Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 280504

Area Forecast Discussion
National Weather Service Raleigh NC
100 AM EDT Mon May 28 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 920 PM AM Sunday...

Deep southerly flow between Subtropical Storm Alberto in the NE Gulf
of Mexico and high pressure centered off the Southeast U.S. coast
will lead to a surge in high moisture content air moving into
central NC overnight, with PW`s expected to around 2-2.25 inches
across central NC by 12-15Z Monday. An associated area of showers
and thunderstorms in a weakly unstable air mass will move into the
area tonight from south to north. Any showers activity late this
evening/early Monday will be light, with intensity and coverage
expected to dramatically increase after 09Z in the south and spread
northward through the morning. As this occurs the threat for heavy
rain and localized flash flooding will increase, especially in the
urban area. However, this initial band of convection will help set
the stage for the main flood potential later on Monday, as
additional heavy showers/storms affect the area. Plan to keep the
Flash Flood Watch as is for now, with the greatest potential for
flash flooding south of the I-85 corridor in central NC. With cloud
cover/topical moisture filtering into the area, expect low temps
will be well above normal again, in the upper 60s to lower 70s.


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 4 PM Sunday...

The remnants of Subtropical Storm Alberto will lift northward
through the TN and OH Valley regions midweek, then into the Great
Lakes region by late week, before drifting back south and eastward
toward our region for next weekend. This will lead to an unsettled
forecast continuing, with high pops through Thursday before
mid/upper level flow becomes more southwesterly as the mid level
ridge pivots more northeastward and becomes more oriented off the
eastern U.S. coast.
This will allow PW`s to fall back to below 1.5 inches from 2.0+
inches by late week (with chances for convection becoming more
diurnal). Timing and strength of the remnants of Alberto is in
question as the remnants sink back south and eastward towards our
region next weekend. However, we should see a good chance of showers
and storms as the remnants of Subtropical Storm Alberto move through
the area.

With regard to possible flooding past the current flash flood
watch... depending on how much rain falls during tonight into
Tuesday morning, we may need to extend in time the flash flood watch
into mid week. Stay tuned as threat for heavy rain will continue
through midweek.

High temps will gradually increase through the week as we should see
less convective coverage by late week. Thus, expect high temps will
be in the lower to mid 80s Wednesday, warming into the upper 80s to
lower 90s by late week, before decreasing some next weekend with a
renewed good chance of convection. Low temps are expected to the in
the upper 60s to lower 70s generally through the period.


As of 100 AM Sunday...

Through 06Z Tuesday: Cigs, which are currently VFR with mostly high
clouds, will gradually lower tonight from South to North as moisture
continues to move in. After 28/10Z, light rain will gradually begin
to move north of the NC/SC border, perhaps reaching KFAY by 10 or
11Z, and our remaining terminals between 12 and 14z. After 28/14Z,
widespread IFR conditions with low clouds and light rain is
expected. Some variability in CAT will be possible late Monday
afternoon/evening as shower coverage decreases, but uncertainty
remain too high to include additional Tempo groups at this time.

After 06Z Tuesday: An unsettled period of adverse aviation
conditions are expected to continue, as central North Carolina
remains in a very moist and unstable airmass. This will allow for
plentiful and variable CIGs/VSBYs associated with intervals of
mainly afternoon and evening showers and thunderstorms. Periods of
morning stratus will also continue for much of the week as the
boundary layer remains extremely moist.


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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