Harvey_Bal/DC

Current: The unysis water vapor loop from the past 12 hours shows upper-level moisture associated with a low pressure near the NC Coast and another system in the middle CONUS. Much colder air can be seen dropping southward in the Plains.  The low of the Coast is producing clouds in the region that are keeping daytime highs in the 70s.  Ahead of the cold front in the central U.S., the winds are southerly with some weak WAA and there is no precipitation in the area currently.
Water Vapor image from weather.unisys.com                                                  
Surface Map from www.hpc.ncep.noaa.gov

Short Range: Overnight the temperatures will be quite mild with some lingering clouds as well as the weak WAA mentioned earlier.  Lows will be near 60 in the DC area with cooler temps further west in higher elevations.  The NAM and the GFS are in good agreement in the timing of the front which should in around the 21-00Z range tomorrow evening.  Precipitation is expected with this front as there appears to be frontogenesis as well as some PVA and WAA in the region.  From the Penn State e-wall site, the CMC shows stronger PVA tomorrow at 500 mb so there may be heavier amounts of rain the predicted with the GFS along the front.  With marginally unstable air and some weak shear, some isolated severe storms could be present along the front tomorrow.  The more interesting part of the forecast happens on the tail end of the front where a cut-off low forms in the FL area and turns into a Nor'easter.  The front moving in tomorrow stalls and continues to bring wet weather overnight Saturday night, but by monday a strong low pressure system will move up from the south bringing much heavier amounts of precipitation.  The environment will favor cyclogenesis with a strong vort max trailing the surface low.  The GFS, NAM, ECMWF, and CMC are in pretty good agreement on the low track that should be right along the coast.  Initially there may be some upscale flow that will enhance precipitation amount before the winds change to westerly and the colder air moves in.  It is likely that it will rain most of the night Saturday as well as all day Sunday into Sunday night.  Higher elevations may just get cold enough to get some snow toward Monday especially where enhanced cooling due to melting will occur; however little to no accumulation is expected as there will be rain mixed in as well daytime highs still well above freezing.  Flooding will be likely with total precipitation amounts will be 2 inches and greater.  Below is a comparison between the CMC and ECMWF as the CMC moves the trough slightly faster and the surface low is further north by this time.
 CMC 500 mb vorticity from http://www.meteo.psu.edu
ECMWF 500 mb vorticity from http://www.meteo.psu.edu

Long Range: The upper trough remains in place before the pattern becomes zonal.  There is great uncertainty associated with the timing of the trough lifting out of the region as well as the strength of the trough by this time. 
GEFS from mag.ncep.noaa.gov

Regardless of timing, precipitation will be possible once the zonal pattern begins to set in sometime in the middle of next week generally associated with strong WAA across a tight temperature gradient.  Any precip that falls will probably be unwanted after the flooding likely from the upcoming nor'easter.  Temperatures will finally rebound once the zonal pattern begins to move in with highs into the upper 60s with slightly cooler temperature at higher elevations in the lower 60s. 
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