Monday, November 19, 2012

Dry Novembers & Years In Philadelphia

2012 has been a much drier year for the Delaware Valley compared to last year. So far, we've picked up 31.01" of precipitation this year. That's still less than half of what fell in all of 2011 (64.33").  This isn't a year that's on pace for the driest year on record -- that record is 29.31" from 1922.  However, we are among the driest on record since the modern climate era began in 1872.  The year probably doesn't end up among the ten driest (31.65" from 1998 is 10th) but a spot in the top 20 is possible (33.58" from 1880 is 20th driest).  Unless we get more than eight inches of rain in the next six weeks, it will end up as the driest year since 2001 in Philadelphia.

Like many months this year, November has been dry -- and this month, thankfully so in the wake of Sandy -- as the month has picked up just a bit more than a half inch of rain so far.  The list of driest Novembers is below.  November is historically among the driest months on precipitation (2.98" is average over the 80's through 2010) so to get a dry November where an inch or two of rain falls is not uncommon.  Getting less than an inch of rain is a bit more uncommon.

November precipitation doesn't have a significant bearing on winter temperatures overall -- there are warm winters (2001-02) and cold winters (1976-77) mixed throughout the data sample.  The temperature correlation is driven on November temperature -- November 2001 was second warmest and November 1976 was third coldest on record.   This November will not finish near either of those extremes.  Snowfall is relatively limited in the dry Novembers listed above -- the snowiest winter of those years listed was 19.5" in 1922-23.  However, November 1917 had 0.86" of precipitation, 9th driest if you exclude this year, and then followed with 38.9" of snow in the subsequent winter so the correlation is tenuous at best.  In a year of mixed bag data, seeing a motley crew of data all over the place is not surprising in the least.

Whether or not we end up with one of the driest Novembers on record is dependent on a storm that looks relatively likely on the 28th and 29th.  Modeling in the longer ranges continues to show this storm as a potential rain and wind maker for the East Coast.

The rainfall outlined above on the GFS for the long range is incredibly speculative (it's over ten days out) but given the projected tranquility of our weather (despite a mainly dry front coming through on Friday night into Saturday) this projected storm system is the last solid chance of rain for our region before December.