Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 271927

Area Forecast Discussion
National Weather Service Raleigh NC
327 PM EDT Sun May 27 2018

The combined flow between a ridge over the western Atlantic and sub-
tropical storm Alberto will drive an extremely moist air mass across
the Carolinas through mid week. The majority of the remnants of
Alberto are largely expected to remain west of the Appalachians
through the end of the work week.


As of 300 PM AM Sunday...

The latest surface analysis shows a Bermuda high pressure system
extending west toward the Southeast coast this afternoon. A
stationary front was noted across the mid-Atlantic while
subtropical storm Alberto was moving north across the eastern
Gulf of Mexico.

The air mass across central NC remains quite moist although
precipitable water values currently ranging in the 1.5 to 1.8
inch range are lower than previous days. Surface dew points
range in the upper 60s to lower 70s across much of central NC.
The 18Z SPC mesoanalysis shows a weak to moderately unstable
air mass with MLCAPE values near 1500 J/Kg near the VA border
decreasing to near 500 J/Kg near the SC border. Mid level lapse
rates range near 5.5 deg C/Km but DCAPE values exceed 800 J/Kg
across the Coastal Plain and Sandhills. With limited forcing for
ascent and warm mid-levels, convection has been limited this
afternoon and convection allowing models have handled this well.
The greatest chances of a shower or storm will be near and
especially north of interstate 85 in a region of weak
convergence in the 850- 700 hPa layer and greatest instability.
The southerly flow will lift the scattered showers slowly
northward with this convection abating fairly quickly this
evening with the loss of heating.

Tropical moisture will surge back northward into central NC
late tonight as the deep layer southerly flow increases.
Precipitable water values will increase late tonight, exceeding
2 inches across the south by daybreak. An area of showers and
thunderstorms in a weakly unstable air mass, driven by a trough
at 925 and 850 hPa will lift north from GA and FL this evening
into SC and eventually southern NC after 06Z. Widespread showers
with heavy rain should reach the Sandhills, Southern Piedmont
and southern Coastal Plain around daybreak, setting the stage
for the potential of some localized flash flooding on Monday.
With clouds and a light southerly flow, low temperatures should
range around 70. -Blaes


As of 325 PM Sunday...

...The Flash Flood Watch has been expanded north to include
Raleigh and Rocky-Mount...

A plume of deep tropical moisture will spread across much of
central and eastern NC on Monday with precipitable water values
between 2 and 2.25 inches, which is up to 2 to 4 standard
deviations from normal. A broad band of showers and
thunderstorms associated with a low-level trough at 925 and
especially 850 hPa will spread north across central and eastern
NC on Monday. NWP guidance has slowed a bit with the northward
arrival of the main area of showers but expect widespread
showers and thunderstorms to reach the Southern Piedmont and
Sandhills around or just after daybreak and the Triangle/Rocky
mount areas during the mid to late morning. The heaviest rain
will surge north during the afternoon and evening and begin to
become more scattered on Monday night. Rainfall amounts will
generally average between 1 to 3 inches with perhaps lesser
amounts across the northern Triad and larger amounts across the
southern Coastal Plain. With high PW values, rainfall will be
locally heavy with impressive rainfall rates resulting in
localized amounts possibly exceeding 4 inches. Most of central
NC is included in a slight risk of excessive rain for Monday.
We have collaborated with adjacent offices and expanded the
Flash Flood Watch northward slightly. Widespread showers and
storms will decreases in coverage on Monday night as the best
forcing lifts north of the area. Scattered convection with the
continued potential for some localized but not as widespread
heavy rain will continue into Tuesday. Highs on Monday will
range in the upper 70s to lower 80s and in the lower to mid 80s
on Tuesday. Muggy lows near 70 are expected both days. -Blaes


As of 320 AM Sunday...

Our rainy period will persist into the late week as a tropical
flavored airmass lingers over the area. Periods of potentially heavy
showers are expected, which will maintain the flood threat at least
through Wednesday, and possibly Thursday as well.

Alberto will be inland in the vicinity of central AL at the
beginning of the extended period. The associated moisture plume off
the Gulf will be focused into the central/southern NC mountains and
Tennessee Valley. Meanwhile, a second, separate moisture plume is
expected along the southern Atlantic coast due to southerly flow on
the western periphery of a strong mid level ridge. The ridge will
block Alberto from making much eastward progress, and will force the
system slowly almost due north, to reach the Ohio Valley on
Wednesday. While central NC will initially be in the `relative` dry
slot between the systems, showers will be likely along with
scattered, mainly diurnal storms which will produce heavy showers
due to deep layer moisture (PWAT`s in this `dry slot` will still
range between 1.8 & 2.1 inches) and strongly diffluent flow aloft.
Highs will be from 80 to 85. Confidence is lower heading into the
mid week, but it looks like we will see a surge of wrap-around
moisture into central NC courtesy of Alberto`s remnants Tuesday
night which will help maintain the anomalously moist airmass. While
forcing will be weak, it won`t take much to produce and maintain
numerous, potentially heavy, showers overnight.

Alberto`s weakening remnants will be in the Ohio Valley Wednesday,
but our subtropical ridging hangs tough along the Atlantic coast.
The deep southerly flow maintains our moist airmass all the way
through Thursday, with widespread (60-70% PoPs at this point) rain
expected, but lower confidence as to pinpointing exactly when and
where the very highest probabilities may lie. Low level thicknesses
will be ticking upward in warm air advection, but tempered by heavy
cloudiness/precip, so highs Wednesday and Thursday will be stall in
the low and mid 80s.

Modest drying (PWAT`s < 1.5 inches at last) ensues on Friday as the
Atlantic ridge finally deamplifies and edges further offshore as a
short wave moves southeast across the area on Friday. Scattered
mainly diurnal convection is expected both Friday and Saturday as
while we won`t be as wet, we will be warmer and more unstable as
highs reach way up into the 80s and very low 90s. Morning low
temperatures through the extended will be mostly persistence due to
the unchanged airmass...within a couple degrees either side of 70.


As of 215 PM Sunday...

Through 18Z Monday: SCT-BKN cumulus in mainly VFR conditions were
noted across central NC this afternoon with a few lingering patches
of MVFR cigs in the Triad. Scattered showers and a few thunderstorms
are expected this afternoon into the early evening, mainly near and
north of interstate 85, which could affect the KINT and KGSO
terminals. Given the limited coverage, will omit a mention of t-
storms in the TAFs but note a tempo period with reduced vsbys and
cigs in a shower. VFR conditions are expected for much of the
evening and early overnight before increasing mid and especially low
cloudiness moves north into central NC after midnight. MVFR and IFR
conditions associated with rain showers spread from SC into southern
NC a little before daybreak affecting the KFAY terminal.
Restrictions continue north on Monday morning reaching the remaining
terminals between 13 and 16Z.

After 18Z Monday: An unsettled period with periods of adverse
aviation conditions are expected associated with intervals of mainly
afternoon and evening showers and thunderstorms and periods of
morning stratus.


KGSO Greensboro upper-air observations will be unavailable
through at least the evening release on Tuesday, May 29th
because of faulty hardware.


Flash Flood Watch from 1 AM EDT Monday through Tuesday morning
for NCZ026>028-038>043-073>078-083>086-088-089.


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