Area Forecast Discussion
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FXUS62 KRAH 250806
Area Forecast Discussion
National Weather Service Raleigh NC
405 AM EDT Tue Apr 25 2017
A strong and deep low pressure system will slowly cross the region
through Tuesday, producing wet and unsettled weather. Behind this
low, a ridge of high pressure will extend into the region for mid to
.NEAR TERM /Through Tonight/...
As of 1000 PM Monday...
Most of North Carolina resides in the left exit region of a jet
streak rounding the base of a compact negatively-tilted trough
across the southeastern CONUS. The center of a compact surface low
pressure can be found across far eastern South Carolina and is
moving northeastward with time. A secondary (and stronger)
subtropical low is approaching the coast of North Carolina, and is
starting to merge with the aforementioned low pressure system to its
west. Closer to home, the region remains in a strong CAD regime
with the wedge front delineating slightly more warm and moist air
and the CAD to its west located across the eastern CWA. As the lows
merge, continued DCVA (as well as frontogenetical forcing, and
isentropic upglide along a wedge front draped across the
southeastern CWA) will continue to enable widespread ascent. Coupled
with an anomalously moist atmosphere across the CWA that increases
toward the east (the 00z KMHX sounding reflects a PWAT of 1.83",
breaking the previous daily record by nearly nearly 0.3"),
widespread rainfall continues areawide.
Currently, the heaviest rainfall is confined to two main bands--one
across Edgecombe to Franklin counties and another across Wayne and
Johnston counties. Both bands seem to combine across Wake, Chatham
counties. While rainfall rates aren`t particularly intense within
the bands (generally a few tenths per hour approaching 0.5"/hr in
the heaviest segments), the long duration of the rainfall is leading
to 6 hourly rainfall rates of 1-2". Unsurprisingly, recent storm
reports indicate a few closed roads across the CWA with rivers
rising in the heaviest bands toward flood stage. However, some
slight drying is being noted across the far NE CWA into the NE coast
of North Carolina (likely in association with vertical mixing of a
pocket of drier air centered around 925 mb) leading to an erosion of
the northern precipitation band. As the combined low pressure system
translates northeastward across the coast of North Carolina, the
bands of heaviest precipitation will likely pivot slightly
northeastward with time. A quick peak at the latest suite of model
guidance indicates a narrow axis of MUCAPE may build into the
eastern CWA enabling slightly heavier precipitation rates as the
band moves northeastward (especially between 03-06z). Indeed, a few
flashes of lightning are occurring across the NC coast within the
aforementioned precipitation band. As the band shifts northwestward,
an additional 1-2" with isolated pockets of 3-4" seems likely across
the NE CWA (e.g. Edgecombe, Nash, Franklin counties) as well as
parts of Wake, Durham, Chatham, and Orange counties. The latest
mesoscale discussion from the WPC lines up well with this reasoning.
.SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/...
As of 400 AM Tuesday...
In the wake of the low pressure lifting up the Mid-Atlantic coast,
low-level southerly winds and short wave ridging aloft will mark the
return of sunshine and warming temperatures. Highs ranging from
upper 70s NE to lower 80s south. Lows Wednesday night in the upper
50s to lower 60s. &&
.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 400 AM EDT Tuesday...
Lead short wave trough will lift from the Mid MS/TN Valley NE into
the Great Lakes on Thursday with trailing sfc cold front expected to
stall out west/northwest of the mtns.
Associated convection is expected to be ongoing Thursday morning
across Tn, Al, and Ga Thursday morning. However with the better
trough dynamics/forcing lifting off well to the north Thursday
afternoon and evening, the showers and storms should exhibit a
steady decline/weakening trend as they cross the mtns Thursday
afternoon/evening. Will leave isolated chances across the far
western zones Thursday afternoon, otherwise it should remain dry
with temperatures steadily rising in the SWLY WAA regime. Highs in
the lower to mid 80s west to upper 80s central and eastern areas.
The synoptic pattern will favor near record heat by Saturday, owing
to a 590+dm H5 ridge, Bermuda highs and H7 anticyclone over SC.
Thickness are progged at 1410m Saturday, 50m above normal and
indicative of upper 80s to lower 90s. The main question is how
quickly another shortwave ejecting out of the Central/Southern
Plains, this one potentially stronger, will cause the ridge to
retreat offshore a bit. Heights are forecast to lower slightly on
Sunday, more so across western NC. Highs in the mid 80s west to near
90 central and eastern area.
A cold front will bring the next chance for rain/storms to the area
on Monday with cooler temps following on Tuesday.
.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
As of 155 AM Tuesday...
24 Hour TAF Period: Adverse aviation conditions are expected to
continue through the 24 hour TAF period (high confidence) as a deep
mid/upper level low moves across the region.
A persistent rain continues to fall across portions of central NC
early this morning, resulting in generally IFR/low end MVFR
conditions. However, periods of LIFR cigs will be possible in areas
of light rain and/or brief lulls in the precip ahead of the
mid/upper low (such as KFAY currently). Sporadic wind gusts of up to
22 kts or so will continue to be possible across the northern TAF
sites early this morning, before diminishing from east to west
through 12Z. Will continue to keep LLWS in the TAFs at
KGSO/KINT/KRDU/RWI as easterly winds at around 2 kft are expected to
be around 45-50 kts or so through 09Z. However, given this will
mostly be speed shear (with generally unidirectional flow) expect
this will only create marginal LLWS conditions (especially given
some of the gustiness expected). Surface winds will then decrease
after daybreak. While the main area of rain will shift northward of
the area by 12-15Z today we can still expect some lingering
showers/rain, especially moisture wrapping around the departing low
pressure system across the northeastern portions of our area
(KRWI/KRDU). In fact we could see a few storm develop if we are able
to destabilize enough this afternoon. While cigs should improve
today, possibly becoming VFR temporarily, think we should generally
maintain MVFR cigs.
Outlook: Cigs should remain MVFR through tonight, although cigs may
drop back to IFR through this evening into the overnight. After the
low shift to our NE tonight, high pressure ridging will take over,
and we`ll see cigs rise to VFR Wed morning. VFR conditions will then
dominate through Sat.
As of 420 PM Monday...
...Flood Watch remains in effect through 12Z Tuesday...
Much of the rainfall so far has been moderate, with most places so
far spared by the heavier rain rates. This is expected to change
through the evening hours as increasing lift and moisture lead to an
uptick in rain rates. Several creeks around central NC are
experiencing rises, with a few approaching minor flooding, and this
should persist through tonight, with retreating water levels
Tuesday. Urban and street flooding, particularly in the typical
flood-prone areas, have been common all across central NC. A few
main stem rivers are projected to approach or exceed flood stage
over the next couple of days in response to this rainfall, including
parts of the Yadkin, Haw, Little, Cape Fear, Rocky, and Tar Rivers.
Rainfall since last night has been around 1 to 3 inches, and an
additional 1-3 inches of rainfall are expected through tonight, with
locally higher totals possible. -GIH
Flood Watch through Tuesday morning for NCZ007>011-021>028-038>043-