California’s searing summer to continue; active East Pacific could bring tropical “slingshot”

Filed in Uncategorized by on July 23, 2017 4,011 Comments

An exceptionally hot summer so far

Daily maximum (and average) temperatures have been exceptionally warm across nearly all of California so far this summer. (Via WRCC. Please note that proposed funding cuts will eliminate all WRCC data access in 2018.)

Despite a mild spring across most of California, summer 2017 has been truly searing across most of California away from the immediate coastline. Numerous, prolonged heatwaves have brought an extended period of well above-average temperatures at the height of summer, with temperature falling to around average for only brief periods. There has been an exception to the unrelenting heat in California’s interior: regions within 5-10 miles of the Pacific ocean have experienced greatly muted effects from these heatwaves, as they (so far) have not coincided with periods of strong offshore flow. Still, ocean surface temperatures have started to rise in response to persistent heat and weak coastal upwelling–and even the beaches have started to experience anomalously warm temperatures in recent days. This is especially true in Southern California, where SSTs into the 70s in some spots have greatly curtailed the typical seabreeze circulation, raised surface humidity into uncomfortable territory, and have prevented overnight temperatures from falling much below 70 degrees.

A persistent ridge has brought relentless heat to much of the American West since the beginning of June. (NCEP via ESRL Plotter)

What’s causing this extreme inland heat and unusual/uncomfortable SoCal humidity? An unusually persistent ridge has developed somewhat to the west of its typical summertime position over the Desert Southwest, which has favored the occurrence of numerous heatwaves across the CA interior. It has been an exceptionally hot summer so far across the entire American West–not just California–though in recent days a robust monsoonal moisture surge has moderated temperatures across Arizona and Nevada. This ridge is not located quite far enough west to bring very hot coastal temperatures, although it has acted to inhibit the northwesterly winds that normally induce cold water upwelling along the coast and has thereby caused coastal SSTs to rise and overnight coastal temperatures to creep upward.


Increasing monsoonal influence; rising heat

An upper low will be in a favorable position for higher elevation thunderstorms across NorCal today and Monday. (NCEP via

Unfortunately, it appears that this pattern over the next 10+ days will bring more of the same–more heat, and rising humidity–though there will be the potential for some interesting weather at times. As of this writing, an upper-level low pressure center was located off the coast of Northern California, and is expected to move slowly inland through Monday. As it does so, sufficient moisture and elevated instability exists over much of the NorCal interior for potentially widespread thunderstorms later Sunday and Monday across the higher terrain. While much less likely, some isolated storms could occur over the Central Valley or North Coast region given the presence of a somewhat moist airmass. The potential for widespread lightning–even if much of it occurs with wetting rains–is a significant fire weather concern, given the existence of numerous large wildfires already throughout California and the lack of firefighting resources available to address new fires.

Yet more strong, California-focused anomalous ridging is in the forecast. NCEP via

Early this week, a stronger push of monsoonal moisture will also move into Southern California. Mountain and desert thunderstorms are a good bet, and there is also a slight chance of storms all the way to the coast. There is still pretty dramatic inter-model disagreement regarding the westward extent of this moisture in SoCal on Monday and Tuesday, which could mean the difference between unremarkable conditions and a pretty active weather period. At this point, it’s not clear which scenario will win–though it’s worth noting that the global models have done an especially poor job simulating westward monsoonal moisture incursions so far this summer. While such events are always difficult to simulate given California’s position at the far western margin of the monsoon region and the highly stabilizing effect of a relatively cold nearby ocean, it seems that the extreme persistence of anomalous West Coast ridging mentioned earlier has inhibited what otherwise might have been more impressive events so far this year.

Very hot weather likely to return across all of the West in coming days. (NCEP via

Regardless of the degree of monsoonal moisture that overspreads California, however, there seems to be agreement that the ridge will re-strengthen over California–and that well above-average temperatures will return later this week after a brief lull.





Eastern Pacific extremely active; a tropical “slingshot” possible
Later this week and into next weekend, all eyes turn toward the eastern tropical Pacific Ocean. This region has recently been producing an exceptional number of tropical cyclones (i.e. tropical storms and hurricanes), and activity is actually expected to further increase in the coming days. Unlike recent storms, which have headed almost due westward into the remote Pacific, additional tropical development over the next 7-10 days is expected to take a more northwesterly track. This will potentially put several decaying hurricanes/tropical storms in a position that has historically been favorable for the advection of moisture into California from the south/southwest.

“Fujiwhara” interaction between two East Pacific hurricanes could “slingshot” tropical moisture toward California. (NCEP via

Intriguingly, the GFS has been consistently suggesting the potential for a very unusual interaction to occur between two of these East Pacific tropical cyclones later this week. The animation below shows what is known as a “Fujiwhara interaction” between two such storms west of Baja California, in which two tropical cyclones move close enough to one another to influence each other’s circulation and begin to rotate around a common center. As this occurs, storm motion can become highly nonlinear and very hard to predict, and occasionally results in the smaller storm being “ejected” from the broader gyre in “slingshot” fashion. For what it’s worth, I can’t personally recall a model forecast calling for such an event to occur so close to California (it’s more common in the West Pacific, where typhoons are quite common).

What does this mean for California? At this point, it’s hard to say–tropical influences in California weather are always hard to project more than a few days in advance, and this is doubly true with the potential for such an unusual multi-storm interaction as discussed above. But it does appear there is a pretty good chance that tropical moisture (perhaps complimented by enhanced monsoonal moisture under stronger southeasterly flow) will eventually make it to California in the 5-10 day period. The most direct consequence of this will be a potentially dramatic increase in humidity–which may make the upcoming heat even more miserable. This moisture, however, will also make mountain/desert thunderstorms likely, and may bring a risk of showers/thunderstorms even to coastal areas. There are presently no ¬†indications that a tropical rainfall event as substantial as that associated with the remnants of Hurricane Dolores in 2015 is in store–but the situation certainly does bear watching. All in all, it appears that the next couple of weeks have the potential to become pretty interesting by California summer standards.

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  • Latest ECMWF is showing 5-day precip totals over 60 inches in the flatlands of coastal Texas. That is getting close to the theoretical upper limit of what can fall from the sky in the absence of elevated topography in that amount of time…

    • Jason Jackson Willamette

      5 FEET of rain! I’m really worried about all of the refineries in the vicinity. God knows if all of that oil and bi-products will get washed out to sea?

      • AllHailPresidentCheetoJesus

        It’s not a matter of if, it’s a matter of when…

      • I read the refineries were filling these to keep them in place. I hope that is Plan A and not just hoping for the best.

    • cthenn


    • AllHailPresidentCheetoJesus

      Wow indeed…

    • weathergeek100

      We often talk about high rainfall totals during flooding events back east and how they can reach our annual precip for cities here in CA. 10″ in 5 days? That’s as much as San Diego gets in a year! 20″? That’s as much as San Fran gets! In this scenario, you cannot compare w/ CA. You have to compare w/ Miami. Miami gets around 60″ of rain annually (I think?). This is a truly incredible event.

    • It’s going to depend if Harvey stalls or moves very slowly. 60″ or more is not out of reach by any means.
      Alvin Texas received more than 40″ in one day from TS Claudette in 1979

      • tomocean

        Check the latest guidance from the NHC as of 4pm CDT.

        In fact, there has been a somewhat notable change in the guidance, with very few of the models showing Harvey lifting out toward the northeast by the end of the 5-day forecast period. As a result, the NHC track forecast has been pulled back a bit and keeps Harvey near or just inland of the Texas coast through the middle of next week. This slow motion only exacerbates the heavy rainfall and flooding threat across southern and southeastern Texas.

    • honzik

      That’s our average winter in the Santa Cruz Mountains in 5 days. Wow.

  • 82/83 El Nino baby (San Jose)

    I wonder what is keeping Harvey from undergoing rapid intensification. It has extremely warm water, no shear. No dry air issues from what I can tell. I know wind is not going to be the story, but staying a low cat 3 might help keep the surge lower., which would be some good news.

    Also, its odd that it has the same pressure as Sandy, but much higher winds.

    • molbiol

      Um, Harvey has undergone rapid intensification. Just 36 hours ago, it was a depression. Also, Sandy made landfall as a large diameter extratropical cyclone. The larger diameter means weaker winds BUT over a larger area

      • 82/83 El Nino baby (San Jose)

        True. Not saying its not impressive already. Just seems like it has a better environment until landfall of any hurricane I can recall. Katrina and Rita interacted with cooler waters and higher shear before landfall I believe.

        Maybe the storm is enlarging and strengthening more gradually vs wrapping up tightly and strengthening fast (like Charley in Florida).

        • molbiol

          Keep in mind that eyewall replacement cycles can modulate the rate at which the storm intensifies. Also, both Rita and Katrina were moving westward when they intensified. As they turned more toward the North and approached landfall, they actually weakened somewhat believe it or not

    • SoCalWXwatcher

      It’s now officially a Cat 3. Max sustained winds @ 120mph. Still appears to be strengthening as the core sits over some very warm waters.

  • Thunderstorm

    One thing that people forget is that all living things will look for safe landings. The fire ants will form huge rafts in the rivers and the snakes will find higher ground and who knows what else, everything grows big in Texas!

    • ThomTissy

      For every poisonous critter we have in California, they have 8.

  • Thunderstorm

    Just read a newspaper article from Houston. Headline – Harvey may hit Texas twice. Moves back to the gulf then reloads and then a direct hit on Houston next Wednesday. The amount of silt that will be deposited will make some areas uninhabitable.

  • tomocean

    It looks like the NHC says the models are changing their guidance and indicating that Harvey will not move off to the northeast and will simply sit and spin just inland of the Texas coast through the middle of next week.
    If that holds, I can’t imagine a worse scenario for that area. The flooding will be biblical.

  • DudeAbides

    I can’t wait for this thing to wreak havoc. “Biblical flooding” and an area turning “uninhabitable” sounds like must see TV. Everyone here knows they’re secretly waiting for the worse case scenario.

    • Fairweathercactus

      The media is dieing and they need events like this to keep ratings up when more and more people are turning off the TV. More events like this = more ad revenue.

  • Thunder98 (Santa Maria Valley)

    Hot weather ahead for Santa Maria! I could see my 1st 90F weather of the year! Meanwhile in Appleton, WI, Early Autumn weather is impacting the Midwest. Stay cool everybody and drink hydrated!

  • Fairweathercactus

    IBM and NBC has done some major damage to Wunderground. They keep making it worse and worse with each update.

    • Nathan

      I agree…just gets slower and more difficult to use.

    • Pfirman

      So they are the Harvey brothers?

  • 805 Weather (Camarillo)
  • cabeza tormenta

    On the wider view of an infrared antlantic loop, storm bands dropping down Florida and curving over Yucatan appear to be caught up in the same circulation.

  • cabeza tormenta
  • hermit crab

    I can hardly believe the forecast for Carpinteria. Due to my health problems I am supposed to avoid really high temps…the coast over 90 degrees? Seriously? If it happens I’m going to bring a cot to the grocery store’s frozen food aisle. But I’m really concerned about people inland.

    And Harvey…can’t think of what to say; it’s so horrible.

    Really fascinating weather going on. So appreciate the opportunity to learn about it. But I wish it were just a simulation.