Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
325 PM EDT Wednesday Jul 23 2014
Synopsis...a cold front will move through the Ohio Valley
overnight...and gradually move southeast through much of the region
Thursday and Thursday night. The front stalls and weakens near the
South Carolina border Friday. Weak high pressure will be located
north of the front Friday into early Saturday.
Near term /through tonight/...
as of 315 PM Wednesday...
Temperatures were hampered early in areas across the south where showers were
a little more widespread during the very early afternoon...but will show
some rebound as the clouds scatter out. The isolated to widely scattered
convection lingering this afternoon will wane quickly with loss of
isolation...and will be trimming probability of precipitation to slight chance with skies becoming
mostly clear through midnight. Expect increasing clouds after midnight in
the west where moisture in the light southwest flow will be pooling in the
cooling airmass along the surface trough.
Short term /Thursday through Thursday night/...
as of 315 PM Wednesday...
The cold front will reach the mountains towards sunrise...with showers
and thunderstorms spreading into the western Piedmont by middle morning...
and across the remainder of the area through afternoon. The front will
be moving across the area in a favorable diurnal time frame concurrent
with better support from aloft as middle and upper flow strengthens with
the associated upper trough pivoting across the area. Upper diffluence is
especially impressive...as we will be under the favorable entrance of
an 80-90kt jet lifting out of the trough. Middle level lapse rates will be
pretty weak...generally <5... instability ranging from ~1k joules
west where highs will be hampered by cloud coverage...to >1.5k east. Risk
of severe is not impressive...with unidirectional deep shear of 25kts...
but the chance of training storms will produce a flood threat...especially
in the east as the front will be slowing by late afternoon. Will have
likely probability of precipitation in the west by 15z...spreading to the remainder of
the area by 18z. Highs will range from middle 80s northwest to 90-92 in the
The upper trough will be blocked by high pressure offshore...and the cold
front will correspondingly slow and then stall as it reaches the coast
Friday morning. As such...probability of precipitation will fall off to slight chance west of i95
by midnight...with lingering chances in the east through morning. Modest
flow of cooler and drier air into the northwest will produce overnight mins
in the middle 60s...with mins over the east still in the low 70s.
Long term /Friday through Wednesday/...
as of 235 PM Wednesday...
Nwp guidance is relatively consistent in bringing the surface cold
front to the coastal plain of the Carolinas by 12 UTC Friday as the
upper level trough axis crosses the Appalachians. The upper level
trough axis makes limited progress east on Friday as the next
shortwave trough over the Ohio Valley early Friday splits with one
piece of energy moving east into the northeast and the other drops
toward the Savannah River valley. The front drifts south and briefly
stalls before moving north into the middle Atlantic as a warm front
Saturday. Deep moisture decreases on Friday compared to Thursday
with the bulk of the moisture confined to the 925 to 700 mb layer in
a region of relatively steep lapse rates. This pattern suggests the
best convective coverage on Friday will be across the far southeast
Piedmont and especially the coastal plain and eastern sandhills
Friday afternoon and evening. Probability of precipitation in the slight chance to low
chance range should suffice. Highs on Friday will range in the middle
to upper 80s. Partly cloudy skies are expected Friday night with
lows in the middle 60s to near 70. Saturday will feature partly cloudy
skies with the bulk of the convection suppressed toward the coast.
There is just a limited threat of precipitation in central NC with
the best chance of storms on Saturday across the eastern coastal
plain with only slight chance probability of precipitation warranted. Saturday should
feature the fairer weather of the weekend with very limited
precipitation coverage and near seasonable temperatures.
Temperatures will moderate a bit on Saturday with highs in the 87 to
The next upper trough approaches from the northwest on Sunday with a
cold front dropping across the Ohio Valley. A Lee trough ahead of the
front across the Piedmont of Virginia and the Carolinas is expected to
become convectively active during the afternoon and evening with
scattered convection. The southerly flow ahead of the upper
trough... combined with deep mixing will result in highs in the
lower 90s. The upper trough axis and cold front will cross central
NC on Monday with good convective coverage anticipated. Will
increase probability of precipitation into the high chance range. With the approaching
trough...the low and middle level flow increases with bulk shear values
of 30-40 kts expected suggesting some high-based storm organization
is possible. Slightly cooler temperatures are expected on Monday
with the cloud cover and precipitation...highs should range in the
86 to 91 range with lows Monday morning in the 69 to 74 range.
The large amplitude upper level trough slowly moves east on Tuesday
with the cold front prognosticated to move into the coastal plain Monday
night and slowly push toward the coast on Tuesday. Cool high
pressure builds into the region with any precipitation on Tuesday
likely confined to the coastal plain and points east. Mainly dry
conditions are expected on Wednesday but it is Worth noting that the
GFS/European model (ecmwf) both indicate the stalled front near the coast may try to
work west. For now will include a dry forecast. The much cooler air
mass with low level thickness values falling into the 1375-1390m
range for Tuesday and Wednesday mornings. This would support lows in
the 60s and perhaps upper 50s in The Triad or near the Virginia border on
Wednesday morning with highs of 79-86. -Blaes
Aviation /18z Wednesday through Monday/...
as of 200 PM Wednesday...
VFR conditions will prevail through the overnight...with the exception
of in the vicinity of isolated to widely scattered showers which will
be focused mainly across the southern tier and diminish by 00z. A late
night period...09-13z...of MVFR visibility is possible at Fay/rwi/rdu...
which could possibly be IFR if showers occur at one of the taf sites.
A cold front will be moving east...with showers and thunderstorms
becoming numerous in the west (gso/int) by around 16z tomorrow morning...
and spreading east to the remaining sites during the afternoon...mainly
beyond the valid taf period. MVFR...briefly IFR conditions are to be
expected in the vicinity of convection.
Beyond the 18z time frame...widespread convection will shift east
to mainly rwi/Fay vicinity by 06z as the front slows near the coast.
There will be lingering convection in the Fay area Friday...otherwise
VFR conditions will prevail through Saturday when our next front will
be approaching from the west. This front will produce increasing
chances for convection on Sunday...but especially on Monday.