West Weather: An increase in pre-monsoon humidity that will bring scattered thunderstorms

The following is an excerpt from a recent Weather West blog post.

By Daniel Swain of Weather West

Much warmer and drier conditions have arrived in NorCal

After a final flurry of cool and choppy conditions last weekend in parts of NorCal, a much warmer and drier pattern is already firmly entrenched as of this writing. Today actually brought a pretty significant heatwave to the NorCal coast, with even downtown San Francisco entering the 90s. Some places are hitting new daily highs as of this writing, although values ​​in traditionally warmer inland locations (albeit toasty) are not so remarkable for June. But the hot, windy conditions are rapidly drying out vegetation essentially everywhere right now, in a marked change from what many places in the north were experiencing just days ago.

Offshore break plus advected subtropical moisture = scattered thunderstorms in southern 2/3 of AC, some dry

The big story this week, and the primary reason for this late-breaking blog post, is the now substantial likelihood of isolated to scattered thunderstorms across much of central and southern California over the next 48 hours ( from early Wednesday to late Thursday). Some of these storms will likely be dry, or near dry, posing a significant fire weather threat.

In somewhat classic form for a California warm season lightning event, a weak low cutoff is currently developing off the coast of Southern California tonight. On its eastern flank, it carries a substantial plume of mid-level subtropical moisture, some of which may even come indirectly from an eastern Pacific tropical cyclone (soon to be Hurricane Celia). It’s a little early to be considered a true monsoon surge, so I’ll call it a “pre-monsoonal moisture incursion.”

A fairly classic, albeit early-season, dry lightning pattern in California: an offshore cut-off trough during the summer pulling monsoon (or pre-monsoon) moisture northwest along its east flank.

Either way, the low’s position will place much of southern 2/3 of California in an upper-level diffluence region, giving some large-scale relief to the inherent mass instability of the low. relatively hot and humid air. . As a result, isolated to scattered thunderstorms are quite likely to occur over part of the region, although the exact location is uncertain. As of this writing, I believe the region most likely to experience a significant lightning event would be the San Joaquin Valley, the surrounding foothills to the west, south and east of the SJ Valley and the southern/central Sierra Nevada. What is less clear is how much lightning can occur closer to shore; if that happens, much of SoCal as well as parts of the Central Coast could be affected. My best guess right now is that most of the San Francisco Bay Area is unlikely to see much or no lightning (although some isolated strikes are possible in the south and east). However, the Monterey Bay area is more likely to see lightning.

In general, these potential thunderstorms will become drier as they move from south to north. So there could be some heavy showers in SoCal, with low to moderate potential for dry lightning (mostly right after the storm starts, before the rain cores develop). Across central CA, however, the risk of dry lightning from some of these cells is likely moderate to high (although coverage is patchy). The air mass is warmer and drier in the north, and the modeled soundings all show classic “inverted V” type profiles (illustrating a very large dew point depression near the surface layer, and therefore layers very dry underclouds favoring the evaporation of precipitation before it reaches the field). Models currently suggest the potential for surprisingly strong instability by California standards over the San Joaquin Valley and adjacent regions (above 1,000 J/kg), so I suspect the convection that develops will have little hard to get big enough to generate lightning.

Some context for this event, as I know many people in California have varying levels of anxiety resulting from the extraordinary August 2020 dry flash and ensuing firestorm: it is highly unlikely that the current situation has such serious consequences, although it could still lead to a major forest fire. -related impacts. Why? Well, first of all, 2020 was an exceptional event in almost every way: the severity of the drought, the record heat that preceded the lightning, the record dryness of vegetation when the lightning struck, and the incredible number of cloud-to-ground strike passes over a relatively concentrated area. Of these three conditions, only the first is nearly as bad right now as it is in 2020.

The California drought is currently most severe in the central California interior, including the Coast Ranges to the west and the central/southern Sierra Nevada (and foothills) to the east.

Central California is unfortunately the drought hotspot right now, and it seems likely that this week’s lightning will focus directly on the band of “exceptional” drought from the eastern slopes of the Coast Ranges to the western slopes of the southern Sierra. It is not good news. However, it is still relatively early in the season – so the vegetation is not as dry as it would be later in the summer or fall and there is still some green grass (read: much less flammable) and scrub at higher elevations and on the north face. slopes. Also, I don’t expect to see as many lightning strikes this week as I did in August 2020 (and those strikes that do occur will mostly occur on a non-overlapping portion of the state). For all of these reasons, I am reasonably optimistic that this dry lightning event will not result in widespread disaster. But I expect there will be at least a handful of lightning-caused wildfire ignitions – possibly more than a handful – and some of those may be in remote areas. with lots of very drought stressed vegetation and forest mortality. So it’s definitely a mixed picture at the moment. I will follow closely on Twitter, as always.

Outlook for the rest of June and early July: warmer than average, but not exceptionally

Once we get through this week’s (dry?) lightning event, I don’t expect any other major weather events on the immediate horizon. Model sets however indicate that warmer than average conditions will be the rule, so the unusually cool and humid conditions recently experienced at NorCal appear to be in the rearview mirror and I would continue to expect modest heat waves during this period.

Multi-pattern sets suggest that the consistently cool and choppy conditions that have plagued NorCal in recent weeks are in the rearview mirror, and above-average temperatures are likely for the foreseeable future (although extreme heat is not currently on the horizon ).

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