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Old 10-12-2007, 09:45   #1
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how accurate are NOAA forecasts?

Greetings
I am hoping that someone here can help clear up some confusion I am having on NOAA marine forecasts.
Yesterday we moved our boat from Long Beach to Dana Point (a very short passage of under 30 miles). As always I checked the NOAA forecast everday for a few days prior to us leaving and in the morning prior to leaving our slip. The weather forecast consistently 10-15 knots, with swells of 4-6. We decided to put all of our sails up since it sounded like a nice easy sail. Boy were we surprised when a few miles out of the break waters of LB we were hit with 20-25 knots with gusts to 30 (this was at 9:30 in the morning). We were able to get some of the sail down fairly quickly but it really was a wake up call for me regarding the accuracy of the NOAA forecasts.
I have seen this happen one other time where NOAA was forecasting 15-20 as it was blowing 35-40 with gusts to 45 at the same time (they did change the forecast about 10 minutes after the winds picked up).
So my question is how accurate is NOAA at predicting wind forecasts? Is there a rule of thumb you go by (figure anything up to double what the forecast is and then play it by ear)? Is there weather information out there that is more accurate than NOAA?
Any information to help me understand this would be great
Jackie
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Old 10-12-2007, 10:40   #2
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Jackie,
I admit that I have often referred to NOAA weather as the "Comedy Channel". It often seems as though they would be better seved if they were to look out the window from time to time or at least listen to their own buoy reports when making forcasts. That said they are really pretty good... with some caveats. In SoCal we are blessed with fairly settled weather (see the recent posts about the storm in Oregon for some perspective). Nevertheless the weather offshore is often boisterous and the weather outside can become the weather inside at any time, which may be what you experienced. It is fairly common for the wind outside the shadow of Pt Conception to be twice as strong as the wind within the wind shadow. Also, this time of year you want to keep your antennae tuned to the possibility of strong NE winds when high pressure moves inland with a low offshore.
Bottom line is any weather report should be taken with a grain of salt and even in SoCal it is good policy to be prepared for a blow when setting out. But you knew that! You mention a "rule of thumb" and I guess mine is to expect the NOAA forcast to be pretty accurate but to prepare for what the offshore forcasts are calling for.
Hope this helps,
Mike
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Old 10-12-2007, 10:52   #3
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I sail out of Dana Point and I have found the NOAA forecasts to be wrong most of the time. We've gone out expecting 10-15 knots and 6 foot swell and ended up with the swell, but not much wind (1-5 knots). Yesterday, we scrubbed our sail because NOAA showed a 50% chance of thunderstorms. Looking out the window, we doubted that could be the case, but there's no reason to risk ending up in a T-storm for a day sail. Next time, I'll try calling the harbor and getting weather from them.
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Old 10-12-2007, 12:23   #4
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An interesting article on weather forecasting "accuracy":
Forecast Verification - Issues, Methods and FAQ
Forecast Verification - Methods and FAQ

Be certain to read (at least): What makes a forecast “good”.
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Old 10-12-2007, 12:55   #5
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I think the answer is in the name "FORECAST" it is a prediction based on gathered evidence, therefore it cannot be exact. As with swell forecasts the give a predicted average, I think the rule of thumb with swells is 20% either sideof the measurement given.
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Old 10-12-2007, 13:12   #6
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This is some really helpful information!
I really like what Mike said about being prepared offshore forecast even while sailing coastal.
I would agree that normally conditions are much less than what is being predicted (except on few occasions...yesterday for example).
The winds were coming out of the NE all day so that added to the discrepancy in the forecast no doubt.
By the way the seas were predicted 4-6 and I doubt we saw much more than 1-2 the entire trip (this is also probably due to the NE wind).
Jackie
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Old 10-12-2007, 16:36   #7
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Just download the WeFax files then look at the text forecasts. They seldom agree and that begins the tale. we did an informal chart of our trip one year down the east coast and charted the forecasts and then recorded the actual conditions. we gave them the benefit of the doubt and used only forecasts 24 and 48 hours out. In a full 85% of the time the forecasts was not only wrong but not even close. In many cases the broadcast current local conditions were not even close. Expect the best and prepare for the worst whenever you leave the dock and you will be OK.
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Old 14-12-2007, 05:50   #8
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METEOROLOGY SIMPLIFIED:

Get a 5-day forecast for any US zip code or city. ForecastAdvisor will also show you the accuracy of the major weather forecasters, including Accuweather, Intellicast, MyForecast, The Weather Channel, and the National Weather Service.
Goto:Weather Forecast and Weather Forecast Accuracy for Your City


METEOROLOGY SIMPLIFIED
:


1. A baroclinic zone should result in rising motion due to density differences. This will result in atmospheric condensation and thus precipitation.
TRANSLATION: A front will bring us rain

2. 6-sided crystalline aggregates are being gravitationally forced to the surface.
TRANSLATION: It is snowing

3. The horizontal pressure gradient* force is quite tight.
TRANSLATION: It is windy

4. Inflow and horizontal vorticity have been tilted into the mesocylone.
TRANSLATION: A tornado is forming

5. Condensation is occurring upon condensation nuclei as the parcel lifts, cools adiabatically and becomes saturated.
TRANSLATION: A cloud is forming*

6. Sensible heat and a high vapor pressure of water molecules are producing extremely high theta-E values.
TRANSLATION: It is hot* and humid

7. Latent heat release is producing bomb-o-genesis and an intensification of the thermal wind.
TRANSLATION: A Nor'easter is developing

8. Isentropic lifting is producing large scale synoptic uplift north of the warm front.
TRANSLATION: Clouds and precipitation are north of the warm front

9. CG 12 o'clock with towering Q.
TRANSLATION: Lightning struck in front of us from a developing thunderstorm*cloud

10. The line echo wave pattern is being fed by warm air advection and moisture convergence.
TRANSLATION: A line of thunderstorms is developing*

11. A veering wind in the vertical indicates warm air advection and synoptic uplift.
TRANSLATION: Warm air rises

Notes:
*3 Isotherms and isobars are even more important than their names sound.

*5 I am not sure how clouds get formed. But the clouds know how to do it, and that is the important thing. Clouds just keep circling the earth around and around. And around. There is not much else to do.

*6 It is so hot in some places that the people there have to live in other places.

*9 & 10 Thunder is a rich source of loudness. You can listen to thunder after lightning, and tell how close you came to getting hit. If you don't hear it, you got hit, so never mind.
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Old 14-12-2007, 08:55   #9
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That accuracy analysis is pretty cool. However, I will also say that if you predicted "precip" in Seattle this time of the year 100% of the time, knowing nothing else, your overall hit rate/false alarm rate would probably be almost as close.

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