Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 190918

Area Forecast Discussion
National Weather Service Raleigh NC
512 AM EDT Thu Oct 19 2017

Surface high pressure over the Mid-atlantic will extend across
the region into the weekend. A cold front will approach the
area from the west early next week.


As of 510 AM Thursday...

A quick updated to the forecast to lower temps across the piedmont
this morning and add some patchy frost to the forecast. Otherwise,
skies will remain mostly clear/clear with seasonably cool/cold temps
this morning.

Previous discussion at 220 AM: A weak mid/upper level disturbance is
currently moving across the region this morning. However, with
generally a dry atmo in place, only a scattering of high cirrus is
accompanying the weak disturbance this morning. Otherwise, surface
high pressure will remain in place with slowly moderating
temperatures. Model guidance has trended cooler with high temps
today, and have such lowered highs by a couple of degrees from
previous forecasts. Thus, expect high temps will generally be in the
lower to mid 70s today.

Mid level ridging will begin to build eastward and into the area
tonight, with surface high pressure holding over the area. This will
result in a continuation of dry and cool conditions with clear
skies. Low temps are expected to range from around 40 in the coldest
rural locations to the mid to upper 40s in the urban areas and far


As of 235 AM Thursday...

Mid level ridge axis will build overhead on Friday into Friday
night, resulting in little change in the weather across central NC,
other than temps will be a bit warmer. Expect high temps will
generally be in upper 70s to around 80. Lows temps are expected to
range from the mid 40s in the cold spots to the lower 50s in the
urban areas and SE.


As of 315 AM Thursday...

Little change in forecast rationale described well by the previous

Mid-upper ridging now over the wrn GOM is forecast to amplify as it
builds east, across and offshore the sern US coast through the
weekend, and downstream of a powerful trough forecast to amplify
across the Rockies and Plains states. The models continue to
indicate that this initially meridional trough will separate and
yield a closing off of the srn portion of the trough, as an
increasingly active srn stream jet in split flow aloft noses across
the lower MS Valley by Sun night. Model spread increases with
respect to the geometry and progression of a new trough forecast to
amplify from the ne Pacific to the Great Lakes vicinity by the
middle of next week, and consequently how quickly this trough may
serve as a "kicker" to cause the aforementioned lead srn stream wave
to lift and accelerate through the Appalachians Mon-Tue.

At the surface, high pressure centered over the middle Atlantic
states Sat will strengthen, beneath the building ridge aloft, and
move off the nrn middle Atlantic and Northeast coast by Sun. A warm
front will retreat nwd, in return flow around the high, across the
Carolinas Sun-Sun night. This will occur ahead of a deepening
surface cyclone and accompanying lead cold front related to the
aforementioned srn stream closed low, forecast to migrate ne through
the srn and central Appalachians late Mon-Tue. A secondary cold
front, one related to the trailing trough forecast to reach the
Great Lakes vicinity by the middle of next week, will likely sweep
east of the Appalachians and across NC late Tue-Tue night.

Sensible weather resulting from the pattern outlined above will
include mild and dry conditions this weekend, followed by the
passage of a band or bands of showers, and probable QLCS
thunderstorms, Mon night-early Tue. Strong and coupled QG and
mesoscale forcing for ascent, and strengthening lower and deep-
tropospheric flow, will support an associated risk of severe storms
over the Carolinas during that time. Much cooler conditions, and
variably cloudy ones, with a chance of "instability" showers in
cold, cyclonic flow aloft, will follow for Tue night-Wed.


As of 515 AM Thursday...

24-Hour TAF period: VFR conditions will generally continue for the
24 hour TAF period. The one exception will be at fog prone KRWI,
where some pre-dawn fog is expected early this morning. Visbys at
KRWI are expected to range from MVFR-LIFR early this morning through
around sunrise. Otherwise, VFR conditions will continue with light
and variable to calm winds and mostly clear/clear skies, as surface
high pressure remains over the region. Patchy sub-VFR visbys will
again be possible on Saturday morning at KRWI, but confidence is too
low to include in the TAFs at this time.

Looking ahead: VFR conditions will prevail through the first half of
the weekend, with brief periods of sub-vfr conditions in the pre-
dawn hours possible near KRWI. Upslope flow in the west late Sunday
night or Monday ahead of an approaching cold front could result in
some sub-VFR conditions. However, the next chance for widespread sub-
vfr conditions will come with pre-frontal convection between Monday
afternoon and Tuesday night as the front moves into and through the




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