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Chester County, PA- Weather Page
Greetings and welcome to the Chester County weather page. This station records continuous weather observations utilizing the Davis Vantage Pro Wireless Solar weather station. This station has detailed daily weather observations for the last 115 years dating back to January 1894 for beautiful Northern and Western Chester County - Pennsylvania. Special thanks to the following National Weather Service Cooperative observers for their efforts in keeping such detailed records since 1894. W.T. Gordon(1894-1930); Howard Pyle(1930-1946) and the National Weather Service 1946-1997. This page is updated frequently so check back often! Please sign my guest book below and feel free to email me with your comments, requests or questions regarding Chester County weather to callapv@verizon.net
Current observations have been observed in East
Nantmeal Township in Northwest Chester County, PA since November 25, 2003.
This Township lies in the Nantmeal Hills within the Welsh Mountain
anticline, The North Valley Hills lie just a few miles south of the Chester
County weather station.
The station is located about 40 miles northwest of Center City Philadelphia and 85 miles west of the
Atlantic Ocean. The general topography of the area is rolling
hills. The East Nantmeal weather station lies at 728 feet above mean sea
level midway between the towns of Glenmoore and Ludwigs Corner,
Pennsylvania. Welsh Mountain is the
highest point in Chester County and rises to 1080 ft above sea level within
8 miles of the East
Nantmeal Station. Exton Pennsylvania just 10 miles south of the station lies at
320 ft above sea level.
Typical of the Piedmont Plateau, the climate is classified as Modified Humid Continental due its proximity to the Atlantic Ocean (85 miles east). However, due to its inland position and relative high elevation many winter storms that mix with or turn to rain in Philadelphia will remain snow in the higher elevations of Chester County. Here in East Nantmeal Township we average 36.0" of snow per winter season. In the lower elevations of Chester County (between 250-500ft asl) the County averages 32.0"of snow each winter based on 112 years of data. Snowfall is quite variable year to year with the snowiest winter recorded in 1898-99 when 95.0" of snow fell. Seasonal snowfall has exceeded 70" 5 times since records began. Statistical analysis shows that Chester County can expect a single storm to exceed 20" on average once every 10 years. During the last 10 years here in Chester County we have experienced 2 snowfalls that have exceeded the 20" mark - February 16-17, 2003 at 21.5" and January 7-8, 1996 when 28.9" of snow fell during the Blizzard of 1996. Snowstorms will exceed 10"every other year. Snowstorms of 6" or more should be expected at least twice each winter.
This area averages between 12" and 16" more snow then points Northeast; East and South toward Philadelphia; NYC; Long Island and New Jersey, all of those locations lie within the Atlantic Coastal Plain. One of the great examples of the climate difference between here and the coastal plain at Philadelphia was the February 23, 1987 storm that resulted in 23.5" of snow here in Chester County - yet only accumulated to 6.8" at the Philadelphia International Airport. Weather systems that affect the Chester County area either develop in the Midwest and are steered eastward by the prevailingly westerly flow aloft or they form in the southeastern United States and track northward along along the Atlantic Coast as powerful "Noreasters" The long overland trajectory of the Artic weather systems in conjunction with a downslope component off the Appalachian Mountains considerably modifies the accompanying cold and dry air masses so that upon reaching Chester County they have warmed and gained moisture either from the Gulf of Mexico or the Atlantic Ocean.
Summers are warm and rather humid. Usually highs reach into the mid-80s with nighttime lows near 60°. However, summer nights in the 50's are to be expected several times each month of the summer season. The occasional maritime influence tends to produce a fairly uniform weather pattern, resulting in occasional prolonged Hot, Hazy and Humid periods. During the period June-August, daytime highs exceed 90° very rarely above 600ft above sea level. However in valley locations around 300 ft in Chester County there is an average of 21 days with temperatures exceeding 90 in the summer. Chester County nighttime summer lows rarely will go lower then 45°. The record high was 107° set back on August 7, 1918. 100 degree readings are quite rare as indicated by the fact that such a reading was not observed at all from 1989 to 1999. Cloud cover is at a minimum during the summer season, as better than 60% of available sunshine is received while nights are generally clear. Prevailing winds flow from the south-southwest averaging 8 mph. Summer rainfall is adequate, falling for the most part as the result of thundershowers which are observed on an average of 22 days.
Cloudiness persists to a much greater degree in the winter due to the increased frequency of cold fronts and coastal low pressure systems ("Noreasters"). Prevailing winds are from the west- northwest averaging 10 to 12 mph. Daytime highs average in the Upper 30's during January with nighttime lows generally around 20 degrees. A normal winter season will have 126 days with subfreezing low temperatures. Around 21 winter days will not make it to the freezing mark for a high temperature. Only one day in recorded history did the temperature stay below zero all day; on January 19, 1994 the high for the day was just -(-1°) below zero after a morning low of -(-11°) below zero.
Summer-like readings have been experienced in the winter, but are rare, the highest being 77° on February 25, 1930. On average sub-zero readings are recorded about twice each winter season, the all time lowest reading was (-19°) below zero on January 14, 1912.
The first snowfall of any consequence may occur as early as November, but is much more likely in December. There is a 35% chance of a White Christmas in Chester County. In a normal winter at least two snowstorms will exceed 6 inches. The largest individual snowstorm (3 days) was the 53.0" inch blizzard that ended on February 14, 1899. The greatest snow depth recorded in Chester County was the 69.8"snow depth recorded on February 15, 1899. The February 1899 snowfall of 69.8" represents the all-time record monthly snowfall for this area. In January 1996 over 36" of snow was on the ground following the Blizzard of January 7-8. Historically however snow cover does not persist for any great length of time, an inch or more of snow is observed on an average of 31 days and six inches or more 6 days annually. The longest period of snow cover an inch or more was 63 days from December 9, 1995 to February 9, 1996, the threat of snow diminishes rapidly after mid-March. However large snowfalls do continue to occur as evidenced by the 13.7" snowfall on March 31 & April 1, 1997, that same system produced only 4" of snow in Philadelphia.
Spring and fall are the transition seasons. Prevailing winds are from the west-northwest at the start of spring, but swing back from the southwest by late spring. As fall progresses the prevailing flow gradually changes back to a west-northwest flow once again. Wind speed averages 8 to 10 mph during both spring and fall. By the latter part of April daytime highs in the 70s are noted more and more frequently; and after the warm summer, these same seventy-degree readings may be observed into October. These mild dry fall days with plenty of sunshine along with the autumnal change in colors amidst the rolling hills of Chester County make the fall season one of the most pleasant times of the year.
The growing (freeze free) season averages 171 days in length although it has varied from 119 days in 1956 to 212 days in 1920. April 25 is the average date of the last spring frost (36 deg or lower) with the latest observed on May 28, 1927. The average date of the first fall frost is October 3 with the earliest September 11, 1924.
A little over one half the annual precipitation usually falls in the spring and summer. The greatest monthly total was 17.81 inches in August 1955, and the maximum 24-hour rainfall was 8.28 inches on September 16, 1999 this resulted from the effects of Hurricane Floyd. While primarily a warm season phenomena, thunderstorms have been observed every month of the year. On the average 9 days with thunder are recorded in the spring, 5 in the fall, and one during the winter, which when combined with the summer statistics gives an annual average of 37 thunderstorm days. As evidenced by Floyd - Hurricanes and Tropical storms have occasionally tracked inland enough to provide heavy rain and wind.