Tropical Depression Nine (FutureImelda) Saturday Update (27 Sep)

As of mid-afternoon, TD9 is getting better organized as Hurricane Humerto lurks nearby …

Here’s links to NHC’s excellent summaries: Key messages regarding Tropical Depression Nine (en Español: Mensajes Claves).

As shown on the above map (which uses raw data from NHC on top of satellite images received direct from GOES here in the office), of 2:30pm, there is a disconnect between what is on the NHC site and other National Weather Service sources. NHC only shows the watch area going up to Flagler County, Florida, whereas the others sites (and the raw digital data feeds I use for my maps) show a warning for South Florida and watches extending to the outer environs of the Frogmore Metroplex (Brunswick) …

Either way, expect more warnings, and watches to expand with the 5 pm advisory, and probably extend to Frogmore proper this evening, and TD9 will likely become Tropical Storm Imelda.

The tracking situation seems a bit clearer. Here’s the obligatory model map with the almost illegible scramble of tracks …n the works

The major models are now pretty well in agreement that FutureImelda will parallel the Florida coast and take a hard right turn just off Georgia on Tuesday. Some of the ensemble members (thin lines) to make landfall, but remember those tracks aren’t of equal weight, you have to look at lots of other information before grasping on to those straws to keep the click rate up.

So what kinds of impacts are we looking at? If the NHC forecast holds up, here is what my TAOS(tm) TC and Lachesis impact models are showing:

This seems pretty realistic based on the trends we are seeing so far. This track would generate gusty winds – below sustained tropical storm force – and waves right on the coast. At the present time it doesn’t look like hurricane force winds, or even sustained tropical storm force winds, will make it past the beaches anywhere in coastal GA/SC. But small wobbles or intensity changes could alter that, so definitely pay attention.

The biggest questions will be rain and coastal flooding due to sustained onshore winds as the storm makes the turn and appears to “hover” offshore. For comparison, let’s look at the GFS and European AIFS (AI enhanced IFS) models. First, here is the 12z GFS outputs for Tuesday at 2pm:

And here is the AIFS view at the same time:

There’s not a lot of practical difference in these two even though GFS has FutureImelda as a hurricane and AIFS does not, the net effect will be that coastal GA/SC would get about two days of sustained onshore winds at 15 knots and up. The good news is tides will be running lower than average (first quarter moon, “neap” tides). The problem on our coast is that the marshes can “store” water in between tide cycles if they can’t drain all the way out due to the onshore winds. So until the winds shift on Tuesday as the storm passes, there will be four high tide cycles that will likely run 2-3 feet above normal. That sound dramatic but it isn’t because, again, astronomical tides are running below average. Still, this will cause coastal flooding around high tide in all the usual places. Will be watching and posting the forecasts as this gets closer.

As for rain, that’s harder to forecast. there is a pool of dry air (shades of brown) just inland, even more than this morning:

that could easily get entrained in the circulation of FutureImelda and choke off a lot of the rain on the West side. The models are showing a lot of rain offshore – 12 to 15 inches! – but onshore not so much, in coastal GA/SC the three day totals are in the 3-4 inch range, with a possible blob of more near Charlotte NC (moisture from FutureImelda enhanced by the remnants of front that is passing today). Here”s the forecast rain swath:

Bottom Line: At the moment, it looks like the Bahamas will see tropical storm conditions in places over the next two days and need to prepare accordingly. The coast of FL, GA, and SC will be brushed by a tropical storm, possible a hurricane, with most of the worst staying offshore. But it will be a near thing, so watches and warnings will go up, and prudence dictates that those on the immediate coast or those normally at risk of shallow coastal flooding take precautions. Those inland who have flooded this year should probably take precautions as well, especially the spots subject to street flooding, just in case the dry air stays pushed to the West. The coast immediate inland areas should also prepare for gusty winds, scattered power outages, etc. just in case. And of course disruptions from preparations!

Following the advice of your local emergency management is almost always a good plan. For most of us other than right on the immediate coast this is looking more and more likely to be inconvenient rather than hazardous (much less dangerous) – but treat it as a potential tropical storm anyway. Impacts in Florida will start tomorrow evening and be spreading in to Georgia by Monday and Tuesday, probably some early Wednesday but it looks like this will be over by Wednesday evening.

At this point you should monitor the NHC site for updates in case something changes, as well as local news (shudder) for things like school and business closings. As for updates, I’ll do one in the morning and tomorrow afternoon to make sure this scenario is still ion track. GA/SC will have Sunday afternoon and Monday to get ready if something changes.

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