Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
645 am EDT Friday Oct 9 2015
Synopsis... high pressure will extend across the region through
this morning... then push offshore later today. A cold front will
move southward through the region late tonight through Saturday.
Cool high pressure will build in from the northwest Saturday night
through Sunday... then settle over the area Monday... as low
pressure tracks just off the North Carolina coast.
Near term /today through tonight/...
as of 245 am Friday...
Earlier forecast is on track and supported by the latest observed
trends and model output. Skies are currently mostly clear over the
area beneath subsidence (noted on water vapor imagery) just behind a
weak wave moving through NE NC/southeast Virginia. Latest surface analysis shows
the cold front extending from just southeast of chi to near stl to just
south of ddc... making swift progress to the east-southeast... attending a
prominent middle level trough. As this trough approaches NC from the northwest
later today with increasing DPVA aloft... clouds will increase from
west to east over the County Warning Area... although the greatest forcing for
ascent will arrive after midnight. Models generally agree on showers
easing into the northwest County Warning Area late today... and have retained the low
chance late-day pop here. Despite the increase in clouds...
thicknesses start the day 15-20 M above normal... so have nudged
temperatures up a degree or so to well-above-normal values of 79-83. Upper
level divergence ramps up overnight over NC from west to east as the
deepening middle level trough arrives... and as the surface front drops
into northern NC with precipitable water values rising above 1.5". Will keep current
trend of ramping up probability of precipitation to likely/categorical range overnight...
highest across the south where DPVA will be most pronounced. Forcing
for ascent rooted in the lower levels will be lacking however...
with little to no mass convergence along the low level frontal
zone... plus low level winds will have a notable westerly component
which will limit moistening in the lowest km prior to frontal
passage... and all of this will curb overall quantitative precipitation forecast. Expect lows in the
Lower-Middle 60s. -Gih
Short term /Saturday through Saturday night/...
as of 300 am Friday...
As the surface front pushes southward and settles just south of the
County Warning Area... high rain chances will continue through much of Saturday...
with anafrontal rainfall supported by above normal precipitable water... strong
upper divergence... and middle level DPVA and height falls. Will have
high likely probability of precipitation across the far north with categorical elsewhere.
Temperatures Sat will follow an atypical pattern with highs in most places
occurring early in the day with steady or possibly falling readings
during the day... as cooler air pours into the region on northeast
low level winds. Expect highs to be not far off morning lows... in
the middle-upper 60s under thick cloud cover and periods of rain. Model
consensus indicates that storm total quantitative precipitation forecast should be under a half
inch. The middle level trough will be slow to exit as it continues to
amplify late Sat into Sat night... potentially closing off over eastern
NC... and will be slow to trend probability of precipitation downward... keeping likely to
categorical in the east and southeast County Warning Area well into the night... although
northwest sections should see at least partial clearing late as the column
dries out with increasing subsidence on the immediate northwest side of the
amplifying shortwave trough. This partial clearing with diminishing
winds and cold air advection supports lows in the 45-50 range Sat
night from the Triangle north and west... with lower 50s in the southeast
where the clouds/precipitation will hold in overnight. -Gih
Long term /Sunday through Thursday/...
as of 245 am Friday...
Fairly quiet forecast for the long term period although Sunday
begins with some uncertainty as a cutoff upper level low sits over
South Carolina potentially brining a few showers to our southeastern
counties. Otherwise cold air advection will have our low
temperatures Sunday morning in the upper 40s to lower 50s. The
cutoff low will begin to weaken and lift out to the northeast later
on Monday while a longwave trough begins digging down into the south
on Monday night. A surface low pressure system over the Great Lakes
will usher a surface cold front through our area on Tuesday but at
the moment...models indicate a mostly dry frontal passage although a
few showers cant be ruled out...especially in the north. Therefore
will maintain slight chance probability of precipitation in the forecast.
Once the cold front pushes through the County Warning Area...the longwave upper
trough will begin to relax and flow will become more zonal. In
addition...a surface low pressure will build over the southeastern
United States. This will keep things dry for the rest of the long
term period and beyond. With few exceptions...temperatures during
the period will top out in the 70s with overnight lows in the 50s.
Aviation /12z Friday through Tuesday/...
as of 645 am Friday...
VFR conditions are likely to hold through 06z tonight/early Sat
morning... but worsening aviation conditions will soon follow
between 06z and 12z. High pressure over the area will move off the
coast this morning... as a cold front approaches from the northwest.
While a few showers are expected ahead of and with the front later
today... most of the rain and low clouds will occur behind the
front... which likely won't pass through central NC terminal sites
until near or soon after the end of the taf valid period. The
current largely cloud-free skies will gradually be overtaken by
clouds from west to east today... with bases thickening and lowering
but likely to remain VFR through 06z. Rain will become more steady
and widespread after 06z especially at int/gso... with ceilings
everywhere falling to IFR or perhaps LIFR. Scattered light showers
will begin to arrive after 20z in the int/gso region... after 22z at
rdu... and mainly after 02z at rwi/Fay... and will be followed by a
better chance of MVFR to IFR visibilities in rain later tonight into Sat
morning. The cold front is expected to drop through rdu/rwi 09z-
12z... causing a shift in winds to northeast and increase in speed.
Looking beyond 12z Sat morning... poor aviation conditions are
likely to last through Sat. The cold front is expected to move south
through the rest of the taf sites between 12z and 18z Sat. Surface
winds will shift with frontal passage from west-southwest to be from the NE...
increasing to 10-15 kts... and ceilings just behind the front will fall
to IFR with periods of rain. Rain will taper off with rising ceilings
late Sat night through Sun morning as surface high pressure builds
in from the northwest with winds becoming lighter and more variable Sun
afternoon through Monday... although brief MVFR ceilings are possible Sun
night as low pressure tracks northeastward just off the NC coast. A
mostly dry and VFR cold front will move through the area Tuesday. -Gih