FXUS66 KMTR 270514 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 1014 PM PDT Sat Apr 26 2025 ...New AVIATION... .SYNOPSIS... Issued at 240 PM PDT Sat Apr 26 2025 Scattered showers and low chance of thunderstorms continues through this evening before diminishing overnight into tomorrow morning. Warmer, seasonal temperatures prevail this upcoming week before the pattern becomes unsettled again next weekend. && .UPDATE... Issued at 908 PM PDT Sat Apr 26 2025 Radar returns show a band of light stratiform rain coming through the East and South Bays and southern San Mateo and western Santa Cruz Counties, with scattered showers behind. There were some relatively intense showers that developed across the South Bay and Central Coast, but the strongest storms remained within the Central Valley and no lightning strikes were detected within our forecast region, with the loss of daytime heating making any further convective activity highly unlikely. As for the parent upper level low, water vapor imagery shows the circulation crossing the California- Nevada border to the east of the southern Sierra Nevada range. Expect isolated to scattered showers to continue through tomorrow morning, concentrated south of the Golden Gate, before those lingering showers too dissipate through Sunday afternoon and evening. DialH && .SHORT TERM... (This evening through Sunday) Issued at 240 PM PDT Sat Apr 26 2025 Scattered thunderstorms remain possible across the interior Central Coast through late this afternoon as a cut-off low enters Southern CA with thunderstorm chances to diminish overnight into tomorrow morning. The 12Z OAK sounding (5AM) showed fairly moist low and mid levels (65-74%) with no real temperature inversion at the lower levels which is beneficial for convective development. When a temperature inversion is present, it acts as a cap for instability/upwards motion. As a refresher, a low level temperature inversion means temperatures are increasing as you go up with height in the atmosphere rather than decreasing. Now assume we have a positively buoyant air parcel at the surface, that air parcel will want to rise as long as it is warmer (i.e. less dense) than the air parcels surrounding it. When an inversion is present, that air parcel will effectively rise until it hits the temperature inversion (where the ambient air temperature is warmer than that of the parcel) causing it to sink back to the surface. The inversion therefore serves to cap the upwards transport of air from the surface and limits convective development. The lack of a capping inversion means that surface based air parcels will be able to rise more freely, with convective activity able to develop. Shower activity increased late this morning with showers currently becoming more widespread over the South Bay and Central Coast. Mid- level overcast skies over the North Bay have kept the atmosphere more stable there resulting in light stratiform precipitation rather than more convective cells developing. Farther south, skies were able to clear for much of the morning (South Bay, Central Coast) resulting in increased surface warming and low-level instability. As seen on both satellite imagery and on KMUX, convective cells have started to develop where clearing was able to occur (generally south of the SF Bay) with light rain showers being observed. So far no lightning strikes have been reported within our CWA but thunderstorm potential will remain elevated through this evening. Thunderstorms are most likely across the interior Central Coast but that depends on mid to upper level clouds continuing to clear across that region. Another limiting factor is the lack of low level wind shear which may reduce our overall thunderstorm risk. Thunderstorm chances drop off this evening (less than 10% everywhere) with scattered showers and light rain continuing through Sunday morning. High temperatures will peak in the upper 50s to mid 60s today and tomorrow. While not anticipating winds to be an issue with this system, if a thunderstorm does develop locally breezy to gusty winds may occur with it. As always, remember, when thunder roars go indoors. && .LONG TERM... (Sunday night through next Friday) Issued at 240 PM PDT Sat Apr 26 2025 Dry conditions will prevail for most of next week with surface high pressure and upper level ridging building back in. The return of upper level ridging will kick off a warming trend with temperatures becoming more seasonal to slightly above average across the region. For interior regions, highs will be in the mid to upper 70s for much of next week while coastal areas stay in the low to mid 60s. Light offshore flow is possible Monday to Tuesday next week as a the cut- off low moving through CA today is expected to deepen and become positively tilted as it passes through the Four Corners region. Minor HeatRisk is forecast Tuesday through Thursday with impacts mainly confined to those who are extremely sensitive to heat. If you are outdoors this week and are sensitive to heat, remember to listen to your body and take breaks/drink plenty of water. The pattern looks to become unsettled again Friday into next weekend with long range guidance showing another deep upper level trough moving into the West Coast. Temperatures are expected to drop back into the mid to upper 60s across the interior and upper 50s to low 60s along the coast starting Friday. Initial ensemble guidance shows this system may bring light rain (similar totals to today's event) to the region. Given that we are almost a week out the specifics may change as we get closer to this event but, for now, be aware that the pattern is likely to become unsettled again with cooler temperatures and some potential for light rain next weekend. && .AVIATION... (06Z TAFS) Issued at 1013 PM PDT Sat Apr 26 2025 MVFR conditions and westerly winds to prevail through most of the TAF period. A late return to low-end VFR is expected by tomorrow afternoon at all but the Monterey Bay terminals. After the rain band goes over the Monterey Bay terminals late tonight, light rainfall will become scattered in nature with the uncertainty lying in the location and timing. If the upper-level low exits quicker to the east, then most terminals will remain dry after 12Z, but if it is slower, that can be delayed to 18Z. Nonetheless, rain showers can be expected in the region with the primary impact being wet runways. Vicinity of SFO...MVFR ceilings to prevail through tomorrow morning with a slow improvement to VFR expected. Westerly winds will prevail through the TAF period. SFO Bridge Approach...The San Mateo Bridge Approach will be slower to clear of MVFR conditions and rain showers tomorrow morning. Monterey Bay Terminals...MVFR conditions and westerly winds to prevail through the TAF period. Light rain will become scattered in nature by the early morning hours of Sunday. && .MARINE... (Tonight through next Friday) Issued at 908 PM PDT Sat Apr 26 2025 Light rain showers will continue through Sunday morning with a less than 10% chance for thunderstorms which would pose the risks of lightning, small hail, and locally gusty winds and heavy rainfall. Strong northwesterly breezes and rough seas will continue for the outer waters, inner waters and bays can expect moderate seas and moderate northwesterly breezes with strong gusts. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory from 3 PM to 9 PM PDT Sunday for Mry Bay. Small Craft Advisory until 3 AM PDT Monday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM. Small Craft Advisory until 9 PM PDT Sunday for Pt Reyes to Pigeon Pt 0-10 nm. Small Craft Advisory from 9 AM to 9 PM PDT Sunday for Pigeon Pt to Pt Pinos 0-10 nm. Small Craft Advisory from 9 AM Sunday to 3 AM PDT Monday for Pt Pinos to Pt Piedras Blancas 0-10 nm. && $$ SHORT TERM...Kennedy LONG TERM....Kennedy AVIATION...Sarment MARINE...Sarment Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, X, and YouTube at: www.facebook.com/nwsbayarea x.com/nwsbayarea www.youtube.com/nwsbayarea