“April showers bring May flowers,” and severe weather

“April showers bring May flowers,” and severe weather

Posted by Kathryn Prociv on April 1, 2013 at 1:03 pm
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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!

Kathryn Prociv Kathryn recently earned her B.A. and M.S. at Virginia Tech in geography with an emphasis in geospatial technology and meteorology. She currently teaches geography and meteorology for Northern Virginia Community College.

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eyeofthebeholder
2:02 PM EDT
Great post, Kathryn! I’m more likely to head indoors when storms are forecast, but I do enjoy the CWG’s analysis and “elecrifying” photos of severe weather.
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Darksideblugrss
12:49 PM EDT
and this is what I will be back for each time I hear there is the possibility of a thunderstorm. assuming human/animals are safe bring on the storms!
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Kathryn-CapitalWeatherGang
12:51 PM EDT
Agreed! Never wish for any injuries to people/animals or property damage, but some good springtime storms are a must!
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jimmytrain21
11:40 AM EDT
Kathryn, why was there a change from the F to the EF scale? And what happens if say a tornado hits above 234 or below 65mph? Just curious.

Thanks for the write up, makes me excited to get some nice thunderstorms (not too severe though!) again!
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jimmytrain21
11:42 AM EDT
And when I say “what happens” of course I mean how does it get rated? : ) Of course I know what would happen if there was 234mph+ around any civilization.
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jimmytrain21
11:58 AM EDT
And one more question too, sorry for the multiples! Your comment about slicing a hailstone to see the layers and determine how many trips up and down the hailstone took… I’ve read before now that may not be the case…take Wikipedia for what it’s worth, it mentions this under “formation” section http://en.wikipedia.org/wiki/Hail

“Former theory suggested that hailstones were subjected to multiple descents and ascents, falling into a zone of humidity and refreezing as they were uplifted. This up and down motion was thought to be responsible for the successive layers of the hailstone. New research, based on theory as well as field study, has shown this is not necessarily true.”

Then it goes into details. Curious if you buy into that or if that’s a less-widely held view.
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Kathryn-CapitalWeatherGang
12:09 PM EDT
All great questions Jimmy! To answer your first question. Even if winds are estimated below 65mph it will be called an EF0. Likewise, even if winds are estimated above 234mph it will still be classified as an EF5. An example, the winds from the F5 Moore, OK tornado from May 3rd, 1999 were estimated at a whopping 318mph which was just above that old 317mph F5 threshold (scary stuff).
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Kathryn-CapitalWeatherGang
12:12 PM EDT
The answer to your second question about hail. You bring up a good point regarding if the layering is solely caused by trips up and down in the cloud. The different layers indicate the hailstone went through layers of different temperatures and thus liquid content within the cloud. The brighter white “rings” are from an area in a cloud with a higher ice crystal ratio (so the white ring is ice) whereas the dark transparent layers are composed of more liquid droplets (so a warmer part of the cloud). Now, more of the ice crystals are higher in the cloud where the colder temperatures are found, so the layering could still be related to “trips up and down.” Long answer haha, hope it helps!
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Kathryn-CapitalWeatherGang
12:16 PM EDT
Oh I didn’t answer your very first question, on why we switched from the F to EF scale in 2007! Quick answer: better understanding of building construction. Building engineers were used to provide information to adjust the wind scale ratings according to a better understanding of what magnitude of winds can cause certain levels of damage.
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jimmytrain21
12:23 PM EDT
Good stuff, thanks!
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Kathryn-CapitalWeatherGang
12:26 PM EDT
You’re welcome!
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jimmytrain21
12:30 PM EDT
National Geographic had a pull out large map a few years ago of the entire nation with a tornado track map like your local one, of the last 50 or so years. It was awesome, every state, all 50 had at least one. I’m sure you probably saw it, but if not then I highly recommend it. Thanks again bring on the storms!
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Kathryn-CapitalWeatherGang
12:40 PM EDT
Bring them on! Oh yes it’s fascinating that you one can say, “No state is immune to tornadoes.” Even Alaska has had 4 tornadoes since 1950. Ian Livingston and I do a lot of tornado climatology research and make national maps for US Tornadoes (ustornadoes.com) if you’re interested!
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Duffman
8:03 AM EDT
One thing I’ve wondered about lately is whether the potential for severe storms in our area has become higher than usual over the past few years. I used to pay no attention to the weather at all outside of winter until a particularly bad t-storm in the summer of 2010 knocked out our power for a few days. Ever since then, I’ve paid a lot more attention to forecasts in the spring and summer months, and it seems like it least once a week there is a chance for severe weather (ok, maybe not that often, but it still does seem to happen quite a bit). Has this always been the case for our area, or has there been a noticeable shift towards increasing potential for severe weather for DC in recent years (even if no storms actually develop)?
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Kathryn-CapitalWeatherGang
9:01 AM EDT
In my opinion we don’t really have enough data to discern if severe weather activity has in fact been on the rise for our area. I’ve seen arguments for the tornado alley shifting east into the Mid-Atlantic, but I’ve also seen arguments that the enormous heat island of D.C. is actually helping to quell storm activity. Like you said, sometimes it just takes one event for people to start paying attention. I’m sure after last summer a lot more people are weather-aware when they hear severe weather is possible. Also, reporting and coverage of severe weather has also gotten much better with the advancement of technology and especially social media.
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Duffman
10:29 AM EDT
Thanks for your response! That really helps a lot.
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Kathryn-CapitalWeatherGang
12:18 PM EDT
You’re welcome! I like your idea of doing an in-depth analysis of finding any conclusive evidence in severe weather. Maybe doing a report on tracking “thunderstorm days” per year!
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Kathryn-CapitalWeatherGang
12:18 PM EDT
*conclusive evidence of an INCREASE in severe weather.
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Peter’s Patter
7:34 AM EDT
Liked the old format with detailed weather forecast for today and days ahead. I now need to go elsewhere for this – dommage!
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Kathryn-CapitalWeatherGang
7:37 AM EDT
Not sure what you mean Peter’s Patter? We had our normal detailed weather forecast posted this morning: http://www.washingtonpost.com/blogs/capital-weathe…
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jaybird926
7:20 AM EDT
Interesting to observe a lack of activity in Hybla Valley in the first map.

Could that be due to the fact it is a lower area/in the bowl? The highest point of the county is north of that at the Beacon Center, about two miles south of the Beltway.
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Kathryn-CapitalWeatherGang
7:40 AM EDT
Interesting observation jaybird. Not sure why there is a relative lack of severe reported there. If anything maybe the nearby water keeps the area somewhat more stable?
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wthrwtchr
6:28 AM EDT
I too get excited about severe weather season. I hope CWG can post more techical discussions regarding possible t-storm events – with SBLI, CAPE and other parameters – shown of course in graphic form, with those easy to understand explanations (for the non-informed) for which the CWG has become so well known!
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Kathryn-CapitalWeatherGang
6:36 AM EDT
We can absolutely do that wthrwtchr. When we see a possible significant severe event coming we will definitely do some write-ups talking about shear, helicity, CAPE, etc. I think Ian has done some reviews like that in the past that were well-written and easy to understand!
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wthrwtchr
6:41 AM EDT
Thank you, and I look forward to it! Now, we just need some better ingredients and antecedent conditions…(I apologize to those on this board who DON”T like severe weather – I don’t want to insite a flame war here (as has happened by other posters in the past on this blog…))
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Kathryn-CapitalWeatherGang
7:20 AM EDT
Haha. Luckily I don’t think the storm lover vs. storm hater is as polarizing at the snow lover vs. snow hater!
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Ian-CapitalWeatherGang
5:38 AM EDT
Hi Kathryn, good stuff. Question on the hail map.. did many of those large reports come from the same day? They sorta track in a line like you might expect from a rogue supercell or something. Assume some is from the La Plata storm at least.
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Kathryn-CapitalWeatherGang
5:44 AM EDT
Ian I thought the exact same thing about the large hail reports. They appear to be in a straight-line pattern as if it was some giant southeast-moving supercell. Surprisingly they’re spread across the board date-wise, but two of them were indeed from the La Plata monster supercell.
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Rick-CapitalWeatherGang
7:08 AM EDT
April 28, 2002 was some kind of day. I can remember it starting with overcast skies and slowly rising temps. The clouds parted after lunch, and it got very warm with bright sunshine. That large supercell developed to the west and approached the Potomac toward 6:45. I tuned in to the local news and was concerned that the various “future track” radar simulations took the tornado into Waldorf, where I lived at the time. We got a brief downpour and gusty winds instead, but very large hail had fallen about a mile to the south. That was an experience I will never forget!
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Kathryn-CapitalWeatherGang
7:19 AM EDT
Awesome personal account Rick! What fascinates me most about that day is how the La Plata supercell survived crossing the Bay. Usually large water bodies serve to disrupt big storms!
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Rick-CapitalWeatherGang
12:17 PM EDT
Yeah, the twin waterspouts were iconic images from that event, maybe moreso than the twister itself.
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Rick-CapitalWeatherGang
12:20 PM EDT
Link to waterspouts: http://www.erh.noaa.gov/lwx/Historic_Events/apr28-2002/tor-and-spout-brightened.jpg
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Kathryn-CapitalWeatherGang
12:27 PM EDT
Amazing! I didn’t know there were twin waterspouts from that event!
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Kevin-CapitalWeatherGang
4:29 AM EDT
Nice write-up and summary of events!
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Kathryn-CapitalWeatherGang
5:43 AM EDT
Thanks Kevin!
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Winter 2012-2013

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30-yr average: 15.4″

Record: 56.1″ (2009-10)

Winter Forecast 2012-13

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TUE
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WED
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THU
partly_cloudy
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FRI
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64
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Last Updated on Apr 2 2013, 5:52 am EDT
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