Brandt: Northern VA

Current Weather Synopsis (as of 18Z):

The most prominent weather feature in the Eastern United States is a 1004mb surface low pressure system currently centered in Southern Michigan. A 1027mb Canadian surface high lies behind, currently sitting across portions of Northern Ontario, and a secondary 1020mb surface high currently resides just off of the coast of Cape Cod. Another weak disturbance is located around 250 km east of Virginia Beach, VA.

Calm surface winds and minimal dew point depressions this led to foggy conditions across much of Northwestern Virginia, this morning. Visible satellite images showed fog over much of the western half of the state, however, this dissipated across most of the area by 14Z. Temperatures this morning were generally in the upper 40s to lower 50s across the forecast area, but have since warmed up into the upper 60s to around 70 degrees, with calm winds evolving into a weak southwesterly flow.

Short Term-- Friday through Monday:

The primary short-term forecasting concern is the low pressure system set to affect the forecast area on Sunday and Monday.

The low-pressure center associated with the cold front currently located in the Eastern MS Valley is forecast to move towards the Northeast, to be located in the northern portion of New England by 18Z Saturday. The direct effects of that system on our area look to be minimal. Of greater concern will be a developing low pressure center on the southern portion of the cold front; multiple models show the 500mb trough in association with this system deepening strongly as well as undergoing a shift from a positive to a negatively tilted trough axis. All major models (at 12Z runs) are in agreement showing the southern portion of the trough cutting off but quickly rejoining the main flow by Sunday afternoon. The GFS and NAM Models are also in agreement showing a very sharply curved jet with 90+ kt cores forcing very strong upper-level divergence over the forecast area; the UKMET and CMC model runs take the upper-level support a bit farther to the east.

The surface low associated with this system is forecast to greatly intensify though it does not yet appear set to approach bomb status at this point... the 12Z NAM is the most aggressive taking the surface low from a central pressure of 998mb at 15Z Sunday down to 980mb by 15Z Monday... thus putting it shy of the bomb criterion but definitely in the category of 'explosive' deepening. The GFS and CMC are slightly more conservative with the deepening, showing the low falling into the upper 980s mb by Monday afternoon.

This system appears to be very strong for both traditional QG forcing terms as a very strong 500mb vortmax approaches the area and a strong SE flow provides strong WAA... however, due to the sharpness of this trough, location will play a very key role... there may be a very steep total precip gradient for this system, especially on an E-W axis...

Precip type appears to be consistent with this system as an all-rain event on Sunday-Monday... strong CAA will bring sufficiently cold air into western parts of our region by late Sunday, however, it appears that locations with sufficiently cold air will have precipitation shut off by that point.

Monday night, however, offers a possibility for mixed precip across the western sections of the forecast area, especially those in high-terrain regions... precipitation wrapping around the low as it stalls out in the NY-PA region bring additional precip chances to portions of Eastern WV... surface temperatures look to be sufficiently warm for most of that forecast time, however, during overnight Mon-Tue there is at least a chance of a few wet snowflakes as temperatures drop off.

In terms of total precipitation over the area... highest totals are expected across the eastern portions of the forecast area. There are some differences in QPF forecast between models, with the NAM forecasting as much as 2.5" of rain over portions of Maryland, while the GFS limits system totals to 1.5-2" maximum precipitation totals associated with the system.

Long Term-- Tuesday through Friday:

As the system departs the area, significantly cooler air behind it will drop temperatures to colder levels. Highs on Tuesday look to be in the upper 50s across the region, though temperatures look to increase throughout the forecast period. Precipitation chances begin again, especially over the western portion of the forecast area, on Tuesday night. The entire period from Wednesday through Friday look to include some chance of precipitation across the entire forecast area.