Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
310 PM EDT Tuesday Jul 22 2014
Synopsis...a surface front along the coast weakens overnight and
early Wednesday ahead of a cold front. This cold front will move
through much of the region Thursday and Thursday night...slowing and
weakening as it approaches South Carolina early Friday. The weak
front then lingers near the South Carolina border and the North
Carolina coast through Friday night.
Near term /through Wednesday night/...
as of 310 PM Tuesday...
Shower converage was limited by the lack of forcing and minimal upper
support this afternoon. While ongoing widely scattered convection will
diminish as the evening sets in...there will be potential for showers
to migrate west into the coastal areas through midnight as the
retrogressing coastal trough will provide a bit of extra impetus with
modest low level convergence. Mins will be persistence from last night...
On Wednesday...looks like more of a conventional summertime convective
pattern with southwest to westerly flow aloft enhancing Lee troffing over
the area. Mixing will be deeper to provide breaks in the clouds tomorrow
as well...allowing highs to reach their potential in the low 90s. We will
remain in the soupy airmass...of course...so instability will be fairly
impressive...around 1.5k joules...so probability of precipitation in the 40 percent range are in
order...graduated slightly higher in the west where differential heating
should provide earlier initiation...and east of the surface trough axis.
Shear will be minimal...unidirectional around 15 knots...so only a marginal
severe threat. Convection will wane with loss of insolation and will
maintain a slight chance of a shower overnight. Mins Wednesday night will
be mild in light southwest flow...72-75.
Short term /Thursday through Friday/...
as of 245 PM Tuesday...
Both the GFS and the NAM show a weakening middle-level wave moving
through central North Carolina Thursday and Thursday night as a
trough axis aloft pushes slowly east. Mean moisture increases
Thursday with good lift forecast by the GFS and the NAM.
Precipitable water values increase to at or just over two inches
over much of central North Carolina Thursday...with the highest
values along and east of U.S. 1. Dynamically...the GFS forecasts the
right entrance region of a 60kt 300mb jet over central North
Carolina by 18z Thursday...and this jet gradually moves east into
Thursday night. Bufr soundings become moist and more unstable during
the day northwest to southeast...and it appears at least good chance
probability of precipitation for central North Carolina are warranted...which may become
likely chances over time over much of the area. 1000-500mb lapse
rates are not really high but decent...to 6... as low-level
winds at both 850mb and 925mb are forecast to actually diminish some
during the day from the northwest...0-3km shear is modest...mostly
in the teens knots to around 20kt toward the Virginia border. The
best cape around 18z is forecast by the GFS just west of U.S. 1...
around 900j/kg...shifting east during the afternoon with the better
downdraft cape certainly better east of U.S. 1. Given a combination
of these modest parameter values...scattered-to-numerous showers and
thunderstorms with an isolated severe weather threat due to wind is
a reasonable forecast. In a relative sense...the better chance of
isolated severe weather may be along and east of U.S. 1 due to the
combination of greater instability and low-level wind. There appears
to be enough wind and shear that storms should move...posing a local
flood threat to the typical urban and poor drainage locations where
local training could occur. 0-3km helicity values are low and
actually get lower...into the lower double digits...during the day.
The NAM is a little faster with clearing and increasing stability
overnight Thursday compared to the GFS...but not greatly so...such
that chances for precipitation based on the bufr soundings should
wane in The Triad by 03z...by 06z toward krdu...and by 09z toward
Interstate 95. Depending on the degree of clearing...with the
weakening front making slow progress south there could be some areas
of fog overnight Thursday...but the confidence in this dependent on
the overall diminishing cloud cover is not high enough to place in
the forecast currently. On Friday...although the weak surface front
should start to get hung up near the South Carolina border and
toward the coast...bufr soundings especially on the GFS become
fairly stable to deep convection. Precipitable water values continue
to gradually diminish...with values approaching an inch in The Triad
by 00z Saturday. Both the NAM and the GFS forecast minimal quantitative precipitation forecast
mainly along and east of U.S. 1 Friday afternoon...and given the
proximity of the surface front and slowly diminishing moisture...
there may be just enough instability and lift to provide for
isolated showers or thunderstorms Friday afternoon mainly outside
The Triad...where it just seems too stable for deep convection
Friday afternoon. NAM bufr soundings have quite high moisture around
850mb while the GFS is somewhat less...and blending them should
result in some areas of good cumulus in much of central North Carolina
for a time Friday especially away from The Triad. Highs Thursday
mostly a blend of MOS...leaning cooler toward The Triad and higher
toward I-95 for values from the upper 80s to middle 90s. Lows Thursday
night 67 to 73...then highs Friday mostly 85 to 90...just a touch
cooler than 1000-850mb thicknesses would imply due mostly to the
potential for cumulus for a period then.
Long term /Thursday through Tuesday/...
as of 300 PM Tuesday...
For the period Friday night through Tuesday...the GFS and the European model (ecmwf)
are fairly consistent in moving the upper-level trough axis just
east of central North Carolina overnight Friday into Saturday. Both
sets of guidance allude to weak shortwave ridging Saturday night...
followed by a middle-level shortwave moving through sometime on Sunday
with the typical timing differences out to the fifth day. As an
upper-level trough deepens over the eastern United States early next
week...backing winds aloft more to the southwest occur. The GFS and
European model (ecmwf) are both dry Friday night and mostly dry Saturday behind the
trough aloft and under weak high pressure especially early Saturday
followed by weak surface trough development Saturday afternoon. For
now...retained the slight chance of thunderstorms along and east of
the Interstate 95 corridor for Saturday with the European model (ecmwf) showing the
presence of some middle-level moisture...but GFS model soundings are
stable and if the current trend of chances below climatology
continues even these small chances may eventually be removed from
1000-850mb thicknesses are warmest Sunday and that may be the
warmest day of the long-term period before lowering heights aloft
take place. The European model (ecmwf) is much more moist and unstable Sunday
compared to the GFS and will maintain the chances for showers and
storms with the higher chances along and north of U.S. 64 closer to
the expected track of any middle-level shortwave and an approaching
surface front. With the deepening trough...both the GFS and European model (ecmwf)
currently move the surface front through central North Carolina
Monday into early Tuesday...lingering a surface trough near the
While chances for precipitation increase closer to climatology early
next week especially on the GFS...general quantitative precipitation forecast is not terribly high
and then higher relative to the rest of central North Carolina along
and east of U.S. 1. This could be...in part...due to more of a
westerly low-level flow on forecast model soundings over the western
Piedmont...at least for Monday. Still...will forecast a chance of
showers and thunderstorms throughout central North Carolina Monday
with the front and then a slight chance in The Triad and chances
elsewhere on the seventh day...higher then toward kfay and
kgsb...based on the expected surface frontal position and the
southwest flow aloft.
Aviation /18z Tuesday through Sunday/...
as of 130 PM Tuesday...
For the current taf period...spotty showers and isolated thunderstorms
this afternoon will linger into the evening...waning with loss of heating.
Conditions will be VFR except for MVFR to briefly IFR in the vicinity of
convection. A typical brief period of MVFR to briefly IFR ceilings and visibility
is expected tonight in the 08-13z time frame...except that conditions will
be lower and develop a couple of hours sooner at sites that receive rain
Winds will be more uniformly out of the southwest tomorrow...but still
light...less than 10 knots. Again expect widely scattered convection...
mainly after 18z...with better potential for showers in the west...
gso/int...and at Fay where conditions will be a little more unstable.
Southwest flow will continue through Thursday as a front approaches from the
west. Showers and thunderstorms will be more widespread on Thursday into early
Thursday night. There will be a lingering chance of convection in the east...
rwi/Fay...into the weekend as the front stalls along the coast...otherwise VFR
conditions are expected Friday through Sunday.