Scientific Forecaster Discussion
fxus62 krah 221417
Area forecast discussion
National Weather Service Raleigh NC
1020 am EDT Mon may 22 2017
a series of low pressure systems will cross central NC through mid-
week, causing periods of showers and thunderstorms. Improving
weather conditions are anticipated by late in the week.
Near term /today through tonight/...
as of 1020 am Monday...
An area of low pressure was exiting the northern Piedmont at
14z, ushering the band of showers associated with it into far
northeast NC and southeast Virginia. Westerly flow and subsidence in the
wake of the system was aiding to temporarily clearing the overcast
skies. As this system pulls farther away from central NC, expect
improving sky conditions across most of the region. The partly
cloudy/partly sunny skies and a steady westerly sfc wind will aid to
boost temperatures to near 80 across the far northern Piedmont, and
the low-mid 80s elsewhere. The heating this afternoon will
destabilize the atmosphere enough to support the development of
scattered showers and thunderstorms, primarily across the southern
and western sections of the forecast area, mainly after 20z. ~Wss
Later tonight, the chance for showers and storms will
increase once again from the SW, as a more potent mid level wave
(now over east tx) approaches, accompanied by a strengthening surface
low with a 35-45 kt low level jet on its southeast edge. Precipitable water will rise
further to over 1.75", and an impressive broad upper divergence
maximum will also move overhead from the SW, in the right entrance
region of a jet streaking from Tennessee/Kentucky into S Quebec. The deep
moisture combined with what is likely to be strong stacked forcing
for ascent will support a large area of rain and embedded storms,
beginning overnight in the SW County Warning Area then spreading NE. Will trend pops
up to categorical after midnight SW to NE. (See below for assessment
of heavy rain threat.)
With a warm and muggy air mass holding in place, expect highs today
from the upper 70s northwest to the low-mid 80s se, with a period of
partial sunshine likely. Lows tonight from the lower 60s northwest to the
upper 60s se, as the frontal zone bisects the forecast area. -Gih
Short term /Tuesday and Tuesday night/...
as of 410 am Monday...
..Flood Watch may be needed late tonight through much of Tue...
Models agree fairly well on a swath of moderate to heavy rain and
embedded storms spreading across central NC late tonight through at
least early Tue afternoon, with abundant deep moisture (pw near 2"
and a deep warm layer of 3.7-4.0 km, favoring warm rain processes)
and the aforementioned strong and vertically stacked forcing
features including mid level DPVA, vigorous upper divergence, and
low level mass convergence prompted by the strong 850-925 mb jet
feeding into the strengthening surface frontal zone. This setup
certainly suggests a risk of steady moderate to heavy rainfall,
especially along the frontal zone, and this added rain along with
this morning's totals presents a risk of flooding particularly in
urban and low lying areas. Would prefer to wait for another couple
of model runs, however, to get a better handle on quantitative precipitation forecast (max amounts
and location) before diving into a Flood Watch. There is also a risk
of a few strong to severe storms late tonight but especially Tue,
considering the dynamic forcing for ascent and model predictions of
marginal to moderate cape along/southeast of the frontal zone, and
strong/shifting winds with height near/northwest of the frontal zone,
yielding long looping hodographs. Will have categorical pops Tue
morning, trending down during the afternoon as the forcing heads to
our east and northeast. Some drying through the column is evident in
forecast soundings Tue evening/night, as the front heads to our southeast
with a weak meso-high settling over the region, so expect just small
chances of light rain or drizzle overnight. Highs Tue from around 70
northwest to near 80 southeast. Lows around 60 to the mid 60s. -Gih
Long term /Wednesday through Sunday/...
as of 400 am Monday...
An initially (slightly) positively-tilted longwave trough from ern
Canada to the srn plains, including an embedded mid-upper low over
the mid MS valley at 12z Wed, will migrate ewd and assume an
increasingly negative-tilt as it pivots across the Carolinas and
Middle Atlantic States by Thu and Thu night. Perturbed and moist
sswly to sly flow aloft will precede the trough, while height rises
and ridging aloft will follow and expand ewd across most of the ern
U.S. Through the weekend.
A complex pattern will result at the surface through the middle of
the week, and one that will likely be somewhat similar to the one
over the Carolinas during the past 24 hours. That is, a frontal zone
will likely be draped swwd from a series of waves passing across and
offshore the middle Atlantic coast, marking the srn periphery of
weak ridging/cad/low ovc extending across interior Virginia/NC, then
extending nwwd as a warm front to a low over the wrn Ohio Valley.
That front, edge of the cad regime, and focus for convection, will
then likely retreat nwd with a narrow wedge of warm sector through
the srn and wrn NC Piedmont during the day Wed, then through the
remainder of central NC early Wed night, all immediately ahead of,
and associated with, a triple point low that will develop and
migrate across the wrn Carolinas (also similar to this morning). The
trailing cold front and preceding warm conveyor belt, and associated
axis of continued likely showers and chance of storms, will sweep
ewd and across the Carolinas later Wed night, with brief drying and
clearing probable through early Thu. Clouds will redevelop with
heating, with an additional round of "instability-type" convection --
in cyclonic and cold air advection-bearing flow aloft-- and result in a chance
showers and/or storms Thu afternoon, despite surface dewpoints that
will likely have fallen into the 50s behind the aforementioned cold
front/warm conveyor belt.
Dry conditions will return by Thu night, as the upper trough lifts
away and weak high pressure expands across the sern states through
early Sat. A warm front will develop newd across the central
Appalachians later Sat and across the Middle Atlantic States by sun;
with a return to warmer and more humid conditions, and a chance of
convection invof the front late Sat afternoon through sun.
Aviation /12z Monday through Friday/...
as of 650 am Monday...
Poor aviation conditions are expected during the first 3-6 hrs of
the forecast period, as a band of showers and isolated storms
crosses the area. IFR/LIFR conditions will persist at int/gso, with
mainly MVFR conditions elsewhere through this morning, with a period
of MVFR to IFR vsbys in showers lasting for a few hours at each
site. Once this band of showers and storms passes east of the area
by noon, cigs will gradually lift to VFR with only isolated showers
expected after 18z today, and possibly a few storms near Fay/rwi
this afternoon into early evening. Rain coverage will begin to
increase again from the SW after 06z, with a trend toward prevailing
MVFR/IFR conditions with showers and storms.
Looking beyond 12z Tue morning, cigs and vsbys will remain MVFR/IFR
at all taf sites as a low pressure system approaches from the SW,
bringing widespread showers and storms. These poor aviation
conditions will dominate through much of Tue. A trend to VFR is
expected Tue night as the low pushes to our NE and offshore, however
sub-VFR conditions will return Wed into Wed night as a cold front
approaches from the west. -Gih