Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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286 FXUS62 KGSP 241728 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 128 PM EDT Tue Sep 24 2024 .SYNOPSIS... A moist and cooler airmass will remain in place across the forecast area through mid-week. Tropical Cyclone Helene is forecast to make landfall in the Florida panhandle late Thursday, with the remnant circulation moving quickly north and bringing wind and heavy rainfall impacts to our area Thursday night into Friday. Calmer weather over the weekend with high temperatures a little cooler than normal. && .NEAR TERM /THROUGH TONIGHT/... 1000 AM Update...The forecast is running rather steady state this morning thanks to a shallow layer of StCu engulfing much of the non/mtns. These clouds will be slow to dissipate as warming surface levels and thermal lift take some to get going. Any precip over the next few hrs will be brief and light, with maybe an isol strike of lightning across NE GA. Temps and td/s are running well within the fcst curve and only made locally minor tweaks. Otherwise, today will be similar to yesterday (Monday) with morning low clouds slowly eroding and lifting from SW to NE thru the morning and into the afternoon. Low pressure will develop to our NW over the Ohio River Valley and help keep a broad area of moist air in place over the region. Moderate instability along with 30 to 45 kts of deep- layer shear can be expected across our area during the afternoon and evening. Expect sct convective coverage during the afternoon with the best coverage more likely during the evening hrs. The severe potential looks better for our area today/tonight with SPC`s current day 1 Con- vective Outlook having most of our CWA under a Marginal Risk for svr thunderstorms. The main threats will be damaging wind gusts and hail. Otherwise, high temps will top-out roughly 3 to 6 degrees above normal with our SW zones approaching 90 degrees again. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... As of 245 am Tuesday: A strong upper low is forecast to be centered near the confluence of the Ohio and Mississippi Rivers at the start of the period, slowly wobbling south through Thursday, as what is expected to become Hurricane Helene makes landfall in the Florida panhandle Thu evening. Throughout Wednesday, an inverted surface trough is forecast to organize across the region...most likely centered across the TN Valley, while increasing southerly mid/upper level flow on the east side of the upper low begins to draw a plume of tropical moisture from Helene into the region. The interaction of this plume with increasingly deep low level convergence in the vicinity of the surface trough. This could result in evolution of a heavy predecessor rainfall event (PRE) from Wed into Wed night, which may incorporate much of southwest NC, eventually including an upslope component as southeast flow accelerates above the surface layer late Wed through early Thu. Whether the axis of heavy rain sets up within, or just west of our forecast area is uncertain at this juncture...it just needs to be noted that widespread amounts of at least a couple of inches could accumulate across a portion of the area before direct impacts from Helene arrive. Regardless, PoPs will increase throughout Wed as spokes of vorticity rotating around the upper low approach the forecast area and interact with a moderately unstable and increasingly moist environment. Other than the potential for lingering upslope showers on Thursday, there may be a bit of a lull in precipitation through much of Thursday, as inverted surface trough shifts well west of the Appalachians. Meanwhile, Helene is expected to lift quickly N/NW through Georgia Thursday night, with the center likely passing near the intersection of GA/TN/NC Friday morning. This will place at least the western part of the forecast area on the favorable right side of the cyclone track for heavy tropical rain bands. Heavy rainfall may pile up quickly Thu night, with excessive rainfall and at least isolated flash flooding becoming likely, especially from northeast GA and the Upstate into southwest NC. By noon Friday, total rainfall amounts (including anything falling prior to Helene`s arrival) are expected to range from 4 to 7 inches across much of the area west of ~I-77. Locally higher amounts as high as a foot are expected across southwest NC...especially along the Blue Ridge escarpment. Confidence in winds and wind gusts remains modest at best. However, a consensus of the latest short term guidance suggests Helene will still be of solid tropical storm intensity as it passes near the CWA Thu night. Our current best guess is that much of the western half of the area will see plenty of 35-50 mph gusts, with damaging wind gusts very much a possibility just to the east of the cyclone`s center. Being on the right side of the cyclone also poses some concern for isolated tornadoes, as shear parameters will be more than adequate for rotating cells within rainbands. However, tornadoes tend to have a strong diurnal correlation in association with inland tropical cyclones, and guidance suggests surface-based instability will be largely absent by the time rainbands arrive Thu night, so the tornado threat appears to be small (but non-zero) at this juncture. Otherwise, wind and rain are expected to steadily diminish throughout Fri morning, with quiet weather returning by evening. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... As of 335 am EDT Tuesday: An upper low is still expected to be wobbling near Memphis at the start of the extended, but should finally weaken and be nudged into the eastern Conus early next week as a strong short wave trough grazes the north central part of the country. After a brief lull in significant precip chances over the weekend, PoPs begin ramping up again early next week, albeit remaining in the chance range. Temperatures begin the period close to normal, but fall below normal early next week, when global model guidance suggests development of cooling E/NE flow in association with an inverted surface ridge/potential cold air damming. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... At KCLT and elsewhere: Low CIGs will persist this afternoon and begin to lift to MVFR/VFR as a line of -shra/tstms move thru the area. Have TEMPO thunder all sites this afternoon and a few storms may be capable of reaching severe levels with high deep shear levels and and increasing instability. The overnight remains active with precip continuing mainly across the mtns and fhills while flight conds reduce to IFR or lower due mostly to CIGs thru the morning hrs with a better chance of LIFR VSBY developing at KAVL. The atmos remains unsettled Wed with precip chances returning more widespread across the mtns as CIGs take a while to improve out of IFR conds. Winds will remain weak and align generally se/ly to sw/ly. Outlook: Fog/low stratus will likely develop over the next few mornings, especially in the mtn valleys. A tropical cyclone will bring heavy rain and strong winds Thursday into Friday before moving out of the area Friday night into Sat. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JDL NEAR TERM...JPT/SBK SHORT TERM...JDL LONG TERM...JDL AVIATION...SBK