Area forecast discussion
National Weather Service Raleigh NC
300 am EDT Wednesday Jun 19 2013
Synopsis...
cooler high pressure will build into the region from the northeast
through the end of the week.
&&
Near term /through tonight/...
as of 255 am Wednesday...
Today: surface observational data at 06z reveal a 1008 mb surface
wave just east of Chesapeake lighthouse Virginia (chlv2)...with a cold
front trailing west southwestward and evident as a krax radar fine
line from Greenville (kpgv) to Sanford NC (ktta)...then
southwestward to near Anderson SC (kand). Nwp guidance suggests this
boundary will drift to a position over southeastern NC...including
across the southern NC coastal plain. This boundary/convergence axis
and northern bound of surface dewpoints at or above 70 degrees will mark the
northern edge of precipitation chances over central NC (sampson
Colorado.)..and mostly cloudy conditions through midday-early
afternoon...with decreasing clouds to partly sunny elsewhere.
However...a chance of showers will be renewed during the middle-late
afternoon hours mainly over the northern/northeast Piedmont and
northern coastal plain (roughly from Raleigh and points north and
eastward)...as a secondary northern stream cold front analyzed over
northern Virginia this morning...and augmented this afternoon by
northeasterly maritime flow and associated sea/Bay breezes...
settles south southwestward into those areas coincident with peak
diurnal heating. While convergence with this feature will be limited
owing to overwhelming northeasterly low level flow behind the lead
front over southeast NC...this area will also be under the influence of
deeper layer forcing for ascent accompanying an approaching
positively-tilted middle level trough axis and pair of embedded
shortwave disturbances from the Ohio and middle MS valleys this morning.
The coupling of this low and deep (albeit relatively weak)
lift...and with forecast k-indices between 25 and 30 c and weak
instability of generally 500 j/kg or less fueled by afternoon
temperatures and dewpoints in the low to middle 80s and 60s
respectively...a small chance of showers is justified in those
areas...with precipitation elsewhere and otherwise less than 15
percent.
Tonight: height rises in northwest to northerly flow aloft...
suggests skies will clear or becoming mostly so...though with few
patches of stratus or fog in NE low level flow toward morning.
Cooler...with lows mostly in the lower to middle 60s.
&&
Short term /Thursday and Thursday night/...
to be updated shortly...
&&
Long term /Thursday through Tuesday/...
as of 215 PM Tuesday...
Thursday through Friday...heights aloft rise over central North
Carolina...with a subsident air mass in place over the area.
Precipitable water values remain higher Thursday relative to
Friday...between an inch and 1.5 inches on the former compared to a
decrease from the northeast to at or below an inch Friday. Both the
GFS and NAM bufr soundings...and particularly the latter...forecast
enough moisture around 850mb Thursday for areas of cumulus to form...more
likely toward the western Piedmont where the 850mb moisture is
highest. K indices fall from the northeast as well Thursday
afternoon continuing into Friday...with k indices well into negative
values by Friday afternoon. In terms of deep convection...ample
overall cin and gradual warming aloft with convergence aloft should
preclude that. There should be a few morning gusts with initial
mixing Thursday in the lower to middle teens kts in the modestly tight
gradient between an inverted trough off of the coast and surface
high pressure building in from the northwest...but overall 925mb
winds are mainly around 10kt. 1000-850mb thicknesses suggest highs
in the lower to middle 80s both days...close to or on the lower side of
MOS guidance. Overnight lows Thursday night around 60 to the middle
60s...mins possibly tempered a little over the southwest Piedmont by
slightly greater upper-level moisture forecast by both the NAM and
the GFS.
Friday night through Tuesday...upper ridging continues to build in.
Around the ridge...there could be a weak middle-level shortwave or two
that attempts to move...or has some Success moving...into central
North Carolina especially late in the weekend and early next week.
The moisture of the mean air mass...and moisture at the
surface...increase Sunday into Monday...with surface dew points
expected to increase to around 70f by Monday...continuing into
Tuesday. With surface ridging over the Gulf currently forecast...it
should be a gradual moisture increase especially at the
surface...and coarse model soundings at least into Sunday afternoon
have a good consensus of at least being weakly capped aloft. Will
maintain the dry forecast Friday night through the weekend...but
with increasing moisture...some guidance suggesting a weakness in
the middle-level heights translating into cooling aloft...at least some
quantitative precipitation forecast by much of the current long-term guidance...and mex MOS probability of precipitation
near or above climatology...will introduce a slight chance of
showers and thunderstorms at the end of the period Monday and
Tuesday. Forcing should be limited to weak surface troughiness...sea
breeze...and/or weak middle-level positive vorticity advection at times...which may be enough
for isolated showers and thunderstorms in a relatively moist air
mass. 1000-850mb thicknesses gradually rise...which...along with MOS
guidance...would suggest highs rising to at or above normal by
Sunday. If the surface dew point is a little higher than forecast
primarily due to slightly less mixing...maximum afternoon heat
indices could rise to around 100 by Monday.
&&
Aviation /06z Wednesday through Sunday/...
as of 145 am Wednesday...
A line of showers lingers along the southern tier of the area
ahead of the surface cold front...which has moved south of all but
the Fay terminal. Ensuing drying in northerly flow has effectively
cleared out IFR/LIFR ceilings over the bulk of the area...and tafs
will show an improving trend through the remainder of the
night...although patchy MVFR ceilings and visibility are possible at
times...especially rdu/rwi/Fay... through an hour or so after
sunrise.
VFR conditions will be prevalent beyond 12z...although a trailing
upper short wave will be sinking south into the area in the late
afternoon. This short wave will be accompanied by broken stratocu or
altocu...and possibly scattered showers across the northern tier
after 21z. Conditions are not likely to fall below VFR anywhere
except possibly rdu and rwi towards 00z...and will not include this
possibility due to the low probabilities at this time.
Looking ahead: high pressure will build into and linger over central
NC Wednesday night through the weekend. This weather system will
bring dry and stable conditions leading to VFR parameters. The
exception will be a brief period of MVFR visibility due to
fog...mainly in the coastal plain.
&&
Rah watches/warnings/advisories...
none.
&&
$$
Synopsis...Badgett
near term...mws
short term...
long term...djf
aviation...mlm
