Posted: 9:04 am Saturday, September 9th, 2017
By Kirk Mellish
Once a firm trend becomes established in weather data the trend is your friend in forecasting. That was the biggest problem with Irma from the beginning, 5-day forecast tracks have been accurate in the Atlantic, but there was never a solid trend for the extended range because of doubt on when and where the North turn would occur.
That’s why the 5 day and beyond track projection kept shifting east and west east and west and back again vacillating all through the week.
I’ve been pointing that out in my blog posts since the start, but it didn’t sink in to some news people and some of the public I guess. I also pointed out how more of a move into Cuba than the NHC was expecting could change things, and it did.
The terrain of Cuba help weaken at least for now the storm and alter it’s path.
Even forecasts considered good/accurate have AT LEAST a 20 mile margin of error one direction or the other, same thing is true of snow/ice maps as long-time readers of mine know.
Once a definitive new movement is established the consistency and accuracy that was seen earlier in Irma’s life should resume, that should happen soon. It could always trend even more to the West sparring more of Florida the worst.
So as people become hysterical in Atlanta and the rest of Georgia I will repeat myself for the millionth time, the normal and expected average track error beyond 3 days grows to hundreds of miles. That is the cone of uncertainty which forecasters constantly show and harp on but people choose to ignore for some reason. In addition, intensity forecasts are notoriously off the mark especially beyond 48 hours.
Trouble to Atlanta’s South, trouble to our East:
The impacts of Irma will be quickly ramping up from south to north at the beginning of the long-term period. With recent model guidance favoring a slight westward shift in the track, much of the area will likely be along and to the east of the center of what will then be a weakening Tropical Storm/Tropical Depression Irma. This track increases the likelihood of more significant wind, heavy rainfall, and isolated tornado impacts in central and north Georgia during the Monday into Tuesday time frame. The most appreciable heavy rainfall, high wind, and severe weather threat will begin to increase across central Georgia Monday morning and work northward through the day Monday.
Preparations for should continue over the next couple of days as it appears increasingly likely that significant wind and rainfall impacts from Irma will affect much of the area. Continue to monitor the latest forecasts through the weekend as expected impacts are refined and forecasts are updated and refined. Forecasts can do and will change. Irma`s remnants will largely lift north of the area by late Tuesday and continue to weaken; however, some lingering unsettled weather remains possible into late week as the remnants do not appear to fully exit the Tennessee Valley until next weekend.
I am currently expecting winds in rain squalls for Atlanta to average 20-35 mph but with some gusts to 45 mph by late Monday then to 55 mph or higher gusts at night or early Tuesday morning, diminishing to 20-30 Tuesday afternoon, with 2-4 inches of rain over a 48 hour period on average with isolated 6 inch totals possible. The risk of a tornado will also increase to the east of Irma’s track. Again, REMEMBER, this is based on the current path and intensity projection from NHC, BOTH are subject to change. So the local forecast is, too.
But as of now anyway, at least some areas of flooding and wind damage can be expected.
The Guikema Research Group has an experimental model to ESTIMATE power outages, use with caution:
It hit Cuba last night
Irma will end the longest “Hurricane drought” for Florida (direct hit) since 1851! Something like 4339 days without a direct landfall in FL.
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