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Tuesday, May 13, 2008

A Record For Records. It's All In The Pattern

Yesterday's expectation proved true. Flooding rain and the bulls eye southern in AA County. Here is the NWS snapshot of rainfall just up through 8AM Monday- over 5 inches south of Annapolis. We hit the record for rainfall at 2.20". In fact, we hit the record 3 out of the last 4 days (Friday, Sunday and Monday). Another 2.63"and we will have the wettest May on record. More on that below. But as for the records, I did a little snooping, and discovered that there are quite a few times when two days in the same month and year hold the record rainfall. It reinforces the fact that extreme weather patterns can build on themselves. So potent storms can pair up and hold the test of time in the record books. While some records may have been eclipsed by later dates, here is a sample of the 'multiple' rainfall records currently on the books:
1939 29th and 30th
1998 15th and 23rd

1890 8th and 14th
1924 5th and 10th
1939 3rd and 28th
1998 4th and 23rd
2003 16th and 22nd
Wettest Jan and Feb in 1979
1910 17th and 24th
1924 6th and 18th

1909 21st and 27th
1921 4th and12th (broken 2008)
1976 1st and 29th
2008 9th, 11th, and 12th

As you can see, it's quite common and goes back over 100 years. Which is why I stopped at May, it would take a while... What is unique is the cluster and more so the 3 days in the same month/year we just had. It's all part of a pattern that is now taking the storm track over Maryland. I expect more in the next week or so. Here is why:

First the North Atlantic Oscillation (NAO). It is a blocking pattern that allows cold air to shift south from Canada into the Mid Atlantic and shift the storm track... In the winter it is promising for snow, but in the spring- just delayed a few months- it brings us frequent rain events, but shifts the severe weather threat from the central plains to the south east USA. When the index is negative, we have a block and the cool stormy pattern. here you can see that we made that shift in mid March- and have been there ever since. That was what I expected to give us a last snow, which did not happen. But my outlook was not entirely wrong. The red line at the end of the chart is the spaghetti plot or multiple model outlook for the end of May trending back to neutral or Zero. That follows a notion I mentioned recently... we can jump right into summer.
Before that happens, we have to consider what is left in this pattern. Here is the 500mb jet stream showing our current ridge, with clearing skies. The trough out west is the stormy pattern, along with a nearly closed Low moving through the desert southwest. This a cold pocket of unsettled air responsible for more snow in Arizona and Colorado today. The slant (up and right) indicates a progressive pattern, which means it is moving eastbound- but that block in the north Atlantic provides an atmospheric northerly flow, preventing that storm from passing to our west. Instead, it gets suppressed and travels almost due east. It should reach us by Thursday night and Friday. While not as impressive as the last storm, it should in it's own right bring a chilly wet Friday to us. Too early to say how much rain we could get, but any more at this point is too much. A guarantee though that more severe weather will develop in it's path in some of the same areas that were hit last weekend.

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