Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 250004
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
804 PM EDT Mon Apr 24 2017

.SYNOPSIS...
A complex and slow moving storm system will bring heavy rain
and the potential for flooding tonight. This system will lift
away from the area Tuesday with a return to dry weather and
above normal temperatures during the mid to late week time
frame. Expanding high pressure aloft and at the surface, will
provide mainly clear skies with warm and dry conditions thru the
upcoming weekend. High temperatures may break into the 90s away
from the immediate coast during this weekend.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 8 PM Monday...Rain shield gradually retreating to the
north with heavy rain and concurrent flooding threat now
confined to Pender County. Expect current trend to continue
with high-res guidance showing flash-flooding threat to end by
around midnight or so, with more scattered showers thereafter.
Previous discussion follows:

Impressive rain event continues to unfold with 5h low and
surface inflection over eastern GA/SC continuing to pump
moisture into the region. Forecast soundings continue to keep
precipitable water values near all time highs, over 1.7 inches,
this afternoon and evening. Water vapor imagery starting to show
hints of another Gulf/Western Atlantic moisture feed. A similar
signature was evident in water vapor this morning as the
moisture tap that ultimately ended up dropping copious amounts
of rain across portions of SC developed. Based on the location
of the surface and mid level low, should this signature develop
further, significant rain and potentially flooding would develop
across northeastern SC and southeastern NC. Strong divergence
aloft and developing surface low will help pull the low level
moisture feed over the front stalled along the coast. The
combination of lift associated with the surface boundary and
dynamical forcing should help maintain rainfall rates of 1-2
inches per hour across portions of the forecast area this
afternoon and evening.

The potential fly in the ointment remains the second surface low
well east of Savannah. Guidance is split with the evolution of this
low. The NAM depicts very limited interaction between the offshore
low and the quasi-stacked low over the southeast. This scenario
would lead to lower rainfall amounts through the late
afternoon/evening hours and an earlier end to the bulk of the
rainfall. The GFS/Canadian show much more interaction between these
features, so much so that the GFS/Canadian have the surface low
moving onshore close to Cape Fear while the NAM moves it onshore at
Cape Hatteras. The scenario will hold onto the heaviest rain a few
hours longer, with the bulk of the activity winding down around
midnight. The 12Z ECMWF is almost a compromise of these two
solutions and maintains heavier rains a little longer than the NAM.
Continue to favor a wetter similar to a GFS/Canadian and ECMWF blend
given satellite/radar trends and the fact that the 12Z NAM was
already too far north (by a good bit)with the offshore low as early
15Z.

Dry slot will eventually rotate around the 5h low overnight which
will bring an end to the heaviest rain. The upper low will be slow
to lift northeast and likely will not move across the area until
midday Tue. Diurnal heating combined with the passage of the cold
pool aloft will likely lead to development of some afternoon showers
and possibly a few embedded thunderstorms. Despite linger cloud
cover and moisture Tue temperatures will be near climo.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 3 PM Monday...Stacked low pressure will be exiting the
northeast NC coastal areas at the onset of the short term period.
500mb height increases will occur Wednesday and Wednesday night as a
mid/upper ridge axis shifts into the Eastern United States. Much
drier weather can be expected with time height cross sections
showing a period subsidence through the column during this time.
Temperatures will recover by Wednesday with cold air advection
lacking in the wake of the aforementioned low. In fact, highs on
Wednesday will likely range a couple categories above normal.
Minimum temperatures will be above normal as well.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 3 PM Monday...Medium range guidance is fairly consistent
with regards to the extended forecast. Longwave troughing will be in
place out west with a cutoff low eventually developing with a
southwest flow and strong ridging developing just off the southeast
coast. At the surface it will be primarily high pressure anchored
offshore. A weak front moving across well to the north will fade
before even getting close to our area so a dry forecast continues.
Temperature trends remain mostly intact well above normal throughout
the period but moreso first half as heights fall slightly late in
the period.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
As of 00Z...High confidence all terminals will be in and out of
the IFR category for the next few hours, with straight-up IFR
and lower for cigs for all locations from about midnight until
late morning on Tuesday. Vsbys will be reduced in rain,
especially between now and midnight. Conditions gradually
improve Tuesday afternoon.

Extended outlook...IFR or lower conditions possible in fog and
stratus Tue night and early Wed.

&&

.MARINE...
NEAR TERM /THROUGH TUESDAY/...
As of 8 PM Monday...Latest obs show Small Craft Advisory
criteria being met over the waters, with 8 ft seas and winds of
20 to 25 kts. Advisory will continue through at least midnight
and there is a possibility that it may be extended a few hours.
Previous discussion follows:

Strong south to southeast flow will continue into the evening
hours before winds become southwest in response to surface low
lifting north. Gradient will remain pinched with speeds hanging
around 20 kt into early Tue. Gradient starts to weaken after
midnight as the surface low moves off to the north. Southwest
flow drops to a solid 15 kt for the bulk of Tue. Seas running 4
to 8 ft may increase slightly during the next 6 hours or so but
then winds shifting to southwesterly and trending down will
allow seas to start following a downward trend. Seas across SC
waters will drop below 6 ft around midnight but NC waters will
take a little while longer. However, by Tue morning all seas
should be under 6 ft running 3 to 5 ft through the end of the
period.

SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 3
PM Monday...Offshore flow will prevail in the wake of exiting
low pressure Tuesday night. The weakening offshore flow will
back to a southwest to south direction during Wednesday as the
sea breeze pushed inland. Generally light southwest flow will
prevail by Wednesday night with high pressure far off the
Southeast U.S. coast and lower pressure across the MS valley.
Expect a larger range of seas initially with the exiting low and
an offshore trajectory. Seas will be 3 to 4 ft, primarily in
swell, through the bulk of the short term period.

LONG TERM /THURSDAY THROUGH SATURDAY/...
As of 3 PM Sunday...Essentially a summertime pattern for the
coastal waters as Bermuda High pressure will be the main feature.
South to southeast winds of 10-15 knots will prevail Thursday
through Saturday. Significant seas will be 2-4 feet.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...Flash Flood Watch through Tuesday morning for SCZ017-023-024-
     032-033-039-053>056.
NC...Flash Flood Watch through Tuesday morning for NCZ087-096-099-
     105>110.
     Coastal Flood Advisory until 11 PM EDT this evening for NCZ107.
     Coastal Flood Advisory until 8 PM EDT this evening for NCZ108.
MARINE...Small Craft Advisory until midnight EDT tonight for AMZ254-256.
     Small Craft Advisory until 8 AM EDT Tuesday for AMZ250-252.

&&

$$

SYNOPSIS...SRP
NEAR TERM...REK/III
SHORT TERM...SRP
LONG TERM...SHK
AVIATION...REK
MARINE...SRP/REK/III/SHK


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