Unusually Intense and Prolonged Early-Season Heatwave Developing Across California

May 22, 2020 at 2:50pm

Weather West, by Daniel Swain

It has been an unusually warm spring so far across nearly all of California, but it has also been a pretty wet spring in southern California. Recent rains finally brought some temporarily relief to northern California, although outside of a few favored areas (which saw several inches of precipitation), accumulations were generally pretty light. Most of NorCal remains under moderate to severe drought conditions, however, following an exceptionally dry winter–and this legacy of curtailed winter rains (and, now, a very warm spring) will likely be with us through the summer and autumn to come.

Fire season–which had been off to a very early start across NorCal–was temporarily damped down by this welcome addition of late-season moisture. But it now appears that this fire weather reprieve will be pretty short-lived, as a remarkably intense heatwave builds across California over the coming week.
Intense, prolonged, and likely record-breaking May heat wave this week

Widespread extreme early-season heat is likely this week across a wide swath of the American Southwest.

A pretty unusual large-scale atmospheric pattern over the northeastern Pacific Ocean and inland over the Southwest will contribute to a strikingly intense and persistent statewide heatwave this week across California. Indeed, extreme and in many cases record-breaking heat will actually extend across a pretty broad chunk of the American SW in the coming days–not just California. Heat will begin building today, but will become noticeably oppressive in some places by Monday. The heat will continue to intensify Tuesday, Wednesday, and possibly through Thursday before reaching a very impressive peak–especially across NorCal. Widespread daily record highs are expected, especially in NorCal, on multiple successive days (likely in some spots from Mon-Thurs). Right now, the high-resolution models are suggesting that this week’s extreme heat will be most anomalous across portions of the San Francisco Bay Area, where I would not be entirely surprised to see some monthly (not just daily) heat records fall. But ensemble forecasting tools are also suggesting that 850mb temperatures could approach/exceed record levels across an even broader region by Wed/Thurs–so there is some potential for this upcoming heat event to “outperform” even these pretty lofty predictions.

A broad region of warmer-than-average ocean temperatures exists offshore of CA. Anomalies are particularly large just off the coast of the Bay Area. (NOAA Coral Reef Watch)

This heatwave will be noteworthy for at least two reasons in addition to its daytime intensity and multi-day persistence. First, this is a very early season heat event that is expected to affect the immediate coastline, which is pretty unusual. Typically, a very robust marine layer and cool seabreezes create strong thermal contrasts between the California coast and inland regions during spring/early summer heatwaves. But the May Gray will likely be nowhere to be found during this upcoming event. Why? Well, ocean temperatures are now well above average levels along nearly the entire CA coast due to a pronounced lack of cold water upwelling in recent weeks. Ocean temperatures are locally as much as 3-5F above average–which will limit the degree of “natural air conditioning” during this heat event and will also contribute to higher than usual atmospheric humidy during this event. These higher-than-usual humidities during an intense heat event will 1) limit the amount of overnight cooling–a key predictor elevated heat-related health impacts, and 2) make daytime temperatures feel even more oppressive than usual. (During many California heatwaves, low humidity makes the “apparent temperature” feel lower than the actual air temperature. Unfortunately, that humidity-related relief will not be available to Californians this week). All in all, this is shaping up to be a potentially high impact heat event across much of California–especially given that a higher-than-usual fraction of folks are staying at home, away from crowded beaches and public parks and in many cases without air conditioning, due to the COVID19 pandemic.

Model ensemble situational awareness tools are suggesting a very strong signal for record-breaking heat across most of California later this week. (NOAA)

Some atmospheric dynamics: “Rex” to “Omega” block ensures prolonged heat event

A classic “Omega block” will develop directly over California later this week–bringing record heat, and then perhaps some thunderstorm activity across NorCal. (NCEP via tropicaltidbits.com)

This upcoming heat event will last from Sunday through at least Thursday across most of California–which is pretty prolonged, especially for such an early season event. The event is also expected to steadily intensify through its duration–also a bit unusual for CA events. So, what’s causing this highly anomalous weather this week?

Well, it all has to do with atmospheric “blocking”–i.e., the disruption of the typical large-scale atmospheric flow by a nearly stationary atmospheric high pressure system. In this instance, the ridge is taking up residence right over California, and will initially take on a (tilted) “Rex” block configuration (i.e., a persistent high pressure system located just poleward of a persistent low pressure system). As the week progresses, this Rex block will transition to an “Omega” block configuration as the ridge remains quasi-stationary–anchored in place by a pair of low pressure systems to the southwest (over the Pacific) and southeast (over the interior Southwest/Texas). Both of these set-ups are very stable–and tend to be associated with persistent and/or extreme weather conditions. (The NWS Bay Area forecast discussions over the past 48 hours have had excellent, detailed descriptions of this).

Read more of the original article here.