Thursday, September 26, 2013

Dry Streak

Today marks the fourth straight day of no rain in Philly. It's not a significant stretch but we're entering into a relatively dry pattern in the region as storm systems aren't apt to move towards us and coastal lows don't develop nearby. September and October are often periods of time where we feature lengthy stretches of dry and usually pleasant weather as humidity levels are low, the Gulf of Mexico exerts less influence on the humidity factor of the eastern half of the country, and high pressure often dominates.  October and November are two of the least rainy months on the calendar...quiet weather tends to perk up more frequently in the last summer through mid autumn time frame.

Or, as I've been apt to call it lately, "boring weather warnings" start to become dominant.

If Sunday's coastal storm, which is unlikely, does not occur, the region may get a stretch of ten days or more of dry weather.  A storm system out to the west that was progged to push through Philadelphia on Monday appears to be drying up on most of the modeling, with clouds and perhaps a brief sprinkle accompanying the frontal passage on Monday.  The front's fizzling is due to the coastal system in the Atlantic, which is blocked by high pressure in the Central Atlantic, which is in turn blocked by a larger coastal storm near Europe.

500 mb pattern (ridges = red, troughs= yellow). Map via

This pattern does eventually break down a bit in the Atlantic early next week as the coastal storm pushes up towards Atlantic Canada, only to be replaced by a ridge of high pressure along the East Coast.  It could end up being the end of next week before we get a shot at any precipitation, IF that drying and dying front on the models does indeed do just that.

This isn't the longest dry stretch of note in recent times though.   We had a 19 day stretch without measurable rainfall between August 24th and September 11th, 2010.  That featured a nice heat wave thrown in there near the Labor Day holiday for good measure. That streak was not the longest on record in Philadelphia -- we have seen streaks as long as 29 days in past history. We likely won't approach that record but if we dodge the weekend and Monday without rain, we could see a double digit streak, which would be the longest in three years of