Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 281646

Area Forecast Discussion
National Weather Service Raleigh NC
1245 PM EDT Fri Jul 28 2017

A cold front will approach from the northwest tonight, and cross our
region overnight through Saturday morning . Cooler and drier air
will filter into the area through the weekend and will linger
through the early part of next week.


As of 1050 AM Friday...

Little change required to the near term forecast. Scattered
convection expected to develop this afternoon with the highest
convective coverage expected between 22Z-06Z.

12Z upper air analysis depicts a couple of features of note. The
first is a 70-75kt upper jet exiting the western Great Lakes, aiming
toward the Carolinas. As this feature gets closer this
afternoon/evening, upper divergence will increase over our region.
The second feature of interest is a strengthening mid level s/w
crossing the Great Lakes with a 850mb boundary stretching wsw-ene
across the central Appalachians into the lower OH Valley. The
approach of the s/w should enhance convergence in vicinity of the
850mb boundary, aiding in the development of scattered-numerous
showers and storms. These showers/storms should begin to affect our
northern Piedmont counties close to sunset.

Otherwise, morning meso-analysis depicts an axis of greatest
instability aligned fairly close to the highway 1 corridor across
central NC. CAM solutions as well as recent runs of the HRRR suggest
that the initial convection will fire in this region by early
afternoon. If this occurs, this may aid to stabilize the atmosphere
over sections of the eastern Piedmont/coastal plain prior to the
arrival of the better forcing aloft late in the day, resulting in
fewer/weaker storms. If convection initiation holds off until 20Z or
later, the risk of isolated severe storms will increase.

The morning analysis also depicts a weak sfc boundary aligned west-
east in proximity of the VA border. As the low-mid level flow
increases later today with approach of the mid level s/w,
convergence will likely increase along this feature, leading to
convective development. Effective bulk shear expected to be in the
neighborhood of 30-35kts late in the day/early evening, fairly
strong for this time of year, and supportive of organized
convection. If this flow is achieved and SBCAPE values climbed into
the 3000-4000 J/kg range, the strong buoyancy coupled with a
favorable shear environment may support rotating updrafts in any
storms that intersect this sfc boundary. This may lead to a risk for
weak, short-lived tornadoes. This risk appears best to the north and
northeast of Raleigh between 22Z-02Z.

The arrival of the dynamics supportive of scattered/numerous
convection will cross our region this evening into the overnight
hours. The dynamics exit our region around 06Z, so should see a
decrease in convective coverage and intensity by that time.

High temperatures this afternoon dependent upon insolation and when
convection develops. Extensive cloudiness and scattered convection
will yield max temps in the mid-upper 80s while some degree of sun
this afternoon and a later arrival of convection will allow
temperatures to reach/slightly exceed 90 degrees.


As of 330 AM Friday...

00Z guidance has begun to diverge with regard to the evolution of an
anomalously deep upper level low and attendant surface low invof the
Mid-Atlantic coast this weekend. The 00Z ECMWF is now quite
progressive, with the upper wave/sfc low tracking offshore the
DELMARVA late Sat night/early Sun and a dry/subsident NW flow
prevailing Sun through mid-week. The 00Z GFS/NAM continue to
indicate a solution in which the upper level low stalls/meanders
along the Carolina/Mid-Atlantic coast for the remainder of the
weekend. Regardless, expect an ongoing chance for showers/storms
along/ahead of a cold front associated with the deepening sfc low
over the DELMARVA through mid-day Sat as the front progresses SE
through central NC. Expect a drying trend from W-E Sat aft/eve into
Sat night, though drying may be delayed and a small chance for
showers may exist in the Coastal Plain Sat night/Sun if the GFS/NAM
solutions verify. Expect below normal temps associated with breezy
northerly flow/cold advection (yes, legit CAA in July) in the wake
of the front Sat afternoon into Sunday.  -Vincent


As of 1245 PM Friday...

Bit of a change in the way ECMWF handles the east coast trof later
this weekend. Latest shows much more progressive movement, with the
trof lifting out quickly early Sunday, in contrast the GFS`s slow
rotation across the area through Sunday night. Not a huge problem,
mainly would affect the sky coverage, with the ECMWF clearin us
quickly while the GFS`s later solution would produce some isolated
lingering showers in the northeast early Sunday. After this trof
exits, a pleasantly dry and cooler airmass will be in place, with
very gradual warming through the midweek. Seasonal conditions return
Thu and Fri as weak return flow nudges humidity and warmer
temperatures up the coast. Highs will start off Sunday from the
upper 70s across the north to lower 80s in the south. These highs
will warm to the mid 80s on Tuesday, then to around 90 Thursday and
Friday. Morning mins will be the low to mid 60s
through midweek, warming to around 70 by Friday morning.


As of 730 AM Friday...

24 Hour TAF Period: Weak disturbances passing through the area
combined and any outflow boundaries could support isolated to widely
scattered showers this morning and into the afternoon, especially
across northern portions of the area. A band/line of  showers and
thunderstorms and associated sub-VFR parameters and gusty winds
appear likely across central NC between 22 to 01z in the west and 00
to 05z in the east, as  a seasonably strong sfc cold front drifts

Outlook: Aforementioned strong cold front will be slow to push south
and east of the area on Saturday. Could see lingering convection
across the eastern/southeastern terminals between 12 to 18z
Saturday. Additionally, a few showers are possible Saturday
night/early Sunday as the main upper trough drifts across the region.
Considerably drier air will eventually filter into the area on
Sunday, resulting in VFR conditions through Tuesday.





SHORT TERM...Vincent
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