000
FXUS62 KRAH 231614
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
1215 PM EDT Sun Apr 23 2017

.SYNOPSIS...
A strong mid and upper level storm system will move slowly east
across the Tennessee Valley into the Carolina`s through Tuesday. A
surface low pressure will track from Georgia east to the Carolina
coast through Tuesday.

&&

.NEAR TERM /This Afternoon/...
As of 1155 AM Sunday...

Flood Watch through 800 AM Tuesday...

Driven by the 1022+ mb surface high pressure that was nosing down
into our region from PA/NY, strong CAA with the low level NE wind
had overspread all our region as of early this morning. The main
cold front had settled into northern SC, and extended to the main
low pressure located over NW Georgia. Temperatures were a good 30
degrees colder today than this time on Saturday. Readings were in
the 40s NW ranging into the 50s elsewhere. Rain was widespread over
the western half of NC west and south through the Tennessee Valley
and northern GA.

Rain continued to develop and overspread areas along and NW of a
line from Albemarle to Raleigh and Roanoke Rapids. This covers all
the Piedmont and portions of the northern Sandhills and northern
Coastal Plain. Rainfall thus far has been generally 0.50 to 1 inch
in the west with between 0.25 and 0.50 east - locally 1+ inches.
This is just about as expected thus far. The maximum rain thus far
has been in a SW to NE corridor from the Mountains along the
northern tier of NC and southern VA where 1 to locally 3 inches has
been observed. The rainfall thus far has simply wet down the dry
ground as many areas had received only 50 to 75 percent of normal
precipitation in the past 1 to 2 months before this rain even began.

Widespread rain is expected to continue to develop and spread
northward this afternoon over much of western and central NC, with
the best lift forecast to be located over the western and northern
part of the state much of the afternoon. This will continue to place
areas along and west of a line from Albemarle to Raleigh to Roanoke
Rapids in the more widespread rain, with areas of light rain and
drizzle over the Sandhills and Coastal Plain. Most of the rain will
be light to moderate, but will accumulate on the order of 0.05 to
0.10 per hour in the western Piedmont. These rainfall rates will not
produce flooding, but simply saturate the ground, priming the region
for significant runoff tonight and Monday. This is when the rain is
forecast to become heavy, with rainfall rates to increase
significantly.

A quick peep at the new model data supports the ongoing forecast and
reflects the current radar and satellite trends very well. The
entire positively tilted mid/upper low near Memphis is forecast to
slowly track SE through the TN valley to over northern GA later
tonight and Monday, evolving into a negatively tilted system aloft
with time. Strong mid level lift is forecast to arrive over much of
southern and western areas later today and tonight, then in the east
on Monday. The negatively tilted system will tap very deep moisture
which will be pulled NW from the Gulf Stream and western Atlantic
into northern SC and much of NC. Thus, the heavy rain with storm
totals of 3 to 5 inches still appears likely if not probable.

&&

.SHORT TERM /Tonight through Tuesday/...
As of 455 AM Sunday...

...Flood Watch through 800 AM Tuesday...

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

&&

.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 Wednesday...then climb into the mid and
upper 80s Thursday through Saturday. Mins will be mostly in the 60s
mid to late week.

&&

.AVIATION /12Z Sunday through Friday/...
As of 655 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 tonight, 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 today through tonight 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 12z 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

&&

.HYDROLOGY...
As of noon today...

...A Flood Watch is in effect through 800 AM Tuesday...

Flooding of creeks and small streams is expected to begin later today
and tonight as the ground saturates and the rainfall becomes
heavier. This is expected to occur over the western Piedmont,
including the Triad region first, then shift east with time (later
tonight and Monday) to bring the threat across all areas.

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 positive note, we were on the cusp of drought conditions due to
rainfall deficits over the late winter and spring, so this event will
turn that around and possibly eliminate much of a drought threat
for the rest of the spring season.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Tuesday morning for NCZ007>011-021>028-038>043-
073>078-083>086-088-089.

&&

$$

SYNOPSIS...Badgett
NEAR TERM...Badgett
SHORT TERM...Hartfield/MLM
LONG TERM...MLM
AVIATION...Hartfield
HYDROLOGY...Badgett/MLM

NWS RAH Office Area Forecast Discussion