September 2016 Gaithersburg 2 WNW Maryland weather summary and statistics for the NJWO blog

by Kevin Shaw, Observer in charge

Average high temperature 80.4° (+ 3.8°)
Average low temperature 63.4° (+ 7.1°)
Mean temperature 71.9° (+ 5.5°)
Precipitation 2.90″ (- 1.30″)
Greatest daily amount (date) 1.54″ (29)
Year to date precipitation 34.13″ (+ 0.02″)
High temperature for the month (date) 93° (10)
Low temperature for the month (date) 52° (25)
Highest barometric pressure (date) 30.32″ (16)
Lowest barometric pressure (date) 29.86″ (1)
Peak wind gust (date/direction): 19 (29/NE)
Heating degree days for the month: 14
Cooling degree days for the month: 220
Greatest diurnal range (date): 28.9° (23)
Least diurnal range (date) 3.8° (30)

Fog days: 4
Thunder: 3
Sunny/clear days: 10
Partly Cloudy days: 8
Cloudy days: 12

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

by Kevin Shaw, observer in charge

September 2016 featured record-breaking warmth with regard
to average mean and average minimum temperatures for my 38
year period of record. The average high temperature wasn’t
quite as extreme but was tied with 2010 as the 5th warmest ever. 3 daily record high temperatures of 92° on the 7th, 8th and 9th were set and 3 daily record high minimum temperatures of 72° were set on the 8th and 10th and 69° on the 19th. No other daily temperature or precipitation records were set during the month. Most of the month was warm and dry as only 0.94″ of rain fell through the 26th, but a late month rally where 1.96″ of rain fell on the last 4 days of the month got the month a lot closer to normal, and not close to any monthly minimum record. September frequently features a lot of heavy daily rainfall amounts from tropical activity, so the maximum monthly amount of 1.54″ that fell on the 29th didn’t come close to the daily record for that date (2.99″ in 2015). The heat during the month coincided roughly with this dry period, and the coolest temperatures of the month also went along with this end of the month wetter period. The lowest temperature of the month (52° on the 25th) is the warmest low temperature ever, surpassing the 51° set just last year in 2015.

Once again this month had a few monthly warm extremes ever
in my 38 year period of record. There were several daily
records set, with 3 maximum low temperature and 3 maximum
daily temperature records. I had the warmest September mean
temperature ever (71.9°) and the warmest average minimum
temperature (63.4°) ever. The maximum average temperature
(80.4°) only came in as tied with 2010 as the fifth warmest ever. The 6 days with maximum temperatures of 90° or greater was
quite a bit above the usual amount for September of just under 2 days. 7 days with measurable precipitation was predictably below the normal of about 9.5 days. The wettest day of the month of 1.51″ on the 29th failed to break a daily record, and no daily precipitation records were set. The highest temperature of the month of 93° was reached on the 10th but was not a daily record. The highest min temperature of the month was 73° on the 9th, but did not set a daily record for that date. No record low temperatures were set.

The average maximum temperature for the month was 80.4°
(+3.8°), the average minimum temperature was 63.4° (+ 7.1°). The resultant mean average temperature for the month was
71.9° (+ 5.5°).

The month’s total precipitation amount of 2.90″ was 1.30″ below the normal September amount of 4.20″. My current year to date precipitation total of 34.13″ through September 30 is 0.02″
below the normal amount of 34.15″.

The month’s highest barometric pressure reading of 30.32″ on the 16th occurred in the middle of the longest (10 days) period of no rain. The modest lowest pressure reading of the month of 29.86″ on the 1st occured on one of our 3 measurable rainy days during the first 26 days of the month.

The distribution of 10 sunny, 8 partly cloudy and 12 cloudy days only modestly fit in with the generally dry conditions found during most of the month, The period from the 2nd through the 25th
contained all 10 of the sunny days during the month.

There were 220 cooling degree days (CDD) and 14 heating degree days. I managed to catch a few foggy days in September (4) but I probably missed a few early morning occurrences when I was asleep. There were 4 thunderstorm days, just a tad above the long-term average amount of 2.3. The peak wind gust during the month was a modest 19 MPH out of the NE on the rainiest day of the month (29). It was a generally calm month as 8 days recorded a PWG of 9 MPH or less, including a very rare day on the 14th when no wind was recorded. Not sure if that really is valid. There were some indications that possibly something got temporarily stuck in the cups. The dominant daily wind direction was 17 days with a NNE to SSE component, followed
by 10 days with a westerly component, 2 days with a northerly component, and the one calm day.

My maximum temperature frequencies included 6 days at or above 90°, 12 days between 80° and 89°, 8 days between 70°and 79°, and 4 days
between 60° and 69°. On the minimum temperature frequency side of things, there were 7 days between 50° and 59°, 19 days between 60° and 69° and 4 days with minimum temperatures of 70° or greater.

The diurnal range average was below normal (17.0° vs the normal of
20.3°). The max daily range of 28.9° occurred on the 23rd on the last day of the month with an 80° max temp (88°-59°). There were an additional 10 days with daily ranges at or above 20°, The lowest daily temperature range of 3.8° (62°-58°) occurred on a cloudy, cool, damp day (coolest max temp of the month on the 30th). There were 4 days during September with daily temperature ranges less than 10°, including the last 3 days of the month.

The tropics appear to be getting active finally as we head into October, with record-breaking Matthew threatening our area for a time here in early October before veering elsewhere. Meanwhile cold fronts are finally becoming more frequent, and the first snows of the season have been appearing in the Rockies and out on the Northern Plains the past few days. Fall color has been slow to develop so far, but should get going soon. Check back in a few weeks to see how October works out. Please continue to watch the tropics closely for the next few weeks but also those stronger cold fronts with snow getting closer.

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