Issued at 225 PM PST Sat Dec 21 2024
Our active weather pattern will continue through this week as several upper weather disturbances continue to slide across the west coast over the next 7 to 10 days. The most impactful system appears that it'll arrive through the day on Tuesday when there will be the potential for widespread shower and scattered thunderstorm activity. In addition to the rain and thunder threat, very hazardous marine conditions today and into at least the first half of the week.
, Issued at 225 PM PST Sat Dec 21 2024 (This evening through Sunday Morning)
Largely zonal flow will be in place tonight and into early Sunday. Gradual ascent atop a diffuse low level frontal boundary (more evident in 850mb theta-e fields) will encourage the re- development of rain showers near and along this boundary. At current time, it's anticipated that this boundary will be draped from along the Golden Gate eastward toward the Delta. Locations near and south of this boundary have the best opportunity for measurable rainfall, especially in areas of higher terrain (Santa Cruz Mountains, eastern Santa Clara and East Bay Hills, as well as the Santa Lucias). Overall rain amounts tonight into midday Sunday are anticipated to range between a few hundredths of an inch, except across the higher terrain. There is some bust potential overnight depending on the location of the stalled boundary. If the boundary shifts north or south, then the axis of "heaviest" rainfall will shift with it. In addition, if the depth of moisture is lower than currently anticipated, we'll likely be dealing with more in the way of drizzle and potentially fog. At present time, the chances for widespread visibility below 3 miles is around 20-30% and may be more tied to brief bouts of heavier precipitation. For now, will defer to the evening shift for any near-term refinements to the Wx grids, but widespread dense fog does not appear probable at this time.
By day break, the greatest coverage of rain showers is anticipated somewhere between San Francisco Bay and Monterey Bay regions. PoPs will be on the order of 30-70% as modest ascent continues along the frontal boundary. Instability appears paltry and the greatest MUCAPE values are forecast to reside across extreme southern portions of the Big Sur Coastline. With the greatest lift remaining to the north, I believe the the thunderstorm threat will be quite low on Sunday, but something we'll monitor. There will be a break in rain shower activity and a good portion of the day on Sunday may be rain-free (with perhaps some lingering drizzle in higher terrain).
..issued at 225 PM PST Sat Dec 21 2024 (Sunday Afternoon through next Friday)
Key Messages: *Parade of storm systems through the end of the week *Most impactful storm appears to be Tuesday *Rain amounts thru next Saturday 2-6" North Bay | 1/2-3" Bay Area/Central Coast | Up to 1/2" Southern Salinas Valley *Wind gusts of 25 to 45 mph on Tuesday
Late Sunday afternoon, a +150 knot 300mb jet noses into the area. In response, surface pressure along the coast will fall. These surface pressure falls will promote low level convergence along a slightly sharper frontal boundary and this will give the area our next chance for rain Sunday evening and into Monday. There's some uncertainty, of course, and with a bulk of the stronger forcing for ascent likely to remain far to the north, there will be a sharp PoP gradient from the north to the south. In addition, the recent wet weather likely has a strong influence on the National Blend of Models via it's statistical methods to bias correct PoPs. Given the location of the strongest forcing being displaced to the north and it appears that some of the more dynamical, though still ensemble based, NWP paint more of a reasonable picture for what is anticipated Sunday and into Monday. Areas across the North Bay will have the greatest chance for late Sunday and into Monday afternoon with PoPs still around 80%. PoPs taper downward dramatically south of the San Francisco Bay with PoPs ranging from 15% to 50% elsewhere (except far southern Monterey and San Benito counties where PoPs are around 10%). Rain amounts between 11am Sunday and 11am Monday (with a bulk of this falling late Sunday into early Monday) range between a few hundredths of an inch to maybe 1/2 inch (mainly across the North Bay). There's around a 50% chance that rain amounts as great as 3/4 inch transpire across the extreme northern portions of the North Bay. All in all, the Sunday and Monday round of rain is anticipated to be less impactful than Tuesday's anticipated rainfall. Checking in on some of the local hydrology, it appears that 1 and even 6 hour rainfall amounts will remain below any sort of Flash Flood guidance through Monday, though quick rises in smaller creeks, rivers, and streams may still be in play. Special thanks to EKA and STO for their coordination on this portion of the short term forecast.
Starting late Monday and into Tuesday, our higher impact system arrives. Global NWP is in good agreement that a deeper trough will amplify and sweep through the area on Tuesday. Taking a look at some more of the traditional synoptic meteorology indicates a deep trough characterized by 130 meter 500mb height falls. The upper trough does exhibit somewhat of a positive tilt and this could minimize the threat for this feature stalling. It'll still pack a punch, however, by early morning Tuesday, as most of the Bay Area and Central Coast will be within the favorable exit region of the upper level jet. The resultant cyclogenesis will help to enhance low level flow which may translate to 25 to 45 mph wind gusts as well as help encourage a larger area of precipitation. Chances of a peak gust or two above 35 mph is around 60-80% mainly across the higher terrain of the North Bay, East Bay, SF Peninsula, Santa Cruz Mountains, and the Big Sur Coastline/Santa Lucia's. At this time, I don't anticipate the need for a High Wind Watches given even our local WRF and 90th percentile NBM has peak wind gusts of around 45 mph along the ridgelines (which is where most of the grid edits were focused). If the system intensifies, stronger winds will be realized at lower elevations (including the larger populations areas). Regardless, those with outdoor decorations will want to make sure they are secure by Tuesday, else they may become airborne with this next system.
850mb theta-e progs show a ribbon of more unstable air along the surface/850mb fronts. The probability of surface-based CAPE at or above 200 J/kg is maximized at around 60-70% across the Bay Area on Tuesday. Given the spatial extent of higher theta-e air, I'll advertise a broad area of isolated thunderstorms that encompasses all of the North Bay, East Bay, SF Peninsula and parts of the South Bay. It's possible that additional areas of thunder will be needed in subsequent forecasts. We will also keep tabs on the potential for any sort of severe weather as 500mb winds will exceed 50 knots and likely support at least loosely organized convective modes. Rainfall amounts late Monday and into Tuesday will range from 1/4" across southern Monterey/San Benito counties up to around 2 1/2" of rainfall across the North Bay. There's around a 40% chance that 24 hour precipitation amounts exceed 3" across the North Bay. Additional rises on area creeks, rivers, and streams are most likely during this time. While deterministic and 50-60th percentile hydrologic output keep most of our mainstem river points in monitor/action stages, we'll need to examine the potential for any more robust/vigorous showers and storms. If instability is a bit greater and/or training activity sets up, the potential for river and flash flooding will increase. For now, we'll forgo issuance of a Flood Watch on this shift, but something to consider, particularly if Sunday into Monday's round of rain overachieves.
Most of Christmas Day will be rain/free with partly sunny skies. Hazardous boating and beach conditions are probable, however, so if headed to the beach, make sure you're aware of any beach hazard or high surf concerns. Christmas Night is when our next storm system arrives as northwesterly flow envelops the area. PoPs are on the higher end (60-80%), but they are rather broad-brushed as the NBM solution likely represents a rather broad range of possibilities. In fact, some models in the days 7-8 time period show higher amplitude ridging over the west coast (keeping storm systems to the north of the area), while others have a slightly flatter ridge/more zonal flow (keeping the storm door at least slightly ajar). For now, we'll continue to keep rain chances in the forecast given projections of IVT still warrant mentionable PoPs in the extended. Additional rainfall amounts of 1-3" are projected across the North Bay, with up to 1 1/2" of rain elsewhere (except across the Southern Salinas Valley). Stay tune for refinements to the forecast.
(00z TAFS) Issued at 349 PM PST Sat Dec 21 2024
Most terminals are reporting MVFR with a few areas of IFR also reported. Periodic wet weather continues during the 00z TAF cycle along with MVFR-IFR conditions.
Vicinity of SFO, MVFR. Light northerly to easterly wind during the period.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, MVFR-IFR, vicinity showers. Mainly a light easterly wind.
(tonight through next Thursday) Issued at 340 PM PST Sat Dec 21 2024
Large, building westerly swell are expected through tonight and Sunday morning with a temporary decrease in swell later Sunday afternoon prior to large to very large westerly swell Monday into Tuesday. Wave heights will remain elevated through late next week with 15 to 20 foot waves Monday through Wednesday. Expect dangerous conditions for small crafts through the weekend through early next week. Elevated seas will continue to produce enhanced shoaling at harbor entrances and bars.
Issued at 111 PM PST Sat Dec 21 2024
High surf advisory remains in effect through Sunday afternoon for elevated surf of 20-25 feet, with breakers up to 30 feet. A long period, westerly swell will continue to create hazardous beach conditions along the coast into Tuesday. Global wave models are forecasting a more intense energetic swell with 20+ feet heights and swell periods of 20+ seconds, this will lead to much higher breakers (possibly 30-35 feet) including localized coastal flooding impacts. These conditions will result in dangerous and life-threatening bay, ocean, and beach conditions lingering into next week.
Ca, High Surf Advisory until 4 PM PST Sunday for CAZ006-505-509-529- 530.
PZ, Small Craft Advisory until 3 PM PST Monday for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.
