“April showers bring May flowers,” and severe weather

Now that we’ve entered April, we can officially start talking about storms and severe weather.  For storm lovers like me, this is exciting!

My article at the beginning of last month on March severe weather climatology was really just to note that severe weather is possible in March, but it’s not close to our most active severe weather month.  Take last month for example, we had *no* severe weather and instead suffered through endless stretches of cold, gray, and wet days.  I’m over it.  (Nationally, there were only 15 tornadoes in March, the fewest since 1969 according to the Weather Channel).

April is the month when severe weather really amps up across the D.C. area and also nationwide.  The clash between arctic air still spilling south from Canada and the warm moist air intruding from the Gulf of Mexico sets up the right ingredients for thunderstorms, severe weather, and tornadoes.

Tornado season is in full-effect by April. Many of the historical tornado outbreaks have occurred during this month.  We know the Great Plains and Southeast light up with severe weather during the month of April, but how does the Mid-Atlantic region, and specifically our area, shape up?

Tornadoes, wind, and hail: Severe weather climatology 1950-2011

Map by Kathryn Prociv. Storm data courtesy of the Storm Prediction Center. Google Physical Map.

Map by Kathryn Prociv. Storm data courtesy of the Storm Prediction Center. Google Physical Map.

Compared to March severe weather maps (shown last month), there is a marked increase in activity.  There isn’t much to talk about regarding major differences in the amount or spatial distribution of severe wind reports, but what should jump out is the increase in hail and especially tornado reports.  Within the map extent there are 67 tornado events from 1950-2011 during the month of April.  By contrast, March only had 14 tornado events!

More on April tornado climatology to come, but first let’s discuss hail occurrence during the month.

Hail climatology 

Referring to the map above, there is a pretty even distribution of hail events across the area.  While hail occurs with convective thunderstorms year round, it can be especially prevalent during the spring months.  Why?  The answer is simple: cold air aloft.

As we slide into spring, often times the surface temperatures rise while air aloft remains very cold, especially if the jet stream is still transporting cold arctic air south.  Cold air aloft is not only an important ingredient in creating the unstable air necessary for thunderstorm development, but it also adds to the number of ice crystals available within the cloud for accretion into large hail stones.

A fun fact about hail: have you ever cut a large hailstone in half?  If the hail stone is large enough it has ice rings that resemble tree rings.  Count the ice rings, and you can see how many trips that hailstone took to the top of the cloud before finally falling to the surface of the earth.

While hail can be a very cool meteorological phenomenon to witness, it can also be incredibly dangerous and damaging to property.  Large hail, typically characterized as quarter-sized or larger, can dent cars and if large enough can damage crops, shatter vehicle windows, and even shatter home windows!  Jason wrote an article about a major hailstorm that impacted portions of Mississippi, Alabama, and Georgia two weeks ago.  

Fortunately for our area, we don’t typically see the baseball, softball, or grapefruit sized hail known for shattering windows. Instead, most of our hail events feature hail of two inches in diameter or less.  The map below shows all documented hail events 1950-2011 symbolized according to size (in inches).

Just because most of our hail is in the smaller range, we are not immune to large hail.  As you can see on the map we have had a handful of events of moderate to large hail.  It’s important to note that size doesn’t always matter; a major hailstorm dropping even small hail can cause major damage when widespread over a large area.  Below is a list of some of the more notable hail events in the past 20 years for D.C. and the surrounding suburbs:

Map by Kathryn Prociv. Storm data courtesy of the Storm Prediction Center. Google Physical Map.

Map by Kathryn Prociv. Storm data courtesy of the Storm Prediction Center. Google Physical Map.

April 1st, 1993

April 9th, 1999

April 23rd, 1999 – I remember this storm vividly.  My parent’s house needed an entire new roof due to the extensive hail damage.

April 21st, 2000

April 9th, 2001

April 28th, 2002 –This was the day of the La Plata, MD F4 tornado which will be discussed in more detail below.

April 3rd, 2006

April 25th, 2010

The largest hail reported in our area according to the Storm Prediction Center database occurred on April 23rd, 1999.  The report was 3.5 inches in diameter and fell near Winchester, Virginia.

Tornado Climatology

While hail is interesting to talk about, the real blockbuster topic when we enter severe weather season is tornadoes.  For storm chasers like myself and CWG’s Ian Livingston, the advent of tornado season is very exciting.  However, many people are afraid of tornadoes and rightfully so.  They are the most violent forces of nature on earth and represent a real danger capable of damage to life and property.

Source: NOAA

Source: NOAA

Climatologically-speaking, April can be quite an active tornado month for our area.  Displayed on the map above are 67 tornado events that have occurred 1950-2011.  The map below displays these 67 tornado events with the tornado tracks rated according to the Fujita and Enhanced Fujita Scales.  (Brief history: the Fujita Scale was used to rate tornado intensity up until 2007, when the Enhanced Fujita Scale, or EF Scale, was implemented in its place.)

Map by Kathryn Prociv. Storm data courtesy of the Storm Prediction Center. Google Physical Map.

Map by Kathryn Prociv. Storm data courtesy of the Storm Prediction Center. Google Physical Map.

Looking at the map, let’s break down a few important points.  Unlike March, where this region has never seen any tornadoes stronger than an F2/EF2 rating, April has featured two F3 tornadoes and one F4 tornado.  The immediate tornado track that jumps out is the orange F4 track cutting straight through La Plata, Md.  This incredibly strong and violent tornado hit La Plata, MD on April 28th, 2002 and remains the strongest tornado to impact the D.C. region on record.  The dark green F3 tornado that tracked just south of Fairfax City occurred on April 1st, 1973 and the second F3 tornado on the east side of the Chesapeake Bay was actually the same tornado as the one that impacted La Plata, just in a weakened state upon crossing the Bay.  Learn more about the La Plata, MD tornado here.

While our area has certainly experienced strong tornadoes during the month of April, the majority of our tornadoes since 1950 have been weak in the F0/EF0-F1/EF1 range (purple and blue, respectively, on the map).  A list of notable tornado events as depicted on the map include:

April 28th, 2002 –There were eight tornadoes total that day, including the La Plata, MD F4 tornado.

April 16th, 2011 – There were six tornadoes on this day.  Interestingly enough, April 16th seems to be a tornado magnet day, with tornadoes also occurring on this day in 1961 and 1993.

April 27th-28th, 2011—The largest tornado outbreak across our area during the month of April, with 19 total that occurred late at night on the 27th and into the early morning hours of the 28th.  You may remember these storms were part of the same storm system that caused the Super Outbreak of 2011 across the southeast earlier that day on the 27th.

2011 was an incredibly active tornado year for our region.

Conclusion 

Now that we’ve entered April, a climatologically active severe weather month for us, it’s time to point our eyes toward the sky and watch for severe weather.  This month we see an increase in the probability of storms causing significant hail as well as an increase in tornado probabilities.  Remember that while the vast majority of April tornadoes are weak (F/EF2 or weaker) we’ve had a handful of strong tornadoes.   Goodbye winter, and bring on the spring-time storms!

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