Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
920 PM EDT Friday Oct 31 2014
Synopsis... a strong low pressure will develop over the
southern North Carolina tonight... then track off the coast
Saturday. Cold high pressure will build in from the northwest
Saturday night through Monday.
Near term /through Saturday night/...
as of 915 PM Friday...
The only update for the evening and overnight period will be to add
the chance of thunderstorms to the western Piedmont and western
sandhills. Rain is expected to overspread the western and central
zones... with more scattered activity in the east overnight.
The 00z/01 November data analysis indicated the closed middle/upper low
over the lower Ohio Valley was digging south-southeast toward the
Knoxville/Asheville areas. Scattered convection... some
producing small hail and frequent lightning... continued to
develop/spread eastward from the southern foothills into the
southwestern Piedmont. Strong lift from the approaching middle level
circulation combined with the residual weak instability has been
responsible for the convection that has made it to very near the
Yadkin River as of 920 PM. As this convection shifts east...
weakening is expected as the boundary layer instability is lost.
However... the large scale strong upward motion will continue to
allow for large areas of rain and showers to overspread much of our
region overnight... in particular the western and central zones...
as the lift area shifts east with time. Temperatures in the west
will continue to fall into the 40-45 range... with readings in the
east and southeast expected to rise from the current upper 40s into
the 50s. Winds will likely not increase in the western zones until
daybreak... and a bit later in the east due to the current projected
track of the middle/upper low to our west and south... and the surface
wave now developing over kclt (which will move slowly east toward
Previous discussion follows below...
This evening through Sat: primary concern remains the brisk and
gusty winds expected to develop Sat afternoon... as the risk of any
frozen precipitation in the far western and SW County Warning Area continues to decline.
Pockets of brief heavier rain are possible over the western Piedmont...
but should cause nothing more than minor street flooding at most.
Overview: highly anomalous (heights dropping to around 5 Standard
deviations below normal) middle level low pressure now dropping into
northern Indiana is still expected to intensify as it drops southeast...
to near avl by Sat morning... crossing SC Sat before swinging NE off
the NC coast Sat night. The greatest height falls will be over SC
(nearly 180 M in 12 hrs)... while they should be muted but still
impressive at 80-120 M over central/southern NC. At the surface...
primary low pressure just off the NC coast will move NE tonight...
as secondary low pressure forms over south central NC. This low will
track east along the central/eastern NC/SC border region Sat morning
while slowly deepening... before moving east/NE off the NC coast Sat
afternoon/evening while strengthening further.
Pops: following the convection-allowing model trends and
extrapolation of existing precipitation... will bring low chance probability of precipitation into
the far western County Warning Area early this evening... spreading slowly east with
probability of precipitation increasing west to east overnight. Given the intense deep layer
forcing for ascent... peak in middle level deformation... and
strengthening low level moisture transport... categorical probability of precipitation still
look good for the western and southern County Warning Area focused on late tonight through
middle afternoon Sat... as this is where the DPVA will be maximized in
conjunction with the greatest upper divergence in the left exit
region of the diving jet to our west... yielding deep and vigorous
lift. However... both explicit and convection-allowing as well as
the sref have trended toward much lower coverage over the NE County Warning Area...
and based on the slightly more southerly track of the middle level low
and associated DPVA and upper divergence... this trend looks
reasonable... and have nudged probability of precipitation downward in areas North/East of
the Triangle to 30-50% through the event. Will be slow to taper probability of precipitation
down in the southeast County Warning Area Sat afternoon... given lingering steep middle
level lapse rates and greater deformation here.
Ptype and thunder concerns: models have been trending warmer with
the boundary layer temperatures in recent runs... and now suggest a smaller
chance of any mixed-in wet snow over a smaller area Sat morning...
primarily along and southeast of a line from int to meb. But even here...
yesterday's indications of a near-freezing isothermal layer with
saturation well up above -10c are looking less certain... with
warmer air around 925 mb and less moisture above -10c. Will continue
to hold any mention of wet snow out of the forecast... given the
very low chances... however wouldn't be at all surprised to see a
little snow mixed in briefly... particularly if any instability can
be achieved which would drive up precipitation rates. Forecast middle level
lapse rates remain steep... 6.5-7.0 c/km tonight slipping just a bit
to 6.0-6.5 c/km Sat... but with the potential for slightly less
moisture through the column (pw values peak near 0.75" which is just
near normal)... any lightning strikes would be few and only where
extremely slim instability is augmented by intense dynamic forcing
Winds: it should take awhile for the surface winds to pick up Sat...
given the initial slack gradient as the secondary low forms over
south central NC. As this low shifts east and offshore and deepens
during the day and Sat night... the rapidly tightening gradient
should result in sustained winds peaking at 15-20 kts (potentially
higher in spots) in the late afternoon through evening. Still expect
gusts of 25-30 kts. This is just under Wind Advisory criteria... but
impact-wise we should still see light objects getting tossed around
and a few branches coming down... especially considering that most
leaves are still on the trees. Wind chills may hold in the 30s
to lower 40s over the western half of the County Warning Area Saturday.
Temps: pattern of earlier forecasts to undercut statistical guidance
substantially still appears prudent... with a chilly Canadian-source
air mass moving in... strengthening cold air advection... steady
precipitation for a good chunk of the day... and abundant cloud cover. But
it may take awhile for the coolest and most dense air to get over
the mountains... especially given the slow/late formation of the
secondary low pressure which would retard cold air advection initially on Sat. Have
nudged highs up a degree or so to 46-55 west to east... still well
below statistical guidance.
For Sat night: model output indicates quickly tapering rain chances
from northwest to southeast Sat evening as the secondary surface low shifts off
the Outer Banks. Will maintain likely probability of precipitation southeast and slight chance at
most west for a few early evening hours... followed by very low to
no probability of precipitation after 06z. Expect gradual clearing west to east overnight
once northwest flow and downslope drying take over. The upper low will
shift east off the NC coast Sat evening with height rises maximized
over SC at 180+ M Sat night. Middle level temperatures will remain rather cool
however as sheared vorticity streaking down through NC on the west
side of the departing trough delays the offshore kick of the mean
trough until after 12z sun... so we're likely to see lingering middle
clouds Sat night... clearing out of the western County Warning Area late. Winds should
be on a steady decreasing trend overnight but the boundary layer
will stay stirred enough to limit full radiational cooling despite
the chilly nature of the incoming air mass and its low dewpoints.
Lows 33-38. A few outlying spots in the western County Warning Area will likely reach
32f... but this should be the exception rather than the rule. -Gih
Short term /Sunday through Sunday night/...
as of 130 PM Friday...
Fair but unseasonably cool weather is expected on Sunday as high
pressure builds in from the west in the wake of the departing
trough. Forecast soundings and cross sections suggest a few
lingering middle-level clouds at the start of the day...but skies
should become mostly sunny by afternoon given dry northwest flow. A little
breezy still in the morning and early afternoon while the high is
building in...until the pressure gradient relaxes later in the day. Low
level thicknesses are prognosticated to bottom out about 50m below
average...around 1300m early Sunday...recovering to 1315m by late-
day. This suggests highs in the lower 50s...about 15 degree below
High pressure is then prognosticated to become centered over the Carolinas
by Monday morning. Given the airmass in place...light winds under
the highs...and mainly clear skies...the expectation is still there
that most of our County Warning Area will see a growing-season-ending frost or
freeze Monday morning. However...will need to keep a close watch
for the potential for some orographic cirrus...given northwest flow and
some moisture spilling over the Middle Ridge axis. While confidence in
this happening is customarily low right now (3 days out)...if it
does materialize...looks like it would be mostly across our far
western zones...so a decent frost/freeze event across most of our
County Warning Area still looks likely. For now...will leave lows as-is...ranging
from the upper 20s west to lower 30s east.
Long term /Monday through Friday/...
as of 130 PM Friday...
Ridging at surface and aloft will provide nice weather across central NC
Monday and Tuesday. Thicknesses will continue moderating during this
time...however with the surface high still centered over US Monday
night...look for lows a couple degrees below normal Tuesday
morning...upper 30s to around 40. Highs Monday around 60 and middle-upper
60s on Tuesday.
By late Tuesday...the high center will move off the coast...but the
ridge axis will remain extended across the Carolinas. With low level
flow becoming more southwesterly and a thickening canopy of cirrus with
enhanced SW flow aloft over US...lows Wednesday morning will be
warmer...in the middle-upper 40s.
The next northern stream short wave will begin approaching our
region during the day Wednesday and it's associated cold front crossing
central NC late Thursday. Thus...Wednesday will see increasing
cloudiness and warming nicely ahead of the front...but remaining
dry. The best chance for any rain with this system will be on
Thursday along and just ahead of the front...but moisture profiles
across our area are modestly moist at best...so for now...will keep
probability of precipitation limited to 20 percent or less on Thursday. Highs climbing above
normal (around 70) for Wednesday and Thursday.
Right now...next Friday looks to be breezy and cooler in the wake of
the front...with decreasing cloudiness through the day. Highs
cooling back down to near or below normal for weeks end.
Record low-maximum temperatures for November 1st
Aviation /00z Saturday through Wednesday/...
as of 800 PM Friday...
24-hour taf period: MVFR/IFR conditions associated with scattered to
widespread showers are expected at the int/gso/Fay terminals
overnight into Saturday morning (06-15z) as a potent upper level low
digs south-southeast through the western Carolinas and a surface low pressure
system begins to deepen offshore the Carolina coast. MVFR ceilings
and brief showers will be possible at the rdu/rwi terminals between
12-18z Sat...though confidence is low enough to preclude mention at
this time. The primary aviation hazard associated/west this system will be
strong northerly winds (nne to nnw) becoming sustained at 15-25 knots
with frequent gusts up to 30-35 knots during the day Sat. Winds will
increase at the int/gso terminals between 12-16z Saturday morning
and between 16-21z Sat afternoon at the rdu/Fay/rwi terminals.
Looking ahead: gusty northwest winds will persist Saturday night...then
gradually weaken during the day Sunday as Arctic high pressure
settles over the region from the northwest. Benign weather /VFR
conditions/ will prevail Sunday through Wednesday as an approaching
upper level ridge anchors high pressure over the region. -Vincent