Area Forecast Discussion
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FXUS62 KRAH 211757
Area Forecast Discussion
National Weather Service Raleigh NC
158 PM EDT SAT MAY 21 2016
.SYNOPSIS...Deepening low lifting up the mid Atlantic coast will drag
a surface front and attendant second wave across the area this
evening. A strong upper level low pressure area will swing over the
region tonight through Monday, bringing continued unsettled weather.
.NEAR TERM /today through tonight/...
As of 955 AM Saturday...
Rest of today: Primary band of steady widespread showers is
currently moving through eastern portions of the Coastal Plain, near
a surface low pressure center over NE NC, and is on pace to be out
of the CWA shortly, as its primary driving mechanisms -- including
mid level DPVA (associated with a vorticity max) and an upper
divergence max -- pivot ENE in the base of the incoming longwave
trough. A second and much smaller wave noted on water vapor imagery
shifting east out of the northern NC mountains is apt to spawn
another band of showers that will sweep eastward across the northern
CWA over the next several hours. Finally, and most notably, models
show yet another vorticity lobe tracking out of the mountains and
across NC/VA later this afternoon into this evening, in tandem with
lowering surface pressures as the primary low over WV fills with new
low pressure forming over the NC Foothills. This upper forcing will
likely have available to it at least marginal diurnal instability,
provided there is sufficient insolation this afternoon. However,
timing of this feature among the hi-res models varies quite a bit,
and thus the presence of sufficient instability to support any
resultant convection is in question. Both convection-allowing and
parameterized models do show good deep layer bulk shear of 40-50 kts
this afternoon/evening, and the HRRR shows organized convection over
the mountains pushing eastward and into the CWA later this
afternoon, a pace similar to most NCAR ensemble members and the hi-
res ARW/NMM. Will retain the general flavor of the earlier forecast
in bringing scattered showers and storms across the area late
afternoon into early evening. The marginal instability will limit
storm severity today, however a few storms could still become strong
with strong wind gusts primarily across the southern CWA (along and
south of highway 64), particularly if the clouds break enough to
allow a couple of hours of good heating. Given the current cloud
breaks already, have made minor temp adjustments upward to a range
of 70 NE to near 80 near the SC border. -GIH
Earlier discussion from 400 am: For this evening, widely scattered
showers will move back in and across central NC from the NW and as
the next short wave assoc with the closed low moves across our area.
While the airmass across the NW Piedmont will not have recovered
from the CAD and is expected to remain cooler and stable, a sfc
boundary trailing the exiting coastal low is progged to extend
across portions of central NC generally along and south of Hwy 64.
South of this boundary, the airmass may be warm enough to support
isolated thunderstorms, some of which could become marginally strong
due to 30-40 kt of 0-6km bulk shear coincident with the area of best
CAPE south of the boundary. Right now, SWODY1 shows much of our CWA
(mainly south of Hwy 64) in a marginal risk for severe tstms later
today, with isolated severe tstm wind gusts the main threat. Lows
tonight in the mid 50 nw to around 60 S/SE.
.SHORT TERM /Sunday and Sunday night/...
As of 400 AM Saturday...
The risk for scattered showers and isolated thunderstorms will
continue through this period as the closed low over PA drifts south
across the Mid-Atlantic region and NE NC. POPs during this time
will be highest NE in closer proximity to the upper low center, and
lowest SW closer to the subsident region of the pattern and
subsequent drier airmass. In fact, there`s a good chance that places
west and south of a line from Lexington to Fayetteville may remain
dry during this time. Temps will continue to run 5-10 deg below
normal. Highs Sunday in the lower to mid 70s and lows Sunday night
in the lower to mid 50s.
.LONG TERM /Monday through Friday/...
As of 300 AM Saturday...
Monday and Monday night: An upper level low over NC will gradually
shift eastward then northeastward along the Mid-Atlantic coast
through Monday night. A slight chance for showers will linger on the
back side of the low as it pulls away. The associated surface low
will strengthen as it moves north-northeastward along the coast.
Surface winds will be mainly NW, shifting to more westerly as the
low departs. As a result, expect highs Monday topping out in the mid
70s and overnight lows in the mid 50s.
Tuesday through Friday: The upper level low and attendant trough
will quickly move away from the area, replaced by ridging aloft.
Similarly at the surface, high pressure will build into the region,
though central NC will still be on the back side of the departing
surface flow Tuesday, resulting in lingering westerly winds. As the
low races off to the NE Tuesday night, surface winds will become
more southerly. With the surface high and upper level ridge over the
region skies will be mostly sunny/clear mid week with moderating
temperatures. Weather will remain dry, likely through Wednesday
night, with a slight chance for showers and afternoon thunderstorms
returning late in the week. Model agreement still is not great at
this time so will keep chances limited to slight. Highs in the upper
70s to mid 80s Tuesday will moderate into the mid to upper 80s by
Wednesday, where they will remain through the end of the week.
Overnight lows will generally be in the mid 60s.
.AVIATION /18Z Saturday through Thursday/...
As of 158 PM Saturday...
24 hour TAF period: Largely in response to strong insolation,
lingering CAD over the region continues to weaken. Ceilings have
lifted to either MVFR or low-end VFR across the area, with most
areas expected to lift to low-end VFR by 21z. As the mid-level
trough begins to move over the area, scattered showers and
thunderstorms are expected to re-develop across central NC between
21z-06z, resulting in local IFR conditions. More widespread IFR
conditions will develop later tonight due to low clouds and some
fog, esp invof areas that received heavier rain. The low
ceilings/stratus will be slow to lift through the day on Sunday
morning, with the potential for MVFR ceilings to persist
at eastern/northeastern NC(KRDU and KRWI)until late in the
afternoon. Meanwhile...western and southern most terminals could see
VFR conditions return by mid morning.
Beyond 18z Sunday: the risk for sub-VFR conditions with scattered
showers will continue through at least mid-day Monday as a closed
low gradually moves south across eastern NC. VFR conditions expected
to return by Tuesday and remain for the mid-week period as the
closed low moves east and short wave ridging moves over our region.