Scientific Forecaster Discussion
fxus62 krah 270757
Area forecast discussion
National Weather Service Raleigh NC
356 am EDT Thu Jul 27 2017
surface ridge axis over the area will shift offshore later this
morning. Meanwhile a stalled surface front draped across SC will
dissipate today with a weak surface trough developing across the NC
Piedmont this afternoon. A cold front will approach from the
northwest Thursday night and cross our region Friday night.
Near term /through tonight/...
as of 350 am Thursday...
Upper level ridge over the southern Appalachians is forecast to
retrograde westward into the central US today, while a shortwave
trough will move across the northeast US. West-northwest mid-level
flow between between these two features will help to steer upstream
convection and associated upper pv anomalies currently over the
mid MS and Ohio valleys southeast into the Carolinas and Virginia late
this afternoon and into the overnight hours.
Meanwhile, weak surface ridge axis extending SW into the area
this morning will move offshore, with the stalled front to our south
expected to wash out/dissipate. The onset of southerly flow today
will mark a return to our typical summertime humidity levels with
highs a few degrees warmer than yesterday, ranging from upper 80s
northwest to lower 90 southeast.
General model timing has the first batch of convection and series
of disturbances knocking on our northwest doorstep between 18 to 21z with
large model spread thereafter regarding whether the convection
will weak and fall apart or hold together as it advances east across
the area through the evening and overnight hours. Further
complicating the convective forecast is the potential development
and uncertainty of additional upstream convection/upper disturbances
that could be apt to move into the region during the evening and
overnight hours, in advance of the amplifying shortwave trough
diving across the Great Lakes, with hopes that the convective
details will become clearer after analyzing upstream radar/satellite
trends later this afternoon. For now will have high/good chance pops
for the northwest Piedmont this afternoon, with chance pops spreading east
during the evening and lingering through the overnight hours and
maintaining highest pops across the northern tier counties.
Any strong to severe storms should be confined to the
northwest Piedmont/northern Piedmont counties, where the arrival
of the forcing, coincident with steep low-level lapse rates/Max
heating may be enough for an isolated severe storm or two.
Widespread convective debris cloudiness and southerly winds should
allow for a warm night with lows in the 70s.
Short term /Friday through Saturday night/...
as of 345 am Thursday...
Overview: an unseasonably strong upper level low over the Great
Lakes will amplify as it tracks slowly southeast into the mid-
Atlantic late Fri/Fri night. An associated surface low is expected
to develop/ intensify along a strengthening frontal zone /cold
front/ progressing into the mid-Atlantic Fri night, with mslp values
~1000 mb over portions of the Delaware-Maryland-Virginia or southeast Virginia by 12z Sat.
The aforementioned cold front is expected to progress southeast through NC
during the day Sat as the low becomes vertically stacked along the
Expect widespread cloud cover as cyclonic flow aloft strengthens
over the region Fri/Fri night, with some clearing possible in the
west/SW Piedmont by ~18z Sat as the cold front progresses into the southeast
coastal plain, however, cloud cover may persist through Sat night
(particularly across the north Piedmont/NE coastal plain) as shortwave
energy digs S/south-southeast on the western periphery of the vertically stacked
/nearly stalled/ low along the mid-Atlantic coast. Highs on Sat
should range from the mid 80s to ~90f, coolest near the Virginia border
and warmest near the SC border. Lows Fri night in the upper 60s to
lower 70s, warmest S/se. Highs on Sat should range from the lower
80s (n) to mid/upper 80s (s), and lows Sat night should be noticeably
cooler, in the lower to mid 60s.
Confidence is high W/regard to above normal cloud cover and above
normal precipitation Fri/Sat. However, convection over central NC
will be modulated by a number of factors, including small amplitudes
waves in strengthening cyclonic flow aloft and upstream convection
and/or convective remnants propagating S/southeast into the area. As a
result, confidence is very low with regard to the timing/location/
intensity of convection Fri/Sat. As a result, the threat for severe
weather and/or flash flooding is unclear at this time. -Vincent
Long term /Sunday through Wednesday/...
as of 350 am Thursday...
Sun/sun night: cloud cover and isold/sct showers may persist in the
wake of the cold frontal passage on sun as shortwave energy digs
S/southeast through central NC on the western periphery of the vertically
stacked low along the mid-Atlantic coast. Expect cloud cover and
potential for showers to be greatest in the north Piedmont and NE
coastal plain. Cloud cover should decrease from SW-NE Sun night as
the aforementioned low meanders northward along the mid-Atlantic
coast. Anticipate below normal temperatures with highs in the
lower/mid 80s and lows in the lower/mid 60s.
Mon-Wed: in the wake of the low lifting slowly NE offshore the mid-
Atlantic coast, expect dry conditions and moderating temperatures
early next week. -Vincent
Aviation /06z Thursday through Monday/...
as of 203 am Thursday...
Weak return flow/warm air advection maximized across western NC, along the western
periphery of the sfc ridge axis centered across eastern NC will
result in 3-5 thousand ft stratocumulus across central NC overnight,
with an isolated/stray shower possible near kint and kgso.
Otherwise, expect predominately VFR conditions this morning and much
of the day on Thursday with a broken stratocu expected owing to
Kint and kgso will see the best chance of convection this afternoon,
with upstream disturbances and associated convection expected to
track southeast into the western Piedmont during 20 to 24z time frame.
Have included a prob30 group for thunder for this time frame.
There after, confidence decreases with the evolution of this
convection whether it will persist and advance east into the central
Piedmont and coastal plain or if it will weaken and dissipate
altogether. There is also the potential for additional convection to
to propagate east/se into the area Thursday night, especially across
the northern terminals.
Outlook for Friday through Monday... MVFR to IFR conditions
associated with numerous showers and thunderstorms late
Friday/Friday night as a cold front is expected to push southeast through
the area. Aside from some isolated convection Saturday night/Sunday
as the upper low/trough digs south over the area, expect
predominately VFR conditions through Monday.