(originally posted at 3 p.m., updated at 6 p.m. and 7:40 p.m.)

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Update, 7:40 p.m.: The National Weather Service has increased its peak wind gust estimate to 70 mph for the region and upped rainfall totals to 5-10”. See: A fiercer forecast for Hurricane Sandy’s effects on Washington, D.C.

Today, forecasts have converged on one scenario for Hurricane Sandy’s impact on Washington, D.C., and it’s severe. In short, between Sunday night and Tuesday, we can expect 4-7” of rain and a long period of sustained winds above 35 mph with peak gusts over 60 mph. This will inevitably result in flooding and power outages.

Consider this post your detailed guide to this storm. It contains a storm timeline, local wind and rain maps, and answers to frequently asked questions.


Important note: Heaviest rain and strongest winds will tend to be east of I-95 when ranges are given. Coldest temperatures will be north and west of I-95.

Sunday afternoon: a chance of showers, mainly east of I-95. Breezy (winds 15-25 mph from the north, gusts to 30 mph) with temps 55-60.

6 p.m. Sunday to midnight: Rain showers becoming likely, steadiest east of I-95. Winds 20-30 mph (from the north), gusts to 35 mph. Temps 50-55.

12 a.m. Monday to 6:00 am.: Rain showers likely, heavy east of I-95. Winds 25-35 mph (from the north), gusts to 40 mph. Temps 48-53.

CWG forecast: Peak sustained winds and gusts

Noon Monday to 6 p.m.: Heavy rain. Winds 35-45 mph (from the north), gusts to 50-60 mph. Temps 47-51.

6 p.m. Monday to midnight: Heavy rain. Winds 35-50 mph (from the north), gusts 50-70 mph. Turning cold, temps 42-47.

Midnight Tuesday to 6 a.m.: Heavy rain. Winds 30-40 mph (from the northwest), gusts 40-60 mph. Cold, temps 37-42. (Outside chance snowflakes western Loudoun and Frederick counties)

CWG forecast: Rainfall totals

Noon Tuesday to 6 p.m.: Showery. Winds 25-35 mph (from the southwest), gusts 35-45 mph. Temps 43-47.

6 p.m. Tuesday to midnight: Showers diminishing. Winds 20-30 mph (from the southwest), gusts 30-40 mph.


How will Sandy compare with the D.C. derecho? Sandy is a slow-moving, large-scale storm capable of strong winds and heavy rain over an extended period. Whereas the derecho devastated locations across the D.C. area in a matter of minutes. Impact-wise, we’d recommend preparing for the same impacts as the derecho - downed trees and the potential for multiple days without power.

On the positive side, utilities and local governments and the public will have had several days warning this time versus several hours (if that) for the derecho. So, there’s a chance the impacts may not be quite as bad or last quite as long. But you should prepare for the worst and hope for the best.

If the derecho was a 10 on a 1-to-10 impact scale here in the D.C. area, we think Sandy will be about an 8. That’s just an estimate though.

Is the storm overhyped? Could it still miss us like some snowstorms do? Models have come into solid agreement on Sandy's general track and strength. So our confidence in the forecast is relatively high compared to some of the trickier snowstorms we’ve seen in the past. Small changes in track could give us less rain and weaker winds than we’re expecting. But at this point it seems unlikely we’ll avoid moderate to major impacts. One wildcard, though, is that the power grid was largely rebuilt after the June derecho, so maybe it fares a little better this time or at least recovers more quickly.

How should I prepare for the storm? See our earlier post with tips on how to prepare for Sandy.

When will we see the worst of the storm? When will it begin and end? There may be some showers Sunday and Sunday night (especially east of I-95) and likely increasing winds, but the strongest winds and heaviest rain should occur Monday into or through the day Tuesday. Wind and rain should ease Tuesday night.

How strong will the winds be and what is the risk of power outages? A High Wind Warning has been issued for Sunday night through Tuesday. Right now we think peak winds late Monday into early Tuesday will be around 35-50 mph with gusts to 55-70 mph (areas east of I-95 are likely to see the higher end of these ranges, while winds west of I-95 may not be quite as strong). Power outages are likely and many could be without power for multiple days.

How much rain will we get? What about flooding? A Flood Watch is in effect. Total rainfall looks to be in the 4-7” range, but locally higher totals are possible especially east and northeast of town, and locally lower totals are possible especially southwest of the city. Flooding of low-lying areas and rivers/streams/creeks is likely.

Who will get the worst of Sandy? Areas along the coast close to and just north of where Sandy makes landfall will get the worst, especially in terms of storm surge. As of now that looks to be from central New Jersey north to Rhode Island, including Philadelphia and New York City.

How will this compare to Hurricane Isabel in 2003? How bad will coastal flooding be on the Bay and Tidal Potomac? The winds may be similar in strength. But Sandy’s likely track just to our northeast is such that we shouldn’t see a storm surge up the Potomac like we did with Isabel, which passed to our southwest. Surge and winds are always worst just to the north and east of where the storm tracks. Isabel raised water levels up to 9 feet along the Potomac.

When winds take on a more southerly component late Monday night, Sandy may raise water levels 1 to 3 feet on the west side of the Chesapeake Bay and on the tidal Potomac (coastal flood warnings are in effect for these areas Monday night to Wednesday warning).

Simulation of blocking flow in atmosphere (Penn State)

Isn’t the storm not supposed to be a hurricane once it gets here? Technically, yes. The storm is losing its tropical characteristics as it moves north into cooler air. But, it’s expected to strengthen again as an unusual pattern featuring multiple jet streaks (rivers of fast-moving winds high up in the atmosphere) pumps the storm with a new shot of energy.

What will schools and governments (federal and local) do on Monday? (from 3:00 p.m.) Seems likely there will be a lot of closings. Many school systems have already decided to close.

6:00 p.m. update: Most school systems are closed Monday and the Federal government is closed.

Here’s our SchoolCast and FedCast:

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Additional closings are likely Tuesday.

One model suggests a little wet snow could fall west of town if enough cold air wraps in. ( StormVistaWxModels )

What about the Eastern shore? Severe coastal flooding is expected for the Maryland and Delaware beaches with water levels up to 4 feet above normal. NWS says that near-shore waves 9 to 15 feet will result in severe beach erosion, and that “WATER LEVELS FROM CHINCOTEAGUE TO OCEAN CITY COULD RIVAL THOSE REACHED IN GLORIA IN 1985.” Also, a Storm Warning is in effect for the Tidal Potomac and Hurricane Force Wind Warning for the Chesapeake Bay.

Where can I find latest tide and surge info? Here and here.

How does this compare to other historic storms? Many have compared Sandy to the Perfect Storm of 1991, and experts have warned it may be even worse. Generally speaking, though, each storm has its own character and sometimes you just can’t compare until all is said and done.