Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Wilmington NC
417 PM EDT Monday Sep 22 2014
an upper low will impact the area tonight through midweek. Light
stratiform rain will become the mainstay across the eastern
Carolinas through middle to possibly late week. A stalled coastal
trough will remain over the Atlantic waters during this time
keeping the threat of storms over land to a minimum. A warming
and drying trend is expected by the upcoming weekend.
Near term /until 6 am Tuesday morning/...
as of 300 PM Monday...the cold front continues to drop south as
dewpoints are now in the upper 50s across most areas. Any showers
associated with the boundary have dissipated for the most part at
least for the moment. This lull in activity will continue for the
next few hours before the deep middle level trough across the
Appalachians drops further south and closes off. Several factors
seem to have come together to forecast a wet overnight period for
the eastern third of the County Warning Area.
First there is a decent plume of moisture emanating from the Gulf of
Mexico via water vapor. Second...the Omega will come in several
forms through a deep layer. Starting at the lowest levels...some
modest isentropic lift exists in the 209-300k layer. Moving
up...some middle level frontogenesis evident in the 700-500mb layer.
Difluence exists at the 500mb layer and finally...a strong jet at
300mb will move up the coast and the favorable regions will move
across the coastal areas. Long story short...I have increased probability of precipitation
to likely for these areas. Quantitative precipitation forecast amounts may eclipse one half inch
especially southern and extreme coastal areas but will otherwise
end up between one tenth inland to one half elsewhere.
I have made a few adjustments for the afternoon temperatures to
address the cloud cover cooled readings but overnight lows remain on
track for lower 60s along the coast to upper 50s northwest.
Short term /6 am Tuesday morning through Wednesday night/...
as of 300 PM Monday...pattern aloft to become somewhat of a
bottle neck across the area. Models in unison with the development
of a closed upper low across the Carolinas on Tuesday...and will
persist across the forecast area through Wednesday. Positioning of this closed
low across western Carolinas will result in S good feed of
moisture from the Atlantic and to a lesser degree from the Gulf of
Mexico. At the surface...strong 1035+ mb high centered over the NE
states will ridge down the East Coast. Just offshore and lining
up nearly parallel to the Carolina coastline south of Cape
Hatteras...will lie a coastal trof/front. During this short term
period...do not anticipate this boundary moving onshore via model
consensus. Will be looking at overrunning or isentropic lift
across the forecast area which will produce stratiform-type light rains. A
sharp gradient will exist with respect to the moisture
distribution across the forecast area...with the heaviest precipitation likely to
occur east of the I-95 corridor to the immediate Carolina coasts.
The positioning of this upper low across the western and central
Carolinas will determine whether additional heavier precipitation will get
pushed inland. Have indicated an isolated thunderstorm possible
for any elevated convection that holds together as it develops in
the vicinity of the coastal front and pushes onshore. As for
maximum/min temperatures Tuesday through Wednesday...have leaned toward the
cooler GFS MOS guidance due to cloudiness and ongoing precipitation threat
as well as the present strong NE flow across forecast area thruout this
Long term /Thursday through Monday/...
as of 300 PM Monday...cool air wedge plus a coastal trough spells
some rather unsettled and probably pretty cool weather Thursday
and Friday. In fact the MOS guidance numbers may be showing their
bias towards climatology that increases with time since they portray
temperatures near climatology both days...while keeping highs near 70 tomorrow
in a similar airmass offering up clouds and rain. Will keep
forecast closer to current values of middle 70s for highs. Rain
chances will be most focused along the coast both days in
proximity to the trough and deepest moisture and the GFS has now
backed off almost completely with inland penetration of quantitative precipitation forecast. This
is now supported by the Gem and so may need to be reevaluated. Middle
level ridging strengthens across the eastern U.S. Suppressing the
deep moisture off to our south likely leading to a rain-free
weekend. While the sense of low level thermal advection over the
weekend will technically be neutral or even weakly cool the return
of sunshine will favor seasonable temperatures during the day. The
lesser cloud cover meanwhile could bring some cooler nights when
compared to earlier in the period.
Aviation /19z Monday through Saturday/...
as of 18z...a cold front will continue to drop southward...with
north northeast winds at all terminals. Patchy light rain may be
replaced with heavier convection...mainly along the extreme
southeastern portion of the County Warning Area...possibly affecting The Myrtles.
Predominately MVFR conditions in the showers. Tonight...activity
diminishing with continued northeast flow. The NAM is very
pessimistic with regard to ceilings...bringing everyone down to IFR.
Fell this is way overdone at the Junction...but keep an open mind.
Tuesday...a low forms along the front and tries to wave the front
back onshore...however feel most of the moisture will stay offshore
at this time.
Extended outlook...unsettled weather through the period...mainly
along the southeast coast of the County Warning Area with periods of rain/showers
and intermittent MVFR/IFR conditions.
near term /through tonight/...
as of 300 PM...the coastal waters will continue to see winds
increase in magnitude and maintain a northerly component through
Tuesday morning. Currently...winds are from the north about 10-12
knots. As high pressure shifts across the Ohio Valley the gradient
will gradually increase to see a robust 15 knots. The direction
will shift a bit to the northeast as well. Significant seas will
slowly build from around the current 2-3 feet to 3-4 feet.
Short term /Tuesday through Wednesday night/...
as of 300 PM Monday...strong surface high centered over the NE
states...will ridge down the eastern sea board through this period.
At the same time...a coastal trough/front will manifest itself
during this period. Models keep this boundary just offshore from
the Carolinas coastlines. This will result in a pinched surface pg
across the local waters for an extended period of time. This will
produce NE to east-northeast 15 to 25 knots winds...with few gusts up to 30 knots.
Somewhat too early for the dayshift...will let either the evening
or overnight shift put up the extended Small Craft Advisory. Significant seas will
build and hold in the 3 to 7 foot range depending on ones location
in relation to the coastline configuration. Overall...sig seas
will become mainly a function of NE to east-northeast wind waves exhibiting
5 to 6 second periods. An underlying 1-3 foot ground swell at 8+
second periods will also remain present.
Long term /Thursday through Friday/...
as of 300 PM Monday...a pinched northeasterly gradient will keep the
region very close to the cusp of scec headlines vs Small Craft
Advisory. The more serious latter headline appears in the offering
for the earlier part of the period while conditions may drop down to
scec come Saturday. This would be in response to a building middle
level ridge and a slight southward push of the main axis of surface
high pressure. There will be a long period NE swell that will
coexist with the wind waves...but both will also experience some
shadowing locally off of Hatteras.