Wednesday, October 09, 2013

Prolonged Coastal Storm Arriving



Radar at 4 PM This Afternoon, 10-9-2013
A prolonged coastal storm will be impacting our region for the next several days. The energy making up this area of low pressure is that of what was once Tropical Storm Karen. With an area of high pressure now positioned over New England and forecast to remain nearly stationary, the area of low pressure will only be able to move slowly northward as the high tries to fight the system. Eventually, it should halt the northward progression of the low which should become centered just offshore of the Delmarva. But with the low at least expected to come as far north as the Delmarva coastline, it will be close enough to throw rain and wind our way. The presence of a strong high in New England and a coastal low moving up from the south will increase the pressure gradient. This will result in a decent northeast wind and this onshore flow will be problematic for the coastline. 

The rain shield associated with the area of low pressure should continue to lift northward throughout the night. Most of the region will probably be within the shield by late Thursday Morning. Even if steady rain isn’t occurring, drizzle is probably going to take over with the flow off of the ocean. We will have to monitor how far north the rain shield lifts before it stops its northward progression. It is still possible that the Poconos misses out on steady rain. The computer model guidance has trended wetter over the past 24 hours and has shifted the rain shield further north. The rain will be heavy at times. On Friday, the rain could be on the occasional side and eventually a transition to showers will occur. Drizzle is quite possible again on Friday when a steady rain or heavier shower isn’t taking place. 

HPC/NWS Rainfall Forecast graphic from early afternoon.
Rainfall totals will probably end up being highest south and east of Philadelphia. 2 to 4 inches of rain is quite possible in portions of Southern New Jersey and Delaware. North and West, amounts will taper with northwest progression. A general 1 to 2 inches is expected here. This will be occurring over the course of several days, so flash flooding isn’t expected. However, some slow rises will likely occur on the smaller creeks and streams, especially if 4 inches is realized. In Southeastern NJ and especially down towards the Delmarva, amounts over 4 inches for a storm total isn’t out of the question.  This is our best guess at the moment. Rainfall amounts will depend on how far north the low moves before retreating later this weekend and also on if the low decides to deepen a bit more than currently forecast. 

The persistent northeast wind element is of concern, particularly for the coastline. This may end up being what the storm is remembered for. Repeated rounds of minor coastal flooding at the time of high tide are likely to continue with some areas of moderate coastal flooding possible. Already, video has shown flooding at the shore points. The prolonged northeast winds will also cause seas to build up to heights of 6 to 12 feet offshore, with waves in the surf zone building to 6 to 8 feet. Minor to moderate beach erosion is likely going to occur from the prolonged battering. Some tidal flooding is also possible along the Delaware Bay. Remember, the coastline is extra vulnerable due to damage from Sandy which caused the displacement of dunes. Rebuilding continues, but many areas still don't have the protection that was once in place.

Overnight, the winds will pick up further. Gusts in the 20 to 35 MPH range across the interior are possible. At the coast, wind gusts could easily reach 40 to 45 MPH, especially along the southeastern part of the New Jersey Coast and along the Delaware Beaches. On Thursday and Friday, wind gusts could exceed 30 MPH across the interior and along the coast gusts could exceed 50 MPH. The winds will begin to relax during the weekend, but they will still be gusty. With the leaves still on the trees, some spotty power outages cannot be ruled out.  We will have to watch the wind speeds carefully due to the aforementioned concerns about the system moving further north than expected and the possibility that the coastal low could deepen a bit more than the models suggest. 

High pressure will slowly build south again and this will push the coastal system back to the south over the course of the weekend. It is looking as though it will be a slow process. As a result, drizzle and showers may linger along with gusty winds into early next week. 

Interestingly enough, if the system was a fast mover and the high wasn't to our north, this would be a nuisance event.  The low pressure center on some of the modeling doesn't even drop down below 1010mb.