Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 301741

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
141 PM EDT Thu Mar 30 2017

A moist ridge of high pressure will build in from the north today
before a cold front sweeps across the forecast area early Friday.
Dry and warm high pressure moves in over the weekend...with
unsettled weather returning early next week.


As of 1100 AM EDT: Convection has developed across the Upstate, with
light rain/drizzle expanding along the Blue Ridge as damming
continues to entrench across the area. CAPE values kind of meager
this morning but expanding up the Savannah River Valley, with some
increasing muCAPE atop the cold dome over the rest of the Upstate.
Have increased pops this morning in areas already experiencing
convection. Adjusted hourly temperatures with a resulting minor
tweak to afternoon highs, but not any significant change.

Otherwise, a strong closed low pressure center continues to move
east into Missouri. Downstream ridging has developed ahead of this
system over the eastern U.S., and surface high pressure stretches
southward from Quebec through the Carolinas. The surface ridge will
gradually migrate offshore today as low pressure approaches from the
west and southerly return flow moisture rides northward over the
surface ridge to set up hybrid cold air damming. Upglide and upslope
forcing will be much better over western NC and points north today,
which leaves the southern tier as the more questionable forecast for
temperatures and associated weather. The northern tier east of the
Blue Ridge should get locked into cold air damming fairly quickly,
with light rain breaking out this morning and expanding this
afternoon. The southern tier could well see drier conditions
initially and more warming, which will in turn pump up instability
for the afternoon hours. SBCAPE values should reach 1000-1500 J/kg
in piedmont sections southeast of I-85 this afternoon. A few 55 dBZ
storm cores reaching reaching 23 kft this afternoon could well
produce marginally large hail and brief damaging wind gusts in a few
places once the cap breaks. This is the early activity that supports
the latest Slight Risk area, at least for the southern tier.

As the upper low pressure system lifts east over the Ohio Valley
tonight, the associated trough will acquire a more negative tilt and
bring a relatively decent period of upper forcing to our region
overnight. Meanwhile, the numerical models have the surface wedge
front retreating northward across western NC overnight. Although
this will uncover a bit more overnight instability, it will also
help to minimize the potential for a focusing mechanism of low level
helicity. Of course, the models are notorious for eroding the wedge
layer too quickly at night, but the offshore high position is not
favorable for entrenched CAD, so the model solutions seem
reasonable. Some uncertainty remains, however, regarding the risk of
nocturnal severe thunderstorms overnight as the 40 kt low level jet
translates through - especially if the better overnight instability
in the NAM is correct. This later activity will thus also support
the Day 1 Marginal to Slight risk severe tstm threat.  The nighttime
rainfall maximum should occur in the southern upslope areas from the
GA mountains to parts of the Blue Ridge and the current belief is
that this area can handle any localized two inch totals with the
main precipitation band without flooding consequences. The better
convection should translate through fairly quickly 06Z to 12Z,
likely crossing the I-77 corridor around daybreak.


As of 2pm EDT Thursday:  Some showers may linger in the mountains
with wrap-around moisture as large upper system exists the region.
Upper flow pattern will be strongly northwesterly on the back side
of the system.  850mb northwesterly flow of 30 to 40kts Friday night
will make for windy and gusty conditions, especially over higher
terrain.  Rain and wind decline Saturday morning as surface low
continues its eastward progress.  Northwesterly dry advection will
lead to clearing skies on Saturday which will last until Sunday
evening when clouds begin to move-in ahead of the next system.  Flow
pattern transitions from northwesterly to upper ridge on Saturday
and the weekend looks to be mostly calm and clear with temperatures
5 to 10 degrees above normal; with next system due in on Monday.


As of 2pm EDT Thursday:  EC and GFS models are in quite good
agreement through Tuesday evening, with the GFS model getting ahead
of the EC with the approach of the system due in around day7.
Extended forecast basically has 2 frontal/spring-time weather
systems rolling through the area.  The first one passing through
Monday night and the second one by Wednesday or Thursday of next week.

Current GFS guidance on CAPE is uninspiring for both systems.  As
the Monday system impacts the area during the CAPE-unfavorable
nighttime period, MLCAPE fails to exceed 400 j/kg, despite a surge
of moisture from the southeast ahead of the system.  Bulk shear,
though, will be quite good at 50 to 60 kts; which will give some
potential for severe, especially in Piedmont areas where deeper
moisture would be expected on southeasterly low-level dewpoint
advection.  System is dynamically potent with well-defined surface
low and cold front/warm front triple point moving along the eastern
edge of the Blueridge Monday night.  Moisture is not completely
scoured following the exit of the first system on Tuesday, but
moisture advection does not really commence in advance of the next
system, as surface low is much further north, giving westerly
low-level advection.  Second system does bring with it some Gulf
moisture, but MLCAPE is not expected to exceed 800 j/kg, even with
peak diurnal warmth.  Dynamical factors are again healthy with the
Wed.-Thu. system with the latest GFS showing 50 to 60 kts of deep
layer shear, so the severe potential should not be ruled-out at this

Temperatures will be running at normal to 10 degrees above normal
throughout the extended with no frost or freeze anticipated.
Following exit of the second system Thurs.-Fri, cold advection may be
sufficient to produce some frost in higher elevations Friday
Morning, though that is pretty speculative this far in advance.
Winds will be expected to be elevated as well with the passage of
the two frontal systems on Monday and Wed/Thu.


At KCLT and elsewhere: Cold air damming entrenched across the area
has led to low MVFR to IFR cigs at TAF time. -SHRA pushing through
the Piedmont should continue to dissipate, but additional -SHRA/TSRA
developing across the Savannah Valley will lift NE. Additional
rounds of -SHRA/TSRA possible through the overnight period. Best
chances for convection this aftn/eve are at KAND and Upstate TAFs,
with PROB30 everywhere overnight as cold front pushes through. Winds
will pick up out of the SE this evening, with low-end gusts possible
overnight, with direction veering SW with the front after daybreak
Friday. IFR to LIFR will persist overnight and into Friday morning,
with rapid clearing/lifting behind the front toward the end of the

Outlook: Drier conditions will return behind the front for the
weekend, but with unsettled weather and associated restrictions
again developing early next week.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High 100%     High  89%     Med   72%     Low   35%
KGSP       Med   78%     Med   78%     Med   72%     Med   65%
KAVL       High 100%     Med   66%     Med   72%     Med   75%
KHKY       Med   78%     High  83%     High  83%     Med   75%
KGMU       Med   78%     Med   78%     Med   72%     Low   48%
KAND       High  93%     High 100%     High  83%     Med   60%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:




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