Area Forecast Discussion
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FXUS62 KRAH 211933
Area Forecast Discussion
National Weather Service Raleigh NC
230 PM EST Sat Jan 21 2017
A mesoscale convective system and associated convectively-
enhanced surface low will track across the Carolinas this
afternoon and early tonight, before lifting away from the
Middle Atlantic coast late tonight. A potent upper level low and
attendant low pressure system will track northeast from the
Deep South into the Carolinas on Sunday, linger over the area on
Monday, then gradually track offshore the Mid- Atlantic coast
.NEAR TERM /THROUGH TONIGHT/...
As of 1125 AM Saturday...
Visibility in fog will continue to gradually improve as the
earlier widespread dense fog lifts into a very low overcast
through early this afternoon. Given this low overcast that is
expected to persist, temperatures will not recover more than
4-8 degrees from 11 AM readings.
Mainly dry conditions are expected through mid afternoon in
central NC. A large MCS (currently in GA) is expected to track
NEwd through the upstate of SC early this afternoon, then track
NE/ENE across central NC this evening. Warm advection, DPVA
associated with a robust MCV and above normal moisture (PWAT
~1.50") will help maintain the MCS as it tracks into/across
central NC. At the same time, a predominately stable airmass
characterized by weak low/mid-level lapse rates and a near-sfc
stable layer should temper the intensity of the system and keep
convection elevated, precluding a severe potential, though
lightning cannot be ruled out. Precipitation chances will be
greatest between ~20Z this afternoon and midnight (~05Z)
tonight, when warm-frontal precipitation --downstream of the
approaching convectively-enhanced surface low related to the
MCS-- and rainfall amounts of ~0.5-1.00", with locally higher
amounts, should be maximized.
The low overcast will likely settle toward the surface and
result in (possibly dense) fog this evening, especially N of
U.S. Hwy 64, where an in-situ wedge airmass is apt to linger.
Lows in the mid 50s tonight.
.SHORT TERM /SUNDAY/...
As of 400 AM Saturday...
Unseasonably rich moisture will surge N/NE from the GOMEX into
the Carolinas on Sunday as low-level flow backs/strengthens in
response to height falls in advance of a potent upper level low
progressing east from the lower MS river valley into the Deep
South. If an MCS develops over the Deep South tonight, it is
possible that convection could affect portions of central NC
Sunday morning, though confidence remains low with regard to
the evolution/track of upstream convection that may develop
later tonight. Otherwise, expect chances for convection to
increase from SW-NE Sunday afternoon as small amplitude waves
rounding the base of the upper low eject NE from the Deep South
into the Carolinas amidst diffluent flow aloft and increasingly
favorable thermodynamics as rich low-level moisture advects
poleward underneath a modified elevated mixed layer
characterized by H7-H5 lapse rates on the order of 7-8 C/km. If
sufficient forcing is available to break the cap at the base of
the EML and/or discrete convection develops upstream and
propagates into central NC during the early/mid afternoon, the
thermodynamic/kinematic environment would support supercells
capable of producing all severe weather hazards. Otherwise, the
primary severe weather threat will not unfold until late Sunday
afternoon/evening when a strongly forced squall line is expected
to propagate into central NC. Expect highs ranging from the
lower 60s (NW) to the mid 70s (SE). -Vincent
.LONG TERM /MONDAY THROUGH SATURDAY/...
As of 230 PM Saturday...
The departing upper low will give way to height rises on Tuesday,
with a brief stint of high pressure at the surface. The ridging
aloft will quickly give way to increasingly cyclonic flow aloft as a
broad trough develops over the eastern US through the end of the
week. A shortwave moving out of the Southwest is forecast to lift
and shear across the Midwest States Wednesday/Thursday, resulting in
limited moisture/lift over our region as the cold front crosses the
area early Thursday. The GFS and ECWMF are in pretty good agreement
on the timing of the front, but the GFS brings a secondary wave
through the base of the trough, resulting in anafrontal precip on
Thursday. Confidence in the longwave pattern and trend from above
normal temps Tuesday/Wednesday toward below normal is high, but the
details of the midweek precip are still uncertain.
.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
As of 125 PM Saturday...
Poor aviation conditions will continue through the 18Z TAF period,
and beyond through the remainder of the weekend...
Widespread LIFR ceilings and mostly IFR-MVFR visibility restrictions
may briefly improve by a category this afternoon, as a warm front
now stretching from SErn NC to central SC to a surface low pressure
over central GA --and following lifting and scattering of low clouds-
- briefly attempts to retreat Nwd this afternoon. However, an area
of warm frontal showers downstream of the surface low will lift NEwd
into central NC between 20-23Z, and this will generally result in
lowering ceilings and visibilities again through the late afternoon
and early evening hours. These showers are expected to continue NEwd
and out of central NC, along with the aforementioned surface low,
between 02-04Z, with mostly LIFR-IFR conditions expected to remain
throughout the night and first half of Sunday. There may also be a
brief period of low level wind shear early tonight (02-06Z) at
RDU/FAY/RWI, as a generally Sly low level jet surges Nwd ahead of
the surface low, atop lingering light surface winds.
Outlook: Generally sub-VFR conditions will persist through Sunday
night, as a powerful upper level low and deepening sfc low pressure
system track across the TN Valley and Srn Middle Atlantic states.
However, a brief lifting to MVFR or scattering to VFR will be
possible south of the Triad terminals early Sunday afternoon, when a
warm front is expected to retreat farther Nwd into central NC. There
will also be episodes of convection, focused around both midday
Sunday and again Sunday evening, the latter of which could become
severe and produce damaging sfc wind gusts and perhaps an isolated
tornado or two. The powerful upper level low and attendant sfc low
will linger over the region Monday and result sub-VFR ceilings and
scattered convection. Improvement should begin by Tuesday as the
system tracks toward the Mid-Atlantic coast, though gusty NW winds
are likely Tue afternoon.