Posts
A
few days old, but never mind.
Hottest
day: Humidity reaches obscene range, heat breaks records
19
July, 2013
The
summer’s longest, most extreme heat wave extends to its fifth day,
cooking the Great Lakes, Mid-Atlantic and Northeast. For Washington,
D.C., under an excessive heat warning, today may well be the most
brutal, with heat indices approaching 110 degrees.
141
million people under heat advisories and warnings
USA
Today reports 23 states and 141 million people are under advisories
and warnings. The most intense heat and humidity coincides with the
I-95 corridor from Richmond to Boston.
Heat
advisories shaded in orange, excessive heat warnings in magenta.
(National Weather Service)
From
Richmond to New York City, the humidity levels are particularly
unbearable, with dew points in the upper 70s to around 80 degrees
Dew
points at 11 a.m. (National Weather Service)
While
the core of the heat has focused in the eastern U.S., the entire
nation has been steamy. On Thursday, at least one location in each of
the Lower 48 states hit 90 or higher.
New
York City’s JFK Airport soared to 100 degrees, setting a record
for the date.
In
Washington, D.C., in the wake of a thunderstorm that dumped over an
inch of rain between 5 and 6 p.m., humidity levels spiked with the
dew point hitting 80 degrees at 7 p.m., just two degrees below its
highest known dew point ever recorded.
Why
is it so humid?
We’ve
explained why it’s so hot: an usually strong area of high pressure
a few miles up (although it has weakened a little) has sat over the
eastern third of the U.S. much of this week, causing the air to sink
and heat up.
The
source of humidity – which became particularly unbearable Thursday
– is an area of high pressure off the southeast U.S. coast, pumping
in extremely moist air from the south. Whereas the prevailing wind
direction was from the north earlier in the week, since Thursday the
air flow has come from the tropics.
Left:
Pressure difference from normal at high altitudes (deep reds indicate
big positive difference). Right: pressure difference from normal at
the surface (yellows and oranges indicate positive differences).
(WeatherBell.com)
When
you combine hot high pressure and moist tropical flow, it’s a
dangerously hot and humid situation.
More
record high minimum temperatures set in D.C.
While
day time high temperatures have held in the mid-to-upper 90s in the
D.C. area – several degrees below daily records, we’ve set or
tied record high minimum temperatures the last three mornings.
This
morning the low at Reagan National Airport was just 81, breaking the
record high minimum temperature of 79 last established in 2011
(assuming it does not drop to 79 or lower before midnight, which is
doubtful).
Low
temperatures the last four mornings? 80 (Tuesday), 80 (Wednesday), 80
(Thurday) and 81 today (Friday).
Through
11 a.m. today, Washington, D.C.’s temperature has not fallen below
80 for 99 straight hours. If D.C. fails to drop below 80 before 4
p.m. Saturday, we will tie our longest stretch on record above 80.
And, if we make it to 5 p.m. above 80, we will set a record of 129
consecutive hours above 80.
Showers
and storms associated with an incoming cold front – that will end
this heat wave – may not arrive until evening, so we have a
legitimate chance to challenge or break this record.
And weather for today - 22 July -
Low
to Ride Mangled Jet Stream From the Ohio River Valley to California
and Back
22
July, 2013
(Image
source: NOAA)
On
about July 11th, I began to report on a strange weather phenomena
involving an upper level low moving backwards against the Jet Stream.
At that time, there were a few reports among meteorologists that such
an event was possible and could affect the weather in the central and
western US, causing more rainfall and storms for a longer period of
time. There was some discussion that the low might end up in Mexico
before moving back east.
All
this prediction was strange enough. Retrograde systems do happen now
and then, but their motion is normally truncated, only progressing a
few hundred miles west at most then returning to the typical pattern
of west to east movement. But this retrograde acted differently,
traveling almost directly against the Jet Stream flow for hundreds
and then, ultimately, for about 3,000 miles. As of last week, the
motion was so abnormal that climate scientists began to comment on
its likelihood. In general, there’s about a 1 in 300 chance of such
an event happening in July, making this retrograde system a 1 in 300
year event.
So
the low did move backwards into Arizona, New Mexico and eventually
California where it dumped rain day after day, alleviating drought
conditions there while also setting off flash floods. Today, flooding
in Phoenix was so intense that numerous motorists had to be rescued
as their vehicles were inundated under flash floods no current
infrastructure could handle. Up to two inches of rain fell within
about a half an hour in numerous locations. Large storms also doused
fires even as they set off localized flooding events in California.
Some areas of New Mexico experienced their first rainfall in over 100
days.
The
flood event in Phoenix today is described in the following video:
Over
the next few days, the low is expected to slowly move northward,
drifting into northern California then Oregon before being picked up
by the Jet and lazily pushed eastward by its feeble flow. Over the
next 7-10 days it will transition once more to the place it started —
the Ohio river valley. Models are too uncertain, at this time, to
determine if the low will again get sucked into the eddy that drew it
all the way to the west coast. But with the Jet Stream so weak odd
weather events such as this one are now becoming the norm.
Links:
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