Most Read: Local

Posted at 08:05 PM ET, 08/26/2011

The latest on Irene and its uncertain forecasts

Hurricane Tracking Center with Tweets from NHC
Q&A transcript: Prepare for Hurricane Irene
Live updates: Follow the storm all day
Tropical Storm Warning for most of D.C. metro area
Hurricane Warning for Va/Md/De Beaches
Irene: Are You Prepared?
Live webcams of East Coast beaches

3 a.m., Saturday: Irene Update:Irene has weakened to a Category 1 storm as it nears the North Carolina coast, the National Hurricane Center said. The enormous storm’s top sustained winds were 90 mph, down from 100 mph. It was moving at 14 mph and the center was about 60 miles south of Cape Lookout, N.C.

11:30 p.m. Irene Update: Irene remains at Category 2 intensity with maximum sustained winds of 100 mph. Landfall is expected along the North Carolina coast in 12-18 hours, and little or no weakening is likely.

Some model guidance coming in has shifted the track a little to the west and increased rainfall totals for the D.C. metro region, projecting 3-5 inches. We’re likely to slightly increase our forecast rain totals and peak winds in our update Saturday morning. Stay tuned.

8 p.m. Irene Update: Category 2 Hurricane Irene continues making progress to the north/northeast at 14 mph. Maximum sustained winds remain around 100 mph and very little has changed with the storm since earlier today - it hasn’t strengthened, but it’s a very large storm with tropical storm-force winds out to 290 miles from its center. Thus far buoys off the North Carolina/South Carolina coast have recorded gusts to near 56 knots (64 mph) as tropical storm conditions spread inland over southeast North Carolina. Says the National Hurricane Center:

DURING THE PAST HOUR...A SUSTAINED WIND OF 52 MPH...84 KM/H...AND A GUST TO 62 MPH...100 KM/H WERE REPORTED AT THE JOHNNY MERCER PIER IN WRIGHTSVILLE BEACH NORTH CAROLINA

For the D.C. area, we continue to expect the worst conditions between late afternoon Saturday and early-to-mid morning Sunday. For more details, see our latest full forecast and Irene FAQs , the latter of which contains forecast details for the Eastern Shore and beaches.

Hurricane Irene: why the uncertainty in forecasts?

As has been well advertised, there has been plenty of uncertainty over the past few days in the predicted track of Hurricane Irene, especially northward of the Outer Banks. Although relatively small, differences in track of only 50 -200 miles can have major consequences with respect to storm impacts, including regions of heaviest rainfall, maximum wind damage, power outages, etc. Especially important now is the interaction between the intrinsic uncertainty of the storm’s track as determined by the steering tropical and subtropical circulation with the uncertainty in predictions of migrating trough (low pressure) and ridge (high pressure) areas embedded in the usual west-to-east flow across the mid-latitudes.

The increased uncertainty with time in the track forecast for Irene is apparent in the sequence of “spaghetti charts” shown in the figure below. The lines represent a range of possible mid-level (~18,000 feet) atmospheric circulation patterns around Irene’s low-pressure center, and the basic west to east flow further north. The further the spacing between lines, the greater the uncertainty.

The top chart is the 42 hour forecast from 2 a.m. this morning, valid at 8 p.m. Saturday evening. In this display, Hurricane Irene and the west-east flow to its north are essentially independent. By 54 hours, valid 8 a.m. Sunday, the uncertainty in both Irene and the west-to-east flow are clearly interacting and compounding the net uncertainty. The continued growth of uncertainty is obvious just 12 hours later (8 p.m. Sunday).

Fortunately, the strength of the trough (amplitude of the dip in the west-to-east flow) is not sufficient to trigger extratropical transition - i.e., transform the hurricane into an intensifying extratropical cyclone. In that case, the energy of the hurricane (initially derived from warm ocean water) is absorbed into the energy which drives cold- season storms (strong temperature contrasts associated with fronts). The most notable example affecting the U.S. of a hurricane transforming into an intensifying extratropical cyclone was the devastation and massive flooding associated with the transition of Hurricane Hazel in 1954.

By and  |  08:05 PM ET, 08/26/2011

Categories:  Latest, Tropical Weather, Tracton

 
Read what others are saying
     

    © 2011 The Washington Post Company