Island, with the European Center model not changing very much.

But now European Center model has moved northward, the GFS model swings inland earlier, so the differences have diminished.

Lets talk a look.  First the U.S. GFS.  Here is a series of surface pressure analyses from this morning's run (solid line are sea level pressure and shading is relative humidity.  A very deep low moves in off the ocean at 8 PM PDT Monday with a pressure of around 950 hPa (the Columbus Day Storm was about 960 hPA), then swings in over Long Island on Tuesday morning, then moves in a circle just inland, and eventually moves northward on Wednesday.  This is extraordinary behavior--to have a deep low just circle about for a day, before heading out of the region.  And deadly.

The winds would be very strong and damaging...sustained winds of 50-70 knots over the water with higher gusts.  Here is the wind forecast at 2 PM on Tuesday morning.  Winds are in knots (see legend).  Lots of sustained winds above 50 knots over the Atlantic and some coastal locations.

 What about the European Center forecasts?  A sub-940 hPa low makes landfall on central New Jersey, circles around, and then moves northward.  The European Center and GFS models tend to overdo the deepening in this situations, so I don't believe we will see anything below 950-960 hPa.

We are talking about huge events, if either of these forecasts are correct--or if the truth is somewhere in between.  Heavy rain, powerful winds over a long period, storm surges north of the low center.   This will make the Perfect Storm look like a zephyr.  Take a look at the cumulative rainfall through October 31st at 11 PM PDT.  5-10 inches over New Jersey and adjacent areas.  And this is a relatively coarse model....could well be more at some locations.  Cascade Mountains do almost as well!

Is there still uncertainty now that the major models are converging?  You bet.  There is some probability that the real storm will moves out to sea.  To get a handle on this, lets take a look at the last European ensemble prediction (left panel below) and the their high-resolution forecast (right panel) for 5 AM PDT on Tuesday.   The shading is a measure of uncertainty...how much the ensemble members disagree.

The high resolution forecast shows a huge, powerful storm making landfall.  Importantly, the ensemble (the mean of many different forecasts) show a low center at a similar position, perhaps a bit farther north.  This provides some confidence in the high-res forecast.  But the shading shows there is considerable uncertainty still, with the shading offshore suggesting a few of the forecasts were for a low farther offshore.  So, this is not 100% now...but a very good bet at this point.

 I think you can appreciate the technology...high resolution forecasts show us some possibilities, with ensemble prediction giving us a measure of the uncertainty in the forecasts.  No forecast is complete without a quantification of the uncertainty.  Nothing is 100%.   And certainly not a complex, sensitive situation like this one.

Funnel and Waterspout Seen Near Everett

A dramatic series of pictures have been reaching the web and sent to me of a waterspout/funnel cloud over and near Hat Island, located between Everett and Whidbey Island (see map).

Here are some amazing shots of a relatively unusual phenomenon around here (this may not be Oklahoma, but every year we get a few).  In the first, you can clearly see a beautiful waterspout that is sucking up water off the surface.

Courtesy Sharon Wandler and KING-5

 Here is a shot on the Scott Sistek Blog site.   There is some evidence of a double vortex (look at water).

Look at these extraordinary images from Chris Evans (you can view and access pictures at his website: 

Courtesy Chris Evans

Courtesy Chris Evans

If you carefully, there appeared to be three funnels.  And here is a close up of the same image.

These funnels were associated with a fairly strong convective cell that was apparent in the Camano Island radar.  The funnels were seen between 4 and 4:15 PM (2300 and 2315 GMT).  The radar reflectively shows some very strong echos (yellow and reds) near the location in question.

A closer in view. Wow..the red dot looks like the position of the funnel!

The radar provides the tops of the convective cell...looks around 20,000 feet (see image).  High for around here, a joke in Oklahoma.

The atmosphere was modestly unstable today, with small amounts of CAPE (convective available potential energy)--the measure of how much energy a rising parcel can acquired from buoyancy.   Here is the plot for around 2 PM--perhaps a few hundred.  Decent for western Washington, comical in Oklahoma.  Lot of instability offshore as the colder air moves in aloft.

There was no real boundary which could cause horizontal rotation (like a front or shear line), so vertical wind shear (winds increased rapidly aloft), must have led to rotation around a horizontal axis that was redirected into the vertical by the strong updraft in the convective cell.  Sounds complicated I know, but one of the great insights meteorologists gained during the past 30 years was how horizontal rotation (rotation around a vertical axis) can result from wind shear in the vertical.

And look what's back in the mountains:

Snow and Cold Returns to the Northwest and California

The atmospheric flow pattern is changing in a big way, developing a deep trough over the eastern Pacific, and driving much colder air south...and I mean south.  The Cascades will get its first real snowfall starting this weekend and then the white stuff will push southward towards the Sierras.  The snow level will drop sufficiently that even our lower passes could get covered with a few inches of the white stuff.   Get your snowball arm warmed up, grease that sled for the kids....the first taste of winter will soon hit the higher elevations.

 By Saturday, deepening upper level troughing will develop over the Gulf of Alaska.  This trough is associated with cold air, and the tight packing of the height lines suggest strong jet stream winds.

During the next few days (Tuesday AM),  the trough plummets southwards, bring much colder air over the NW (see graphic below)

The Climate Prediction Center's 6-10 day forecast shows the result of this pattern:  a high probability of below normal temperatures (dark blue colors) over the NW and California.   This air will be cool and unstable, and the result will be snow in the mountains. How much?   Lets check the model forecast!

 First, for the 24-h ending 5 PM on Saturday.  Up to 8 inches in the north Cascades and mountains of southern British Columbia, with lighter amount spreading down into norther Oregon.

 The next 24 hrs (ending Sunday at 5 PM), the same thing.

One more day later, snow starts spreading into southern Oregon, and lightens up in Washington.

 Then early next week the snow picks up again in the Cascades and extends over the Sierras.

 The snow level by the end of the weekend will drop to around 3000 ft, so even Snoqualmie should see some white stuff.  Stevens is a sure thing.   This is good news for the Ski folks...they are going to get a good start to a base on their higher runs.  El Nino has weakened and in any case the first part of the autumn is not influenced by it much anyway.

This is all great impetus for us to get our application Seattle SNOWWATCH ready for this winter, with new capabilities that I will describe in a later blog.

How high do weather balloons rise?

This week we were all thrilled by the jump of Felix Baumgartner from a balloon that ascended to 128,000 ft.  At that height he was above the vast majority of the atmosphere (99% of the atmosphere was below him!), the sky was nearly as black as space, and the curvature of the earth was obvious (see picture).  Simply amazing.

But what about weather balloons...also know as radiosondes...how high do they fly?

Radiosondes are launched twice a day at 92 locations around the U.S. (see map and picture).

As shown by the picture, these units include a balloon, a parachute, and an instrument package that radios back temperature, humidity, pressure, and sometimes position (for units with GPS).    Winds can be computed by tracking the radio signal and some trigonometry.  Radiosondes originated in the late 1920s and have been an important part of the weather observing network ever since.  The biggest problem with them is that they are mainly limited to land stations.

The pressure near sea level averages around 1013 hPa (hPa is unit of pressure and is the same as a millibar).  Most radiosondes rise to 5-15 hPa before the balloons burst (the balloon expands as the pressure decreases around it...finally the balloon gets so large it breaks up).  Then the parachute brings the unit back down to earth in an controlled way.

What heights are these balloons reaching?  Roughly 100,000 ft or 20 miles.  A few of the balloons ascend to 5 hPa, reaching around 115,000 ft---not quite as high as Felix Baumgartner.

Pressure tells you about the amount of atmosphere above you, so if you go from roughly 1000 hPa to 10 hPa, you are rising above 99% of the atmosphere.    Felix was virtually in space--without his space suit he would not have survived.

Many people wonder...how many of the radiosonde are recovered and reused?  According to the National Weather Service less than 20% of the 75,000 radiosonde launched by the U.S. are recovered.  Considering that they cost several hundred dollar each, this is not a trivial cost (plus gas, parachutes, labor costs).

A funny story...several years ago I gave a lecture on radiosondes to my 101 class. The next day a grinning student walks in with one.  Where did you get it, I asked?  Turns out he was playing basketball with his friends when the sonde drifted down into their game.  The radiosonde had been launched at McChord and then drifted north under southerly winds.    His friends were mighty impressed with his knowledge of the device!

Here is the plot of the latest radiosonde data from a nearby launch location:  Quillayute (or Forks) on the Washington coast.  This unit got to 10 hPa.  The black line to the right is temperature, left line is dewpoint, and the y axis is pressure.  The bottom (x axis) is temperature.  Temperatures dropped off with height until the radiosonde reached the stratosphere, after which they increased.  The winds were weaker up there too.

One of my favorite type of videos are made by folks launching a camcorder on a weather balloon--essentially doing remotely what Felix did this week.  Here is a good one (click on image)

http://www.youtube.com/watch?feature=endscreen&NR=1&v=1h0ptpimm04