Sprinklers or Not?

Possible heatwave May 16th-18th…..

Low amplitude ridging will take place this weekend over California as a system in the northeast Pacific deepens. The result will be a nice weekend with a weaker onshore flow on Saturday (northwesterly flow aloft). Highs in the mid-70’s inland with morning low clouds. Sunday, onshore flow strengthens as a trough approaches the north.

Friday: Morning low clouds with temps in the mid-50’s. Mostly sunny and breezy with daytime highs in the upper 60’s. Overnight lows in the low 50’s.

Saturday: Morning low clouds and patchy fog clearing to mostly clear skies and calm conditions with highs in the mid-70’s.

Sunday: Morning low clouds clearing to mostly sunny skies with highs in the low 70’s with soutwesterly afternoon breezes.

UPDATE (Monday at 9pm): Showers brought 0.05″ to portions of the San Gabriel valley Monday night.

AS FORECASTED: A cluster of cumulonimbus clouds are seen on satellite heading west towards the LA basin. These could provide some sprinkles or a brief shower later this evening! See previous posts about this setup.

UPDATE(Monday 4pm):  Thunderstorms seen in southern Sierra and also near Santa Barbara heading west (image below is as of 3:45pm).

500mb temperatures are just below -20°C as expected. The marine layer is clearing out a little later than anticipated (see animation below) and convection over the San Gabriels is getting a late start. Upper level winds are on target to blow from the east this evening. They are about at 20knots right now and cells may have a hard time holding together at that velocity. However, UL winds should decrease throughout the afternoon. I am still hopeful that some cumulus will develop and form into mature cumulus congestus. Drainage flow is not in full swing but should start at lower levels this evening.

Thunderstorms pop over the Sierra Nevada on Sunday as seen below. Why? The upper level low which is currently moving down the coast has a cold pool which allowed for convection to fire from elevated surfaces.

Monday, the action shifts southward to southern California. We will be under the left-front quadrant of this low Monday afternoon. Due to the cold upper level temperatures and wind flow, I believe convection will fire in the local mountains and deserts Monday afternoon. The low passes to the south late Monday and winds blow from the east. The upper level winds could steer some of this convection over Orange County and coastal areas in the late afternoon. So, we could see some sprinkles or a brief shower from the TOPS of these thunderheads.

Forecast: Monday morning low clouds with a morning low of 52°F. Clearing to partly cloudy skies during the late morning with highs in the upper 60’s. Convective activity for San Diego, San Gabriel and San Bernadino mountains is likely after 11am. Increasing clouds late afternoon with a chance of sprinkles or a brief shower from westward drifting cumulonimbus cloud tops between 3-6pm.

Forecast is working out nicely as skies have been partly cloudy trending to clear lately. Satellite shows marine layer pulling out to sea. Moderate westerly flow aloft seen over the San Diego county mountains.

Highs Sunday on target (low 70’s) with low 50’s Sunday night with low clouds. CONFIDENCE IS LOW FOR MONDAY AND TUESDAY: The upper level low will deepen and begin to move over us on Monday. Models show some low-level moisture beginning Monday morning. This will warrant mentioning a chance for drizzle or light rain Monday morning. The cold pool will de-stabilize the atmosphere over our deserts on Monday afternoon (-23C 500mb temps). Thundershowers are a possibility for the desert regions with the tops of some of these thundershowers moving northwest over the valleys Monday evening. This will have to be watched.

No coastal eddy as of noon Saturday. However, a strong onshore flow is expected late Saturday afternoon.

A spring low is projected to deepen off our coast between now and Monday. This has the potential to create some strange weather on Monday. Stay tuned….

Synopsis: Sounding 1000-500mb thickness 5648m with adiabatically extrapolated surface temp of 84F. Stagnant conditions currently with a temp of 66/43 dewpoint. Stagnant conditions expected through Friday night. Developing southerly flow Saturday morning.

Saturday: Morning low of 56°F with calm conditions. Increasing westerly winds during the late afternoon with highs in the upper 70’s (inland Irvine). Increasing clouds late afternoon/eve with southwesterly breezes. Low clouds overnight with a VERY slight chance of drizzle or a light shower. Overnight lows in the mid-50’s/dewpoints close to 54°F.

Sunday: Clouds in the morning becoming partly cloudy during the afternoon. Clouds lingering against the foothills and mountains with highs in the low 70’s. Temperatures could dip into the low 50’s overnight.

Overnight lows were right at 56°F as predicted.  Sounding shows a steep subsidence inversion and a projected high temperature Friday of 78°F. However, I believe the atmosphere will warm throughout the day as the upper level ridge builds. This should push high temps just past 80°F. Stagnant conditions Friday night and early Saturday as the weak offshore flow subsides. Onshore push strengthens Saturday afternoon.

Synopsis: A warming trend is on the way beginning Friday due to ridging along the west coast. A shortwave will pass through central California late Friday night and is expected to disintegrate over the Sierras. Onshore flow will build through Sunday keeping temperatures in the 70’s. A trough is projected to deepen off of southern California beginning late Saturday. Models show this deepening significantly and moving over us on Sunday. This will bring a pretty cold pool of air over us on Sunday. Possible deepening of the marine layer with middle and high level clouds are a possibility on Saturday night/Sunday.

Friday: Morning low of 56°F with low clouds along the coast in the early morning hours. Fair in the afternoon with highs in the low 80’s (inland Irvine).

Saturday: Cooler with highs in the mid-70’s with some high clouds. Increasing clouds overnight with the slight possibility for some drizzle or light rain late.

Sunday: Slight possibility of drizzle or light rain in the morning becoming partly cloudy with highs in the low-70’s.

UPDATE (Tuesday 9:00am): This morning’s sounding shows thickness down to 5700m with strong westerlies at most levels. Marine layer has moved inland as seen in the sounding (strong inversion with a moist surface layer (see sounding below, or click here)). Low of 62F Tuesday morning as expected. Much cooler for Tuesday.

Synopsis: 1000-500mb thickness now 5753m (weighted avg between San Diego and Nevada’s sounding). As a shortwave passes to our north, the atmosphere will maintain it’s thickness due to warm air advection. However, after this evening the atmosphere will continue the cool down from this past weekend. Clouds will be associated with the passing shortwave as seen near the setting sun. Thickness will drop below 5700m before Tuesday morning.

Forecast:

Monday Night: Decreasing high clouds Monday evening with temps heading into the low 60’s. Moisture rebounds with dewpoints approaching the 40’s.

Tuesday: High temps will depend on when the onshore push begins. The onshore push should be underway after 12:30pm. However, with clear skies temps could jump into the mid to upper 80’s before it gets started. The highest expected temperature for inland Irvine is 87°F. Confidence is low.