Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
918 am EST Sat Feb 6 2016
low pressure will develop offshore of Florida this afternoon
then rapidly intensify and track northeast offshore of the
Carolinas late tonight through Sunday. This system may bring
snowfall to inland locations Sunday. The strong low will track
farther out to sea Sunday night through Monday. Weak high pressure
will prevail Monday followed by a cold frontal passage Monday
night. A dry Arctic air mass will follow Tuesday and Wednesday.
Near term /through tonight/...
as of 918 am Saturday...adjustments were needed for sky cover
into partly cloudy range based on the extensive cirrus and middle
level clouds around Cape Fear. This will result in a slightly
slower warm up this morning...but for this update...maximum
temperatures were held in check...a couple degrees either side of
50 degrees by mid-afternoon. No other changes at this time. New
model data may result in further adjustments with the midday
update for tonight and early Sunday...as a coastal storm impacts
NE SC and southeast NC. The previous forecaster discussion follows.
A middle-level trough will tap the Gulf of Mexico moisture and along
with Atlantic moisture...have overrunning/isentropic type precipitation
starting out later this evening across the southern portions of
the ilm County Warning Area. It should overspread the ilm County Warning Area during the pre- dawn
Have identified the threat for a mixture of light snow with the
rain for the far northwest portions of the ilm County Warning Area...running from
northern Bladen...northern Robeson and northern Dillon counties
and including much of Marlboro and Darlington counties. This will
occur during the pre-dawn Sunday hours. 850mb temperatures are right at
0 across the mentioned area...with 1000-850mb thickness ranging
from 1305-1310m which is still higher than the recommended 1300m
for snow. Earlier in the day...the 1000-850mb thickness across
this region will lie between 1295 and 1300m...favorable for snow.
Enough warm air advection in the lower levels from off the Atlantic will keep
the majority of the area liquid rain overnight...and will only
advertise snow mixing in across the far northwest portions...with a
trace for accumulations through daybreak sun. Tonights temperatures to
remain slightly above freezing...except the northwest portions where it
could drop to around 32. With maximum temperatures today expected to
reach the upper 40s to low 50s...plenty of cooling is needed for
that snow mixture to be realized late tonight. The dynamics from
the developing system will likely supply some of that cold air
but will it be enough for the snow to occur.
One tenth to one half inch of rainfall is expected west of the
I-95 corridor. Along and east of the I-95 corridor and to the
coast...one half to 1.25 inches are expected. The higher end of
this range will occur across the immediate coastal counties. The
rain will fall in a 12 to 18 hour period and therefore do not
anticipate any flood watches. Temporary nuisance type flooding may
occur across those areas that received 2 to 5 inches of rain
several days ago.
The surface pg will tighten ahead of the rapidly deepening low off the
Florida coast late tonight. North-northeast to north winds will increase to 15 to
25 miles per hour late tonight along the immediate coast...with gusts up to of
30 miles per hour possible by daybreak Sunday. This remains below wind advection
criteria...however subsequent periods may flirt with gusts up to 40
to 45 miles per hour...depending on the strength and positioning of this low.
Short term /Sunday through Monday night/...
as of 3 am Saturday...all eyes remain on the system that will
already be affecting the forecast area as the period beings.
Southern stream shortwave over the eastern Gomex will induce surface
low development off the Florida coast late tonight. The low will then
track northeast...passing off the Carolina coast sun into Sun night.
There will be an abundance of deep moisture during the first half of
the period and strong upper level jetting will help deepen the low
and provide dynamics. This should result in precipitation falling across
much of the area. Have high confidence in precipitation falling sun into
Sun night but confidence drops with respect to the type of precipitation
expected during the event.
Currently all but the coldest of the guidance is pointing to a
rain/snow event for the area with potential for all snow as it ends
across inland locations. This however does not take into account
dynamical processes that can contribute a significant amount of
cooling as a low rapidly deepens. There will be plenty of ice in the
cloud as strong upward motion with the dendritic growth zone will be
present and forecast soundings show the column below freezing above
roughly the 850mb level (again discounting the coldest outlier).
Given the high level of uncertainty and the sensitivity of the p
type forecast to minor adjustments in the lows track do not plan to
alter the inherited forecast significantly. After discussion with
neighboring offices decided on a rain/snow mix for inland areas Sun
morning turning to all rain in the afternoon. However it must again
be mentioned that confidence remains extremely low and changes to
the forecast will likely be required.
Second half of the forecast period is no less of a challenge. Brief
period of drying late Sun night and early Monday comes to an end as
shortwave/cold front combo approach from the northwest. This feature
also has plenty of lift and moisture within the dendritic growth
zone. Cold air will again be the issue but latest guidance is
showing temperatures a bit cooler and the system will have strong dynamics.
The limiting factor will be a fair amount of dry air below 850 mb.
So while there is arguably a better chance at getting snow to fall
with the second system the low level moisture remains a limiting
factor. Have maintained inherited chance pop for late Monday and Monday night
but whether precipitation ends up rain...rain/snow mix...or even all snow
the quantity that reaches the surface will be little more than a
trace for most areas.
Long term /Tuesday through Friday/...
as of 3 am Saturday...impressive longwave 500 mb trough over the eastern
Continental U.S. Tuesday will usher in Arctic air for middle week. First shot of cold
air arrives early Tuesday as cold front moves across the area with a
reinforcing shot expected on Wednesday. Initial round Tuesday drops 850 temperatures
to around -8c in the afternoon with potential for 850 temperatures
approaching -13c on Wednesday. By Thursday the middle level trough starts lifting
north with an increase in both 850 temperatures and 1000-500 heights in
store. Unfortunately little of this warming translates to the
surface where partial thicknesses suggest Arctic air will hold on
through Thursday before air mass starts to modify Friday and temperatures approach
normal. Temperatures for much of the period will be below climatology with
Wednesday and Thursday having temperatures 10 to 15 degrees below. Presence of cold
advection throughout the event is a double edged sword. It will help
prevent strong radiational cooling...which would yield lows in the
teens...but it will lead to wind chill values which will make it
feel like it is in the teens Tuesday and Wednesday nights.
Any precipitation chances during the period would be linked to shortwaves
rounding the base of the 500 mb trough. There appears to be only 1 with
any shot at squeezing out precipitation and this is currently timed to pass
Tuesday afternoon and evening. Forecast soundings do not show a lot of
moisture to work with...precipitable waters remain well below half an inch...but
stronger shortwaves always seem to squeeze out what little moisture
is available. At best it would be some patchy light flurries and
does not currently warrant a pop over 5%.
Aviation /14z Saturday through Wednesday/...
as of 12z...high confidence for VFR through tonight...with the
potential for MVFR overnight into Sunday morning as a developing
storm system brings low ceilings and the potential for mixed precipitation.
With high pressure overhead...VFR prevails this morning with light
winds and middle to high clouds. Light northeast winds this morning
will increase to around 10 kts later this morning. A low pressure
system will develop this evening and track up and along the Carolina
coast...bringing low ceilings and increasing chances for rainfall
overnight into Sunday morning. Would not entirely rule out a
wintry mix inland...but confidence is low at this time...thus have not
introduced in going forecast. Will see an increase of north-
northeasterly winds tonight...mainly along the coast...as the
pressure gradient tightens. Sustained winds will becoming around
15 kts with gusts around 25 kts into Sunday morning.
Extended outlook...rain and or possible snow Sunday with IFR
conditions possible. Conditions improve to VFR late Sunday night
into Monday. A chance of precipitation with MVFR conditions possible
late Monday into Tuesday. Predominately VFR Wednesday.
near term /through tonight/...
as of 918 am Saturday...Gale Warning in effect all waters
commencing 600 am Sunday.
A potent middle-level short wave trough will cross northern Florida late
tonight...and aid in the development of a surface low just offshore
from the Florida East Coast by daybreak sun. A modestly tightened
surface pg currently across the local waters...will further tighten
later tonight once the surface low commences off the Florida coast.
The 1003-1005mb low at sunrise will rapidly deepen to 995mb Sunday
morning...and deeper yet 986-990mb Sun afternoon. This will result in a
small period of Small Craft Advisory conditions developing later tonight...and
rapidly transition to gale conditions by sunrise Sunday and there-
after. For today...looking at NE 15 to 20 knots backing to north to north-northeast
15 to 25 knots with gusts to 30 knots tonight. Significant seas will be
governed by wind driven waves at 5 to 7 second periods. Looking at
2 to 5 feet seas today...building to 4 to 8 feet late
tonight...highest off Cape Fear and romaine. The north to north-northeast winds
will result in a lower fetch for which significant seas to build
upon...and thus the reason why the large range of seas being
Short term /Sunday through Monday night/...
as of 3 am Saturday...gale conditions through sun evening relent
Sun night and early Monday as low exits and weak surface ridge
builds in.The reprieve is short lived and southwest to west flow
exceeding 25kt is expected to develop late Monday afternoon and
continue through the end of the period. Seas follow a similar path
with highest values on sun before dropping under 6 feet late Sun
night and early Monday. However by Monday evening seas will be at or
above 6 feet and will continue to build Monday night.
Long term /Tuesday through Wednesday/...
as of 3 am Saturday...anticipate continuation of headlines for
much of the period due to strong cold advection and offshore
flow. West to northwest flow will remain in the 20 to 25 knots range
from the start of the period through Wednesday evening with seas
running 4 to 7 feet for all but the very end of the period. Cold
advection and winds start to weaken Wednesday evening with speeds
dropping to near 15 knots by the end of the period.
Marine...Gale Warning from 6 am to 10 PM EST Sunday for amz250-252-254-