Searing, prolonged heatwave developing across much of California & Desert Southwest

Filed in Uncategorized by on June 16, 2017 2,184 Comments

Prolonged, dangerous heatwave imminent for areas away from coast

Very warm temperatures will encompass all of California during this heat wave. (NCEP via tropicaltidbits.com)

After a relatively mild May and early June period across most of California, a rather remarkable heatwave is already underway and will likely persist for many days. Today will be the first of potentially 7-10 very hot days across inland portions of California and interior portions of the American Southwest, and numerous high temperature records are likely to fall in the coming days. The upcoming heat will be of a dangerous intensity and duration in a significant number of highly populated regions, including the Phoenix, Tuscon, Las Vegas, Bakersfield, Fresno, and Sacramento metropolitan areas. Cities closer to the Pacific coast will still see hot to very hot temperatures over the next week, but not to quite as exceptional a degree as these inland areas.

 

Dynamics of a heatwave with some unusual characteristics

The present heatwave strongly resembles a similar event which occurred just last year (in June 2016), which brought record temperatures to inland desert portions of the Southwest. The event over the next 7-10 days, however, has the potentially to slightly more intense and significantly more prolonged–meaning that overall impacts could be considerably greater. It’s interesting to note that both the GFS and ECMWF model ensembles did a good job capturing this event–with strong signals for anomalous warmth appearing 10-14 days in advance.

A very large and unusually strong mid-tropospheric ridge will be in place over the entire Western U.S. (NCEP via tropicaltidbits.com)

A strong and broad high pressure ridge is developing over the West Coast and interior Southwest, and will shift slowly westward over time. As it does so, the hottest temperatures will slowly shift from the southern Arizona deserts this weekend to the California interior early next week. Though it’s often California’s good fortune to experience “dry heat”–during which low surface dewpoints actually decrease the “apparent temperature” and take the edge off otherwise searing temperatures–that won’t necessarily be the case during the present heatwave. An unusually moist airmass is currently in place over much of California, which can be linked to the passage of an unusually strong atmospheric river over the Pacific Northwest yesterday (which brought heavy June rain to Washington but just clouds to Northern California). Linger moisture will become trapped within the stagnant airmass under the building ridge–which ultimately means that the extreme temperatures associated with the upcoming heatwave will coincide with a rather moist airmass and unusually high surface dewpoints.

This added moisture has several implications. First, it will make the intense heat feel even more oppressive (i.e. “muggy”), as apparent temperatures will not fall below the actual temperature as they often do during California heatwaves. Second, higher dewpoints will greatly inhibit overnight cooling across inland areas. In fact, portions of the Central Valley could see overnight lows staying as high as 70-80 degrees. Lack of overnight cooling is one of the strongest predictors of heat-related illness during prolonged heatwaves, so this presents significant concerns. Finally, the relatively “juicy” airmass by California standards will allow afternoon thunderstorms to develop over the Sierra Nevada on most afternoons during the heatwave. While not currently depicted in model forecasts, previous experience with events like these suggests that there will be at least a small chance of some convective build-ups and perhaps some isolated thunderstorms outside of the Sierra Nevada. These would be most likely over the San Joaquin Valley, where the confluence of extremely hot temperatures and outflow boundaries propagating away from Sierra Nevada storms could act as a localized trigger. The chance of this happening is low, but it’s worth mentioning as an indicator that this heatwave is a departure from the norm in California.

An unusually moisture airmass will be associated with the developing heatwave across most of California. (NCEP via tropicaltidbits.com)

 

Sweltering inland, but merely “hot” near the coast

Slightly cooler than average SSTs are currently in place off the CA coast. (NOAA Coral Reef Watch)

This heatwave will behave more typically in another respect, however: immediate coastal areas will not experience exceptionally hot temperatures during this event. This is a characteristic shared with nearly all June heatwaves in California. Why? The near-shore Pacific Ocean is still too cold enough in May and June to generate strong near-shore sea breezes when things heat up inland–acting as a natural air conditioner for places within about 10 miles of the coastline. Later in the summer (by August and September), sea surface temperatures are warmer, and this effect is less pronounced. This is the reason that San Francisco and the beaches of Southern California typically see their hottest temperatures of the year in late summer and autumn. On the other hand, the upcoming heatwave will likely be prolonged enough to curtail coastal upwelling–meaning that near-shore SSTs may begin to rise by next week and provide progressively less relief as the event drags on.

While the coastal plain of California experiences highs mostly below 90 degrees next week, most of the Central Valley will see temperatures approaching 110 degrees (and perhaps even getting close to 115 in some portions of the San Joaquin). California’s southeastern deserts will see temperatures of 115-120 degrees. Incredibly, the Phoenix, AZ region may see temperatures rising into 122-123 degree territory. Except along the immediate coast, these values will likely break daily temperature records in many places, and will approach monthly June records in some spots. In the hottest desert regions, all-time high temperature records (for any month) may be at risk.

 

Relief in sight? Maybe not.

The ECMWF suggest the searing heatwave may persist for 7-10+ days. (NCEP via tropicaltidbits.com)

Heatwaves in California rarely last longer than a few consecutive days, or a week at the most. Yet it’s not clear at this point that the developing heatwave will be quick to dissipate. In fact, the ECMWF ensemble now suggests that the ridge may re-strengthen considerably next weekend after weakening only slightly late next week–turning this already remarkable heatwave into a 10+ day event. The GFS is a bit more merciful in showing a partial breakdown of the ridge in 8/9 days, but at this point it’s hard to say which solution is more likely. Either way: this will likely be a high-impact and very uncomfortable event for those living away from the ocean for the next week at least. Stay tuned.

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  • Tyler Price (Monterey/Seaside)

    I’m super excited for the upcoming first monsoonal intrusion that is to come to SoCal soon!!! It’s looks to be quite an extended period of monsoonal Convection will occur across the southwest coming in with a bang for July! The potential exists for monsoonal moisture I even make it to the valley’s and possibly coasts at times with the influx of monsoonal moisture making quite a westward extension off the desert southwest and northern Baja/Mexico! Can’t wait to see the radar and the pics next week!

    • happ [Los Angeles]

      Entirely agree but am mindful of dew points; esp enjoying low humidity/ temps recently. Hope showers are widespread w/ lots of great cumulonimbus

  • 805 Weather (Camarillo)

    Special weather statement just hoisted up for parts the southland IRT heat potential for the latter half of this week & a small mention of chances of thunderstorms… Here is the latest AFD from NWS LA/Ox regarding monsoon potential this weekend as well.

    “Area forecast discussion National Weather Service Los Angeles/Oxnard California 346 PM PDT sun Jul 2 2017

    As the upper level flow turns more to the south and southeast, the NAM and GFS models showing some increase in mid level monsoonal moisture across our region… with most of this moisture at 600 mb or above. The 12z NAM model shows some elevated instability/cape so cannot rule out isolated light showers late Tuesday night into Wednesday. At this point, only carrying ghost pops (in the 5-10 percent range). On Wednesday, there is a 10 percent chance of a thunderstorm over the mountains and Antelope Valley. If any thunderstorms do develop on Sunday, there will be the potential for isolated dry lightning strikes and gusty downdraft winds due to a dry lower atmosphere.

    Long term… (thu-sun)

    The Four Corners high will remain entrenched through the extended period bringing increased monsoonal moisture across southeast California. At this point, it looks like the best threat of thunderstorms will remain to our south and east, but still carrying a slight chance of thunderstorms for the eastern san Gabriels and Antelope Valley on Friday. The slight chance of thunderstorms will then extend westward into the mountains of Santa Barbara and Ventura counties over the weekend, as the GFS cross section shows a better surge of moisture into our area.”
    https://uploads.disquscdn.com/images/99e88a123c976a4424c3b2f65acd1114d10bd3638df4ed160202fbef18863aca.png

    • Bring some to the Bay Area. I know you guys are tired of me complaining about this everyday. ?

      • 805 Weather (Camarillo)

        Maybe something will come across the SJV to say hello from the Sierra’s who knows lol.