Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 221936

Area Forecast Discussion
National Weather Service Raleigh NC
335 PM EDT Sun Apr 22 2018

High pressure will extend south through the mid Atlantic states
through tonight. Low pressure will track slowly eastward over the
lower Mississippi Valley and Mid South region before tracking
northeast over the East Coast through midweek.


As of 335 PM Sunday...

High pressure will continue to nose into the area from the N through
tonight, although it will begin to give way to a sheared lobe of mid
level vorticity pushing into the Carolinas from the SW. The
corresponding high and mid clouds will slowly increase and thicken
from SW to NE through tonight, a scenario reflected well in the RAP-
adjusted sky cover forecasts, so will have a trend to partly (NE) to
mostly (SW) cloudy. The low levels will remain fairly dry through
late tonight, with weak and somewhat shallow moist upglide confined
to the SW CWA late. Will bring in low pops late over the SW, with
minimal QPF. Low from the mid 40s NW (beneath the heart of the
surface ridge, where clouds will be last to move in) to the mid 50s


As of 300 PM Sunday...

Overview: Compact closed upper level low over the Lower MS Valley
Monday morning will drift slowly NNE across the TN Valley and into
the southern/central Appalachians through Tuesday night. Primary
surface low associated with this system will remain beneath the
mature cyclone and eventually fill with a secondary low at the
triple point expected to track NEWD through the Savanna River Valley
and into eastern NC Tuesday afternoon/evening, before lifting up
along the Delmarva coast Tuesday night and into Wednesday.

Rainfall Amounts/Impact: While strong mid/upper level ridging
extending along the western Atlantic/Mid-Atlantic coast may
initially delay the spread of rain NEWD into the area Monday, rain
will become widespread  over the entire CWA by late Monday
afternoon/early evening. Owing to the slow movement of the upper low
across the region, along with a very strong Atlantic maritime feed
between the offshore high and the mature cyclone to our west, a
widespread soaking of 1 to 2 inches, with localized higher amounts
can be expected. The heaviest rainfall is expected to occur Monday
night through Tuesday morning, with another round of convective
rainfall possible Tuesday afternoon as the surface low and attendant
warm sector retreats north into southern/southeastern NC(more on
that below). With area rivers and creeks running high from recent
heavy rainfall over the past week, this additional rainfall may be
enough to result in some localized/minor flooding.

Severe Potential: Models are in good agreement that the
secondary/triple point low and attendant warm front/sectors(sfc
dewpoints surging into the mid 60s) will lift north into
southern/southeastern NC Tuesday afternoon. CAD in place across
central NC should keep the the warm sector confined to southeastern
portions of NC, with mid-level dry air intrusion likely to result in
sufficient breaks in cloud cover to allow for partial insolation by
the afternoon. Models indicate warm sector destabilization of 500 to
1000 J/Kg of MLCAPE across the eastern Sandhills and southern
Coastal Plain counties. Effective shear of 35-40kts will be
sufficient to support a few strong to severe storms Tuesday
afternoon/early evening with the presence of the dry air aloft
(models showing dCAPE values of 750 to 1000 J/Kg) likely to enhance
downdraft potential with any strong updrafts that develop.
Additionally, with the low tracking NE along the warm/wedge front,
cannot rule out an isolated tornado. Stay tune.

Temperatures: Rain spreading SW to NE into the area, along with
widespread and lowering cloud deck and the development of
cold air damming over the NC Piedmont will will keep temps on the
cool side Monday. Highs ranging from mid to upper 50s across the NW
Piedmont to upper 60s/near across the NE coastal plain counties
where rain will be last to set in. Highs Tuesday will be complicated
by the ongoing CAD and the exact location/track of the secondary low
and attendant warm sector lifting NE into the area.  Highs ranging
from upper 50s/ near 60  over the northern Piedmont to mid 70s
across the southern Piedmont, southern Sandhills and southern
Coastal Plain counties.


As of 245 PM Sunday...

Unsettled weather is expected through the middle and end of the work
week, with mean mid level troughing dominating over the eastern
states, and generally more clouds than sun, at least through Fri.

The first shortwave trough (responsible in part for the Mon/Tue
rain) will be sitting overhead early Wed before lifting off to the
NE, culminating in a phasing with a strong polar stream wave over
the Northeast states Wed night. Models suggest that we may see
enough low and mid level moisture (focusing on 850-700 mb) for a few
late-day showers with heating, however the overall forcing for
ascent will be small, and expect no more than chance pops. Expect
temps to stay slightly below normal, with thicknesses about 10 m
under normal and reduced insolation.

The next shortwave, which will be located over NE/KS early Wed, will
cross the Gulf states and swing NE over the Carolinas Thu night into
Fri. As this feature will be deeper and provide stronger forcing for
ascent along with an opportunity for moisture return, we should have
higher rain chances, targeting late Thu through Thu night, and some
of this may be heavy, as the GFS shows a PW over 1.5". After dry
weather late Wed night through the first half of Thu, will bring in
good chance pops Thu afternoon through Thu night. Temps should again
be a bit below normal figuring in the cloudiness and cooler

Then, a deep mid level trough from central Canada through the Upper
Midwest early Thu should be our next weather-maker. The GFS/Canadian
are in fair agreement on a deep low just to the W or NW of NC late
Fri, crossing the Carolinas Fri night or early Sat, although
uncertainty and model spread grows at this range. Will keep a chance
pop, higher N closer to the cooler mid levels and better lapse rates
aloft, Fri/Fri night extending into early Sat. We should see
subsidence and drying behind this wave for Sat afternoon through
Sun, and will trend toward dry weather and fewer clouds as high
pressure builds in from the west. -GIH


As of 1120 AM Sunday...

VFR conditions are likely through much of tonight, as surface high
pressure noses into NC from the N. But high and mid clouds will
steadily thicken with lowering bases late today through tonight,
ahead of a slow-moving deep low pressure system crossing the mid and
lower Miss Valley. MVFR cigs are expected after 06z tonight in the
SW part of the forecast area toward CLT, and cigs are likely to drop
to MVFR at INT/GSO after 10z late tonight/early Mon. Vsbys are also
expected to drop to MVFR at INT/GSO after 13z Mon with rain
spreading in from the WSW. After 15z Mon, IFR conditions are
possible at INT/GSO with MVFR at RDU. Surface winds will increase
from the SE and become gusty late tonight and through Mon,
particularly over S and W sections (including INT/GSO/FAY), with
stronger and shifting winds with height, potentially resulting in
handling difficulties with smaller aircraft starting early Mon.

Looking beyond 18z Mon: Poor aviation conditions will dominate all
sites through Tue as a strong and slow-moving storm system crosses
the area. Periods of showers and sub-VFR conditions are likely. On
Tue, FAY and perhaps RDU/RWI may see some breaks in the sub-VFR
conditions with a risk of storms. Rain should taper off Tue night
but sub-VFR stratus/fog may linger into Wed morning. VFR conditions
should return Wed, although a few showers are possible Wed
afternoon, and again Thu into Fri, as a series of upper level
troughs cross the region, bringing unsettled weather and perhaps
brief periods of sub-VFR conditions. -GIH





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