A surge of low clouds comes just before rain and possibly thunderstorms return to the Central Coast this week

Morro Bay with marine layer arriving
Posted at 4:39 PM, Apr 30, 2023
and last updated 2023-04-30 21:43:17-04

The Central Coast experienced a nice weekend with ample sunshine and just a few clouds here and there, but all of that is about to change.

As of Sunday afternoon, a significant surge of low clouds was already racing at the Central Coast and more clouds are likely overnight into Monday.

These low clouds look significant and look to surge into the interior as well. Pockets of mist/drizzle/light showers are possible with the cloud surge, but this is not really the essence of the next system, which looks to hold off until early Tuesday morning for the leading edge of that system.

Highs will drop dramatically Monday with the low clouds and a trough replacing the ridge of high pressure which was dominant since the mid-point of last week.

This pattern shift has been long anticipated, showing up on models for the last week or two. It is such a dramatic shift, and not very typical of May, that there was some initial doubt it would develop. However, the models appear to have been right about the development.

The cooler air and onshore flow associated with the trough will drive low clouds into the coast thru Monday. As the trough continues to dig the surface low-pressure features will start to provide mist and drizzle at first with the thick marine influence but then some real showers begin early Tuesday with more robust rain Tuesday thru Thursday.

In terms of how much rain is possible: I think "up to 1 inch" is a good broad-brush expectation. Models actually show anywhere up to 1.5 or even 2" but there are also models which have low expectations. There are some significant factors here: such as where the surface low sits and spins, if it is 25 miles different from current expectations the potential rain amounts can change a lot. I think most folks will see less than 1" but I think a few places like SW faces or the SLO County north coast could exceed an inch. Stay tuned for refinements on expectations.

Winds will also be out of the SW on Tuesday and Wednesday, and some gusts past 30 or 35mph are possible, especially Wednesday. Don’t be surprised if some wind advisories are posted for this system.

Also, snow levels do drop to about 5000ft. Locally, we don’t have much above 5000 other than the peaks of the SB County Mountains but it is an interesting development this late in the season.

This system is not only cool at the surface but also over our heads. This means air can rise quickly causing the potential formation of thunderstorms. Thunderstorms can produce lightning, hail, locally strong winds and downpours, and occasionally weak tornadoes. There is nothing like a severe thunderstorm watch in place, but there is a “marginal risk” posted by the Storm Prediction Center Tuesday into Wednesday.

Their outlooks do not take us past Wednesday. Still, the trough does look to linger into Thursday so I could see that outlook extended (but possibly further south into the Southcoast and SoCal as that is where the activity is more likely to be Thursday).

Highs all week to be in the 50s and lower 60s as the entire area is under the trough influence.

I think the trough exits Thursday with Friday thru the weekend likely being dry, but it does appear another upper-level low spins in the region to keep temps from any fast rebound.

This is a forecast where the details are certain to change quickly and more advisories are certainly possible, so please stay tuned this week.