Marine Weather and Tide Forecast for Florence, SC

KEY MESSAGES
1) Confidence continues to increase in a significant winter weather event this weekend. Expect impacts to travel and infrastructure, ranging from hazardous to dangerous conditions and closures.

2) Temperatures well below normal will result in periods of dangerously low wind chill temperatures below 16 degrees through at least Monday night and potentially into the middle of next week.

DISCUSSION
KEY MESSAGE 1:... Confidence continues to increase in a significant winter weather event this weekend. Expect impacts to travel and infrastructure, ranging from hazardous to dangerous conditions and closures.

Low pressure aloft moving through the TN Valley and into GA/SC Sat will interact with low pressure off the coast later Sat into Sun.
Strong divergence in the favorable exit region of the 5h low/trough will lead to rapid strengthening of the low off the NC Coast. The location of the 5h low and the surface low continue to be critical to the forecast and will determine where the heaviest precipitation falls. Timing continues to look like Sat night into Sun, so confidence with respect to timing is increasing. The rapidly strengthening low also increases winds with gusts approaching 40 mph not out of the question and gusts in excess of 30 mph all but guaranteed for much of the forecast area.

The bigger questions remains how much QPF will fall, where will it fall, and what will the end result be. Plausible QPF range is on the order of 0.30"-0.70" with pockets of higher amounts possible.
Looking at the ensemble data there is very good agreement between the 3 main suites (CMC/EC/GFS) with high probabilities of QPF > 0.50" along the coast and probs a bit lower farther inland. However the probability of QPF > 1.00" is just about zero for the entire forecast area.

Precipitation starts late Fri night into Sat morning, mainly inland, as weak isentropic lift develops. May start out as a mix of rain/sleet/snow before a quick transition to snow/sleet after midnight. The precipitation then expands to the coast during the morning hours. As we saw with the last system the low level arctic air takes a while to saturate when the forcing is weak, which it will be at the onset. May see a tenth across inland areas Fri night, although wouldn't be a shock to see less. Bulk of the QPF occurs late Sat afternoon into Sat night with precipitation starting to taper off around daybreak Sun as the low starts moving away from the coast.

Biggest question remains snow ratios. Forecast soundings show a "warm" layer between 5k and 10k ft. This layer is around -10C which suggests snow ratios somewhere in the range of 12:1 to 15:1. Surface temperatures would suggest ratios much higher, but with the crystals forming and moving through the somewhat warmer layer not sold on the high ratios suggested by surface temps. The simple fact is close to the coast it is hard to remove all of the ocean influence. Could see slightly higher ratios farther inland, but still think 15:1 is probably the upper bound. Taking the expected QPF range and an avg snowfall ratio suggests snowfall range of 4-8" with locally higher amounts. That said, there remains uncertainty with respect to the axis of highest QPF. Guidance continues to show pockets of 1"-1.5" QPF in the general vicinity, mainly offshore.
A subtle shift in the location of the synoptic features could lead to the higher QPF moving onshore. The dry slot continues to be a concern as well. Latest solutions continue the slight southward shift of the 5h low/trough Sat night into Sun which would tend to limit the northern push of the drier air, but still a source of uncertainty and something that will have at least a slight affect on QPF/snowfall.

KEY MESSAGE 2...Temperatures well below normal will result in periods of dangerously low wind chill temperatures below 16 degrees through at least Monday night and potentially into the middle of next week.

A very cold air mass arrives tonight and early Saturday. This air mass will linger through the middle of next week, enhanced by expected snow cover.

A Cold Weather Advisory has been issued for wind chills dipping below 15 degrees tonight and continuing through much of Saturday for non-coastal counties plus Pender and Northern Horry. As northeasterly winds increase tonight and cold advection amplifies, wind chills will rapidly fall below this threshold with some areas along and west of I-95 falling into the single digits before sunrise. After sunrise, temperatures will warm slightly and briefly climb above the cold weather advisory threshold east of I-95. As precipitation begins to fall inland and northerly winds increase, wind chills will cool starting near midday Saturday. The strengthening gradient and falling temperatures lead into the Extreme Cold Watch on Saturday evening.

The Extreme Cold Watch remains in effect. There is potential for an extended period of Extreme Cold running from Sat night through Sun night and into Mon. Will depend on the strength of the wind and how much snow falls vs when skies clear. Looks like an extended period of temperatures well below normal flirting with Cold Weather Advisory thresholds into next week, somewhat dependent on lingering snow pack.

AVIATION /12Z FRIDAY THROUGH TUESDAY/
High confidence in VFR through late tonight. Mid level moisture advection will see VFR CIGs descend late this afternoon and this evening. MVFR CIGs will gradually start to engulf the region after midnight with a rapid deterioration to IFR thereafter. Isentropic lift may see the early onset of light snow inland (FLO/LBT).

Extended Forecast... Flight restrictions quickly develop across the region Saturday afternoon as wintry precip increases in intensity and spreads eastward. IFR is likely for a long period starting Saturday evening and continuing through Sunday morning. Visibilities in moderate to heavy snow will approach minimums. Gusty northerly winds will accompany snowfall throughout this time period. VFR returns on Sunday and continues through early next week, but cold temperatures will retain surface snow pack.

MARINE
Through Tonight... Light winds turn northeasterly today. The pressure gradient will increase this afternoon and tonight as low pressure develops offshore. Conditions deteriorate overnight with strengthening winds and increasing seas. SCA conditions are likely by the end of the period, but stronger winds will approach Gale force by Saturday morning.

Saturday through Tuesday... Treacherous marine conditions Sat and Sun requiring prolonged gales gradually improve into Mon with headlines likely ending by daybreak. Offshore winds Mon will remain breezy into the afternoon before dropping near 10 kt as winds back to west- northwest. Offshore flow will drop double digit seas Sun to 3-5 ft by Mon morning and 2-3 ft by Mon night. Impressive northerly wind wave this weekend 7ft at 5 seconds will transition to a northeast swell early next week and will be the dominant wave.

ILM WATCHES/WARNINGS/ADVISORIES
NC...Cold Weather Advisory from 1 AM to 6 PM EST Saturday for NCZ087-096-099-105.
Winter Storm Warning from 1 AM Saturday to 4 PM EST Sunday for NCZ087-096-099-105>110.
Extreme Cold Watch from Saturday evening through Monday morning for NCZ087-096-099-105>110.
SC...Cold Weather Advisory from 1 AM to 6 PM EST Saturday for SCZ017-023-024-032-033-059.
Winter Storm Warning from 1 AM Saturday to 4 PM EST Sunday for SCZ017-023-024-032-033-039-054>056-058-059.
Extreme Cold Watch from Saturday evening through Monday morning for SCZ017-023-024-032-033-039-054>056-058-059.
MARINE...Gale Watch from Saturday morning through Sunday evening for AMZ250-252-254-256.