Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
145 am EDT Sat Apr 25 2015
Synopsis...a series of upper disturbances will cause unsettled
weather across central NC through the weekend.
Near term /through tonight/...
as of 1030 PM Friday...
Increasing high cloudiness through the dampening middle level ridge has
slowed the diurnal fall and dew points have recovered in light
southerly flow to the low and middle 40s across the southern tier.
Progression of precipitation spreading into the area remains on track with
light rain poised to enter the southwest corner of the state by
midnight and reaching the far southwestern County warning forecast area around sunrise.
Evening update will be to reflect latest temperature and humidity trends.
Previous discussion...mid-upper level short wave ridge will drift across
the forecast area tonight. This weather feature will initially cause
mostly clear skies overhead. The attendant dry air mass (sfc
dewpoints this afternoon down into the 20s) will allow the
atmosphere to cool efficiently after sunset. While not as cool as
last night...late evening temperatures should be in the 40s to near 50.
Most places will likely achieve their min temperatures shortly after
midnight. During the overnight hours...increasing middle-high level
clouds associated with an area of low pressure crossing the lower MS
valley will increase west-to east across central NC. This
increasing/thickening cloud cover should cause temperatures to hold steady
or possibly slowly rise during the pre-dawn ours. Cannot rule out
the possibility of a shower or two skirting along our western
periphery before 7 am. Elsewhere no rain should occur. Min temperatures
near 40 far NE to the middle-upper 40s west.
Short term /Saturday and Saturday night/...
as of 310 PM Friday...
Increasing low level warm air advection associated with an upper
level disturbance moving east across the deep south into the southeast U.S.
Will cause areas of rain to increase in coverage and intensity
Saturday across central NC. Expect the bulk of the rain to occur
west of the Highway 1 corridor through middle day...then overspread the
remainder of the region in the afternoon. The rain falling through
the dry sub cloud layer may cause an in-situ or weak hybrid damming
event to occur...mainly limited to the northwest Piedmont. The
timing of the rain will have an impact on high temperatures for the day.
Expecting the rain not to arrive in the rdu area until close to middle
day which should allow for some temperature recovery. In The Triad...expect
the rain to start between 8 am and 12 PM. Due to the rain arriving
shortly after daybreak...temperatures in The Triad may struggle to warm
much above the lower 50s.
Late in the afternoon...the passage of the 850mb warm front across
our southern counties may erode the stable layer enough to allow for
the development of a few thunderstorms. With better instability
lying to our south...low level flow will be advecting this slightly
unstable air mass into our southern counties into the evening hours.
Shear will be sufficient to support organized convection and latest
cam depictions suggest a few storms crossing the SC border Saturday
evening. While cannot rule out the potential for a severe storm
near the SC border Saturday night...confidence not high enough at
this time to mention in forecast or the severe weather potential statement.
The continued northeastward progression of the 850 mb warm front
should push the rain out of the NE Piedmont/northern coastal plain
Saturday evening...with scattered shower highly probable in its
wake. With the warm air aloft above the relatively cool surface
layer...will likely see areas of fog and/or low clouds over the
Piedmont and northern coastal plain Saturday night.
Maximum temperatures Saturday low-middle 50s northwest to the middle 60s south. Min temperatures
Saturday night upper 40s/near 50 north and the middle-upper 50s south.
Long term /Sunday through Friday/...
as of 300 PM Friday...
Sunday: expect warm moist west-southwesterly flow to start the
period...with the retreating warm front in vicinity of the NC/Virginia border. An
upper level shortwave will traverse the region during the day
Sunday. At the surface...a west-east oriented low over the Carolinas
will slowly be pushed south and east as a cold front slides through
the region. Expect some rain to move through central NC from north
to south ahead of the front...ending late Sunday evening...although
the exact timing remains uncertain. The 12z GFS solution is close to
the 00z European model (ecmwf) solution with respect to timing of the front...the 12z
Canadian lags 6-12 hours behind. Will continue to lean more toward
the European model (ecmwf)...with the front making its way through the area by Sunday
night. Highs will depend on the timing of the front and if and how
much skies clear across the south during the day. For now...expect a
wide range of highs...middle 50s north to potentially low 70s south.
Sunday night through Tuesday night: as previously stated...the rain
should be out of the region by Sunday night...and expect the weather
to remain dry through Tuesday night under northwest flow and subsidence
aloft. At the surface...cold but weakening high pressure will
gradually build into the area through the period. Highs will
generally be in the middle 60s to low 70s...lows in the low to middle 40s.
Wednesday and thursday: there is quite a bit of divergence between
the models during this part of the forecast...thus there is not a
lot of confidence with respect to timing and intensity of various
features. Generally speaking though...expect a surface low to
develop over the Gulf Coast on Wednesday and slowly move east-
northeast toward the Carolinas through Thursday. Aloft...though
there is uncertainty on whether it will be cutoff or open...expect
an upper level low to develop over OK/Texas on Tuesday and move east
toward the Carolinas Tuesday night/early Wednesday. The European model (ecmwf) seems
to be more intense/amplified and quicker moving into the area than
the GFS at this time. Regardless...expect increasing rainfall
chances beginning late Tuesday night/Wednesday morning through
Thursday and possibly Thursday night. With the low level flow
dependent on the position of the low...and the temperatures
dependent on the low level flow...there is a high degree of
uncertainty in the temperatures for Wednesday and Thursday and there
could quite possibly be a large range of temperatures from northwest to southeast
across the area. Highs in the middle 60s to near 70 degrees and lows
upper 40s to low 50s.
Aviation /06z Saturday through Wednesday/...
as of 145 am Saturday...
VFR conditions early this morning with just increasing middle and high
clouds through 12z. An area of rain associated with warm advection
across the lower tennesse valley region will move into the western
Piedmont between 12z and 15z...though the rain should be light
initially given how dry the airmass is over central NC this morning.
Ceilings will steadily lower to MVFR from west to east by late
morning and early afternoon...followed by IFR conditions are the
rain becomes more widespread. A warm front is forecast to lift
into the southern portions of the area...possibly approaching kfay
during the afternoon. This may offer some support for an isolated
thunderstorm from krcz to kfay during the late afternoon and evening
hours. The probability of thunderstorms is much lower to the north.
Poor aviation conditions will continue overnight as ceilings are
likely to lower to LIFR and some areas of fog develop
Outlook... adverse aviation conditions are expected to continue into
late Sunday before improving by Monday. The next chance of sub-VFR
conditions will be midweek. &&