Weather Forecast: 4 April
Saturday 4 April 2009, 12:00 GMT
High pressure will continue to expand north today, dominating the weather conditions for the in-port races this weekend. In fact, the synoptic map is quite straightforward, with a moderate E'ly to ESE'ly pressure gradient developing around the top of the building high. In addition, the conditions on course will be heavily influenced by any thermal gradient.
Looking a little closer, any overcast will have a big impact on the thermal gradient. In the unlikely case that cloud cover lingers through the day, the breeze would remain out of the E, struggling to reach 10 knots. On the other hand, the more clearing there is, the more the thermal gradient will influence the winds. In the morning, any clearing will cause a more left shifted drainage flow. Then, look for a decreasing trend through the morning as the drainage flow fades. Around noon, the winds will be at their lightest as a convergence zone between the fading drainage and any developing sea breeze moves through the course area. The more clearing there is, and the more significant the midday lull is, the more right shifted the sea breeze will be when it fills. Through the late afternoon, look for a left trend as the sea breeze fades.
Forecast Discussion: A weak 3-6 knot NNE'ly drainage flow will fade into a midday lull, before building again from the SSE for the afternoon. The longer the midday lull, the more right shifted the winds will be when they fill. In this case look for a SE'ly flow to fill to between 10-14 knots mid afternoon, backing through the day. By the end of the afternoon the breeze will be nearly E'ly, likely dropping below 8 knots. On the other hand, stronger and more E'ly flow through the morning will indicate less variability through the day. This is a less likely scenario, but if it develops, you could see the winds veer slowly right (to ESE'ly) for the early afternoon peaking in the 10-12 knot range, before fading again and shifting left (E'ly) for the afternoon. If the winds in the Bay are too light, sailing outside the Bay will offer a better chance at a building ESE'ly flow through the afternoon. Outside the bay the winds could reach 15 knots.
Weather: Some lingering morning overcast, clearing through the day. In fact, there could even be a light morning shower; however, by midday the sky will be clear. In the afternoon there may be some cumulus development onshore. The high temperature is expected to be just shy of 30C.
Detailed Wind Forecast for (4/04/2009) for the in-port racing area:
Wind Speed Wind Direction (true)
Time(LT) Mean Range Mean Range
0900 05 03-08 060 000-090 . more N early, veering to E'ly
1000 04 02-07 100 080-120
1100 03 01-06 120 090-130 . midday lull, winds veer right
1200 03 01-06 125 100-140
1300 07 06-11 160 120-170 . filling, more right, more build
1400 12 09-15 155 130-170 . Peak gusts in the mid teens
1500 11 07-12 150 120-160
1600 10 06-11 145 110-150 . left shift towards ESE'ly
1700 10 06-11 135 110-150
1800 09 05-10 120 110-150
HEDGE: Forecast Confidence -Average.
1. There is very good model agreement with regard to the synoptic scale features; however, pinpointing the influence of the thermal breezes will give the locals the advantage. I have high confidence that the background flow will stay ESE'ly today, with a slight left shift from the morning drainage flow and a right shift from the afternoon sea breeze. The strength of the thermal gradient is the only question that remains.
2. The more clearing you have in the morning the more left shifted the morning breeze will be with the early drainage flow. In this case, it will take longer for the drainage flow to fade to light and variable, and there will be a more significant convergence zone as the sea breeze starts to develop. As a result you will see a longer midday lull on course.
3. The more overcast you have early, the more likely the E'ly pressure gradient will fill across the sailing area. In this case the winds will be lighter all day, staying closer to 10 knots, but they will also be more consistent.
4. With clearing for the afternoon, you can expect a building sea breeze. The more clearing you have (over the course and offshore) the stronger the thermal gradient will be and the more dramatic a right shift you can expect when it fills. Any cumulus clouds that develop onshore will also be an indication of a stronger more right shifted thermal. This is the best case scenario for winds that build over 12 knots in the course area. In this case you could see the winds peak just shy of 15 knots, before they start to back left and decrease as the thermal gradient fades.
5. If you were to sail outside the bay, the winds will be stronger (near 15 knots) and there will be a more stable ESE'ly flow. As the thermal gradient fades through the afternoon, more of this breeze may fill into the bay.
Forecast for Sunday: Very similar sailing conditions, with mostly clear skies. Once again the background gradient will be E'ly but any developing thermal will heavily influence the winds. If anything the winds are expected to be slightly lighter and more right shifted. Look for a high temperature near 30C.
- Jennifer Lilly, Race Meteorologist