September 18th, 2007 Forecast

After a cool start with a tied record low at Allentown (39 degrees) we are going to see temperatures climb slowly throughout the remainder of the week. Temperatures will again be cool tonight and tomorrow night but with plenty of sunshine we should see a nice jump in temperatures before the day is out.

Tonight: Record low temperatures appear not to be in the forecast but it will be pretty chilly tonight nonetheless with lows in the 40's in the burbs to around 53 in Philadelphia with clear skies. Winds will be light and from the northeast.
Tuesday: With sunshine and a few clouds during the afternoon we will see temperatures climb up into the lower and middle 70's in the Delaware Valley. It will be cooler at the Shore Points, probably in the upper 60's to around 70 during the afternoon with northeast winds coming in off the Atlantic.

Climate Corner: Despite the recent run of colder than normal weather Philadelphia is running with temperatures about 2.5 degrees above normal since the beginning of the month. We also have been quite dry as we are almost two inches below normal on rainfall since the beginning of September. While there is not much of a correlation between September weather and winter weather patterns overall there is a bit of a connection between September weather and December temperatures. Since 1970, September temperatures and December temperatures flip flop from normal 64% of the time (23 of 36 years, with 1 year having a 'normal' month). That means if you have a warm September then it would be colder than normal December and conversely a cold September would equal a warm December.

Year	SEP	SEP DEP	SEP PRE	DEC	DEC DEP	DEC PRE	DEC SNOW

1970	72.0	3.2	0.82	35.8	-1.6	3.27	1.1
1971	71.6	2.8	5.83	41.6	4.2	1.21	0.1
1972	69.2	0.4	1.12	39.9	2.5	5.20	T
1973	70.7	1.9	3.39	38.6	1.2	6.34	4.6
1974	68.1	-0.7	4.68	39.4	2.0	4.04	0.8
1975	66.6	-2.2	7.21	36.9	-0.5	2.89	1.1
1976	67.3	-1.5	2.44	30.3	-7.1	1.63	2.8
1977	69.9	1.1	3.44	32.6	-4.8	5.19	0.2
1978	68.5	-0.3	1.59	38.6	1.2	5.61	T
1979	68.5	-0.3	4.89	38.2	0.8	1.67	4.9
1980	72.2	3.4	2.79	32.5	-4.9	0.77	1.4
1981	66.8	-2.0	2.83	34.6	-2.8	4.14	2.8
1982	67.6	-1.2	2.32	41.3	3.9	1.80	6.8
1983	69.0	0.2	3.45	33.2	-4.2	7.37	T
1984	64.7	-4.1	1.96	41.9	4.5	2.17	0.2
1985	69.1	0.3	5.78	33.3	-4.1	0.98	1.5
1986	68.3	-0.5	2.33	37.9	0.5	5.89	0.4
1987	68.8	0.0	2.78	39.2	1.8	1.68	1.5
1988	66.7	-2.1	2.62	35.4	-2.0	1.00	0.4
1989	69.7	0.9	5.03	25.5	-11.9	1.21	5.3
1990	68.0	-0.8	1.71	42.1	4.7	3.79	6.4
1991	69.5	0.7	3.58	39.6	2.2	3.86	T
1992	68.4	-0.4	3.04	38.8	1.4	4.61	T
1993	69.7	0.9	6.66	38.4	1.0	3.69	0.9
1994	68.3	-0.5	1.64	41.9	4.5	2.11	0.0
1995	70.4	1.6	3.55	31.6	-5.8	2.15	7.3
1996	68.7	-0.1	4.95	40.2	2.8	8.47	T
1997	67.1	-1.7	1.59	38.5	1.1	3.09	0.2
1998	71.8	3.0	1.86	42.0	4.6	0.82	2.0
1999	69.9	1.1	13.07	39.9	2.5	2.99	T
2000	66.4	-2.4	8.28	31.3	-6.1	2.82	10.5
2001	68.4	-0.4	2.58	43.7	6.3	2.11	0.0
2002	72.2	3.4	3.67	35.4	-2.0	4.05	8.4
2003	70.0	1.2	4.66	36.8	-0.6	5.46	6.0
2004	70.4	1.6	5.19	37.7	0.3	3.17	0.4
2005	73.8	5.0	0.21	34.4	-3.0	2.97	7.0
2006	67.1	-1.7	5.97	42.7	5.3	2.15	T

The breakdown by above and below normal in September:

Above normal Septembers: 14 feature below normal December, 5 do not (74% flip)
Below normal Septembers: 9 feature above normal December, 7 do not (56% flip)

If we end up with an above normal September, which looks pretty good, we could quite conceivably see a cold start to winter. The question is how long will that cold last? That's simply too early to speculate on...for now!

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