Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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000 FXUS66 KMTR 240543 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 1043 PM PDT Thu Mar 23 2017 ...Rain and gusty southerly winds return on Friday... .SYNOPSIS...Rain and moderate southerly winds return to the North Bay overnight as a frontal system approaches. Rain and wind will spread southward Friday as the front moves across the region. A break in the rain is expected Saturday with showers anticipated to return Sunday afternoon and night as a low pressure system moves through the region. && .DISCUSSION...as of 9:03 PM PDT Thursday...Radar starting to slowly fill in with a few bucket tips just starting to show up in northern Sonoma county. Rain event is on track with 00z models keeping similar timing with rain gradually increasing over the North Bay tonight reaching the greater Bay Area for Friday morning commute. Looking at satellite ir imagery there are cooling cloud tops well offshore and would expect to see some baroclinic leaf structure development as trough digs and jet streak passes over the region. AMSU/SSMI blended satellite scans show that the moisture plume has tpw values from 2.0-2.3 inches, well in excess of 200% of normal for late March. This depth of moisture with a potentially slow moving front does raise concerns. As the front moves southward through midday the models have been consistent in developing a wave with an associated surface low just west of Half of Moon Bay. As this occurs the band of rain will likely slow with latest projections showing the Santa Cruz mountains once again taking the brunt of the rainfall with the Santa Lucia range above Big Sur taking a close second. Heavy rain rates will once again inundate small streams and any compromised hill sides will once again be tested due to short term heavy rain rates. The one bit of good news is of course we`ve had a little bit of dry weather since the rain earlier this week and the main stem rivers are running low. The Friday afternoon commute will be a mess as well from San Francisco southward to the Central Coast with winter storm warnings up in the Sierra as well so make your weekend travel plans accordingly. In addition to the rainfall, winds will once again be problematic ahead of the front with gusty southerly winds from 35 to 45 mph in many coastal and hillside locations. It appears that many trees are still stressed from the long term drought and now saturated soils so more downed limbs, trees and power lines seem likely. Precip will turn showery by Friday night and end in the pre-dawn hours of Saturday. Saturday looks dry but seasonably cool. Rain chances return, and look likely by Sunday afternoon for the Bay Area and North Bay as another cold upper trough passes across the state. This system wont have near the moisture tap but any showers will bring moderate to heavy downpours with numerous hail reports likely and snow levels down to around 4000 feet by the tail end. Rain totals for the Sunday afternoon/night system look to be around 0.25-0.50 with local amounts 0.75 inches. All rain looks to be east of the area by Monday morning. Beyond that much of next week now looking dry. && .PREV DISCUSSION...as of 1:40 PM PDT Thursday...After an active weather day yesterday, today -- as expected -- has been tranquil with just scattered clouds. Temperatures have been running close to values from Tuesday with most spots in the upper 50s to the lower 60s. Hard to believe in less than 24 hours we will be dealing with another round of active weather. Synoptically a cold front associated with a 988 MB low heading toward British Columbia will move through our region on Friday. Models have shown very little variation from previous runs, so the forecast was changed very little during the shift. Rain will move north-to-south starting overnight in the North Bay then down to the SF Bay Area early in the morning and to Monterey Bay Region close to noon. Rain rates will pick up with the approaching front and could be moderate to heavy at times. Current timing brings the main band through SF Bay late in the morning or early in the afternoon. In fact, if the current NAM verifies the bulk of the rain will fall after morning rush and before the afternoon commute. Along with the rain southerly winds will increase with many spots forecast to see gusts of 25 to 35 mph. Winds for higher elevation locations will be stronger -- gusts over 45 mph are likely. Rainfall totals still look like 1.5" to 3" for the North Bay with 2/3" to 1.5" around SF Bay with generally less than 2/3" for points to the south. All of the coastal ranges are expected to be in the 2-4" range with a few local spots possibly seeing slightly higher amounts. Rain will switch to showers behind the front with light precipitation possible through Friday night. However, by mid-day Saturday all of the rain should be well to our south and east. Similar to today`s time period, that break will be short as another system moves across on Sunday. This one should be quite a bit weaker and will have less moisture to utilize. In general amounts will be around half of what we get out of the first round. Rain will switch over to showers going into Monday. A ridge of high pressure will build back to our region starting next Tuesday, although a system will dive down from the north late Wednesday/early Thursday possibly bringing a few light showers. However, after it departs a more pronounced pattern change will take place as a stronger ridge of high pressure returns to our region. This should give us dry conditions with warmer temperatures toward the end of next week possibly through the following week. && .AVIATION...
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As of 10:40 PM PDT Thursday...For 06z TAFs. VFR conditions are expected to persist the next several hours across the Bay Area through about 10 to 14Z before showers associated with an approaching front push onshore. Rain is expected to reach the San Francisco Bay Area terminals by Thursday morning with a steady rain mid/late morning that will continue into the afternoon and evening hours. Expect ceilings to become MVFR as the rain reaches the terminals. Medium confidence on timing of the rain. Winds are becoming southerly tonight and can expect breezy conditions by mid morning at many locations. Gusts are likely to exceed 20 to 25 kt by 18Z. High confidence on wind direction, medium confidence on the timing and gust magnitude. Vicinity of KSFO...VFR through about 9 to 10Z, and then transitioning to MVFR ceilings and with rain by Friday morning. Southerly winds will increase by morning with gusts exceeding 20 kt by 18Z and continuing into the afternoon and evening. SFO Bridge Approach...Same as KSFO. Monterey Bay Terminals...VFR overnight with generally light winds. Rain showers are expected to begin impacting after 18Z with MVFR ceilings and increasing southerly winds.
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&& .MARINE...
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as of 10:25 PM PDT Thursday...Occasional gale force gusts are possible late tonight and into tomorrow as a storm system impacts the region with strong southerly winds. Rough seas possible just ahead of and behind the frontal passage, but should begin to decrease by Saturday. Expect northwest winds to return by Saturday as well before briefly becoming southerly on Sunday as another weak system moves over the waters.
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&& .MTR WATCHES/WARNINGS/ADVISORIES...
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.Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm from 5 AM SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 5 AM SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...Pt Reyes to Pigeon Pt 0-10 nm SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm SCA...Pt Arena to Pt Reyes 0-10 nm SCA...SF Bay from 5 AM
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