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Posted at 11:00 AM ET, 04/21/2008

Recap: Yesterday's Stormy Situation

By Jason Samenow
  • Headlining yesterday's stormy weather were the tornadoes that struck Charles and Prince George's County. The two tornadoes were produced by the same thunderstorm as it moved from south to north during the 2 p.m. hour. The National Weather Service (NWS) provides a detailed account of the tornadoes' evolution, intensity and damage in a public information statement. The Washington Post provides additional summary information and a video detailing the damage.

  • At exactly the wrong time yesterday, the NWS's doppler radar for our region went caput due to "hardware failure." The NWS uses this radar to detect rotating thunderstorms capable of producing tornadoes. The NWS has access to other radars but I wonder if the problems it had with its radar might explain why tornado warnings were not issued for this storm.

Keep reading for more News & Notes, and see our full forecast through the weekend. Programming note: Andrew Freedman's column will appear tomorrow.

  • In addition to the twisters, the storms caused some flash flooding and produced hail. See the NWS local storm reports for more detail.

  • Because of the localized nature of the heavy rain yesterday, totals were quite variable across the region. Reagan National recorded 1.4" of rain, but Baltimore Washington International got just 0.65". Dulles Airport was the winner among the three local airports, measuring 2.9" of rain, a new record for the date (shattering the previous record of just 0.46"). Most locations, according to the NWS, have received 1-3" since the rain began. How much rain fell where you live?

  • With the deep fetch of moisture from the southeast, the intermittent bands of locally heavy rain, and the rotation in some of the storms (and the isolated tornadoes), it almost seemed like we were dealing with a tropical storm yesterday. But we weren't. Yesterday's storm, a cold core system or cut-off low, had sufficiently cold air aloft for hail. You would never/seldom see that with a tropical storm, which is a warm core weather system -- meaning the freezing level is usually too high for hail to form.

By Jason Samenow  | April 21, 2008; 11:00 AM ET
Categories:  Recaps  
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Nice capsule Jason, I used to live in that area of Waldorf (St Charles) in the 90's. Rain was just non-stop out west sunday, just a great day to relax and do absolutely NOTHING, which I excelled at! lol

Posted by: Mike from the Blue Ridge | April 21, 2008 11:19 AM | Report abuse

Does anyone on this board know how common NWS Doppler radar "hardware failures" are? Also, can NWS tap into any TV station radars in the area as a backup, or just the airport doppler radars (along with adjacent NWS radars)?

Posted by: Andrew Freedman, Capital Weather Gang | April 21, 2008 11:22 AM | Report abuse


I'm pretty sure that in one of their Area Forecast Discussions yesterday afternoon, Sterling, mentioned that they were using neighboring Dopplers and radars at the local airports, as back-ups.

Posted by: Chris R. | April 21, 2008 11:50 AM | Report abuse

"might explain why tornado warnings were not issued for this storm."

Around 10 pm last night, I saw a tornado warning. Or are we talking about different events?

Posted by: Jake in Reston | April 21, 2008 12:11 PM | Report abuse

"might explain why tornado warnings were not issued for this storm."

Around 10 pm last night, I saw a tornado warning. Or are we talking about different events?

Posted by: Jake in Reston | April 21, 2008 12:11 PM

There were two hits in MD yesteday afternoon.

Posted by: FYI | April 21, 2008 12:24 PM | Report abuse

Different events, the tornadic maryland storm happened in the afternoon (2ish p.m.)

Although the NWS Doppler radars are on occasion taken down for maintenance or upgrades, an actual hardware failure (or other unexpected downtime is quite rare). The contingency plan, as I understand it, is to use radar from neighboring NWS offices. This was pretty bad timing for the Sterling radar to go down though, yikes.

I'm not really sure that the downed radar played into this scenario regarding the lack of the warning issued. I'll have to go back and check, but I suspect this tornadic cell didn't have particularly strong low-level rotation. I also wonder if anyone was even able to see this thing, given the copious amounts of rain that was likely in the immediate vicinity.

Posted by: D | April 21, 2008 12:30 PM | Report abuse

NWS definitely can and does analyze other radars when their own goes down.

D: You're probably right that the low level rotation on yesterday's storms was pretty weak and difficult to detect. The question is: if NWS had had its radar working -- a radar presumably more powerful compared to its backups -- would the rotation have been discernible?

Posted by: Capital Weather Gang | April 21, 2008 12:42 PM | Report abuse

Not only a daily record, but that 2.90 at Dulles yesterday is an ALL-TIME record for the month of April...of course all time is 1963, but still. My wife checked our gauge at 2pm and its just under 5 inches.

Posted by: Dulles ARC | April 21, 2008 2:26 PM | Report abuse

It behaved very similar to the tropical system in Sept., 2004 associated with Hurricane Ivan. The radar signatures were very similar. One of the tornadoes associated with that system struck the Chantilly/Dulles area.

Hail usually is not asociated with hurricanes but exceptions have been recorded. Apparently hail was observed with Hurricane Hugo both in the Dominican Republic and in South Carolina. This would be highly unusual since Hugo was a well-organized warm-core system. (It is possible that large hailstones formed at very high levels did not melt completely during their descent to earth. Hurricane hunter aircraft flying at high levels of a hurricane have noted intense blizzard-like snow at these high cloud levels.)

Posted by: El Bombo | April 21, 2008 4:28 PM | Report abuse

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