Monsoonal Surge to Continue Through Thursday
A very moist plume of monsoonal moisture surging northward from southern California and the desert southwest has nosed into northern California as of Monday, and will continue to surge north over the next few days.
The amount of moisture this plume or surge is quite significant, with precipitable water values in excess of 1.5 – 1.7″, boosting dewpoints across California’s lower elevations into the 50s and 60s for the most part, with even some 70s during the afternoon Monday in far southern California’s deserts. This amount of low-level moisture combined with bands of deeper moisture extending into the mid and upper-levels and decent warming of the surface, as is common during the summer, allows decent instability to form over the mountains and deserts where inversions and stable layers aren’t as strong as in coastal areas and in valleys, such as the Sacramento/San Joaquin valley.
12km NAM valid for 14z Tuesday (early morning), with 500mb relative humidity on left, and precipitable water on the left. The 500mb RH helps depict areas of clouds, with the higher the humidity the more cloud coverage (usually/in general).
Thunderstorms are once again expected to blow up over the central and southern sierra during the late morning and afternoon Tuesday as temperatures warm and instability increases, with MLCAPEs of 200 – 800 j/kg forecast from around Tahoe south. This, combined with the more than fair amount of moisture present, will support the development of diurnal convection. Some weak upper level winds streaming around weak cutoff low off the southern California coast will support some weak mid/upper-level wind shear to perhaps support some stronger thunderstorms as seen Monday afternoon in the central sierra. Hail, gusty winds, frequent lightning, and flooding rain are all possible from thunderstorms over the coming days.
Overnight tonight (Monday) into early Tuesday, a band of mid-level moisture and widespread cloud deck will slide overhead from central California, which may make for a rather warm night, but could also limit overall daytime temperatures Tuesday where it sticks around. However, if it isn’t too thick it enough sun could get through the clouds to increase low/mid-level instability a bit in the northern sierra, foothills, and valley. Monday, showers/thunderstorms were mainly confined to the mountains and foothills east of the San Joaquin valley. Believe each day through Thursday storms will develop further northward a bit, but if and how far into the lower elevations convection can make it is the big question.
In general, each day through Thursday, bands of moisture and weak little vorticities/pieces of energy rotating around the main upper low off the southern California/Baja California coast should spark off a good amount of mountain convection, and if timing is right, perhaps some late afternoon/overnight valley action given it’s hard for daytime storms during the summer due to stronger inversions/thermal gradients and dry patches in certain levels of the atmosphere, which can be a bit weaker at night when temperatures drop off a bit and night time moisture recovery occurs… it can just be hard to hold enough instability at night to support nocturnal convection, and that’s where those little disturbances would come in. For now, I’d say the chance is low. Best chance actually looks to be Tuesday at the moment.
Water vapor satellite imagery combined with 500mb heights analysis via the RAP, from 9:45pm Monday evening. You can see the upper low off the S CA/Baja coast via the 500mb heights quite easily.
By Friday/Saturday, upper-level flow should begin to flip to the west as a trough/upper low slides into the Pacific Northwest, which should shift storms into the highest of elevations to the east, with storms moving north to east.
