Weather Forecast for 15 August 2009

SAILING WEATHER FORECAST
VERVE CUP OFFSHORE
Issued: 0645 LT SATURDAY, AUGUST 15, 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.

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REVIEW OF YESTERDAY: The day started out more or less as predicted with a light SSW/SW offshore breeze backing into the SSE/SE as a sea breeze on all course circles. Winds slowly increased and filled through the late morning. However from about mid-day onward, the wind became more unsettled than expected. The reasons for this were two-fold:
1) Being later in the season, the lake is at or near its seasonal maximum temperature, nearing the mid-70sF (Lake Temperture Plot). Yesterday inland temperatures where mid-/upper-80sF. That is enough temperature contrast to support a lake breeze, but not nearly as strong a thermal difference as one would observe late spring/early summer. Therefore, a weaker thermal component.
2) The gradient was more right (more SW than SSW) and slightly stronger than expected. This meant that we would need more thermal effect to offset the breeze and, as described in (1), that was not the case.

As a result of these two factors, the lake breeze falter and veered right as a convergence zone moved back out off the shore and into the course area – First on southern Course B, then onto middle Course C. It seems that northern Course A got the best of the Lake breeze – both in persistence and strength.
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TODAY’S SYNOPSIS: We will once again get a lake breeze today, but like yesterday, its eventual strength and persistence will depend largely on the power of the gradient relative to the late-season thermal component.

The weather map shows high pressure over the northeastern US, moving very slowly to the east. Low pressure continues to deepen and organize over the upper Plains/Dakotas. There is a developing S/SSW gradient between the eastern high and western low. An upper level ridge is protecting the Great Lakes from cloud or rain impacts from the western low.

Thermal conditions will be excellent today and the gradient should support a SE sea breeze. The question will be whether or not the gradient overwhelms the lake breeze. At the moment, it appears that a slightly weaker gradient than yesterday and a subtle left trend in the gradient direction will make it easier for the lake breeze to arrive and hold most of the afternoon. It will however, be a very close call and any increase in the gradient could pose problems for the lake breezes holding.

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 reported ranging from 3-6 knots along the shore to 10 knots offshore. There has been a slight left trend and easing over the last couple hours.

The satellite and radar are clear. At the moment, clouds associated with the low pressure out to the west are evaporating before getting to northern IL.

FORECAST DISCUSSION: S/SW winds this morning around 3-6 knots at the lake front and up to 10 knots offshore, but easing. Backing breeze by 1000 all areas, becoming SE late morning/mid-day and slowly becoming less patchy with a lake breeze developing. Continued backing to SE/ESE through the early afternoon along with a building breeze. Lake breeze could increase to 8-12 knots during the afternoon.

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

WEATHER: Mostly sunny and hazy again today. Like yesterday, a few minor cumulus will develop on the lake front, perhaps slightly more vertically developed than yesterday, but still not heaping clouds. Some high altitude debris cloud may spill over the area in the later afternoon.

TEMPERATURE: Similar temperatures to yesterday but with more humidity. Warming up to the mid-/upper-80s by mid-day, then cooling to mid-70sF on the afternoon lake breeze.

SEAS: Flat water this morning. A slight chop around 1-2 ft developing in the afternoon lake breeze.

Detailed Wind Forecast for (15 AUGUST 2009):
Time Wind Speed Wind Direction
(LT) Mean Range Mean Range
0900 06 03-08 200 185-230 …some puffs of SSW wind to 10 knots
1000 04 02-06 185 170-190 …easing with a generally left trend
1100 05 02-07 160 140-180 …lake breeze onset; could be delayed
1200 07 04-09 140 125-150
1300 09 07-11 130 115-140
1400 10 08-12 120 105-135 …should be holding/building lake breeze
1500 10 08-12 110 100-125 …watch for any softening bringing righty
1600 10 08-13 110 100-125 …stronger, more right north course
1700 09 07-12 120 105-135
1800 08 06-11 135 120-150

HEDGE: 1) Forecast Confidence: AVERAGE. Models are in good agreement and where virtually unanimous in going with a lake breeze developing by late morning and then building to hold all day. If the lake breeze is able to get going, it should be a little stronger and steadier than yesterday.

2) Assuming the lake breeze kicks in to the SE/ESE this afternoon, watch for the following difference in direction and speed between the race courses:
a) North Course A will have a slightly more right-shifted and stronger lake breeze once it fills.
b) South Course B a filled lake breeze will be more left-shifted (E/ESE) and lighter on average compared with the north.
c) Middle Course C will be split between these two winds.

3) In the event the lake breeze does not develop as well as forecast and/or battles with a more SSW gradient this afternoon, this battle will probably be most dramatic on southern Course B once again. Any softening of left-shifted lake breeze could indicate a possible return onto the course of warmer, right-shifted gradient air. Such a change would make its way up from south to north through the course areas, with northern Course A probably holding onto any left-shifted lake breeze longer.

4) All in all, South Course C has the potential for the most unstable breeze and biggest transitional shifts between land breeze and lake breeze.

5) Once the SE/ESE lake breeze is established, it should be a fairly filled and steady breeze. Any patchiness to the pressure will be a good indicator that the wind may be going lighter and you need to watch for possible gradient influence, especially on the southern course area.

6) Hazy again today, but there should be some cumulus developing in the thermal over the land from late morning through afternoon. I believe they will stay onshore today. However keep an eye on them. If there is any move by the clouds onto the course area, you’ll need to watch for possible gradient return.


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SUNDAY (16 AUGUST 2009): This promises to be a tricky day with a gradient vs lake breeze battle possible. At the moment, I’m favoring the gradient, but it could be a very puffy, shifty and unstable offshore wind. The gradient will veer more SSW tomorrow and build in strength as the upper level ridge breaks down. While there will be some backing possible with thermal effect late morning through early afternoon, expect building gradient and mixing to push right-shifted, warm gradient air back out onto the race course in the afternoon.

Detailed Wind Forecast:
Time Wind Speed Wind Direction
(LT) Mean Range Mean Range
1000 10 08-12 200 195-215
1200 08 06-11 180 160-200 …easing and backing with thermal
1400 11 08-14 185 165-200 …thermal wanes; gradient kicks in
1600 14 10-16 190 170-205
1800 15 12-17 205 185-220

Weather: Mostly sunny and hazy in the morning. A few high clouds. Clouds will likely increase from the west late morning through afternoon. There is a chance of afternoon showers and thunderstorms moving toward the Lake. At the moment, it appears that most of this action will hold off until after is sailing is done, but it could be close.

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