Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Raleigh NC
1020 am EDT Sat Apr 19 2014
Synopsis...low pressure will slowly move east off the the southeast
US coast through the weekend.
Near term /through today/
..as of 1020 am Saturday...
The slow moving middle/upper low was located over northern Florida this
morning. A middle level dry punch has worked NE into the southeast part of NC.
However... the deep layered east-northeast flow was continuing to transport
moisture off the Atlantic across all of NC. This moisture was being
lifted up and over the coastal boundary extending from the surface
low near jax along the SC/NC coast creating large areas of rain over
NC. The latest radar trends indicate a filling in of the area in the
southeast where the middle level dry slot affected earlier. Although the rain
is not as heavy as the first round that has moved to the Outer Banks
and immediate NC coast - periods of mostly light rain are expected
to continue to develop/fill back in over the region throughout the
day as the deep layer lift in the lower to middle layers continues due
to the very slow moving low. Radar trends also support the latest
operational GFS in depicting the main area of rain to slowly begin
to pivot southeast (and out of far northwest nc) by middle to late afternoon.
However... periods of light rain will affect the rest of the region.
Quantitative precipitation forecast of an additional 0.25 to 0.50 appears reasonable on top of the
0.25 north to 1.50 totals south thus far. Due to the NE flow...
rain... and cad developing... we will lower temperatures back - to show the
struggle to reach 50 in the interior of central NC.
Short term /tonight through Tuesday night/...
as of 255 am Saturday...
For sun/sun night: the middle level low is expected to shift slowly off
Savannah early sun then slowly fill as it heads east-southeast through Sun
night. Models indicate abundant moisture hanging on in the 850-700
mb layer over NC Sunday... with a persistent cool and brisk
northeast low level flow feeding in from the chilly but progressive
and modifying surface high to our NE. Low level thicknesses are
expected to be around 15-20 M below normal... indicating highs
nearly 2 categories below normal. Expect highs of 64-68... with
partial sunshine late in the day as middle level ridging starts to
build in from the west. Clearing skies Sun night... although there
may be enough residual moisture in the northeast County Warning Area with a
continued Atlantic fetch to bring about patchy low stratus
development overnight. Lows 40-47. -Gih
The slow moving cut off low impacting the region this weekend is
forecast to be east of the Carolinas Sunday night...with an
elongated surface ridge extending from the Middle-Atlantic States into
the NC. As the low moves off to the northeast...winds should back
around to more northerly than northeasterly and allow some drier air
to work into central NC from the north. The NAM isnt quite as dry
as the GFS and European model (ecmwf)...showing some moisture lingering around 850mb
east of I-95. While the airmass wont begin to moderate significantly
until Tuesday...stronger heating should support highs reaching their
potential of upper 60s and lower 70s.
A progressive trough moving through the central US will approach the
region Monday night and early Tuesday...with the associated frontal
zone projected to cross central NC Tuesday afternoon and evening.
Ahead of the system...lows will be much more mild Monday night with
increasing middle/high clouds and a weak return flow across the western
Piedmont. Lows in the lower 50s west and upper 40s east. South to
southwesterly flow will increase on Tuesday ahead of the approaching
front. Models currently forecast a dewpoint recovery into the middle
and upper 50s...which combined with highs in the upper 70s to around
80 and weak middle-level lapse rates would suggest weak
instability...mainly across the west. Modest shear may support a
few strong storms...but without better upper support the threat of
severe weather looks isolated at this point.
Long term /Wednesday through Friday/...
as of 255 am Saturday...
Behind tuesdays system...Canadian high pressure will build across
much of the eastern US on Wednesday and Thursday. Thursday looks to
be the coolest day as thicknesses fall back below normal...with
highs maybe 5-10 degrees below normal...but then quickly rebound on
Friday as another progressive upper trough approaches from the west.
This next system resembles the early weak system...though
potentially with a little more moisture available from the Gulf.
The current forecast will remain dry...with the better chance of
precipitation just beyond the end of the forecast on Friday night and
Aviation /12z Saturday through Wednesday/...
as of 740 am Saturday...
An expansive storm system centered on the Florida Gulf Coast will affect
the southeast states and Carolinas throughout today and tonight.
Fairly widespread light rain has lingered through the night... but
visibilities have vascillated among VFR / MVFR / IFR through the night.
Ceilings are likely to vary between VFR and MVFR (as MVFR clouds shift
from scattered to broken and back to scattered again) throughout the day.
Unfortunately... confidence in the occurrence of MVFR conditions at
any particular time today is very low. Visibilities may briefly drop to
MVFR in any heavier rain bands through 19z. After around 19-21z at
int/gso/rwi/rdu... ceilings are expected to improve to prevailing VFR as
drier low level air noses in from high pressure to the north... with
just patchy drizzle at most from 21z through the end of the taf
valid period (12z sun). Expect improvement to MVFR at Fay 06z-09z.
Looking beyond 12z sun... expect improvement at Fay to VFR by 12z.
Expect VFR conditions to dominate through Monday night. Passage of a
middle level trough and surface cold front from the northwest will bring a
risk of sub-VFR conditions Tuesday/Tuesday night. VFR conditions return for