03 October 2015

Hurricane Joaquin finally moving northeast away from the Bahamas

Enhanced infrared satellite image of Joaquin and the U.S. east coast on Saturday morning.
Joaquin has devastated parts of the central Bahamas with nearly 2 feet of rain, destructive storm surge,  tropical storm force winds for 3 days, and hurricane-force winds for 1.5 days.  It will begin to clear up there today, so I'm sure we will start seeing more photos and videos of the aftermath.  The anticipated turn to the north happened on Thursday night, and a turn more to the northeast occurred later Friday.  The track history, as well as the current NHC forecast, is shown here:

The NHC forecast with the cone of uncertainty included is also shown for reference... note that Bermuda is now under a hurricane watch and a tropical storm warning.  Although the cone is exactly the same size it has been for every storm all season, the current model guidance consensus makes this forecast much more confident than it previously was.  A fixed-size cone is not capable of illustrating the evolving forecast confidence (see "Why the forecast cone of uncertainty is inadequate for Hurricane Joaquin").

Joaquin is now a Category 3 storm with maximum sustained winds of 125 mph, with a possibility of some strengthening today.  It will then weaken gradually over the coming days as it encounters higher shear and cooler water.  By the end of the week it is expected to transition to a strong extratropical cyclone.

Of course, the main event which has always been part of the forecast is the enormous rainfall totals along the U.S. east coast.  An endless feed of deep tropical moisture from Hurricane Joaquin is streaming into the Carolinas and mid-Atlantic states, and won't be stopping anytime soon.

South Carolina has been the bullseye for incredible rains, which isn't surprising when you see the graphic above showing the moisture plume persistently centered on that state.  The forecast through the end of the weekend is not good either... the flooding that is already happening will get much worse. Locations from South Carolina up to Maine are also experiencing coastal flooding due to onshore winds, but that is being caused by a mid-latitude storm system, not Joaquin.

The U.S. "dodged a bullet" with this storm.  If a major hurricane had stalled over south Florida for 3 days, it's hard to even imagine the outcome.  Or if it followed Sandy's path from three years ago, etc, etc.  The prospect of a U.S. landfall was never presented as a certainty by anyone (I hope).  There were times when the majority of model guidance as well as NHC indicated that the center could turn back west and hit the coast, but that should not be interpreted to mean that it was a sure thing.  Times like this are when scientists and forecasters hope to educate the public more about probability and uncertainty, and that a single forecast is not always enough.  Nature is just sometimes inherently less predictable than other times, the hard part is communicating that to everyone.

With Joaquin, this season has now had 10 named storms, 3 hurricanes, and 2 major hurricanes (Cat3+). The average for this date, using a 1981-2010 climatology, is 9 named storms, 4 hurricanes, and 2 major hurricanes.  However, in terms of Accumulated Cyclone Energy, or ACE, this season is running at just a little over 50% of average for this date because most of the storms that formed were so short-lived.

01 October 2015

Why the forecast cone of uncertainty is inadequate for Hurricane Joaquin

Another post today... it was just too much to cram into one.  This covers an extremely important and timely topic: uncertainty in forecasts.  In particular, the shortcomings of NHC's "cone of uncertainty".

Why the forecast cone of uncertainty is inadequate for Hurricane Joaquin