September 2015 Weather summary and data – Gaithersburg Maryland by NJWO member observer Kevin Shaw

by Kevin Shaw, Observer in charge

Average high temperature 80.9° (+ 4.3°)
Average low temperature 61.4° (+ 5.1°)
Mean temperature 71.2° (+ 4.8°)
Precipitation 3.63″ (- 2.40″)
Greatest daily amount (date) 2.99″ (29)
Year to date precipitation 32.10″ (- 2.05″)
High temperature for the month (date) 92° (3)
Low temperature for the month (date) 51° (14)
Highest barometric pressure (date) 30.40″ (26,27)
Lowest barometric pressure (date) 29.65″ (12)
Peak wind gust (date/direction): 23 (20/NNW)
Heating degree days for the month: 7
Cooling degree days for the month: 191
Greatest diurnal range (date): 31.5° (16)
Least diurnal range (date) 7.1° (21)

Fog days: 2
Thunder days: 0
Sunny/clear days: 11 Partly Cloudy days: 10 Cloudy days: 9

Days with measurable precipitation (=>.01″) = 6
Days with maximum temperatures of 90 or more = 7
Days with minimum temperatures of 70 or more = 4

by Kevin Shaw, observer in charge

September 2015 was basically very dry and quite warm, but skewed significantly by short periods that accentuated the trend in temperature and diminished the very dry tendency most of the month. The month started out hot, with 7 of the first 9 days posting maximum temperatures of 90 or greater. From the 10th to the end of the month, no further 90 max temps were to be reached. So this initial period was the biggest reason we had much above normal September temperatures. Record highs were set on the 8th with 91 and tied on the 9th with 90. The month’s high of 92 did not set or tie a record. The minimum temperatures of 74 on the 9th and 70 on the 29th both set daily high minimum temperature records. No other temperature records were set. On the precipitation side, the month was very dry on 29 of the 30 days of the month. Only 0.64″ of rain fell from the 1st-28th and then on the 30th. However, because of torrential late afternoon and evening rains on the 29th when over 80% of the month’s rain fell (2.99″) we almost rallied to a normal precipitation level for the month. Needless to say that 2.99″ amount set a daily record, but with numerous tropical systems occurring several times in the past, a monthly daily record was not set.

The average maximum temperature for the month was 80.9° (+ 4.3°), the average minimum temperature was 61.4° (+ 5.1°). The resultant mean average temperature for the month was 71.2° (+ 4.8°). This mean average temperature was the third warmest ever, only surpassed by 1998 (71.6°) and 1980 (71.3°). The 7 days with max temps of 90 or greater was above normal (normal just under 2 days) but not a record (11 days in 1980 and 8 days in 1998).

The month’s total precipitation amount of 3.63″ was 0.57″ below the normal September amount of 4.20″. The 6 days of measurable precipitation during the month was below the long term average of 9.6 days. The year to date (YTD) precipitation total of 32.10″ now puts us 2.05″ below the normal YTD average amount for this time of year of 34.15″.

The monthly barometric pressure extremes were a modest 30.40″ on the 26th and 27th near the end of the dry spell most of the month with the 29.65″ lowest pressure value being recorded on the 12th, during one of our very few cloudy, light wet days during the month. Average barometric pressure for the month was 30.07″, within .04″ if you averaged the two extreme values (30.40″ and 29.65″). So far with seven 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 and 0.04″ in September. These values look very promising for developing a strong relationship there. Further study continues through (at least) the rest of the year.

The distribution of 11 sunny, 10 partly cloudy and 9 cloudy days coincided roughly with the slightly below normal monthly precipitation 3.63″ amount.

There were 7 heating degree days and 191 cooling degree days. There were 2 days with fog observed and no thunder days. The peak wind gust during September was a modest 23 MPH out of the NNW during a cold frontal passage on the morning of the 20th.

My maximum temperature frequencies included 7 days at or above 90°, 9 days between 80° and 89°, 13 days between 70°and 79° and 1 day from 60° to 69°, On the minimum temperature frequency side of things, there were 14 days below 60°, 12 days between 60°-69° and 4 days with minimums of 70° or greater.

The diurnal range average was a bit below normal (19.5° vs the normal 20.3°). The max daily range of 31.5° occurred on the 16th when we were in a warm, dry period with good radiational cooling. The lowest daily temperature range of 7.1° occurred on the 21st during our coolest max temp day of the month (65.9°) with lots of clouds and light rain. We had a total of 2 days with a diurnal range less than 10° (the aforementioned 7.1° day and then on the 29th when our big rainstorm of the month occurred) while there were a considerable 13 days with ranges of 20° or more and 3 additional days with ranges of 30° or more, distributed fairly evenly through the month.

September 2015 was headed for the record books in both temperature and precipitation till we had that big rainstorm on the 29th that broke our drought, with more rain falling in early October. We all needed the relief! Fall has progressed nicely in October so far, though still no frosts recorded as yet. That looks to possibly change very soon, but we shall see. Details in a few weeks, Enjoy the fall weather and the colorful changing foliage, hopefully sooner rather than later!

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