textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Snow chances continue to increase for the weekend, but the forecast remains far from certain.

KEY MESSAGES

1. The forecast low temperatures tonight have trended up a bit more, so a Cold Weather Advisory will not be needed over the mountains. However, patchy black ice Wednesday morning will result in slippery spots that could cause hazardous travel. 2. Extended cold wave continues, as temperatures will remain well below normal through the end of the week. Dry weather will also continue through Thursday. 3. Snow chances continue to gradually increase for this weekend in association with likely development of low pressure off the Carolina coast. If snow were to fall, impacts to travel can be expected due to the cold temperatures prior to onset.

DISCUSSION

Key message 1: The forecast low temperatures tonight have trended up a bit more, so a Cold Weather Advisory will not be needed over the mountains. However, patchy black ice Wednesday morning will result in slippery spots that could cause hazardous travel.

The relatively cold weather will continue through mid-week as we will remain under a broad mid/upper trof. A short wave dropping down through the bottom of the trof, on path that will take it south of our fcst area, will still drive a reinforcing cold front down across the mtns tonight and east of the fcst area Wednesday morning. Nothing too controversial here, but the front will bring some shallow moisture to the nrn mountains along the TN border, with cloud top temps cold enough for snow showers. A slight chc of snow was added there, but with no significant accumulation. Meanwhile, the modification of the arctic air mass, and some increased cloud cover, will help to keep the low temps tonight on the order of ten degrees warmer than this past morning. Thus, we may be merely ten degrees below normal for late January, but wind chill values should stay far enough above our Cold Weather Advisory criteria such that an advisory should not be needed.

We will be considering the issuance of another Special Weather Statement for patchy black ice across parts of the region for tonight/early Wednesday morning. There could be slippery spots in shady places and roads that still have melting snow or sleet piles on the shoulders.

Key message 2: Extended cold wave continues, as temperatures will remain well below normal through the end of the week. Dry weather will also continue through Thursday.

A deep longwave trough will remain entrenched across much of the eastern CONUS heading into tomorrow night and Thursday. Northwest flow will be in place tomorrow evening with continued gusty winds across the mountains. This will lead to low wind chill values overnight, but just shy of cold weather advisory criteria outside of the highest peaks and ridges. An isolated northwest flow snow shower cannot be completely discounted, but will be the exception and not the rule owing to a rather dry airmass and scant moisture. Well below average temperatures continue through the day Thursday and Thursday night as a period of qausi-zonal flow sets up from the Southern Plains into the Southern Appalachians.

Key message 3: Snow chances continue to gradually increase for this weekend in association with likely development of low pressure off the Carolina coast. If snow were to fall, impacts to travel can be expected due to the cold temperatures prior to onset.

The main focus of the forecast will be for increasing snow potential this weekend. Guidance is in good agreement in depicting several key synoptic features, but their eventual timing and evolution will heavily dictate what transpires across the region. Starting off on the hemispheric scale, our primary feature of interest is a strong Tropopause Polar Vortex (TPV) currently on the south side of the Hudson Bay as noted by anomalously high pressure on the 2 PVU surface. This feature is rotating around a much larger gyre and is forecast to move west towards the Canadian Prairies tomorrow into Thursday before dropping across the Northern Plains into the Midwest late Thursday into Friday. This potent arctic trough will quickly evolve from a strong positive tilt to a neutral tilt by Saturday as an intense upper low closes off. At the same time, a tall western ridge builds from the Great Basin through the Cascades while a second piece of energy drops out of the Rockies and into the Southern Plains. A subtropical jet will also extend from the eastern Pacific across the Gulf States. Eventually, a coastal low will likely develop along the East Coast.

Cold air will be readily available and supportive of a mainly snow event with any precipitation that occurs, however, the timing and evolution of these features will be key as to the magnitude of any snowfall. To realize a higher-end snowfall, snow lovers will want to see the arctic trough trend farther west and dig over the Deep South and phase with the second piece of energy dropping over Texas. This would also favor the development of a Gulf low along coastal baroclinic zone and a classic Miller A event. Another scenario exists where the troughs do not phase, the arctic trough is too far east, and the coastal low develops too far off the east coast and remains far enough offshore to leave us moisture starved. And thirdly, should the upper low close off over our area it could support a few inches of snow regardless of how the coastal low evolves. Expected temperatures will be cold enough that even an inch or two of snow could prove hazardous on roads. Thus, while the trend is towards at least some degree of accumulating snowfall, the forecast is far from certain and could trend either way through the week. The reader is strongly encouraged to check the latest forecast each day for updates.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: VFR at all terminals through the period, with wind being the only concern. Starting to see some deeper mixing across the region early this afternoon, so occasional low end gusts from the SW are still expected at most places through the afternoon until the boundary layer decouples around sunset. Clear sky. Tonight, a weak reinforcing cold front will drop down across the region, bringing a wind shift to WNW or NW late this evening or in the early morning hours. Some low level moisture could result in cloudiness along the TN border, but this is not expected to reach KAVL. After the front goes through, wind will go light/variable for several hours through mid/late morning, and then will come up from the WSW.

Outlook: VFR conditions are expected to continue thru most of the week under a cold and dry air mass. Guidance is beginning to trend toward a possible coastal low bringing a chance of snow to the region Friday night thru Saturday. Confidence remains low, but trends bear watching.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.