Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
726 PM EST Wednesday Dec 24 2014
Synopsis...a warm front will lift slowly northwest through the
interior Carolinas through this evening... as a deepening low
pressure system tracks from the Ohio Valley northward into the Great
Lakes. A cold front will push east across our area tonight. High
pressure will build in from the southwest for Christmas day through
Near term /through tonight/...
as of 305 PM Wednesday...
Most of the heavier precipitation we saw this morning has shifted to the
east with the warm conveyor belt/low-level jet. Meanwhile...The
Wedge and coastal warm fronts...which made a strong push inland
early this morning have moved very little in the past few hours.
The Wedge front stretches from Monroe to Asheboro to Roxboro...while
the coastal front is closer to Laurinburg-Smithfield-Tarboro. Both
should begin to retreat inland more aggressively this afternoon as
the precipitation comes to an end and stronger pressure falls begin ahead
of the approaching upper trough and synoptic cold front.
The severe threat is decreasing over the coastal plain as the
forcing departs the area....so the bigger question now will be
whether or not there is renewed development along the cold front.
Moisture return and warming with the retreating warm front(s) may
allow for some weak destabilization ahead of the front. The NAM
shows more instability than the GFS...up to 400 j/kg MLCAPE in the
east on the NAM this evening. Given the meager lapse rates and lack
of additional warming today...the NAM may be overdone. There are
already some signs of showers near the cold front over northern
Georgia/Alabama...though the character appears more jet driven than
via low-level convergence. However...with time...showers may
develop and expand along the front as it encounters the slightly
more unstable airmass over central/eastern NC...which the cams
weakly hint at. If any deeper showers/storms were to
develop...strong unidirectional deep layer shear would support a
wind threat...but this doesnt seem likely at the moment.
The cold front cross will the area between roughly 03z and
09z...with significant drying in the low-levels and an end to any
precipitation. There is still plenty of middle/high level moisture streaming
southwest to northeast just ahead of the upper trough...so some of
this cloud cover may linger into Thursday morning and limit some of
the cooling behind the front. Lows should range from the middle 40s
west to middle 50s east.
Short term /Friday and Friday night/...
as of 305 PM Wednesday...
As the cold front exits the area...high pressure will build in from
the Gulf Coast region. Given the source of the airmass and westerly
flow...highs should be slightly above normal in the middle to upper
50s. Winds within the mixed layer cap out at around 25-30mph
midday...so there should be some gusts into the lower 20s earlier in
the day. Otherwise...quiet weather will continue on through
Thursday night as high pressure builds overhead and lows dip into
the low to middle 30s.
Long term /Friday through Wednesday/...
as of 335 PM Wednesday...
For Friday/Friday night: calm and mostly clear as surface high pressure
drifts overhead... and middle level ridging builds aloft over and just
off the southeast states. The middle level and surface ridges will
shift east late Friday into Friday night. As a result of strengthening
upper level winds from the SW into the midatlantic region... high
thin clouds will spread in Friday evening/night. In addition...
increasing return flow on the west side of the exiting low level
ridge will bring a chance of fog and/or stratus late Friday night as SW
winds increase in the 925-850 mb layer. Low level thicknesses start
out close to normal... although much of this warmth will be
contained below the subsidence inversion just off the surface.
Regardless... given the mild source region of the air mass... expect
highs about a category above normal... and lows near to just below
normal with minimal cloud cover and light winds.
For Sat-Sun: as the surface high shifts further offshore Sat... the
frontal zone to our south is expected to start moving northward...
pushing into the NC Piedmont by sun. The GFS/European model (ecmwf) begin to spread
precipitation into NC from the SW late Sat night but especially sun... a
function of the strengthening SW flow in the 850-500 mb layer from
the northwest Gulf riding up and along the surface frontal zone. With good
model agreement... will spread in good chance probability of precipitation from the SW
starting early Sun morning... and continuing through sun and Sun
night... as weak middle level vorticity and a surface low track up
along the front across western and northern NC. Highs in the upper 50s to
lower 60s both days... likely a little cooler sun as the rain
arrives. Lows in the 40s Sat night and middle 40s to around 50 Sun
For Monday-Wed: expect a drop in temperatures and at least a couple of rounds
of light rain during this period. The GFS/European model (ecmwf) take the front back
southward in a backdoor fashion Monday... and the front should settle
just south of the County Warning Area Monday night... as a portion of expansive high
pressure centered over the northern High Plains extends into NC from the
north. We maintain a fairly flat SW middle level flow from Texas to the
Carolinas through middle week... as positively tilted longwave
troughing digs from the upper Midwest toward the Desert Southwest.
It appears likely that we'll keep an overrunning SW flow at 850 mb
up and over the surface frontal zone... so we should keep a cool and
fairly cloudy and stable air mass over central NC. It's difficult to
try to time individual weak perturbations in this sort of pattern
this far out in time... so will need to keep in a chance of rain
through the period... although we should see the best chances on Monday
(with a more prominent wave passage)... and perhaps early Wednesday.
Expect early-day highs Monday in the middle 50s to low 60s... then highs
should drop to the 40s for Tue/Wed. -Gih
Aviation /00z Thursday through Sunday/...
as of 720 PM Wednesday...
Primary aviation concerns this period: IFR/LIFR conditions through
this evening... and low level wind shear potential through this
24 hour taf period: varying conditions across the County Warning Area this evening
as dense fog continues for The Triad sites with dense fog vlifr
conditions under the cold air damming wedge. To the
east...conditions are generally better but light rain will continue
for eastern sites for the next several hours. As the front works
through the area conditions will become drier but some residual
elevated instability could cause light rain to redevelop over the
area during the overnight hours. Eventually the cooler and drier
airmass will move in and conditions will improve to VFR and should
remain there through the end of the taf period. As conditions
improve...attention will turn to the winds which are beginning to
gust now in some locations to 15-20 kts. Gusting will be episodic
overnight but be more prevalent after 15z on Thursday through
sunset. In addition to gusting...winds will shift from a more
southerly direction this evening to a west or northwesterly
direction by sunrise. Some afternoon clouds near MVFR levels could
develop but probably will be scattered in nature.
Long term: a series of storm systems in the long term will keep the
pattern unsettled. The next chance for adverse aviation conditions
will be on Sunday afternoon when a southern stream system affects
the area. Periods of sub-VFR conditions are possible through Tuesday
night or Wednesday before a brief lull in the precipitation before
another storm moves in for the end of the work week.
Hydrologic discussion /through Friday/...
as of 400 PM Wednesday...
There have been three fairly significant rainfall events over
central North Carolina during the past two weeks...with heavier
rainfall amounts (>1 inch total) generally from the Triangle and
Fayetteville areas east to the coast. As a result...relative soil
moistures prior to the rain over the past 36 hours were uniformly
high through a 200 cm deep column...which promotes faster runoff as
well as a higher percentage of rainfall-to-runoff from todays rain.
Widespread significant rainfall of 1.5 to 2.0 inches has fallen
across the area in the past 24 hours or so...with some areas...
particularly in Wake County... receiving over 2 inches. Additional
rainfall is expected through this evening and possibly into the
early overnight hours...however totals from 4 PM through 3 am will
be mainly a quarter of an inch or less. The highest amounts are
expected across the eastern half of the area.
Many of the main Stem rivers in the hydrologic service area have
seen increases as a result of the rain that has fallen yesterday and
today. In Wake County...Crabtree creek is out of its banks at old
Wake Forest Rd and is likely high in most other places. The Neuse
River at Clayton is currently in flood...with at least one section
of the Greenway near NC 42 under water. The Neuse at Smithfield has
increased rapidly today and will likely go above flood early this
evening. River flood warnings have been issued at both locations.
The Neuse River at Goldsboro will likely flood Friday or Saturday in
response to the rises at the other two locations upstream. Other
locations that may flood in the next few days include Fishing Creek
at Enfield and the Tar River at Tarboro.