Issued at 921 PM PST Wed Nov 20 2024
The highest 24 hour rainfall reports so far are very focused in the North Bay, varying from 1-3 inches over Marin and eastern Napa counties to as much as 2-7 inches Sonoma county with as much as 8-10 inches of rain so far in the normally wettest spots of northwestern Sonoma county. Expect rapid rises in creeks, streams and rises in the mainstem rivers (rivers are handling rain well so far), including mudslides and road closures in the North Bay. Flood advisories have been issued through the evening in the North Bay with more expected overnight and Thursday. A Flood Watch remains in effect for the North Bay through late Friday night.
Rainfall amounts fall off sharply so far south across the San Francisco Peninsula and the East Bay where amounts are a few to several tenths of an inch to less than a tenth of an inch, even less so far across the Santa Cruz Mountains where amounts are generally less than two tenths of an inch.
Issued at 214 PM PST Wed Nov 20 2024
A strong Atmospheric River has arrived in the North Bay and will bring significant impacts to the region over the next several days bringing chances for heavy rainfall and strong winds.
, (This evening through Thursday) Issued at 214 PM PST Wed Nov 20 2024
Key Messages:
-Flood Watch is effect for the North Bay through Saturday morning -5-12" of rainfall in the North Bay through Tues -1-6" of rainfall south of the Golden Gate, with the heaviest rain expected on Friday -Wind Advisory in effect for the North Bay, San Francisco Bayshore, and Pacific coast through early Thu for gusts up to 45-50 mph -A High Surf Advisory is in effect through Friday morning for breaking waves of 14-22 ft. -Beach Hazard Statement in effect through late Thursday night.
The well anticipated "bomb cyclone" continues to sit off the coast to the north. Radar and satellite shows a prominent band of subtropical moisture flowing through the Bay Area, especially north of the Santa Cruz Mountains. Observations show that most of the rainfall within the last 12 hours are within the North Bay with totals ranging from 0.6 to 2 inches, with higher terrain seeing up to 4-5 inches! The latest model runs and guidance continues to show agreement of an uptick in intensity bringing heavier rain bands going into the late afternoon and early evening. In addition, IVT values continue to show support of an atmospheric river event at least through Thursday.
Expect another 1-4 inches of rain in the Bay area, with North Bay seeing the higher rainfall values going into Thursday. With the expected continuous rain and the front becoming semi-stationary, Flood Advisories have been issued for parts of the North Bay (mostly Sonoma County), with a possibility of additional products or extensions being issued as we continue to get reports of flooding in small streams, creeks, and poor drainage areas.
In addition, strong southerly winds are well underway with this front. Most of the mountain ranges are reporting strong gusty winds with several places seeing up to 55-65 mph with expectations that winds will continue to increase or remain strong as we approach the late afternoon and evening. As the front continues to bring heavy rainfall and strong surface winds, there is a risk for downed trees and branches, ponding and debris on roadways, and power outages. Therefore, Wind Advisory will remain in affect for North Bay, San Francisco Bay shoreline, and San Fransico through early Thursday evening.
..(Thursday night through next Tuesday) Issued at 214 PM PST Wed Nov 20 2024
After Thursday, the parent low that sits off of British Columbia will lift northward making room for another low pressure system to dominate. Cluster analysis shows agreement that the low has a good chance to moving inland. This system will keep us in an active pattern through Tuesday, bringing additional rainfall to the Bay Area, with most of the impact continuing over the North Bay. Rainfall this weekend will transition from a continuous pattern to a more occasional shower with winds slowly diminish Friday. Some higher terrains, coastal areas, and some isolated spots may becoming gusty up to 40-50 mph on Friday but winds will continue to decrease going into the weekend. With the mix of subtropical moisture and the cold front, there will be a increase in instability in the atmosphere, making way for chances of thunderstorm late Friday into Saturday. In addition, there is a continued risk for flooding as we continue to expect rain through Tuesday, which will promote saturation of soil and overflow of creeks and streams.
(06z TAFS) Issued at 951 PM PST Wed Nov 20 2024
A nearly stalled frontal boundary associated with a strong atmospheric river continues to bring moderate to heavy rain to the North Bay. North Bay conditions include strong lower level southerly to southwesterly winds and lower level wind shear, KSTS and KAPC weather is persistence for the 06z TAFs. Surface to lower level southerly winds are upsloping winds for the North Bay, enhancing rain rates not only through upslope cooling, air parcel volume expansion and lower pressure, but enhancing frontal development (frontogenesis) via ingesting initially drier more distant surface air, air parcels partially cooled via evaporation in contrast to the incoming lower level warm air advection with the nearly stalled atmospheric river. South to southeast surface winds across much of the rest of the forecast area are resulting in downsloping (drying) winds limiting rainfall away from the North Bay causing a sharp gradient to the current copious rainfall/no rain line, though this line is shifting slowly southward a bit with surface winds easing tonight (increasing nocturnal stability). It's a high confidence IFR forecast for the North Bay, to the south a moderate to high confidence MVFR-VFR forecast.
Vicinity of SFO, MVFR, periodic light rain and gusty southeast wind through tonight and Thursday.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, VFR, a passing shower/sprinkle or two possible otherwise periodic gusty southeast winds, especially in the Salinas Valley.
(tonight through next Tuesday) Issued at 842 PM PST Wed Nov 20 2024
Heavy rain and strong winds continue primarily across the northern waters tonight into early Friday morning before pushing southeastward Friday. Very rough seas up to 20 feet in the northern zones are propagating south through the coastal waters. Gusty gale-force winds to near storm force winds are expected across the waters tonight and linger into late week.
Ca, High Surf Advisory until 6 AM PST Friday for CAZ006-505-509-530.
Wind Advisory until 6 AM PST Thursday for CAZ006-502>506-508-509.
Flood Watch through late Friday night for CAZ502>506.
Beach Hazards Statement through late Thursday night for CAZ529.
PZ, Gale Warning until 3 AM PST Friday for 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 Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.
Small Craft Advisory until 3 AM PST Friday for Mry Bay.
Gale Warning until 3 AM PST Thursday for Pt Arena to Pt Reyes 10- 60 NM.
