Scientific Forecaster Discussion

000 fxus62 kgsp 172004 afdgsp Area forecast discussion National Weather Service Greenville-Spartanburg SC 304 PM EST sun Feb 17 2019 Synopsis... a moist warm front will continue to move north from Georgia tonight as low pressure crosses Tennessee. Expect Canadian high pressure ridge down into our area from North Dakota by late Monday giving US a short dry period. Rain returns on Tuesday as a series of low pressure systems will track from southwest to northeast across the region, resulting in a persistent moist pattern through most of the week. && Near term /through Monday night/... as of 251 PM EST sunday: starting to see an uptick in the coverage of light precip moving quickly east-northeast across the region, perhaps a sign of the improving low level isentropic upglide. Fcst appears to have this taken care of, but some minor adjustments are in order. Temp trends still look OK. Cold air damming wedge will remain locked in across the region into the overnight hours, now that a chunk of the parent high as broken off over the mid-Atlantic region. That part of the fcst remains unchanged, as does much of the thinking about the rain tonight. Upper forcing is not particularly strong, and some DPVA will move thru later this evening, but what we lack at mid/upper levels will be made up by excellent low level isentropic lift. The upglide continues to improve into the evening hours and gets strong in the 295k-300k layer after 00z, which should drive the development of a large area of rain that is expected to move across the fcst area in the 00z to 06z time range. The improving SW flow should focus the heavier precip over the SW-facing mtns, meaning the biggest threat for excessive rain will be in the little Tennessee River valley. There is some concern about precip amounts and how that might relate to flood potential. As it stands right now, our quantitative precipitation forecast is only on the order of half the 1hr ffg, so think we should be able to tolerate what falls. That being said, it will only serve to further increase our potential for flooding later this week. The precip should end steadily from the west with the passage of a sfc wave and the ending of the isentropic lift. Will hold onto some higher chances on the Tennessee border a bit past daybreak, but that should also end quickly. Downslope quickly ensues by midday Monday, so we should clear out and warm up nicely for Monday afternoon. Might be the only decent weather we get all week. && Short term /Tuesday through Wednesday/... as of 255 PM EST sunday: the fcst will start off dry across the area as a broad Canadian high builds south across the glakes region and begins to ridge down the East Coast. Temps will drop to arnd freezing across the NC mtns while a developing warm front begins to approach the area arnd daybreak. There is some uncertainty with the level of moisture with this front as the NAM continues to have very little qpf response by 12z Tue. Likely a null chance of any wintry precip Tue morning as any light precip shud not arrive until temps warm abv freezing. Temps wont rise much during the day...probably remaining 40s all locales as sfc-based cold air advection continues and cloud cover increases. The fcst becomes quite tricky Tue night as a parent 1040 mb high sets up across the ern glakes. The high config looks to be classical in nature and bad for wintry precip across the colder NC mtn areas as a stg llvl SW/ly jet brings in a sigfnt warm nose aloft. Model temp guidance has trended down overall so mixed in lower values with the previous fcst numbers. This gave more areas for accum fzra over the NC mtns and even a -ra/fzra mix across the nrn fthills and nrn NC Piedmont. Temps look to hover just north of freezing generally outside the nrn mtns...but a degree or two difference in sfc tw/S could create wintry/icy precip. The amt of precip is not totally certain as there could be robbing convec south and the overall depth of saturation will likely be limited to arnd h7. Therefore...ice nuclei may not be activated esp as the morning progresses...per the latest GFS/NAM soundings. Thus...will anticipate an onset of freezing precip arnd 02z with likely a switch to -fzdz...before a changeover to all rain occurs arnd 14z. Expect a light glaze of ice at most across the non/mtns...and the best ice accum potential of a couple tenths over the nrn NC mtns and high elev nrn fthills...and perhaps even further south across the black mtns. Right now...this event is certainty adv level and as the fcst changes warning level conds could develop. A light cold rain will continue thru period with temps only reaching the l40s north and l50s over the far SW/rn zones as The Wedge is slow to lift out of the area. Another issue Tue night into Wed will be the potential for stream flooding across the SW/rn NC mtns. Wpc has the area in a day 3 slight risk of flooding the favored upslope SW-facing regions will likely recieve arnd 2 inches to near 3 inches by then in continued high saturated soils. && Long term /Wednesday night through Sunday/... as of 200 PM Sun: an active pattern is still depicted across the County warning forecast area throughout the medium range, still with a strong ridge centered over the eastern Caribbean and deep troughing over the southern rockies and vicinity. As low pressure traverses the Great Lakes region early Thursday, a cold front will move thru the lower Mississippi Valley, then pivot across the Appalachians as the sfc low moves thru New England. That will promote erosion of the midweek cad affecting our area, though with the boundary lingering, unsettled weather will persist. Furthermore, global models depict yet another seasonably strong sfc high moving eastward north of the boundary, suggesting cad will return by Friday. Temperatures within the weekend cad event currently do not look cold enough for wintry precip, so the main impact initially may just be to prolong the already cloudy/damp conditions. Model consensus shows pattern change finally occurring next weekend as a shortwave ejects from The Rockies, cyclogenesis occurs somewhere to our northwest, and a cold front swings thru the southeast. This front is most likely to reach our area on Sunday. The European and Canadian solutions depict a stronger low and better defined cold front than does the GFS, though all of them show at least modest instability in the warm sector preceding the front, while cad persists for some of our area. These trends are Worth watching as they imply some severe weather threat may result for those parts of our area along/south of The Wedge boundary. All in all it is difficult to identify any meaningful break in precip chances between Wednesday evening and Saturday morning, so we will advertise at least likely pops for nearly the whole area during that timeframe. A limited diurnal range is expected due to cloud cover and/or wedging, but overall temps will remain near climo. Chances were allowed to drop a bit by Saturday afternoon as warm front shifts northward ahead of the developing low. && Aviation /20z Sunday through Friday/... at kclt and elsewhere: MVFR ceiling firmly locked in with the cold air damming wedge at kclt, and suspect that it will transition to the lower half of the MVFR range in the next hour or two. Wind should stay northeast. First question is when ceiling will finally drop down to IFR. Guidance has been too fast with this lowering of the ceiling and have been pushing it backward all day. Leading edge of IFR just reaching kgsp as of 18z, so have decided to play the trend and keep the IFR out until this evening. What remains to be seen is if we will get a transition to a light southeast wind late this afternoon as guidance indicates. Am skeptical. If that happens, it is likely that light wind will eventually come back around to something with a north component. After sunset, with the resurgence of the low level upglide, expect ceiling to come back down to IFR around 01z. In all likelihood, we will see periods of LIFR and that will be indicated with a tempo around sunrise. Could have a problem with low level wind shear during the early morning hours as strong low level jet passes just to the south. Will hold off for now. The passage of the low will bring a quick improvement to the ceiling during mid-morning Monday, with wind strengthening from the northwest. Outlook: after brief improvement Monday, a series of low pressure systems will move through or near the region through much of the week. This will produce precipitation and associated restrictions for long periods of time during each wave of precipitation. Confidence table... 20-02z 02-08z 08-14z 14-18z kclt Med 75% high 83% Med 78% high 81% kgsp low 55% high 92% Med 71% high 100% kavl high 86% high 81% Med 74% high 100% khky Med 70% high 81% Med 76% high 100% kgmu Med 61% high 94% Med 68% high 100% kand Med 76% Med 79% Med 61% high 91% 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 && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...deo

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