Scientific Forecaster Discussion
Area forecast discussion...corrected synopsis
National Weather Service Wilmington NC
755 am EDT Thursday Aug 21 2014
hot and humid conditions can be expected over the next couple of
days as an upper ridge expands into the Carolinas. A heat advisory
is in effect for portions of South Carolina today. Slight cooling
this weekend will arrive as a cold front moves through. High
pressure building from the north early next week will keep rain
chances on the low side.
Near term /through tonight/...
as of 755 am Thursday...upper ridge axis will extend between Florida
pan handle north to the Great Lakes. What models indicate is
amplifying of this upper ridge during this period and continuing
into the next. Models indicate the majority of the middle-level
vorts/impulses moving within the northwest flow aloft on the Lee side of
the upper ridge will stay just north of the forecast area. Their closest
approach and any possible precipitation will occur across the northern
portions of the ilm County Warning Area during the pre-dawn Friday hours.
Otherwise...the best chance for convection today will be along the
sea breeze and possibly along the inland pseudo Piedmont trough.
Subsidence from the ridging aloft will be felt across the majority
of the forecast area and will tend to keep convection at a minimum even along
the sea breeze and inland trough. Probability of precipitation up to 30 at best.
The weather headline for the ilm County Warning Area today and into the short term will
be of the heat...with Summer saying that its not done with US yet.
Heat advisory remains in effect for portions of the ilm SC
County Warning Area...with no deletions or additions. Temperatures in the middle to upper
90s combined with low to middle 70 dewpoints will result in 105-108
heat indices across the heat advisory area...and 100-104 outside
the advisory. Whats somewhat discerning as the guidance wants to
paint an area of 60s dewpoints along and west of the I-95
corridor. Will not jump on this idea yet...since it would likely
eliminate the need for a heat advisory.
Short term /Friday through Friday night/...
as of 3 am Thursday...forecast continues to be playing catch-up. The
relatively low amplitude but nevertheless Stalwart middle and upper
ridge over the lower MS valley will remain in place. For days now
guidance has been showing that some energy tops the NE of this ridge
and dives into the Carolinas. In fact now with the ridge expected to
gain amplitude into the Ohio Valley and upper MS valley through the
period it seems that any of the aforementioned energy will tend to
bypass US by to the north. A Piedmont trough will be displaced
towards the coast and the seabreeze will make little inland progress
and both may fire isolated convection...but now the period appears
to be largely characterized by a continuation of unseasonably hot
and humid conditions although a slight abatement of both may creep
in on Saturday. Heat advisory appears possible for part of the area
on Friday. This will hinge on the cold front that may come through
Saturday or not until Saturday night. Will show a slight decrease in
temperatures Saturday as most models are a little faster. The slow GFS
though implies that many areas may Cook again Saturday afternoon.
Take mav highs for lbt for example Thursday, Friday, saturday; 97,
96, 98. Don't want to show much of a cool down in case something
like that comes to pass which given recent model errors certainly
seems possible...as the slower GFS could imply that some locales
will be looking at yet another heat advisory.
Long term /Saturday through Wednesday/...
as of 3 am Thursday...upper trough off the NE and middle Atlantic coast
finally sharpens enough to drive cool front south of the area by
Sunday. The remainder of the period will see the middle level ridge
expand eastward with a NE to SW elongation from the New England
states to the Gomex. At the surface high pressure will spill down
the East Face of the Appalachians in a pressure pattern more
reminiscent of the cool season. Little to no real rain chances and
temperatures seasonable. It also bears mentioning at this point that the
system just east of the Windward Islands will likely be a tropical
system well off the East Coast late in the period. Swells from this
system could be affecting area beaches by Wednesday or soon
Aviation /12z Thursday through Monday/...
as of 12z...pretty dense fog at cre and ilm should mix out by
13z...or shortly thereafter. Need to keep an eye on lbt as it may go
down briefly. A weak thermal trough will reside over the region
today. Less moisture to work with than previous days...nevertheless
should see isolated/scattered activity beginning around
18-19z...probably along the resultant. Southwest flow will continue
today...with temperatures again in the 90s.
Extended outlook...chance afternoon/evening rain showers/thunderstorms and rain through the
Sunday. VFR Monday.
near term /through tonight/...
as of 625 am Thursday...the north-S surface trough extending across the
eastern Carolinas and weak Bermuda ridging extending west to the
southeast U.S. Coast...will combine efforts to continue with a SW wind
across the area waters...possibly veering slightly to the west-southwest
northern portions tonight. The surface pg remains rather relaxed with
wind speeds around 10 knots thruout this period. The sea breeze
circulation may temporarily produce 10-15 knots wind speeds near
shore during this afternoon and early evening. Look for significant
seas of 2 to 3 feet...with 3 feet possibly being a bit too generous.
Nevertheless...an east-southeast 1-2 foot 8-9 second period ground swell will
remain prevalent and likely the driver of significant seas during
this period except for the wind chop from the sea breeze.
Short term /Friday though Saturday night/...
as of 3 am Thursday...the Piedmont through will be displaced towards
the coast from its normal position on Friday with some models even
portraying it right along the coastline. Either way this will favor
very light southwesterly winds and seas of no more than 2 to 3 feet.
The seabreeze will not be very strong due to offshore winds just
above the surface layer. Saturday is one of the more uncertain
periods of both the short term as well as the long term. A cold
front will be backdooring its way south across the area but its
timing is poorly agreed upon by models and this has been the case
for several days now. Frontal passage timings can range from Saturday to
Saturday night. Being an east-west oriented cold front it should
lead to a pretty abrupt wind change from west to east via clockwise
veering. Fortunately wind speeds will remain minimal through the
frontal passage and waves will not grow in size even though they may steepen
Long term /Sunday through Monday/...
as of 3 am Thursday...east or northeast winds in store for the
period as cold front sags off to the south. High pressure builds in
from points north and east as more typically seen in cooler weather
seasons. Seas will build and become rather choppy and an advisory is
not out of the question by the end of the period at least according
to wna guidance. Any effects from the would-be tropical system that
will likely remain well offshore to be slated for the middle week at
SC...heat advisory from noon today to 6 PM EDT this evening for