Seasonal monsoonal moisture from the Southwest is moving across California today, bringing with it some chance of thunder and lightning, most likely concentrated closer to the Sierra and in the North Bay.
Northern California remains nervous about any chance of lightning after the disastrous dry lightning storms of August 2020 that sparked three major wildfire complexes across the region. But the current monsoonal moisture pattern is not as likely to produce lightning as that scenario was, because then there was tropical moisture that combined with the monsoonal flow from the Southwest.
Another, similar pattern took shape a week ago, prompting a Red Flag Warning for parts of the region, but that was soon lifted and no thunderstorms materialized.
Still, neither lightning nor the possibility of rain are common occurrences in the Bay Area, with the stable marine layer moisture usually keeping unstable air masses away. But the moisture is headed our way, and some of it caused major thunderstorm activity in Las Vegas last night.
Who says overnight thunderstorm complexes only happen in the Plains. This one that blew through Vegas last night with very active weather is moving across the southern half of CA. With it holding together so well, some widespread showers and thunderstorms continue moving west. pic.twitter.com/PZNtbOVQwC— Jim Cantore (@JimCantore) July 26, 2021
"It’s still a slight chance," says National Weather Service forcaster Geri Diaz, speaking to SFGate. "It boils down to how much of that moisture makes it up north." Diaz adds that this can be tricky to predict.
Satellite water vapor imaging below shows the dry air (in red and black) circulating toward the Pacific Northwest, while the mass of monsoonal moisture passes over Southern Nevada and begins to breaks up as it moves into California.
Gorgeous water vapor satellite imagery early this morning.— NWS Bay Area (@NWSBayArea) July 26, 2021
Very dry air denotes the edge of the clockwise flow around the monsoonal ridge (dark red - black).
Large convective complex moves from Vegas towards LA/SD (white-blue).
Moist monsoonal flow elsewhere w/ weaker cells. pic.twitter.com/l0rSqq80kk
The National Weather Service's Bay Area office says that the moisture is definitely headed for the Central Coast and the Bay Area, but "there is low confidence that there will be enough instability to kick off thunderstorms."
The chance of rain and possible lightning activity here extends from Monday afternoon into Tuesday, and the most likely place it may be seen is in the North Bay. According to the weather service, isolated showers tonight as well as a slight chance of lightning are predicted.
This system brought rain to Los Angeles Monday morning, marking the third wettest July in L.A. since records began being kept in 1877 — with 0.09 inches total.
This monsoonal moisture is normal this time of year for the Southwest, as weather scientist Daniel Swain explains on Twitter — and not part of the ongoing climate apocalypse discussion with heat domes and floods. But the movement of it toward the Bay Area two years in a row seems concerning.
"'#Monsoon' refers to a seasonal reversal of wind patterns over a region; it does not refer to individual storms/downpours! So in most contexts, 'monsoon' is singular (i.e., 'the North American Monsoon'), not plural ('the monsoons will bring downpours today')," Swain writes. "There are several monsoonal systems globally (most notably the West African, and South/East Asian-Australian monsoons). The North and South American monsoons are less distinctly defined, but are nonetheless regionally important climate features."
"The summer monsoon is indeed a normal, if somewhat sporadic feature, of California's summer climate (especially mountains & deserts)," Swain adds. "It's much more pronounced in the interior Southwest, e.g. Arizona and New Mexico, where heavy summer downpours occur every year."
As for whether this is part of a larger, concerning pattern of more frequent lightning in NorCal just when our vegetation is at its crispiest, Roger Gass of the National Weather Service tells SFGate, "The August 2020 event was uncommon given the amount and widespread coverage of lightning strikes. However, not completely unheard of. All the right ingredients must align for such an event and therefore occurs much less often/infrequently. Additionally, there are no indications that events such as these will become more frequent at this time."