textproduct: Columbia

WHAT HAS CHANGED

No significant changes to the overall outlook for the incoming winter storm. Confidence gradually increases in potential need for Cold Weather products early in the week.

KEY MESSAGES

- 1) Freezing rain stating early tonight, continuing into Sunday is expected to cause hazardous travel and power outages with the greatest impacts north of I-20.

- 2) Very cold air funnels into the region behind the weekends winter storm with values near Cold Weather Advisory criteria possible multiple days next week.

DISCUSSION

KEY MESSAGE 1: Freezing rain stating early tonight, continuing into Sunday is expected to cause hazardous travel and power outages with the greatest impacts north of I-20.

Overview: No significant change has been made to the Ice Storm Warning along and north of I-20 or the Winter Weather Advisory in the southern forecast area this forecast cycle. Freezing rain continues to be expected across the FA starting early tonight, continuing into Sunday morning where significant ice accumulations of one quarter to a half of an inch are expected along and north of I-20 (especially toward Lancaster, Fairfield, and Newberry Counties), while lesser amounts are expected in the southern part of the forecast area due to warming temperatures Sunday morning and afternoon. Some sleet accumulations up to around half of an inch to three quarters of an inch could be possible as well toward the northern FA.

Potential Impacts: An ice storm will track across the Southeast starting early tonight, continuing into Sunday. Significant ice accumulations are expected to cause hazardous travel, downed tree limbs, and power outages across the FA. The greatest threat for these hazards will be along and north of the I-20 corridor where accumulations over one quarter of an inch are most likely. Isolated power outages and downed tree limbs cannot be ruled out further south, but less accumulation should aid in lessening the chances for these impacts. Hazardous travel should still be expected in the Winter Weather Advisory area, though travel conditions here may improve through the day Sunday as the wedge front lifts a bit north and temperatures warm to above freezing.

To further emphasize, very challenging travel conditions are expected Sunday morning, especially along and north of I-20. If light freezing rain or freezing drizzle continues during the afternoon or evening hours, these hazardous conditions will persist as well so travel is strong discouraged.

Forecast Challenges: As the push of WAA and isentropic lift moves over the surface wedge late Saturday afternoon and into the evening, periods of sleet or rain could be possible, but recent high-res model guidance indicates dewpoint depressions in excess of 15-20F will need to be overcome initially, likely leading to a start time of wintry precip after 7-10 PM as temperatures fall closer to the initially 28-31F wetbulb temperatures. Once precip begins reaching the surface, the dominant precip type is expected to be freezing rain as the 10C warm nose moves in with a shallow sub-freezing layer. As the push of WAA aloft initially builds in, periods of sleet could be possible north of the I-20 corridor early in the night, but this is expected to transition to freezing rain as the inversion grows stronger. Freezing rain then likely continues into Sunday morning across the FA.

Greater uncertainty remains as we continue through the late morning hours and the remainder of Sunday as to how far north the wedge front will lift as a weak surface low pushes in, and thus if precipitation will transition from freezing rain/freezing drizzle to rain/drizzle. The bulk of guidance agrees that areas mainly south of the I-20 corridor will gradually rise above freezing through the day Sunday, aiding in melting ice accumulations here and transitioning precip to rain. More uncertainty exists along the I-20 corridor and north where large IQR spreads for temperatures remain. With this said, the 00z HRRR and recent 00Z HREF mean solution have begun hinting at the wedge holding strong in the aforementioned area (which lines up closer to climatology) with temperatures barely reaching above freezing. Have continued to lean toward this cooler guidance as high-res soundings indicate saturated profiles will remain through the afternoon and into the evening Sunday with ongoing 285-295K isentropic lift, which generally leads to continuing light precip to reinforce the wedge. With this thinking, periods of drizzle or freezing drizzle could continue into the afternoon and evening hours Sunday. This could bring further ice accumulations for areas that remain near/below freezing throughout the day. The cold front then is expected to push in Sunday evening where thunder remains possible in the southern FA with weak elevated instability on top of the pronounced warm nose.

KEY MESSAGE 2: Very cold air funnels into the region behind the weekends winter storm with values near Cold Weather Advisory criteria possible multiple days next week.

Surface high pressure and an overall troughing pattern funnels in arctic air into the region over the work week, bringing well below temperatures. Afternoon highs much of the work week are expected to struggle into the 40s while multiple chilly nights are expected. The first concern comes Monday night and into Tuesday morning where overnight lows into the low to upper teens are possible (probabilities for temperatures less than 15F are up to 40%). This has led to increasing confidence that a Cold Weather Advisory may be needed and could possibly bring surface refreezing concerns, though confidence in this is not as high. After this, multiple nights into the mid week could have temperatures/wind chills near Cold Weather Advisory criteria, but values are expected to remain above this at this time. Thursday night into Friday morning may be the next chance where probabilities have increased for nearing this criteria.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

VFR conditions will continue for at least the next 6-8 hours but beyond that MVFR and IFR conditions look increasingly likely.

Our well-advertised cold-air damming setup has arrived across the area. This hasn't yielded our typical ceiling restrictions just yet as the low-level airmass is extremely dry. However, mid-level moisture continues to increase as the system shifts eastward towards us from the west and cloud cover is widespread across the area. Northeasterly winds are noted at all sites, with winds 8-10 knots common. This will continue today with some gusts up to 18-22 knots likely at times today given a strengthening pressure gradient. By this evening, lift and low- level moisture are forecast to continue increasing, with ceilings falling to MVFR levels shortly after 00z. Precipitation will quickly develop, especially near the Augusta/Columbia sites, and sleet and freezing rain should become the dominant p-types for the night. Sleet is probably something that will only impact the sites early in the evening as the column will saturate quickly while a robust warm nose remains in place. The rain rates look fairly light - similar to what we get with wedges often times. It is counter intuitive but these lighter rains are more dangerous in freezing rain setups as that kind of rainfall accumulates on surfaces very efficiently. All in all, after 00z, aviation interests in the area should prepare for impactful weather, including accumulation freezing rain and MVFR/IFR ceilings. Orangeburg will likely see the ceiling issues but the timing of freezing rain/sleet is a little more uncertain. Expecting it to get going there sometime in the 05z-08z timeframe, with freezing rain becoming impactful as the forecast period comes to a close.

EXTENDED AVIATION OUTLOOK...The winter storm ramps up Saturday night through Sunday and likely to bring restrictions as sleet and freezing rain are likely to impact terminals and restrictions in cigs/vsbys likely.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Ice Storm Warning from 1 PM this afternoon to 1 PM EST Monday for SCZ016-018-020>022-025>031-037-115-116. Winter Weather Advisory from 1 PM this afternoon to 1 PM EST Monday for SCZ035-038-041-135>137. GA...Ice Storm Warning from 1 PM this afternoon to 1 PM EST Monday for GAZ040-063>065. Winter Weather Advisory from 1 PM this afternoon to 1 PM EST Monday for GAZ077.