Saturday, June 15, 2013

Rainy Junes & Their Aftermath

We're currently sitting in 8th for wettest Junes on record -- 7.33" of rain has fallen so far. With today marking the halfway point in the month, the likelihood of getting another couple of inches of rain is pretty high and it would not shock me if we made a run at 1938 for wettest June on record before all is said and done.

How do these other years stack up as we move forward through the summer? What kind of rainfall can we expect...and how about the connection to the tropics?

First, years that are as rainy as this one is in June have a tendency to produce another substantially rainy month through September. 1887, 1906, 1938 (twice), 1962, 1969, and 1975 (twice) all featured months with six or more inches of rain between July and September.  2006 barely missed that cut.

Why is that? For one, a wet June along the East Coast is often caused by a main upper level trough position in the Midwest or Great Lakes, with high pressure in the Atlantic or over New England. This provides an avenue for moisture to travel up the East Coast along slow moving or stalled frontal boundaries.  This pattern often will set up a second or third time during the summer months. There are exceptions -- 1973 is a notable one, but by and large years such as this run into a rainy pattern of some sort again during the course of summer or early fall.

500 mb (mid level) anomaly chart for Junes since 1948 that are among rainiest in Philly.

500 mb (mid level) anomaly chart for June-September since 1948 w/ rainy June.

In terms of tropical connections, some of the years you see above also ring bells for past hurricane and tropical storm impacts along the East Coast.

  • 2006 featured Ernesto, which brought two inches of rain to Philly as well as gusty winds over 70 mph at the Shore.
  • 2003 brought Isabel, which brought not much rain to us but a lot of wind to our region as Isabel's center tracked through the Carolinas, West Virginia, and into Western Pennsylvania.
  • 1906 brought a hurricane into South Carolina in September. This had little impact on our weather locally.
  • 1938 brought the Long Island Express to New England. We missed the worst of the wind impact as the storm's center went to our east but because a frontal boundary was nearby we picked six inches of rain in Philadelphia over a three day period between the front and some thrown-in moisture from the hurricane.
  • 1962 and 1969 also brought storms that paralleled the coast (Alma in 1962) or struck eastern New England (Gerda in '69).
Most years tend to have some level of tropical threat along the East Coast, so seeing this isn't a correlation/causation type thing but given the pattern (Midwest/Lakes trough, ridge in Atlantic) is setting itself up, tropical systems later in the summer or into the fall can get pretty dang close to the coast in such scenarios.  I've listed a motley crew of storms -- from Ernesto to Isabel to 1938 -- as an example that various scenarios may unfold.  Then there are years like 1973 where a quasi-tropical system hits Massachusetts and does nothing in terms of impacts locally, and 1975 with a remnant system (Eloise) bringing four inches of rain in September.  None of this is a guarantee, of course, but years like this do have that tendency as we move through the balance of the summer.