fxus66 kmtr 191621 

Area forecast discussion 
National Weather Service San Francisco Bay area 
921 am PDT Tue Mar 19 2019 

Synopsis...a cooling trend will begin today as cloud cover 
increases over the region ahead of an approaching weather system. 
Rain showers will develop over the region tonight into Wednesday 
with an unsettled pattern likely to continue through late week. 
While dry conditions are possible this weekend, unsettled weather 
will return heading into next week. 


Discussion...as of 09:20 am PDT Tuesday...a negatively tilted 
trough will approach our area through the day, bringing increased 
cloud cover, cooler temperatures, increase onshore flow, and by 
late this evening, widespread beneficial light to moderate 

The thickening cloud cover overnight insulated the region and 
mitigated the nocturnal radiational cooling rate, allowing for 
temperatures this morning to be slightly warmer than yesterday 
morning. That said, temperatures are expected to remain cooler 
throughout the remainder of the day due the same cloud cover and 
cold air advection aloft. Afternoon temperatures will be in the 
low to mid 60s along the coast, or 3-5 degrees below yesterday, 
while inland temperatures will remain in the upper 60s to low 70s, 
or a 6 to 12 degree cool off. Southerly winds will pick up through 
the day and become breezy to locally gusty over the higher 
terrain, coastal areas, and down the favorably oriented valley. 
Winds will turn westerly and remain breezy later Wednesday into 
Wednesday night. Given the orientation of the frontal boundary, 
expect to see rainfall appear along to the entire coast from Point 
Arena southward through Santa Barbara around the same time this 
evening. Lighter warm sector showers May Spring up in time for the 
evening commute while the heavier rain will pick later in the 
evening through the overnight period. Isolated thunderstorms are 
possible in proximity to the core of the upper disturbance late 
tonight into Wednesday. A second disturbance in the broader upper 
trough will extend showers throughout the day Wednesday and into 
Wednesday night. Short term models are struggling to identify the 
trajectory the secondary disturbance will take by Thursday. Some 
models, such as the NAM, delay the departure of this feature and 
instead keep it directly over head, increasing the uncertainty and 
possibility that lingering showers may persist to Thursday. 
Overall, precipitation amounts for the upcoming storm system will 
the heaviest on southwest facing slopes over the Santa Cruz and Big 
Sur mountains, picking up between 2-3 inches over the higher 
peaks. Elsewhere, precipitation amounts for urban areas will 
generally range from 1/5" to 1/2", with rain shadowed areas on the 
lower end of the spectrum. 

The current forecast looks on track so no changes are planned for 
the morning update. Please see previous discussion for more 
details on the current forecast and extended period. 

Previous discussion...as of 02:58 am PDT Tuesday...no major 
changes to the forecast this morning with low clouds near the 
coast and widespread mid/high level clouds advecting inland across 
the region. Increased onshore flow and cloud cover will make 
temperatures notably cooler this afternoon with highs in the 
Lower/Middle 60s at the coast to upper 60s/near 70 deg f inland. 
Rain showers will likely hold off through much of the day before 
spreading inland late this evening and into the overnight. This 
will be the result of an approaching mid/upper level trough and 
surface low that will approach the coast through Wednesday 
afternoon. Breezy to locally gusty winds will be possible with 
scattered showers producing periods of light to moderate rainfall. 
Cannot rule out isolated thunderstorms as well late in the night 
into Wednesday morning, yet widespread coverage is unlikely. 
Rainfall totals through Wednesday will likely range between 
0.25"-0.50" for most urban areas. Some of the region's drier 
locations may see lesser amounts however. Meanwhile, the Santa 
Cruz Mountains and Santa Lucia Range could see amounts of 
0.75"-1.50" with isolated amounts upwards of 2.00". Showers are 
forecast to diminish late Wednesday night into Thursday morning 
and persist through Thursday night. 

Dry conditions are looking likely heading into the Friday morning 
commute ahead of the next system approaching from the northwest 
late Friday into early Saturday morning. This system will move 
through more quickly and produce less rainfall than the previous 
system, especially south of the Golden Gate. Temperatures through 
late week will generally be near to slightly below seasonal 
averages given the progressive pattern and cooler troughs passing 

The upcoming weekend looks mainly dry with a weak short-wave 
ridge building over the region in between weather systems. The 
ensembles continue to point toward the return of yet another 
unsettled pattern early next with periods of rainfall through 
midweek. While details remain limited this far out; there is the 
potential for heavier rainfall during this time frame. 


Aviation...as of 4:51 am PDT Tuesday...between breaks in the 
high clouds satellite imagery reveals patchy low clouds along the 
coast as well as locally over North Bay valleys and down the 
Salinas valley. Mainly VFR conditions will prevail with local IFR- 
LIFR cigs possible for ksts and ksns through the early morning 
hours. Low clouds and fog will clear fairly quickly this morning 
while high clouds continue to increase. An low pressure system and 
associated cold front will approach the coast from the west today 
with light rain forecast to develop between 03z-05z tonight. 

Vicinity of ksfo...VFR conditions will prevail with increasing 
high cloud ahead of an approaching weather system. Cigs will lower 
to MVFR overnight. Winds will be light offshore through the 
morning then switch around to the southwest this afternoon. Winds 
will return to the southeast and strengthen overnight as the front 
moves through the region. 

Sfo bridge approach...similar to ksfo. 

Monterey Bay terminals...VFR conditions will prevail with local 
LIFR cigs anticipated over ksns this morning. Low clouds and fog 
will clear fairly quickly this morning while high clouds continue 
to increase. Cigs will lower to MVFR tonight. Light southeasterly 
winds will prevail through the morning then switch around to the 
west this afternoon. Winds will return to the southeast and 
strengthen overnight as the front moves through the region. 


Marine...as of 09:00 am PDT Tuesday...southerly winds will 
continue today and tonight ahead of an approaching low pressure 
system and cold front moving in from the west. Expect winds to 
increase and veer to the west northwest on Wednesday. A long 
period west swell will arrive today increasing shoaling on bars 
and Harbor entrances. 


Mtr watches/warnings/advisories... 
... Pt to pt pinos 0-10 nm from 3 PM 
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 3 PM 
Small Craft Advisory...mry Bay from 3 PM 



Public forecast: drp 
aviation: CW 
marine: CW 

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