Tuesday 8/28 Update

Good afternoon everyone!  It's Isaac update time again.  With yesterday's weak cold front on to our southeast, what happens with Isaac is now the primary focus for our forecast heading through the rest of the week.  Isaac was upgraded to a hurricane just before noon today and looks healthier now than ever.

As of the latest update from the National Hurricane Center, hurricane force winds (74 mph or higher) extend out 60 miles from the center of the storm, with tropical storm force winds (39-73 mph) extending out 185 miles from the center.  Here's the latest track from NHC:   

The hurricane warning runs from Morgan City, LA, to the MS/AL border.  Storm surge could reach 6-12 feet in this area...rainfall totals of 7-14 inches are expected, with localized totals as high as 20 inches...hurricane & tropical storm force winds will last for a long duration (some locations near the center could deal with winds for 24-36 hours)...and isolated tornadoes could occur with storms north and east of the center.

Locally, I wouldn't be surprised if we see a couple of isolated showers from the outermost rain bands as early as Thursday.

Looking ahead, the most challenging part of the forecast for Isaac is what will happen as the storm moves inland, and thus, what impacts it will have locally.  It seems like forecast models are starting to line up on a solution...and unfortunately, if current trends continue, we may be missing out on the heaviest rains.  Above, you can see the official NHC track takes the remnants of Isaac over north central Arkansas by Friday afternoon, near St. Louis by Saturday afternoon, and near Chicago by Sunday afternoon.  However, nearly all of the forecast models are in agreement on a track much farther to the west:

Here's what some of the individual models are showing:

The 12Z run of the European model shows the center over western MO at 7AM Saturday, bringing some rain to SEMO & S. IL, but little to KY & TN. 

The 12Z run of the GFS above shows a very similar picture at 7AM Saturday.

By Sunday morning at 7AM, the two models are a little farther apart:

 European model shows the center around the MO/IL/IA border.

GFS at 7AM Sunday is a little farther to the northeast.

The fact that these models are trending farther to the west and north are making it very tough to get excited about our rain chances....and tough to nail down a forecast.  Based on these trends, it looks like SEMO & S. IL have the best chances of noteworthy rainfall, while we're gonna need an eastward shift in things to bring the heavier rains to KY & TN.  Here's a couple of rainfall estimates I've seen today:

This estimate is from NOAA, and is highly dependent upon the NHC track.  Personally, I think it's too far east based on what most models are showing us.  Here's the GFS estimate, which lines up better with other models:

With the uncertainty, I think it's still too early to give any specific rainfall estimates.  I know there have been some numbers floating around today, but I think we need to see another day's worth of model run agreement to start getting real specific.  Still think the Friday-Saturday time frame is best as far as timing...with rain clearing from west to east by Sunday.  I'll hopefully start talking about totals tomorrow.