Hurricane Sandy's Transformation

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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.

Heavy Rain and Massive Snow

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It was a warm, cloudy day here in the Northwest with large contrasts of rain (see figure below of the observed 24-h precipitation ending 7 PM Thursday): over 3 inches in the Olympics, but only light rain over the lowlands, and a few sprinkles across the basin of eastern WA.   Why so little over the Cascades but lots over the southern slopes of the Olympics?  The key reason is that the flow aloft was southerly (from the south) and you can't get good upslope on the Cascades from the direction.  But you can get good upslope over the south-facing slopes of the Olympics.

But the real precipitation story is to our south, over southern Oregon and northern CA, where some locations got over four inches (pink color) and over two inches was commonplace.  But this is just the beginning.

 With a large low center center over the NE Pacific and persistent strong flow pushing eastward to our south, northern CA is going to get hammered during the next several days.
Flooding is inevitable.

Take a look at the latest UW WRF model precipitation forecast for the next 72 h:  huge amounts, including over 10 inches in "favored" upslope locations.  The Olympics and north Cascades are also wet.

 From weather satellites that measure atmospheric moisture, we can see the narrow plume of water vapor...an atmospheric river..that has been feeding the California rainfall.

 Tonight I will premiere a new figure created by UW WRF modeler Dave Ovens:  72hr snowfall!  Lots (feet) of snow over the mountains of BC, and the higher elevations in the Cascades do very well indeed (foot or more).  Most of the accumulation at pass and ski area elevation will occur later in the weekend as the freezing level drops and the wind direction becomes more westerly.   The Sierra Nevada range goes to white (literally)--more than 45 inches of snow at higher elevations!  The folks at Tahoe must be salivating. 

Here is a close up of the 72h snowfall ending on Monday at 4PM for Washington.  Baker and Rainier will be buried.  Stevens, at least a foot.   Snoqualmie will pick up a half-foot or so.  Not sure whether it will be enough to allow them to open.

Can Smartphone Observations Revolutionize Weather Forecasting?

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I believe that one of the biggest revolutions in weather forecasting is literally in your hands:  your smartphone.

The large Galaxy S3 has a decent atmospheric pressure sensor.

It turns out that a number of new smartphones, such as the Galaxy S3, have relatively good atmospheric pressure sensors in them.  Intended to provide elevation information for a variety of apps, these pressure sensors--if collected and quality controlled--could provide thousands, if not millions, of pressure observations each day across the nation.  The implications are huge, particularly since surface pressure is a uniquely valuable meteorological observation.  As described below, a private-sector company has already created an app to collect and display these pressures, and meteorological research (including myself and others at the UW) are doing research on getting maximum benefits from these pressure observations.

First, which smartphones have pressure sensors in them?  Right now they are mainly Android 3 phones, including: the Galaxy Nexus, Galaxy S3, Galaxy Note, Galaxy Note II, Nexus 4, Nexus 10 and Xoom.  Tens of millions of these phones have been sold worldwide, so there are undoubtedly millions in the U.S. alone.  Millions of weather observatories, if only if they were collected in real time.

One company has stepped up to the plate:  a small Canadian firm called, appropriately enough, Cumulonimbus Software.  They have developed a nice app for the Android 3 phones called PressureNet2.1  that can be downloaded for free.  This app accesses your

pressure observation and your location (smartphones use GPS and/or cell phone tower triangulation) to a central database, and allows you to view the observations taken by others.  Here is a screenshot of the graphics provided by the app:


I have been talking to the developers of PressureNet2 and they said that thousands of folks signed up for their app before and during Hurricane Sandy hit the East Coast.  Here is a plot of the smartphone pressure observations for an area of New York City during landfall.  Amazing....you can see the pressure minimum and the calibrations were really quite good.

Pressure is a uniquely valuable observation...in many ways better than temperature or wind.  Surface pressure expresses the state of the deep atmosphere, since it depends on the weight of the air above.  Pressure doesn't have the same exposure "issues" as temperature (is the thermometer in the sun?) or wind (is the anemometer behind a tree or house?).  Pressure sensor can be inside a house or outside...doesn't really matter since the pressure is essential the same.   Pressure errors are generally easy to remove (e.g., elevation differences or errors in a particular barometer).  And you can also use the  pressure change information.

Even better a number of research groups include those at the UW are studying the use of new data assimilation methods to take advantage of pressure observations. (data assimilation is the combination of models and observations to produce the best possible three dimensional descriptions of the atmosphere.  Data assimilation provides the starting point of weather forecasts done by computers).   With Professor Greg Hakim and Graduate Student Luke Madaus we are testing the use of advanced ensemble-based data assimilation using surface pressure, with sophisticated quality control of the pressure observations.  Here is an example of a Puget Sound convergence zone.  You can see the radar an area of heavy precipitation over Seattle.

These figures show the analysis of our data assimilation system with (left) and without (right) additional pressure observations.  No contest...pressure really helps.


So imagine the possibilities.

Millions of surface pressure observations across the country EVERY HOUR, allowing us to define local scale features, such as the cold pools and dry lines associated with the initiation of severe convection.    Using these observations we could radically improve our definition and description of fine-scale atmospheric features and short-term forecasts.

This could be a major advance in weather prediction technology.  And then we can use the smartphones to provide folks with highly detailed and accurate forecasts at their location.  so the benefits flow both ways.

Exciting times ahead!

Does it REALLY rain all the time in Seattle?

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You have heard the complaint--in fact, you may have made it yourself:
It seems like it is ALWAYS raining around here during the winter.  Endless precipitation.

The funny thing, it really isn't true.  Let me prove it to you.
Let's start with a plot of the probability of getting at least a trace of precipitation (.01 inch) anytime during the day for Seattle Tacoma Airport:

There is roughly a 50-70% chance from early November through April 1.  Sothe  odds are, if you stand out there all day, you will get wet. Duh!

But you don't do you?  You are only outside for a short errand or walk or run or whatever.
So let's ask a more interesting and important question.  What is the chance you will get wet during a particular hour?

Ready to be shocked and amazed?  According to a study by Professor Phil Church and Mark Albright (published in Weatherwise Magazine, December 1974) only 18% of hours had measurable precipitation in November, 19% in January, and 15% in February. 

You heard it right. Even during our rainiest months, more than 80% of the hours are dry.

You don't believe me, do you?   Well, lets look at a plot of cumulative rain at my department over the last 72 hr...a very wet period indeed.  (the rain is the blue line in the upper panel, the lower one is solar radiation).  During rainy periods the blue line moves upwards, during dry periods it is level.  You can see there are plenty of  level (dry) periods (at least 60% of the time), and heavy precipitation intervals (when the line moves up quickly) are relatively rare (perhaps10-15% of the time).

Now most Northwest natives understand this--you can find a few dry hours on almost any day to have fun or do your chores (like remove the leaves from the street drains!).  Few days are complete wash outs.  If you use the weather radar effectively, you can see the dry periods and time them for maximum advantage.  Or befriend a meteorologist!

Talking about wash outs, there was a relatively rare, very intense, rainfall that hit north Seattle around 2 AM last night.  Several of you have sent me emails about it.  Streets turned to rivers and the rain was deafening.  Some of you got .3-.5 inches in less than an hour.  Take a look at the radar image..you can see the intense band (red line).


 Enjoy the rain...there will be a lot more tonight and tomorrow morning...

Major Weather Transition Ahead

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During the past week or so, the Northwest coast of the U.S., from the central California to Seattle has pummelled by one Pacific Storm after another.  Looking at the percent of normal precipitation (see image below) for the last 7 days, you can see northern California has been particularly hard hit, with some places getting 800% of normal precipitation.  The reason:  a persistent trough over the northeast Pacific and an amazing frequency of atmospheric rivers extending into the northern part of the Golden State.
But there is a very positive side to this deluge:  with large precipitation additions recently, both snowpack and reservoir levels are near average over most of California, for a good start to the water season. California needs water.  Eventually, when human-forced global warming hits, they will have  real water problem.  But not this year.


But things are going to change soon, since virtually all the major models are forecasting a major flow transition this weekend, with a high-amplitude  upper-level ridge developing over the eastern Pacific (see example for Saturday at 4 PM).  This ridge is far enough offshore so that we will be in cool, occasionally showery flow, but it will be WAY drier than the last week. 

 This is a favorable pattern for snow, with far lower freezing levels.  Finally, it should be cool enough so that Snoqualmie will get enough white stuff to open.  Here is the 72 h snow total from the UW WRF model.  Over a foot in the Washington Cascades, a feet over some favored locations.

This pattern continues into early next week.  Take a look at the upper level map for Monday at 4 PM... an even bigger ridge.  But don't expect that we will be cloud free and completely dry...there is plenty of flow over us and weak disturbances will southward down the eastern flank of the high.

Anyway, this will be an opportunity for things to dry out a bit and winds should be more modest for the entire region.

KUOW Hike of the Week- Granite Lakes

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Updated 12-4-2012
You can hike on this new hiking trail that has been converted from an old logging road. This is next to Mailbox trail. This is a mostly snowfree (maybe at dusting) hike to the bridge and I do not suggest going beyond the bridge as the bridge is a log (maybe some ice) across the river.
Most  photos by Alan Bauer- thanks
Photo below is Granite Lake with Mt. Defiance
Check out Green Trails Maps for directions. The map for this hike is 206S. Click here for complete list of maps. 
Photo below Track hoe working it's way down the road result no road - trail.
Photo below is new trail
Photo below is a good stopping point at 3.2 miles one way. This log crossing can have ice on this time of the year. A bench for lunch no less, Alan is waiting for room service lunch. Photo @ Dennis Long
Photo below is view of the ridge running from about Green Mt to Bessemer photo @ Dennis Long
Photo below is for those that want to take risk of going to lake need to cross. About 30 feet long.
Driving Directions: From Seattle, drive east on I-90 to exit 34 . Turn left (north) onto 468th Street and follow it to the junction with the Middle Fork Snoqualmie Road (Forest Road 56). Turn right and continue up the Middle Fork Snoqualmie Road to the end of the pavement (about 3 miles from the I-90 exit). Turn right onto a gated road and park, being sure not to block the gate.
Trail Directions: Trail starts off as road walk about .50 mile now and then you can hike 3.2 to log crossing. This is about 1000 foot gain to bridge. This part is usually snowfree.

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Weather extended outlook- Wash. Cascades

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Updated 12-5-2012

Forecast models seem to making a shift to cooler conditions for the Washington Cascades with snow levels averaging close to 3000 for the week of 12-9-2012. So below normal temperatures with average to above snowfall. The map below is the 500 mb chart (18,000 feet) and shows a cool and sometimes moist northwest flow. Thus above normal snowfall with this pattern through 12-18-2012.

For custom forecast anywhere in the world or if you need an expert witness in a legal weather related matter contact usClick here For our weather services or call toll free for information 877-969-4786.