Wednesday, March 9, 2011

Flood Watch Expanded - Now Includes Metro

The flood watch has been expanded farther south and east to include much of central, southern, and eastern VA. Expect 1 to 3 inches of rain across the watch area through early Friday morning. This additional rainfall will result in moderate flooding along the James River near the Westham gage Friday into Saturday.

Wednesday Morning Update On Approaching Storm System

A strong storm system with a history of producing severe weather is heading toward the East Coast today and Thursday. Rain has been moving into southwest Virginia early this Wednesday morning, and will gradually expand north and east into central Virginia this afternoon and tonight.

I do not expect rain to reach the Richmond Metro area until after dark, closer to 9 p.m., and then rain overnight into most of Thursday.

A Flood Watch is in effect for the green counties highlighted on this map, where the highest accumulations are possible (because it will rain the longest here):

However, here is a snapshot of the potential rainfall accumulation in central Virginia by late Friday night as the rain ends, which shows most locations should receive at least 1"-2" of rainfall (dark blue shade):

In addition to the potential for heavy rainfall, we'll also have the threat for severe weather. Thursday afternoon and evening, conditions will be ripe for strong to severe thunderstorms in part of central and southeast Virginia (yellow on this map below):

The primary threat will be damaging straight-line winds (like what we saw on Sunday), but the environment will also have enough wind shear to potentially produce brief tornadoes. A potent upper vorticity maximum will dig into the region, producing significant lift and instability in eastern North Carolina and into Virginia:

At the same time, strong southerly flow will force moisture to surge northward into the Mid-Atlantic ahead of the system's cold front Thursday. This moisture (look at the bright green shades on the map) will serve as fuel for storms:

The warmer air mass that will also surge northward into the Commonwealth Thursday afternoon will combine with the wind shear and moisture to generate instability, shown below in purple as CAPE (Convective Available Potential Energy):

The areas in purple, you can see, align closely with our yellow Slight Risk map above. This is the part of the region most likely to receive severe weather Thursday afternoon and evening because of this optimum combination of factors.

After the heavy rain ends late Thursday night, the James River will rise Friday and Saturday higher than it did on Tuesday night as a result of Sunday's heavy rainfall. Click here for the latest James River forecast for Richmond-Westham. You can also check the level a little farther downstream at Richmond Locks by clicking here.