Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
652 am EDT Monday Apr 27 2015
Synopsis...high pressure will build in from the northwest today and
linger over the Carolinas through Tuesday night. Meanwhile...an area
of low pressure will develop along the Gulf Coast...then track up
the southeast and middle Atlantic coast Wednesday through Thursday.
Near term /through tonight/...
as of 350 am Monday...
Today will be warmer than recent days...but still below average owing to
the influence of below normal heights around a deep...mean trough
centered near Nova Scotia. A couple of notable shortwave troughs
rotating around the west-southwest quadrant of the upper low will pivot across
the Middle-Atlantic States today. The first...initially over central
in...will track east-southeast across the Appalachians and northern NC...to
near the NC Outer Banks by 18z. The trailing one...initially over
Lake Michigan...will take a slightly more northerly track across the
virginias tonight. While moisture with both will be limited...
marginally cooling temperatures aloft...in conjunction with diurnal
heating this afternoon...will likely result in a brief uptick of
shallow cumulus based around 7-8 thousand feet over the NE Piedmont and
northern coastal plain today. Although the hrrr suggests that cumulus
will deepen sufficiently to generate a few measurable showers with
the lead vorticity maximum across those same areas through early
afternoon...a deep and dry sub-cloud layer -one increasingly so with
time with diurnal heating- should cause any associated sprinkle to
evaporate before reaching the surface...if any even develop amidst light
northwest downslope flow as high pressure ridges in from the northwest...and
otherwise deep dryness. Highs in the middle 60s to lower 70s.
Surface high pressure will continue to ridge southeast into the Middle
Atlantic States and result in light northwest stirring or brief periods of
calm...with lows mostly in the lower 40s. Some upper 30s will be
possible if/where winds go calm for any length of time...however...
given projected ll thickness values near 1325 M and just passing
Short term /Tuesday and Tuesday night/...
as of 405 am Monday...
An upper low initially over nm will migrate east across the Southern
Plains and lower MS valley...while short wave ridging aloft briefly expands
across the southeast states in advance of the low. Dry surface high pressure
will consequently build across and linger over the Middle Atlantic
States through early Tuesday night...then yield to top-down moistening
ahead of surface low pressure forecast to develop and gradually deepen
from the western gom and along the Gulf Coast through Tuesday night.
Associated cirrus on Tuesday will progressively thicken and lower
overnight and culminate in a chance of rain over the southern and
western Piedmont prior to daybreak Wednesday. Highs generally 65 to 70
degrees. Low temperatures...initially driven by radiational cooling before
clouds thicken and lower and cause temperatures to steady or rise
slightly after midnight...only to evaporationally cool where rain
occurs prior to daybreak...in the middle 40s to lower 50s.
Long term /Wednesday through Sunday/...
as of 300 am Monday...
An unsettled start to the long term as Wednesday will feature a deep
longwave trough across the Ohio Valley with the possibility of a
cutoff upper level low helping to develop an associated surface low
in the southern stream which will move northeast along the coast
affecting central NC from Wednesday through Thursday and possibly
into Friday. A somewhat non-traditional in-situ cad scenario will
set up as overrunning precipitation falls into a drier airmass below
which will cool the low levels significantly. This will deter any
severe threat despite plenty of forcing on multiple levels. While
details on the timing are still coming into agreement...the general
scenario will have rain moving into the SW late Tuesday night or
early Wednesday morning and continuing through at least Thursday
afternoon as the surface low moves up the Carolina coast. Both
models are producing 2 day quantitative precipitation forecast totals of close to an inch so some
periods of moderate to heavy rain will be possible. Details start to
blur Thursday night into Friday as both models show an upper level
low diving southward out of Canada but the GFS produces a much
stronger cutoff low than the European model (ecmwf) but the upshot will generally be
the same which is some lingering lighter precipitation into
Friday...especially across the north.
The low moves offshore Friday night and ridging builds in from the
west...helping to dry things out for the weekend. The progression of
the surface high is yet to be determined but suffice to say that
there will be a drying and warming trend through the weekend.
Therefore temperatures start out midweek in the upper 50s to middle 60s
for highs under the cad airmass but will climb into the upper 70s by
the end of the weekend and will feel more like may and less like
Aviation /11z Monday through Friday/...
as of 645 am Monday...
Patchy sub-VFR visibility restrictions...and associated vlifr
ceilings/vertical visibility restrictions will quickly disperse
through 13z. Otherwise...calm to light west to west-northwest winds this morning
will become northwest and increase into the 8-14 knots range this
afternoon...with occasional gustiness between 15-20 kts in the
several hours prior to sunset. Skies will be relatively clear
throughout the day...aside from mainly few-scattered shallow cumuliform
clouds around 7-8 thousand feet...with relative greatest coverage from
krwi and points northeastward...where a brief broken ceiling at that
height will be possible middle-late afternoon owing to the approach of
an upper level disturbance and associated relative strongest lift
and moisture over NE NC and southeast Virginia.
Outlook: the passage of a coastal low pressure system will result in
a good chance of sub-VFR conditions and rain Wed-Thu...with the
relative highest probability of occurrence and lowest and longest
sub-VFR conditions...at eastern taf sites.