Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
250 PM EDT Friday Aug 22 2014
Synopsis...upper level disturbances will continue to move southeast
across the region through the early part of the weekend. A cold
front will move south through much of the Carolinas by Sunday
morning. High pressure will then south behind the front at least for
the early part of next week.
Near term /through tonight/...
as of 250 PM Friday...
Perturbed northwesterly flow continues over the region today
between a middle-level ridge over the deep south and a trough over the
northeast. The couple of disturbances moving in from the northwest
appear to have weakened considerably today and all of the
associated showers have essentially dissipated before crossing the
NC/Virginia border. Clouds have been increasing across the north since
late morning...but strong heating has still resulted in MLCAPE of
1000-2000 j/kg. Winds over the area are relatively weak...with
slightly higher shear across the northern coastal plain as a
northwesterly speed maximum crosses the central Appalachians. Outside of
weak disturbances aloft...much of the area is void of any strong
forcing. Any area of convection currently over WV may bring a
better chance of showers/storms later this evening...so will
continue probability of precipitation into the overnight hours before trending down after
midnight. If convection can get going...especially by early
evening...dcape will be fairly high across the southern half of the
area where better mixing has taken place and a wind threat may still
exist. Lows tonight in the lower 70s with the potential for some
stratus on the northeast as a weak boundary slips into ther area
from the northeast.
Short term /Saturday through Saturday night/...
as of 220 PM Friday...
A weak boundary...potentially outflow induced...forecast to move
into the area tonight will settle across the southern portions of the
area...while the true cold front remains to our northeast. Ahead of
the front...850 mb temperatures remain above 18c...though mixing should stop
short of that level with precipitable water near 2 inches. Periods of high clouds in
northwest flow aloft should also temper highs a bit...especially
across the northeast as the front begins to push into the area late
in the day. Cooler guidance gives a wide range of temperatures from lower
90s southwest to middle 80s northeast. Due to this...instability will
be greatest across the southeast...with marginally stronger shear
back to the northeaster closer to the frontal zone. There is still
the potential for another disturbance to approach from the
northeast...which combined with increasingly convergent low level
flow and high precipitable water...should lead to a better chance of showers
The backdoor cold front will make its way into the area late
Saturday and overnight...with drying aloft behind the front zone.
Moist northeasterly flow will likely lead to fairly widespread
stratus...and possibly a few light showers as a shortwave rotates
south across the area. Lows will be in the upper 60s northeast to
lower 70s southwest.
Saturday and Saturday night: high pressure over New England will
begin to ridge south down the middle-Atlantic Seaboard which should
push the back-door front slowly south into the area on Saturday. As
usual...timing of these back-door fronts this time of year is
suspect(usually slower is better)...with diurnal heating and
resultant destabilization aiding in scattered coverage. The late
arrival of a shortwave trough rotating southward down the back-side
of the upper trough Saturday night may even support a few anafrontal
Cooler on Saturday with clouds and northeasterly flow north of the front.
Highs ranging from Lower/Middle 80s north to lower 90s south. Lows
Saturday night in the upper 60s to lower 70s.
Sunday and Sunday night: as the upper air pattern amplifies (ridge
building over the Mississippi River valley...trough deepening across
the western atlantic) on Sunday...central NC will be under
northwest/northerly flow aloft. At the surface...a cold front will
be shunted south of the region as high pressure begins to ridge down
from the northeast. As this occurs...drier air will filter into the
region throughout the day...but some deep moisture will remain in
place...mostly across the south and west where precipitable water values are
prognosticated to hover between 1.5-1.75 inches through Sunday night. Given
this and the lingering front...and the possibility of any weak
disturbances moving over the area within the northwesterly flow
aloft...will maintain slight chance to chance probability of precipitation...highest across
the south and west. In fact...northeastern portions of the forecast
area may stay dry. Temperatures will range from near 80 degrees to the middle
80s...with overnight lows generally in the middle 60s.
Long term /Sunday through Friday/...
as of 1250 PM Friday...
An amplified upper pattern will persist through early next week...
with an upper ridge gradually building east towards the region. A
cold front will be suppressed south of the area by late Sunday as
dry and cooler surface high pressure becomes firmly entrenched
down the Atlantic coast. There will be a lingering chance of showers
Sunday as the front surges south...followed by dry conditions
through Thursday. The upper ridge will deamplify/flatten out as the
next upper trough and associated surface front approaches the area
late Thursday into Friday...bringing increasing precipitation
chances by the end of the week.
Post frontal conditions will be pleasant...with considerably cooler
temperatures and lower humidity. Low level thickness values will
fall through early next week...to the tune of 30 meters equating to
around a 10 degree cool off. Highs Sunday through Tuesday will be
well below...5-7 degrees...normal with upper 70s to lower 80s over
all but the far southern tier which may see middle 80s. Temperatures will begin
to moderate Wednesday through Friday as the upper ridge shifts east...
with middle/upper 80s expected by the end of the week. Overnight lows
will follow the same pattern...with mostly low to middle 60s early
next week warming to middle and upper 60s by late week.
Aviation /18z Friday through Wednesday/...
as of 155 PM Friday...
VFR conditions across central NC this afternoon. Showers and storms
streaming across WV and Virginia in northwest flow aloft should eventually
move into the northern and north eastern portions of NC this
afternoon and evening. While a shower or storms cannot be ruled out
at any site...probabilities are highest at krdu and krwi after 20z.
Later tonight...a weak boundary is forecast to surge into NC from
the north...ahead of a cold front set to move into the area on
Saturday night. Northeasterly wind behind the boundary should lead
to some low clouds...mainly north and east of krdu...though it isnt
clear from guidance how widespread any stratus would be. Mostly VFR
conditions and a continued chance of showers/storms are expected
during the day on Saturday.
Outlook... a cold front will push into NC from the northeast on
Saturday night...with widespread low ceilings expected behind the
front on Sunday morning. A drying trend will ensue heading into
next week as high pressure builds into the area behind the front.