Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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750
FXUS62 KRAH 270830
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
430 AM EDT Thu Apr 27 2017

.SYNOPSIS...
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 /Today and Tonight/...
As of 345 AM EDT Thursday...

A strong subsidence inversion /cap/ and dry mid-level airmass in
place over the region this morning will persist through the majority
of the afternoon. A squall line associated with a deamplifying
shortwave lifting NE through the TN/OH valley (and an attendant cold
front) is expected to track eastward from central TN into the
southern Appalachians by early afternoon. Forcing assoc/w the
aforementioned wave will weaken at this latitude as it lifts NE
through the OH valley this afternoon and the thermodynamic
environment becomes increasingly hostile (dry/capped) east of the
mountains (in closer proximity to the upper level ridge building
along/offshore the SE coast). As such, one would expect the squall
line to weaken and gradually dissipate as it progresses east of the
mountains late this afternoon.

However, mesoanalysis data this morning shows a S/SW LLJ advecting
rich low-level (925-850mb) moisture poleward from the GOMEX into the
Deep South, directly in advance of the squall line. This plume of
low-level moisture will advect northward into the Carolinas
immediately in advance of the approaching squall line late this
afternoon and evening, beneath steep mid-level lapse rates attendant
a modified elevated mixed layer over the region (see 00Z GSO RAOB).
With rapid destabilization expected immediately in advance of the
approaching squall line, probs are increasing that the MCS will be
able to maintain itself as it progresses east of the mountains
during the late aft/eve. The bulk of 00Z guidance indicates as much,
showing the line progressing through central NC in the 21-04Z time
frame. Expect highs in the upper 70s (west) to mid 80s (east).
Temperatures may fall 10-15F as outflow attendant the squall line
progresses east of the I-77 corridor into the western Piedmont
during the late afternoon (21-22Z). With increasing low-level
moisture, expect overnight lows in the mid/upper 60s. -Vincent

&&

.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
temps.

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
this time.

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
region.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
As of 145 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

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SYNOPSIS...Vincent
NEAR TERM...Vincent
SHORT TERM...Vincent
LONG TERM...CBL
AVIATION...Vincent



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