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Wave So Long to the Heat Wave

Guest Blogger @ 12:00 AM

By Megan Linkin

DC's looking to have a good weekend in store, with sunny skies and drier conditions. A front pushed through the region on Friday, taking the oppressive heat and stifling humidity with it; the high pressure system behind the front will be accompanied by drier air and cooler temperatures, setting us up for a nice weekend.


Forecast Confidence: Medium-HighPartly cloudy to begin, clearing throughout the day. The exiting frontal system may leave some lingering clouds for Saturday morning, but these will dissipate throughout the day. By afternooon, the sun will be shining brightly resulting in a very high UV index near 9. Northerly winds (around 10mph) will bring in much drier air and temperature will be at or below the average of 88°F for August. Expect highs between 84°-88°. Overnight, clear skies and calm winds will result in low temperatures reaching around 70° in the city, upper 60s in the suburbs.


Forecast Confidence: Medium-HighPerfect DC summer day. Sunday looks to be better than Saturday (if you like hot), with mostly sunny skies and highs just above average. The high pressure system to the west will remain relatively inert, dropping slightly south, bringing northerly winds and more dry air. Expect highs to be at or above 90 with the UV index remaining at 9. Overnight, clear skies will allow temperatures to drop, with a low temperature of about 71 in the city and 65-68 in the suburbs.

Wednesday's Wild and Wacky Weather

This Wednesday saw the most oppressive temperatures in the district all summer, severe thunderstorms rolling through the New York metro region and even an EF2 tornado touching down in the borough of Brooklyn. And what do we have to thank for this weather? The jet stream all the way north along the Canadian-US border, a frontal system to the north, a high pressure centered over the Tennessee Valley and persistent southeasterly flow that brought in Atlantic moisture and humid conditions.

The jet stream was flowing through southern Canada, allowing warm tropical air to move on in. Low pressure over the Great Lakes resulted in southerly flow to dominate much of the region. The official high at Ronald Reagan National Airport on August 8th was 102°F, the warmest in 8 years, with a sultry dewpoint of 78°F that morning. The wind was from the south during much of the morning, bringing plenty of moisture and humidity with it. By 10AM, the temperature already reached 92°F. As the afternoon progressed, the low moved to the east, sitting over Quebec province by the evening. This allowed for the high in the Midwest to move closer to our region, bringing west winds and drier air, but an abundant amount of heat. The temperature first hit 100°F at 1pm, and remained near 100°F until 6pm that evening. The low exited overnight and the high moved further east, giving us northwesterly winds and slightly drier conditions as the dew points dropped to 70°, setting us up for the remainder of the week.

The tornado in Brooklyn and much of the severe weather experienced in the northeast during the early hours of Wednesday was the responsibility of a mesoscale convective complex (MCC) that rode along the northern flank of the high. MCCs are clusters of thunderstorms that thrive during the overnight hours, when the low level jet is active; providing a constant influx of moisture (as shown in the image below). Upper air support is critical to the maintenance of an MCC as well; the jet stream must remove air from the top of the thunderstorms to allow for additional rising motion to occur.

Image courtesy of USA Today.

On Wednesday morning, the MCC had strengthened and was moving through Pennsylvania. As it approached New Jersey, both the necessary upper air support (jet stream immediately to the north) and low level source of moisture were present (southeasterly flow off the Atlantic). This allowed for the MCC to both be sustained and to strengthen. Powerful thunderstorms flooded much of New Jersey and New York, affecting commuters by flooding roadways, railroad tracks and underground subway tunnels. As the system moved over Brooklyn, rotation in the backside of the MCC resulted in a tornado touching down in the Bay Ridge section a short time after 7AM EDT. The tornado was an EF2 on the new Enhanced Fujita Scale, damaging several homes and uprooting trees. A tornado in Brooklyn is a rarity; this is the first EF2 tornado to touch down in Brooklyn since official tornado records began in the 1950s.

Megan Linkin is a Ph.D. candidate at University of Maryland (UMD), expecting to receive her Ph.D. in atmospheric and oceanic science in May 2008. She received her M.S. in atmospheric and oceanic science from UMD in 2006 and was granted her B.S. in meteorology from Rutgers University in May of 2004.

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