Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 230903
Area Forecast Discussion
National Weather Service Raleigh NC
500 AM EDT Sun Apr 23 2017
A cold front will push southward through North Carolina overnight,
reaching the South Carolina border by daybreak. A strong low
pressure system will slowly cross the region through Tuesday.
.NEAR TERM /Today/...
As of 355 AM Sunday...
A significant period of wet weather starts today. The latest surface
map shows that the cold front is now settling over far northern SC
and SE NC, where it is projected to linger through tonight.
Meanwhile, the potent mid-upper low centered over the MO Bootheel is
expected to drift across W TN into N AL today, with a series of weak
perturbations in the downstream SW steering flow tracking across NC
helping to instigate numerous to widespread showers over our area,
where PW values that are already approaching the 80th percentile
will rise further today. Given this deep moisture, also observed on
experimental GOES-16 water vapor imagery and blended TPW imagery,
the waves of weak DPVA, strengthening moist upglide through the 290-
300K depth up and over the stable cool pool nosing in from the
north, and increasing low level moisture transport, categorical pops
are more than warranted. Model guidance is in fairly good agreement
on a scenario of steady light to periodically moderate rain today,
and expect rainfall amounts through nightfall of around a third of
an inch in the SE CWA to an inch and a third in the N and W CWA.
While any flooding today should be localized and mostly minor, this
rainfall will help saturate the soils, increasing runoff potential
for later rainfall, and a few rises on area creeks are likely. With
a trend to thick cloud cover areawide and cooler air wedging in from
the north, expect highs no better than the mid 50s NW ranging to the
upper 60s SE, a diurnal rise of just around 5-10 degrees in most
.SHORT TERM /Sunday night through Tuesday/...
-- Changed Discussion --As of 455 AM Sunday...
...Flood Watch remains in effect from midnight tonight through 800
Tonight-Mon night: The areal extent and intensity of showers will
ramp up beginning tonight, peaking midday Mon through Mon evening,
as we see a continued increase in PW (reaching near daily records,
around 1.5"), upper divergence (on the east side of the upper
cyclone), and low level moisture transport (nearly 200 g/kg,
associated with a strengthening 925 mb long-fetch ESE jetlet,
originating with a jet near the Bahamas), with falling mid level
heights and increasing DPVA. These factors appear to be the most
ideally juxtaposed in the vertical over southern NC Mon morning
through Mon evening, focusing first on the southern Mountains and
Foothills, then shifting to the central and southern Coastal Plain
and SE NC. While the models differ somewhat on exact amounts, they
all agree on this overall pattern and evolution, including the NCAR
ensemble, MPAS, and SREF. Will continue with the flood watch in
place, with categorical chances for periods of moderate to heavy
rainfall, with the heaviest rainfall potential where we see
contributions from convection, most probable near the lingering
frontal zone where moist upglide will be strongest and moisture most
available. Have raised storm total rainfall amounts to 3.8-4.7",
based on latest model guidance and increasing confidence in heavy
rainfall totals. While our antecedent conditions are somewhat dry,
the potential for deep layer ascent compounded by the anomalously
high PW, along with today`s rain to help saturate the soil, will
likely cause many of our creeks to rise enough to allow water to
spill onto roads, and urban/street flooding is likely to be a big
problem through Mon night, irrespective of initial soil conditions
and unremarkable flash flood guidance ahead of the event. Regarding
the chance for storms, models are suggesting minimal CAPE confined
to the SE CWA through Mon, shifting to the Coastal Plain Mon night,
although deep layer shear values will be favorable (30-40 kts) with
rich low level vorticity near the boundary, where surface-3 km
helicity values are projected to exceed 300 m2/s2. So if we can
manage to achieve higher potential instability, a few storms could
certainly become strong to severe with a risk of damaging winds,
hail, and perhaps a brief tornado, focusing Mon evening into the
early overnight hours. Expect lows tonight not too far below today`s
highs, from around 50 to the lower 50s. Highs Mon from the upper 50s
NW to the lower 70s SE, which should get into the warm sector. Lows
Mon night 50-60, with decreasing precip chances late from west to
east as the low exits. -GIH
The upper low begins to lift up the coast early Tuesday, with wrap
around rainshowers, mainly in the east, due to strongly convergent
low level flow. These showers will be gradually diminishing
throughout the day as the low departs, ending over nearly all but
the northern coastal plain by sunset. Cloudiness and northerly low
level flow will hamper wedge airmass scouring, especially in the
west, so temperature moderation will be delayed another day, with
highs in the mid 60s across the north to perhaps very low 70s along
the southern tier counties. -mlm
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.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 330 AM EDT Saturday...
Lingering showers will be ending in the northeast by around midnight
Tuesday night as the upper low lifts northeast and off the Delmarva
peninsula by Wednesday morning. The lows will be in the mid 50s. We
then see dry weather with a rapid warmup though the end of the week
as shortwave ridging and return flow settle along the eastern CONUS.
Highs will approach 80 on Wednesdsy...then climb into the mid and
upper 80s Thursday through Saturday. Mins will be mostly in the 60s
mid to late week.
.AVIATION /06Z Sunday through Friday/...
As of 155 AM Sunday...
High confidence in poor aviation conditions over the next 24 hours,
With IFR conditions common in all locations except mostly MVFR at
FAY. A cold front dropped southward into and through NC this past
evening, and is now settling near the NC/SC state line. This front
will remain stretched across northern SC and southeast NC through
the next 24 hours, while surface low pressure drifts slowly from
northern AL toward southwest NC, and strong mid level low pressure
passes over the Mid South. This will all keep unsettled weather over
central NC, with deep moisture leading to low cigs, lowest at
INT/GSO, through Sunday night, with periods of MVFR to IFR rain,
most widespread at INT/GSO but likely everywhere. Surface winds will
be mainly from the NE at 10-15 kts gusting to 15-20 kts infrequently
through Sunday night. While winds aloft (4000-5000 ft AGL) will be
strengthening from the SE late Sunday through Sunday night to 25-35
kts, however the shift of winds in the vertical from NE to SE
through the low levels will not meet low level wind shear criteria,
although it may make handling of small aircraft difficult.
Looking beyond 06z Mon: Poor aviation conditions will continue to
dominate through Monday night as the deep low pressure system slowly
crosses the region, with periods of rain through Mon, tapering down
from south to north Mon night, and persistent low MVFR/IFR cigs and
vsbys. Cigs should rise gradually to MVFR areawide Tue as the mid
level low slowly departs the area, then trend to VFR Tue night
lasting through Fri with a drier air mass and deep SW flow,
although a few afternoon showers are possible Thu/Fri. -GIH
As of 310 AM Sunday...
...A Flood Watch is in effect through 800 AM Tuesday...
Rain has begun to saturate soils in the northwest Piedmont with 1/2
to 1 inch of rain on Saturday in the Triad. Rain will continue to
spread across the area as isentropic upglide (looking at the 295K
isentope analysis as representative) increases with strengthening
southeast low level flow today. Even modest rain rates will produce
hefty amounts, and given that we have categorical PoPs for ~48 hrs,
our 3-4 inch storm total forecast looks reasonable...perhaps even
Flooding will be likely on smaller creeks and streams since we will
have periods of convectively enhanced rain covering smaller areas.
Our larger mainstem river basins will take longer to saturate, and
will begin to rise more quickly after we prime the basins with the
first couple of inches of rain. Thus river flooding will become more
likely on Monday and could linger into midweek. Based on our
expected rainfall distribution at this time, the Haw river would be
particularly prone to minor flooding above Lake Jordan, but any of
the mainstem rivers could reach minor flood stage, especially given
just a bit more rain than currently forecast. On a postive note, we
were on the cusp of drought conditions beginning to have more
significant impact across the area due to rainfall deficit over the
late winter and spring, so this event will provide at least a couple
of weeks` respite.
Flood Watch through Tuesday morning for NCZ007>011-021>028-038>043-