Monday, October 29, 2012

October 29, 2012: Ex-Sandy Undergoing Rapid Transformation

8 pm:  Vortex of Change

The central vortex is now inland over New Jersey, and rapid structural changes are taking place.   Most importantly, the central pressure is increasing...the storm is filling from below, which is starting to weaken the system.  Second, the system is now highly asymmetric, with most of the heavy rainfall on the west and north sides of the system.  Heavy rainfall is being maintained on the north-western side by jet stream energy aloft, and this is a very typical asymmetry that develops in tropical systems that interact with the jet stream.  Third, a punch of dry air aloft off the continent has entered the storm from the southwest, and has wrapped all the way around into the storm's interior.   The eastern semicircle is highly eroded and open.

Radar:  WeatherTAP
Water Vapor:  WeatherTAP
Additionally, warm air has invaded the storm to its north, cold air to its south - a highly wrapped-up, ying-yang type of pattern that spells early demise of the storm:
The strong temperature differences that helped to maintain the storm for the past 24-36 hours - warm oceanic air juxtaposed with a cold air mass - are starting to mix out.

Is the worst over?  Not yet...but conditions should slowly start to improve after midnight.  These enormous wind engines take their time to wind down, even as they lose their principal sources of energy.

October 29, 2012: Sandy Coming Ashore


5 PM Track and Intensity Forecast:

National Hurricane Center

Forecast positions and max wind speeds:

5 pm Monday 90 mph
2 am Tuesday 75 mph York, PA
2 pm Tuesday 60 mph Altoona, PA

Check out the NWS warning coverage for the Mid Atlantic and Northeast - I am amazed to see both a hurricane warning and a blizzard waring, from the same storm, across a distance of only a few hundred miles!


4 pm, an amazing pressure gradient:

5 PM Heavy Rain and Snow:

5 pm radar composite, showing heavy rain bands and the western eye wall along the Delaware coastline.  Yes, that blue zone over the Appalachians is a blizzard!  What is this thing...a snowcane?  Blizzardcane?  White hurricane?


5 PM Wind Forecast

As of 5 pm, sustained winds are over 30 mph area-wide, with gusts into the mid-upper 60 mph range are being reported in the DC-Baltimore region.  The core of highest winds is still to come, tonight, at the point of closest approach of vortex near York, PA (due north of Baltimore) approximately 2 am Tuesday morning.

***From Previous Post Today***
High Wind Forecast:

We are still on track for a prolonged period of high, sustained winds, from the west, not shifting around to south winds until Wednesday.   As Sandy approaches the coast today, the winds will rapidly ramp up in intensity and gustiness.

I am introducing a new wind prediction product here, courtesy of Quantum Weather and my colleague Professor Robert Pasken (based in St. Louis, MO).   For several years Quantum has produced a highly sophisticated regional forecast model, designed to help area utilities plan for worst-case weather scenarios.   The max sustained wind plots, generated for the greater Baltimore region, show a significant ramp-up in high winds (> 45 mph sustained) after 11 AM this morning expanding westward across the region (4 pm), with widespread 50-55 mph sustained winds by 8 pm:

The wind model takes into account the detailed terrain and topography of the Bay.  Focus on the bottom panel, at 8 pm tonite, with an intense west wind sweeping across the entire region.  The areas of greatest wind damage potential (red colors) become concentrated (1) along the high terrain (Catoctin Mountains) west of Baltimore, and off the western shore of the Bay, southeast of Baltimore.   You can literally see streaks or corridors of high wind that accelerate over the Bay's waters and then impinge along southern Maryland.  The greater Baltimore region lies in a bit of a "wind depression" between mountain- and Bay-enhanced regions.

Sunday, October 28, 2012

October 28, 2012: Staying The Course

The storm remains at 75 mph intensity.  The inner core region is holding onto its tropical characteristics including intense thunderstorms and a partial eyewall.   Limited additional intensification to 80 mph is possible overnight and tomorrow morning.  The storm will make landfall in southern NJ and weaken to 70 mph over Wilmington, DE.  The vortex will track NW across southern PA, weakening to 50 mph in the vicinity of Harrisburg, PA:

National Hurricane Center
The cold front has pushed east of our region, and will be a player during the continued transition of this hurricane into an extratropical storm:

As I discussed earlier, this front is serving to focus a heavy rain band along the Delmarva coast:

One aspect I neglected to mention earlier, regarding wind damage to trees:   Many tree crowns have shed their leaves, and once the winds pick up on Monday, additional defoliation will occur.  Leaves  act as tiny "sails" which catch the wind and greatly magnify the wind stress exerted on tree limbs.   This may mitigate some of the tree failure during the high wind period.

Oct 28, 2012: No Change In Forecast Conditions

Sandy is well into its extratropical transition process.  It has an absolutely enormous wind field spanning nearly 1000 miles across.   At 5 pm, the intensity was still 75 mph - a minimal Cat 1 storm, but the hurricane hunters have had a difficult time finding actual surface winds in excess of hurricane intensity.  This does not mean the storm is weakening...rather, it is holding its own as it moves over a river of warm water (the Gulf Stream) in the western Atlantic, and the jet stream continues to feed energy into the circulation.   Point of landfall still appears to be central-southern N.J. with a long transit across Pennsylvania.  Tomorrow, Sandy is expected to turn toward the west and execute a complex "S"-shaped turn that will arc it first across the Mid Atlantic and then into New England:

Tropical Prediction Center
The expected weather impacts in the Baltimore region are covered in my earlier posts today.   Early Monday morning the winds will begin to really pick up, increasing throughout the day.   Periods of heavy rain will fall.  By Monday night, and overnight, expect the winds to be sustained in the 40-50 mph range with gusts in the 60-70 mph range.   It is going to be a rather frightening night, with the roar of the wind, and heavy pounding of rain.  The worst of it will continue into Tuesday.  Then, on Tuesday evening, winds and rain both will begin to taper across the region.   On Wednesday, conditions will rapidly improve.

This storm is going to hit the Mid Atlantic extremely hard.  It is unlikely to experience a sudden shift in track, nor a sudden lapse in intensity.  I see no miracle nor "dodging the bullet" here.  This will be a historical, and freakish, meteorological event.  Over 50 million residents will feel significant impacts from this storm.   But we have gotten through the Hazels and the Isabels and the Irenes.  We will weather this one and recover.   I believe that much of the dread we feel is due to anticipation.   Once the event is underway, we take solace in the fact that the worst will soon be over.  Knowledge is power.   Preparation is power.   Getting together and helping our neighbors and our community recover during the aftermath is power.  

Next Update:  11:30 PM tonight. 

October 28, 2012: 11 AM Forecast Update

1.  Track.  Here is the 11 AM track forecast update from the Hurricane center:

National Hurricane Center
This has essentially remained unchanged.

2.  Intensity.  The storm continues to maintain 75 mph in the core, and the radius of strong winds has expanded since earlier morning.   The Hurricane center is calling for a bit of additional intensification later today and tonite, before weakening at landfall.

I just received an email from my colleague Dr. Frank Marks, of the NOAA Huricane Research Division, who is presently flying a recon mission inside the eye of Sandy, and he reports that the dry air has not yet penetrated the inner core (dry air would help weaken the system).

National Weather Service
3.  Impacts.  All impacts and timing still on track, based on my earlier morning post.  See the map of current NWS warnings above.  This may be the only time in your life you will see a hurricane warning on one side of the mountains, and a winter storm watch on the other side, for the same storm.

4.  Next Blog Update:  Early this evening.

October 28, 2012: Time To Make Final Preparations

1.  Track:  The models continue to converge on a point of landfall along the South Jersey shore...but please note there continues to be generous uncertainty:

National Hurricane Center
2.  Intensity:  The storm has held together overnight, as a hybrid (combined tropical-extratropical) system rated overall as Cat 1 (75 mph sustained winds).  A core of thunderstorms continues to feed energy into the center.   The storm may get an energy boost as it passes a narrow strip of warm (80 F) ocean water later today.   North of this region, off New Jersey, the water is much cooler, which argues for the storm weakening somewhat prior to landfall.  However, bear in mind that the jet stream will also continue its interaction with this storm, feeding its own energy into the system.  Thus, the storm's surface winds may show very little weakening prior to landfall.

3.  Weather Impacts.  The basic principles I discussed in yesterday's post (October 27) still apply.  Here are the updated rain accumulation and wind gust maps from this morning, provided by the National Weather Service:

The wind gust forecasts indicate strong winds ramping up early Monday morning, peaking Monday night into Tuesday morning.   High Wind Warnings go into effect area-wide Monday morning, for sustained winds up to 40-50 mph and gusts to 60 mph.   Expect high winds for a sustained period, 18+ hours. 

This blog will be updated later today.

Saturday, October 27, 2012

October 27, 2012: The Beast of the East

We're starting to get down to details:

1.  Model Track/Landfall Location

With a bit of waffling north and south (as expected) the models appear to be converging on a solution that brings the storm into the strip of real estate from Delaware-New Jersey.   I expect that the track forecast will tighten up in the next 24 hours.   As long as this is the case, Baltimore-D.C. stay in the less energetic left-of-track semicircle:

National Hurricane Center 11 AM Track Forecast
Obviously, we would prefer to be farther away from the center...but this vortex is so large, you'll need to be 100-150 miles away before significant effects are no longer felt!    Here is the afternoon spread in track forecast (still highly divergent):

South Florida Water Management District
2.  Intensity Forecast

I believe this aspect will remain problematic.   The tropical core itself has become disorganized and has opened up;  Sandy is barely clinging to hurricane status.  The energy-yielding thunderstorms are no longer sitting over its core.   The storm is becoming tilted over by strong vertical wind shear, and dry air is invading from the south and west.   The storm is likely in its early phases of extratropical transition.   The models historically do not demonstrate as much skill with intensity as they do with track.  I'm going along with the Hurricane Center's general idea, which suggests a bit of intensification is likely prior to landfall. This is because of the anticipated jet stream interaction, which adds energy, and also the energy contained in the strong contrast between air masses (cold continental air abutting with warm tropical air).  Will the storm remain organized enough to efficiently tap this new source of energy?  I would still plan on ex-Sandy maintaining tropical storm status at landfall.

Here is a portrayal of the three elements beginning to interact, creating a hybrid vortex out of post-tropical Sandy:  (1)  strong cold front moving across the Mid Atlantic;  (2)  large trough (southward dip) in the jet stream (over the Upper Midwest);  (3) the tropical vortex itself:
Surface Weather Map, 11 AM Saturday (Intellicast)
Jet Stream Chart, 8 AM Saturday (NCAR)
3.   Wind Forecast

The left semicircle usually experiences weaker winds, because of several factors:  (1)  the storm's forward movement, or translation, which is in a direction opposite the counterclockwise swirl of wind (the storm is moving W-NW, but the winds are blowing from the W-NW - the opposing effects cancel somewhat);  (2) in the right semicircle, E-SE winds add to the direction of movement, which is from the E-SE;  (3) on the left side, winds are blowing over land (where surface friction is large) but on the right side, winds continue to scream in off open ocean, where they have built up momentum;  (4)  gustiness is reduced on the left side, because of the cooler, stable air mass in place over land.   When the air layer near the ground is stable, it resists vertical stirring, meaning less of the higher momentum air aloft gets mixed down to the surface.

Here is the NWS maximum gust forecast during the window of maximum storm impact (Tuesday morning-evening).  Notice how the Bay really amplifies the wind speed, and the highest winds shift to north-central MD as the storm tracks by to the N&W:

National Weather Service
National Weather Service
How fast will the winds diminish?  (1) If the storm stalls inland over PA, strong winds remain longer;  (2) if the storm makes progress toward the N&W, the winds will begin tapering Wed AM.   Winds will start to taper once inland, as the hybrid vortex "occludes" (the low pressure fills in and loses its jet stream support) and the supply of oceanic heat and moisture is lost.   However, this large vortex will take a while to wind down, because of its tremendous angular inertia (think of it as an enormous atmospheric flywheel).

4.  Big Rains

We're still on track for significant rainfall, in the 6" range.  This could begin falling as early as Sunday afternoon.  When large, landfalling storms approach the East Coast, the strong onshore flow of warm ocean air induces a frontal boundary along the coast.  In Sandy's case, the cold front presently moving across the Mid Atlantic will set up shop along the coast, and will become reinforced...becoming the coastal front.  There are myriad factors contributing to heavy rain:  (1) Heavy rains tend to concentrate not just around the core of this storm, but also focus along the coastal front, where there is strong, sustained uplift of air.  Heavy rains will likely spread all the way up to New England well in advance of landfall, along this front;  (2) if storm movement slows, the higher the local rain accumulation; (3) uplift of tropical air along the east slopes of the Appalachians will create "orographic enhancement" of heavy rain;  (4) the strong uplift of air in the jet stream trough will generate heavy rainfall over a wide area;  (5) a mitigating effect for Baltimore may arrive Wednesday, in the form of a surge of dry air from the southwest...shutting down precipitation over parts of the interior Mid Atlantic.


5.  Tidal Surge

Surge effects, coinciding with astronomical high tide, should be in the 3'-4' range, mainly in the northern reaches of the Bay and large main stems feeding toward Baltimore and Annapolis.   As the storm approaches from the east, W-NW winds will initially push water out of the river mouths and toward the eastern shore of the Bay.  As the storm passes to the N&W, the water will slosh back, and E-SE winds will push water into the mouths of main stems, and along the Bay's western shore.
This is NOT another Isabel, provided the storm track remains to the north of Baltimore.

Friday, October 26, 2012

October 26, 2012: Sandy's Baltimore Impact Becoming More Certain And More Severe

What The Prediction Models Say:

The prediction models continue a westward trend in the point of landfall;  the official hurricane center forecast issued at 11 AM today calls for landfall of a Cat 1 storm over Delaware early Tuesday morning:
National Hurricane Center
As we discussed yesterday, note the very broad cone of uncertainty.  The various models still remain widely divergent:
South Florida Water Management District do the various ensemble runs of individual models:
Weather Underground
Nonetheless, because of the consistent wholesale westward shift in track guidance - closer to Baltimore - confidence is growing for increasingly severe impacts in our region.   Bear in mind a couple things:  (1)  don't focus on the center point of the hurricane center prediction, as this is a very large storm, and the most severe effects will not be confined close to center;  (2) we still have another 5-6 cycles of model runs to go before landfall.  These runs are made every 12 hours, and I would not be surprised to see further shifts in the landfall point, perhaps even back toward the eastern side of the guidance spread.  Some back-and-forth movement of tracks is typical for this timeframe in the numerical track guidance.

Likely Impacts In the Baltimore Metro:

Again, because we are 2-3 days out from passage of the vortex past our region, we can't get too specific here.   But some generalities are starting to emerge.

(1)  Tropical Storm force winds.  If we remain on the LEFT SIDE of the track, the overall energy will be less than on the right side.   This means lower wind speeds and gusts.   This applies as long as the storm is moving briskly.  Here is a very preliminary estimate of the sustained winds on Tuesday morning and early afternoon, the likely period of peak activity:
National Weather Service
These winds are tropical-storm intensity (40+ mph sustained, with higher gusts, perhaps in the 50-60 mph range).  The highest winds will be over the Bay and along the eastern shore of the northern Bay.  These winds will initially push water OUT of the tidal mainstem rivers draining along the western shore of the Bay, but pile up a surge of several feet along the Bay's eastern shores and mouths of major tributaries.  Winds will also be stronger in the high elevations of the Maryland Piedmont and Blue Ridge;

(2)  Heavy rain is expected, to the tune of 6"-10".  Here is the latest graphic illustrating likely amounts:
National Weather Service
These rains will be fairly steady with intermittent, heavy bursts, for a 24+ hr period.  So there is certainly the possibility of urban and small stream flooding, as well delayed flood crests in the main stem rivers

(3)  Timing of impacts.  Because this is a large storm, impacts will begin long before the center moves onto Delaware.  Conditions will begin deteriorating Sunday night...meaning arrival of rain, and a freshening of the breeze.   Winds and rain intensity will continue to ramp up through Monday, peaking early Tuesday AM.   As long as the storm remains on the move, winds should begin to abate early Wednesday AM, and rain will slacken.    The winds may die down particularly quickly, from a combination of the storm moving away from Baltimore, and fairly rapid weakening of the storm once inland.

All of this information is predicated on the latest (11 AM) hurricane center track and landfall information, which is the best guess offered by the hurricane prediction pros - a mental consensus of the dozens of track and intensity forecast models.  There are models that suggest alternate scenarios, including (1) a stronger storm at landfall than the hurricane center is forecasting;  (2) storm tracks further north and east of our region;  (3) the storm becoming stationary for a time along the coast, and even executing a tight loop - which would greatly prolong the window of severe weather.   So much of the outcome depends on how the post tropical phase of Sandy interacts with the large, intense jet stream trough, and the vagaries of its poorly-understood extratropical transition process.  This interaction has not yet started.  There are few historical precedents for this type of unusual tropical-extratropical merger to gain insight from.  There is one model (the European Center model) that continues to portray an absolutely frightening storm, the intensity of which has never been experienced in our region - but there is some reason to discount this scenario as unlikely.

Thursday, October 25, 2012

October 25, 2012:  Countdown to Hurricane Sandy

After a long hiatus (I have been on sabbatical) I have decided to re-activate the UMBC Storm Page, for obvious reasons.  I will be updating this daily as long as Sandy remains a threat.

Track Uncertainties 


Let's start with the Hurricane Center's 11 AM advisory:

NOAA Hurricane Prediction Center

Sandy is a very late season hurricane, and an intense one at that - presently a high-end Cat 2, with tropical storm force winds extending across a 200 nmi diameter vortex.  The forecast is for Sandy to accelerate northward this weekend and gradually weaken (from a combination of cooler ocean water and increased wind shear).  It is expected to dip just below hurricane intensity once it passes north of the Carolina Outer Banks.

But DO NOT focus on the exact center-line of the forecast track, but rather on the large envelope of track uncertainty, which expands with time.   The numerical prediction models have been struggling for days to reach consensus on exact track, and point of landfall.  Some of this is to be expected, since there are a great many different types of track models being used, and large errors are typical in the 5-7 day range of prediction.   To provide an example of the disagreement, the plot below (called a "spaghetti diagram") illustrates the possible track solutions from a large number of these models, compiled at 2 pm today:

South Florida Water Management District
Most models curve Sandy, as a post-tropical storm, back TOWARD the U.S. mainland.   But there is tremendous divergence, or spread, in where the storm will make landfall.

Another type of track prediction comes from what's called an "ensemble model".  This is where a single forecast model is run with many different meteorological starting points, in order to better characterize the error inherent in the simulation.   An ensemble of individual runs is generated.  An example of this is shown below, which is the ensemble model forecast for a widely used model called the GFS:

Weather Underground

Once again, lots of possibilities exist for various points of landfall, ranging from the Mid-Atlantic, New England and Canada.   The white track indicates the "center of mass" of all these various ensemble solutions.  Notice the odd kink over Long Island.  This is created when the storm slows to a stop (stalls) and essentially loops on itself, before making final landfall.

What Will The Impacts Be? 


With all the uncertainty in the track, it's still premature to nail down specific impacts for the D.C.-Baltimore region.   Right now we can only speak in general terms:

1.  Timeframe will be Sunday evening through Tuesday, with greatest impacts on Monday;
2.  Expect, at a minimum, periods of heavy rain (squalls) with sustained windy conditions

It's still too early to foretell (1) exact amount of rain, and (2) speed of maximum winds - there are too many variables involved without knowing (a) exact track;  (b) vortex intensity as it enters the post-tropical phase;  (c) storm speed (forward motion);  (d) vortex size.

Suffice it to say, given the POSSIBILITY of a landfall as close as Delaware (bringing the storm center very close to Baltimore), it's prudent to begin preparing for rapidly deteriorating conditions Sunday night, the possibility of urban and small stream flooding, and extended power outages.
The area utilities (Dominion, BGE, Pepco) have already started their initial preparations.

The Nature of The Beast 


It's important to understand that the storm that impacts the Mid Atlantic and New England is not expected to be purely tropical, but rather a hybrid type of system that combines elements of a hurricane and a nor'easter (nor'easters are intense wintertime coastal cyclones).    As Sandy approaches our area, it is expected to interact with a vigorous trough in the jet stream (currently over the western U.S.).   So while we have to not only forecast the track and intensity of the tropical storm, we also must accurately predict how the jet stream trough evolves over the next few days, and the manner in which it will phase with post-tropical Sandy.   Some hurricane-trough interactions can intensify a vortex, while others can lead to its demise.   Sandy's interaction with this trough is expected to (1) maintain a strong post-tropical storm against factors that would otherwise cause it to spin down (decay);  and (2) draw the storm westward toward the East Coast.

The process by which a tropical cyclone morphs into an extratropical system is termed extratropical transition, and a lot about this process remains poorly understood.   Transitioning storms change character in many ways - for instance, they (1) acquire weather fronts;  (2) develop strong asymmetries in the wind and precipitation (heaviest rains shift to the north and west side, highest winds develop on the east side);  and (3) expand considerably in diameter.   Some storms accelerate rapidly once they become embedded in the jet stream.  If a constructive type of phasing between a trough and vortex occurs, the storm can even re-intensify, as it begins feeding from dual energy sources (both the warm ocean and the jet stream);  this process is called rejuvenation.  These transitions are tricky to forecast because meteorological data near the surface and aloft is non-existent, or sparse, over the open ocean.

Some of you may remember Hurricane Hazel from 1954, which created quite a mess across the Mid Atlantic.  While the meteorological situation with Sandy is different in many respects, the Hazel that struck Washington-Baltimore was a storm that underwent extratropical transition in mid-October, interacting with a very strong trough in the jet stream.   The post-tropical vortex over land drew a tremendous amount of energy from the jet stream, and allowed Hazel to generate hurricane-force winds all the way to Toronto, Canada (the storm made landfall in the Carolinas).  Again, I stress that what we will experience with regards to Sandy is not a Hazel repeat, but Hazel serves as a reminder of how potent a tropical-extratropical interaction can be.

For additional meteorological perspectives on this storm, check out the blog of my colleague and hurricane expert Dr. Michael Folmer at NOAA: