December 2019 was warmer and a bit wetter than normal. My maximum temperature average (45.9°) was 0.7° above normal but my minimum temperature average (33.0°) was much more significantly above normal (+ 4.8°), resulting in the mean temperature average of 39.4° to be 2.7° above normal. My total precipitation of 3.26″ was 0.16” above the normal December value for my station of 3.10”. There were 12 days with measurable precipitation, roughly 2/3 of which occurred between the 9th and 17th (more on that later). All 1.4 “ of the monthly snow also occurred during this period (0.3” on the 11th and 1.1” on the 16th) A trace of snow also occurred on the 4th. The year-to-date (annual final) total of precipitation of 49.37” was 7.46” above the normal average annual total of 41.91”.

There were two small, moderately cold spells during the month, but there was only one really cold day when the max temp stayed below freezing (30.6° on the 19th). It was the coldest max temp day since March 6 (28.7°) The high of 30.6° and low of 20.0° on the 19th were the coldest readings of the month. The two moderate cold spells were from the 11th-12th and 16th-21st. No really notable high temperatures occurred during the month, with the two highest max temperatures of the month occurring on the 28th (60.6°) and 30th (61.8°).

The monthly maximum daily precipitation of 0.70″ on the 9th was at the beginning of the generally wettest period of the month between the 9th and 17th, a 9 day event when 2.10” of precipitation fell (roughly 64% of the monthly total) expressed a bit differently than above. Another significant occurrence took place within this period. The highest (30.79” on the 12th) pressure reading of the month (and highest since 30.80” was recorded back on 2/19/2019) was followed by the lowest pressure on the month about 50 hours later on the 14th when 29.35” was recorded. This extreme pressure difference within a short period of time occurs more frequently than you might think, particularly during the colder, stormier months.

The great amplitude in the pressure variations can’t help to increase the precipitation around the time of this particular type of occurrence. The only other wet periods during the month were near the beginning of the month (0.43” total on the 1st and 2nd) and near the end of the month (0.69” on the 29th and 30th, which also was the warmest day of the month).

The highest dew point temperature reached during the month was 61° on the 30th. With the colder temperatures dew points continued much lower than previous months, and never high enough to produce any discomfort.
The lowest dew point temperature value was reached on the 18th and 19th with a 7° reading, on two of the coldest days of the month.

December had 11 cloudy days, 13 partly cloudy days, and 7 clear/sunny days, probably about normal for this time of year. There were 791 heating degree days and no cooling degree days during the month. There were 11 foggy days, 9 windy days, 3 days with snow and 1 day with glaze. The peak wind gust noted during December was 45 MPH from the NW on the 5th.

My maximum temperature frequencies included 2 days between 60° and 69°, 9 days between 50° and 59°, 12 days between 40° and 49°, and 8 days below 40°. On the minimum temperature frequency side of things, there were 10 days below 30°, 17 days between 30° and 39°, and 4 days between 40° and 49°. The coldest daily max temperature day was 30.6° on the 19th while the warmest daily min temp day of 45.3° was reached on the 29th.

The diurnal range daily monthly average was only 12.9° (- 4.1°). Our greatest daily range of temperature was 23.2° on the 26th (54.2°-31.0°). We had a total of only 4 days with daily ranges above 20°. Our lowest range of temperature was 3.5° (40.8°-37.3°) on the 2nd. There was a whopping 12 days with diurnal temperature ranges of less than 10° during the month, mostly during the first half of the month.

December 2019 was a bit wet and mild month with our first measurable snow of the season occurring on the 11th. Amazingly enough the monthly minimum temperature did not reach into the teens at all. There has been no real ice
formation on local ponds yet. My new Davis Vantage Vue console has yet to be made operational, but I have been using Marty’s spare VP2 console to do the trick so far. Marty has resolved a possible glitch in the Vue console, and we are waiting to get together to set that up, along with my new Dell computer, which needs to get my Thunderbird email program to be loaded onto. I am not sure where we all can find the time to do all this and much more.

2019-12 Rockville 2.8 WNW

Average high temperature                          45.9° (+ 0.7°)
Average low temperature                            33.0° (+ 4.8°)
Mean temperature                                      39.4° (+ 2.7°)

Precipitation                                               3.26″ (+ 0.16“)
Greatest daily amount (date)                      0.70″ (9)
Greatest storm total (dates)                      1.03” (9-11)
2019 annual precipitation total                 49.37″ (+ 7.46”)

High temperature for the month (date)      61.8° (30)
Low temperature for the month (date)       20.0° (19)

Highest barometric pressure (date)           30.79” (12)
Lowest barometric pressure (date)            29.35” (14)

Peak wind gust (date/direction):                45 (5/NW) (USG data across the street)

Heating degree days for the month:          791
Cooling degree days for the month:             0

Greatest diurnal range (date):                    23.2° (26)
Least diurnal range (date)                            3.5° (2)

Number of days:

Fog                      11
Windy                   9
Glaze                    1
Cloudy                  11
Partly Cloudy        13
Sunny/Clear          7
Max temp ≥ 60°     2
Min temp <= 32°   14
Max temp <= 32°   1
Precip ≥ 0.01”       12