Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 201 am EDT Friday Jul 3 2015 Synopsis... a broad upper level trough will remain over the southeast through the weekend...steering a series of weak low pressure systems over the region. These systems will increase rain and thunderstorm chances each afternoon and evening. Slight drying will occur on Monday and Tuesday...before another front pushes through late next week. High temperatures will be near normal through next week...with lows just above normal. && Near term /through today/... as of 200 am EDT Friday...light showers continue to work their way into the NC high terrain early this morning. Earlier convection across the SC upstate also continues on...however with some weakening experienced. No sig changes made to the immediate near term. Did tweak temperatures/dews to better reflect observation along with probability of precipitation to account for most recent radar trends. Full forecast discussion to follow within the hour. Previous discussion... As of 245 PM EDT...an outflow from earlier convection working southward over the western and southern part of the County warning forecast area has encountered increased instability...with 1500 to 2000 j/kg of SBCAPE over the upstate and NE Georgia this afternoon. Probability of precipitation have been ratcheted across the southern third of the forecast area for a few hours this afternoon...and have continued to back away from the higher probability of precipitation previously indicated by the mesoscale models for convection coming off the Blue Ridge into the NC foothills and Piedmont. This is possibly just the first wave of many...as additional shortwaves embedded in the 500 mb trough will cross the forecast area later this afternoon and early this evening. Yet more vorticity lobes will cross the base of the southern Appalachians later tonight through Friday as the trough deepens to the west from the lower Ohio Valley to the MS River Valley. Westerly 850 mb flow will increase tonight through Friday across the southern tier as the 850 mb low center deepens slightly over the Ohio Valley. The forecast area will stay solidly warm sector through the near term period as a surface wave moves along the stalled boundary from the lower Ohio Valley through the central Appalachians. Anticipate a smaller than usual diurnal range on temperatures with plenty of clouds/debris...and will need to shotgun high chance to low end likely probability of precipitation for much of the area beyond the immediate near term given the multiple shortwaves and moist profiles. Expect gusty winds with mixing again on Friday afternoon. Heavy rain will pose a localized Hydro threat just about anywhere that training occurs...with the SW mountains standing the best chance of flooding. Confidence remains too low for a Flash Flood Watch since coverage will be highest in the western mountains but rates will be better in more unstable areas. && Short term /tonight through Sunday/... as of 250 PM Thursday...upper trough axis will remain positioned west of the forecast area through the weekend...with unseasonably high precipitable waters (generally 1-2 Standard deviations above climo) through the period. A series of weak short wave moving through the trough axis will support higher-than-climo probability of precipitation through the period...with at least good chances for convection persisting into the nocturnal hours. This will especially be true on Sat night...as the most significant short wave (possibly developing into a closed low) nears the area. High chance to likely probability of precipitation will be carried in all areas... mainly from Sat afternoon through Sunday afternoon. Moist thermodynamic profiles and rather extensive cloud cover will limit cape through the period...and the severe threat should remain highly localized through the period. However...with precipitable waters remaining elevated... the threat for locally heavy and possibly excessive rainfall will be possible through the period. This threat is expected to be most prevalent across the far western NC mountains...where westerly low level flow will support upslope enhancement to coverage of convection and precipitation rates. Antecedent condition will probably also gradually deteriorate in those areas over the next 2-3 days as well. && Long term /Sunday night through Thursday/... as of 230 PM Thursday...the medium range begins 00z Monday with the area in an upper level split-flow regime. A quasi-stationary boundary will be located just to the south of the forecast area...providing a focus for diurnally enhanced probability of precipitation that extends across the southern Appalachians. The influence of the Bermuda high and persistent southwesterly flow will keep deep layer moisture plentiful throughout the period...though there does appear to be some relative slight drying of the airmass on Monday and Tuesday. Diurnal probability of precipitation near to just above climatology will be present both days. On Wednesday...weak upper level troughing will begin to build back into the forecast area as an upper low and associated frontal boundary take shape over the southeast...and precipitable waters climb back over 2 inches. Probability of precipitation are therefore above climatology toward the end of the period...and do not wane much overnight. Instability does not appear spectacular for any day in the medium range...but plentiful deep layer moisture throughout the entire forecast period will mean that 7-day quantitative precipitation forecast totals bear watching. Despite height rises throughout the medium range...moist profiles and associated cloud cover will keep maximum temperatures hovering around climatology. Min temperatures will be near to just above climatology. && Aviation /06z Friday through Tuesday/... at kclt...unsettled taf period ahead as rounds of restrictions prevail through the overnight and through the day on Friday. Initialized taf amidst broken IFR ceilings and light SW winds as stratus continues to spread in the very moist planetary boundary layer. Models forecast scattered rain showers around the region shortly after daybreak associated with an approaching upper shortwave and associated surface low to the north. Thus...introduced vcsh at 13z with a slight improvement to MVFR for ceilings. By late morning winds are expected to increase ahead of low level jet intrusion leading to gusting...along with prevailing rain showers and a prob30 for afternoon thunderstorms. Elsewhere...much the same as kclt above however with slight timing adjustments. Initialized all tafs with light/calm SW winds under mixed stratus decks with the SC sites getting IFR scattered layers. Intruding convection to the west at this time warrants overnight mention of showers therefore carried vcsh at all sites by 8z-9z aside for khky where onset will be closer to daybreak. Beyond that...precipitation chances increase further with all sites seeing prevailing rain showers with thunderstorms in the vicinity and or prob30s for thunderstorms in the afternoon. Winds will be quite gusty on Friday at the SC sites and kavl...less so at khky...all thanks to strong low level jet development which guidance favors at around 40kts/5kft. Outlook...a lingering frontal boundary will persist through the weekend into the start of the next work week. This will lead to on/off chances for showers each day...enhanced diurnally. Afterwards...the pattern becomes increasingly diurnal for the remainder of the weak. Restrictions will be possible associated with convection...and also each morning atop rain soaked areas. Confidence table... 06-12z 12-18z 18-24z 00-06z kclt high 83% high 85% high 97% high 100% kgsp high 84% high 85% high 99% high 100% kavl high 97% high 97% high 98% high 100% khky high 80% medium 71% high 97% high 100% kgmu high 88% high 83% high 99% high 100% kand medium 70% high 84% high 99% high 100% 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: (must be lower case) Www.Weather.Gov/gsp/aviation_tables && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...level near term...cdg/hg/wjm short term...jdl long term...level aviation...cdg

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