MAY 2015 – WEATHER STATION HIGHLIGHTS – GAITHERSBURG 2 WNW MARYLAND
by Kevin Shaw, Observer-in-charge/NJWO member
This May was a very warm and significantly dry month. But daily records were sparse, except for several
record high minimum temperatures, mostly in the middle third of the month. Despite being one of the
warmest Mays ever, no record high temperatures were set, and I did not hit 90 all month, which has now
happened during 19 Mays in my 37 year period of record. The mean temperature of 69.6 is now the
second warmest May mean temperature ever in my POR (period of record), just missing the 69.7 record
set back in 2004. The average max temperature of 80.3 came in third place, only surpassed by the record
80.5 (2004) and 80.4 (1991). The average min temperature of 58.8 came in a tie with 2004 for second
place, only topped by 59.2 in 2012. One renegade, non-pattern-fitting record was set on a very cool, damp
day towards the end of the month with a 57 high temperature on the 21st, a daily record low maximum
temperature for the date. The 1.54″ of precipitation during the month was the fourth driest ever May in my
37 year POR. With only 8 measurable days of rain this May which was more than 4 days below normal and
fit in well with the much below normal rainfall total. One last bit of information – May 2015’s lowest temp
(46) was the highest monthly May low temperature ever.
The average maximum temperature for the month was 80.3° (+6.0°), the average minimum temperature
was 58.8°(+7.4°) with the resultant mean average temperature for the month of 69.6°(+6.7°). The highest
temperature recorded during the month was a modest 88° on both the 19th and 28th, which did not come
very close to setting or tying a daily record. While we had no days with max temperatures at or above 90,
we had a lot of 80° or over readings (22). Unfortunately I don’t keep track of that particular statistic through
my POR, so I don’t know where it ranks historically, but I am sure it is very close if not at the top of the heap
of the 37 Mays in my POR.
The month’s total precipitation amount of 1.54″ was 2.92″ below the normal May amount of 4.46″. The 8
days of measurable precipitation during the month was well below the long term average of 12.3 days. The
year to date (YTD) precipitation total of 15.42″ now puts us 2.72″ below the normal YTD average amount for
this time of year of 18.14″. No snow fell in May, so the final seasonal snowfall total of 35.8″ was 7.3″ above
the normal total seasonal snowfall amount of 28.5″.
The monthly barometric pressure extremes were a modest 30.48″ on the 23rd and 29.82″ just a few days
before on the 19th. Average barometric pressure for the month was 30.13″, not the same value you would
get if you averaged the two extreme values (30.15″) I had for the month but pretty close. So far with three
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. It looks promising for developing some sort of relationship there. Further study needed
the rest of the year (at least).
The distribution of 9 sunny, 14 partly cloudy and 8 cloudy days did not align with the below normal
precipitation pattern, nor the number of days with measurable precipitation, but the preponderance of
warm, humid days during this May did align somewhat.
There was 40 heating degree days and 182 cooling degree days. There were 3 days with fog (below
long-term average) and 4 days with thunder (about average).
My maximum temperature frequencies included 22 days at or above 80°, 6 days between 70°and 79°,
2 days between 60° and 69°, and 1 day from 50°- 59°. On the minimum temperature frequency side of
things, there were 5 days between 40°and 49°, 10 days between 50°-59°, 14 days between 60°-69°,
and 2 days with a 70° min.
The diurnal range average was a bit below normal (21.5° vs the normal 22.9°). The max daily range
of 36.3° occurred on the 4th as headed into our first May heat wave. The lowest daily temperature
range of 6.8° occurred on the 21st during that renegade, isolated cloudy, cool day in between hot
periods discussed earlier. This was the only day of the month with a diurnal range lower than 10°
while there were just 3 days with ranges of 30° or more.
The month of May had persistent warmth and dryness that reflected in the monthly averages but not
in the records, save for the large amount of record high mins as mentioned above. Precipitation deficits
were starting to be of concern by the end of May, but a comparatively wet beginning to the month of
June so far has alleviated that worry for the time being. When will I hit my first 90 maximum temperature?
Hopefully it won’t be for a while. I don’t see it for the foreseeable future as of June 4. My personal
preference would be not to hit 90 all summer. But unfortunately that is not very likely. Details on June
coming in about a month!