Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 231503 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1103 AM EDT Wed Aug 23 2017 .SYNOPSIS... A cold front will move through the region today, allowing for increasing precipitation chances. High pressure building in behind the front will introduce an extended period of unseasonably cool conditions that is expected to last through at least early next week. Although active tropical weather is anticipated near the Southeast early next week, this is unlikely to have any impact on the weather over our area for at least the next 7 days. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1030 AM EDT Wednesday: Front is working its way across the area at AFD time, and actually a tad faster than expected, so it`s kind of unclear how quickly temperatures will respond. Looking at new hourly data, guidance is wanting to keep temperatures down a little more than previously forecast, so did make some minor adjustments to hourly temperatures as well as afternoon highs, but only by a couple of degrees. NAM remains quite a bit higher with forecast CAPE this afternoon than the GFS (like 4x higher); Marginal Risk just barely scrapes extreme eastern Union County NC but the rest of the area is in General Thunder. Not overly excited about severe potential with minimal shear, but CAMs are progging some semi- organized multicell clusters moving across this afternoon, with another round after sunset. Made some minor adjustments to pops with this, but nothing too different at this time. Otherwise, cloud cover should steadily increase into the afternoon as the upper trough axis and associated cold front move eastward today and tonight. The front is still expected to transition from the Blue Ridge to the typical lee trough position by this evening, but some of the current guidance is hinting that it could be a bit sooner. Ahead of the front, the operational models are still generating up to roughly 1000 J/kg of sfc-based CAPE over the higher terrain while the rest of the CWFA has more like 1500-2000 J/kg during peak heating. Shear still looks weak with no more than about 25kts of bulk shear evident on model soundings. PoPs were lowered just a bit from the previous forecasts, mostly after 00z Thurs. The overall severe threat still appears low, as all the better ingredients remain north and east of the area, but a few damaging wind events could materialize today. High temps will be near to slightly above normal with values closer to normal over the higher terrain.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 320 AM Wednesday: An upper trough will dominate the synoptic pattern across the East Coast through the short term, with confluent mid/upper flow anchoring an expansive area of surface high pressure from the Great Lakes through New England, with associated ridging nosing into the Mid-Atlantic and the Carolinas. As such, cold front is expected to be south of our area by the start of the period. Although there are some hints in numerical guidance that weak surface wave formation will be a possibility, which could result in some shower development across the southern/southeast zones Thursday, this appears to be an outlying/low probability scenario, so only token slight chance pops will be maintained across the southern tier of zones, mainly in the morning. Otherwise, drier and cooler air will filter through much of the forecast area. Max temps should be around 5 degrees below climo (lower to mid 80s across the Piedmont and foothills). In fact, Thursday will signal the onset of a protracted period of seasonably cool conditions, which should last well into the medium range. Friday should be rather similar to Thursday, with mostly sunny/partly skies, relatively low dewpoints and sub-climo temps. By the end of Friday, the low level flow is expected to become more pronounced out of the E and SE, with increasing low level moisture supporting gradually expanding sky cover, as well as increasing pops, especially near the Blue Ridge, although pops should be no higher than 20 percent through Fri night. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 330 am Wednesday: The pattern will become somewhat stagnant during the medium range, although it will not at all be the type of stagnant pattern that one would typically associate with late summer. Cool surface high pressure is expected to maintain its grips across much of the eastern Conus through at least the start of next week. In fact, temps are only expected to trend downward through the period, as easterly flow becomes increasingly moist, while some degree of cold air damming could become established by the end of the weekend, as the surface high across the northern Conus moves into a more favorable position. Thus, while max temps begin the period almost 10 degrees above climo, they are forecast to be as much as 15 degrees below normal by the end of the period. The increasingly moist easterly flow will also result in gradually increasing precipitation chances through the period, with generally 20-40 pops advertised across much of the area from Friday afternoon through the end of the period. Of course, the elephant in the room is the goings-on in the tropics. As has been the case for the past 24 hours, it is pretty clear that the Southeast will be sandwiched between two tropical cyclones early next week: a reinvigorated Harvey somewhere in the vicinity of the northwest Gulf of Mexico, and something else off the Southeast coast. The synoptic pattern being what it is, this latter storm would bear no threat to the interior Carolinas. Harvey is a different story. However, it continues to be the case that the GFS is the only model that is allowing the cyclone to be picked up by a short wave trough digging into the Mississippi Valley early next week, resulting in tropical moisture overspreading the Southeast by Day 7. Pretty much all other global model guidance, including the 00Z GEFS mean surface fields, suggest that Harvey will plague the Texas and Louisiana coasts well into next week before making any significant inroads inland. Thus, there is a strong consensus that any potential tropical impacts for are our area would occur after Day 7 (if at all). && .AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
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At KCLT and Elsewhere: Only change to the KCLT TAF for the 15z AMD was to delay the VCSH from 16 to 17z. Timing for convection still looks good. Otherwise, showers and embedded thunderstorms are expected to redevelop over the region from the west as the boundary moves slowly across the fcst area today/tonight. Thus, I kept TEMPOs and PROB30s for TSRA at all sites beginning in the afternoon and going well into the evening. Convective activity should be done for most locations by late tonight, however a few lingering showers could persist into the overnight. Winds are expected to turn NWLY and become gusty at KAVL after roughly 21Z, but the fropa could be a bit more delayed toward the end of the period for the foothills and piedmont sites before finally transitioning into a piedmont trof setup. Outlook: The front should push through the region by early Thursday and allow drier air to work in from the north. Thus, the probability of restrictions and diurnal convection should diminish Thursday and Friday. Low level moisture may make a return toward the Blue Ridge over the weekend. Confidence Table... 15-21Z 21-03Z 03-09Z 09-12Z KCLT High 100% High 100% High 100% High 100% KGSP High 100% High 100% High 100% High 100% KAVL High 96% High 100% High 100% Med 71% KHKY High 100% High 100% High 100% High 100% KGMU High 100% High 100% High 100% High 100% KAND High 100% High 100% High 100% 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: www.weather.gov/gsp/aviation
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&& .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JDL NEAR TERM...JPT/TDP SHORT TERM...JDL LONG TERM...JDL AVIATION...JPT/TDP

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