textproduct: NWS Wilmington

WHAT HAS CHANGED

Aviation discussion updated for 12z TAFs.

Despite the event drawing closer, forecast confidence is not increasing. Accumulations, both ice and total rainfall, have decreased, but there remains significant uncertainty with respect to the timing and evolution of this weekend's storm.

KEY MESSAGES

1) Significant ice accumulations, in excess of 0.25" well inland, are possible across the area Saturday night through Sunday. Heavy ice accumulation will make travel hazardous and could lead to power outages.

2) Temperatures well below normal arrive Monday and persist through the end of the week, leading to dangerously cold temperatures.

DISCUSSION

KEY MESSAGE 1: Significant ice accumulations, in excess of 0.25" well inland, are possible across the area Saturday night through Sunday. Heavy ice accumulation will make travel hazardous and could lead to power outages.

Unfortunately the overall picture has not become much clearer. Thinking has not changed, but the important forecast details remain highly uncertain. What we know is the arctic air coming in is potent and will drop temperatures to or below freezing for much of the area. Coastal South Carolina may escape the freezing temperatures, but even that is somewhat uncertain. The residence time of temperatures at or below freezing is another issue. Along the coast much of the guidance shows temps bouncing back above freezing quickly, taking a bit longer inland. At the same time weak isentropic lift will be working to moisten up a dry boundary layer. On Sat it will be a race against temperatures warming and precipitation managing to reach the ground. At this point it seems like Sat is more likely to be dry than wet/icy and precipitation chances have further decreased.

The real forecast problems begin Sat night. The layer of very dry air in the lowest 2-3k ft in place on Sat is all but gone by Sat night and the column is fully saturated. Away from the coast surface temperatures will be well below freezing, but the depth of the freezing layer is less than 2k ft (and gradually decreasing). The problem Sat night will be down the strength of the isentropic lift. Inland forecast soundings still indicate decent lift, but closer to the coast the lift weakens and it could be the dynamics from the developing/passing low take over. Still expect a prolonged period of freezing rain across the watch area, which may start as a mix of sleet and freezing rain. However, as the warm nose strengthens any sleet will transition to freezing rain. Still some uncertainty regarding how long temperatures remain below freezing. Inland, suspect the guidance may be too quick to break down the wedge, meaning freezing rain lingers longer than currently forecast. Sun night any lingering pockets of freezing rain should transition to rain just prior to the arrival of the cold front.

Closer to the coast Sat night and Sun the wedge becomes a bigger forecast problem. The cold air within the wedge is always stubborn and resists the warmer air. Partial thickness within the coldest of the guidance(basically the GFS/NAM) do suggest low level warming for much of the coastal counties sufficient to transition any frozen precipitation (freezing rain and sleet) to all rain Sat evening through Sat night. For these areas, once the rain arrives the frozen precipitation threat decreases. The caveat here is the far inland portions of Pender county in NC and Horry County in SC. These counties extend far enough inland that the coastal warming is not likely to cover the entire county and freezing rain may linger into Sun morning for these areas. Of course a lot of this hangs on the location, track and strength of the coastal low Sat night into Sun.

The areas with the lowest confidence are those in between the far inland watch and the coastal areas that transition to all rain Sat night. This is where the denser cold air will try and resist the warmer air. It's possible some areas could see multiple transitions between rain and freezing rain Sat night into Sun before the transition to all rain slowly spreads inland ahead of the second low/cold front trailing the coastal low. Confidence in the timing and the inland extent of this transition is low. The generally poor performance of guidance in wedge setups increases the uncertainty.

The overall trend in the guidance (operational/ensemble/AI) has been for less total QPF, less ice accumulation and in general warmer temperatures. However, this system still has potential to bring significant impacts to a large part local area (even those outside the current watch). To give an idea of the level of uncertainty, at Wilmington, NC ice accumulation in the NBM (NBM 10 vs NBM 90) ranges from 0 to almost 0.2" while at Hartsville, SC, well inland, the range is 0.05" to almost 0.50". So again, while the trends are for less ice overall, confidence is low and much of the area still has the potential to see significant ice accumulations Sat night and at least part of Sun.

KEY MESSAGE 2: Temperatures well below normal arrive Monday and persist through the end of the week, leading to dangerously cold temperatures.

A succession of arctic highs will move into the Southeast next week. Daytime highs will be in the 30s and 40s Monday through Friday with lows ranging from lower teens to lower 20s depending on the night. Dangerously cold temperatures are likely, with significant impacts likely in any areas where power outages from the weekend storm linger.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Overall VFR conditions will continue through much of today with light rain and lowering ceilings mainly between 22z-04z tonight. Then ceilings will raise a bit with surge of NNE winds picking up to 15 kts with gusts to 20 kts after 06z.

Extended Forecast...A winter storm will impact the area this weekend with prolonged restrictions expected. KFLO and KLBT in particular stand the greatest chance at recording FZRA, especially late Saturday into Sunday. VFR should return by Monday afternoon or so, continuing through Tuesday.

MARINE

Through Tonight... Light NNW winds will begin to veer to the NNE and increase up to 10 to 15 kts with higher gusts followed by a further increase through tonight. Expect NNE winds up to 25 kts after midnight tonight as gradient tightens with approach of storm system. Seas will build up to 3 to 4 ft through this evening but will ramp up considerably overnight reaching 5 to 8 ft by Sat morning. This will lead to a Small Craft Advisory beginning at 1 am tonight (early Sat morning).

Saturday through Tuesday... Treacherous marine conditions in place for the bulk of the forecast period. Combination of arctic high and coastal low will increase winds to near gale force Sat into Sun. Brief reprieve from the winds Sun as the coastal low exits, but then the gradient tightens back up ahead of the trailing low/cold front combo. Cold advection in the wake of the cold front Sun night into Mon helps keep headlines going through Mon night. Decreasing offshore flow leads to quick drop in seas with headlines likely ending Tue morning. Seas could briefly touch 10 ft Sat night and will remain above 6 ft into Tue.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Winter Storm Watch from Saturday afternoon through Monday afternoon for NCZ087-096. SC...Winter Storm Watch from Saturday afternoon through Monday afternoon for SCZ017-023-024. MARINE...Small Craft Advisory from 1 AM Saturday to 7 AM EST Tuesday for AMZ250-252-254-256.