Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
Issued by NWS San Francisco Bay Area, CA
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363 FXUS66 KMTR 122345 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 445 PM PDT Sun May 12 2024 ...New AVIATION, MARINE... .SYNOPSIS... Issued at 146 PM PDT Sun May 12 2024 Slight decrease in daytime highs today and tomorrow as a result of weak troughing. Temperatures warm little into the mid week as ridging resumes. Coastal stratus will continue to bring overcast skies for coast and bay shorelines in the early portions of the day, then clearing in the afternoons. Hints at a cooling trend in the late week. && .SHORT TERM... (This evening through Monday) Issued at 146 PM PDT Sun May 12 2024 Its a quiet day, with a relatively quiet forecast in store, so lets have some fun with this morning`s discussion and learn a little bit about why we saw the cloud cover we did near the coast. The 12Z sounding from KOAK this morning revealed a strong inversion, a condition in which air temperature rises with increasing height instead of falls, as well as a deeply saturated layer reaching just shy of 950mb beneath this inversion layer. What is 950mb you ask? Well, as you go higher in the atmosphere, pressure begins to fall. 950mb simply describes in the atmosphere a certain pressure level. However, the actual altitude at which particular pressures are found is always changing as a result of the atmosphere continuously evolving as troughs and ridges build and decay. But enough about that, lets move on. If you were to glance at satellite imagery this morning, you would have seen abundant cloud cover near the coast, as well as it penetrating further inland into coastal valleys and bay communities than in mornings previous. This cloud cover is represented by the saturated layer within the sounding where the temperature and dewpoint lines meet, with the inversion acting as a lid or a cap on top, limiting the altitude to which these coastal clouds could reach. A glance at the Ft. Ord profiler which examines the depth and altitude of the marine layer shows the maximum altitude of the marine layer reaching only to about 1400` approximately. Therefore, if you live at a higher elevation near the coast, say 1500` and above, you did not see overcast skies this morning from the marine stratus, but rather, probably looked down on a blanket of grey and white below. Meanwhile, coastal communities lying below 1400` feet or so were enveloped in clouds, leading to grey skies. However, at this time, coastal cloud cover has begun to recede as daytime solar heating helps to warm air near the surface and the clouds. As this warmer air rises, it will help to sap moisture, or humidity, from the saturated layer, aka the clouds, and effectively evaporate them until clear skies are achieved. So what does this all mean in short? We saw extensive cloudy and grey conditions near the coast, bays, and even reaching into coastal valleys this morning as a result of total saturation being reached in the lower levels of the atmosphere. These clouds reached their maximum altitude at about 1400` before they hit a cap and could not lift any higher. As the sun rose and began to warm the Earth, clouds began to dissipate as warmer air was mixed into them and helped to evaporate or rid the lower levels of moisture. As this warming continues, cloud cover will begin to recede, leading to clear skies for much of the region. So there you have it! Perhaps you just wanted to hear what the temperatures were going to be today, or what the *potential* aurora viewing conditions might be tonight. Instead you got an unsolicited meteorology lesson. Well, don`t fret, we`ll cover those other things too. Warm temperatures continue today, though will be just a few degrees cooler than what was seen yesterday as weak troughing aloft begins to build into the region. Expect daytime highs in the mid to lower 80s today for interior regions, and upper 50s to low 60s along the coast. Tomorrow, those daytime highs drop to upper 70s to very low 80s, and mid to upper 50s, respectively. As this troughing regime begins, this should allow the marine layer to deepen and for coastal stratus to push a bit further inland tonight as air rises and is able to surpass coastal mountains and elevated terrain easier. That being said, if you`re looking to do some aurora scouting, you`re going to need to go higher up in elevation, and further inland. For more on geomagnetic storms and aurora forecasts, please visit the Space Weather Prediction Center`s website at spaceweather.gov. Or visit their Twitter/X account @NWSSWPC. && .LONG TERM... (Monday night through next Saturday) Issued at 146 PM PDT Sun May 12 2024 High pressure, or ridging, will begin to build towards Tuesday, allowing temperatures to begin to warm up once again. However, with the main ridge axis staying out over the northern Pacific ocean, we are not likely to see temperatures warm significantly, and likely not become as warm as we saw them Friday and Saturday of this past week. For interior regions of our area, daytime highs will likely be in the low 80s, topping out into the mid 80s. Coastlines and bay shores will see high temps in the upper 50s to low 60s as onshore winds help to keep things cooler at the coast. Minor heat risk for the most sensitive populations will be present through much of the week. Stratus is expected to bring cloudy conditions at the coast most mornings, then generally clearing into the afternoons each day. Towards the late week and next weekend, the end of the forecast period, it appears the ridge begins to break down, and troughing begins with decent agreement, though not perfect, amongst models, hinting at a bit of a cool off for most locations. All for now. && .AVIATION... (00Z TAFS) Issued at 445 PM PDT Sun May 12 2024 VFR transitioning to MVFR-IFR overnight. Widespread stratus will move inland with satellite currently showing an impressive stratus bank located just offshore. Stratus coverage largely expected to fill in slightly earlier tonight than last night with stratus already moving inland along the coast. Clearing times remain similar to today with conditions improving to VFR by mid-morning around 16- 19Z. Low to moderate confidence that stratus will reach LVK with both persistence and guidance showing a higher probability of stratus reaching LVK. Southerly winds across the North Bay, west to northwest at all other stations. Moderate winds continue through the evening before becoming light and variable overnight. Vicinity of SFO...VFR becoming IFR overnight. Gusty, moderate winds continue into the evening with gusts up to 25 knots expected. Gusts should drop off and winds weaken as the sun sets with light to moderate west winds overnight. Stratus will move inland heading into the late evening with IFR conditions returning. Conditions will improve by mid to late morning with moderate west to northwest winds and clear skies expected. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...VFR transitioning to IFR-LIFR overnight. Moderate to high confidence that stratus will return early this evening with IFR to LIFR expected at both airports. For MRY - moderate to high confidence that LIFR CIGs will develop and persist through the night. For SNS - moderate confidence that IFR CIGs will develop with guidance indicating LIFR CIGs become more likely after 10Z as RH increases and winds weaken. Visibilities are expected to drop overnight but not currently expecting fog to develop. Light to moderate NW winds continue through the TAF period. && .MARINE... (Tonight through next Friday) Issued at 445 PM PDT Sun May 12 2024 Fair weather and stratus continues for the waters of Central California through the week as high pressure builds over the Eastern Pacific. Northwest winds begin to increase this evening to a fresh breeze. Seas build to 12 to 15 feet by Tuesday before gradually abating through the rest of the week. && .BEACHES... Issued at 719 PM PDT Fri May 10 2024 Long period southwest swell will lead to slightly elevated chances for sneaker wave and rip current development this weekend. Conditions are expected to stay below Beach Hazards Statement criteria as swell height will only be between 1-2 feet. However, in light of anomalously warm temperatures and increased likelihood of people traveling to the coast, visitors to the beach should remember to take safety precautions to prevent being caught by an unexpected large wave and to never turn your back on the ocean. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory from 3 PM to 9 PM PDT Monday for Mry Bay-Pt Pinos to Pt Piedras Blancas 0-10 nm. Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm. Small Craft Advisory until 3 AM PDT Monday for Pt Pinos to Pt Piedras Blancas 0-10 nm. Small Craft Advisory until 9 AM PDT Tuesday for Pt Arena to Pt Reyes 10-60 NM. && $$ SHORT TERM...AC LONG TERM....AC AVIATION...Kennedy MARINE...Behringer 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