ChescoWx

I thought I would share a post from my Chester County PA Facebook Weather Site that arose from some farmer questions around the growing season and the relations to the relative elevation differences across the county. Along with an understanding of the differences between frost and freezes. This is going to be a pretty lengthy post as I started looking at some questions I received on frost vs. freeze and the growing season here in Chester County. First off I want to be sure everyone knows the difference between frost and freeze. Frost occurs when the temperature on the ground level reaches 32 degrees. However, the National Weather Services and trained COOP observers and spotters are required to measure the actual air temperature at 5 to 6 feet above the ground level. Therefore, it is easy to measure freezing temperatures based on weather observation records. Frost however can occur with temperatures in the upper 30's so records are not as robust for frost observations....unless it was reported in the notes section of weather observer reports. There are also significant differences based on elevation above sea level. Higher elevations across our county experience less frost and fewer freezing days as cold air is heavier and sinks into valley locations. This is why on calm and clear nights there can be a 5-to-10-degree difference over a mile...with temperatures lower and frost and freeze conditions more common at lower spots vs. higher locations. Here in Chester County, we range from the highest elevation of 1,071 ft. in the Welsh Mountains across Honey Brook Township...to the lowest elevation of 70 feet above sea level along the Schuylkill River at the Montgomery County line. The primary weather records I use are from Coatesville NWS Coop observers (1894-2003) and East Nantmeal Township (2003 to Present). The weather records from the NWS COOP observers between 1894 through 1982 were recorded in Coatesville at an elevation between 350ft and 400 ft above sea level. All observations since 1983 at both Coatesville and East Nantmeal have been taken between 650 ft to 685 ft above sea level. Due to this when I compare actual to average temperatures I use the higher elevation temperature averages for Chester County from 1983 to Present. This will be the 40th year of high elevation records. For all-time records (including temperature, rain, and snow) I use the combined records from 1894 to Present. Overall, the average annual temperature readings from 1894 through 1982 compared with 1983 through 2021 period are almost exactly the same! This, despite our current warming phase of our cyclical climate changes. As expected average high temperatures are more than 1 degree colder when analyzing the higher elevation observations from 1983 through today - with low temperatures averaging almost 2 degrees warmer than the lower spots ran from 1894 to 1983. Overall average annual temperatures from the first 89 years of obs from 1894-1982 had an average annual temperature of 52.2 degrees - compared with the 1984 through 2021 period having an annual average temperature of 52.5 degrees...so only 0.3 degrees warmer. Now due to the differences in elevation the growing season has shown a much greater difference compared to the overall temperature profiles when you compare the higher spots over 650 feet to the lower spots below 400 feet in our County. The near 2 degrees of low temperature warming due to the higher elevations along the ridges has resulted in a 2 week increase in the growing season comparing ridge vs. valley locations (172 days in the valley vs 186 days on the ridges). There is a one week or greater difference between both the last freeze of the spring (April 18th (ridge) vs April 25th (valley) and the first freeze of the fall (October 14th (valley) vs. October 24th (ridge). Of the 89 years of valley observations from 1894-1983 our last freeze in spring occurred in April a total of 60 times (68%) vs. 29 times in May (32%). On the ridges since 1983 our last spring freeze has occurred in April a total of 33 times (84%) compared to 3 times in both May (8%) and March (8%). Let me know if any questions or follow ups to any of the above information. Thanks again for your tremendous support and participation on this site!! It is deeply appreciated!! Sincerely, Paul

The record Chester County High for today is 91 set back in 1960, our record low is the 24 degree reading in 1922. Record rain is the 1.39" that fell in 1921

Record high for Chester County today is 88 set back in 1985 with the record low being 22 from 1922. Record precipitation is 1.26" in 1927.

The Chester County record high for today is 90 degrees set back in 1976 with the record low of 24 from 1928. Record rainfall is the 1.32" from 1991. So far this month we are averaging exactly 1 degree below normal.

We hit freezing again this morning for the 95th and possibly last time this season in Chester County. Warming from now through early next week before a change to below normal temps again to both finish out April and as we open up May across the County.

We ended up with 2.19" of rain with 1.79" yesterday - that sets a new daily rainfall record for the 18th breaking the old mark of 1.43" back in 1924. Also some snow showers this AM

We have tied the record for coldest April 18th max temp here in Chester County...equaling the previous cold max of 45 set back in 2003 with records back to 1894! Also we continue with a mix of rain and wet snow with a temp at 37.9

Continuing to see quite a few reports on my Chester County page and via email from my website of snow falling in spots over 650 feet here in Western Chester County. Continues to rain steadily here in East Nantmeal with frequent mangled snowflakes at 685 ft ASL. Temp at 38.5

A few wet snowflakes mixed with some rain here in NW Chester County - temp 43.7

1983

We did reach a morning low of 30 degrees here in East Nantmeal this morning - our 1st freezing temperature since back on March 30th. As mentioned today is the average date of our last freeze in the highers spots of Western Chester County. The record high for today is 95 back in 1896; with the record low being the 21 degree reading on 1928. Record rain is 1.43" in 1924. Largest snow was 0.3" on this date that began the largest late season snow in Chester County history...when over the 3 days ending on April 20, 1983 we received 6.7" of snow here in Chester County. Snow began at 1120pm on the 17th and continued until the early AM on the 20th. Kind of unusual with a late season snow such as this...was the NWS COOP observer reported localized drifting of the snow with the strong winds and most schools in the area were closed.

The record high for today is 95 set back in 1896 - the record low is 20 degrees from this date in 1928. We do have a Freeze warning up for Western Chester County and a Freeze watch for Eastern Chesco. Of interest tomorrow the 18th is the average date of our last freeze here in Chesco...we shall see.

Today marks the start of the earliest heat wave (at least 3 consecutive 90+ days) in Chester County PA history. As on this date way back in 1896 the National Weather Service coop observers across Coatesville, West Chester and Phoenixville recorded high temperatures exceeding the 90 degree mark starting on the 16th at Coatesville and the 17th at both West Chester and Phoenixville. The heat wave lasted till the 19th across all locations. The next earliest heat wave would not occur across Chester County until April 17, 1976.

The record high today is 86 set in 1967 and the record Chester County low is 24 from back on this date in 1981. Record rain was the 1.71" that fell in 1925.

With today's 1st 60+ day of April we are tied for the 2nd latest date in Aprll Chesco weather history to record our 1st 60+ day. Tied with 1896 and 1938 which also recorded their first such day on the 12th day of the month.

With today's high of only 47.2 here in NW Chester County this is the latest we have gone into any April without a 60 degree reading in the last 61 years! In 1961 the 1st such day was on the 14th of the month - that was also the latest in history for our first 60 degree reading in April.

The record high today is 86 set back in 1922 with the record low of 23 recorded on this date back in 1985. Our largest April snowstorm began on this date at 10am on April 10, 1894 and continued until ending at 230pm on April 12th. By that time 29.2" of snow had fallen. Of note temperatures never fell below freezing the entire time during that historic snowstorm. During the snowfall temps stayed between 33 and 34 degrees so you know it was a very heavy wet snow.

Indeed....when I tell locals we average the same annual snowfall as Chicago or Providence RI they are surprised - they know it snows a lot more here compared to philly or south jersey but....still they are surprised

20.5" for the season here in East Nantmeal - which is about 16" below our annual seasonal fall in the western philly burbs. Largest was the 5.8" on Jan 28/29 and the 4.3" on both March 12th and January 7th.

Record Chester County high temperature today is 86 degrees in 1922 the record low is 22 in 1896. Record rain is 2.23" back in 1906 with the daily record snow being the 6.0" of snow on this date back in 1916. Since last evening another 0.26" of rain has fallen.

Happy Phillies Opening Day (This should be a local holiday!!) Storm Total - 2.91" Tues (0.12") Wed (0.99") and Thurs (1.80")

Of note this is the latest we have gone in April without a 60 degree reading in Chester County in 33 years since way back in 1990! This is only the 11th time since 1894 we have gone this deep into April without a 60 degree reading.

1.04" of rain so far today with more on the way this evening. We are now above normal rainfall for the season - year to date = 12.27" normal thru today 12.24" - the Tempest is only at 0.81" so far with the VP at 0.94" today....

Record high for today is 88 degrees back in 2010 and the record low is 20.0 set in 1982. Record rain is 1.16" in 1930 and the record snow is 2.6" in 1990

Total storm rainfall through 7am for East Nantmeal Township 0.96" with 0.84" since midnight. Now up to 91% of normal year to date rainfall through today 11.08" (12.16") Record high today is 86 degrees back in 1942 - record low is 15 degrees from 1982. It also snowed 3.4" on this date in 1982.