Weather West: Strong Storm to Bring Widespread Flooding and Wind Impacts

The following is an excerpt with permission from the Weather West Blog. 

By Daniel Swain of Weather West

Wild times in the California weather world

Welp, here we go again. After a long period (a full season, really) of different kinds of exceptional weather conditions all around California, there’s yet one more big storm to come in the immediate future (discussed below). But what has transpired in the past, oh, 48 hours or so? Well:

  1. A “Pineapple Express”-type atmospheric river brought very heavy precipitation (heavy low-mid elevation rain, including atop an existing snowpack in many places, and extremely heavy high elevation snow) to central California. Widespread and locally serious flooding occurred in the Southern Sierra watersheds (including Kern and Tulare watersheds) as well as in the agricultural centers of Monterey/Santa Cruz Counties (especially the community of Pajaro, near Watsonville, that has been completely inundated by rising waters caused by a nearby levee break on the Pajaro River).
  2. Despite lower and middle elevation snowmelt, the addition of new high elevation snow and absorption of rainwater by the snowpack at middle elevations has resulted in what appears to be a new record regional snowpack (in terms of SWE) for the Southern Sierra, and possibly also for the Central Sierra. These snowpacks will likely grow yet more this coming week–exceeding all-time record levels by an even wider margin. Although this record SWE will most likely not contribute substantially to flood risk this week, I am becoming increasingly concerned what it may imply regarding flood risk from late March through April or May. (Stay tuned.)
  3. The “break” between storms in NorCal has been anything but in some localized spots. Yesterday, several areas of thunderstorms developed in the Central Valley, including isolated true “supercell” thunderstorms that ultimately produced damaging large hail, flash flooding, and at least one confirmed tornado in the Central Valley and lower Sierra foothills. Additional isolated severe storms are possible this afternoon, posing hazards in their own right but also keeping soils saturated ahead of the next big storm.

Another moderate to strong atmospheric river Mon-Tue may produce “outsized” flood and wind impacts due to extremely wet antecedent conditions

There is an unusual amount of uncertainty for such a short-range forecast regarding exactly how the Mon-Tue atmospheric river will look at the point of landfall, and this has some pretty consequential implications for flood and wind damage-related impacts. Here’s a quick overview:

A moderate to strong atmospheric river, once again with deep subtropical origins (i.e., the “Pineapple Express”) will make landfall in northern and/or central California–bringing a period of widespread moderate to heavy rainfall to a wider swath of the state than the Friday event (i.e., some moderate to locally heavy rainfall will extend into most of SoCal as well). Unlike the previous event, this AR will be associated with a rapidly deepening surface low rather close to the NorCal coast. This will raise the potential for widespread and possibly damaging winds well beyond what was observed in the previous storm. It will also potentially amplify the AR on final approach, depending on precisely how much the surface low deepens.

ECMWF depiction of strengthening surface low attached to strong atmospheric river making landfall in California late Mon into Tue.

The GFS and ECMWF ensembles do not currently agree regarding how strong the AR will be, and the difference stems from a disagreement over the strength and exact position of the above-mentioned surface low. The ECMWF is strong and closer to the NorCal coast with the surface low, and hence its depiction is of a much strong AR affecting primarily NorCal (and, to a lesser extent, Central CA). The GFS is weaker with the surface low and therefore more diffuse with the IVT plume–making for a broader but weaker AR and a precipitation bullseye along the Central Coast and into parts of SoCal. (Why are there such big difference between models? The CW3E had an interesting discussion on Twitter this morning.)

If the ECMWF is right, expect a more powerful AR overall with a more northward focus. If the GFS is right, expect a somewhat weaker AR overall but with a broader swath of heavy precipitation across already flood-affected areas of Central CA.

Either way, this storm appears to have better dynamics (thanks to overhead jet streak and proximity of deepening nearby surface low). While I do not expect precipitation totals from this storm to be exceptional, peak hourly rainfall *rates* may well be higher with this storm (including the possibility of embedded thunderstorms in a subtropical airmass). Additionally, soils are now supersaturated everywhere across the northern 2/3 of CA (at least) and rivers are running high/actively flooding. So the higher rain rates, plus exceptionally wet antecedent conditions, pose a thread of more widespread and serious urban, stream, and flash flooding on small rivers than the Friday storm in most places. Additionally, I expect a larger number of northern and central CA rivers to flood on Tue or Wed as a result of the heavy precipitation (along with a small amount of additional snowmelt) from this storm. I *still* don’t expect widespread major river flooding with this event, though we are getting to the point that there may be some isolated major river flooding along the Central Coast and in the San Joaquin Valley (especially places that are already having problems).

Snapshot of ECMWF-predicted position of strengthening surface low near NorCal coast early Tuesday morning that could portend storm “overperformance” potential in the northern half of the state.

I would also expect to see pretty widespread wind-related issues in NorCal with this event, especially if the ECMWF’s deeper surface low is correct. Pattern recognition suggests this kind of setup tends to “overperform” in the wind and rain rate department even if 24 hour totals are not that extreme, so this storm could well have notable impacts in some areas (the very wet soil damage also compounds potential tree damage/power outage potential when combined with strong winds at the end of a storm cycle).

The ECMWF ensemble suggests that the Mon-Tue atmospheric river will be a moderate to strong event, with impacts bolstered by very wet antecedent conditions and presence of a strong nearby surface low.

Hints of relief in the long range: still active, but less wet and stormy

I don’t have much time to discuss the long range except to say that it will probably still be active to some extent, but there are decent ensemble signals suggesting attenuation of the high amplitude flow pattern and a slowing/weakening of the California snow parade. Following the Mon-Tue storm, I don’t see any obvious candidates to produce major flood/wind impacts for the foreseeable future despite ongoing episodes of light to moderate precipitation at times–so storm and flood recovery prospects improve quite a bit after Wednesday. Stay safe out there!

Next live YouTube “Virtual Office Hours” session Mon @ 9am PT


Daniel Swain

Written by Daniel Swain

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  1. An excerpt from:
    By Esprit Smith,
    NASA’s Jet Propulsion Laboratory
    A new NASA-led study shows that climate change is likely to intensify extreme weather events known as atmospheric rivers across most of the globe by the end of this century, while slightly reducing their number.
    The new study projects atmospheric rivers will be significantly longer and wider than the ones we observe today, leading to more frequent atmospheric river conditions in affected areas.
    “The results project that in a scenario where greenhouse gas emissions continue at the current rate, there will be about 10 percent fewer atmospheric rivers globally by the end of the 21st century,” said the study’s lead author, Duane Waliser, of NASA’s Jet Propulsion Laboratory in Pasadena, California. “However, because the findings project that the atmospheric rivers will be, on average, about 25 percent wider and longer, the global frequency of atmospheric river conditions — like heavy rain and strong winds — will actually increase by about 50 percent.”
    The results also show that the frequency of the most intense atmospheric river storms is projected to nearly double.

    • “The study relied on two resources — a set of commonly used global climate model projections for the 21st century developed for the Intergovernmental Panel on Climate Change’s latest assessment report, and a global atmospheric river detection algorithm that can be applied to climate model output. ” I’m sure that model put forth by the Intergovernmental Panel on Climate Change has no bias whatsoever, and has been ever so accurate predicting things like CA’s drought being the “new normal”, and “the world is going to end in 5 years” every 5 years…. How often do the models need to be wrong before people question their accuracy? So climate change is responsible for our “new normal” CA drought but climate change is also responsible for increased atmospheric rivers which end / prevent droughts? There aren’t many things you can have both ways but I guess climate change is one of them.

    • Actually, the climate projections relied on by the IPCC have been on the conservative side, as the data coming in are showing. Also, climate change is not one sided – you can have it both ways with extreme weather. For example, as the arctic warms, the jet stream is displaced, bringing colder air farther south
      Some clues for you:
      Denier Myth #6 – Models Are Unreliable
      Denier Myth #24 – Extreme Weather Isn’t Caused by Global Warming

  2. Look up “The Great Flood of 1862” in California. Nothing like it has occurred since. Half the Central Valley was turned into a lake, and Sacramento was under 30’ of water. The entire region was affected by that incessant atmospheric river event.
    Here’s just one link to that significant event in California’s history:

  3. This looks like a substantial amount of rain in a day for this area. The county and schools seems to be very inconsistent with how they order evac or warnings or cancel schools. Sometimes it’s too much; then it’s too little. Objectivity is hard to find. Well, hopefully folks in the danger zone get ahead of time and be prepared.

  4. Hardly a rare event. One clue as to the rareness of the event is when there is a nickname (Pineapple Express) associated with said event. The nickname means it’s fairly common and has happened thousands of times in the past, and will happen again, and again and again in the future. It’s really amazing how folks can get all upset about this non-rare event when people do not accept their wrong view that it is rare…when it’s not….LOL!

  5. The storm that hit in 1862 is a regular event for the state. Here’s an interesting article about it:
    The past occurrences are known based on sediment deposits, and here’s an excerpt:
    “Sedimentological records show that extreme storms and flood events occur every 150-200 years.”
    “Silt deposits record megafloods that occurred in AD 212, 440, 603, 1029, 1300, 1418, 1605, 1750, 1810 and 1861-1862—ranging from 51- to 426-year intervals with a mean return period of 150-200 years.”
    “Storms in 440, 1418, 1605 and 1750 were more intense than the 1861-1862 event; the storm in 1605 deposited 2 in (5 cm) of silt in the Santa Barbara Basin, indicating that it was 50% more powerful than any of the others.”
    When the next one happens it will be an unimaginable disaster.

    • “regular” and “extreme” are different things.
      Anyway, it’s completely irrelevant–it’s a fallacy of relative privation and a strawman fallacy. And you don’t even have the courage to make an explicit argument–you just mention these outliers as if their mere existence contradicts climate science. But climate science does not ignore or deny natural variability. Of course there is natural variability–*and* there is anthropogenic global warming … the existence of the former doesn’t contradict the latter–to think so reflects the poverty of logical reasoning among climate science deniers.
      (from page 702)
      Frequently Asked Question 9.2
      Can the Warming of the 20th Century
      be Explained by Natural Variability?

      Although natural internal climate processes, such as El Niño,
      can cause variations in global mean temperature for relatively
      short periods, analysis indicates that a large portion is due to
      external factors. Brief periods of global cooling have followed
      major volcanic eruptions, such as Mt. Pinatubo in 1991. In the
      early part of the 20th century, global average temperature rose,
      during which time greenhouse gas concentrations started to
      rise, solar output was probably increasing and there was little
      volcanic activity. During the 1950s and 1960s, average global
      temperatures levelled off, as increases in aerosols from fossil
      fuels and other sources cooled the planet. The eruption of Mt.
      Agung in 1963 also put large quantities of reflective dust into
      the upper atmosphere. The rapid warming observed since the
      1970s has occurred in a period when the increase in greenhouse
      gases has dominated over all other factors.
      Numerous experiments have been conducted using climate
      models to determine the likely causes of the 20th-century climate change.
      These experiments indicate that models cannot
      reproduce the rapid warming observed in recent decades when
      they only take into account variations in solar output and volcanic activity.
      However, as shown in Figure 1, models are able
      to simulate the observed 20th-century changes in temperature
      when they include all of the most important external factors,
      including human influences from sources such as greenhouse
      gases and natural external factors.

  6. Nope. This is ordinary weather variation for California. I will always remember the severe droughts of the mid 1970s up in Northern California which were brought to a sudden end just like this year. Massive snow accumulation then warm atmospheric river events melting a lot of the snow in a short period.

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