textproduct: Columbia

WHAT HAS CHANGED

Snow totals have increased, especially in the northern portion of the forecast area where a major snow storm is possible. Extreme Cold Watch is in Effect Saturday evening through Sunday morning with increasing confidence in wind chills near 0F.

Aviation discussion updated for 18z TAF issuance.

KEY MESSAGES

- 1) A Winter Storm Watch is in effect as a major snow storm is possible to impact the area this weekend.

- 2) Well-below normal temperatures likely continue into early next week. An Extreme Cold Watch now in effect Saturday night into Sunday morning.

DISCUSSION

KEY MESSAGE #1: A Winter Storm Watch is in effect as a major snow storm is possible to impact the area this weekend.

Model guidance overall has tended to converge on a solution for he 500mb trough that has been mentioned as the main feature to watch for the potential for significant snow across the area. Models continue to favor a deepening trough to the west which favors increasing snow totals across the forecast area as the trough moves from positively tilted to neutral and finally negatively tilted. This will allow moisture to pull in from the Atlantic as a low deepens off the coast of the Carolinas. This is a favorable climatological setup for our most notable historical snowfall events with cold air in place and upper troughing over the eastern US, reinforced by the much colder air mass to the west moving into the area. As a result, unlike last week, this expected to be a mostly snow event (although at the onset some brief mixing is possible which will quickly turn to all snow). Strong frontogenesis expected just north of this upper low and the accompanying 850mb low pressure system will lead to some stronger banding of snow leading to higher rates and thus more accumulation. While it is likely that much of the area will receive accumulating snow reaching Winter Storm Warning criteria (2 inches), have opted to maintain the watch, mainly because impacts will likely not start until Saturday morning, giving us more than 24 hours.

Highest confidence in major impacts from accumulating snow remain in the northern portion of the area with 5-8 inches in the forecast and higher amounts not out of the question at least in part of the area with the aforementioned banding. Another thing to note is that typically our climatology snow-to-liquid ratio is around 8:1 but with the cold air moving in, blended guidance is favoring SLR closer to 15:1 to maybe even 20:1 which means that the snow will be a lot fluffier than we typically see. This means that less rainfall will attribute to more snow accumulation. With the strengthening low off the coast, wind gusts above 30 mph will be possible which would lead to blowing snow and reduced visibilities as well as very cold wind chills (mentioned more in key message 2). While there remains a bit more uncertainty in the snowfall amounts in the southern portion of the forecast area, still think it is likely that the entire forecast area sees at least 2 inches of snow.

Timing: Snow is expected to begin late Friday night into Saturday, although a mix is possible at the onset when precipitation will be light. A transition to all snow is expected Saturday morning with snow continuing through Saturday and into Saturday night for much of the area.

KEY MESSAGE #2: Well-below normal temperatures likely continue into early next week. An Extreme Cold Watch now in effect Saturday night into Sunday morning.

Ahead of the incoming winter system, troughing is expected to continue over the area with below normal temperatures as overnight lows tonight and tomorrow night likely fall into the 20s. The main concern with this extended period of cold weather is the reinforcing shot of cold air that moves in behind this weekends winter storm. With the increasing likelihood that snow will be on the ground Saturday night into Sunday morning and gusty winds between 25-35 mph, a frigid night is expected. Blended guidance currently sits near the NBM 75th percentile for overnight lows (in the mid to upper teens) but even the 50th percentile brings lows down into the low 10's to mid teens, which seems more likely. With the breezy winds in place as well, wind chills between 0F to 5F will be possible across the area, with spots dipping as low as -3F. Due to this, an Extreme Cold Watch is now in effect Saturday evening, continuing into Sunday morning. Another night with temperatures into the low 10s, to mid teens is possible Sunday night and into Monday where cold weather products will likely be needed again.

Overall, prolonged cold weather persists into this weekend, but cold weather products are not likely tonight or Friday night. A Extreme Cold Weather watch is in effect for the whole FA Saturday evening and into Sunday morning where another product could be needed Sunday night and into Monday morning. Problems with poorly insulated pipes can be expected and be sure to minimize time outside for people and pets in these very cold conditions.

AVIATION /19Z THURSDAY THROUGH TUESDAY/

VFR conditions expected through the TAF period.

Light and variable winds are being seen this afternoon as some cirrus move into the region. Overall, this trend will continue overnight and into Friday with VFR conditions and winds that go calm in some spots overnight tonight. Cirrus will continue to move over the region through the end of the TAF period but winds should begin to pick up to out of the southeast to south by the late morning to early afternoon on Friday. EXTENDED AVIATION OUTLOOK...Impactful aviation weather becomes more likely late Friday night into the weekend as a low pressure system moves into the region with the possibility of wintry precipitation mainly Saturday.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Winter Storm Watch from Friday evening through Sunday afternoon for SCZ016-018-020>022-025>031-035-037-038-041- 115-116-135>137. Extreme Cold Watch from Saturday evening through Sunday morning for SCZ016-018-020>022-025>031-035-037-038-041- 115-116-135>137. GA...Winter Storm Watch from Friday evening through Sunday afternoon for GAZ040-063>065-077. Extreme Cold Watch from Saturday evening through Sunday morning for GAZ040-063>065-077.