Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 221145

Area Forecast Discussion
National Weather Service Raleigh NC
745 AM EDT Sat Jul 22 2017

Hot and mainly dry sub-tropical high pressure will expand across the
mid-South and southern Appalachians through the weekend, then
gradually weaken by early next week.


As of 400 AM Saturday...

...Dangerously hot conditions will expand across central NC

Excessive Heat Warning now in effect for the cities of Raleigh
and Fayetteville (counties of Wake and Cumberland) from 11 AM
until 7 PM today...

Heat advisory in effect for the rest of our region, including
all of the Piedmont, Sandhills, and Coastal Plain from 11 AM
Saturday through 7 PM today.

Excessive Heat Watch remains in effect for the cities of
Raleigh and Fayetteville (counties of Wake and Cumberland) from
7 PM this evening through 7 PM Sunday.

Beneath and downstream of a 321 dm/595 dm sub-tropical ridge
at 700mb and 500 mb, respectively, centered from the central
Plains to the lower OH and TN Valleys, around persistence high
temperatures are expected throughout central NC today - mostly
middle 90s to around 100 degrees.

A difference between the two days, and an important one, will be
that the Appalachian-lee/Piedmont trough is forecast to sharpen
over the srn and wrn Piedmont today, as opposed to from near
KASJ to KJNX to KMEB like Fri. Consequently, surface dewpoints
in the lower to middle 70s are expected to encompass a larger
portion of central NC, on the ern side of the trough axis; and
associated heat indices should likewise be higher farther nwwd.

Observed heat index values from Fri, the ones with a relatively
high degree of confidence of reliability, and the idea of a
persistence forecast for today, suggest maximum heat index
values will be very near 110 degrees across the ern Sandhills
and srn and central Coastal Plain, and an upgrade to an
Excessive Heat Warning was considered for the counties between
the current Warning for just Wake and Cumberland Co., which
should be noted was set up and well placed with "outside the
box" thinking by the day shift on Fri. This would include
Scotland, Hoke, Harnett, Johnston, Wayne, Sampson, and Wilson.
Since it appears the handful of sites that reached or exceeded
110 degrees on Fri was for only an hour, and Warning criteria
is two hours, we elected to maintain the Heat Advisory as is.
Frankly, the difference between a degree or two and an hour or
two is more for bookkeeping than actual affects of the heat, so
it`s important to not get caught up in criteria and definitions.
It will be dangerously hot throughout central NC today.

Also like Fri, isolated convection will initially be possible
in the vicinity of both the Piedmont trough, likely over the
srn and wrn Piedmont, and the sea breeze. Later, upstream
convection from the upper Midwest and OH Valley, or at least
associated MCV`s and/or outflow, may propagate into the nwrn and
nrn Piedmont late this afternoon through this evening - an idea
many of the CAM solutions favor. Strong to marginally severe
wind gusts will pose the primary hazard owing to a strongly and
deeply mixed boundary layer characterized by surface dewpoint
depressions of around 25 degrees, amidst a weakly-sheared
environment on the srn rim of the belt of westerlies focused
over the nrn Middle Atlantic states.

It will be mild/warm, and muggy, overnight, with lows mainly 75
to 80 degrees. The associated continued relatively warm and
moist boundary layer, and probable remnant outflow over at
least the nrn half of the forecast area, will favor a slight
chance of a shower or storm all night.


As of 415 AM Saturday...

Excessive Heat Watch remains in effect for the cities of
Raleigh and Fayetteville (counties of Wake and Cumberland) from
through 7 PM Sunday.

Generally persistence, though with a higher probability of
afternoon convection, as heights aloft fall over the srn Middle
Atlantic states and NC consequently becomes increasingly
susceptible to passing perturbations, some resulting from
upstream convection, in wly to nwly flow. Another Heat Advisory
will likely be needed for all of central NC, with a good chance
of another Excessive Heat Warning over at least Wake and
Cumberland counties once again. The coverage and initiation time
of convection, and preceding high level "blowoff" cloudiness
from upstream convection, may result in slightly lower high
temperatures and associated heat index values, however, but the
overall cumulative affects of the heat will persist regardless.


As of 335 AM Saturday...

A weak but fairly progressive upper level trough will move over the
area on Monday and then continue eastward, taking a surface low off
of the Long Island coast and out to sea. Behind the exiting low, a
high pressure system will move over the northeastern CONUS that will
eventually drop a cold front through the area and bring some relief
to the heat. A general cooling trend will begin on Monday with
temperatures easing back into the lower 90s, but dropping further
into the mid to upper 80s by Wednesday afternoon, after the passage
of the front. Upper 80s to low 90s will prevail through the
remainder of the forecast period.

The Piedmont trough and a lingering frontal boundary
across the southern part of the area will be the focus for
convection for much of the work week as a series of disturbances
tracks along this boundary. The strongest of these disturbances
could come on Tuesday night into early Wednesday morning but will be
short lived before either weakening or moving out to sea. Otherwise
expect diurnal showers and thunderstorms. The best chance for severe
weather may be across the north on Monday as the upper dynamics are
more favorable for supporting severe thunderstorms. As such the SPC
has included northern portions of the CWA in a marginal to slight
risk of severe thunderstorms.


As of 745 AM Saturday...

Aside from a small chance of showers and storms, and associated
gusty winds and brief sub-VFR conditions, VFR conditions with a
mainly light, 7-13 kt surface wind, are expected through the TAF
period. The relative highest probability of convection will be at
INT and GSO, where upstream convection and associated outflow, from
a couple of MCS`s over the OH Valley and/or from the Appalachians,
may propagate late this afternoon and this evening. Otherwise and
elsewhere, an isolated "pop-up" storm will be possible, but
predictability and timing of any such occurrence is too low to
include for any other TAF site at this time.

Outlook: Mainly VFR conditions will continue through the middle of
next week, with the primary sub-VFR risk coming courtesy of showers
and storms that will grow in coverage each successive day through


RDU Records:
Date     | High   Year  |  High Min  Year
22 July  |  103   2011  |     78     2011
23 July  |  105   1952  |     78     2011
24 July  |  101   2011  |     79     2011

GSO Records:
Date     | High   Year  |  High Min  Year
22 July  |  101   1926  |     77     2011
23 July  |   99   1952  |     78     2010
24 July  |   99   1914  |     78     2010

FAY Records:
Date     |  High  Year  |  High Min  Year
22 July  |  107   1932  |     79     2011
23 July  |  103   2011  |     79     1998
24 July  |  105   1952  |     79     1945


Heat Advisory from 11 AM this morning to 7 PM EDT this evening
for NCZ007>011-021>028-038>040-042-043-073>078-083>086-089.
Excessive Heat Warning from 11 AM this morning to 7 PM EDT
this evening for NCZ041-088.
Excessive Heat Watch from this evening through Sunday evening
for NCZ041-088.


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