Area Forecast Discussion
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FXUS62 KRAH 212048
Area Forecast Discussion
National Weather Service Raleigh NC
348 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.
.NEAR TERM /THROUGH TONIGHT/...
As of 345 PM Saturday...
The warm sector supportive of severe convection is bounded to the
west by an effective cold front related to a fast moving QLCS/squall
line now stretching S and then SWwd through Srn GA and the FL
panhandle, from a 1004 mb surface low near Augusta GA, and to the N
by a warm front retreating slowly Nwd across central and E-central
SC. This air mass is expected to be shunted Ewd and offshore as the
related surface low races to the SC coast during the next several
To the north, an old surface low --but nonetheless a vigorous one
with wake-low pressure falls of 5-8 mb in just minutes-- has become
elongated from near Atlanta to Greenwood SC. This low will continue
to move NEwd across south-central NC this evening, along a preceding
warm/wedge front, while likely weakening as the earlier deep/severe
convection "transfers" energy Ewd to the aforementioned lead frontal
So while severe weather is not expected in central NC through
tonight, and probably not even any thunder for that matter, an area
of deformation-type showers --driven by both strong DCVA
accompanying the primary MCV from the upstream MCS, and 925-850 mb
WAA-- will pivot NEwd across central NC through mid-late evening.
Upstream 3-hourly precipitation totals in this area of deformation
showers suggests rainfall amounts in central NC are expected to
range from around a quarter to half inch roughly from Albemarle to
Raleigh, with lighter amounts on the NW edge of the precipitation/
over the NW Piedmont, and also in the dry slot across much of the
Sandhills and Srn Coastal Plain. However, a renewed area of showers,
and possibly a couple of thunderstorms, are likely to arrive in the
Srn NC Piedmont around daybreak, in association with the residuals
of what is likely to be another upstream MCS from the Deep South.
Once this lead area of rain lifts NEwd, and probably out of the NE
Piedmont and Nrn Coastal Plain by around 03Z, the majority of the
night is expected to be measurable rain-free. However, the passage
of the associated low pressure and rain, and following light Nly
surface flow, will serve to reinforce the wedge airmass that has
persisted over most of central NC well into this afternoon. As such,
widespread very low overcast, areas of (some dense) fog, and
mist/drizzle will envelop central NC overnight. Temperatures will
hold within a few degrees of current readings, mostly in the 50s, to
lower 60s along the Srn tier of counties.
.SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/...
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
.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.