textproduct: Columbia

WHAT HAS CHANGED

Aviation discussion for 12z TAF issuance. Otherwise, the previous update still holds true: The main timeframe for greatest impacts from the upcoming winter storm is coming into better focus. This includes a slightly later onset of precipitation Saturday afternoon or evening. Additionally, there remains quite a bit of uncertainty regarding temperatures Sunday afternoon, which will impact the magnitude of precipitation type variation across the area.

KEY MESSAGES

- 1) A winter storm is expected to bring periods of freezing rain, sleet and rain to the area late Saturday into Sunday, with hazardous travel, downed tree limbs and power outages possible, particularly along and north of I-20.

- 2) Behind this weekends winter system, very cold air funnels into the region with values near Cold Weather Advisory criteria possible.

- 3) Light rain, not associated with the incoming winter system is possible today.

DISCUSSION

KEY MESSAGE 1: A winter storm is expected to bring periods of freezing rain, sleet and rain to the area late Saturday into Sunday, with hazardous travel, downed trees/tree limbs and power outages possible, particularly along and north of I-20.

Overview: A wintry mix of precipitation is expected across much of the forecast area this weekend as a strong system moves across the Southern Tier. Of most concern is the potential for significant ice accumulations, in addition to the potential for periods of sleet, particularly north of I-20. Models have continued to converge on the onset of precip late in the day Saturday. This timing would favor frozen (sleet) or freezing precipitation for most of the forecast area when conditions begin to deteriorate, though rain is still expected across the southern FA at least initially. Surface cold advection will be stronger earlier in the day Saturday for the northern Midlands, and these conditions would support higher accumulations of sleet and freezing rain for this region across the duration of the event.

Potential Impacts: Impactful ice and sleet accumulations remain the main story across the northern Midlands and Pee Dee. The latest forecast indicates potential for over 0.25" of freezing rain approximately north and west of the I-20 corridor. Sleet amounts of up to 1" are also possible, though accumulations are expected to be a bit more limited to along the SC/NC border and toward the Upstate in Fairfield, Newberry and Lancaster Counties. Ice amounts for areas along the I-20 corridor, including the Columbia and Augusta metro areas, are still expected to range between 0.1" to 0.25". Lesser amounts of a few hundredths up to 0.1" are forecast for the southeast CSRA and eastern Midlands. These conditions are likely to cause very difficult if not hazardous travel, power outages and downed trees/tree limbs.

Forecast Challenges: Models continue to hold on to dry weather through much of the day on Saturday, with a few solutions holding off on precipitation until as late as 00z Sunday (7 PM EST). Although some light wintry precip may occur earlier, models now depict precip overspreading the region from northwest to southeast while cold surface air filters in from the northeast, setting up a classic Cold Air Damming scenario. Strong warm advection is expected to develop over top the cold surface air, though how fast this occurs and how deep the warm air is in contrast to the surface layer will determine when areas transition from a sleet p-type to freezing rain Saturday night. More questions arise by Sunday, as surface warm advection from the south will try to push the wedge front north and transition southern areas back to rain. As of now, we expect the sub- freezing cold air will likely hold steady in the northern half of the FA, and the transition zone is likely to feature a very sharp temperature gradient. We may see highs around freezing near the SC/NC border with portions of southern Burke County pushing 60 degrees or more. That said, there remains a lot of uncertainty of where this gradient will set up and how far north it will travel Sunday evening. For example, the NBM interquartile range for high temp at CAE Sunday ranges from 38 to 62; for AGS this range is 45 to 68 degrees. The other thing to note is that highs Sunday should occur during the overnight period when warm advection is strongest and southwest to southeast winds have developed across the southern FA. Lows for the day will likely have already occurred in the morning around 12z. One more thing to note is that the warmer solutions such as the EC allow for some weak elevated instability to develop across the southern FA, leading to a brief period of thunderstorm chances which do make their way into the blend from 00z to 03z Monday. From this period through the end of the event, temperatures should remain warm enough for liquid precip across the bulk of the Midlands and CSRA, with the coldest temps and potential for continued mixed wintry precip along the SC/NC border and northwest of I-20. Precip ends from southwest to northeast across the region Monday morning.

Summary: Areas along and north of I-20 should be prepared for an impactful winter storm, with freezing rain likely late Saturday through Sunday afternoon. Attention should be paid for the potential of expanded watches, warnings or advisories issued ahead of the storm.

KEY MESSAGE 2: Behind this weekend's winter system, very cold air funnels into the region with values near Cold Weather Advisory criteria possible.

Anomalously strong troughing will persist across the region much of the upcoming week behind this weekends winter system. This coupled with strong high pressure building into the area should bring a very cold air mass with temperatures well below normal, especially Monday night and into Tuesday. Temperatures are expected to be around 10-20 degrees below normal into the mid week period where multiple nights with low temperatures falling into the teens or 20s is possible. Right now, the chilliest night is expected to be Monday night and into Tuesday where LREF probabilities for temperatures less than 15F are now between 20-40%, highest north of I-20. Due to this the need for a Cold Weather Advisory will continue to be monitored, especially once this system has passed. Near Cold Weather Advisory criteria lows/wind chills cannot be ruled out other nights next week as the area hangs onto to this arctic airmass through the end of the current forecast period.

KEY MESSAGE 3: Light rain, not associated with the incoming winter system, is possible today.

Persistent southwesterly flow with a weak shortwave nearing the region is already bringing light rain showers to the FA, especially along and north of the I-20 corridor (this rainfall is not associated with this weekends system). These isolated to scattered light rain showers could continue through at least the late afternoon before tapering off during the evening hours. QPF Amounts should generally be between 0.10-0.20" from this activity.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

VFR conditions to start the TAF period with MVFR conditions moving in late this morning into afternoon.

Low clouds, but still VFR, are blanketing the region this morning with some light shower activity. Expect these clouds to hang around throughout the TAF period, with vicinity showers through about 06z. MVFR ceilings are anticipated to move over AGS/DNL around 15z, with periodic MVFR ceilings prior to that. The MVFR ceilings there are anticipated to remain through the rest of the TAF period, but could improve late. CAE and CUB are anticipated to see MVFR ceilings from about 20z to 00z, but not confident that they will be prevailing, so left the tempo group for now. OGB is expected to remain in VFR throughout the TAF period. Northeast winds around 5-8kts shift a bit more northerly for the afternoon. Winds shift back northeasterly around 06z and increase to near 10 kts by the end of the TAF period. Some higher gusts are also possible at the very end of the period.

EXTENDED AVIATION OUTLOOK...The next storm system is likely to bring restrictions Saturday and Sunday as sleet and freezing rain are likely to impact terminals.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Winter Storm Watch from Saturday morning through Monday afternoon for SCZ016-018-020>022-025>031-037-115-116. GA...Winter Storm Watch from Saturday morning through Monday afternoon for GAZ040-063>065.