Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
405 am EDT sun Jul 5 2015
a series of disturbances aloft will move overhead and interact with
a surface front that will waver across Virginia and NC through Monday.
Near term /through tonight/...
as of 1000 PM Saturday...
Convection has been isolated to widely scattered this evening.
Modest instability (500-1000 j/kg) is still depicted on the Storm Prediction Center
mesoscale analysis but cin is on the increase. Convection across the
northern tier of the forecast area may persist a bit longer as an
upper level shear axis helps to focus the precipitation. In addition...a
surface boundary is also located in this vicinity. The shear axis is
prognosticated to lift northward through 06z...however another vorticity maximum is
expected to track east-northeastward from SW Tennessee through 12z.
However...at this time it appears it will remain far enough west to
have too much influence on our area (except possibly over the far
northwest piedmont). Nevertheless...given the moisture rich air
mass...a few showers may persist through the overnight hours.
Overnight lows will be similar to previous nights...generally upper
60s to lower 70s.
Short term /Sunday and Sunday night/...
as of 310 PM Saturday...
Middle-upper level trough over the Tennessee Valley amplifies...causing
heights over our region to slowly build/expand northward. This weak
subsidence may initially inhibit convective development...especially
across our southeast counties. Perturbations aloft crossing western NC into
western Virginia will aid to focus low level convergence over the northwest
Piedmont...leading to scattered convection by afternoon-early
evening. Since majority of model guidance has backed off on upper
support...have limited probability of precipitation to solid chance across the northwest...trending
to small chance across the southeast.
Maximum temperatures Sunday afternoon will follow similar trend with middle 80s northwest
due to mixture of sun/clouds to around 90 southeast.
Long term /Tuesday through Saturday/...
as of 405 am Sunday...
With the departure of the increasingly sheared upper low over the
south-central Appalachians on Tuesday...the pattern over the
southeast US will shift back into a Oregon typical summertime
regime...with the subtropical ridge building across the southern US.
Meanwhile...progressive flow across the northern US will allow a
series of shortwaves to cross the Great Lakes region... and to some
degree keep heights aloft from rising too quickly over the
Carolinas. The first shortwave will cross the Great Lakes on
Wednesday...with a trailing cold front sinking south into the middle-
Atlantic region...but given the fact that models have trended a
little stronger...and warmer...with the subtropical ridge....expect
the front to remain north of our area. A Lee trough will be the main
focus for isolated to widely scattered showers and storms each
day...with the potential for disturbances tracking west to east near
the frontal zone to provide some additional forcing as the front
effectively tried to sink south into the area by the weekend. This
will be dependent on how the ridge evolves...and the models arent
in full agreement on how quickly the center of the ridge will build
west over the Southern Plains...which would allow heights to fall
again along the East Coast. For now...will continue to show mainly
slight chance of low end chance probability of precipitation each day...with highs
increasingly in the 90-95 range.
Aviation /06z Sunday through Thursday/...
as of 225 am Sunday...
Generally light south to southwest low level flow will prevail over
central NC...on the south side of a surface frontal zone forecast to
settle to near the Virginia/NC state line later this morning. The front is
then forecast to become quasi-stationary and focus scattered showers
and storms this afternoon...initially most probable at triad taf
sites and rdu...in closest proximity to both the front and an upper
level trough forecast to drift slowly northeast across the Tennessee
Valley. The trough aloft...and preceding weak impulses in
southwesterly flow aloft...will probably trigger a north to south-
oriented band or two of additional showers and storms through this
evening...but confidence in location and timing is low at this time.
Otherwise...the hi-res rapid refresh model (hrrr) suggests pockets
of IFR-MVFR stratus will expand from Alabama/Georgia northeastward across
SC...and into southern NC (including fay) between 10-15z. Although
the consensus of other model guidance maintains VFR conditions at
Fay and elsewhere...the recent good track record of the hrrr versus
other guidance will be followed; and a tempo group for MVFR ceilings
has been included at kfay for a few hours later this morning.
Outlook: the aforementioned upper level trough over the Tennessee Valley
will continue to lift slowly northeast through the Appalachians and
direct a moist and weakly perturbed southwest flow aloft across NC
through Tuesday. Above average probabilities of mainly afternoon-early
nighttime showers and storms --and associated sub-VFR conditions--
will result...with the relative better chances on Monday. Sub-tropical
ridging will then expand north through middle-late week...with
associated mainly dry and warmer/hot conditions.