Scientific Forecaster Discussion
fxus62 kilm 210727
Area forecast discussion
National Weather Service Wilmington NC
327 am EDT Sat Apr 21 2018
Canadian high pressure will build into the Carolinas through
the weekend, bringing dry weather with chilly temperatures.
Slow-moving low pressure will cross the southeastern U.S.
Monday through Wednesday, bringing widespread rainfall with
continued below-normal temperatures. Another system may affect
the Carolinas Thursday into Friday.
Near term /through tonight/...
as of 3 am Saturday...surface high over the Ohio Valley this morning
will drift southeast today, ending up just north of the Outer Banks
late in the day. The high will be reinforced by Canadian high late
tonight into early sun. Northerly flow continues through the period,
but does become a little more onshore in the afternoon. Low level
moisture return will be limited, even with developing onshore flow.
Pattern aloft also works against any significant moisture return
with weak shortwave ridge keeping flow above 10k ft from the
northwest. Precipitable water under half an inch through the period
shows just how dry this air mass is. Mid-level ridge does start to
weaken/flatten later tonight with flow aloft becoming westerly. This
might lead to an increase in upper level moisture and a bit of high
cloud above 20k ft overnight. However, precip chances remain zero
through the period.
Highs should end up a little warmer than yesterday, although the
slightly cooler start this morning will keep the region below climo.
Coastal areas will struggle to hit 70, especially with onshore flow
starting to materialize. Outside of the coastal influence highs
should be in the lower 70s. Weak east to northeast flow overnight
along with air mass modification will keep lows slightly warmer than
the last few nights, but temps still end up well below climo.
Short term /Sunday through Sunday night/...
as of 300 PM Friday...high pressure slides east of the area Saturday
night, passing by well to our north. Temperatures will be held a bit
below climatology in the NE flow with a mainly clear sky brought by
the downsloping mid level flow. Wit the high progressing further off
the midatlantic coast on Sunday the door will open up to moisture
influx. Cloud cover will increase from SW to NE. Some of the
guidance that has been suggesting that rain could start breaking out
soon thereafter has been discounted as being too fast. This will
eventually start to change heading into Sunday night as the
moistening deepens in response to approaching upper troughiness
tilting flow to the southwest. Probability of precipitation will increase though rainfall
amounts expected to remain light and generally confined to SC zones
Long term /Monday through Friday/...
as of 300 PM Friday...cutoff low will continue to move east
along the Gulf Coast states Mon into tues spreading clouds and
rain across the area from south to north. Increasing moist on
shore flow will push pcp water values up to near 1.5 inches late
Mon into tues. The center of the low should reach the SC coast
tues night and then track up along the Carolina coast lifting
off to the northeast late Wed. The best moisture and lift should
come together to produce the greatest quantitative precipitation forecast Mon night through
tues with fairly widespread amounts between a half inch and an
inch with locally higher amounts.
Some dry air should wrap in with westerly flow on the back end
of the system on Wed, but lingering upper level energy could
keep some lingering clouds and showers around. By thurs, another
cutoff low will track across the appalachian mtns, with
increased potential for clouds and pcp late thurs into Fri.
Overall a damp week ahead with a good amount of clouds and pcp
through the week.
Temps running on the cooler side on Mon will return toward
normal tues into Wed. Overall temps will remain near or just
below climo through much of the week.
Aviation /08z Saturday through Wednesday/...
as of 06z...although a period of MVFR or IFR fog is possible at
the coastal sites during the next few hours, especially at
kcre, these restrictions are unlikely to last for any length of
time. As such have carried VFR for all sites through the entire
forecast period. Northeast winds this morning become more
easterly later this morning and eventually east to southeast by
Extended outlook...VFR. Mon periods of MVFR/ra. Tue periods of
IFR/rain becoming MVFR. Wed VFR.
near term /through tonight/...
as of 3 am Saturday...surface high over the Ohio Valley will
gradually move southeast today. The high will end up centered
just north of the Outer Banks later today. Slight increase in
the pressure gradient later today into tonight results in a
slight increase in east-northeast flow this afternoon but speeds
are not expected to exceed 15 kt. Winds will become more
easterly later today and overnight. Seas will run 2 to 3 ft
though the period.
Short term /Saturday night through Sunday night/...
as of 300 PM Friday...high pressure progressing eastward and
staying well north of the area all period. This will turn flow
onshore Saturday night with wind and seas remaining quite
light/small. This will hold true most of Sunday as well save for
a gradual increase in wind speed. Things then ramp up with the
approach of low pressure from the west Sunday night. Expect
conditions to deteriorate to near advisory levels, though the
flags will likely go up this period to incorporate further
deterioration later on/early in the long term.
Long term /Monday through Wednesday/...
as of 300 PM Friday...low pressure system will move east across
the Gulf Coast states to the coast of SC by tues night and then
will track NE parallel to the Carolina coast. A strong on shore
flow will persist Mon into tues in a tightened gradient flow
between this low pressure system and high pressure moving slowly
off the northeast coast. Winds will diminish as they back from
east-southeast to north-NE tues night as the center of the low tracks
northeast becoming elongated just off the Carolina coast.
Eventually an off shore west-SW flow will develop late Wed as the
low exits off to the northeast.
Overall, expect this onshore push of easterly winds up to 25 to
30 kts to push seas up through Mon into tues, from near 6 ft Mon
morning up to possibly 10 to 12 ft by tues morning and then
subsiding slowly as winds back around further becoming off shore
Wed. Expect Small Craft Advisory conditions through much of the period.