Tropical Depression Florence should begin its turn toward the north tonight and eventually track more to the northeast on Monday. The latest spaghetti charts below show some uncertainty in exactly how close the center of circulation will pass to the DC Region (as a turn toward the east north-east is possible). Wind shear is forecast to increase during the afternoon hours across the Washington Region on Monday as will the risk of severe thunderstorms, some of which could produce tornadoes.
Latest North American Model (NAM) shows Tropical Depression (TD) Florence moving northward into West Virginia during the day on Monday with very heavy rainfall beginning to impact southwestern Virginia and the southern and central Shenandoah Valley. On Monday afternoon TD Florence may begin to move toward the northeast or east-northeast (exact track is important regarding evolution of impacts here in DC). As the system tracks closer to the DC Region, wind shear will increase (winds that change in direction and speed with height in the atmosphere) and any convective lines of thunderstorms that form could spawn several tornadoes across the region.
The images below shows the latest European operational run with a general one to three inches expected however, rainfall totals could be much higher in areas that receive training convective thunderstorms which are better forecast utilizing higher-resolution models. The latest NAM high-resolution model for example shows some locations along the Blue Ridge and Shenandoah Valley picking up to 7″ of rainfall (locally) and this will need to be monitored during the afternoon hours on Monday.
Significant Tornado Parameter (NAM Model):
With low-level veering winds increasing as well as CAPE (Convective Available Potential Energy) there is a threat of a few isolated tornadoes on Monday afternoon and evening across the Washington Region. While the threat at this time is low, it needs to be monitored on Monday as the details in track and surface instability (the strength of updrafts at the surface) become clearer.
CAPE (Convective Available Potential Energy):
Current NAM output shows moderate instability on Monday afternoon with 1500-2000 joules per kilogram (J/kg) of CAPE.
Monday afternoon and early evening need to be monitored for the potential of a few severe thunderstorms, some of which could be capable of producing tornadoes.
High pressure builds in across New England and the Mid-Atlantic on Tuesday night. Winds will shift to the north bringing lower humidity and some widespread sunshine on Wednesday!