textproduct: Columbia

WHAT HAS CHANGED

Confidence is increasing in the potential for significant snowfall this weekend. Confidence is highest in the northern half of the forecast area.

KEY MESSAGES

- 1) Probability of impactful snow continues to increase this weekend.

- 2) Well-below normal temperatures will continue through early next week.

DISCUSSION

Key Message #1: Probability of impactful snow continues to increase this weekend.

Upper low pressure system centered near the southern tip of the Hudson Bay will be the main feature we will be watching into this weekend for the potential for significant snow, the probability of which continues to increase.

Strong agreement among global models and ensembles that this upper low will dig southward, ushering in a significant cold air mass Friday night into Saturday for the Mississippi Valley, where EC EFI indicates a large area of below -0.8 to -0.9 for minimum temperatures with a highly anomalous cold air mass settling in to our west. By Saturday morning, the very deep 500 mb trough (NAEFS mean indicating that heights will be near climatological minimum) is likely to be positively tilted and its progression through the day Saturday is where uncertainty is introduced into the forecast. This upper trough will lead to strong lift and strengthening cyclogenesis along the coast of the Carolinas. Generally, there remains good agreement that there will not be a lot of mixed precipitation types with this event. Guidance continues to favor mostly a rain-snow mix at the onset (some brief period of mixed precipitation is possible), which would likely transition to mostly snow as the reinforcing very cold air mass and the upper trough shifts over the area, limiting any warm air aloft. There is increasing confidence in moderate impacts from accumulating snow, which now sits at greater than 50% over most of the forecast area. There remains uncertainty as to some of the mesoscale details but increasing confidence in an area of potentially major impacts from accumulating snow where heavier banding sets up, although the exact location of this is difficult to determine (it may remain north of the area).

Bottom line: Prepare for the potential for accumulating snow beginning as early as Friday night and continuing through Saturday and into Saturday night. In the synoptic scale, guidance continues to converge on a solution and confidence remains relatively high in the potential for accumulating snow. The mesoscale details will still need to be worked out in the coming forecasts. Highest confidence in a significant snow event remains north of Columbia where blended guidance indicates high probability (especially for several days out) of 60-80% of Winter Storm Warning level (2 inch) snow amounts. While we have held off on issuing any watch products for the afternoon package, it is increasingly likely that a Winter Storm Watch will be issued tonight for at least portions of the forecast area. While this has been mentioned in the other key message and earlier in this discussion, it bears repeating that the air mass will be very cold that moves over the area. This will lead to greater impacts for any snow that does accumulate as surface temperatures will be on the low side and the potential for higher than climatology snow to liquid ratios.

Key Message #2: Well-below normal temperatures will continue through early next week, with the possibility of reaching extreme cold criteria.

Cold weather will persist through the weekend and into early next week. Deep and broad mid and upper level troughing will keep temperatures below normal for much of the eastern CONUS including the SC Midlands and CSRA. This will result in highs in the 40s (perhaps 50 by Friday) and lows in the 20s through Saturday morning. An additional shot of cold air is expected to accompany this weekend's winter system, particularly during the Sunday-Monday period. Additionally, wind gusts of 15-20 mph Saturday night/Sunday morning could lead to wind chill values in the single digits. The 72- 96 hour EC EFI for min temps shows -0.7 to -0.9, while the 96-120 hour period is even stronger at -0.9 to -0.99 over most of the area. For high temperatures, these values are -0.6 to -0.8 and -0.8 to -0.95 for these periods, respectively. While we still don't expect to be near any cold weather headline criteria until this coming weekend, the persistent cold weather could present a compounding problem with poorly insulated pipes and anyone without access to sufficient heating.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

VFR conditions expected through the TAF period.

High pressure centered to our west remains in control through the TAF period. This will bring mostly clear skies, with only passing high clouds from time to time. Westerly winds may be a bit breezy with occasional gusts of 15-20 kts. Gusts diminish after sunset, but remain elevated through early Thursday morning. Then, expect northwest to northeast winds through much of the rest of the period at around 4-7 kts. No restrictions expected.

EXTENDED AVIATION OUTLOOK... VFR conditions likely through early Friday, then impactful aviation weather becomes possible as a low pressure system likely moves somewhere near the region bringing the possibility of wintry precipitation.

EXTENDED AVIATION OUTLOOK... VFR conditions likely through early Friday, then impactful aviation weather becomes possible as a low pressure system likely moves somewhere near the region bringing the possibility of wintry precipitation.

CAE WATCHES/WARNINGS/ADVISORIES

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