8/13 Weather Forecast

Dear Sailors:

As you know, today is the first day of the J109 North Americans being held at the Offshore Verve Cup Regatta. The following weather forecast was prepared by Sailing Weather Services for today, and includes an outlook for the rest of the weekend.

See you all tonight at the skippers' meeting!




SAILING WEATHER FORECAST
VERVE CUP OFFSHORE
Issued: 0700 LT THURSDAY, AUGUST 13, 2009

WARNING AND ADVISORIES: At the time of issuance, there were no warnings or advisories posted for the racing area. See the Chicago Regional Nation Weather Service Office web page (link) and monitor NOAA All Hazards Radio on your VHF for up to date OFFICIAL warnings and weather advisories.

NOTE: Your observations and notes regarding actual on-the-water weather are extremely important for improving future forecasts. Please send forecast feedback to cbedford@sailwx.com.

SYNOPSIS: A beautiful weekend coming up, weatherwise. High pressure is building both at the surface and aloft, with Chicago expected to be on the western periphery of the associated sunny and warming weather pattern. At present, it appears that the lake breeze will be the dominant wind-maker, though some gradient may mix in to enhance the breeze over the weekend.

On this morning’s weather map, high pressure is located over northern IL and moving slowly east. Gradients across the area are essentially slack, so the best forcing for wind will be the developing thermal differences between heated land and cooler lake waters from later this morning through the afternoon. With little or no gradient assistance, even the lake breeze is expected to be weak today – but it certainly does appear that it will develop enough to get sailing in.

There is a minor upper level trough over eastern Iowa this morning that we’ll be watching closely. There are some isolated showers and thunderstorms associated with this feature that are slowly moving toward the east. At the moment, the most impact expected from these storms are some debris cloud moving overhead this morning through early afternoon. The storms should dissipate this morning and any threat for rain will stay over northwestern IL and southern WI. Still, will monitor the radar this morning in case there are any surprises.

Click the following for useful images related to the Synopsis:
Close-up Satellite Loop
Regional Radar Loop
Surface Weather Map

CURRENT CONDITIONS: Mainly calm winds reported around the area. Some random puffs of 2-4 knots at times with no consistent direction reported. Clear conditions. Some high cloud is off to the far west and radar shows dying showers/thunderstorms just morning out of eastern IA into NW IL. These should be gone in the next couple of hours.

FORECAST DISCUSSION: Light and variable winds through mid-morning. By 1000 or 1100, hints of lake breeze are expected – developing at the shore first, then slowly filling out into the lake with time. E/ESE lake breeze filling slowly late morning through early afternoon perhaps up to 5-8 knots for a time. In the middle afternoon, there is a chance that the lake breeze will ease and trend left, perhaps 4-7 knots with mainly E or possibly ENE. In the late afternoon, more right trend to ESE and a recovery in speed up to 5-9 knots is possible.

Useful model graphics and forecast animations can be found at the SWS Verve Cup weather data web portal (link).

WEATHER: Mostly sunny at first. Some scattered or perhaps broken middle and high cloud will overspread from the west/northwest mid-/late-morning. This cloud should be minor and will thin and clear mid-day through early afternoon. Otherwise, watch for cumulus developing over onshore mid-morning through afternoon. The cumulus will push inland with the lake breeze. It should remain clear and sunny over the water.

TEMPERATURE: Warming to around 80F through late morning, then cooling to 70-75F on the afternoon lake breeze.

SEAS: Flat water this morning. A slight chop around 1 ft developing in the afternoon lake breeze.

Detailed Wind Forecast for (13 AUGUST 2009):
Time Wind Speed Wind Direction
(LT) Mean Range Mean Range
0900 02 00-04 Variable
1000 03 01-05 Variable to 090-120 …developing lake breeze
1100 04 02-06 100 085-115 …building close to shore first
1200 05 03-07 100 085-115
1300 07 05-09 095 080-110
1400 06 04-09 090 070-105 …slight chance easing with left trend
1500 05 03-07 080 065-100
1600 06 04-08 090 070-110 …building if trending right
1700 07 05-10 100 080-115
1800 06 04-09 105 090-120 …easing quickly toward evening

HEDGE: 1) Forecast Confidence: AVERAGE. There shouldn’t be any big surprises this today. Models are in excellent agreement with respect to the forecast of the major weather features this morning. There are some differences in the lake breeze forecasts, with peak wind speeds between 5 knots and 11 knots among the model suite. Also, some models are quite steady with a E/ESE lake breeze while others are oscillating between ESE and ENE. In the forecast table, I have hedged toward the lighter side of the model spread for wind speed and played a median for the wind direction.

2) Lake breeze could develop a little more quickly this morning and, if so, a couple knots of extra wind speed may be added to the forecast table through early afternoon.

3) This will be a “fetch limited” lake breeze – decreasing in strength with distance from shore. Odds for strongest wind will generally be within 1-2nm of shore. Wind speeds will decrease offshore and may back slightly left on direction.

4) The easing with left trend in the mid-afternoon is not a sure thing. In general, the more the breeze stays on the right side of the wind direction range, the stronger it will hold. If the sea breeze backs left, the more likely you will see some easing pressure on the race course.

5) Not expecting a lot of cloud today. Cumulus developing onshore will indicate the thermal component. The cumulus should stay over the land and not venture offshore. Clear conditions offshore and cumulus cloud over the land are good news for the lake breeze. In the unlikely event any cloud moves toward the lake and out over the sailing area, it won’t last long, but it would briefly soften the lake breeze.

6) As mentioned in the synopsis there is a minor upper level trough out to the west which should be monitored to avoid any surprises today. At the moment, it is behaving as expected and weakening/washing out as it moves east into northwest IL. All I expected are maybe a few fragments of debris cloud moving overhead – but even that should be evaporating with time. The most likely impact may be to slightly weaken the thermal component today. There is only the slimmest of long-shots that any showers/storms over eastern IA at present will hold together to impact Chicago.


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FRIDAY (14 AUGUST 2009): Excellent weather and light winds again tomorrow, though slightly more favorable lake breeze conditions are expected. High pressure moves to the eastern Lakes allowing a weak SSW gradient to form over the racing area. Thermal conditions will support a lake breeze from late morning through afternoon – probably slightly stronger and more right on average than today.

Detailed Wind Forecast:
Time Wind Speed Wind Direction
(LT) Mean Range Mean Range
1000 04 00-06 SSW > SSE
1200 07 05-09 135 115-150
1400 08 06-10 130 115-145
1600 06 04-08 125 110-140
1800 08 06-10 130 115-145

Weather: Mostly sunny. Some cumulus develop onshore during the morning and stay inland all day.

Seas: Calm in the morning, becoming light chop on the afternoon lake breeze 1-2 ft.

********** EXTENDED FORECAST ***********

SATURDAY (15 AUG 2009): Continued fine weather. High pressure will be holding strong over the eastern Lakes and NE US. A ridge will hold back across the central Lakes. Barometric pressure falling way out to the west will effectively increase the S’ly pressure gradient component. It will still be good for thermal conditions to develop and a slightly enhance SE lake breeze is forecast by Saturday afternoon.

Morning winds from the SW at 3-6 knots will back left to the SE through early afternoon and build to 8-11 knots. Outside chance it could be stronger for a time. Then easing slightly in the mid-/late-afternoon down to 6-10 knots before a final push after 1700 back up to 10 knots.

Mostly sunny except for cumulus over the land. Slight chance of some high cloud late in the day.

SUNDAY (16 AUG 2009): High pressure moves further east into the NE US and the next trough starts to shift east toward the western Lakes. Any threat of weather associated with the trough should hold well to the west of Chicago. The main impact will be to further enhance the S’ly gradient component and offshore moderate SSE winds in the afternoon.

Morning S/SSW at 5-10 knots, backing to SE and increasing to 8-13 knots with and outside chance of stronger early/mid-afternoon. Winds veer toward the SSE through the afternoon.

Mostly sunny at first, then increasing cloud onshore in the afternoon. Some high clouds may overspread the area with time. Hot.



--
Chris Bedford, CCM
Chief Meteorologist
Sailing Weather Service, LLC
T: +1 888 4SAILWX (472 4599) ext. 100
E: cbedford@sailwx.com

Sailing Weather Service. We know wind.