September 2020 was actually a tad cooler but a bit drier than normal. My maximum temperature average (74.5°) was actually 2.6° below normal which was enough to overcome my minimum temperature average of 59.4°
which was only 2.3° above normal, resulting in the mean temperature average of 67.0° to be 0.1° below normal. My total precipitation of 3.49″ was 0.54” below the Prism-derived normal value for my station of 4.03”. I still am having reasonable doubts about that average being too
low and will be checking into the records further in the coming weeks ahead. There were 12 days with measurable precipitation, with 2.64” falling on the first 10 days of the month. This amount was just about 80% of the monthly total amount. There were 2 six day completely dry
periods (11-16 and 19-24) that were separated by the amazingly light 2 day total on the 17th and 18th of 0.08” that fell from the remains of tropical storm Sally that passed very weakly through our area during that time. Sally was the only tropical system that came close enough to have any measurable effects on my location during the month. There were only two notable thunderstorms that fell during the month on the 2nd and 3rd. More details to follow on these noteworthy stormy days which fed off a sub-tropical moist SW flow that followed late August
Hurricane Laura activity.

There was a significant 4 day cool period from the 19th-22nd when max temps stayed in the 60s and mins bottomed out in the 40s (My lowest monthly value was 42.8° on the 22nd). More rural, favorable locations mostly to my N and W actually had mins in the 30s with pockets of frost on their lawns and fields during this period, helping
to usher in astronomical fall appropriately. Conversely the modest streak of 6 days of 80 of more during that first 10 days of the month was not significant except to note that I did not reach 90 for a monthly max temperature. My high for the month was 88° (87.5° actually) on the 3rd.

On the two thunderstorm days during the month, the two highest daily PWG readings recorded on the University of Shady Grove anemometer across the street included the highest reading of 50 MPH on the 3rd and 33 MPH on the 2nd.  After a 29 MPH gust on the 4th winds really calmed down on most of the remaining days of the month, with 17 days having PWG less than 20 MPH, The predominant general direction was from a westerly component (also 17 days).

The highest dew point temperature reached during the month was 79° on the 3rd, reached just prior to our biggest thunderstorm of the month. The dew point remained at or above 70° all day on 3 days during the month, and reached the 70° mark (at least) on 10
days. The lowest dew point temperature value was 32° recorded on September 19. Five days in a row (18-22) featured comfortably low dew point values in the 30s.

The highest barometric pressure reading during September was a respectable 30.62” on the 21st, during our previously described most significant cool spell of the month. The lowest barometric pressure reading for the month was a modest 29.61” on the 30th during our month ending light rainy pattern featuring 5 of the last
6 days of the month with measurable rainfall amounts.

September had 10 cloudy days, 15 partly cloudy days, and 5 clear/sunny days. There were 61 heating degree days and 119 cooling degree days during the month. There were 3 foggy days, 3 windy days, and 2 days with thunder.

My maximum temperature frequencies included 0 days of 90° or greater, 7 days between 80° and 89°, 16 days between 70° and 79°, and 7 days less than 70°. On the minimum temperature frequency side of things, there were 4 days in the 40s, 11 days in the 50s, 10 days between 60° and 69°, and 5 days 70° and above, The coolest daily maximum temperature day was 62.4° on the 19th while the warmest daily minimum temperature day of 73.4° was reached on the muggy, rainy 10th.

The diurnal range daily monthly average was 15.1°          (- 4.9°). Our greatest daily range of temperature was 29.3° on the 23rd at the end of the month’s major cool snap (79.8°- 50.5°). Our lowest range of temperature was 5.7° on the cloudy, rainy 10th (79.1°- 73.4°). There were a total of 5 days during the month with single digit daily diurnal ranges, 4 of which occurred during this period (9-12) that featured mostly cloudy, muggy, rainy conditions.

On my computer/hardware weather station front, I continue to get my tipping bucket clogged frequently with tree and other debris but was very recently cleaned up and a new ISS battery installed by Marty. I still partially rely on the tipping bucket data from the USG station across the street to give me a good indication on how much storm event rainfall I have received plus immediate rainfall rates as it is occurring before I get my final amount from my daily CoCoRaHS rain gauge reading. Also during Marty’s visit I got the display for the data from across the street changed over to a Vantage Vue model, which I am enjoying reading daily. I look forward greatly to the coming cooler autumn days with the usual fall foliage changes. Tropical storm activity and intensity records continued to fall in parts of the Eastern and Gulf Coast states during this 2020 hurricane season, though our immediate area was spared any heavy activity during September.

Data file – Spreadsheet PDF file – September 2020

2020-09 Rockville 2.8 WNW

SUMMARY STATISTICS

Average high temperature 74.5° (- 2.6°)
Average low temperature 59.4° (+ 2.3°)
Mean temperature 67.0° (- 0.1°)

Precipitation 3.49″ (- 0.54“)
Greatest daily amount (date) 1.00″ (3)
Greatest storm total (dates) 1.78” (1-3)
2020 year-to-date precipitation total 41.42″ (+ 10.69”)

High temperature for the month (date) 87.5° (3)
Low temperature for the month (date) 42.8° (22)

Highest barometric pressure (date) 30.62” (21)
Lowest barometric pressure (date) 29.61” (30)

Peak wind gust (direction/date(s)): 50 W (3)
(USG data across the street)

Heating degree days for the month: 61
Cooling degree days for the month: 119

Greatest diurnal range (date): 29.3° (23)
Least diurnal range (date) 5.7° (10)

Number of days:

Fog 6
Windy 3 (PWG => 30 MPH)
Hail 0
Thunder 2
Cloudy 10
Partly Cloudy 15
Sunny/Clear 5
Max temp ≥ 90° 0
Min temp < 50° 4
Precip ≥ 0.01” 12