Area Forecast Discussion
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FXUS62 KRAH 271432
Area Forecast Discussion
National Weather Service Raleigh NC
1030 AM EDT Thu Apr 27 2017
A cold front will approach the mountains from the west late this
afternoon and evening, then stall out and dissipate over the Mid-
Atlantic tonight. A moist southerly return flow on the western
periphery of a strengthening Bermuda high will prevail Friday
through the weekend.
.NEAR TERM /through Tonight/...
As of 1030 AM Thursday...
The 12Z/27th GSO raob. indicated a strong, dry capping inversion
near 780mb, while visible imagery, and a glance out the window
reveals mostly clear skies east of the Foothills. Big question for
this afternoon will be destablization, with RAP soundings and SPC
mesoanalysis showing sufficient moistening from the south between
700mb and 925mb to remove the cap, and despite the approach of
debris cloudiness from the southern Appalachians, insolation and
southerly winds should allow heating to yield at least 500 j/kg
MLCAPE. The richest theta e air looks to be immediately ahead of
the prefrontal trough and ongoing convection over eastern Alabama,
and guidance suggest the mid 60 dewpoint air will make it into thew
Piedmont this evening. Could envision a scenario where mid-afternoon
convection cross the mountains makes it into the western and
northern Piedmont areas as the better jet forcing associated with
the shortwave lifting into the Great Lakes passes by, and then some
additional showers or isolated storms pop up in the south withing
the axis of higher theta e. Thus, there is still some uncertainty
in how much of the area will see convection, but the overall trend
should be downward in coverage and intensity late this evening and
into the overnight. Accordingly, SPC has western NC in a marginal
risk for damaging winds. Highs today will be in the upper 70s to
mid 80s based on a 1380m thickness at GSO this morning.
With increasing low-level moisture, expect overnight lows in the
Severe Weather: The MCS progressing through eastern TN this morning
is progged to cross the mountains into the western Piedmont invof
peak heating this afternoon. The potential for severe weather is
difficult to ascertain for several reasons, mainly 1) uncertainty in
the evolution of the MCS as it crosses the southern Appalachians and
2) environmental uncertainty associated with rapid modification of
thermal profiles progged to occur in advance of the system. With the
above in mind, an isolated instance of damaging winds cannot be
ruled out in the western Piedmont late this afternoon and early this
.SHORT TERM /Friday and Friday Night/...
As of 400 AM EDT Thursday...
Moderate to extreme destabilization (~3000 J/Kg MLCAPE) is expected
during the day Friday in assoc/w strong insolation and seasonable
low-level moisture (Td lower 60s) beneath steep mid-level lapse
rates /a modified elevated mixed layer/. Despite an anomalously
favorable thermodynamic environment, weak forcing and a strong ~800
mb cap may preclude convective development over central NC. Highs
will range from the mid 80s (NW) to ~90F (SE). As such, a gradient
in CINH (highest west / lowest east) will exist across the state.
Unless a well-timed shortwave/MCV in SW flow aloft progresses over
western NC or upstream convection propagates downstream into western
NC (neither appear likely at this time), dry conditions will
prevail. In eastern NC where afternoon temps will reach ~90F and
CINH erosion will be maximized, isolated convection cannot be
entirely ruled out if sufficient convergence is present (e.g. a
robust seabreeze penetrates inland). If the cap breaks and deep
convection develops (e.g. updrafts survive entrainment) in the SE
Coastal Plain 21-00Z, thermodynamic/kinematic profiles would support
robust supercells capable of producing destructive hail/microbursts.
If the cap holds, dry conditions will prevail through tonight.
Expect lows in the mid 60s to lower 70s. -Vincent
.LONG TERM /Saturday through Wednesday/...
As of 345 AM EDT Thursday...
The Bermuda high`s influence will be strongest over the area on
Saturday with low-level thicknesses progged to peak between
1420m(40m above normal) with H8 temps warming to around 18C. Highs
Saturday in the upper 80s to lower 90s on Saturday, when we could
challenge the record high max of 91 at RDU. Heights are forecast to
lower slightly on Sunday, with highs Sunday similar to Friday`s
Rain/convective chances through the weekend will be limited by the
dominate ridge with best chance across western NC, where convection
is most apt to fire where owing to differential terrain heating.
Will retain a slight/small chance pops across the western Piedmont.
If storms do develop or propagate east into the area on Saturday,
strong instability ~3000 J/Kg aided by steep lapse rates aloft
associated with a modified EML, coupled with deep layer shear of
~25kts could support some vigorous updrafts capable of producing
large hail and damaging thunderstorm winds.
Models have slowed down with the timing of the front on Monday,
delaying fropa until after 00z Tuesday. This unfavorable nocturnal
timing will not bode well for severe potential, with models
suggesting only weak instability across central NC Monday
afternoon/evening. When coupled with the better trough dynamics
lifting off to the north, the potential for severe storms is low at
Cooler/near normal temps will follow Tuesday and Wednesday in the
wake of the cold frontal passage Monday night.
Late next week, medium range models are actually in good agreement
in showing the development of a deep trough east of the MS, with
unsettled/active weather expected to accompany this dynamic system
late Wednesday night through Friday as it progresses east into the
.AVIATION /12Z Thursday through Tuesday/...
As of 630 AM EDT Thursday...
24 Hour TAF Period: VFR conditions associated with a dry/capped
airmass will prevail through mid to late afternoon. Scattered
convection (remnants of a squall line expected to cross the
mountains this afternoon) will be possible 20-00Z at INT/GSO
terminals and 00-04Z at eastern terminals, with the relative best
chance at INT/GSO terminals given an increasingly dry/hostile
airmass with eastern extent. SW/WSW wind gusts as high as 20G35KT
will be possible in assoc/w any convection. Otherwise, calm or light
SSW winds early this morning will increase to 10-15 knots a few
hours after sunrise, with gusts as high as 20-25 knots this
afternoon (after ~15Z), highest at the INT/GSO terminals.
Outlook: Though VFR conditions will prevail through the majority of
the upcoming weekend, adverse aviation conditions are periodically
expected. Examples include IFR/LIFR ceilings expected to develop
each morning Sat/Sun/Mon between 08-12Z, lifting/scattering out to
VFR by 12-15Z. Additionally, isolated to scattered afternoon/evening
(21-03Z) convection will be possible Fri/Sat. Should convection
develop and/or propagate into the region Fri/Sat, environmental
conditions would strongly support severe weather. Otherwise, expect
the best overall potential for convection Monday evening/night in
assoc/w a cold front /squall line/ progged to track eastward through
the Carolinas. -Vincent