Area Forecast Discussion
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FXUS62 KRAH 301907
Area Forecast Discussion
National Weather Service Raleigh NC
305 PM EDT Thu Mar 30 2017
Weak high pressure will extend into the area from the north today,
then move offshore tonight. A storm system will approach from the
west tonight, cross the area Friday, then move off the coast Friday
night. High pressure will build in from the north over the weekend.
Another storm system will affect the region Monday into Tuesday.
.NEAR TERM /through tonight/...
As of 305 PM Thursday...
The latest surface analysis has the departing surface high shifting
over and just off the NE and Mid Atlantic coast, with the residual
stable pool covering much of central and eastern NC, and the warm
frontal zone extending from central OH through WV and W NC to SW SC.
A band of showers and storms along this frontal zone, from near SAV
to CAE to just west of CLT/INT, is being supported in part by moist
upglide, low level mass convergence, and weak upper divergence,
situated along an 850 mb moisture ridge. This convection will
continue shifting to the ENE over the next several hours, decreasing
in coverage and intensity as it encounters the drier and more stable
air over the eastern Piedmont/Sandhills and Coastal Plain, beneath
the eastward-moving mid level ridge. Will retain a trend to good
chance/likely pops but with light QPF over the far western CWA only
into the early evening, followed by a short relative lull before the
more substantial mid level DPVA, now supporting convection over S AL
into the Gulf, and the attendant upper divergence maximum (in the
left exit region of the 120+ kt STJ) shift to the NE into the
Carolinas later tonight. Based on extrapolation of these features
and the timing of high-res models, coverage is expected to ramp up
to categorical over the western and far southern CWA overnight, with
lower-end likely in the NE CWA. While forecast hodographs tonight
will be long and curved with very high 0-1 km helicity of 200-300
m2/s2 and strengthening 850 mb SSW flow, the lingering low level
stable air and marginal mid level lapse rates may yield too little
CAPE to work with overnight. But with the strengthening dynamic
forcing for ascent late tonight, including a band of strong upper
divergence and DPVA, there may be enough overlap with the retreating
surface frontal zone to bring about a concern for isolated tornadoes
with any discrete or embedded cells, most likely after 3 am. Expect
lows of 51-60. -GIH
.SHORT TERM /THURSDAY NIGHT AND FRIDAY/...
As of 420 AM Thursday...
Attention will turn to the southwest on Thursday night as a warm
front surges in and temperatures should actually begin to rise
overnight at most locations. Good isentropic lift in the west will
begin to encourage precipitation over the western portion of the CWA
between 0-6Z Friday. Model forecast soundings are virtually
saturated throughout the column with PW values near 1.3 inches. The
low level jet structure is fairly unorganized early Friday morning
and there are several small disturbances in the flow so expect
several smaller waves of precipitation which could be scattered in
nature at times. As the precipitation pushes eastward it could
become a bit more organized into more multicellular or broken lines
but shear will be decreasing at this time particularly in the lower
levels as winds become southwesterly and more unidirectional with
height. Instability will increase slightly with peak heating and a
dynamic push from the left exit region of a 500 mb jet could cause a
storm or two to become severe. SPC has the eastern half of the area
in a marginal risk for severe wx with damaging winds the most likely
threat. With decent moisture in this system expect rainfall totals
anywhere from a half of an inch on the low end to about an inch on
the high end.
Temperatures will be tricky on Friday given the convection but with
central NC in the warm sector for much of the day expect highs in
the low to mid 70s in most locations.
.LONG TERM /SATURDAY THROUGH THURSDAY/...
As of 215 PM Thursday...
An energetic Pacific jet will send a series of significant
shortwave perturbations into the west-central U.S., where the
models indicate they will amplify toward the Four Corners/Srn
Plains region and interact with an equally energetic southern
stream. That interaction will ultimately shear the
perturbations ENEwd into the Middle Atlantic states, with
associated episodes of precipitation/convection, about every 72
hours in central NC.
Saturday into Sunday: Beneath longwave ridging bulging northward
from the Caribbean to central Canada, the weekend is expected to be
dry, and mild, with a reinforcing dry frontal passage (the leading
edge of a Pacific high) Sat night. Periods of stratocumulus may
bubble beneath a strong, 5 k ft subsidence inversion, but not with
enough coverage to significantly impact temperatures through the
period. Highs in the upper 60s to 70s, warmest ahead of the
reinforcing front on Sat. Lows, coolest Sun morning in the 40s, and
about a category higher in return SEly flow and increasing deep
moisture by Mon morning.
Monday into Thursday: The next couple of strong shortwave
perturbations will follow on schedule and cross central NC late
Monday into Tuesday morning and again on Thursday. The result will
be another batch of showers and thunderstorms late Monday into
Monday night and again Thursday into Thursday night. It will
otherwise be mild through the period, with maximum temperatures
mostly in the 70s and minimum temperatures in the 50s.
.AVIATION /18Z Thursday through Tuesday/...
As of 1230 PM Thursday...
MVFR cigs over the far W (INT/GSO) will lift to VFR during the mid
afternoon, while farther east at RDU/FAY/RWI, VFR cigs will hold for
the rest of the daylight hours. Scattered showers will move into
INT/GSO after 21z, however VFR vsbys and cigs should persist, with
only brief MVFR conditions within short-lived heavier showers
through this evening. Cigs are likely to drop back to MVFR then IFR
around midnight (04z) at INT/GSO, after 2 am (06z) at RDU/FAY, and
after 4 am (08z) at RWI, all as a storm system approaches from the W
and SW tonight. Widespread MVFR to IFR vsbys within showers and
thunderstorms are expected starting late tonight, tracking eastward
through early Fri afternoon, ahead of an eastward moving cold front.
The showers/storms and sub-VFR conditions are expected to push east
of INT/GSO after 15z Fri, and RDU around 18z. Ahead of the front,
through tonight, surface winds will be from the SSE, shifting to SW
Fri morning, then to WSW with frontal passage at western sites late
in the TAF valid period.
Looking beyond 18z Fri: The trend from MVFR to VFR will progress
eastward through RDU/RWI/FAY prior to 21z, as a cold front crosses
the area. VFR conditions are expected to dominate through Sun as
high pressure builds in from the north, although the moist low level
flow from the NE may bring a short period of MVFR cigs mainly at RWI
03z-12z Sun (Sat night). Another storm system will bring a good
chance of sub-VFR conditions and strong/shifting winds with height
starting after 06z Mon (Sun night), dominating until Tue morning,
when the system will move to our east with a trend back to VFR by
Tue afternoon. -GIH