Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
1021 am EDT Thursday Jul 31 2014
Synopsis...the atmosphere across central NC will become very moist
and slightly unstable. This will lead to an unsettled weather pattern
Friday...lasting into the weekend.
Near term /through tonight/...
as of 1015 am Thursday...
For the middle morning update...forecast is on track for increasing
cloud cover and light precipitation from the southwest into the
northwest Piedmont. This will be aided by an exiting upper level 80
knots jet streak and a couple of middle-level disturbances...the first of
which will be a weaker one...moving through the area by late
afternoon...followed by a stronger vorticity maximum overnight which will
move through the western Piedmont by early Friday morning. While
these disturbances could initiate a few thunderstorms...instability
is modest at best and shear values of 35-40 kts may be too far east
to interact with increasing moisture which should first arrive in
the west...thus making severe thunderstorms fairly unlikely. Cloud
cover should keep maximum temperatures this afternoon in the middle 80s despite
thickness values suggestive of near 90 degrees.
The stronger disturbance this evening...while diurnally unfavorable
for convection...will have some help with forcing from an increasing
upslope component and thus continued cloud cover and rain chances
will continue through the night...especially in the western Piedmont
where the wave is anticipated to track. Increased clouds will help
to moderate low temperatures into the middle 60s to near 70 degrees.
Short term /Friday and Friday night/...
as of 310 am Thursday...
Middle-upper level flow backs in response to
the digging l/west trough over the Tennessee Valley/deep south. This places
central NC in the favorable right entrance region of upper jet plus
taps abundant moisture residing over the Gulf. A rather potent vorticity
maximum currently associated with convection across the Southern
Plains/Red River valley projected to cross our region Friday. Lift
associated with this feature along with jet dynamics and available
deep moisture suggest likely probability of precipitation for Friday-Friday evening. Locally
heavy rain may occur across the west where training of
showers/storms appears more probable. Convective parameters over the
sandhills and coastal plain are a bit more supportive of an isolated
severe storm as MLCAPE approaches 1000j/kg and bulk shear 40-50kts
Considerable cloudinesss and numerous showers-storms should hold
temperatures down several degrees below normal. Maximum temperatures will likely not
break 80 degrees over parts of the Piedmont. Maximum temperatures Friday middle
70s northwest to lower 80s southeast. If shower coverage turns out to be more
scattered than numerous...then maximum temperatures may end up being 3-4
Anticipate a brief lull in convective activity sometime Friday night
as one vorticity maximum lifts out of the region but another follows behind
it...but difficult to say for certain if/when this occurs. Plan to
show a gradual decrease in probability of precipitation Friday night with chance probability of precipitation (40
percent) for most locations after midnight. Min temperatures 65-70.
Long term /Saturday through Wednesday/...
as of 310 am Thursday...
Early in this long-term period...a southwest flow aloft is forecast
over central North Carolina with a series of middle-level shortwaves
moving through the area ahead of a trough axis extending from the
Great Lakes through the Ohio and Tennessee valleys. While there are
timing differences...the long-term guidance is fairly consistent in
lifting the trough north by early next week with a modest increase
in heights aloft...although a weak southwest flow aloft tends to
persist over central North Carolina ahead of a weakness in the middle-
level heights to our southwest in vicinity of Mississippi and
Alabama. Extending to the northeast of this weak low aloft may be a
shear axis near or over central North Carolina at least through
early next week.
On Saturday...the NAM moves the surface front farther inland than
the GFS and European model (ecmwf)...such as that former model has much higher quantitative precipitation forecast
toward The Triad than the others. Nevertheless...MOS guidance probability of precipitation
are high throughout central North Carolina Saturday...and given the
inland progression of the front...mid-level waves moving through the
area in the southwest flow...and ample moisture...will continue with
the likely chances for showers along with scattered thunderstorms.
Instability on the NAM and European model (ecmwf) is not as great as on the GFS...
which actually paints a somewhat concerning picture Saturday with
surface-based cape near 1000j/kg...0-3km helicity near 100m2/s2...
and shear in the 20s knots. As local heights should be low given the
degree of low-level moisture...depending on the instability a
rotating cell or two in vicinity of the surface front would be a
possibility if the GFS verifies. For now will focus on the rainfall
but trends will need to be monitored. Basin-average rainfall amounts
late in the week into the weekend could be around 1.5 inches.
While the surface trough slowly weakens...its position relative to
central North Carolina for now demonstrates little change through
the rest of the long-term period. Good chances for showers and at
least isolated thunderstorms continue into Sunday...then as the middle-
level trough lifts north and heights increase modestly aloft...
chances for showers and thunderstorms should start to take on more
of a typical diurnal nature...aided by weak surface convergence...
lingering moisture...and a likely shear axis nearby or overhead. The
precipitation chances for Monday through Wednesday will tend to be
lowest in the northwest Piedmont and highest toward the Interstate
95 corridor...the latter where the deeper moisture and better
instability should be each day. While the European model (ecmwf) really trends toward
a typical diurnal pattern of chances during the day and virtually
dry overnight...the latest GFS forecasts periodic waves moving
southwest-to-northeast along the weak surface boundary which keeps
some chances even into the overnight hours. For now...will forecast
for early next week lower probability of precipitation overnight but only show below slight
chance probability of precipitation in the northwest Piedmont overnight Tuesday when both
the GFS and the European model (ecmwf) agree in a relative minimum of precipitation
1000-850mb thicknesses and expected cloud cover will result in highs
leaning toward the cooler end of guidance...and in many cases a
little below. Some of the bias-corrected guidance has been doing
well of late and will lean toward those which are consistently on
the cool end of guidance. Some areas could start to return close to
90 by Wednesday.
Aviation /12z Thursday through Monday/...
as of 715 am Thursday...
Pockets of MVFR/IFR fog will lift/dissipate prior to 14z. Otherwise
expect considerable cloudiness today though cloud bases will remain
VFR. Surface winds will be light (6kts or less) and variable.
There will be isolated to scattered showers and possibly a
thunderstorm this afternoon due to surface heating and a marginally
moist air mass. The highest threat for this scattered convection
will be south of The Triad terminals and west of kfay.
For tonight into Monday...expect periods of adverse aviation weather
conditions to begin later tonight as the atmosphere over central NC
becomes increasingly moist and unstable. A series of upper level
disturbance will traverse the region through Monday...causing
scattered to numerous showers and thunderstorms. In addition...there
will be an increased probability for prolonged MVFR-IFR ceilings.