Tuesday February 18th… Dear Diary. The main purpose of this ongoing post will be to track United States extreme or record temperatures related to climate change. Any reports I see of ETs will be listed below the main topic of the day. I’ll refer to extreme or record temperatures as ETs (not extraterrestrials)😉
Main Topic: A New Drought With Added Fire Danger For California?
Dear Diary: Yesterday we touched on the soggy South where Jackson, Mississippi is being severely flooded due to ramifications from a warmer than average winter. The associated jet stream pattern has become familiar over the last few years producing very wet times for most areas east of the Rockies. Conversely, the pattern at times has led to the dipole, or a ridge over California and the West Coast. Such a pattern has led to drought, then summer heat waves with extreme fire danger nearly year round, which has made world news and led to much tragedy. Dry weather has reared its ugly head in California this winter, so my question is whether or not that state is doomed for a repeat of similar events from 2013-2019? Others noticed this from yesterday’s post:
As a reminder, global warming does not cause flooding or drought. Theses weather phenomena have been with us since we evolved into being humans. Global warming accentuates or exacerbates jet stream patterns leading to both much worse flooding and very severe drought.
My go to guy for all things West is Dr. Daniel Swain, or “Weather West.” I’m taking the liberty of reposting his latest blog in hopes that we can start to answer questions concerning what may happen in California later in 2020.
A bone-dry February compounds dry start to winter
Autumn 2019 was record dry across parts of northern California. Despite an early December reprieve (especially in southern California), conditions have increasingly become increasingly dry across most of the state since that time. Parts of California have now gone 3-4 consecutive weeks without any meaningful precipitation during what is typically the wettest month of the year. As a result, increasingly wide swaths of the state have fallen well below typical seasonal precipitation accumulations–many locations in the northern half of the state are running below 50% of the Oct 1-Feb 12 average. Recent record warmth in some areas has compounded this dryness, further reducing Sierra Nevada snowpack and even allowing regional vegetation to support a (low) level of potential wildfire activity.
Next 2 weeks look very dry (still)
Unfortunately, there’s very little (if any) relief on the horizon over the next 10+ days. The next 7 days may feature completely dry conditions across 85% or more of California; the following week may feature a chance of showers in some spots, but will very likely be mostly dry in most places. That will bring us to the end of February without any widespread significant precipitation–meaning that existing precipitation and snowpack deficits will continue to grow.
What’s going on over the Pacific Ocean? (Regular readers will already have a sense of where this is going). Well, a persistent ridge of high pressure has been holding relatively steady over the far northeastern Pacific over the past 5-6 weeks, and is expected to persist for at least 1-2 more. The mean ridge axis has been shifting slowly eastward over the past couple of months, but still hasn’t quite made it all the way to the West Coast. As noted in the previous blog post, this particular ridge position is favorable for dry (but not completely so) and generally cool conditions. While such a ridge effectively blocks the primary Pacific storm track, it does allow cold/dry systems to drop southward from Canada and bring mostly cold temperatures and wind to California, outside of occasional scattered mountain and southern California showers. Until there is a major pattern shift, this same kind of recurring pattern can be expected to continue for the foreseeable future.
Right now, there are perhaps modest signs that the prevailing ridge could weaken in early March, though there are no clear signs of impending breakdown. Interestingly, California is not alone in its high pressure plight at the moment–nearly all of the Northern Hemisphere mid-latitudes are experiencing similarly “ridgey” conditions right now. Much of this can be blamed on the presently record-strength positive phase of the Arctic Oscillation. This essentially means that (in great contrast to recent years) both the stratospheric and tropospheric polar vortex are exceptionally strong at the moment, which is keeping Arctic air locked up tightly in the high latitudes. One consequence of this record-strength AO is a broad displacement of atmospheric mass from the Arctic to the mid-latitudes (in other words, unusually low pressure in the Arctic and unusually high pressure essentially everywhere else). The ECMWF model suggests that the AO may go even higher over the next 7 days–shattering the previous record (set just last week) by an even wider margin.
Has the Ridiculously Resilient Ridge returned?
Understandably, I’ve started to see this question floating around with increased frequency in recent days. Drought anxieties are running high, given such recent experience with the worst drought in California history (during 2013-2016).
My answer, right now, is that we’re not quite there yet–but we’re getting closer. Multi-week dry spells are neither that unusual nor particularly worrying in California–mid-winter lulls are just part of the intrinsic climate here. But now that we’re seeing multi-month persistence of high pressure and far below average precipitation, the story’s starting to change a bit. At this point, it’s exceptionally unlikely (given the current forecast, and historical precipitation climatology for the rest of the season) that we’ll be able to erase the accumulated precipitation deficit by the end of the rainy season–so it is virtually certain (even with a “Miracle March”) that we’ll end up drier than average this year. But if we do see a wet spring, some of those impacts will be temporarily mitigated.
Presently, storage in California’s surface water reservoirs is sufficient to carry the state over until the next wet season even if the precipitation spigot were to completely shut off as of today (which is unlikely). That should ease drought concerns for urban and agricultural areas, at least for this year. On the other hand, the natural landscape does not benefit from water stored in man-made reservoirs–so if the rains don’t pick up soon California will start seeing significant impacts to ecosystems, and therefore vegetation and subsequent wildfire risk this summer and coming autumn. It’s still too early to discuss those potential impacts in detail, since much will hinge on March and April. Stay tuned.
New research on atmospheric rivers!
For the presently precipitation-starved: you might be interested in checking our recently published research on improving atmospheric model simulations of extreme atmospheric rivers. Some of you might be able to pick out your favorite California (or West Coast) storms from the past ~4 decades from the lineup! Ultimately, this work forms the basis of additional research focused on how the most intense winter storms in California will change in a warming climate. (That paper is still currently in revision–I’ll be sure to write a full blog post on that one when it becomes available). For now, check out the public Twitter thread below or visit the journal website directly: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JD031554
Since Valentines Day when Dr. Swain posted his blog I can’t offer much in the way of good news for California. A zonal flow is typical this time of the year through March crashing weather systems into California, bringing moist conditions, which is not forecast on met models:
That “Ridiculous Resilient Ridge” will be making a comeback as the jet amplifies later this month allowing cold air masses to penetrate as far south as Florida, finally giving much of the South a reprieve from wet times. Unfortunately such a change will reinforce dry times in California.
We will all be keeping an eye on California, one of the states most severely affected by the climate crisis, through this year.
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Here is some more weather and climate news from Tuesday:
(As usual, this will be a fluid post in which more information gets added during the day as it crosses my radar, crediting all who have put it on-line. Items will be archived on this site for posterity.)
Here is more European warmth:
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Guy Walton “The Climate Guy”