Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
Issued by NWS San Francisco Bay Area, CA
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901 FXUS66 KMTR 222347 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 447 PM PDT Sun Sep 22 2024 ...New AVIATION, MARINE... .SYNOPSIS... Issued at 1209 PM PDT Sun Sep 22 2024 Temperatures peak on Monday and Tuesday with a Heat Advisory in effect for the interior East Bay, the South Bay, and the Santa Cruz Mountains. Temperatures return to near the seasonal average on Wednesday through the rest of the week. && .SHORT TERM... (This evening through Monday) Issued at 1209 PM PDT Sun Sep 22 2024 The warming trend continues today as an upper level ridge continues to build into the Pacific Northwest, although with a developing shortwave trough helping moderate the influence over California. The latest RTMA analysis shows inland temperatures running around 5 to 10 degrees higher than this time yesterday, and high temperatures across the inland valleys are expected to reach the lower to mid 90s. Elsewhere in the region, the Bayshore and coastal Santa Cruz County will see highs reach into the mid 70s to the low 80s, while the Pacific coast sees highs in the low to mid 60s. Low temperatures on Monday morning will generally hover in the mid 50s to lower 60s across the lower elevations, with some lower 50s in the Sonoma and Marin County valleys, while the higher elevations see lows reach the mid 60s to lower 70s. The warming trend peaks on Monday, when the upper level ridge crosses into the Pacific Northwest and the shortwave trough spins off a weak upper level low. A Heat Advisory has been issued for the interior East Bay, South Bay, and the Santa Cruz Mountains, as highs reaching the mid 90s to near 100, or around 15 to 20 degrees above seasonal averages, create widespread Moderate HeatRisk (heat that impacts sensitive individuals, especially those without adequate hydration and/or cooling) with some patches of Major HeatRisk (heat that impacts anyone without adequate hydration and/or cooling). The Bayshore will see highs reach the mid 80s to lower 90s, coastal Santa Cruz and downtown San Francisco see highs near 80, while locations along the Pacific coast see a slight uptick to the mid to upper 60s. While some offshore flow is possible during the overnight and morning hours, onshore winds are expected to prevail during the afternoon and evening hours. The stratus deck has retreated to the immediate coastal regions, with some limited inland development overnight into Monday morning, especially in the Monterey Bay region into the Salinas Valley, but overall stratus coverage will be limited by the influence of the upper level high. && .LONG TERM... (Monday night through next Saturday) Issued at 1209 PM PDT Sun Sep 22 2024 A slight cooling begins on Tuesday with temperatures dropping a couple of degrees across the region. The Heat Advisory continues through Tuesday in the interior East Bay, where potentially dangerous heat lingers. Elsewhere there is some uncertainty over how severe the heat impacts will be. The NBM ensemble products are showing significant spread in the low temperature forecast along and adjacent to the coastal ranges and San Francisco Bay on Tuesday morning, with the spread between the 25th and 75th percentile forecasts reaching 5 to 10 degrees across the coastal ranges. The main forecast issue is with the evolution of the marine layer which would enhance onshore flow and thus the overnight cooling wherever it sets up, whereas weakly offshore flow could keep temperatures warm above the marine layer inversion. Colder low temperatures will limit the risk of heat- related illnesses, as the body is able to "cool down" better overnight than in a scenario where low temperatures remained above seasonal averages. For now, the low temperature forecast has kept the region near the cooler side of the forecast range, but this is something to monitor over the next day, and should confidence in a warmer scenario increase, Heat Advisories may need to be extended into the Bayshore region. More significant cooling is expected on Wednesday, as an upper level trough moves into the West Coast and temperatures return to around or somewhat below the seasonal average. The current forecast calls for a slight warming closer to the end of the week, and CPC outlooks suggest a lean towards temperatures above seasonal averages through the first week of October. && .AVIATION... (00Z TAFS) Issued at 446 PM PDT Sun Sep 22 2024 Currently VFR at all terminals. The TAFs were constructed using persistence and a reasonable worst case scenario as guidance/models are not conclusive on what is going to happen tonight. My educated guess as to why this could be is due to offshore flow developing overnight, and while it is not expected to be strong, it could be just enough to keep clouds closer to the coast and increase dew point depressions if the surface is able to dry out. The marine layer is currently at 1,000 feet and is expected to further compress through the TAF period as high pressure continues to build. This will result in the ceilings that do develop being LIFR in addition to sub-VFR visibilities. Widespread VFR will prevail by late-morning tomorrow. Vicinity of SFO...Currently VFR with westnorthwesterly flow. Low to moderate confidence on a ceiling developing through the TAF period. Winds will begin backing early tomorrow morning to become offshore before the afternoon sea breeze prevails tomorrow afternoon. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Currently VFR with northerly flow at MRY and VFR with northwesterly flow at SNS. High confidence in LIFR conditions at both terminals this evening through late-morning tomorrow. && .MARINE... (Tonight through next Friday) Issued at 446 PM PDT Sun Sep 22 2024 Breezy northwesterly flow will continue over the northern outer waters early Monday. The locally stronger winds will continue to generate a steeper fresh swell. Northwesterly winds will ease on Tuesday with a decreasing swell. Seas restrengthen and become elevated in the outer waters towards the end of the forecast period. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...Heat Advisory from 11 AM Monday to 8 PM PDT Tuesday for CAZ510- 515. Heat Advisory from 11 AM to 8 PM PDT Monday for CAZ512>514. PZ...Small Craft Advisory until 9 AM PDT Monday for Pt Arena to Pt Reyes 10-60 NM. && $$ SHORT TERM...DialH LONG TERM....DialH AVIATION...Sarment MARINE...MM Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea