Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 220010

Area Forecast Discussion
National Weather Service Raleigh NC
710 PM EST Sat Jan 21 2017

An elongated convectively-enhanced surface low will track across the
Carolinas through 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 on Tuesday.


As of 710 PM Saturday...

Recently updated the forecast, mainly to advertise a slightly
quicker ending to the categorical PoPs southwest to northeast across
central NC this evening.

A strong line of convection and a attendant meso low was exiting off
the SC coast at 00Z. This system has disrupted the inflow of
moisture into central NC, leading to less intense rainfall rates
than previously thought. In addition, a mesoscale convectively-
induced vorticity center was lifting northeastward across our
southern Piedmont. As this system lifts newd and exits our northern
coastal plain counties by mid-late evening, overall rain shield will
diminish. The thunder threat (a very remote chance) will be confined
to the far southeast counties through 03Z.

The rain has aided to improve the visibilities across central NC. As
the rain ends, expect to see visibilities turn south with the
potential for pockets of dense fog. Another dense fog advisory may
be needed later tonight into the first half of Sunday morning.
Little change required/needed to the temperatures at this time.


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


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.


As of 705 PM Saturday...

24-Hour TAF period: High confidence that poor aviation conditions
will persist through at least 15Z Sunday and likely through the TAF
period at KINT and KGSO. The showers will continue to move NE
through and out of central NC by roughly 04Z, with a brief and
slight improvement in visibilities where rain is falling. Once the
rain ends, expect mostly LIFR/IFR conditions to dominate the
remainder of the night and into the day Sunday. There may also be a
brief period of low level wind shear tonight between 00Z and 09Z at
RDU/FAY/RWI, as a generally Sly low level jet surges northward ahead
of the surface low, atop lingering light surface winds. Expect
another round or two of rain to move through Central NC on Sunday,
with some thunderstorms possible late in the day (best chances at
KFAY and KRWI, worst chances at KINT and KGSO). There is still
uncertainty with respect to if/when/and to what extent KRDU, KRWI,
and KFAY will improve Sunday afternoon. -KC

Looking ahead: 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. -MWS




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