Geography and Topography

Chester County is located in the Northeast U.S in the Northern Mid-Atlantic Region. The County topography features rolling hills and valleys, characteristic of the Piedmont Plateau Region, with a high point of about 1,071 feet at Welsh Mountain and a low point along the Schuylkill River at approximately 70 feet. The land is shaped by major watersheds like the Brandywine and Octoraro creeks, contributing to a landscape that historically supported fertile agricultural land and is now a blend of suburban development and rural open spaces, including significant areas of woodlands and streams.

Much of Western Chester County PA lies in elevations over 600 ft ASL. The National Weather Service split out this zone (PAZ101) from the relatively lower valley elevations seen across much of Eastern Chester County (PAZ102) several years ago to better distinguish the climate differences attributed to relative elevation and more inland location from the Atlantic Ocean. We have taken care to ensure an appropriate mix of these higher and lower elevation locations to support the most accurate climate data and analysis for Chester County.

Climate Classification

Typical of the Piedmont Plateau, the climate is classified as Modified Humid Continental due to 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 as snow or another form of wintry precipitation in the higher elevations of the County. In the relative higher elevations, here in Chester County (above 660ft above sea level) the County averages around 35” at higher elevations above 550 ft and closer to 30” of snow at lower valley locations in an average winter season based on data from 1983 to Present. Snowfall in Chester County is quite variable year to year with the snowiest winter recorded in 1957/1958 at Devault 1W when 106.7” of snow accumulated that winter season to as little as absolutely no snow at all recorded in Coatesville 1SW in the winter of 1972-1973.

Historic Snowfall and Major Storms

The Chester County area is home to 2 of the largest snowstorms ever recorded in the state of Pennsylvania. The largest being the 52.4″ snowstorm at Coatesville 1SW that ended on February 14, 1899 and the second greatest being the 50″ of snow that fell in Morgantown on the Berks/Chester County border that ended on March 21, 1958. Statistical analysis shows that Chester County should expect a single storm to exceed twenty inches (20″) on average once every 10 years. However, just since 2003 the County has experienced 6 snowfalls that have exceeded the 20″ mark -January 23, 2006 with as much as 27.4” at Coatesville 1E; February 9-10, 2010 with as much as 26.8″ at East Nantmeal Township; February 18, 2003 with as much as 22.4″ at West Chester; February 14, 2014 with Morgantown/Elverson recording 22.2”and February 12, 2006 with Coatesville 2W reporting 21.1”. Snowstorms will exceed 10″ every other year. Snowstorms of 6″ or more should be expected at least twice each winter. The Chester County area averages significantly (12″ to 16″) more snow then areas on the Atlantic Coastal Plain such as Philadelphia; New York City; Long Island, New Jersey, and Delaware.

Weather Systems and Airflow

Weather systems that affect the Chester County area either develop in the Midwest and are steered eastward by the prevailing westerly flow aloft or they form in the southeastern United States and track northward along the Atlantic Coast as powerful “Nor’easter’s The long overland trajectory of any arctic high pressure systems in conjunction with a down slope component off of the Appalachian Mountains to the west of Chester County 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 America or the Atlantic Ocean.

Summer Climate

Summers are warm and humid. Usually, highs reach into the mid-80s with nighttime lows near 60 degrees. 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 uniform weather pattern, resulting in occasionally prolonged Hot, Hazy and Humid periods. During the period June-August, daytime highs will only reach the 90-degree mark on average less than 10 times per summer season across the higher ridge locations. This is in sharp contrast to the more than 30 such days reported at the urban heat island that is the Philadelphia International Airport (PHL). While the airport is only 20 to 28 nautical miles to the southeast of Chester County it does lie close to the urban center of Philadelphia and at only 59 feet above sea level and on the Atlantic Coastal Plain. Here in Chester County on the Piedmont Plateau summer low temperatures will rarely go lower than 45 degrees. The all-time record high for Chester County (though some dispute the reading) was 111 degrees at Phoenixville on July 10, 1936. The next hottest day was 107 degrees set back on August 7, 1918 at both Sadsburyville and Coatesville. 100-degree readings are almost non-existent across the higher elevations of Chester County. and has in fact only occurred one time in the county, this occurred on Augusts 2, 2002. 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 as the result of thundershowers which are observed on an average of 22 days.

Winter Climate

Cloudiness persists to a much greater degree in the winter due to the increased frequency of cold fronts and coastal low-pressure systems known as Nor’easter. Prevailing winds during the wintertime are from the west- northwest averaging 10 to 12 mph. Daytime highs average in the Upper 30’s during January with nighttime lows around 20 degrees. A normal winter season here in Chester County will have around 108 days with sub-freezing low temperatures, this ranges from as many as 124 days at low elevation spots like Spring City to as few as 76 such days at higher elevation stations such as London Grove. Around 23 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 at Coatesville 2W, the high for the day was just one (1) degree below zero after a morning low of eleven (11) degrees below zero.

Temperature Extremes and Snow Cover

Summer-like readings have been experienced in the winter, but are rare, the highest being 80.2 degrees on February 21, 2018 at the Warwick Township DEOS station. The all-time lowest reading was nineteen (19) degrees below zero which happened 3 times the first being at Coatesville 1SW on January 14, 1912, the 2nd being at Phoenixville on January 22, 1961 and the last time occurring at Octoraro Lake on January 22, 1984. 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-inch blizzard that ended on February 14, 1899 at Coatesville PA. The next largest snowstorm occurred on the Chester County/Berks County border in the Morgantown/Elverson area when 50″ of snow fell between March 19 through 21 back in 1958. The greatest snow depth recorded in Chester County was the 69.8″snow depth recorded at Coatesville 1SW on February 15, 1899. The February 1899 snowfall of 69.8″ at Coatesville 1SW represents the record monthly snowfall for this area. In January 1996 over 36″ of snow was on the ground following the Blizzard of January 7-8 in 1996. 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 of an inch, or more was 63 days from December 9, 1995 to February 9, 1996 at Coatesville 2W, 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 at the Philadelphia International Airport.

Transitional Seasons

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.

Growing Season and Freeze Dates

The growing (freeze free) season averages 188 days in length although it varied from only 119 days in 1956 to 233 days in 2005. April 18th is the average date of the last spring freeze, with the latest observed on May 29, 1936 with 32 degrees at Phoenixville. The average date of the first fall freeze is October 25th with the earliest September 17, 1986 at Phoenixville and the latest on December 2, 1985 at Devault 1W.

Precipitation and Thunderstorms

A little over one half the annual precipitation usually falls in the spring and summer. The average annual precipitation (melted snow and rain) across the County is 45.83”. The greatest monthly total was 18.15” at Morgantown/Elverson in August 1955, and the maximum 24-hour rainfall was 8.28 inches at Thorndale on September 16, 1999 this resulted from the effects of Hurricane Floyd. While primarily a warm season phenomenon, 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 occasionally have tracked inland enough to provide heavy rain and wind.