Isaac Remnants to Impact Valley
Isaac is slowly moving through the Mississippi Valley as we head into the weekend, the storm system will start to move eastward and we will start to see the impacts on our area as early as Saturday.
Rain should be more spotty to start with as the outer bands of the storm make their way into the area.
But as we head into the end of the weekend and more into the Labor Day Holiday, more moisture will move into the area. The remnants of Isaac are also meeting up with a frontal system over the Great Lakes and that will help steer all that tropical moisture into our area.
Now that Isaac is no longer a Tropical system, he is becoming a post-tropical low, the National Hurricane Center has basically passed off any updates to the Hydrometeorological Prediction Center (HPC). Which is basically updating rainfall totals and flood threats. The Storm Prediction Center, as always, will monitor any tornado threats. As of Friday night there have been over 25 reports of tornadoes since Isaac's Gulf coast landfall.
Here is the latest rainfall amounts updated by HPC as of Friday morning.
That's about 1.5” of rain in the northern Valley to about 2.3” in parts of the southern Valley.
This is through Wednesday morning.
Our current thinking is that this might be a bit conservative.
This also doesn't account for any convective, or thunderstorm rain. Which in a thunderstorm you can easily see a half to an inch of rain in a short period of time.
Here is a term you may hear a lot of this weekend if you haven't already heard it. Training. This is what we call when a band of rain, or even thunderstorms “train” over the same area. So basically the same area sees a lot of rain as showers and storms continue to move over the same area.
Something else we will be watching for is the potential for flash flooding. For that, the National Weather Service uses something called Flash Flood Guidance. Basically it's an update of ground conditions and it gives them a guide to go by as to how much rain we need to see before flash flooding may become an issue. There are many different categories, how much rain we need to see an 1 hour, 3 hours, 12 hours, etc. I checked for area and it was updated Friday morning.
This is the 3 hour flash flood guidance.
Basically for Rockingham county, Page, and Shenandoah in the dark blue we would need to see 3” of rain in 3 hours for flash flooding to become likely.
Not out of the question if we average that out to an inch an hour.
Especially if we see a particularly heavy band “training” over the same area.
For the other counties it would be about 3.5” of rain in 3 hours.
Then we have some of our own forecast models we look at.
This is the RPM model, just another forecast model we can use for guidance.
This one will only give us 48 hours of data but it tends to do pretty well especially with our current line of thinking for this particular system.
This particular model thinks by Sunday evening most areas should see around 0.50" to 1” of rain. With some areas already closing in on nearly 2” of rain.
One thing to remember is that mother nature does not see county or town lines. Parts of Harrisonburg could see heavy rain while the other half of the city may not see much. As I mentioned earlier, we are thinking generally we could see at least around 2” of rain generally. I think some areas could see as many as 3”+ of rain possibly. A lot also depends on the track of the storm and the rain bands. Post tropical systems in mountainous areas can also be extremely tricky, and dangerous because of the terrain and heavy rainfall potential. That also comes with a flash flooding threat. So heads up for this Labor Day weekend.