Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
330 PM EDT Tuesday Sep 23 2014
Synopsis.. upper level low over the Carolinas tonight will
slowly weaken over the area through mid-week. Meanwhile...at the
surface...an area of low pressure will form and track north along a
stalled coastal front Wednesday and Wednesday night as high pressure
remains wedged across the Piedmont through late week.
Near term /tonight/...
as of 330 PM Tuesday...
Overview: upper level low will continue to drift northward over
western NC tonight. Shortwave energy rotating around the upper low and
strengthening divergence aloft underneath an upper jetlet on the east
side of the middle/upper level circulation center will aid in
cyclogenesis along a coastal front/inverted surface trough as high
pressure over the middle-Atlantic wedges southward through central and
western portions of the Carolinas. The
Remainder of the afternoon: light precipitation/sprinkles continue to
steadily fan north across central NC the nice upper diffluence
pattern showing up on WV imagery. This precipitation should remain light
through the afternoon/early evening...with a tendency to dry
up/dissipate as it advances northward into the Piedmont as it
encounter drier air to the north while becoming farther removed from
the low-level easterly onshore flow. Will carry low-end likely probability of precipitation
across the sandhills/southern coastal plain...decreasing to slight
chance into the western Piedmont where precipitation will likely be of the
non-measurable variety. Precipitation amounts will remain light.
Tonight: precipitation/lift will start to crank up late tonight/Tuesday
morning with initial sharpening of the inverted coastal trough and
low pressure wave development. Resultant enhanced Atlantic
transport/isentropic lift inland will yield increasing probability of precipitation after
midnight with motorists east of the Triangle expected to contend
with a wet morning commute into work. Expect a sharp cut-off in probability of precipitation
and quantitative precipitation forecast west of the Triangle with rainfall amounts Wednesday morning
ranging from a third of an inch over the southeast to near a tenth
across eastern portions of the Triangle.
Lows tonight in the middle 50s northwest to near 60 east.
Short term /Wednesday through Thursday night/...
as of 330 PM Tuesday...
Overview: upper low over western NC will undergo gradual filling
Wednesday-Wednesday night as the main circulation center lifts north
up the Lee-side of the Appalachians to near the Mason-Dixon line by
daybreak Thursday. The coastal low will follow suite...tracking
inland across NC coastal areas during the day on Wednesday...
reaching the Delaware-Maryland-Virginia coast Thursday morning.
There still remains some considerable spread with respect to the
details surrounding this system. Forecaster confidence is well below
average...especially with respect to to quantitative precipitation forecast amounts. The European model (ecmwf) has been the most
consistent solution over the GFS and NAM...and thus will be the
model of choice. Moderate to heavy rain will spread north across the
NC coastal counties through the day with a sharp east-west gradient
in quantitative precipitation forecast...though how far inland that gradient will setup is unclear
at this time. For now will have quantitative precipitation forecast amounts ranging from three-
quarters of an inch across the coastal plain...a quarter of an inch
in the Triangle...to only a few hundredths in The Triad. Of
note...if the hi-res cam models verify...rainfall amounts across
central and eastern portions of the forecast area could easily be
cut in half. Precipitation chances should decrease from SW-NE
Wednesday evening/night as the coast low lifts north of the area.
Temperatures: similar to today...expect little diurnal variation in
temperatures. Highs in the lower to middle 60s. Lows in the 57 to 61. -Cbl
For Thursday/Thursday night: the coastal low is expected to be to our
northeast over southern Delaware-Maryland-Virginia by Thursday morning. The surface high
centered well north of the low... out over the northwest Atlantic... will
continue to ridge west and south into the interior midatlantic...
eventually closing off a weaker high over the Great Lakes by late
Thursday night. Heights aloft over NC will steadily rise as the remnant
trough leftover from the upper low continues to slowly dampen and
lift out... supplanted by a westward-easing ridge aloft off the
southeast coast. We expect some daybreak fog/stratus... but the
limited moisture depth... mild mid-levels... low precipitable water... and only weak
dynamic lift should support more clouds than sun during the day...
and just small probability of precipitation. A lingering 850-700 mb shear axis running from
SW to NE into central NC could bring about enough upslope-directed
low level flow for enhanced stratocu Thursday night over the northwest Piedmont.
But mainly dry weather is anticipated dry weather given the overall
deficiency of moisture. The NAM is quite wet Thursday night over SC into
the southwestern Piedmont/foothills of NC... in contrast to the much drier
GFS/ECMWF... and this drier solution is supported by the sref which
depicts very low probabilities of measurable rain over central/western
NC... holding these higher chances at the coast where upper
divergence will be greatest (yet still modest). Thicknesses Thursday are
projected to be close to normal... so with at least partial sunshine
through the day... have bumped up highs slightly to 74-79. Lows 56-
62 under partly cloudy skies... a bit greater sky cover south and southeast
than north. -Gih
Long term /Friday through Tuesday/...
as of 230 PM Tuesday...
For Friday/Friday night: the weakening middle level pattern over the eastern
Continental U.S. Leads to greater model differences and reduced confidence in
the details later in the week. High level flow speeds up a bit...
sourced from northern Mexico and the northwest Gulf... which should mean a
continuation of some middle-high cloudiness streaming over central NC.
But given the weak steering flow... lack of dynamics... and marginal
moisture flux into the area... the drier GFS remains favored over
the wet NAM... as the former has some support from both the European model (ecmwf)
and the sref members. Will keep the forecast dry. Still expect a
veil of clouds through the day... but thicknesses hold near or just
a tad below normal... so have brought highs up slightly to 74-77.
For Sat through Tue: looks like a mostly dry period with temperatures not
far from normal... but notable model differences with the overall
pattern will lead to lower-than-usual confidence in the daily
details of cloud cover and temperatures. Surface high pressure looks to sit
over the interior midatlantic/Ohio Valley over the weekend... with
both the GFS and European model (ecmwf) showing little to no precipitation over central NC
as we remain cut off from any substantial moisture source. Looking
aloft during this time frame... a baggy north-south elongated middle
level low extends over the central Continental U.S.... while weak troughing
holds just off the East Coast... leaving a weak pseudo-blocking high
over the eastern Continental U.S.. the GFS breaks this down by Monday via a
positively-tilted polar trough digging over New England and down
through the eastern states... whereas the European model (ecmwf) maintains this ridge
axis overhead. In either case... models show central NC remaining
dry with no good source of moisture into the area and little to no
dynamic forcing for ascent. If the polar stream does dip enough
southward Mon/Tue... we may see a backdoor front dropping into the
vicinity... however it looks like the much chillier air mass will
almost certainly hold to our north through Tuesday. -Gih
Aviation /18z Tuesday through Sunday/...
as of 230 PM Tuesday...
24-hour taf period: despite light rain overspreading the area...VFR
conditions will continue through the evening and into the first half
of the overnight hours. Conditions are expected to rapidly
deteriorate east to west between 06 to 15z Wednesday...via strong
easterly Atlantic moisture transport in advance of a wave of low
pressure tracking NE along a coastal front. As the steadier precipitation
pushes inland...expect at least IFR at east terminals(kfay/krwi)with
at least MVFR across the piedmont(krdu/kgso/kint).
Additionally...expect breezy northeasterly wind of 20kts by the late morning
and into the afternoon hours as pressure gradient tightens in
response to sharpening coastal front and low pressure development.
Looking ahead: sub-VFR conditions will persist Wednesday night with a
return to VFR conditions on Thursday as the surface low tracks north
away from the area. Otherwise...expect VFR conditions to prevail
through the remainder of the week as high pressure extends into the
Carolinas from the north.