From North Sails
Sailing Weather Service Forecast
Sponsored by North Sails & Southern Spars
2010 Three Bridge Fiasco Race
Issued 1200 LT 29 January 2010
NOTE: This is an OUTLOOK FORECAST. Conditions will likely evolve and change overnight and before the start tomorrow morning. Please make sure you check local observations and official National Weather Service forecasts before setting out tomorrow morning in case conditions have change significantly.
WARNINGS AND ADVISORIES: At the time of issuing this forecast, there are no active warnings of advisories for the sailing area. Please monitor official forecasts, warnings, and advisories from the National Weather Service via your VHF or on the web at:
http://www.wrh.noaa.gov/mtr/
FEEDBACK: Unlike our normal forecasting practice, the race organizers have asked for this forecast to be sent early – the day before your regatta starts. Normally, we strive to make the best possible forecast for race day using the most up-to-date information we can. However, that final update will be your responsibility tomorrow morning before the start. We are interested in improving all our forecast and analysis products, so we respectfully you send any feedback to
[email protected]. Thanks in advance.
SYNOPSIS: Low pressure will continue toward the Pacific northwest pushing an associated cold front through SF Bay this evening. This will bring rain moving from north to south across the area overnight. The rain is presently forecast to clear the bay by mid or late morning tomorrow, but there may be a few showers still lingering. As the front moves east tomorrow, high pressure is expected to build over the eastern Pacific and ridge north-northeast into northern CA and OR.
A SW gradient ahead of the front today is forecast to shift to the W/NW tonight as the front passes through. The high pressure building offshore will cause a pressure gradient as the barometer rises across SF through tomorrow afternoon. With flo w generally from high pressure to low pressure, an onshore wind is expected by morning perhaps building to moderate strength in the favored areas east of GG Bridge in the morning.
By tomorrow afternoon, the barometric rise will be more to the north of SF and this will veer the gradient right to NW and possibly even N by later afternoon and evening. This will tend to veer winds right during the day, with a more aggressive right shift expected Saturday night and Sunday. Also, the wind will likely weaken with slacker pressure gradient developing in the afternoon and overnight.
FORECAST DISCUSSION: Winds becoming WSW in the early morning and building to moderate strength (9-14 knots) from GG Bridge to Alcatraz and Treasure Island. The north bay will likely hold lighter and more left SW at 5-10 knots. Some holes of no breeze can be expected in the lee of Angel Island and north.
During the early afternoon, a very slight right trend is indicated in the south-central Bay to W, while SW/WSW winds hold elsewhere. Little change is expected in the wind speed, though a peak is expected 1100-1400 across the southern part of the race area. This peak MIGHT approach 15 knots, but could very well be lighter.
From about 1400 onward, winds will be easing and may veer more aggressively right, especially in the northern part of the racing area. Wind speeds will likely decrease under 10 knots and may be less than 5 knots in some areas by later afternoon/evening.
WEATHER: Mainly overcast in the morning with a few mainly light showers lingering past sunrise and into the mid- and possibly late morning. After 1000/1100, the risk of rain should diminish and the clouds may brighten and break to partly cloudy conditions during the afternoon. Clearing will be more likely on the inshore (Berkley) side of the bay while overcast may persist elsewhere.
TEMPERATURE: Seasonably cool. Generally in the upper 40sF to near 50F in the morning. Slow warming up to mid-50sF in the afternoon.
SEAS: Waves will be ma inly 1-2 ft on the bay from Golden Gate to Alcatraz and Treasure Island. From Angel Island north, generally 1 ft or less.**
Detailed Wind Forecast:
*** SOUTHERN ZONE – GG BRIDGE to ALCATRAZ to BAY BRIDGE ***
Time* Wind Speed Wind Direction (MAG)
(LT)* Mean Range* Mean Range
0900** 11* 08-14* 255* 240-265 …maybe lighter at first
1000** 13* 10-15* 255* 240-265 …possible build filling west to east
1100** 14* 11-16* 255* 245-270
1200** 13* 10-15* 260* 250-270
1300** 13* 10-15* 260* 250-275
1400** 12* 09-14* 265* 250-275 …watch for easing if breeze veers
1500** 11* 09-13* 265* 250-275
1600** 10* 08-12* 270* 255-280 …trending lighter
1700** 10* 07-12* 270* 255-280
1800** 09* 07-12* 270* 255-280
*** NORTHERN ZONE – ANGEL ISLAND to RICHMOND ***
Time* Wind Speed Wind Direction (MAG)
(LT)* Mean Range* Mean Range
0900** 06* 04-08* 235* 220-250 …very light and patchy in spots
1000** 07* 05-09* 240* 225-255
1100** 08* 06-11* 240* 230-255 …some modest fill, but still patchy
1200** 11* 09-13* 240* 230-255
1300** 11* 09-13* 250* 235-265 …max breeze; holes in lee of islands
1400** 10* 08-12* 260* 245-275 …significant veering trend POSSIBLE
1500** 09* 07-11* 270* 255-290
1600** 09* 07-11* 275* 260-295 …easing again and veering right
1700** 08* 06-10* 280* 265-300
1800** 07* 05-09* 290* 275-310
HEDGE:* Forecast Confidence: BELOW AVERAGE.
1) This is a complex forecast for a complex venue and a complex race. Unfortunately, the cold front crossing the area will leave a marginal gradient of onshore flow in its wake. With building high pressure offshore of SF, it’s a near certainty that winds will be trending onshore in the morning and through much of the afternoon. However, the various forecast models are not consistent on how strong this onshore flow will be. I’ve played an average to slightly stronger than modeled forecast for winds speeds. For wind direction, the flow will be generally onshore from the WSW/W, perhaps trending right through the day. However, with the uncertainty, confidence in the overall forecast package is lower than normal and some variations should be expected, especially in the area north of Angel Island and up to Richmond.
2) Of course, the geographic effects of the bay are critical and will be important. Winds are generally channeled through the bay, with winds more right-shifted downwind of GG Bridge, and more left shifted into the northern bay area from Angel Island to Richmond. The exception may be with a more right-shifted wind in the northern-most area mid-/late afternoon.
3) If I have a concern, its that winds could be much lighter than forecast. At the moment, it looks like lighter winds are more a risk than stronger winds. So you may want to hedge your preparation toward a lighter air race if anything. Still local variations could create zones of stronger than forecast wind speeds.
4) There may be some winds impacted by showers in the morning, but it looks like the rain action should be more or less completed by start time. So this won’t be a worry. Otherwise, cloud effects should be minimal. If anything, the possible breakup of cloud cover may lead to lighter winds.[Image] [Image]
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