textproduct: Columbia

WHAT HAS CHANGED

After the system passes, cold air moves into the region to start the workweek.

KEY MESSAGES

- 1) The chance for an impactful winter storm continues, focusing north of I-20 late Saturday into Sunday.

- 2) After the weekend winter system, very cold air settles into the region.

- 3) Ahead of the weekend system, mild temps expected with some rain chances into Friday.

DISCUSSION

KEY MESSAGE 1: The chance for an impactful winter storm continues, focusing north of I-20 late Saturday into Sunday.

Overview: The threat for an impactful ice event north of I-20 continues, as guidance has trended more north and warmer with this system. While confidence remains that areas along and south of I-20 will see some freezing rain, the chances for a significant ice storm have decreased. However, a reminder that only a few hundredths of an inch of freezing rain can cause notable travel impacts.

Potential Impacts: A spectrum of impacts are expected across the forecast area with a steep gradient in freezing rain amounts likely. The potential remains across the northern Midlands and Pee Dee for an impactful ice storm from this system despite the trends in guidance. Primarily late Saturday and into Sunday, notable impacts remain possible in these locations, with potential for over 0.25" of freezing rain causing very difficult travel, power outages and downed trees. Confidence has decreased in this potential just along and south of the I-20 corridor, but at least some freezing rain does still appear likely leading to some travel concerns; again, only a few hundredths of an inch of freezing rain can quickly cause driving and travel impacts.

Trends and Forecast Challenges: The warm, northerly trend in guidance that the ECE started to capture 36 hours ago appears very legitimate, as guidance universally has trended persistently and significantly warmer and northward in temps and qpf totals; additional recon data in the 00z data increases confidence that this trend has staying power. This trend is driven by the more aggressive phasing of the southern stream ejecting trough and the northern stream diving shortwave, which then dramatically amplifies the pattern downstream and greatly increases flow below 500mb. This increased flow allows for a truly remarkable WAA setup, extending well into the Mid-Atlantic late Saturday and into Sunday; typically guidance under-does WAA leading up to an event, so this is another factor favoring the legitimacy of this trend. Strong CAD will still develop initially late Saturday as this WAA drives substantial moisture advection north, with a period of freezing rain expected across much of our area but with a range of impact potential; its important to note that even the warmest guidance has this period of freezing rain and this CAD will be impressive, at least initially. So even if some northerly trend continues, areas north of I-20 are still expected to see impactful freezing rain. Areas along and south of I-20 are where questions arise and the potential for a significant ice event has decreased, but at least some icing is still probable for much of the Midlands. By later in the day Sunday, the warm trend really comes to fruition with the 60-70 knot 850mb jet eroding the CAD and driving a very strong warm front northward, likely into the southern extent of our area. Questions remain in how much the CAD dome erodes, especially along and north of I-20, and whether or not those areas get above freezing. A tremendous temp gradient Sunday is consistent in all guidance now, with even some CAPE developing south of the warm front but the northern Midlands still around freezing.

Summary: Areas north of I-20 should remain prepared for an impactful winter storm, with freezing rain likely late Saturday and into Sunday. A period of freezing rain, with impacts to travel, remains likely south of I-20 but confidence is decreasing in the potential for a significant ice storm in those locations.

KEY MESSAGE 2: After the weekend winter system, very cold air settles into the region.

Strong high pressure builds into the Southeast under large scale troughiness, leading to very cold air to begin the work week ahead. Monday and especially Monday night are expected to be the coldest of the first half of the week, which could produce a couple of different hazards. One hazard to be aware of is the potential refreezing of any residual water on the roads and surfaces, which would make for tricky travel in spots. The other hazard is simply the cold as forecast lows Monday night are hovering around Cold Weather Advisory criteria.

KEY MESSAGE 3: Ahead of the weekend system, mild temps expected with some rain chances into Friday.

Southwesterly flow over the region into Friday keeps the area relatively mild ahead of the weekend system. A weak shortwave is forecast to move over the region, which is expected to bring periodic isolated to scattered showers this afternoon into midday Friday. Most of the activity is expected in the western Midlands and upper CSRA, gradually moving to the I-20 corridor overnight into Friday.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

VFR conditions expected through much of the TAF period with MVFR conditions possible late.

Mid-level clouds have moved over the western half of the forecast area, while the eastern half remains mainly clear for the time being. Lower clouds are expected to move over the terminals overnight with MVFR ceilings likely reaching all TAF terminals except OGB. There is potential for some IFR ceilings as well, but confidence in on the lower end. Isolated to scattered showers are also anticipated overnight, sliding eastward and potentially affecting the terminals after about 12z. However, this activity is not expected to reach OGB, so left showers out of their TAF for now. West to northwest winds 5-8 kts expected through 00z before becoming light and variable overnight. Winds shift to more northeasterly after daybreak, but remain light through 18z.

EXTENDED AVIATION OUTLOOK...The next storm system will likely bring restrictions as early as tomorrow, but especially Saturday and Sunday.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Winter Storm Watch from Saturday morning through Monday afternoon for SCZ016-018-020>022-029-115-116. GA...Winter Storm Watch from Saturday morning through Monday afternoon for GAZ040.