You Asked the Meteorologist and Here Are Your Answers

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Last week we offered to take any type of weather question from readers and honestly I thought we’d receive more questions along the line of, “I live in D.C. and my birthday is March 21st. Can we have the party outside?” Or “Why do TV weather people use such exaggerated hand gestures?”  In fact, each question submitted was scientific in nature and for that I am grateful . . . and a little relieved.  Obviously I underestimated the studiousness of our readers and I’m pleased to share the questions and our answers.

So here we go.

A question from Keith via email:  “I have seen it reported on TV newscasts and I’ve heard many people say that the storm surge from a hurricane is primarily determined by the strength of the hurricane. This term seems to loosely refer to the barometric pressure in the center of the storm and the wind speed of the storm. Over the course of the past 6-8 years with Katrina, Ike and now “Superstorm Sandy” it is very clear to me that the significance of the storm surge has a lot to do with not only the strength of the hurricane but also the size of the wind field, wind speed and the distance that the storm has traveled over open water with these significant factors in place prior to making landfall. My question is: Have meteorologists now just recently recognized these factors or are TV meteorologists trying to simplify their explanations for the viewing audience?  I.e. Katrina – Category 5 from the Caribbean, then a 4 and then a 3 at landfall with storm surge of 23 feet, Ike – Category 4 from the Caribbean, then a 3 and then a 2 at landfall with a storm surge of 18 feet, Sandy – Category 3, then 2 and then 1 with a storm surge of 12 feet at landfall compared to Hurricane Andrew at Category 4 but only traveling from Florida to Mississippi with a 12-15 feet storm surge and others like Celia in 1970 that was a Category 3 but only made up tight a few hundred miles from the coast with a storm surge of about 8 feet.”

Here’s the answer provided by ImpactWeather TropicsWatch Team Manager and Lead Hurricane Forecaster Chris Hebert:

Chris Hebert. Image: ImpactWeather

In my experience, most TV meteorologists don’t put nearly as much time into the study of surge as  we do here at ImpactWeather.  It’s easier to say that surge is a function of strength vs. going into a lengthy explanation  of all of the variables associated with storm surge.  We’ve known what produces the storm surge for many decades.  Nothing new has been learned in recent years.  Storm surge height is primarily a function of the following variables:

  1.  Where the storm hits.  Coastal bathymetry is one of the prime variables.  By bathymetry, I’m talking about the shape and depth of the sea bottom.  Shallow water closer to the coast can significantly enhance storm surge.  This is called the “shoaling factor.”  It’s a multiplier we use in calculating storm surge.  In some areas, that shoaling factor may be as low as 0.4.  In other areas, it’s at 1.8.  The same exact storm striking the coast where the shoaling factor is 1.8 would produce a surge more than 4 times higher than if it struck the coast where the shoaling factor is only 0.4.  Areas with the highest shoaling factor are the mid-Louisiana coast, the Mississippi coast and near Tampa, FL.   Pensacola, on the other hand, has a shoaling factor of only 0.6 due to the very deep water not far offshore.   A storm that produced a 30 ft storm surge on the Mississippi coast (like Katrina) would produce only a 10 ft surge in Pensacola.  That’s how important coastal bathymetry is.
  2. Coastal shape.  This sort of goes along with the first factor, where a storm hits, but it is different.  Bays tend to enhance storm surge.  The farthest inland point of many bays may experience twice the storm surge that is produced near the mouth of the bay.  This is a function of the funneling of the water into a narrower point combined with the shallowness of most bays.   Some coastal areas have a shape that tends to funnel water toward a point, greatly increasing the potential for a large storm surge.  Think of southeast Louisiana and Isaac. Isaac struck from a direction that produced a long fetch of wind (and waves) directly into southeast Louisiana between the mouth of the Mississippi River and the Mississippi coast.  This significantly increased the surge into Lake Pontchartrain, flooding areas that were not flooded by Katrina seven years earlier.  Sandy struck New Jersey from a direction almost unheard of – the east or east-southeast.  With Long Island and southern New England oriented east-west to the north of Sandy’s landfall, its surge had nowhere to go but western Long Island and northern New Jersey.   This is a big reason why Sandy produced a much higher surge than Irene did last year.  Sandy was also quite a bit larger than Irene.
  3. The size of the storm’s wind field.  It’s not the Saffir-Simpson category that determines potential surge height, it’s the size of the wind field.  That’s why Sandy and Isaac, two relatively weak hurricanes this past season, produced such large storm surges.  They both had a very large wind field.  It’s the large wind field that generates the large volume of water moving toward the coast, not the peak wind that may occur over only a few square miles of the storm (Saffir-Simpson rating).
  4. Speed of movement.  A slower-moving storm at landfall will produce a larger storm surge as its winds blow toward the coast for a longer period of time.
  5. Angle of impact.  A more direct impact (90 deg) will typically produce a larger surge than a glancing blow.

You mention a number of past storms that were stronger well away from the shore then weakened significantly before impact.   Let’s take Katrina, for example.  It was a Category 5 hurricane in the east-central Gulf but weakened to a Category 3 hurricane at landfall in SE LA/MS.    An incorrect conclusion would be that because it was a Cat 5 well offshore that it carried that large storm surge with it as it impacted the coast.   The storm surge is generated at the coast, not offshore.  Instead, let’s examine what happened with Katrina’s wind field.   While it was a Category 5 hurricane, it was larger than average, with hurricane force winds extending out as far as 75 miles from the center.  But prior to landfall, Katrina underwent what is called an eye wall replacement cycle.  This happens as the eye of a very strong hurricane shrinks so much that it becomes unstable.  It then forms a NEW eye with a much greater diameter.  Basically, the storm grows in size significantly during such a cycle.  But it also weakens significantly during an eye wall replacement cycle.  Since the storm surge is more a function of the size of the wind field vs. its peak intensity (Saffir-Simpson), Katrina produced a very large storm surge at landfall. In addition, it struck a section of the U.S. coast with a shoaling factor of 1.8.  So it wasn’t the fact that Katrina had been a Category 5 hurricane prior to landfall that produced the large surge, it was the large wind field and coastal bathymetry that were the main reasons.

Same story with those other hurricanes you mentioned.  When Ike dropped from a Category 4 to a Category 2 hurricane, it more than doubled in size.  Hurricanes often conserve energy that way.  Impacts with land generally disrupt a hurricane’s core and result in both weakening and an expansion of the wind field.  That’s what happened with Andrew, Ike and Wilma.  Rita was also changing structure as it made landfall (wind field was growing).  Weakening hurricanes quite frequently expand in size.  And it’s the size of the wind field that produces the larger surge.

Steve wants to know why derechos form. “I’ve read that for some reason, strong upper level winds get redirected down to the surface.  I’ve already read that we don’t really know why they happen.  Last year’s major derecho from the Midwest to the east coast is a case in point that they can be very damaging.”  Answered by ImpactWeather StormWatch Team Manager Fred Schmude:

The derecho earlier this season that you’re referring to was one of the strongest seen in many years, stretching about 800 miles from northern Illinois  to Delaware and lasting nearly 18 hours from 10am June 29 to 4am June 30.   Peak wind gusts of 91 mph were reported with this derecho, with widespread reports of 60-80 mph winds, which resulted in hundreds of millions of dollars in damage.  Fatality estimates by this extreme weather event were reported at 22, mainly due to falling trees and large tree limbs.

Shelf cloud of developing derecho over Chicago, June 29, 2012. Image: NWS

The term derecho comes from a Spanish word meaning straight or directional as opposed to the terminology tornado, which refers to twisting motion.   A derecho would be best described as a widespread and long-live wind storm associated with a fast moving line of showers and strong thunderstorms.  Even though a derecho can produce damage similar to tornadoes, the damage is typically one dimensional (as the word implies) along relatively straight paths, and this is mainly due to the type of environment that the derecho forms in.   By the National Weather Service definition the peak wind gusts in a derecho must be at least 58 mph in a path at or greater than 240 miles and lasting at least 6 hours.

As for what causes a derecho, there’s some truth in what you said about stronger upper-level winds being directed downward, but the main source of energy actually comes moist air interact with drier air aloft and some basic laws of thermodynamics.  As a thunderstorm starts to develop, warm moist air is drawn upward by strong updrafts where the interaction with drier air ensues resulting in evaporation and cooling.  The drier the air is directly proportional to how much evaporation and environmental cooling will result inside the thunderstorm.  Since colder air is heavier and more dense, this results in the air column being directed downward toward the surface of the Earth where it will be directed outward at speeds sometimes exceeding hurricane force in some of the more extreme situations like we saw earlier this year over the Ohio Valley and Mid Atlantic.  In addition to the drier air aloft, a strong upper-level jet stream is typically associated with the stronger derechos since these streams of energy can provide further environmental instability resulting in stronger storms and momentum directing the storm at speeds sometimes greater than 50 mph.

Note that Doppler radar has been a huge help in identifying those thunderstorm complexes capable of producing derechos since this type of radar can penetrate inside of a storm and depicting wind motions and precipitation rates.  Sometimes these features can be observed moving from the upper part of the thunderstorm to the lower part, which is a clear sign that strong downburst winds could occur in the very near future.

Another hurricane question we received was about “whether the extreme low pressure in the eye of a hurricane sucks the water up and creates a ‘bubble’ of water that comes in with the hurricane if it makes landfall.  With Sandy still being fresh, there is a news connection.  If there is a bubble, how much of an impact is it to the storm surge in comparison to the wind pushing water ahead of the right side of the storm?”

Chris Hebert again with the answer:

In a landfalling tropical cyclone, there are two components of storm surge, wind-driven and pressure-driven.  The vast majority of a landfalling tropical cyclone’s storm surge is wind driven (95%).   The pressure-driven surge, that “bubble of water” in the eye of a hurricane is typically much lower – on the order of about 5% of the total storm surge.  To put this in perspective, for a landfalling tropical cyclone that produces a 10 ft. surge, about 6 inches of that surge is due to the pressure drop in the eye.  If the surge was 30 ft., then that pressure-driven surge would be about 1.5 ft.   In general, the height of the pressure-driven component of storm surge varies between 6-18 inches, which is quite a bit lower than the wind-driven storm surge.

Here’s a question about what this winter holds in store.  “I do a lot of skiing and I have heard that this winter is supposed to be snowy in the Northeast. Is that true, and if so, what is the basis for that prediction?”  Fred Schmude again:

Yes, we are expecting to see near to above-normal average snowfall for much of the Northeast this winter season.  A lot of that has to do with a complete change in the atmospheric  flow pattern.  Recall that last year we had a west-to-east flow pattern move across the Lower 48 for most of the winter season bringing milder and drier weather conditions over many areas, including the Northeast U.S., resulting in well below normal snowfall for many areas.   The pattern was mainly contributed to by a moderate La Niña condition over the Tropical Pacific aided by a stronger than normal Polar Low pressure area which combined to enhance that west-to-east flow.  This season were seeing the opposite weather pattern staring to take shape over the Northern Hemisphere with a weak Polar Low and considerably more high latitude atmospheric blocking, which tends to shove colder air southward.   In addition over the Pacific, last year’s La Niña has dissipated and has been replaced by some slight warming over the central Tropical Pacific, which typically correlates  to unsettled and colder weather conditions over the Central and Eastern U.S.   As a result I would expect weather conditions to become considerably more stormy across a large part of the Lower 48, including the Eastern Seaboard during the last week to 10 days of December followed by an unsettled and potentially very snowy weather pattern as we move into January and February.   Don’t be surprised if we see one, two or maybe even three major winter storms affect the major metropolitan areas of the NE this season from Washington D.C. up through Boston.

From the comments section of YWB: “What were the dates of the Little Ice Age?”  Another answer from Fred Schmude.

Fred Schmude. Image: ImpactWeather

The Little Ice Age peaked during the years of about 1560 to 1660, but actually lasted for several centuries in between 1500 and 1800.  It was characterized by very low solar activity with very few sunspots and extended solar cycle periods.   Solar scientists have made some remarkable strides in predicting these types of events by using current and projected solar cycle trends, which, by the way, I am a huge fan of.  Based on some remarkable finds, that unfortunately is not making some of the major media sites, we are seeing a dramatic change towards a much quieter phase in the evolution of these solar cycles during the past 15 years, and it does appear the quiet trend will continue through the 2020s, 2030s and 2040s.  Even though the total ramifications of a quieter sun have yet to be determined, there’s strong correlation to the past suggesting the Earth’s total temperature will take a nose dive over the next 20 to 30 years, possibly even evolving into another Little Ice Age.

This was fun and educational (even for me, and I’ve been here for 21 years!) and we’ll do it again. Thanks to everyone who contributed and feel free to drop us a question about weather and how it affects business continuity, travel, industry in general, dispatching, deploying . . . you name it . . . any time you’d like by emailing me at frogers@impactweather.com.

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