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 4^th-5^th, 2.04” on the 14^th and 16^th, and 1.71” on the 24^th-25^th). 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 15^th-19^th 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 11^th-13^th holding forth with max values around the 60° mark, the highest of which was 62.3° on the 12^th.

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 29^th-30^th) 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 24^th. 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 25^th. 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 19^th while the warmest daily min temp day of 45.0° was reached on the 12^th.      

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 16^th. 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.

Data spreadsheet for December (EXCEL PDF) 

2020-12 Rockville 2.8 WNW

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 (date)            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