Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 212023
Area Forecast Discussion
National Weather Service Raleigh NC
323 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 THROUGH SUNDAY NIGHT/...
-- Changed Discussion --As of 320 PM Saturday...
...Severe storms possible, especially over southern and eastern
NC, late Sunday....
There should be ongoing showers and possibly a few
thunderstorms Sunday morning, associated with increasing upper
diffluence ahead of a deepening upper trough over the Deep South
and possibly an MCV from Lower Miss Valley convection tonight.
Given the stable airmass over central NC today will hold
tonight, the convection should be elevated and not pose a severe
threat. The biggest concern will be how the early morning
precip impacts destablization during the afternoon, as the upper
trough closes off and a surface low deepens toward the southern
Appalachians. The primary surface low will initially be over
the mid-Miss Valley region, while a secondary low develops over
northern Georgia. The two lows are then forecast to consolidate
and deepen sub-990mb (-4 to -5 STD) somewhere in the vicinity of
the TN/NC/VA border area Sunday night, an fairly typical
pattern. Thus, confidence is still below average in the details
of potential severe weather on Sunday evening.
Strong upper level forcing will arrive after 20Z under
increasing diffluence and 100+ meter height falls, not to
mention the couple left exit/right entrance regions of the upper
jet. Convection will increase from the south during the evening
as low-level warm advection increases and a 50kt LLJ crosses SC
and southeast NC. It seems pretty certain that a QLCS will
cross northern FL, GA and SC, and the question continues to be
how far north the line will extend as the warm front penetrates
inland in the lingering wedge airmass over central NC. The
deepening of the surface low just west of the mountains and the
isallobaric response should help to draw the low/mid 60
dewpoint air into the the Piedmont, at least as far as US-1,
resulting MLCAPE that could approach 1000 J/KG. This combined
with a 100kt flow at 500mb and strong backed low-level flow
would support some discrete supercells and a straight line
wind/tornado threat, especially in the vicinity of the
retreating warm front. It is also possible that the QLCS to our
south disrupts moisture transport and mass fields to the north,
and renders convection less robust.
The main takeaway is that there is a lot in place for severe
weather, but tonight`s precip and the mesoscale evolution to
our south still leads to a fair amount of uncertainty.
QPF on Sunday and Sunday night will be heavy in spots given the
unseasonably moist airmass, but widespread flooding is not
expected. One area to watch may be the northwest Piedmont Sunday
night as the upper low pivots over the mountains and moderate is
is potentially sustained for several hours. otherwise, the bulk
of the deep convection is expected to depart to the east by 06Z
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.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 /20Z 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.