Hartman - Sterling, VA

Sterling, VA forecast region ...


Surface obs. show temperatures ranging from the high 60's to low 70's along the coast and mid to low 70's along and west of the Appalachian Mts. Much of the region is is reporting partial to full cloud cover with coastal dewpoints (dwpts) in the 50's and mountain dwpts in the 40's. Visible satellite shows a disturbance off the coast, east of DE/NJ. The IR sat shows low clouds covering the area with high clouds coming in with that disturbance. WV shows good degree of moisture availability. Radar does not show any currently falling precip but Morehead City, NC radar site shows precip off the coast with reflectivities reaching 50 dBZ. Winds are variable along the coast but are generally SSW. The synoptic driver is the deep trough digging in across central US and how it will interact with the low off the coast.

SHORT TERM (4/20 18Z - 4/24 00Z):

A look at the near term using RUC 4/20/2012 16Z run showed a generally quiet scenario for the forecast region. This model run covers 16Z 4/20 - 10Z 4/21. No PVA for the area, no jet. WAA is seen and is associated with the inter-low flow. RH is low in the fcst region, really following the front associated with the central US trough that reaches into the Gulf. The NAM 12Z run, ending on 4/24 at 0Z shows the vort field starting to advect into the area on 4/23 around 03Z, bringing with it storm generated over the Gulf. The system continues to wrap up and become occuled by 15Z. The models (WRF, GFS,  are in good agreement at this time (fhr 72-75). Previous to hitting the fcst region, PVA from the base of that trough spurrs up storms over the Gulf region and over the Gulf itself. The low's general track progresses NE-ward with PVA the whole trip. Additionally, there is jet support from the curvature of this trough. Precip is definitely expecting with this storm and should reach the eastern edge of forecast area around 9 or 10 am. Skew-t's show surface temperatures are well above freezing however mt. location may seem snow. By 03-09Z 4/23, both the GFS and NAM show freezing surface temps for the mountainous areas. These get as low as -5 with a good freezing layer for a few hours. The SE portion of the fcst area will have a pause in their precip but the wrap around will continue to affect the northern part of the fcst region. The tail end of the trough will bring with it another patch of rain discussed in the longer term.

LONG TERM (4/24 00Z - 4/27 12Z):

Picking up on 4/24 12Z, PVA and upper level jet support are strong according to the GFS and CMC. Both show good moisture advection as well. There is some disagreement regarding the spatial and temporal extent of the next rain event, which is expected to enter the fcst region by Wed, 4/25. Where the models agrees is on the pattern of the system: the trough is elongating out over the Atlantic mimicking a zonal flow, all models advect in a good amount of moisture, and all models show rain on Wednesday. This will be a westerly system traveling rather zonally. Therefore, downslope winds cutting across the Appalachians will make valley surface temps cooler than the models show.