The Upper Midwest’s latest extreme rainfall event produced record flood levels on some northeast Iowa Rivers today. Floodwaters surged out of river banks and into towns like Cresco and Decorah, Iowa.
— KWWL (@KWWL) August 24, 2016
Downstream from Decorah, the Upper Iowa River surged 14 feet to reach record levels at Dorchester today after last night’s torrential rainfall.
Floodwaters surged out of the river banks on the Turkey River west of Decorah.
— KWWL (@KWWL) August 24, 2016
The Turkey River at Spillville also set a new ‘flood of record’ Wednesday.
— MPR Weather (@MPRweather) August 24, 2016
Incredible rainfall totals of 6″ to 8″+ have been reported across northeast Iowa. It’s just the latest extreme rainfall event to hit the waterlogged Upper Midwest this summer. Flood warnings continue into Thursday morning.
1-in-500+ year flood?
Additional rainfall totals are still coming in, so far the highest observed totals on the ground are in and around Decorah.
The early estimates appear to show the 8.46″ rainfall total in 6 to 9 hours near Decorah will qualify as somewhere close to a 1-in-500+ to 1-in-1,000 year rainfall frequency event.
Forecast: Drying out
The good news in the forecast for the waterlogged Upper Midwest is high pressure building south from Canada through Friday.
That should give us 2 solid dry days across the Upper Midwest, before the next wave of rain arrives Saturday.
In the meantime the sump pump symphony continues with saturated soils across the Upper Midwest.
Florida: Watching hurricane potential
We’re still watching the latest models runs which suggest the development of a potential tropical storm or hurricane for Florida and the Gulf Coast next week. There has been a slight shift to the right, or east in some of the latest model trends. Florida is still definitely ‘in play’ as of Wednesday night.
The latest European model has shifted considerably east, and now brings Florida’s west coast into play early next week. Let’s hope this solution doesn’t verify for residents of Fort Myers and Tampa. My former MPR weather colleague Craig Edwards reports the Fort Myers Airport staff has been told to review preparedness plans for the area.
Forecast uncertainty still high
Think forecasting snowstorms is hard? Try predicting landfall and intensity of a potential hurricane 5 days out from a storm that doesn’t even have a name yet. Weather Underground’s Jeff Masters and Bob Henson elaborate on why it’s so hard to pinpoint hurricane forecasts.
Track forecast: Still uncertainty beyond The Bahamas
Since 99L had not yet formed a well-defined circulation center as of early Wednesday, it has been difficult for models to agree on its future track and intensity. Moreover, it appears that upper- and lower-level circulations are not yet well aligned. As a result, there remains a good bit of spread in how models are foreseeing its potential track.
A strong upper-level ridge now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through the weekend, keeping 99L on its general west-northwest track for the time being. Among our three most reliable models for tropical genesis and track forecasting, the ECMWF has been the most consistent over the last couple of days. The ECMWF continues to bring 99L across or near South Florida on Sunday or Monday and moves it onward into the Gulf of Mexico. More than 90% of the 50 ECMWF ensemble members from 00Z Wednesday–and all four members of the “high probability cluster” (those that performed the best on 99L during the preceding 24 hours)–bring 99L into the Gulf. The system could make landfall next week anywhere from Texas to the Florida Panhandle, according to various ECMWF ensemble members.
Intensity forecast: Watching and waiting
Models are now in close agreement that 99L will spend roughly 2 to 3 days over the very warm waters surrounding The Bahamas, where sea-surface temperatures are running around 29-30°C (84-86°F). The latest output from the SHIPS model (12Z Wednesday) indicates that wind shear will remain light to moderate over 99L, holding around 10-20 knots for the next couple of days and then dropping to the 5-10 knot range by this weekend. These factors support intensification, which would become more probable as soon as 99L forms a more consolidated center. The longer 99L takes to organize and the further south it tracks, the better the chance of interaction with the mountains of Hispaniola, which would interfere with its development. If the track remains further north, 99L will have a better shot at intensifying sooner. The 00Z Wednesday run of the HWRF–our top intensity model in recent years–keeps 99L as a weak wave or depression until Friday, then brings it to hurricane strength just east of South Florida on Sunday. The 06Z Wednesday HWRF run is similar, though less intense and even slower.
Even our best models are not very skilled at forecasting rapid intensification. If 99L makes it into the Bahamas with a well-organized center of circulation, the possibility of a burst of strengthening prior to any encounter with Florida would need to be monitored very closely, given the prime conditions that may be present. Should 99L enter the Gulf, it could have a longer period of potential intensification before any landfall.
The bottom line: it may well be another day, or several, before 99L manages to become a tropical storm or hurricane (assuming it does). If 99L does intensify by this weekend, there is still the possibility for tropical storm or even hurricane-level impacts in Florida. We also have the chance of a potentially serious hurricane in the Gulf of Mexico at some point next week.
The bottom line here? Stay tuned for likely changes in forecast track and intensity.
Programming note: Weather Lab with Paul Huttner at 10:30 am Friday on MPR News stations
We’ll update the latest hurricane tracks and talk hurricanes Friday morning at 10:30 am on MPR News in a special edition of Weather Lab with Paul Huttner. We’ve got some great guests lined up. I hope you can join us.