8/14 Weather Forecast

SAILING WEATHER FORECAST
VERVE CUP OFFSHORE
Issued: 0645 LT FRIDAY, AUGUST 14, 2009

WARNING AND ADVISORIES: At the time of issuance, there were no warnings or advisories posted for the racing area. See the Chicago Regional National Weather Service Office web page (link). When on the water, monitor NOAA All Hazards Radio on your VHF for current 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: Once again, lake breezes are expected to be the main wind maker for today, though S’ly component gradients are showing some signs of increasing compared with yesterday. This trend of increasing S/SSW gradient is forecast to continue right through the weekend and will play an escalating role in the forecast.

This morning’s weather map shows the high pressure which moved across Chicago yesterday is now centered over the eastern Great Lakes and continuing to move gradually to the east. Low pressure is over the northern Plains region – rather disorganized but working with the departing high to initiate a strengthening S/SW gradient from the Dakotas east to Lake Michigan.

Mainly sunny conditions should supply ample thermal input for a lake breeze.The lake breeze will develop on a local scale first, initially filling from the SE late morning and holding into the early afternoon. This lighter sea breeze may fade for a time early afternoon before a left-shifted fill arrives from the E with a modest increase in pressure mid-/late-afternoon. Winds will ease off quickly toward evening once the thermal dies away.

One side note – There is a complex of thunderstorms over central WI this morning that is moving slowly to the east. This area of thunderstorms is more or less isolated to WI and is not showing any signs of expanding south toward IL. Cloud and rain associated with this complex will remain well north of IL and is not a concern. There may be a few fragments of debris cloud caught in upper level winds which move across the area today, but even this is not expected to be of significance.

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

CURRENT CONDITIONS: Reports show winds 0-3 knots at the coast, increasing offshore to 5-8 knots from the S/SSW. There has been little change over the past few hours.

Satellite shows considerable cloud over WI with radar confirming thunderstorms over northern and central WI and scattered showers over southern WI. This activity is moving generally to the east and is not a threat to Chicago. There are a few fragments of middle and high level debris cloud just brushing the northern Suburbs, otherwise it is mainly clear over Chicago, northern IL, and southern Lake Michigan.

FORECAST DISCUSSION: Light S or SW winds are forecast this morning – mainly less than 5 knots and practically calm along the immediate coast. Expect light and variable conditions through mid-morning, but a light S’ly around 5 knots may hold a few miles offshore. Lake breeze will start developing and backing breeze late morning, very light at first, but filling to 4-7 knots through mid-day. Could be a little stronger, up to 8 or 9 knots, but doubt it will. Hold. In the early, expect a fading SE lake breeze and becoming patchy for time. Between 1400 and 1500, a refreshed lake breeze is expected to build from the left and increase to 7-10 knots – possibly up to 12 briefly through late afternoon. The E lake breeze will die quickly toward evening.

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

WEATHER: Mostly sunny except for a few minor fragment of high cloud mainly north of the area this morning. A few small cumulus will develop just inland on the lake front late morning/mid-day, moving inland with time as the lake breeze develops.

TEMPERATURE: Warmer than yesterday. Up to the mid-80s by mid-day, then cooling to mid-70sF 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 (14 AUGUST 2009):
Time Wind Speed Wind Direction
(LT) Mean Range Mean Range
0900 04 00-06 180 165-200 …lightest at the beach
1000 03 01-05 180 160-190
1100 04 02-06 Backing to SE …initial lake breeze fill
1200 07 03-07 130 085-150
1300 06 05-09 125 080-140
1400 05 03-08 110 070-125 …fading breeze before left shift
1500 09 07-11 095 080-110 …left-shifted lake breeze with pressure
1600 10 08-12 090 075-105
1700 09 07-11 090 075-110
1800 07 05-10 095 080-110

HEDGE: 1) Forecast Confidence: AVERAGE. Reasonably good agreement among the various forecast models today. Most uncertainty revolves around whether or not the gradient will allow the lake breeze to build a little more than yesterday. Also, like yesterday, expecting an initial SE fill with the “local” lake breeze followed by a left shift and increase with the “lake-scale” lake breeze. Timing of this shift is not certain. I have put it a little later than yesterday to account for more momentum in the gradient holding it out a little longer. However, if the gradient is weaker or less important, could see the left shift and build earlier in the afternoon.

2) Changes between yesterday and today are subtle, but it will be a little different on the race courses today. Expect a bit more S or even SSW fill through mid-morning. This will back fitfully to the SE late morning through mid-day.

3) That peak of 12 knots at 1600 is probably optimistic! Realistically, doubt wind will top 10 knots today. However there is an outside chance that the gradient will help this lake breeze along a bit and help it build over 10, albeit briefly.

4) Expect a generally left-trending lake breeze from late morning through early afternoon. That left shift to the E noted in the forecast table could come through abruptly along with some pressure. I expect you may see it coming on the water if indeed the SE’ly goes light. There may be an initial push of good pressure with that shift, but look for it to ease off in the late PM.

5) Clouds will not be significant today. There shouldn’t be much other than a few cumulus onshore marking the development of thermals to support the lake breeze.


****************************************

SATURDAY (15 AUGUST 2009): Continuing a slow increase in the gradient strength from the SSW tomorrow. This will have the dual effect of delaying the onset of the lake breeze by an hour or so. But in the end, the lake breeze will probably be a little stronger and hold more from the SE/ESE rather than seeing the left shift to the E like is forecast today. Peak lake breeze speeds could easily get up to 12 knots and maybe slightly more.

Detailed Wind Forecast:
Time Wind Speed Wind Direction
(LT) Mean Range Mean Range
1000 04 02-06 200 175-215
1200 07 05-09 140 120-155
1400 10 08-12 125 110-140
1600 11 09-13 120 105-135
1800 10 08-12 125 110-140

Weather: Continued mostly sunny and warm. Becoming more humid and hazy. Cumulus will develop onshore late morning and afternoon. It should remain mainly clear over the lake.

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

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

SUNDAY (16 AUG 2009): This is looking like the trickiest day in terms of the weather forecast and perhaps on the water activities. The low pressure and attendant fronts over the northern Plains will be moving closer bringing a stronger SW gradient to the racing area. In addition, it will be quite warm and mixing of stronger momentum air to the surface may hold that lake breeze out offshore.

Expect a sheared and puffy start to the day, with SSW/SW winds increasing through the morning to 10-15 knots. Quite gusty. It will go light a few miles offshore with the lake breeze working hard to turn the breeze to the SSE/SE. I think the Lake breeze will eventually get into the sailing area, but it could be a fitful battle between right-shifted gradient pressure and left-shifted lake breeze pressure. Late afternoon surging of the gradient may assure an early end to the lake breeze.

Mostly sunny at first, then increasing cloud onshore in the afternoon. Some high clouds may overspread the area with time. There is a slight risk of showers/thunderstorms in the afternoon, but most action should hold out to to the west in the afternoon.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.

Learn more at sailwx.com and windpower.sailwx.com