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    Elliot Abrams

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    Storms to Miss the Northeast

    April 4, 2012; 7:22 AM ET

    Wednesday 10 AM

    Today's video outlines what we might expect from today through early next week. Even though you may be at just one place, being able to see what is forecast nearby can give you a sense of how much concern you could have about alternate outcomes.

    I sometimes refer to the "GFS Ensemble Means." So, what does that mean? The GFS is a National Weather Service product that goes out 16 days. I often show maps of what the operational version of that model is predicting.

    However, the model is run multiple times, each time starting with slightly different data. The justification for using slightly different data is the realization that we cannot sample the atmosphere for all points. This means the data we do collect can only be considered as very close approximation.

    Since the equations of motion for the atmosphere are second-order nonlinear differential equations, they really have no one exact solution. In fact, by running the model many times, there is a chance that some extreme event will be generated... a feature that might never be picked up or considered if the model was run just once.

    The ensemble mean is what you get if you blend all of the different runs together to obtain just one solution. The mean may do a better job of showing us the most likely outcome for the general flow. This is its strong point. However, in generating the mean solution, potentially useful outliers are obliterated. This may be its weak point. On some charts, an attempt is made to include the outliers as in various colors and shades that represent the amount of spread in the set of solutions from which the mean was derived.

    Anyway, one output from the ensemble means is a table of predicted values for things like temperature, relative humidity, amount of precipitation, etc. I am not going through all of those now. What I will point out is if you look at the leftmost three columns, you see day names, times and dates. The day names and times are keyed to Greenwich Mean Time, which is 4 hours ahead of Eastern Daylight time. This means the first entry, Wednesday 00z 04 April. is actually 8 p.m. Wednesday April 3 if you live in the East (7 p.m. central, 6 p.m. Mountain... check local listings, as they say). The next column has Celsius temperatures that are forecast at 6-hour intervals. The prediction for 18Z is the one closest to the daytime high, and the 12 z forecast is closest to the overnight low. As you scan the list (which is for NYC, as stated on the top line), you realize that the temperature forecast for 2 p.m. today is the highest reading listed April 14! The 850 temperature column is a forecast for the temperature at an altitude of about 5,000 feet. In winter it will often snow when the 850 temperature is below -2 or -3.

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com


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