Friday, April 27, 2012

How Can the Past Winter Pattern Influence Hurricane Season?

Earlier in the year, I explained the drivers of this winter's record setting warmth and lack of snow. At the top of the list are the strongly "positive" behavior of the North Atlantic and Arctic Oscillations. (Read my earlier post which explains the specifics of each oscillation) The effects that each oscillation produces can linger for months even after the numbers positive or negative have stabilized. In most seasons when one or more teleconnection is abnormally strong (as was the case the last 3 winters), the change in air pressure, temperature and ocean temperature, the dynamical imprint on the atmosphere can be much greater.  These imprints seem to trickle down into the large scale wave changes and stochastic processes that drive our weather patterns.

A great example of these changes are seen in the Atlantic Ocean sea surface temperatures. Remember the last few winters consistently cold and snowy pattern?  This was largely driven by record low Arctic and North Atlantic Oscillation levels.  Remember that the North Atlantic Oscillation are caused by changes in pressure of the Icelandic Low and the Azores High. If the Azores High becomes weaker, the Icelandic Low stays north/stronger, the NAO is negative, more frequent cold air outbreaks/more clipper snows.  When the Azores high becomes stronger, the NAO goes positive, not as much cold air.  

What does this have to do with hurricane season?  Look at the pressure patterns over the Atlantic in the spring following a strongly NEGATIVE NAO winter. The Azores High (shown in light green) is smaller and weaker.

Dec, Jan and Feb 2009-2010
Dec, Jan and Feb 2010-2011
The steering currents around the high stay weak.  These weak trade winds from the east don't cause upwelling of deeper colder water. Here are the steering winds over the last 2 winters into early spring.

Now this early spring

The turnover of water is much smaller which allows the sea surface temperature to increase.  Here are the Atlantic sea surface temperatures in early spring in 2010 and 2011 after a STRONGLY NEGATIVE NAO winter.  Very warm water is located in the tropical storm development areas.

Now look at this winter's NAO signature.  Remember our mild pattern?  Look at the Icelandic Low and the Azores High.  The High is very strong which kept cold air locked up in Canada.   

The Icelandic Low stays further north which kept the flow across the US and the Great Lakes out of the west NOT THE NORTHWEST.  The Azores High allowed the steering currents to stay strong which is now causing the upwelling of water off of Africa and into the central Atlantic development region.

Currently, the Atlantic Sea Surfaces are much cooler than in springs past covering a larger area.  
So the positive NAO this past winter was a result of a stronger High in the central Atlantic. This strengthened the trade winds which caused upwelling of cooler water which has led to a cooler Atlantic development area.

Will this result in fewer tropical storms developing earlier in the season?  Will this result in less tropical storm or hurricanes affecting the east coast or Gulf coast?  We shall see...

Monday, April 23, 2012

Late Season April Snow On The Way!

Late April snowfalls are indeed rare birds.  The last time Northeastern Ohio had measurable snowfall after April 20th was in 2005 when more than a foot of wet snow fell between the 23rd and the 25th. Normally, it takes me 35-40 minutes to drive home. That evening after the 10PM news, I didn't get home until 1:45AM!  25 mph along the interstate straddling tracks in the snow made by the semis ahead of me. By far the worst drive through snow I've ever had.


The biggest concern will be for downed powerlines across central PA. Leaves on the trees in early spring have the ability to hold more than leaves in late fall. Copious amounts of heavy wet snow will no doubt bring power lines down.  Luckily in northern Ohio, the amounts of snow will be far less.  We have several factors working against widespread snowfall:  1) The track of the storm steers the moisture north, away from northern Ohio.  2)  The ground temperatures are in the lower 40s.  3) The Lake Erie water temp is 50.  Still, evaporative cooling (cooling process within the cloud as precipitation falls--the chill you feel when you step out of the shower) will cause temps to drop sufficiently enough during the night low enough to allow snow to accumulate in spots.  Here are projections through midday Tuesday.

Here are the times in Cleveland when we had snowfall after April 20th since 1948:
2005     April 23-25th  12.4"
1971     April 24            0.1"
1968     April 25            0.2"
1967     Apri 24             0.1"
1963     April 30            0.3"
1953     April 20-21st    1.7"

Wednesday, April 18, 2012

Do We Need More Spring Rain?

The quick answer is a surprising YES!

The frequent rains of February proved to be a little misleading. Thoughts of another record setting spring like last April through June have proved to be premature.  The hottest March on record with several days of 80+ rapidly dried the ground out across northern Ohio. Soil conditions were still too wet for farmers to turn over the fields. April has proved that weather patterns can change drastically.  Only 0.33" of rain at Hopkins Airport with average temperatures running a degree above normal. (Expect the cool weekend to lower the monthly average). As of April 18th, as I drive into rural Lorain and Medina counties, many farmers' fields seem ready to go.

Look at last year's April rainfall compared to average.  Most areas from upstate New York through Ohio to northern Mississippi were more than 10" above normal. The trend continued through June.

Now this April. Quite the opposite but not to extreme. Rainfall is slightly below normal going back 60 days.

Soil moisture over the last 3 weeks has dropped significantly in the east.
Our pattern that shows a somewhat persistent zonal to WNW flow aloft still concerns me. The last two weeks of April should feature near normal temperatures with a few shots of cooler air. If the cooler air is accompanied with clear skies, expect a few more frost advisories to be issued so keep an eye on the nighttime forecasts. The WNW/NW flow could bring weak fronts with less frequent hard rains.

Should you plant your garden in early May? I wouldn't.  No way.  I was burned one year as a late May freeze wiped everything out.

How will the rest of the spring shape up?  Here are the questions to ask:

* No more La Nina. Does the neutral ENSO conditions prevail?
* Where will the warm/cool pools develop in the Atlantic
* Will the PDO continue to weaken?
* Will the northern Pacific warm pool back to the west and cool?
* How much variability will the MJO introduce into the pattern?
* Will any widespread precipitation extremes in the middle of the continent (wet or dry) enhance/weaken storm systems before they can impact Ohio?

Monday, April 16, 2012

Weekend Tornado Outbreak Videos

The weekend tornado outbreak was not record setting but it did feature some long track tornadoes. One report had a tornado with a track at 60 miles! Typically, the tornado number is adjusted lower once the National Weather Service examines the reports weeding out multiple reports. The video from storm chasers is breathtaking. Special thanks to all storm chasers who make these videos possible.

Share photos on twitter with Twitpic

Friday, April 13, 2012

High Risk Severe Storms/Tornadoes Saturday

A HIGH RISK for severe storms and tornadoes Saturday and Saturday night for the middle of the country was issued by the Storm Prediction Center.  High risks are very rare, normally reserved for drastic severe weather situations. No doubt many long path tornadoes will develop tomorrow.

Historically, high risk areas are relegated to the traditional "tornado alley" areas. Below is the HIGH RISK climatology put together by University of Oklahoma graduate student Patrick Marsh who runs a blog about meteorology data, severe storms and a slew of other meteorology related topics.

Upon first inspection, the map seems straight forward.  Yet the work involved in creating this easy-to-read map is not so simple. Normally I steer clear of detailing complex descriptions due to their esoteric nature. However this time, I made an exception. Here is Patrick Marsh's explanation on how he took almost 20 years of HIGH RISK data and morphed it into the above graph.

 I took the polygon outlining the first Moderate [High] risk during a given year, regardless of time of issuance. This means I treated a 12 UTC issuance the same as a 1630 UTC issuance and 2000 UTC issuance. I took the risk polygon, placed it on a 4km grid (specifically grid number 240), and activate all grid points that fell inside the risk polygon. This left me with a grid of 1′s (inside outlook) and 0′s (outside outlook). I created a grid for each year and then summed all the grids together. This gave me a grid containing the number of times each grid point was within the first Moderate [High] risk of the year. I then divided each grid point by the number of years I was examining. This left me with the probability that a grid point would be contained in the first Moderate [High] risk of the year.

Got all that?

What about the areas where the FIRST HIGH RISK is issued each year? The same technique was applied.

The same system will move through OHIO and CLEVELAND on Monday.  Even if the parent cold front is a shadow of its Saturday self, we can anticipate some severe storms. More updates this weekend

Monday, April 09, 2012

Tornado Statistics

Severe weather season is here.

As this winter's La Nina weakens, the strength of the southern jet stream won't be as strong this spring.  So the frequency of tornado outbreaks over the next 3 months shouldn't be as high.  Yet the middle levels of the atmosphere remain strongly cool.  While we don't anticipate tornado outbreaks like what we had last year, a few outbreaks will occur.

Here are some historic tornado statistics. We start with the time of day tornadoes occur.

Tornadoes By Month...

Strong tornado trends since 1950 (EF3-5)
EF0 Tornado Reports to Total Reports
Remember these statistics when the next outbreak occurs.

Wednesday, April 04, 2012

Dallas Tornadoes & Global Warming

The footage showing the Dallas tornado tossing tractor trailers like toys is fascinating and scary.  Thankfully no one was killed yesterday

Evidently a CNN meteorologist said that the Dallas tornadoes were brought on by Global Warming.  The meteorologist stated further:

“That’s kind of the climate change we are seeing. You know, extremes are kind of ruling the roost and really what we are seeing, more become the norm.”

Normally, I stay calm and cool when erroneous statements like this make headlines.  Yesterday evening, something inside me went off. So I went on a twitter rant. Social media venting in general rarely results in anything positive.  The pot become stirred.  Words are taken out of context. Objective analysis becomes obscured in a cloud of emotion.  Passion and enthusiasm fade into a mire of negativity.  Once you are there, it becomes very difficult to extricate ones self.  Last night on twitter, I danced on the precipice of the mire. I noticed the err in my way so I stopped dancing and cooled down.  All of this happened in roughly 10 minutes.  Did you follow all of that?

Let's be clear. The statements by the CNN meteorologist needs to be separated into two subjects which need to be addressed individually. 

1.  Tornadoes and Global Warming.
2.  Extreme weather events and Global Warming.

The portion that I am concerned about here is correlating Global Warming to tornadoes.  The Storm Prediction Center has an excellent FAQ section on their website. One of the questions is simply stated:  

Does "global warming" cause tornadoes?  Here is the answer word for word from the SPC site:

Does "global warming" cause tornadoes? No. Thunderstorms do. The harder question may be, "Will climate change influence tornado occurrence?" The best answer is: We don't know. According to the National Science and Technology Council's Scientific Assessment on Climate Change, "Trends in other extreme weather events that occur at small spatial scales--such as tornadoes, hail, lightning, and dust storms--cannot be determined at the present time due to insufficient evidence." This is because tornadoes are short-fused weather, on the time scale of seconds and minutes, and a space scale of fractions of a mile across. In contrast, climate trends take many years, decades, or millennia, spanning vast areas of the globe. The numerous unknowns dwell in the vast gap between those time and space scales. Climate models cannot resolve tornadoes or individual thunderstorms. They can indicate broad-scale shifts in three of the four favorable ingredients for severe thunderstorms (moisture, instability and wind shear), but as any severe weather forecaster can attest, having some favorable factors in place doesn't guarantee tornadoes. Our physical understanding indicates mixed signals--some ingredients may increase (instability), while others may decrease (shear), in a warmer world. The other key ingredient (storm-scale lift), and to varying extents moisture, instability and shear, depend mostly on day-to-day patterns, and often, even minute-to-minute local weather. Finally, tornado record keeping itself also has been prone to many errors and uncertainties, doesn't exist for most of the world, and even in the U. S., only covers several decades in detailed form.

As my tweets said last night, I am not against Global Climate Change. As a meteorologist with a focus in physics and mathematics, my stance has been luke warm on blaming any and all "extreme" weather event on global climate change.  Why not mention the connections to the change in southerly jet stream position and strength to severe weather and how La Nina can drive this? How about mention that the changes in temperature with height as primary driver of thunderstorm complexes not just warming at the surface? 

If the CNN meteorologist made an argument that global climate change is a driver for upper level wind/temperature changes over time which make conditions much more favorable severe weather/tornado outbreaks,  I'd listen.  Yet the met jumped right into blaming global warming for the tornadoes in Dallas.  How can anyone make the jump from "A" to "Z" without addressing components "B" through "Y"

This all might sound like a game of semantics.  After all, if the conclusion is correct, who cares how we get there. Why bother with what lies in between "A and Z" if we know the endgame as fact?  In this case, the conclusion, or endgame, drawn on CNN--later refuted earlier in this post from the Storm Prediction Center website--is way off course.


How about Metropolitan tornadoes?  These tornadoes occurred in a downtown location.  That rarely happens!

According to the National Weather Service Dallas/Fort Worth office:

"Over the last 60 years (since 1952), a total of 172 tornadoes have been reported in either Tarrant or Dallas Counties, Texas. Of these, 42 have been rated at least EF2 (wind speeds over 110 mph). The strongest, rated EF4, hit Dallas on April 25, 1994, killing 3 people and injuring 48. Neither county has ever reported an EF5-rated tornado."

An incomplete list of downtown/metro tornadoes from the Storm Prediction Center in the link below:

Tuesday, April 03, 2012

Xenia Tornado - April 3, 1974 - A Look Back

On April 3rd back in 1974, the 2nd deadliest tornado outbreak in US history (2nd only to the outbreak last year).  The Xenia tornado touched down around 4:30PM and was on the ground for more than 30 miles killing 32 people.  All told, 12 tornadoes touched down in Ohio in less than 24 hours with 148 tornadoes across 13 states!  Half of the buildings in Xenia were destroyed

Imagine television weather reports like this in 2012. The archaic radar loop hardly has any detail!

Original 8mm footage of the tornado as it approached

The photos of the damage below tell it all.


The "Ohio History" website has even more great photos and some video.