Weather Forecast for Sunday August 16

SAILING WEATHER FORECAST
VERVE CUP OFFSHORE
Issued: 0620 LT SUNDAY, AUGUST 16, 2009

WARNING AND ADVISORIES: While no warnings or advisories are active at the time of issuing this forecast, there is a risk of thunderstorms with frequent lightning, brief wind gusts, and heavy rain this afternoon. See the latest official information on 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.

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REVIEW OF YESTERDAY: Winds on most race courses seemed to develop about as expected yesterday. Again, southern Course Area B had the most fitful breeze being much closer to shore and, therefore, closer to the convergence between lake breeze and gradient winds.

The shifts, lulls, and pressure seemed to correlate well will cumulus cloud moving offshore into Course B. In general, clouds were getting offshore just to the course area, then evaporating in the sinking air on the offshore side of the lake breeze. However, when clouds held together and/or were slower to evaporate, this indicated a more right-shifted influence from the gradient. Course B had directions ranging between 110 and 150 and pressure differences of a couple knots across the course.

Further to the north on Course Area A, the lake breeze was able to hold more consistency in pressure and direction – though it was still a bit shifty, especially in the bottom of the race track, closer to shore.
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TODAY’S SYNOPSIS: I suppose it was too much to ask for 3 near-perfect days of weather this year! Indeed, the weather for today will deteriorate and chances for a complete transition to lake breeze appear very low. A frontal system which has been developing out to the west of IL the last few days finally makes a very slow move to the east.

This morning’s weather map shows high pressure over New England and down to the middle Atlantic States. This high is shifting slowly east and south while gradually weakening and losing the control its had on the Chicago weather the last few days. More important for today is a low pressure area over south-central Canada with a cold front extending south through central MN and western IA. The cold front is forecast to move to the IL/IA border by this evening.

A well developed S’ly gradient is in place and building ahead of the front. This gradient will likely dominate today, though backing with some thermal effects may be noted late morning into early afternoon. Morning winds will generally increase and shift slightly right through 0900. Beginning 1000 or so, a left trend should be noted along with some lulls as a thermal component gets involved. Left tending breeze will last though 1300 before a stronger, right-shifted fill of gradient air returns for the balance of the afternoon.

Of course any showers/storms moving into the area from the west this afternoon will substantially impact local winds. These will tend to pull the wind right as outflow from the cells will work its way from SW to NE.

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

CURRENT CONDITIONS: Mainly S winds 5-12 knots reported this morning – lighter at the shore and windier offshore. Winds have been decreasing along the shore the last couple hours, but have been holding offshore. No direction trends have been noted recently.

Satellite shows considerable cloud out to the west over NW IL and IA. This cloud is spreading east with time – though appearing to thin as the parent rain cells are presently weakening.

There is rain and few embedded thunderstorms IA and just moving into NW IL, but for the most part these are moving northeast and not an immediate threat. They are also moving relatively slowly. The action forecast for this afternoon will be new cells developing southwest of Chicago in the daytime heating.

FORECAST DISCUSSION: Mainly S at 6-11 knots at first this morning. A slight veer to SSW/SW for a time mid-morning with winds increasing and becoming more puffy with average speeds 9-14 knots. By 1000, some left-trending shifts and lulls develop and persist through the about 1300. Winds becoming mainly S/SSE at 6-10 knots. Occasional right-shifted puffs into the mid-teens are possible, especially south course and the lakefront side of the middle and northern course. More left-shifted breeze may be found offshore. From 1300 or so, a return to right-shifted gradient will work up from south course to north course. Becoming S/SSW and building to 10-15 knots with some stronger gusts possibly coming off the shore.

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

WEATHER: Mostly sunny at first, but clouds will increase through the morning with a combination of developing thermal cumulus and higher altitude debris cloud overspreading the area from the west. There is a slight risk of showers or maybe a thunderstorm late this morning, however the highest risk will be this afternoon, mainly after about 1300LT. Note that some of the thunderstorms could bring briefly heavy rain, frequent lightning and some brief wind gusts.

TEMPERATURE: A warm start and more humid afternoon coming up. Mid-70s this morning warming to mid-80s this afternoon.

SEAS: Light chop mainly 1-2ft.

Detailed Wind Forecast for (15 AUGUST 2009):
Time Wind Speed Wind Direction
(LT) Mean Range Mean Range
0900 10 07-13 190 180-210 …becoming puffy and unstable
1000 08 06-12 185 175-200
1100 07 05-11 170 155-190 …left trend with thermal
1200 08 06-10 160 140-180 …more left possible Course A
1300 07 05-11 170 150-190 …south course first to get right shift
1400 11 08-15 190 165-205 …right shift and filling everywhere
1500 13 10-16 205 190-220 …watch for right-shifted rain cells
1600 15 12-18 210 195-225 …stronger gusts possible
1700 12 10-15 220 200-240
1800 13 10-16 225 210-240

HEDGE: 1) Forecast Confidence: BELOW AVERAGE. While the large scale forecast has fairly high confidence, it will be difficult to get the details of the forecast table correct today – therefore lower confidence is assessed to the forecast. At some point today, there will very likely be different breezes blowing on different course area.

2) Through mid-morning, look for right-trending puffs coming off the shore. By late morning, lulls will likely take the wind left as a thermal influence. The thermal left shifts may be more pronounced further offshore.

3) As a general rule today, expect left-shifts to be lulls and right shifts to be puffs.

4) The north course (Course A) area will have the best chance of seeing the maximum left with thermal lake breeze influence on the gradient. Even still, its unlikely to get as far left as the last couple of days and there course be some unstable right-shifts, especially after about 1300.

5) The south course (Course B) will have the largest and earliest exposure to the right-shifted gradient. This will probably be the least stable area with right shifted puffs moving across the course from right to left anytime after about 1200, but especially after about 1300.

6) The center course (Course C) will split the difference and could be more likely the south course today, with more right-shifted breeze and puffs.

7) The more cloud there is today, the more the breeze will tend toward the gradient direction. So, if debris cloud spread across Chicago earlier than expected this morning, the forecast left thermal may not arrive at all.

8) Watch for showers/thunderstorms this afternoon. I don’t think this will be a real risk until after about 1300/1400 this afternoon, but could be earlier. Any cells will tend to move from southwest to northeast and bring right-shifted gusts as they move out over the water. Hopefully, most of this will hold off until racing is done today!