Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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406 FXUS62 KGSP 231751 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 151 PM EDT Thu May 23 2024 .SYNOPSIS... Temperatures will remain several degrees above normal each day through Monday as humid subtropical air remains over the Southeast. Chances for showers and thunderstorms will remain higher than normal, with some chance lingering each night. A cold front passing late Monday will bring cooler and drier conditions in the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... As of 1236 PM EDT Thursday: Unexpected convection over eastern TN has wrought havoc on the forecast for today, as although it shouldn`t incite any activity for us, it appears to be somewhat stabilizing parts of east TN. The result is that some of the rapid-refresh guidance is slowing down the progression of convection later this afternoon and evening. Weak cells are just firing up over the NC-TN border, though, so PoPs didn`t require too much tweaking. The trend is toward the severe risk coming later in the day, however. Evidence in water vapor imagery of a shortwave making tracks across the upper Mississippi Valley, which when it arrives here should still uncap profiles as originally expected and set us up for thunder from there on out. Today still looks to be the start of a more convectively active pattern, as the Southeastern upper ridge gets flattened and allows a series of embedded shortwaves/MCVs to cross the region. The most pronounced impulse appears to be migrating into the upper Mississippi Valley attm, and this feature will reach central to eastern TN by early aftn. Convection is expected to fire with this impulse just west of the forecast area. Meanwhile, there may be a lot of cirrus over the area that may help delay convective initiation until mid aftn. From there, mid-level cooling is expected atop the forecast area, as heights continue to fall thanks to the flattening ridge. This should erode the cap and allow convection to fire across the high terrain, with steering flow taking activity into the Piedmont late aftn thru the evening. The CAMs are not in good agreement on the exact timing or the storm mode. The 00z HRRR was very active and showed some strong multicell clusters tracking thru the eastern half of the FA. But the last few runs have been more subdued. The two HiResWindow CAMs are quite active, while the NAM is in between. So all that to say, confidence is below average on convection/PoPs. Will stick with the hourly NBM, which is a little lower than previous fcst. As for severe potential, the new Day 1 SPC outlook keeps the entire area in a marginal risk. This seems reasonable, although convective coverage across our NE GA Piedmont and southern Upstate is expected to be low. Moderate instability of 1500-2000 J/kg of sbCAPE with 0-6 km bulk shear of 35-45 kt should support some organization of multicells and possibly some rotating storms. So damaging winds and large hail will be a threat. The tornado threat should be low, however, as the low-level shear will remain weak. Temps are expected to be similar to yesterday, about 5-7 degrees above normal. Tonight, convection should drift east of the area, with some clear expected west to east overnight. If precip coverage is decent today, we could see some areas of fog form as winds should be light. This will mainly be in the mountain valleys. Lows will be mainly in the 60s, a few degrees above normal. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 230 AM Thu: Confidence has not really increased in the forecast for Friday and Saturday. The Southeast states will remain in the warm sector of a low pressure system in the northern Plains as it matures and begins to occlude, and very large diurnal instability is expected to develop in the lower Mississippi Valley. For our CWA, temperatures will remain several degrees above normal and dewpoints generally in the lower to mid 60s and we too should be able to generate respectable CAPE each afternoon. One of the factors limiting confidence is the quasi-zonal, slightly anticyclonic upper pattern, which will allow any weak shortwaves generated upstream to drift over our area with relative ease. Models remain in some disagreement as to the nature and timing of these waves. The weak waves, upstream instability and westerly steering flow together suggest MCSs could develop in the Cumberland or TN Valleys and make a run at our area either Friday night or Saturday night, which might suppress convection the following days. PoPs remain slight-chance to chance overnight both nights this period as a nod to that possibility, particularly in the western CWA. There remains decent agreement for a fairly well-defined wave to cross the area Friday afternoon, which still suggests PoPs above climo (40-50% Piedmont and 60-70% mountains). Shear has trended a bit downward but still enough to suggest loosely organized clusters of storms producing marginally severe hail and/or wind. Upper ridging may build slightly over the area Saturday, and midlevel drying looks to occur if the shortwave does pass Friday as anticipated. That permits stronger sfc-midlevel delta-theta-e and an increased risk of damaging wind. However, without the enhancing effect of the shortwave and with post-MCS subsidence or cold pool somewhat more likely, PoPs overall are 15-20% less on Saturday afternoon compared to the previous day. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 310 AM Thu: A cold front will linger in the mid-Mississippi and Ohio Valleys into Sunday, being left behind by the dying low then having moved into central Canada. The next low spinning up in the lee of the Rockies will reactivate this front and push it toward the Mid-Atlantic states, reinforcing the already hot and humid airmass in place over our CWA. Heights fall slightly as a shortwave rides over Sunday invof the warm front, and upper trough in the Plains will take on a negative tilt. PoPs trend upward again on Sunday although the best DPVA should occur to our north. Shear presently looks likely to increase over 40 kt and risk of organized severe storms thus increases as well. The developing cyclone looks likely to bring a cold front to the area perhaps as soon as Sunday night, but more likely Monday (Memorial Day). We reintroduce likely PoPs in the mountains Sunday night and across the area Monday. Severe risk would appear to continue until the front passes. Upper trough over the region Tuesday and Wednesday suggests small chances for showers beneath it. Temperatures trend back to about normal Tuesday and slightly below normal Wednesday. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... At KCLT and elsewhere: The TAF forecast is difficult, with every model basically choosing its own version of reality and presenting a different picture of how the afternoon and evening will evolve. Currently (c. 1745z) cells are just starting to fire up over southeast TN and northeast GA. The expectation is that these cells will translate eastward, becoming greater in coverage, as the afternoon wears on. In general, the timing of best coverage looks to be later - post-4pm - for most terminals than advertised in previous forecasts. Guidance has also doubled down on the idea of widespread fog tonight outside the mountains, in response to rain from this afternoon/evening`s thunderstorms. For now, have added mention of low-VFR/MVFR vis at KCLT and the Upstate terminals, and IFR at KHKY and KAVL. For KHKY and KAVL, it`s unclear if things will clear out enough to permit stratus development...but IFR mountain valley stratus is certainly not out of the question. Winds will be out of the SW for most of the period (variable, of course, during thunderstorms) but may turn slightly N of W during the evening or overnight hours after convection slides east. Expect overall less coverage of storms on Friday, but at least some convection seems likely. The evolution of such activity remains in question, and confidence on either timing or location is very low. Outlook: A low-confidence pattern emerges for the weekend, with some rain likely on Saturday. Sunday and beyond, an unsettled pattern will set in with diurnal rain/thunder chances and mountain valley fog possible each night. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...Wimberley NEAR TERM...ARK/MPR SHORT TERM...Wimberley LONG TERM...Wimberley AVIATION...MPR