Area Forecast Discussion
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FXUS62 KRAH 260810 CCA
Area Forecast Discussion
National Weather Service Raleigh NC
410 AM EDT Sun Mar 26 2017
A backdoor front will settle into and stall over central VA today,
while a weakening upper level disturbance pivots northeast across
our region. Bermuda high pressure will otherwise extend west into
the southeastern U.S. through Mon.
.NEAR TERM /THROUGH TONIGHT/...
As of 345 AM Sunday...
On the sern periphery of an upper low that will become an open wave
as it lifts from the Middle MS Valley NEwd across the Great Lakes
through tonight, a lead mid level trough axis now extending across
the TN Valley will edge Ewd into the wrn NC Piedmont this afternoon.
An associated axis of moisture aloft, and weak forcing for ascent,
will likely result in the slow NEwd maintenance of a band of showers
now moving through W-central GA. While these showers will likely
weaken with Ewd extent into the W-central Carolinas, as they near
the wrn periphery of a sub-tropical ridge and associated dry and
capping influence centered near Bermuda, a few may drift into the
wrn Piedmont by mid-late afternoon. While CAM`s also indicate the
potential for much shallower showers to the east, ones capped above
by the aforementioned subsidence inversion sampled by proximity
RAOBS around 5-6 k ft last evening, it`s unlikely that barely
positive CAPE would support deep enough cumulus to indeed support
any showers elsewhere.
Otherwise, like Sat, diurnal heating of a seasonably moist BL
characterized by surface dewpoints generally in the lower 50s should
again support a scattered to broken cumulus field by midday-early
afternoon, but with enough breaks of sun to allow temperatures to
again reach the middle to upper 70s.
While shortwave ridging aloft behind the departing mid level trough
will likely yield dry conditions tonight, the mid level moist axis
accompanying the trough will be left behind; and this may support a
lingering shower or two over the wrn Piedmont tonight. Low
temperatures centered in the middle 50s.
.SHORT TERM /MONDAY THROUGH MONDAY NIGHT/...
As of 405 AM Sunday...
A kicker shortwave trough now over the Four Corners, and a trailing
lower amplitude perturbation near the Baja of CA, will both move
generally Ewd across the MS Valley on Mon, and into the Carolinas
Mon night. Lapse rates aloft will steepen into the 6.5-7 C/km range
first Mon by a modified EML plume preceding the troughs, then as the
associated cool pool aloft accompanying the troughs moves overhead
Mon night, with an associated increase in (still-weak) instability
of around 500-1000 J/kg over the Carolinas Mon afternoon and night.
The models indicate the perturbation now over the Baja will slightly
outpace the main shortwave trough over the Four Corners, such that
the associated forcing for ascent accompanying the perturbation will
spread E and interact with the weak instability axis over the
Carolinas, and consequently result in scattered showers and storms
Mon afternoon, particularly along and west of U.S. 1, where diurnal
timing of the forcing for ascent will be maximized.
The chance of showers and storms will increase overnight, as the
parent shortwave trough and preceding forcing for ascent cross the
Appalachians. The aforementioned steep lapse rates aloft, and
relatively cool thermal profiles/low freezing level around 10 k ft,
and 30-35 kts of mid level flow, may support the development of
small hail in the storms. Instability would appear to be too limited
to support any severe hail threat, however.
It will again be warm, in the middle 70s to around 80 degrees, with
lows in the middle to upper 50s.
.LONG TERM /Tuesday through Saturday/...
As of 355 AM Sunday...
Tue/Tue night: The mid level shortwave trough and its accompanying
surface trough is expected to cross the area during the first half
of the day, during which time there should be enough low level
heating in combination with the thermal trough aloft to prompt
marginal to moderate instability. Forecast soundings show subsequent
warming and drying aloft during the latter half of the day as the
shortwave trough shifts to our east and 850 mb SW flow slowly
decreases. With above normal PW decreasing in the west in the
afternoon, expect a good chance of showers and storms in the
morning, tapering down west to east through the afternoon. The
lagging weak cold front will drop SE through the area Tue night with
low and mid level winds becoming northwesterly and westerly
respectively, and with heights rising aloft behind the wave, expect
a trend to mostly clear skies overnight west to east. Much above
normal thicknesses in the warm sector support highs of 77-82. Lows
52-58 with marked cooling not arriving until late.
Wed-Thu: Skies should be partly to mostly sunny Wed with generally
dry air in place, although a weak inverted surface trough extending
up through central NC -- ahead of an approaching backdoor front --
along with a col area at 850 mb and a rebounding of PW values across
southern NC may yield broken clouds in the afternoon. The backdoor
front is expected to drop NNE to SSW through central NC Wed night
with cooling low levels and increasing moisture in the 900-800 mb
depth, which is likely to bring a trend to mostly cloudy skies
overnight. These clouds will disperse somewhat with heating and
mixing across the Coastal Plain Thu, but the Piedmont will see
strengthening moisture influx with an increasing 850 mb SW flow atop
the intruding cool wedge nosing in from the NNE, and this
overrunning should lock in clouds especially in the western CWA
through the day, with some light drizzle possible in the far western
Piedmont. High uncertainty with temps in this period given the
lingering mild air Wed ahead of the backdoor front and the incoming
wedge air mass for Wed night/Thu. Prefer the milder ECMWF/Canadian
thicknesses Wed, indicating highs in the mid 70s to near 80. Will
have lows of 46-53 Wed night and highs Thu 60 N to 70 S.
Thu night-Sat: A deep low over the Four Corners late Tue will track
eastward and gradually fill late in the week, in response to yet
another powerful storm system crashing onto the West Coast and
crossing the Rockies, but the degree of weakening of the lead system
and the tilt of its trough differs between the ECMWF/GFS. A
deepening polar low just SE of the Canadian Maritimes will help
drive the cool surface ridge southward into the NC Piedmont, and as
the mid level low/trough approaches late week, moist isentropic
upglide atop the cool air mass should lead to increased rain
chances, considerable cloud cover, and cooler temps. The open wave
will then shift eastward through the region with ample deep moisture
in place and an occluded frontal system, so have kept in a good
chance of rain Thu into Fri, and we may see a few thunderstorms Fri
especially across the SE CWA, which could get into the warm sector.
The trailing cold front is expected to push through and SE of the
area sometime Sat, so will show a trend toward lower pops. Expect
temps to be below normal Fri and near normal Sat, but uncertainty in
these details is high, given the strength of this frontal system and
model timing differences. -GIH
.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
As of 155 AM Sunday...
Mainly VFR. Morning to midday heating of a seasonably moist air mass
around Bermuda high pressure will favor the development of a field
of stratocumulus that will initially develop around 2500-3000 ft,
and may be briefly broken in that range this morning in the few
hours surrounding 15Z, before lifting into the 3500-5000 ft range
this afternoon. The passage of an upper level disturbance and
associated moistening aloft may allow a few showers to move into and
affect INT and GSO this afternoon and evening. Otherwise, SSWly
surface winds will increase into the 8-13 kt range after 14-15Z,
with occasional gusts up to 20 kts, then lessen after sunset.
Outlook: Late night-early morning fog and/or stratus, then MVFR
range cloudiness as that moist layer is heated through midday, will
be possible each day through the middle of next week. Otherwise, a
chance of showers and associated sub-VFR conditions will exist late
Mon through Tue. The probability of any such shower at any given TAF
site will be relatively low, however, given limited overall
coverage. Lastly, a backdoor cold front will settle into SC Wed
night; and "overrunning" flow atop the boundary will result in the
development of MVFR-IFR conditions and associated cold air damming
as early as late Thu, but more likely just beyond the outlook TAF
period - Thu night-Fri.