Gaithersburg 2 WNW Maryland – December 2015 climate summary

by Kevin Shaw, Observer in charge

Average high temperature 57.1° (+ 12.8°)
Average low temperature 41.2° (+ 14.9°)
Mean temperature 49.1° (+ 13.8°)
Precipitation 5.12″ (+ 1.75″)
Greatest daily amount(date)1.71″ (25)
Annual 2015 precipitation 43.27″ (- 1.57″)
Snowfall 0.0″ (- 5.2″)
High temperature for the month (date) 71° (12)
Low temperature for the month (date) 25° (20)
Highest barometric pressure (date) 30.73″ (5)
Lowest barometric pressure (date) 29.45″ (14,15)
Peak wind gust (date/direction): 22 (3/WSW)
Heating degree days for the month: 485
Cooling degree days for the month: 1
Greatest diurnal range (date): 29.6° (12)
Least diurnal range (date) 6.3° (1,30)

Fog days: 10
Thunder days: 0
Sunny/clear days: 7 Partly Cloudy days: 7 Cloudy days: 17

Days with measurable precipitation (=>.01″) = 16
Days with maximum temperatures of 70 or more = 3
Days with minimum temperatures of 32 or less = 8


December 2015 closed out the year in spectacular style. Not exactly the style I like, the fourth such wet and warm December in my 37 year period of record (POR). The eighth wettest December (5.12″) but more notably easily the warmest December I have ever recorded. I am not unique in this finding, all of the stations in the region had that same record set. Warmest record average maximum temperature, warmest record average minimum temperature, warmest average mean temperature, and fewest 32° or lower minimum temperatures (8). And all these records smashed the old records by several degrees or days. The zero amount of snow for the month also set a minimum record for December. Several individual days set daily records for maximum temperature and for high minimum temperature. There were 17 Cloudy days, including the 10 days in a row between the 21st and 30th that were quite remarkable also. I don’t have long time records for cloudy days in a month, but 17 tied March 2014 for most ever over the past 3 years. Another notable streak was the 9 straight days of measurable rainfall, between the 22nd and the 30th. The warmest day of the month, 71° on the 12th, did not set a record, but the 63° minimum temperature on Christmas Eve set an all-time highest minimum temperature record for December. The 1.71″ rain on Christmas Day set a daily record for the date and was the fifth wettest December day ever.

The average maximum temperature for the month was 57.1° (+12.8°), the average minimum temperature was 41.2° (+ 14.9°). The resultant mean average temperature for the month was 49.1° (+ 13.8°). Just about every day of the month featured above to much above normal temperatures. The coldest days of the month (around the 19th-20th) were actually about normal values for that time of year, just prior to the first day of astronomical winter. The 3 days of 70° or greater maximum temperatures
were notable enough, but the persistent overnight warmth around Christmas (6 days in a row between the 22nd and 27th that were 50° or greater) was the most notable.

The month’s total precipitation amount of 5.12″ was 1.75″ above the normal November amount of 3.37″. The total precipitation for 2015 of
43.27″ put us 1.57″ below the normal annual total of 44.84″. Precipitation fell fairly evenly for the first 2/3 of the month (1.63″) then we had the streak of 9 straight days of measurable rain mentioned earlier from the 22nd through the 30th when the remaining 3.49″ of rain fell. 16 days of measurable rainfall for the month was quite a bit above the long-term normal amount of 11.

The monthly highest barometric pressure reading of 30.73″ on the 5th was again quite high but not as high as it got in November when 30.80″ was reached. This month’s 30.73″ occurred in the middle of our longest dry spell of the month (a modest 6 days) between the 3rd and 8th. The lowest pressure reading of the month was 29.45″ around midnight between the 14th and 15th during a small, slightly wet period that was transitioning from the extreme warmth of the 12th and 13th into a gradual cooldown towards our coldest period of the month on the 19th and 20th. Average barometric pressure for the month was 30.12″, within .03″ if you averaged the two extreme values (30.73″ and 29.45″). So far with nine months study under my belt, this relationship has shown up as the same in March, 0.06″ difference in April, and 0.02″ in May and June, 0.005″ in July, 0.01″in August, 0.04″ in September, 0.06″ in October, 0.01″ in November and now 0.03″ in December. These values continue to look very promising for developing a strong relationship there. We are now at the end of the year and the end of the study, so conclusions will be forthcoming very soon.

The distribution of 7 sunny, 7 partly cloudy and 17 cloudy days correlated well with the above normal monthly precipitation total of 5.12″.

There were 485 heating degree days and 1 cooling degree day. There
were an above normal amount of 10 days with fog observed and no thunder days. The peak wind gust during December was a modest 22 MPH out of the
WSW on the 3rd on the backside of the retreating rain system at the end of November into the first few days of December. This 22 MPH value breaks a streak I had going of four straight months with a peak wind gust value of 23 MPH.

My maximum temperature frequencies included 3 days between 70°and 79°, 7 days from 60° to 69°, 15 days between 50° and 59°, and 6 days between
40° and 49°, On the minimum temperature frequency side of things, there were 4 days between 20° and 29°, 9 days between 30° and 39°, 11 days between 40°-49°, 6 days between 50° and 59° and 1 day with minimums of 60° or greater.

The diurnal range average was a bit below normal (15.9° vs the normal
17.9°). The max daily range of 29.6° occurred on the 12th on our warmest day of the month after a relatively cool morning to start that day with. The lowest daily temperature range of 6.3° occurred on the first and next to last day of the month, during our initial wet period that started at the end of November and extended into the early first few days of December and then at the end of the month at the end of our longest wet spell of the month described earlier. We had a whopping 12 days with a diurnal range less than 10° (mostly in the wet periods at the beginning of the month and then around Christmas) amd 11 days with ranges of 20° or more, all occurring before the wet Christmas period.

December 2015 increased the notable extremes greatly from November’s total, and easily was the most spectacular month, especially temperature-wise, of any month during the 2015 year. There was a real lack of cold air during the month of December. About half of the days had a warm WSW or SW flow with very few days with the more normal wintertime colder NW flow. It looks like our El Nino winter has kicked in with big storms out west, but a change in the pattern for us so far in January has at least got our temperatures back down to more seasonable levels. Whether that will translate into any snowstorms for us remains to be seen, so far there have been none or even any real close calls. Stay tuned in a few weeks for the January report and perhaps a year in review before that, depending on how busy my schedule is the rest of the month.

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