Major lightning outbreak/fire weather event likely in California, part of very active summer monsoon in 2014

Filed in Uncategorized by on August 10, 2014 760 Comments

Short-term outlook: NorCal lightning outbreak likely

California’s very active summer pattern (see below) is set to continue over the next few days. As of Sunday afternoon, a well-defined cutoff low had set up shop about 150 miles northwest of San Francisco.

A well-defined cut-off low was centered ~150 miles northwest of San Francisco. (NOAA)

A well-defined cut-off low was centered ~150 miles northwest of San Francisco. (NOAA)

This low–which is rather deep for the late-summer months–has actually begun to retrograde (move back westward) while sending some spokes of energy northward around the east side of its circulation center. This low is quite moisture-starved, with convective activity thus far mostly confined to the west slopes of the Sierra Nevada and near Mt. Shasta. However, a few lightning strikes have recently been detected near the low center ~100 miles NW of San Francisco over the open ocean, which suggests that significant elevated instability is starting to overcome the very dry environment near the low center.

In many ways, the current setup closely matches the classic scenario for widespread late summer/early fall lightning events across Northern California. The deep, slow-moving offshore low is positioned favorably to maximize both instability and upper-level divergence, though (as mentioned previously) moisture appears to be severely lacking. The atmospheric forecast models have a relatively poor historical track record with these sorts of events, and this pattern is often recognized by California meteorologists one that has the potential to produce fairly widespread convection even if model forecasts do not explicitly project such activity. Thus, it’s tricky to say exactly where lightning will occur over the next 48-72 hours, but there’s an excellent chance that it will be fairly widespread and be associated with relatively little precipitation.

The NAM depicts a well-defined low pressure center and associated vorticity maximum just west of San Francisco. (NCEP via Levi Cowan)

The NAM depicts a well-defined low pressure center and associated vorticity maximum just west of San Francisco. (NCEP via Levi Cowan)

All of Northern California appears to be at risk of dry or marginally wet thunderstorms over the next 72 hours, and it now also appears that Southern California will be at risk of (mostly wet) storms on Monday and Tuesday (possibly including coastal and valley locations) as a subtropical moisture starts to become entrained into the far eastern periphery of the circulation. The San Francisco Bay Area may actually be sandwiched in between these two more favorable regions for convective activity and thus actually stands a lower chance of lightning over the next 72 hours, but I still think isolated storms will be possible given the proximity of the low and the tendency for the models to underestimate spatial coverage of convective activity in situations like this.


High risk of major wildfire outbreak next 72+ hours

Energy release components in region of maximum dry lightning threat are at record-highs. (GACC/INFC)

Energy release components in region of maximum dry lightning threat are at record-highs. (GACC/INFC)

Wildfire activity in California has increased pretty dramatically over the past several weeks, especially in the far northern portion of the state. Most of these starts have been caused by lightning, and I expect to see many new ones spring up during and after the impending lightning event. Since vegetation in NorCal is near or exceeding record-dry levels and wildfire risk is exceptionally high for this time of year due to California’s extreme drought, this event could easily lead to a very significant and widespread wildfire outbreak in Northern California and also over much of Oregon. Wildland firefighters in the American West often speak of “lightning sieges” that occurred in various years, and it’s starting to look like August 2014 is headed in that direction for California and Oregon.


The North American Monsoon in 2014: unusually active so far

Recent weeks have featured many consecutive days of enhanced thunderstorm activity over mountain and desert regions in California (and occasionally elsewhere), bringing locally very intense rainfall and in at least once instance a remarkable and deadly flash flood event. A few localized regions in the Sierra Nevada and in the mountains of Southern California have picked up very substantial precipitation over the past two months–enough to locally and temporarily reduce wildfire risk in these regions. It should be noted that this recent precipitation, while locally impressive, has done very little  to ease long-term drought conditions in California (and none of California’s major reservoirs have responded at all to these localized thunderstorm downpours).

Observed precipitation so far during monsoon season 2014 (relative to normal). Some mountain/desert regions have received >300-400% of normal, while other regions have received almost no precip. (NOAA/NWS)

Observed precipitation so far during monsoon season 2014 (relative to normal). Some mountain/desert regions have received >300-400% of normal, while other regions have received almost no precip. (NOAA/NWS)


Persistent ridging returns: a West Pacific legacy?

Nonetheless, this very active monsoonal pattern in California has been fairly remarkable for several reasons. First, it appears that a persistent middle-atmospheric ridge is once again favoring the West Coast of North America as it has for the better part of 18 months now. While the ridge amplitude has not recently been as high as during the heyday of the Ridiculously Resilient Ridge, the persistence of the present ridging over weeks (and even months) is once again eyebrow-raising. How has this RRR-like feature managed to stick around into the summer months this year, unlike last summer when it temporarily dissipated during the warm season?

Persistent ridging has developed--yet again--along the West Coast. (NCEP via ESRL)

Persistent ridging has developed–yet again–along the West Coast. (NCEP via ESRL)

Easterly wind anomalies have occurred--on average--for the past 5+ weeks over California. (NCEP via ESRL)

Easterly wind anomalies have occurred–on average–for the past 5+ weeks over California. (NCEP via ESRL)

Atmospheric water vapor has been much more prevalent than usual this summer. (NCEP via ESRL)

Atmospheric water vapor has been much more prevalent than usual this summer. (NCEP via ESRL)

Anomalous upward motion--favorable for precipitation--has been persistent thus far this summer over California. (NCEP via ESRL)

Anomalous upward motion–favorable for precipitation–has been persistent thus far this summer over California. (NCEP via ESRL)

The answer may lie in the far western tropical Pacific, where a remarkable series of powerful typhoons have managed to develop and recurve into the extratropical mean westerly flow over the past couple of months. The injection of latent energy into upper levels of the atmosphere by West Pacific tropical cyclones is a fairly well-known mechanism for exciting an amplified atmospheric Rossby wave train over the downstream North Pacific Ocean, and this appears to be the reason why we’ve been so consistently locked into the now-infamous “North American Dipole” (Warm West/Cold East) pattern since late June.

Interestingly, the actual effects of persistent large-scale ridging in summer is quite different than in winter over California and across much of the Desert Southwest. Observations since July 1st suggest the presence of a very coherent pattern of increased monsoonal activity in these regions, including strong east-to-west wind anomalies, large positive moisture anomalies, and a remarkably large region of anomalous upward vertical motion. While the easterly/anticyclonic anomalies are similar to those observed in winter, the positive moisture and upward motion anomalies are opposite in sign, which is exactly what one might expected given the seasonal reversal in wind patterns that defines the North American Monsoon.

New typhoons are continuing to develop and recurve in the West Pacific, which suggests that a continuation of the North American Dipole pattern is likely for at least the next few weeks. Thus, I would expect there to be a continued chance for more a more active monsoonal pattern for the rest of summer (especially when combined with the fact that northeastern Pacific sea surface temperatures are far above normal).

Keep an eye (or two) to the sky!


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  • I’ll have a new post tonight or tomorrow, despite the relative lack of new things to talk about (we’re rapidly closing in on 1000 comments in this thread!). Our weather remains–for the most part–remarkably unremarkable, and there are no immediate indications that this will change over the next 2 weeks. I’ll have a look ahead to the coming fall, though don’t expect an explicit forecast of when the rains will return (spoiler alert: no one really knows!).

    • Ian Alan

      Accuweather 30 day forecast knows!

      (Sarcasm alert LOL)

      • Blaze202

        Surprised you didn’t mention Old Farmers Almanac Forcast. They know more lol.

        • Dan the Weatherman

          I never even bother to look at the AccuWeather 30 day forecasts.

          • Kamau40

            Exactly Dan-
            Never pay attention to accuweather’s 45 day forecast, they are certainly not reliable and accurate when it comes to forecasting weather.

          • inclinejj

            The only reason why I look at AccuWeather is for the records they keep.

      • Utrex

        AccuWeather has what I like to call “outdated” forecasts since they hardly update their forecasts out.

        Haha… I remember one day we had a decent thunderstorm outbreak here, but nothing severe. We had some rounds of hail and 50 mph winds, loud thunder and erratic lightning. It was night…

        AccuWeather had cloudy with a chance of sprinkles. They should have said “Cloudy with a chance of sprinkles on steroids!”

        It’s basically ghetto.

    • Dan the Weatherman

      I am really surprised how boring the weather pattern is right now at the end of August, when there is usually more monsoonal or tropical remnant-related activity going on at this time.
      I wish it would either become more active or that it would cool off for awhile! I am really getting tired of the heat this summer because it was so hot in May and so much warmer than normal during a good part of the winter. We haven’t had the highest summer temperatures that I have ever seen for this area, but rather the heat has been very persistent in the upper 80’s to near 90 degree level for almost two months without much break (it was even hotter last week, well into the mid-upper 90’s). We need a good trough from the NW to increase the onshore flow for a little while, and hopefully that happens next week as is forecast.

      • craig matthews

        Yup. Wake me up when it gets interesting again. I thought we’d be seeing a lot more x tropical activity up here with the way things looked back in July.

      • Kamau40

        I’m really surprised too of how stable the weather pattern has been lately. We are now though getting into the REAL teeth of the dry season. I thought we would see more monsoonal or E. Pac. tropical activity this month in terms of moisture, but as far out as I can see the same ole dry boring weather is likely to continue for quite sometime. Climatologically speaking, usually the first shot of rain comes around the Fall Equinox. September is the month when our chances of us seeing precipitation gradually start to slightly increase. We shall see.

        • Mike Stephenson

          I was hoping for more monsoon activity this month….. Usually there is activity the first 2 weeks at least of september. I guess the monsoon was above average but i wasn’t too impressed for Socal. That last event was cool with the offshore low but other than that there was just a couple mountain t-storms all summer. There were just some nice tropical cloudy days but not many easterly waves….. I guess the next thing to look foreword to would be a nice convective cutoff low with widespread t-storms- lots of cool air with a nice sun angle still for some daytime heating. I don’t think we have had a good fall set up like that in a while

          • Kamau40

            My point is don’t put all of your hopes that Ca. will have a wet season this year, you and many others could very well be disappointed!

  • Cachagua1

    Starting to get that fall-like spread in the temperature now. Low last night of 46, high today of 98. Love this time of year. Looking forward to the rains hopefully coming sooner then later. Been reading all the posts on this blog, and I am sure learning a lot from all of you and Weather West. Looking forward to another insightful blog post.

  • RSpringbok

    SST anomaly now vs. one year ago. It’s an animated gif that flips between then and now. Not sure it will upload and display properly; apologies if it doesn’t work…

  • inclinejj

    Wow! Pay attention to the top and the bottom video’s. Notice the lights. The blue light looks like a PGE pole or transformer exploding but the others are weird.

  • lightning10

    Anywhere els but So Cal summer is about over. Sadly we have another 8 more weeks left as we get the warmest weather late summer/early fall.

    • BlackRoseML

      But you at least have cooler nights and shorter days. At least there is no more monsoon flow increasing humidity.

      • xeren

        yeah, this heat wave has been a lot more tolerable- cooler for longer in the morning and far less humid. nothing worse than not being able to sleep because it’s so hot at night

    • Ian Alan

      Not me in the mountains 😛 yesterday was the warmest day in a month at 76F – it always amazes me in the late summer into fall when Santa Ana’s pick up and coastal valleys can hit 105+ (downtown LA 113 a few novembers ago) and it struggles to hit 75 up here if that.

      Going down the hill to San Bernardino / Redlands reminds though summer still has it’s grip on the area – 90’s feels so stifeling!!

      • BlackRoseML

        Saint Louis is still hot and humid, but it will rapidly drop off as September progresses. (I don’t live there.)