Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
110 am EDT Wednesday Aug 20 2014
Synopsis...a series of upper level disturbances in northwest flow
aloft will cross the region this week. A back door cold front is
forecast to slip into the area over the weekend.
Near term /overnight/...
as of 925 PM Tuesday...
With loss of heating and weakening support aloft as SC vorticity maximum
drifts east...bulk of convection as diminished across central NC at
mid-evening. Isolated showers continue to persist over the western
Piedmont along a low level Theta-E gradient with some weak support
aloft from a vorticity maximum crossing The Highlands of WV/VA. Scattered
convection now occurring in the vicinity of Roanoke, Virginia may follow
along the surface Theta-E gradient and drift into the northwest Piedmont
counties after midnight. Otherwise abundant low level moisture and a
light-calm wind regime should allow areas of low clouds and/or fog
to develop late this evening or overnight. Think low clouds will be
more prevalent versus fog as surface ground quite warm from heating
earlier today. Min temperatures near 70-lower 70s.
Short term /Wednesday and Wednesday night/...
as of 310 PM Tuesday...
Northwesterly flow aloft continues on Wednesday...with models
showing a disturbance currently associated with convection over
southern Illinois sinking south and crossing Virginia/northern NC during
the day. After the erosion of any morning stratus...conditions are
are expected to be similar today with guidance giving highs in the
upper 80s/lower 90s. Dewpoints may mix out a little more int he
western Piedmont with more northwesterly flow...but 1000-1500 j/kg
of MLCAPE is still expected in the east. Shear will again be weak
and the severe threat is relatively low. Lows in the upper 60s and
Long term /Thursday through Monday/...
as of 240 PM Tuesday...
Expect a meridional upper level pattern...with ridging over the
Midwest and troughing along the NE and Atlantic coast through the
period. This will result in northwesterly flow aloft over central
NC. The GFS and European model (ecmwf) both indicate the chance for showers and
storms throughout the period as a series of short waves move over
the area around the base of the upper level trough. Although the
convection should be diurnal in nature...coverage and location are
still fairly uncertain at this time. At the surface...expect a
trough to linger along the coast...with weak ridging to the north-
northwest. The latest GFS still pushes a backdoor cold front into
the NE portions of the area this weekend...resulting in highs 80-85
degrees across the north and 85-90 degrees across the south for
Saturday through Monday. The advection of warm moist air from the
Gulf around the high and the resulting subsidence in the Lee of the
mountains could mean increasing temperatures for the middle of next
week. Also...the GFS increases precipitable water values from around 1.5 to around
2 inches through the weekend...before decreasing again to near 1.5
inches. Thus...expect the convection over the weekend to be capable
of producing some locally heavy rainfall. Overnight low temperatures should
gradually decrease...from the low to middle 70s Thursday night to the
middle to upper 60s Sunday and Monday nights.
Aviation /06z Wednesday through Sunday/...
as of 110 am Wednesday...
Skies have cleared off this evening. Area metars are already
reporting 2 to 3 degree dewpoint depressions. Combined with light
winds...this allow for the development of mostly MVFR fog. Northern
coastal plain/northeast piedmont(krwi and krdu)could also see some
stratus advect into the area as north-northeasterly flow develops. Expect
conditions to improve to VFR between 14-15z.
Shortwave disturbances traversing the region will once again fuel
isolated to scattered convection this afternoon/evening... with the
highest probability across the northern taf sites. However...
confidence is not high enough to include any mention of convection
in the tafs at this time.
Outlook... early morning stratus/fog...along with afternoon showers
and storms will be possible each day this week into the weekend.