December 2020 was warmer and wetter than normal.
My maximum temperature average (45.7°) was 0.5°
above normal but my minimum temperature average
(32.7°) was much more significantly above normal
(+ 4.5°), resulting in the mean temperature average
of 39.2° to be 2.5° above normal. These are quite
similar to my December 2019 temperature averages
and departures. My total precipitation of 4.93"
was 1.83” above the normal December value for my
station of 3.10”. There were 10 days with
measurable precipitation, roughly 96% of which
occurred in three periods (0.98” on the 4th-5th,
2.04” on the 14th and 16th, and 1.71” on the 24th-
25th). All 2.3“ of the monthly snow occurred on
that wet 16th date, with a trace of snow (flurries)
on Christmas Day . The year-to-date (annual final)
total of precipitation of 53.41” was 10.37” above
the normal average annual total of 43.04”.
There were two short, moderately cold spells
during the month, one around the mid-month snow
event from the 15th-19th and the other right
around Christmas when max temps held in the 30s
and lows were mostly in the low-mid 20s. The
coldest day of the month was the day after
Christmas when the max barely got above the
freezing mark (32.2°) with the lowest min temp
of the month (22.0°) also being reached. No
really notable high temperatures occurred
during the month, with one moderate 3 day
streak from the 11th-13th holding forth with
max values around the 60° mark, the highest
of which was 62.3° on the 12th.
The monthly maximum daily precipitation amount
of 1.38" was measured on Christmas Eve, with
an additional 0.33 falling on Christmas Day.
The two day total of 1.71” was the greatest
rainstorm of the month. The highest barometric
pressure (30.60” on the 29th-30th) occurred
only a few days after the lowest pressure
reading of the month on Christmas Day (29.41”
at the end of the month’s biggest rain event).
The highest dew point temperature reached
during the month was 59° during the Christmas
Eve rainstorm on the 24th. With a strong cold
front following immediately thereafter, the
lowest dew point temperature of the month of
9° was reached the very next day, on Christmas
Day, the 25th. This is a pattern we often see
between the highest and lowest pressures of
the month, but not this time.
December had 9 cloudy, 19 partly cloudy, and
3 clear/sunny days, probably about normal for
this time of year. There were 797 heating
degree days and no cooling degree days during
the month. There were 9 foggy days, 11 windy
days, 2 days with snow and 1 day (each) with
glaze and sleet. The peak wind gust noted
during December was 40 MPH from the NW on
the 2nd.
My maximum temperature frequencies included
3 days between 60° and 69°, 4 days between
50° and 59°, 16 days between 40° and 49°,
and 8 days below 40°. On the minimum
temperature frequency side of things, there
were 10 days below 30°, 16 days between 30°
and 39°, and 5 days between 40° and 49°.
The coldest daily max temperature day was
32.2° on the 19th while the warmest daily
min temp day of 45.0° was reached on the 12th.
The diurnal range daily monthly average was
only 13.0° (- 4.0°). Our greatest daily range
of temperature was 32.5° on the 25th (55.1°-
22.6°). We had a total of only 3 days with
daily ranges above 20°. Our lowest range of
temperature was 4.5° (47.7°-43.2°) on the 4th.
There were 11 days with diurnal temperature
ranges of less than 10° during the month,
spread fairly evenly throughout the month.
December 2020 was a bit wet and exhibited the
usual mild month pattern largely due to warm
minimum temperatures with our first measurable
snow of the season occurring on the 16th.
Amazingly enough once again the monthly minimum
temperature did not reach into the teens at all.
There has been no real ice formation on local
ponds yet. The wet month added to what was
generally a wetter than normal year. Amazingly
enough these characteristics were strangely
similar to last December, and I used much of
the same text with just a little bit of
editing needed.
DECEMBER 2020 – ROCKVILLE 2.8 WNW MARYLAND
SUMMARY STATISTICS
Average high temperature 45.7° (+ 0.5°)
Average low temperature 32.7° (+ 4.5°)
Mean temperature 39.2° (+ 2.5°)
High temperature for the month (date) 62.3° (12)
Low temperature for the month (date) 22.0° (26)
Precipitation 4.93" (+ 1.83“)
Greatest daily amount (date) 1.38" (24)
Greatest storm total (dates) 1.71” (24-25)
2020 year-to-date precipitation total 53.41" (+ 10.37”)
Snow 2.3” (16)
Highest barometric pressure (dates) 30.60” (29,30)
Lowest barometric pressure (date) 29.41” (25)
Peak wind gust (date/direction): 40 (2/NW)
(USG data across the street)
Heating degree days for the month: 797
Cooling degree days for the month: 0
Greatest diurnal range (date): 32.5° (25)
Least diurnal range (date) 4.5° (4)
Number of days:
Fog 9
Windy 11
Glaze 1
Sleet 1
Snow 2
Cloudy 9
Partly Cloudy 19
Sunny/Clear 3
Max temp = 60° 3
Min temp <= 32° 17
Max temp <= 32° 1
Precip = 0.01” 10