Posted at 02:41 PM ET, 10/27/2012

Washington, D.C. braces for Hurricane Sandy: the bad and the ugly storm scenarios


Latest models are converging on landfall in New Jersey, but then the storm bends back towards northern Virginia.
Confidence is now high that the monstrous and menacing Hurricane Sandy will bring serious to severe wind and rain to the Washington, D.C. metro region.

At this point there are essentially two scenarios for the area: a worst case scenario (less likely) and a very bad scenario (more likely) - but the differences between the scenarios are not significant for most of us.

Before we break the scenarios down, here’s what we can likely expect, irrespective of the storm track scenario:

* The worst of the storm will hit Monday into Tuesday

* There will be a prolonged period of strong winds, potentially sustained at 25-45 mph, with gusts to 45-60 mph (likely higher east of the Chesapeake Bay) for a period of 12 hours or more during the height of the storm. Power outages are likely.

* Rainfall totals of at least 3-6” are a good bet. Flood prone areas are likely to flood.

Start preparing now.

The differences in the two scenarios are most significant for the Maryland and Delaware beaches, where the exact storm track will have a big impact on the severity of coastal flooding. For inland areas (including Washington, D.C. and Baltimore), because this storm is so large, we will experience heavy rain amounts and strong winds in either case. It doesn’t matter if the storm first crosses land in central New Jersey or Chincoteague.

SCENARIO 1: Near-direct hit - Landfall from northern Delmarva to northern New Jersey (70 percent chance, very bad case)


GFS model shows storm making landfall Monday night in central New Jersey with torrential rain and strong winds (note tightly packed isobars) over the northern mid-Atlantic
In this scenario, the storm comes ashore just north of Washington, D.C.’s latitude, above the Maryland, Delaware beaches. Because the worst effects of the storm will probably be north and northeast of the center, much of the region would have somewhat weaker winds in this scenario and the beaches less coastal flooding. (This is potentially the worst case scenario for the New York City area, positioning it in the most intense northeast section of the storm, with a punishing flow off the ocean for up to two days. This could result in a devastating storm surge in the range of 5-10 feet, possibly flooding their subway system among other things).

However, there is likely to be a secondary area of torrential rain and strong winds on the southwest side of the storm (where cold air wrapping into the storm clashes with warm, moist air transported by the storm from the tropics) which would likely batter our region if the storm moves inland to our north.

Here’s what we would expect:

* A slight chance of rain west of I-95 Sunday, with a better chance east of I-95 with winds slowly increasing out of the northeast, especially during the afternoon. Rain becoming likely everywhere at night, with winds continuing to increase but not yet to hazardous levels.

* Rain and wind increase Monday. Heavy rain and strong winds by Monday afternoon and night, continuing into Tuesday before diminishing Tuesday night or early Wednesday.


Winds (in knots) at an altitude of about 3,000 feet indicated by different color shades Monday night around 8 p.m. Surface winds would be about 20 percent less. Given the red shades over D.C. area, this would suggest winds to about 60 mph, conservatively. (from the GFS model) (WeatherBell.com)
* Peak winds late Monday into early Tuesday: west of I-95 sustained at 25-45 mph, with gusts 45-60 mph; east of I-95 sustained at 30-50 mph, with gusts 50-65 mph. Power outages likely.

* Breezy conditions would not completely subside until Thursday.

* Rainfall totals 3-6” west of I-95, and 4-8” east of I-95 resulting in flooding of low lying areas, and rivers/streams/creeks.

* Major coastal flooding possible for Maryland and Delaware beaches, but worst case scenario avoided as winds will shift to an offshore direction after the storm moves north. Nevertheless, this area would experience sustained winds over 40 mph with gusts to hurricane force for up to 48 hours from late Sunday to late Tuesday - resulting in widespread power outages and minor structural damage.

*
European model shows rainfall totals of 4-7” in the D.C. area, highest amounts east of I-95. (StormVistaWXmodels.com)
Turning cold Monday night into Tuesday morning, even as low as the 30s in some spots (probably closer to 40 in downtown Washington), with an outside chance of some brief snowflakes especially above 1,000 feet in Loudoun and Frederick counties. Heavy snow likely in the mountains of West Virginia and southwest Virginia (and maybe western Maryland), with some accumulations over a foot combined with wind gusts over 50 mph resulting in power outages.

SCENARIO 2: Direct hit - Landfall from southern to central Delmarva (30 percent chance, worst case)

If the storm center tracks over or (worse) just south of the region, this brings the storm’s most severe conditions to the Maryland and Delaware beaches and then inland over the Washington, D.C. and Baltimore metro region. I would be particulary concerned about severe to historic coastal flooding for the Maryland and Delaware beaches in this scenario, due to the combination of storm surge and high tide, raising water levels 10 feet above normal.

Here’s what we would expect:

* Rain increasing across the region Sunday, first east of I-95.

* Rain and wind increasing everywhere Sunday night.

* Heavy rain and strong winds Monday through Tuesday morning before diminishing Tuesday afternoon or night.

* Peak winds mid-Monday to early Tuesday. West of I-95 sustained at 30-45 mph, with gusts 45-60 mph. East of I-95 sustained at 35-55 mph, with gusts 55-70 mph. Power outages likely.

* Breezy conditions would not completely subside until late Wednesday or Thursday.

* Rainfall totals 3-6” west of I-95, and 4-8” east of I-95 resulting in flooding of low lying areas, and rivers/streams/creeks.

* Severe to historic coastal flooding possible for Maryland and Delaware beaches with 48 hours or more of onshore flow. The combination of a storm surge of 5 feet and astronomically high tides (due to full moon) could raise water levels 10 feet above normal. Sustained winds over 40 mph for 48 hours, with gusts over hurricane force.

* Snow confined mainly to high elevations of West Virginia in this scenario as mild ocean air wraps into the aea.

By  |  02:41 PM ET, 10/27/2012

Categories:  Latest, Tropical Weather

 
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