Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
751 PM EDT Tuesday may 5 2015
Synopsis...Bermuda high pressure will extend into our region
through middle week. An area of low pressure will form near the Bahamas
and meander off the southeast coast during the middle of the week.
Near term /through tonight/...
as of 230 PM Tuesday...
High pressure will continue to ridge westward into central NC...even
as the high shifts offshore. Skies will generally be clear...with
some increasing clouds in the far southeast toward daybreak
Wednesday. Expect winds to become light and variable overnight. Mild
lows tonight...mainly in the middle to upper 50s.
Short term /Wednesday and Wednesday night/...
as of 240 PM Tuesday...
A weak surface trough will stall to the north on Wednesday night...
with high pressure over the NE U.S. And off the Carolina coast. The
bigger feature will be the closed low developing off the east
Florida coast...which is expected to move north-northeast toward the
Carolinas through Wednesday night. For now...it appears that the
system will stay to the south through Wednesday night...with the
main impact over central NC being increasing cloudiness from the southeast
and an increase in after/evening winds. There is a slight chance for some
showers/storms to develop in the after/evening along the stalled boundary
to the north...but expect most of the activity to stay to the north
and northwest of central NC. With little to no change in airmass...
and Bermuda high pressure continuing to ridge into the area...
expect highs in the low to middle 80s. With the increasing clouds
overnight and some weak warm air advection...lows will be a few degrees warmer than
previous nights...mainly in the upper 50s.
Long term /Thursday through Tuesday/...
as of 415 PM Tuesday...
The main forecast problem through the period remains the evolution
of an area of low pressure...with possible sub-tropical
characteristics...forecast to form near the Bahamas and drift near
the southeast US coast through the end of the week. While two model
camps remain...with solutions ranging from the NCEP ones on the
western side of the guidance envelope to more eastern ones depicted
by the non-NCEP models...the spread has significantly decreased in
the past 24 hours. The decreasing model spread results from a
westward shift in the non-NCEP guidance...including the 12z op
European model (ecmwf)...which yields relative increased forecast confidence in a
closer to the Carolinas coast meander of the low through the
However...no solution is truly off The Table...since the steering
flow for the low is forecast to become quite weak on the southern
periphery of a persistent ridge aloft over the East Coast. In
fact...the ridge...in conjunction with an elongated upper low
analyzed this morning from the southeast states to the Yucatan
Channel...will favor the development of at least a brief Rex
block...which would indeed favor slow and erratic motion of any
cyclone trapped within it. It will likely take a kicker shortwave
trough to migrate slowly from the Pacific northwest today to the MS valley
early next week to finally cause the low to lift NE and away from
the NC/Virginia coasts early next week.
Will follow the wpc-preferred "compromise" solution between the two
model camps indicated above...best illustrated by a 12z GFS/ec
blend...whose respective mass fields are really not too dissimilar.
The resultant sensible weather impacts will be increasing clouds and
probability of precipitation from the south and east by late
Thursday...with the latter probabilities ranging from chance over the
coastal plain and eastern sandhills to slight chance over the
Piedmont. With the low nearby and forecast to move little...likely
just to the south and east of central NC...a persistence forecast of
similar pop distribution -highest chances southeast and lowest northwest- will be
hard to beat through the weekend. Provided the timing of the kicker
wave remains consistent...Monday may be the relative driest day of the
period in brief subsidence behind the low and in advance of the
frontal system associated with the kicker wave...forecast to arrive
Temperatures have been adjusted downward...into the 70s mainly over
eastern areas...given the expectation of more clouds and higher
precipitation chances with the more westward forecast solutions for
the low. If the low were to stay offshore and farther southeast of NC...the
air mass would support lower to middle 80s...and some upper 80s
probable by Tuesday.
.Aviation /00z Wednesday through Sunday/...
As of 745 PM Tuesday...
24-hour taf period: high confidence that VFR conditions will prevail
through the taf period as high pressure continues to dominate the
weather over the Carolinas. Diurnal cumulus will continue to dissipate
this evening with the loss of daytime heating...leading to mostly
clear skies overnight. Some cumulus will likely develop once again on
Tuesday during the afternoon hours. Winds will be light and variable
overnight...before becoming more southeasterly/easterly on Wednesday.
Looking ahead: the main aviation concerns will come later in the
week and/or into the weekend as an area of low pressure develops off
the southeast U.S. Coast. Models still differ on the track/location
and timing of this system as it progresses northward...possibly
meandering off the southeast coast. Therefore...conditions beyond Thursday
still remain fairly uncertain. Regardless...this system could result
in some adverse aviation impacts late in the work week and into the
weekend...especially across eastern NC. Additionally...enhanced
easterly onshore flow from this system could bring an increase
threat for morning stratus/fog.