nice snippet from LWX AFD... mentions the lows
.CLIMATE...
A rather notable streak of heat has been ongoing in the immediate
Washington DC area for nearly three weeks.
As of this afternoon, 20 consecutive days had reached at least 90 F
at Ronald Reagan Washington National Airport (KDCA). While it is
certainly not unusual to hit 90 F in July in the DC area, multiple
weeks without a break are relatively rare, as evidenced by the all-
time record which is 21 days (it happened twice in the 1980s -- once
ending on August 14th 1980, and again ending on August 18th 1988).
The ongoing streak is notable not only for its longevity, but its
earlier occurrence in the summer season.
If the temperature reaches 90 F on Thursday, the record will be
tied. But it`s going to come down to the wire.
While the background air mass will be warming, it will also become
increasingly humid. The more humid it is, the longer it takes for
the air to heat up (humid air is denser and requires more
energy to raise its temperature). In weather speak, 850 mb
temperatures will be about 1 C warmer than today, and 1000-500
mb thicknesses will be increasing several dm. But with the
increasing humidity will come a deck of clouds. These clouds
should lift and scatter by late morning and may actually help to
trap heat tonight leading to a warmer start. But if the clouds
linger too long, the air might not have enough time to heat up
to 90 F, especially at the airport with an onshore wind right
off the Potomac River (the water temperature at nearby
Washington Channel is 85 F).
Which of these competing factors will win out? It`s not clear just
yet, and probably won`t be until at least later tomorrow morning.
Guidance available so far today suggests highs will range anywhere
from 85 to 93 F at KDCA, with a best-guess consensus of 89 F. But if
it does hit 90 F tomorrow, it`s seeming increasingly likely it will
continue to hit 90 F for at least several more days after that as
the warming trend continues.