Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
153 am EDT Tuesday Jul 28 2015
Synopsis...a series of perturbations aloft interacting with a moist
and unstable air mass will trigger scattered thunderstorms...mainly
during the afternoon and evening hours...through the remainder of
the work week.
Near term /through tonight/...
as of 925 PM Monday...
Diurnally induced convection is beginning to dissipate with the
exception of an area of persistent rainfall in the southwest
Piedmont coincident with two converging surface boundaries...aided
by some divergence aloft...as seen clearly in the water vapor
imagery. While this activity could persist for a little
while...generally expect it to weaken over the next couple of hours.
While central NC remains on the back/front side of an upper level
trough/ridge...water vapor shows another surge of dry air aloft
making a southward progression through Virginia at this time. To the west
of this however...a steady stream of moisture should continue to
feed into the western counties of the County Warning Area. High resolution models
weaken convection through the overnight hours but the hrrr holds
onto some rainfall in the SW Piedmont for longer than the WRF
simulations which is not out of the question at this point. Will
start to see pockets of fog and low stratus in rain moistened areas
as well as possibly in the eastern counties closer to sunrise. Low
temperatures tonight in the low to middle 70s...coolest in the NE
counties where upper level cloudiness may cease once/if dry air
Short term /Tuesday and Tuesday night/...
as of 325 PM Monday...
A weak surface trough will remain stalled in vicinity of the western
Piedmont. Aloft...an upper ridge centered over the Southern Plains
briefly amplifies into the western lakes...veering flow aloft over
central NC to a northwesterly direction. Weak perturbations embedded in this
flow aloft interacting with the stalled surface trough and available
moisture/instability should trigger scattered convection...mainly in
the afternoon and evening hours. Probability of scattered convection
appears best in vicinity of the stalled surface trough in the western
Piedmont...and in vicinity where the perturbations aloft track
through the region. Thus...probability of precipitation will vary from solid
chance(scattered) across the northern Piedmont-northern coastal
plain to slight chance(isolated)over the far southern counties.
Bulk of convection will diminish with loss of heating Tuesday
evening though a few showers may occur well into the night due to
perturbations in the northwest flow aloft interacting with the moisture
rich air mass over our region.
While a strong or severe storm cannot be ruled out Tuesday
afternoon...bulk of convective parameters point toward most of the
storms remaining below severe criteria.
Maximum temperatures Tuesday highly dependent on extent of early morning clouds
and timing of afternoon convection. Favoring partial sun to occur
for most of the late morning-early afternoon...boosting temperatures well
into the 80s-around 90. If low clouds linger past noon or convection
develop by noon-2 PM...than maximum temperatures may end up being 3-5 degrees
cooler than forecast. Min temperatures Tuesday night 70-74.
Long term /Wednesday through Monday/...
as of 245 PM Monday...
Medium range models are in general agreement in showing a middle level
ridge centered across the central/Southern Plains and lower MS River
Valley to start the period (wednesday)... then retrograding it
westward across the western U.S. By late week. The middle level ridge
is forecast to build north/northwestward as it retrogrades...
helping to amplify the middle/upper level flow pattern across the
Continental U.S.. this should yield broad middle level troughing for central NC
late week into early next week... with a surface cold front pushing
into the area from the north and west late Thursday into Friday.
However... the stronger flow aloft will remain to the north of
central NC... yielding generally weak deep layer shear. Questions
still remain if the front will push through the area or settle
across southeastern portions of central NC... and resultant how long
precipitation chances will continue across central NC. For now will to
trend the forecast towards an unsettled pattern for late Thursday
through Sunday... with the potential for high moisture content air
to advect into the from the northeastern Gulf of Mexico along with
potentially a few weak disturbances lifting north-northeastward and
across south/eastern portions of the forecast area. However... depending on how
far south and east the boundary makes it will determine how much of
the area see a good chance for showers and storms over the
weekend... along with the potential for heavy rain. Thus... will
limit probability of precipitation to chance for now but raise them a bit... with the
highest probability of precipitation south/southeast this weekend.
High temperatures ahead of the front on Wednesday and Thursday are expected
to be near normal... generally around 90 to the lower to middle 90s.
High behind the front will generally be in the middle to upper 80s...
with a few locations touching the lower 90s... depending on the
precipitation. Lows ahead of the front area expected to be in the lower to
Aviation /06z Tuesday through Saturday/...
as of 150 am Tuesday...
Convection has diminished over central NC and the only remaining
storms are north of the NC-Virginia border. The rest of the night should
be quiet with regards to convection...with stratus possible in a
fairly well established southeasterly return flow. Given this
return flow and low-level moistening...along with earlier clusters
of storms over the western Piedmont...stratus may end up being more
widespread west of krdu and impact kgso/kint. However...these
signals are mixed in model guidance...and confidence is still low
given the lack of development so far. Patches of high clouds will
also continue to streaming from the northwest...with some MVFR/IFR
visibilities possible in areas where skies are clear for an extended amount
What stratus that forms will burn off quickly this morning and
scattered storms will be possible again this afternoon...with
slightly higher confidence in storms in the vicinity of
Outlook: early morning stratus and/or fog will be possible Wednesday
and Thursday with afternoon storms...mainly west...each day. A cold
front will then move into the area on Friday...potentially stalling
over eastern NC this weekend.