Update on the North American Monsoon and active California summer weather

Filed in Uncategorized by on July 19, 2014 43 Comments

Quick summary of recent conditions

An active monsoonal pattern over the past 1-2 weeks has brought some active summer weather to California’s mountain and desert regions. Strong (to even severe)

GOES satellite water vapor imagery depicting monsoonal surge and offshore low. (NOAA/NWS)

GOES satellite water vapor imagery depicting monsoonal surge and offshore low. (NOAA/NWS)

thunderstorms have occurred nearly daily over parts of the California interior, bringing locally intense downpours and resulting in some minor flash flooding in a few highly localized areas. Thousands of lightning strikes have occurred over California’s tinder dry forested regions over the past week, though most of these strikes have been accompanied by considerable rainfall. This is in stark contrast to the current situation in Oregon and Washington, where dry lightning strikes have led to massive and destructive wildfires over the past week.

 

Why are there thunderstorms in California in July?

The primary reason for all of this active weather throughout the West in recent days is the seasonal reversal of large-scale wind patterns associated with the North American Monsoon. This reversal occurs every summer, and is typically associated with occasional convective activity (often in the form of thunderstorms) over mountain and interior regions of the Southwest. Occasionally, this pattern can expand northward and westward to include California in the region of deep southeasterly flow around the characteristic mid-atmospheric high pressure area centered near the Four Corners region. Due to the stabilizing influence of the cool Pacific Ocean near the coast of California, convective activity associated with the monsoon is typically confined to mountain and desert regions, and thus tends to stay away from California’s major population centers. This gives rise to the perception that it never rains in California during the summer months (which is, indeed, a rather accurate characterization if one lives in the coastal metro regions or in the Central Valley).

Recent 500mb geopotential height anomalies depicting persistent ridging along the West Coast and a dipole pattern over North America. (NCEP/ERSL)

Recent 500mb geopotential height anomalies depicting persistent ridging along the West Coast and a dipole pattern over North America. (NCEP/ERSL)

The current large-scale pattern is a bit different than usual, though. A region of anomalously strong and persistent high pressure in the middle atmosphere–which in recent weeks has once again become increasingly reminiscent of the much-maligned Ridiculously Resilient Ridge–has become prominent near the West Coast of North America. This ridge is part of an even broader region of highly-amplified atmospheric flow on the continental scale–a pattern that has led to some remarkable weather conditions across much of the United States. In recent days, record heat, thunderstorms, and extreme fire weather conditions have occurred in the Pacific Northwest while drastically cooler than normal temperatures have occurred throughout the eastern half of the United States. Part of the reason for this recent amplification may be an injection of energy into the Westerlies by a series of powerful “super typhoons” over the West Pacific Ocean.

This persistent ridge near the West Coast has allowed southeasterly flow to remain in place for an extended period of time, with a quasi-stationary cutoff low southwest of California remaining in place for similar reasons. The net effect of these persistent flow patterns has been to allow for the continuous advection of moist air into California over the past two weeks, which has led to the recurring mountain thunderstorm activity and rather humid/muggy conditions that have been experienced elsewhere.

 

Widespread high-based convective activity possible this weekend

After several bursts of very weak high-based convection over valley and coastal regions in recent days (most of which did not result in any lightning), there finally appears to be a substantial risk of high-based thunderstorms tonight into Sunday across much of California.

The GFS shows large precipitable water anomalies over California this weekend. (NCEP via Levi Cowan)

The GFS shows large precipitable water anomalies over California this weekend. (NCEP via Levi Cowan)

A weak disturbance will rotate around the east side of the offshore cutoff low this evening, which will likely be sufficient to trigger a band of elevated showers and possible thunderstorms. Recent high-resolution model forecast show that this band will lift northwestward across most of Northern California in the overnight hours and during the first half of Sunday, bringing scattered showers and very possibly scattered thunderstorms to a broad swath of California (likely including the Bay Area and Sacramento region). While the atmospheric column is actually quite moist and mountain thunderstorms could bring torrential downpours, storms elsewhere are still likely to be high-based and so any lightning will have the potential to start wildfires despite probable co-occurrence of some light showers. As of this writing, a few high-based cells with cloud-to-ground strikes had already developed in the San Joaquin Valley. These sorts of nocturnal warm-season convective events in California are notoriously difficult to forecast, and so uncertainty remains high even at this late hour. It’s entirely possible that lightning will not occur in coastal/valley locations tonight, but given the extreme wildfire risk in place across all of California due to the drought, it’s prudent to keep a close eye to the sky this weekend.

High-resolution NAM depicts an organized band of convection sweeping northwestward late Saturday into Sunday. (NCEP/NOAA)

High-resolution NAM depicts an organized band of convection sweeping northwestward late Saturday into Sunday. (NCEP/NOAA)

 
Medium-range outlook

The offshore cut-off low will finally eject to the northeast by Monday as a more zonal pattern develops along the West Coast. This will cut off the supply of monsoonal moisture to California, bringing about drier and cooler conditions for much of next week. However, long range model predictions are strongly suggesting that the Sonoran high will strongly rebuild by next weekend, bringing hot conditions and probably a return of monsoonal moisture, at least over the mountain and desert regions.

Special note: I’ll have an additional Weather West post late tomorrow on the ongoing and exceptional California drought and its consequences. Tune back in tomorrow for this extensive update!

© 2014 WEATHER WEST

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  • TheNothing

    Time lapse of cumulonimbus cloud formation over the Sierra Nevada mountains near Truckee, click on link to play video.

    https://www.flickr.com/photos/100185958@N02/14715121493/

    • Xerophobe

      Love watching clouds formulate. At about 30 sec (25 sec in) remaining the left formation looked like a puppy. Thanks!

      • Bandini

        Great display! I was in there somewhere yesterday. Did a mountain bike ride on the Emigrant trail out of the Prosser area and got hammered by hail with thunder n lightning, actually ended the ride short it was too wet. Looks like a nice line of storms firing up south of the lake right now.

        • john patrick

          Seriously, what fascinates me most is we now have something to “talk, talk, talk” about. Media attention is on the rise, news papers selling like hot cakes and you can bet that pretty soon there will be mass evacuations occurring out of the entire Los Angeles basin do to swimming pools that are emptied by mandatory declorination plants installed in wealthy backyards.

          No doubt thousands lining up to swan dive off the Golden Gate Bridge as mass hysteria blankets the SF Bay Area.

  • JibJab2

    Thanks for the update!

  • redlands

    No measureable rain for 74 days in a row in Redlands, Ca — No monsoon surges here where I live –

    • Dan the Weatherman

      This has been one of the most boring years weatherwise that I have ever seen living here in Orange County. The weather during the last few weeks has been about normal as it is usually dry this time of year west of the mountains with occasional thunderstorm activity in the mountains / deserts, but I am referring to the entire year starting with winter.

      • redlands

        Dan — I would have to 100% agree with u — just plain boring — even when its active its not exciting —- I don’t understand a lot of this recent El Nino talk by everyone on this board – but hopefully it will be a whole lot rainier – wet

        • Dan the Weatherman

          We had better have a wetter winter coming up or CA is going to be in huge trouble if we suffer another bone-dry season. I am absolutely rooting for El Nino to strengthen so we don’t have to endure yet another ENSO neutral or weak ENSO year.

  • redlands

    Last July 2013 Redlands, Ca got 0.26 — nothing for the July 2014 season

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  • snow755

    This was a bust event so far in fact this was a bust ever where in Northern California what happened?

    • Kamau40

      I agree, this indeed was a bust for Nor. Cal. Nothing happened other than just a lot of clouds. As Dan rightly posted though these types of events can be trickey.

    • http://www.weatherwest.com/ Weather West

      It’s not over yet! The main convective potential may end up being this afternoon when the vorticity maximum swings through. Still no guarantees, but today still has potential.

  • Zepp

    I’m just hoping for the promised break in the heat beginning today. For the past three weeks, temperatures have run about 8-10 degrees above normal, setting various daily records, with highs routinely above 95 in a region where hardly anybody has home A/C because it’s not normally needed.

    • Stereolab

      Well here near San Jose it’s been > 80 degrees daily for basically the past two months nonstop. And starting Thursday it will apparently jump to > 90 for an indefinite period of time. What the hell is going on? If I wanted to live in Texas I’d have moved there.

      • Dan the Weatherman

        I’ll take the climate of the coastal counties of CA anytime over the climate in TX. It gets very hot and humid in most of TX and stays that way for much of the summer (and remains hot throughout the night requiring the use of air conditioning night and day) and the Gulf of Mexico doesn’t provide the cool sea breeze like the Pacific because its waters are so much warmer. Here it cools down enough in the evening during most of the summer to open the doors and windows to let in fresh air from outside at night.

  • Xerophobe

    Thanks for the thorough commenting on the whys and wherefores, Daniel.

  • Lycanthus

    On and off sprinkling in northern Marin for the past couple of hours! Very unusual for summer here, but I’ll take what we can get! Does anyone know if this has happened before in the Bay Area, and if so, with what frequency?

    First post here – been following for several months and learning a lot about weather and climate – thanks to everyone for the great discussion!

  • TheNothing

    Clouds are really starting to thicken down here in the Rocklin area, looks promising.

  • Dan the Weatherman

    My Internet connection is extremely slow right now and took me forever to get logged in on this site. Anyone else experiencing the same problem or is it just me?

  • Utrex

    Wow, clouds have really thickened up this afternoon. Citrus Heights has also been hit by a heavy shower, and it looks like more of this is coming through the valley.

  • 60blue

    Just beginning to rain in Foresthill. According to the radar there is more coming!

  • http://www.weatherwest.com/ Weather West

    Intense cell developed in San Benito County earlier. A new cell is developing in far eastern Santa Clara County. Given the instability in place and the incoming vort max, would not be entirely surprised to see some lightning in/near the Bay Area over the next 2-5 hours.

  • Kamau40

    There have been some rain drops here in the North Bay. It was nice to see some drops from the sky. There are still lots of clouds in place.

  • Bandini

    Chased from Kings Beach to Incline Village, pretty epic storm cells going on. I thought this thing was going to form a water spout. Very intense rain in Incline. Working on a video.

    • TheNothing

      Excellent photo Bandini, this whole day has left a smile on my face.

      • Bandini

        Thanks. Yeah it has been an exciting day, in fact the past 8 days seem to have been more exciting than all of last winter.

    • http://www.weatherwest.com/ Weather West

      That’s a fantastic photograph!

    • craig matthews

      Absolutely blown away by this one. Sure does look like some type of rotation on that lower cloud shield. Beautiful!

    • Kamau40

      Wow, that is a fantastic shot. Looks like something we would see in Florida yet it is here in Ca. Thanks for the photo.

    • so.cal.storm.lover

      Wow looks like something you’d see in the Midwest!!

      • Dan the Weatherman

        It is certainly nice to see some heavy precip in the Sierras for a change!

  • craig matthews

    No activity in Big Sur, but is sure feels humid. Sea surface temps approaching 67 degrees at buoy 42, Monterey bay. That’s getting pretty warm for this area of the central coast.

    • Kamau40

      That is very warm for this time of year. The seas along the Ca. coast is definitely warming up!

      • craig matthews

        Surface flow has been almost consistently southerly up the coast for over a week now. This has shut off the upwelling that normally occurs periodically along the immediate northern and central California coast. Combine this lack of upwelling and a large warm pool in the north pacific, and we are getting some unusually warm ssta’s now, which are running around 10 degrees above average for July between Monterey bay and Santa Maria. And now computers are keeping a weak flow along the coast for the next week. We could be looking at more humid days on the coast this week.

        • Kamau40

          The weather pattern I have observed so far this Summer in Ca. are as follows:
          1. Warmer than normal sea surface temperatures(as you have pointed out).
          2. More humid than normal.
          3. More moisture surges west of the mountains, which also started earlier than usual.
          4. Lots of breezes.
          5. Hot temperatures.
          6. More marine layer than I have seen compared to the previous Summers.

    • Kamau40

      We got pretty much similar cloud formation up here in Nor. Cal. It really is great to see this in July. I just posted that the models are showing yet another moisture surge for Ca. about a week from today. The models are also showing that the 4 corners high which the models showed about a week ago will be sticking around for quite sometime. I think if we get some tropical moisture from a decaying E.Pac system along with a cut-off low we will have a much better chance for dynamics and lift to trigger stronger convective activity. Now that we are approaching Aug that is the start of the prime time season for seeing such events. What are your thoughts?

  • craig matthews

    Nothing compared to the sierra photos(Bandini), but these clouds were interesting yesterday when I was traveling between Colinga and King City in the Diablo Range. No lightning though, but some sprinkles.

  • TheNothing
    • Bandini

      Blazing monsoonal sunsets, classic.

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  • redlands

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