Excessive Heat in the West Eases Off; Fire Danger Increases

Nick Wiltgen
Published: August 19,2015
Within the past week, excessive heat warnings were issued hottest region of the country, a sign that it's definitely hot. And when Phoenix ties its all-time August record high, you know it's really hot.
Last week and this weekend, the hottest weather targeted the Southwest. So far this week, it was the Northwest that felt the heat. While more seasonable conditions are forecast to settle in, the fire danger is expected to ramp up in the coming days.

Forecast Highs

Several daily record highs have either been tied or broken across Oregon and Washington this week. The heat reached a climax on Tuesday and continued to bake some areas into Wednesday.
Portland easily topped out in the 90s once again on Wednesday. This gave Portland's airport its record 26th day with highs in the 90s in a calendar year. This breaks the previous record of 24 days, set back in 2009 and records for the airport date back to 1940.
To the north, Seattle, Washington may have just missed reaching the 90-degree mark on Wednesday, but their high of 89 degrees was enough to break the daily record of 88 degrees, set back in 1967.
Cooler air settles in for Thursday, dropping temperatures closer to average.
Although the heat may be tapering off, the fire danger is expected to increase on Thursday. Much of the northern Great Basin, including a large portion of eastern Washington, is under a red flag warning for both Thursday and Friday.
(MORE: Latest on Western Wildfires)

Southwest: August Records Challenged

Actual high temperatures measured in selected locations Aug. 14-16, 2015, along with the all-time August record highs for each place.

Observed low temperatures for selected locations in the Southwest from Aug. 14-16, 2015. It was the first time Phoenix has ever recorded three consecutive days of lows at or above 93ºF (34ºC).

At least one major heat record has been established in this heat wave. On Friday, Phoenix Sky Harbor International Airport recorded a high of 117 degrees, tying the Arizona capital's all-time record high for the month of August previously set Aug. 26, 2011. Like that date, Friday's low was 93, so it also tied for Phoenix's warmest daily mean temperature on record for the month of August.
Phoenix topped out at 115 degrees on Saturday, which is a new daily record for Aug. 15 (previous record was 112 degrees in 1992).
(FORECAST: Phoenix | Scottsdale | Mesa)
The health dangers of heat waves are not just a function of high temperatures, though. Very warm overnight lows can cause problems for anyone living without access to air conditioning, or for those who cannot afford the electricity required to take full advantage of their AC.
In Phoenix, the lows Friday, Saturday and Sunday were a torrid 93 degrees. This is the first time Phoenix has ever recorded three consecutive calendar days without falling below 93. The mercury fell to 91 Monday morning and into the upper 80s on Tuesday morning. Phoenix's record for most consecutive lows in the 90s is seven straight days from Aug. 7 through Aug. 13, 2012.
In a way, to call this a "pressure cooker" could be considered more than just a metaphor. The high-pressure zone has literally exerted a downward influence on the air over the Southwest. Since clouds and rainfall require rising air motions (not to mention adequate moisture), large-scale sinking air tends to prevent or inhibit precipitation from developing.
That means more sunshine, which in turn, means more heat. In addition, the air mass itself is warmer in the first place, so the sun is heating up air that's already toasty to start with.
Relative to historical norms, the epicenter of the heat has been over central and southern Arizona and southeastern California.

Three-Day Forecast

Phoenix's 117-degree high Friday made it the hottest day of 2015, surpassing the 115 recorded June 18. Of the past 20 years (1995 through 2014), Phoenix's annual high temperature has only occurred in August three times: 2012, 2004 and 1999. In 2004 and 1999 August shared the crown with July.
Death Valley National Park, already the world record holder for highest official temperature, hit 122 degrees Friday and 123 Saturday, easily clinching the nation's hot spot both days. Saturday's reading was the highest August temperature in Death Valley since 2012. Sunday and Monday the high topped out at 122 degrees in Death Valley.

Current Temperatures

The lower elevations of the Las Vegas area and parts of the Colorado River Valley are under excessive heat warnings through Wednesday.  If you are in any of these areas, take precautions against the heat. Drink plenty of water, stay in the shade as much as possible, and take a break now and then in an air-conditioned building.
(FORECAST: Tucson | Las Vegas | San Diego)
And it's not just the Southwest that's been feeling the heat. Along the central coast of California, Santa Cruz topped out at 101 degrees Saturday, crushing its daily record high for Aug. 15 by 13 degrees. Weather records in Santa Cruz go back 120 years, which makes the margin by which this record was broken very impressive. It is also only the sixth time in those 120 years that Santa Cruz has recorded a triple-digit daily high temperature in the month of August.
A daily record high of 92 degrees was also set at San Francisco International Airport on Saturday. Sunday's high reached 90 at SFO, breaking another daily record (86 on Aug. 16, 1960). It's only the fourth time SFO has had back-to-back 90-degree days in August since records began there in 1945, and only the fifth time it's had two 90-degree days at all in August. San Francisco's airport has never had three 90-degree August days in the same year, whether consecutive or not.
(FORECAST: Downtown L.A. | LAX Airport | Encino)
Excessive heat warnings for inland areas near Los Angeles and San Diego have now expired, as a cooling trend began Monday.
For cooling over the weekend, Angelenos had to head to the beach. Highs Saturday soared to 107 in the Woodland Hills, Van Nuys and Chatsworth neighborhoods of the city, while Los Angeles International Airport only hit 81, and the famous pier in the neighboring city of Santa Monica only hit 73.
Highs in the inland valleys will cool into the 90s and eventually 80s this week. More importantly, overnight lows will recede back into the lower 60s, allowing for a break in the warmth at night.
MORE: Western Wildfires, July 31 - Aug. 11, 2015