Technical Discussion

Synopsis...

High pressure will continue to break down as storm system organizes over Tennessee Valley today.  Rain expected tomorrow and Tuesday across the region with more tranquil conditions on Wednesday and Thursday.

Near Term (Sun-Sun Night)...

High clouds overspreading entire region currently.  This allowed morning lows to not drop as much as forecast in many locations.  Temperatures will rebound a bit today but high clouds, and late September sun angle will keep readings near normal.  To our south, a 500mg shortwave trough will cutoff and slowly spin northeastward later today.  We expect a few showers across southern zones by dawn tomorrow.  This could need to be tweaked in the afternoon given the 12z model suite. 

Short Term (Mon-Tues)...

Rain will overspread all of the Commonwealth tomorrow morning as low pressure begins to organize and move northeastward.  Interesting thing to note (also see previous tech discussion for more detail) is a shortwave trough over southern Canada tomorrow.  The GFS wants to phase this trough with the cut off low which could result in a stronger storm system.  The WRF on the other hand does not phase the two systems.  Either way, the region is in for some beneficial heavy rainfall during the early part of the week.  SREF plumes show 1"+ across the majority of the state with more in the east.  The easterly component to the wind should help to enhance overrunning precip during the day tomorrow as is usual in PA.  Low will move into OH and then across Lake Erie and Ontario by Tuesday.  Tricky part of forecast was dealing with temperatures Tuesday due to large difference between GFS and NAM.  Difference is due to warm sector ahead of cold front and whether we see sunshine during the day on Tuesday before FROPA.  Decided to take a blend of the two models at this time.  Showers will become more scattered by evening and overnight across the entire area as the disturbance departs.

Long Term (Wed-Thurs)...

Much more tranquil weather will be on tap for Wed/Thurs with partly cloudy skies and seasonable temperatures.  Next area of interest is the tropical disturbance currently located in the western Caribbean.  Most models have it developing into a tropical storm and moving into the SE US by the middle/end of the week.  Could bring us some moisture Thurs/Fri but uncertainty is large at this point.  To add to the already complex situation, the first signs of the polar vortex are seen in southern Canada by the end of the week and some models bring the 0C 850mb line as well as the 540dm thickness line into central PA by the weekend.

-Dan DePodwin