Western weather: a taste of April’s improvement?


Here is an excerpt from a recent article on the Weather West Blog.

By Daniel Swain of Weather West

A taste of April’s improvement? Recurring Western Pacific typhoon may help direct substantial late-season rainfall to parts of NorCal, though SoCal remains dry

The driest January to mid-April periods and among the hottest on record in California

January 1 to April 11, 2022 was the driest period on record for most of California and Nevada, as well as parts of adjacent states. (climatetoolbox.org)

It was an extraordinary start to the calendar year in California from a weather and climate perspective, and not in a good way. The rainfall was so low that it broke all previous low rainfall records from January through mid-April for virtually all of northern and central California, and therefore also statewide. Additionally, record-breaking warm spells pushed average temperatures during the period to very high levels relative to the all-time high – many parts of California also experienced the top 5 warmest calendar year starts. warmest on record in addition to record low rainfall. . As a result, the Sierra Nevada snowpack fell at a record rate – briefly falling to 22% of the average for the statewide date. Wildfires have been happening basically statewide for a few months now, and the NWS in Sacramento issued its first recorded spring red flag warning for the NorCal interior earlier this week.


But at least in the short term, I have noticeably better news to report (for the northern part of the state, at least)!

Some (unexpected) short-term good news in NorCal: heavy end-of-season rainfall could be coming to some areas

The current overall consensus is that widespread drench precipitation is likely in NorCal, especially near/north of the I-80 corridor, over the next 10 days. Depending on the details, total accumulations along the North Coast and Northern Sierra could be quite impressive for mid to late April (in the range of 2-3 to locally 5+ inches of liquid equivalent).

The multi-model ensembles have now converged to a North Pacific weather pattern that will favor wet conditions over the northern third of California for the next 7-10 days. In fact, the sets trended slightly wetter as the potential event unfolded – a welcome change from the trend for much of this winter. As it stands, it looks like an unusually deep northeast Pacific trough will set in off the west coast later this week, allowing 2-3 weather systems to flow over the next 7 days. approximately. While these systems will likely be confined to NorCal, with almost all precipitation remaining north of Monterey Bay, some locations along the North Coast and in the Northern Sierra could end up seeing fairly substantial accumulations of 2 to 3 inches of liquid equivalent (locally 5+ inches in the most humid places for 7-10 days). While these precipitation totals are still quite small compared to the accumulated annual precipitation deficit (which reaches 25 to 50 inches in some of these wetter places in NorCal), they will be quite remarkable for an event occurring so late in the season. . And while these won’t be particularly cold storms, they won’t be particularly warm storms either: it looks like there could be a pretty decent snowpack above about 5,500 feet. . In fact, I’m expecting statewide SWE to recover (albeit modestly), perhaps picking up as much as 30-35% of average for the date around April 20th (this which is still, needless to say, very low – but better than it was).

Lower elevation areas away from the north coast will be a bit more of a wild card. Much of the Sacramento Bay and Valley area also has a good chance of absorbing rainfall from this pattern – likely 0.5 to 1 inch in many places (although with the caveat that there could be heavy rain in some interior valleys). SoCal will remain mostly dry – possibly completely – throughout the period, so it’s primarily the northern third (and to a lesser extent the northern half) of CA that will reap the benefits of this pattern shift to to come.

The culprit: a recurving western Pacific typhoon (yes, another one of those)

What is the culprit behind this decidedly unexpected mid-April reprieve (perhaps leading, given the seeming imperative to give it an alliterative name, to the April improvement? As far as I can tell, it seems the Unusually deep west coast trough/depression that will be proximally responsible for wet conditions in NorCal can be attributed to the indirect effects of a recurring West Pacific typhoon (Malakas).

This may be a familiar refrain for long-time blog readers: a large fraction of “surprising” early and/or late season (fall and/or spring) rainfall events (even in extreme drought years) in California appears to be due to disruption of the North Pacific jet stream by injection of moisture and vorticity well upstream. Often these ingredients are brought in by powerful tropical cyclones (deriving their energy from warm oceans) which rapidly weaken and transform into extratropical cyclones (deriving their energy from latitudinal temperature gradients in the atmosphere) as they move over much colder waters and become injected. in the usually powerful jet stream over the Pacific Northwest. In doing so, these moisture/rotation injections can pump the ridge over the central Pacific in such a way that it can cause either a strong ridge (with hot and dry conditions on CA) or a strong trough (with conditions cool and humid on CA), depending on the exact longitude at which it occurs. Luckily, in this case, it looks like a wetter/holier result will occur. It should be noted that such events – where a large fraction of seasonal precipitation (in this case, spring) falls essentially during a single event – are virtually impossible to predict on the scale of seasonal forecasts. (So ​​there can be, and probably always will be, the potential for these kinds of events to “surprise” us at the last hour, even in the driest years.)

In this animation, you can see the Western Pacific’s Typhoon Malakas recurving in the Westerlies just northeast of Japan and then acting to amplify the downstream flow, producing an active storm pattern in the PacNW and northern Asia. AC.

The long term still suggests intensifying drought and a poor fire season, but the pattern may still provide tangible short-term relief to NorCal


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