FXUS62 KRAH 211508

Area Forecast Discussion
National Weather Service Raleigh NC
1008 AM EST Sat Jan 21 2017

A mesoscale convective system will track across the region late
this afternoon and evening. 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 on


As of 400 AM Friday...

...Dense Fog Advisory now in effect through 11 AM for the Srn
half of central NC...

Widespread dense fog developed shortly after Midnight this
morning and will be slow to dissipate due to the presence of low
ceilings and unseasonable low-level moisture assoc/w 925-850 mb
warm advection. Expect a slow/gradual improvement a few hrs
after sunrise, with fog lifting by late morning.

A few elevated showers cannot be ruled out late this morning
and early this afternoon in the Sandhills and SW Piedmont
(primarily near the SC border) in association with low-level
warm advection and small amplitude waves in SW flow aloft.
Otherwise, dry conditions are expected through mid afternoon in
central NC. A large MCS (currently in LA/MS) is expected to
track NE through the Deep South this morning, progress through
upstate SC early this afternoon, then track NE/ENE across central
NC late this afternoon through 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, and rainfall amounts of ~0.50" are
entirely reasonable, with locally higher amounts possible.
Expect highs in the mid 50s to lower 60s (south) this afternoon
and lows in the mid 50s tonight. -Vincent


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


As of 300 AM Saturday...

Sunday night through Monday night:

Based on latest model consensus, ongoing squall line/QCLS associated
with the deepening upper low moving into the Southern Appalachians
and attendant cold front moving through the area should be in the
process of exiting the area late Sunday evening ~06z, bringing an
end to the damaging wind and isolated tornado threat.

The deepening upper low will take it`s sweet time lifting north into
the Mid-Atlantic region on Monday, resulting in another
unsettled/wet day across the area. So after the potential for a
brief lull Sunday night as the dry slot overspreads the area, rain
chances should increase once again as instability showers
accompanying the upper low is expected to move through the area
during the day on Monday. Cooler behind the lead front, aided by low
clouds and showers. Highs Monday ranging from mid 50s NW to near 60
SE. As the system finally begins to pull away from the region Monday
night/early Tuesday, lingering moisture/clouds are expected to clear
out from SW to NE Monday night as the system exits the region. Lows
Monday night in the 40s.

Tuesday through Friday: Height rises over the region Tuesday and
Wednesday will result in a return to near or just above normal
temps. Shortwave energy moving through a positively-tilted trough
setting up/sharpening over the Eastern US, could lead to scattered
showers across the area late Wednesday night and Thursday as the
attend cold front moves through the area. A colder pattern with
below normal temperatures on Friday and into the weekend with good
model agreement of a deep trough developing over the Eastern US.


As of 500 AM Saturday...

     Extremely Poor Aviation Conditions This Weekend...

24-HR TAF Period: Adverse aviation conditions are expected to
prevail through the entire TAF period. VLIFR/LIFR visbys assoc/w
dense fog and low ceilings are likely to prevail through ~15Z
before conditions begin to improve. Observational trends along
with the latest model guidance suggest dense fog will begin to
lift by late morning, with visbys improving to IFR/MVFR and
ceilings to IFR (500-1000 ft AGL) by early to mid afternoon,
however, showers (and perhaps isolated thunderstorms) expected
to track across central NC (from WSW- ENE) later this afternoon
and evening will hamper further improvement. Conditions will
deteriorate to LIFR/VLIFR in the wake of precipitation late this

Looking Ahead: Poor aviation conditions will persist Sunday/
Sunday night as a powerful upper level low and deepening sfc
low pressure system track across the Deep South into the
Carolinas. Low ceilings will persist at most terminals with
episodic convection, some of which could become severe and
produce damaging sfc wind gusts late Sunday afternoon/evening.
The powerful upper level low and attendant sfc low will linger
over the region Monday resulting in 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. -Vincent


Dense Fog Advisory until 11 AM EST this morning for NCZ041-042-



NWS RAH Office Area Forecast Discussion