MAY 2016 – GAITHERSBURG 2 WNW MARYLAND – Report for the NJWO Blog

by Kevin Shaw, Observer in charge

Average high temperature 70.8° (-3.5°)
Average low temperature 54.0° (+2.6°)
Mean temperature 62.4° (-0.5°)
Precipitation 5.66″ (+1.20″)
Greatest daily amount (date) 1.58″ (2)
Year to date precipitation 18.42″ (+0.28″)
Seasonal snowfall (2015-2016) 37.9″ (+9.1″)
High temperature for the month (date) 90° (26,27)
Low temperature for the month (date) 41° (16)
Highest barometric pressure (date) 30.31″ (10)
Lowest barometric pressure (date) 29.67″ (4,5)
Peak wind gust (date/direction): 23 (14/WSW)
Heating degree days for the month: 161
Cooling degree days for the month: 81
Greatest diurnal range (date): 33.0° (25)
Least diurnal range (date) 2.7° (6)

Fog days: 8
Thunder: 1
Sunny/clear days: 7 Partly Cloudy days: 7 Cloudy days: 17
Days with measurable precipitation (=>.01″) = 18
Days with maximum temperatures of 90° or more = 2
Days with maximum temperatures of less than 60° = 7

by Kevin Shaw, observer in charge

May 2016 was remarkable with regard to the extent of
cloudy, cool days the first half of the month, and the
streak of 14 straight days with rain on the first 11
days of the month that was an extension of the last 3
wet days of April. I don’t have records for this
particular type of streak but so far in my preliminary
research this is the longest streak of its kind in my
37+ years of records for my current location. The 18
total days with rain during May was quite a bit above
normal but was not a record for May. The record for
May is 20 days set back in 2003, with second place a
tie at 19 days in 1996 and 2009. The 17 cloudy day
total is probably not a record, but once again I don’t
keep close tabs on the records for that statistic.
This May was my 10th wettest May in my 38 year period
of record. Amazingly as cool and cloudy as it was,
the mini-heat wave the last third of the month
balanced out my temperature averages into below normal
but only with the average max temperature and the mean
temperature (only slightly). The only daily
precipitation record set during the month was on the
wettest day of the month on the 2nd with 1.58″. It was
also my only thunderstorm day of the month. There were
two 90 max days (not records), but seven days with temps
holding in the 50s (three of these days set new minimum
maximum daily records).

The average maximum temperature for the month was 70.8°
(-3.6°), the average minimum temperature was 54.0° (+ 2.6°). The resultant mean average temperature for the month was
64.2° (- 0.5°). The cool first part of the month was almost
balanced out by a warm finish, but not quite.

The month’s total precipitation amount of 5.66″ was 1.20″
above the normal May amount of 4.46″. My current year to
date precipitation total of 18.42″ through May 31 is 0.28″
above the normal amount of 18.14″. Precipitation was
concentrated somewhat during the first 23 days of the month, with over 90% of the total precipitation and 17 out of the
18 measurable days of rain falling. The 18 days of measurable precipitation for the month well above the the long-term
normal amount of 12.3 days. I think it is very interesting to note on one particularly rainy day on May 22, the relative
humidity only varied the entire day by one percent, from 98% to 97%. I believe that probably at least ties my long time
record, I don’t ever recall any day with no variation in
relative humidity values since I started to record this data daily back in November 2009, and probably before that too.

The month’s highest barometric pressure reading of 30.31″ on the 10th occurred near the end of our 14 day precipitation
streak. The lowest pressure reading of the month was 29.67″
on the 4th and 5th occurred in the middle of our big early
month rainfall and cool, cloudy streak.

The distribution of 7 sunny, 7 partly cloudy and 17 cloudy
days fit in the wet May 2016 pattern. The sunny and partly
cloudy days mostly occurred during the last part of the month.

There were 161 heating degree days (HDD) and 81 cooling degree days. There were an above normal amount of 8 days with fog and the lower than normal only 1 thunderstorm day. The peak wind gust during the month was a modest 23 MPH out of the WSW on the 14th, behind the strongest traditional cold frontal passage of the month. It was not a very windy month, with only 3 days
registering peak wind gusts over 20 MPH. The cool first period generally featured easterly component winds and the warmer drier last week or so featured more of a westerly component wind, particularly WSW.

My maximum temperature frequencies included 2 days of 90°, 6 days between 80° and 89°, 9 days between 70° and 79°, 7 days between 60° and 69° and 7 days between 50° and 59°. On the
minimum temperature frequency side of things, there were 6 days between 40° and 49°, 20 days with minimums between 50° and 59°, and 5 days with minimums of 60° or greater.

The diurnal range average was quite a bit below normal (16.8° vs the normal of 22.9°). The max daily range of 33.0° occurred on the 25th at the beginning of the mini-heat wave at the end of the month (88°- 55°). There were an additional 11 days with daily ranges at or above 20°. The lowest daily temperature range of 2.7° (54°-51°) occurred on the 6th on our second wettest day of the month. right in the middle of the 14 day consecutive wet day period from April 28-May 11. There were a total of 9 days during May with daily temperature ranges of less than 10°.

May 2016 was a reversal to the trend of warmer and drier than normal months so far this year. We did have the usual warmup at
the end of the month which helped to knock down the below normal departures quite a bit. Convective activity continued to run below normal in what normally is a very active month. The
predominant easterly flow helped to keep thunderstorm activity to a minimum. But June has started out more summery, not too hot yet but quite muggy and more active convectively so far. Check back in a few weeks to see how June finishes out. It can be quite a steamy, wet month in this area as we head into summer.

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