WRAL WeatherCenter Blog

Rain from Ida?

Posted November 9, 2009

Forecast patch of Hurricane Ida issued 7 am 9 Nov 09, from the National Hurricane Center web site. Open circle at the final forecast point near the AL/GA/FL border indicates an expected transition from tropical to extratropical characteristics.

After a stretch of weekends with less than ideal weather, we really turned things around with a beauty just behind us, with deep blue skies, a few scattered cirrus clouds, and temperatures that topped out at 66 on Saturday and 76 Sunday, all thanks to a departing high pressure center over the Atlantic that continues to extend back into the state from the east as we start the week today.

The forecast becomes more active, and much more difficult, with the approach of Hurricane Ida and associated moisture from the south, a cold front to our west, and a developing upper level trough also approaching front he west and northwest over the next couple of days.

Ida continues to move north across the Gulf of Mexico, but is already starting to weaken in the face of strengthening wind shear aloft as it closes in on the Gulf Coast, and may barely hold hurricane strength as it makes landfall Tuesday morning (first graphic, showing the NHC forecast path). Once it makes landfall, it will begin to interact with a frontal system that should rapidly change the storm from tropical to extratropical and take a right turn, but the scenario from that point on becomes quite complex, with a lot of variety amongst computer model projections and theoretical evolutions that could follow.

The spread in computer model solutions exists between different models and also between different runs of the same model with slightly altered initial conditions, and you can get a bit of a feel for that by looking at the second image I included, which shows 3 different runs of three different models, all of which are a subset of something called the Short Range Ensemble Forecast system, or SREF. All 9 panels are a projection of the pressure and precipitation pattern for 1 am Wednesday morning, and if you look closely you'll see a variety of outcomes regarding the location and intensity of low pressure at the surface, and how much rain has fallen in the 3-hours leading up to that time.

In a general sense, it appears the low-level circulation center of Ida will turn east and perhaps even southeast and weaken some as it is absorbed into the approaching cold front, but some upper level circulation will move northeast and become part of an approaching upper level trough. This may help initiate a second low pressure wave development along the South and North Carolina coasts, increasing north to northeast winds,  which could become quite blustery at times, and pulling some cooler air into the region.

The bigger question, given the moderate drought in place across central and parts of eastern NC, is how much rain this system will produce. As you see on the attached model panels, there is some small potential for anything from a very heavy rainfall event from Tuesday afternoon into midday Wednesday, to having the bulk of the rain shunted south and southeast of our viewing area. Right now, the most likely course of events would seem to be a pretty wet period from later Tuesday into early Wednesday, with rainfall totals on the order of .5 inch to around 2 inches for about the northwest half of our area to around 1-3 inches toward the southeast. This is pretty close to the estimate of rainfall totals for the area shown on the National Centers for Environmental Prediction 3-day rainfall estimate ending Thursday morning, shown as the third attached image.

So, be prepared in the next 2-3 days for anywhere from a little rain to the more likely scenario of quite a bit, along with a decline from high temperatures in the 70s today to 50s on Wednesday, and some stiff northerly winds that could gust as high as 25-35 mph later Tuesday night into Wednesday.

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  • pyrosinthesky Nov 18, 2009

    Looks like they nailed it. Good job, all.

  • miked6500 Nov 9, 2009

    Starting NOW! You can only forcast 3 hours ahead.