Weather West: Brief But Intense Inland Heatwave

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

By Daniel Swain of Weather West

May and June so far: Cool and damp North Coast, but very dry and windy everywhere else

Northern California – including the latest just this past week, which brought quite a lot of rainfall to the North Coast and northern mountains (and some lighter showers as far south as the SF Bay Area and I-80 corridor). Meanwhile, the southern 2/3 of California has been almost completely dry during this period (including the Southern Sierra, where some places had their driest May on record).

May (and, especially, June) are not usually very wet in California – but the “haves” and “have nots” of this recent precipitation distribution have created some pretty stark contrasts between the northwestern corner of the state vs the rest of California. May and June have been notably cool along the North Coast – one of the cooler such periods on record. It has also been wetter than usual in this state, and that combination has kept vegetation unusually green there now well into June. But even just 75 miles inland, it has generally been quite a different story in the Central Valley and adjacent foothills. The resulting gradient between the NW CA – where vegetation levels are actually * above average * and the north-central CA (where there is a poor vegetation level in some places) is therefore quite remarkable.

Recent precipitation up north has definitely tamped down in the short term, and in some places probably for the rest of June (in places that have seen more than an inch or so in recent days). That is good news, especially given that some of these spots have been expected to start early (but will probably now occur for the next few weeks). But across the state, vegetation and wildfire conditions are already running 1-2 months ahead of schedule. Here, I would expect a feature above average fire activity. And in the northern spots that have seen some extra moisture lately, it is likely to be more common than the summer season. season, combined with considered anomalous mid-late summer heat, could actually “add more fuel to the fire” by increasing fine fuel loading). Meanwhile, Sierra Nevada snowpack is basically gone now (except for a tiny bit in the northern reaches),

To sum it all up: recent cold and damp conditions along the North Coast and northern mountains will give that part of the state a fire season reprieve for at least a few more weeks. But there will be no reprieve elsewhere, and even far more than average fire conditions later this season.

Fri-Sat heatwave: Interior CA / Central Valley focus, with record warm overnight temps (but coast stays mild)

There has been much buzz on social media regarding an upcoming California heatwave. And while there is a substantial one coming later this week — at least to some parts of California — it will not be the Earth-shattering event portrayed in a handful of viral web posts. In fact, the average ensemble of magnitude and duration of this event has been slowly but steadily trending downward since it first appeared about 2 weeks in advance (though that’s still a pretty solid predictive signal, if you ask me!).

This heatwave will be quite typical of the coastal region 2/3 of California – excluding most coastal areas from significant heat (though there will be a very sharp west-east temperature gradient). It will be somewhat less typical of California heatwaves in the Central Valley. While Friday may also bring some daily record high temperatures for mid-June (in the 105-110F range across the Central Valley), this heatwave will be more notable than the hot daytime temperatures alone might bring. In addition to this, recent precipitation in parts of NorCal may bring elevated surface dewpoints during the first half of the event – helping to bring “muggier” than usual conditions during the last part of the event and further amplifying how hot it will actually feel. (Drier north east may develop towards the end of this event, likely dissipating any accumulated mugginess.) Excessive heat watches and advisers throughout the Southwest during this period.

The heatwave does not appear likely to have been originally suggested by the ensembles – so the heat will probably be quite definitively later this weekend (although it may be later . But it certainly will feel quite hot in recent times and damp conditions up north.

After the heatwave: a fairly progressive summer pattern, with (mercifully!) Unremarkable weather

Muti-model ensembles suggest a fairly progressive pattern along the West Coast by June standards, with occasional droughiness and no real persistent ridges.

The West Coast will continue to experience a fairly progressive pattern over the next couple of weeks – with a relatively weak pattern of persistence, and a periodic alternation between ridging and coughing. …

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