Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

447 am PDT Tue jul 17 2018

Synopsis

Seasonably warm weather will continue inland through the rest of the week as high pressure remains over California. Monsoonal moisture is projected to begin moving into our area from the southeast starting late Wednesday. Model solutions vary in the details, but there is some possibility of high-based showers or thunderstorms for our area beginning around Wednesday evening in the far south and across the entire forecast area by Thursday.

Discussion

As of 3:10 AM PDT Tuesday, A strong upper ridge with 500 mb heights in excess of 594 dm remains centered over California early this morning. Despite the presence of this ridge, the marine layer has not compressed much since yesterday and the depth of the marine layer has been holding steady at around 1500 feet since yesterday afternoon. In addition, onshore surface pressure gradients remain virtually unchanged since yesterday. Consequently, low clouds have developed well inland once again overnight. The amount of warming expected yesterday fell short of expectations and indications are that additional warming today will be limited to just a few degrees. Therefore, high temperatures today were adjusted downward slightly in the early morning forecast update. Even so, the inland valleys and hills are expected to be very warm today and tomorrow. Coastal areas will remain mild thanks to persistent onshore flow. Projected Heat Risks are low in coastal areas and moderate inland. Only isolated locations in the hills are expected to be in the high Heat Risk category through midweek due to warmer nights at the higher elevations.

The upper high currently centered over California is forecast to shift to the northeast over the next few days, resulting in southeasterly flow aloft across central and southern California by Wednesday. This southeasterly flow will bring monsoon moisture back into our area starting late on Wednesday across the far southern portion of our area, and then north across the remainder of our forecast area by late Thursday. In addition, a weak trough rotating around the southwest side of the upper ridge may provide enough mid-level instability for isolated shower and thunderstorm development, particularly across the southern portion of our area from late Wednesday into Thursday morning. The GFS forecasts scattered light precipitation over southern Monterey and San Benito counties during this period of time, as well as over our southern coastal waters. While the NAM does not forecast precipitation, it does forecast sufficient mid level moisture and instability for the development of high-based shower and thunderstorm activity. Therefore, have decided to add a slight chance of showers and thunderstorms to the far southern portion of our forecast area Wednesday night into Thursday morning.

After about midday Thursday the models begin to diverge. The NAM shows the highest instability shifting offshore as the moisture plume moves northward on Thursday afternoon. The GFS forecasts rapidly decreasing moisture and instability Thursday afternoon, but then shows another pulse of monsoon moisture and convective precipitation arriving from the south by Friday morning. Given the uncertainty due to these model differences, have left shower and thunderstorm chances out of the forecast after midday Thursday, at least for the time being.

Any precipitation that develops late Wednesday into Thursday will likely be high-based with enough dry air in the lower levels to evaporate most, if not all, rainfall. Thus, the potential for dry lightning exists with any isolated thunderstorms that may develop.

Slight daytime cooling is expected Thursday and Friday as the center of the upper high shifts away from our area and as mid and high level clouds increase from the south. However, nights will be warmer due to increased atmospheric moisture limiting diurnal radiational cooling.

The upper flow over California is forecast to gradually shift from southeast to southwest over the upcoming weekend, which will bring in a drier airmass aloft and end precipitation chances. Temperatures are forecast to remain quite warm inland during the weekend while coastal temperatures remain near seasonal averages.

Aviation

As of 4:46 AM PDT Tuesday, For 12z TAFs. Almost a carbon copy of what was occurring this time yesterday. Fog product satellite shows coastal stratus in the typical spots, around 1,400 feet deep, impacting most TAF sites at this hour with flight categories ranging from VFR to LIFR. Similar to yesterday morning, a hole in the stratus over KSFO is quickly filling in over the terminal. Clearing times are forecast to occur between 16-18z this morning. Moderate onshore winds this afternoon/evening, generally 10-15 kt. Stratus is forecast to return this evening/overnight again.

Vicinity of KSFO, IFR/MVFR cigs will impact the terminal through 16-17z, then VFR. Breezy onshore winds starting late this morning, through this evening, sustained 15-20 kt.

SFO Bridge Approach, similar to KSFO with similar clearing times.

Monterey Bay Terminals, IFR/LIFR cigs through about 17z, give or take a few minutes, then VFR through the afternoon. Light to moderate west/northwest winds, 10-15 kt. Stratus return again early this evening.

Marine

As of 2:47 AM PDT Tuesday, A relatively weak pressure gradient along the northern California coast will strengthen slightly today and increase winds over the northern outer waters through the rest of the week. Westerly winds will also be breezy through the Golden Gate Gap and West Delta each afternoon this week. Winds are forecast to remain light over the remainder of the coastal waters through the rest of the forecast period. Northwest swell will increase by this evening and continue through the week. A long-period southerly swell is forecast to arrive this weekend.

Watches, Warnings, Advisories

Today, sca, pt arena to pigeon pt 10-60 nm from 3 pm

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