textproduct: Columbia

WHAT HAS CHANGED

Confidence continues in a potentially impactful winter storm this weekend, with some small tweaks in the forecast thinking.

KEY MESSAGES

- 1) After very cold start Wednesday, temps steadily moderate through Friday along with some rain chances.

- 2) The trend continues towards a potentially significant winter storm this weekend.

DISCUSSION

Key Message 1: After very cold start Wednesday, temps steadily moderate through Friday.

As high pressure traverses the area, temps will slow warm up through the end of the week following very cold temps this morning. Temps will push back to around average both Thursday and Friday, and with building heights and moisture, showers are possible Thursday and Friday. These will generally be isentropic, upslope flow type showers so they generally favor the Upstate and down into the western Midlands.

Key Message 2: The trend continues towards a potentially significant winter storm this weekend.

Overview: The forecast thinking has not changed much for the weekend system with potential for a highly impactful winter storm continuing. The overall synoptic pattern is quite consistent across all guidance with a deep digging trough off the NE CONUS, strong confluence and an associated strong surface high in the central- eastern CONUS, and an ejecting cutoff shortwave in the SW CONUS. This patterns sets up a broad overrunning scenario as the arctic high digs southeastward presenting an all-hazards impactful winter storm potential for much of the southern and eastern US, including GA and SC as strong cold air damming sets up.

Trends and Forecast Challenges: While some disagreements continue, guidance remains in above-typical agreement over the potential impacts from this system given the 72-96 hour window, with historic analogs and climatology concurring; this setup distinctly favors mixed precip with snow-sleet-freezing rain-rain potential. A subtle shift in some of the 18z guidance and more evident in much of the 00z guidance is a northerly shift in the axis of heaviest qpf, thanks to a slightly more amplified flow and associated low pressure development; this also pushes the surface edge of the CAD dome north. This trend in guidance is seemingly driven by a very subtle slowing of Pacific cutoff ejection and positioning of the broad surface high; aircraft recon early Wednesday morning should help improve this sampling, but not until the 12z guidance. Thanks to this trend, the GEFS has steadily become more and more of a cold- southerly outlier compared now with the ECE, Canadian Ensemble, and AI suite being warmer- northerly. As noted previously, shifts in the handling of both of the low level cold air and the Pacific cutoff are expected as these are two of the most notoriously difficult meteorological features for guidance to handle. The current trends, and anticipating typical model biases, reinforce the thinking that the I20 corridor will potentially lie along a steep gradient zone for qpf and therefore potential winter impacts.

Potential Impacts: A range of impacts continues to be possible from this system, ranging from a notable ice storm to a inconvenience- nuisance event with a gradient of impacts across SC and eastern GA looking more likely. A mix of precip-types is expected, but trends over the last 24 hours suggest snow is less likely for central SC and eastern GA. Confidence remains high that much of, if not the entire, area will see at least some wintry weather from late Saturday-Sunday, however specific geographic impacts remains unclear. Based on the guidance trends, confidence is increasing that the I20 corridor will lie along the gradient of impacts; the probabilistic WSSI for moderate impacts summarizes this well, with ~60-80% chance north of I20 and then ~20-40% chance south of I20. While details in the spatial impacts are still unclear, the high end ceiling for this system remains, with notable ice storm potential and lingering cold weather behind the system extending impacts in time.

Summary: Guidance remains in fairly good agreement over much of this forecast but subtle differences result in a range of impact potential. Confidence is already fairly high that many areas of SC and eastern GA will see at least some winter weather this weekend but a gradient of impacts is likely geographically. Unlike many southern winter events, the ceiling for this event is very high with significant impact potential.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

VFR conditions expected through the TAF period.

SKC or a FEW passing cirrus clouds this morning. A slow uptick is possible in cloud cover this afternoon, with cirrus becoming BKN and SCT-BKN clouds developing around 10kft MSL. Otherwise, surface winds light and variable to calm expected this morning before picking up from the south or southeast around 4 to 6 kts early this afternoon. A shift to a predominantly southerly direction is expected late in the afternoon, but speeds remain less than 10 kts. Light and variable winds return tonight. Surface conditions remain too dry to support fog concerns.

EXTENDED AVIATION OUTLOOK...VFR conditions likely continue into Thursday. The next storm system will likely bring restrictions as early as Friday, but especially Saturday and Sunday.

CAE WATCHES/WARNINGS/ADVISORIES

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