Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
332 am EST Thursday Mar 5 2015
a powerful cold front will bring sharply colder temperatures
beginning this evening. There is a chance of some light frozen
precipitation Friday morning. The weekend will feature slowly
moderating temperatures. The slow warmup will continue next week
with showers possible.
Near term /through tonight/...
as of 3 am Thursday...lots of interesting weather over the next 24
hours. It begins with moist southwest winds producing sea fog across
the coastal waters and adjacent land areas. ASOS/AWOS observation at kcre
and ksut have shown low visibilities all morning...and this should continue
through about noon. As we saw yesterday...this same airmass also
happens to be exceptionally warm. Even though middle-level clouds will
be increasing along the I-95 corridor this morning and spreading
down to the coast this afternoon there should still be enough
sunshine to push temperatures into the upper 70s to around 80 degrees east
of I-95. Along the south- or southeast-facing beaches...cool onshore
winds will keep temperatures ~10 degrees cooler.
A powerful cold front still sharpening up through the southern
Appalachians and across southern Virginia will sag southward into
the area this afternoon. Model trends over the past few days have
been for a slower arrival of this front...and it now appears the
front should arrive in Bennettsville-Lumberton-Elizabethtown around
2 PM...Florence-Marion-Southport and Wilmington between 3-4 PM...and
southward across Myrtle Beach-Georgetown-Kingstree between 4-5 PM.
The temperature drop behind this front will be phenomenal: upwards
of 12-15 degrees in the first hour after frontal passage with a 30-35 degree
drop over a 6 hour period going into this evening.
Behind the surface front light to occasionally moderate rain should
develop and last into the evening hours. This rain will be due to
isentropic lift along the 300k surface which should have a fair
amount of slope over the increasingly cold air nosing southward into
the area. Moisture on this isentropic surface clears out by midnight
with a substantial reduction in precipitation expected by this time.
After midnight...shallower moisture will remain over the area as the
atmosphere dries from the top down. By 3 am Friday morning forecast
isentropic motion is downward throughout the entire moist layer...
which usually spells the end of precipitation. However an impressive
300 mb jet streak off the middle-Atlantic coast will remain favorably
oriented to squeeze every little bit of moisture out of the
remaining cloud deck. The 00z NAM is quite a bit more moist than the
00z GFS...and 00z sref members show 30-50 percent chances for
lingering light precipitation as surface temperatures plunge to near
freezing. Forecast soundings at 09z-12z Friday reveal a healthy slug of
cold air below 3000 feet above ground level which should be sufficient to freeze
falling hydrometers into sleet pellets. Anything falling during
that time should take the form of sleet...or perhaps rain where
low-level temperatures are warm enough to remelt precipitation.
Forecast quantitative precipitation forecast after 06z is quite low (.02" or less) and with very
warm ground temperatures over the past 48 hours it appears this
should be not be a significant winter weather event.
Short term /Friday through Saturday night/...
as of 400 PM Wednesday...Spring is typically a season of
extremes...and Thursday by itself will be a microcosm of the
season as a whole. A very dynamic system and associated cold front
will bring very warm temperatures...very cold temperatures...convection...and
even wintry precipitation. The grids required use of both Summer time
convective procedures...and winter weather procedures. In other
words...it is not an easy forecast.
Very warm temperatures will start Thursday. High pressure off the
coast will continue to pump warm return flow into the area...and
temperatures will soar the first half of Thursday. Guidance continues to
slow timing of the cold front...thanks in part to the persistent
ridge and the drawn out/weakening driving vorticity impulse. This
suggests that guidance may be too cool with highs Thursday...and
expect most of the area to again rise to 70 or greater by late
morning. Sref plumes suggest temperatures rising as high as the middle 70s
late Thursday morning...and plan to hedge towards the warm end of
the sref plume envelope thanks to a warm start and delayed frontal passage.
The cold front will cross the region Thursday during the late afternoon
through early evening...but most of the precipitation associated with it will
be Post-frontal. Storm Prediction Center has most of the area outlined in a "general
risk" for swody2...mentioning potential elevated convection behind
the front. Forecast profiles do suggest a small amount of
instability within the 700-500mb layer...but this is accompanied by
very weak middle-level forcing and only marginal jet-level diffluence.
Additionally...the atmosphere above 700mb looks to remain quite
dry...so driving saturated parcels into the charge separation zone
will be difficult to achieve...and thus will carry just rain shower and leave
out any thunder mention Thursday evening. What will not be difficult to
achieve will be rapidly cooling temperatures...as very strong cold air advection follows
behind the cold front. Temperatures between Thursday evening and Friday morning
will drop some 40 degrees across the County Warning Area...and expect the entire County Warning Area
to drop to freezing or below for Thursday night mins.
Temperatures below freezing will be accompanied by light...but still
occurring precipitation. Thus...freezing rain is looking more likely late
Thursday night into Friday. Guidance differs into how long precipitation
will persist on Friday...but with the vorticity strung out and hanging
well back to the west...could see precipitation linger into Friday
afternoon...especially along the coast. Westward graphics paint a large area
of up to 0.10 ice accrual...and this seems reasonable at this time. Cold air
chasing moisture is never a good setup for wintry precipitation...but the
strength and rapidity of the cold air dropping south should overcome
the typical bias...and frozen precipitation is expected for at least a
short duration. Fortunately...any freezing rain will be falling onto wet and
warm grounds so only elevated surfaces may actually see
accumulation. The highest ice accretion is expected along the Cape
Fear area where the longest crossover of cold temperatures and precipitation is
Drying will occur Friday afternoon/evening as cold high pressure ridges
eastward...and a clear Friday night is expected with light north
winds. With highs Friday afternoon expected to be only in the low 30s
(!!)...mins Friday night should drop well into the 20s...and may
approach record lows for the date.
Long term /Sunday through Wednesday/...
as of 400 PM Wednesday...overall the middle level pattern for the
extended has changed little from this time yesterday. Still
expecting considerable troughing over the eastern third of the
Continental U.S.. through early Tuesday with a relaxing of the pattern
thereafter. Some details with surface features have changed
Any probability of precipitation with a glancing shortwave system for Saturday have been
removed. The system emanating from the Gulf of Mexico should occur
Sunday and some of medium range guidance is now keeping any affects
well offshore. A more significant system with slightly better model
agreement develops late Monday and Tuesday. I have maintained the
slight chance probability of precipitation for this time and the new day Wednesday.
Temperature trends remain intact as well with a deliberate warming
trend basically due to airmass modification under a very weak
pressure pattern after a cold start Saturday.
Aviation /09z Thursday through Monday/...
as of 06z...sea fog has pushed into kcre and is looming just east of
kmyr. Through sunrise confidence is moderate to high sub-IFR
conditions will occur at the coastal airports. Highest confidence is
at kcre and kilm (mainly ceilings at kilm) with lesser confidence at
kmyr. Low stratus ceilings 500-1000 feet will be possible at kflo/klbt
generally after 11z. Low level wind shear is a good possibility with
30-35kt winds around a 1k foot inversion.
IFR or lower ceilings should break up 14-15z with generally VFR
expected. A strong cold front will move through the terminals
during the afternoon with Post frontal MVFR ceilings/-ra at the coastal
terminals and Post frontal IFR/-ra at the inland terminals. Winds
will shift to the north with gusts t0 18-25 knots.
Extended outlook...IFR conditions on Thursday...and possibly into
Friday along the coast. Otherwise expect VFR.
near term /through tonight/...
as of 3 am Thursday...steady southwest winds continue to bring
subtropical moisture north across the area. Where this moisture
encounters the very chilly ocean water we have a long-lived dense
sea fog event ongoing. Airport weather observations from North
Myrtle Beach & Southport confirm the fog is still very dense.
Very little should change in the synoptic wind setup this morning
into the early afternoon...until a powerful cold front now taking
shape across southern Virginia sinks southward into the area. This
front will bring an instantaneous shift to north winds during the
middle to late afternoon hours...with wind speeds increasing to 25-30
knots tonight. Gusts to gale force should occur during the period of
strongest cold air advection late this evening and overnight...and
we have turned the gale watch into a Gale Warning beginning at 9 PM
Seas currently 3-5 feet are nearly steady-state with the southwest
winds...but should increase to 6-9 feet overnight...highest east of
Cape Fear and east of Winyah Bay where north winds will have the
longest effective fetch to work with.
Short term /Thursday through Friday night/...
as of 400 PM Wednesday...high pressure offshore and a cold front
approaching from the west will keep a pinched gradient with gusty SW
winds the first part of Thursday. Wind speeds of around 15 kts are
expected to continue into Thursday afternoon before veering more to the
west/SW in the evening. This will keep seas 3-5 feet...but will also continue
the potential for area of dense marine fog as the warm moist air
advects across still very cold shelf waters. Have opted not to
extend the marine dfa at this time...but a local extension into Thursday
afternoon may be required tonight.
Cold front approaches and then crosses the waters from north to
south Thursday night. This is a powerful front...and will be
accompanied by an abrupt wind shift to the north and a quick rise in
speeds to 20-30 kts Friday morning. A gale watch has been issued
beginning at 11pm Thursday night for frequent gusts above gale force
during this time period...and lasting into late Friday morning. High
pressure will build east during Friday and Friday
night...maintaining the north winds but with slowly decreasing
speeds. Seas...which will rise to as high 5-9 feet overnight Thursday
into Friday...will fall through Friday...becoming 2-4 feet at the end
of the period.
Long term /Saturday through Monday/...
as of 400 PM Wednesday...some residual stronger winds continue across the
waters early Saturday with a northerly flow of 10-15 knots. After
this a very weak and prolonged pressure pattern develops under a
relaxing middle level pattern. Difficult to even pinpoint a predominant
wind direction but northeast backing to westerly seems like it will
work. Speeds will be ten knots or less and probably much less in
some cases. Seas will be 2-4 feet for a few hours Saturday then
subside to 1-2 feet Sunday and Monday.
SC...dense fog advisory until 10 am EST this morning for scz054.
NC...dense fog advisory until 10 am EST this morning for ncz108-110.
Marine...dense fog advisory until noon EST today for amz250-252-254-256.
Gale Warning from 9 PM this evening to 10 am EST Friday for