Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 230447

Area Forecast Discussion
National Weather Service Raleigh NC
1245 AM EDT Mon Apr 23 2018

Strong high pressure will remain nearly stationary or slowly drift
off the New England coast through tonight. Low pressure approach
from the Tennessee Valley today and tonight, then track slowly east
over SC on Tuesday, then NE across eastern NC late Tuesday and
Tuesday night.


As of 935 PM Sunday...

Multiple signs pointing to the inevitable advance of moisture from
the west this evening as mid-level cloud decks are expanding across
central NC at this time. In addition, observations showing 50s
dewpoints now creeping into the southwest Piedmont. Cloud cover will
continue to increase overnight as high pressure remains offshore and
and a low pressure system works its way across the Deep South. Some
lighter returns also starting to show up on radar in the southwest
but this is evaporating before reaching the ground. Observed
precipitation is still well off to the southwest, closer to Columbia,
SC with more significant rainfall still back over Georgia at this
time. Lows in the upper 40s across the northeast Coastal Plain where
thicker cloud cover will hold off the longest to mid 50s in the
southwest Piedmont.


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 1245 AM Monday...

MVFR to VFR CIGS will lower to IFR TO LIFR CIGS and IFR VSBYS with
rain and fog later today and tonight. Winds will increase from the
east at 15-20kt with gusts to 30kt this afternoon and tonight.

Looking beyond 06z Tue: Poor aviation conditions will dominate all
sites through Tuesday. Mostly IFR CIGS and IFR to MVFR VSBYS with
areas of rain and showers are likely on Tuesday, before tapering off
Tuesday night. 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.





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