Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
631 PM EST Friday Nov 27 2015
high pressure will maintain its influence across the area through
Sunday...continuing warm and dry conditions. A cold front will
approach early next week bringing showers Monday through
Wednesday. Temperatures will drop to below normal following the
passage of the cold front late Wednesday. A coastal low may bring
increasing rain chances Friday.
Near term /until 6 am Saturday morning/...
as of 630 PM Friday...an area of surface high pressure across the
Carolinas will weaken dramatically overnight as the supporting
upper ridge deamplifies and sinks southward. This will allow the
southern branch of the jet to begin streaming overhead with
gradually increasing cirrus cloudiness expected overnight.
Pressures should fall by about 3 millibars overnight which will
lead to a weaker pressure gradient and somewhat lighter NE winds
than we experienced last night.
The primary forecast concern tonight is the potential for fog.
Lighter wind is always a plus...but increasing cirrus advecting
overhead from the northwest is a minus. Boundary layer moisture is
currently very good relative to where we expect temperatures to
end up in the 3-7 am timeframe...however both the GFS and NAM show
dry advection cleaning out a good deal of the moisture almost down
to the surface after midnight. This should still leave enough
moisture to produce patchy fog and this will remain in the
forecast...particularly inland from the coast late tonight.
Very few changes are needed to forecast lows...still anticipated
to dip into the upper 40s inland and lower 50s at the coast.
Short term /6 am Saturday morning through Saturday night/...
as of 300 PM Friday...above normal temperatures and dry
weather will continue through the weekend...but a slow weakening of the
upper ridging will bring increasing rain chances late in the period.
Large upper ridge across the east will very slowly decay through the
weekend in response to anomalous trough digging into the plains.
Between these two features...a slow moving baroclinic boundary with
a Pacific moisture tap will drift to the east and southeast. This
slow moving boundary is responsible for the very heavy rainfall
occurring across the Midwest...but as it encroaches up high pressure
both aloft and aligned along the East Coast...it will slow and
weaken. Forecast soundings suggest moisture will remain very low in
the column Saturday before increasing on Sunday...and especially
Sunday night. This suggests dry and warm weather with abundant
sunshine Saturday...before clouds begin to increase Sunday. GFS
brings in much higher precipitation chances Sunday night than the other
guidance...but this is likely too fast and will maintain the silent
pop through the end of the weekend.
As mentioned...temperatures will remain well above normal for the end of
November. Highs Saturday will rise into the low 70s most of the
area...with highs around 70 expected Sunday due to slightly lower
thicknesses and increased cloud cover. Mins both nights will stay
within a degree or two of 50...about 10 degrees above the usual lows
this time of year.
Long term /Sunday through Friday/...
as of 300 PM Friday...high pressure wedge building in from the
north paired with some overrunning moisture to bring a seasonable
but rather cloudy Monday. Some light rain will be tough to rule
out due to the overrunning but zonal middle level flow will preclude
anything more substantial. The Wedge breaks down on Tuesday
allowing for a few degrees of warmth but the low level moisture
and its associated clouds and minor rain chance will be tough to
rout out. A large occluded system moving through the Great Lakes
will drag a cold front through the area on Wednesday. Given that
the main system is so far to our north rain chances will be rather
minimal with frontal passage. Temperatures will take a hit on Thursday but
there should be a brief decrease in the amount of cloud cover
though likely a short-lived one. A fairly strong low may develop
on the offshore frontal boundary on Friday bringing back clouds
and perhaps good rain chances.
Aviation /00z Saturday through Wednesday/...
as of 00z...potential for areas of MVFR due to fog development late
overnight into the early morning hours. Otherwise expect VFR.
VFR prevails tonight with some passing middle clouds and cirrus...few
to scattered in nature. Expect this trend to continue through the
overnight hours with light winds. Guidance does indicate the
possibility of fog development overnight...though confidence remains
fairly low given dry air aloft seen in forecast soundings and latest
trends the past few days. For now...have kept tempo MVFR at all
sites for the late overnight hours. On Saturday...VFR will prevail
with few/scattered cirrus and light east-northeasterly winds...becoming
southeasterly along the coast in the afternoon hours.
Extended outlook...unsettled weather Monday night through late
Tuesday with possible MVFR/IFR conditions from ceilings and reduced
visibility from precipitation and fog. Otherwise expect VFR.
near term /through tonight/...
as of 630 PM Friday...weakening high pressure across inland North
Carolina will continue to send northeast winds across the coastal
waters tonight...although wind speeds should be lighter than we've
seen over the past couple of days. There is currently a large
range in wind speeds from barely 10 knots on the beaches to over
20 knots in gusts out at The Frying Pan Shoals buoy. This is
probably due to the wide difference we are now seeing in sea
surface temperatures: the warmer offshore water promotes deeper
vertical mixing in the atmosphere with stronger winds coming down
to the surface. Little change in wind speed is forecast overnight.
Seas are quite a bit lower than the previous forecast indicated.
Adjustments downward are as large as 2 feet in the Cape Fear
area based on 4-foot wave reports from both nearshore buoys near
Wrightsville Beach and Southport. Recent data from the edisto
buoy 40 miles offshore from Charleston indicates seas may continue
to be quite large for the waters off the central and southern SC
coast including east of Georgetown/Winyah Bay. No changes are
being made to the Small Craft Advisory with this update although
conditions are quite marginal at present.
Short term /Saturday through Sunday night/...
as of 300 PM Friday...high pressure elongated along the coast just
inland from the waters will maintain NE flow through the period.
Although the high pressure will generally push NE during the
weekend...the gradient over the waters will remain somewhat
pinched...and thus NE winds of 10-15 kts will be common Saturday and
into Sunday. Late Sunday the gradient across the area will begin to
pinch in advance of a slow moving cold front approaching the
Appalachian Mountains this will cause a slight uptick in wind speeds to
10-20 kts...still from the NE. Seas will be formed through a
surprisingly long period easterly swell in conjunction with the NE
wind chop...at stay above Small Craft Advisory thresholds of 4-7 feet into Saturday
evening. The ongoing Small Craft Advisory remains unchanged...and as the easterly
swell abates a bit late Saturday...headline should be able to be
dropped as seas fall to 3-5 feet Saturday night and Sunday.
Long term /Monday through Wednesday/...
as of 300 PM Friday... surface wedge in place on Monday to keep
the gradient moderate across the waters. Tough to say whether or
not a scec will be needed this far out in time but wave shadowing
of the near shore waters probably means headlines won't be needed.
The Wedge breaks down on Tuesday but according to wna guidance
seas will be rather slow to abate. Wednesday may offer many
different wind directions. There will be a small prefrontal
southerly component early on followed by veering associated with
the approach of a cold front. More pronounced veering will be
associated with the late day cold front.
Marine...Small Craft Advisory until 5 PM EST Saturday for amz250-252-