textproduct: NWS Wilmington

WHAT HAS CHANGED

Small Craft Advisory issued for the NC coastal water zones starting at noon on Sunday.

Severe weather risk has increased for late Sunday night into Monday morning. Confidence in freezing temperatures for Tuesday night has increased.

KEY MESSAGES

1) A strong cold front will bring impacts due to strong winds and severe thunderstorms on Monday. A few storms are possible ahead of the front on Sunday afternoon and night as well.

2) Near-freezing temperatures are expected on Monday night and subfreezing temperatures on Tuesday night are likely.

DISCUSSION

KEY MESSAGE 1... A strong cold front will bring impacts due to strong winds and severe thunderstorms on Monday. A few gusty storms are possible ahead of the front on Sunday afternoon and night as well.

Significant pattern amplification will take place as a very deep and sharp trough in the jetstream takes shape over the middle of the country while shifting eastward on Sunday and Monday. Strengthening surface low pressure tracking across the northern US through the Midwest and into the Great Lakes will deliver a major snowstorm on its north side while anomalously warm and moist air is drawn out of the Gulf and western Atlantic ahead of its cold front. Closer to home, this mass response will pull a stalled front over the southwest Atlantic back to the coast on Sunday, lifting it through late in the day. A cold front will follow later on Monday with increasingly breezy conditions and bands of storms expected as it approaches. Some thunderstorms may bring severe winds of 60mph or greater and isolated fast-moving tornadoes. Stay tuned to the latest updates regarding this severe weather threat as details become clearer in the timing of the threat.

On Sunday, high pressure northeast of the area is expected to result in a cold air damming wedge setup which will be overrun by increasing moisture advection out of the southeast. This strengthening low-level isentropic ascent will result in a period of rain, especially near the coast. The warm front is expected to lift northwestward late in the day or during the evening, with east winds during the day turning southeasterly during the evening, then southerly behind the front. Temperatures and dewpoints will subsequently hold steady or slowly rise through most of Sunday night. This will allow for at least weak surface-based instability to develop, especially as height falls and mid-level cooling take place ahead of the trough. In addition, as increasingly divergent ageostrophic flow at the jetstream level overspreads the area, strengthening low-level convergence will likely lead to a band or two of showers and thunderstorms developing and tracking northward late in the night. These bands could bring a threat for strong to locally damaging wind gusts and possibly a tornado, particularly if supercell structures take shape due to increasing low-level shear.

On Monday, the approaching jetstream trough and cold front will lead to a mild and windy day as very strong flow just above the ground will not need much surface warming to get to the ground. In fact, a Wind Advisory may be needed if some clearing of the clouds occurs, as gusts exceeding 45mph are quite possible ahead of the front. In addition, strengthening convergence will likely lead to bands of showers and storms forming and lifting northward ahead of the cold front, which may limit the amount of instability that can develop ahead of the front itself. Nevertheless, with an abundance of low- level shear in place, isolated low-topped supercells may take shape in the pre-frontal convection, which could bring their own threat for damaging wind gusts, fast-moving tornadoes, and perhaps some hail. The front itself should feature its own band of convection, although the latest CAMs suggest this will decay as it draws nearer to the coast, likely owing to the south winds bringing in stable marine air nearer to the coast. Nevertheless, inland areas will be most likely to see severe winds and possibly line-embedded tornadoes associated with this frontal band, even with limited instability. SPC maintains an Enhanced risk (level 3 out of 5 or at least a 30% chance) for severe weather for the forecast area, with significant wind gusts (>75mph) possible. The front is expected to cross the area from west to east during the afternoon and evening hours, with winds turning westerly behind it.

KEY MESSAGE 2... A period of near- to subfreezing temperatures along with frost will pose a threat to unprotected plants and crops from Monday night through Wednesday night.

A broad zone of high pressure will build in behind the cold front on Monday night with strong cold advection bringing down temperatures from the 70s on Monday into the 30s by sunrise Tuesday morning. With winds still expected to be a steady 5-10 kts by sunrise, whether or not temps reach the freezing mark will depend strictly on the advection of air that is cold enough. Typically, the first night behind cold fronts end up warmer than forecast due to the winds remaining elevated, but this will need to be monitored closely.

The much more impactful night is anticipated to be Tuesday night as high pressure shifts east of the Appalachians and supports much lighter winds amidst very dry dew points in the low 20s or even the teens. Skies are also expected to be clear as very dry air should be in place throughout the troposphere across the area. While the current forecast indicates mostly upper 20s to low 30s across the area, these may need to be revised downward, especially if winds are expected to become calm. Freeze warnings will likely become necessary to address this threat as the recent stint of very warm temps has sparked a significant green-up, leading to our frost/freeze program starting earlier than usual.

Another night of light or calm winds is forecast on Wednesday night as a lingering surface high pressure wedge will keep the pressure gradient weak. This will likely lead to one more night of near- freezing temps, which could yield extensive frost as dew points rise on northeast to east winds during the preceding day. However, there is a chance that cloudiness may hamper cooling and prevent frost in some areas, so this will need to monitored closely as well.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

With the exception of a moderate potential for some temporary ground fog at KCRE (North Myrtle Beach) overnight, VFR conditions have a high likelihood of continuing through 12z Sunday.

An approaching warm front from the south is expected to lead to deteriorating ceilings during the day Sunday and an increasing chance of showers. Ceilings are expected to dip into the MVFR range Sunday afternoon with a a low to moderate potential of temporary IFR ceilings, particularly at KCRE and KMYR.

Sunday's showers should be falling from clouds with tops low enough to produce small droplet sizes that can reduce surface visibility even without significant rainfall rates. I'm including tempo TAF vis down to 4SM Sunday afternoon but it's possible brief IFR visibility could occur.

The warm front should begin to lift north of the area by Sunday evening, perhaps accompanied by fragmenting low cloud ceilings and a reduction in shower coverage.

Extended Forecast...Low clouds may redevelop Sunday night. An approaching strong cold front on Monday will be preceded by increasing surface winds gusting to 35 kt and scattered showers and thunderstorms. The front itself should reach the area Monday afternoon and could be accompanied by a squall line of severe thunderstorms with gusts to 50 knots. Behind the front VFR conditions should return by late Monday night.

MARINE

Through Sunday...Variable winds start building out of the ESE around 10 kts by late this evening. Gradient winds continue to build overnight into Sunday morning, with sustained winds around 15 kts and gusts up to 20 kts. Winds gradually veer more to the southeast throughout the day. Seas build up quickly, starting from 1-2 ft to 2- 4 ft by early Sunday morning, all the way up to 4-6 ft by Sunday afternoon. Highest seas will remain over the coastal waters from Surf City, NC to Little River Inlet, SC. As such, a Small Craft Advisory has been issued over these waters, which starts officially at noon EDT Sunday.

Sunday night through Wednesday Night... A warm front will lift through the waters early Sunday night with southeast winds turning southerly behind the front. A batch of showers and storms will likely accompany the warm frontal passage, which could bring locally heavy rain, strong winds, and higher seas. South winds will increase markedly through the night into Monday morning with sustained winds expected to reach 20-25 kts by sunrise as a strong cold front approaches from the west. Additional rounds of storms should affect the waters from late in the night through Monday evening as confluence bands take shape ahead of the cold front, with strong winds and waterspouts possible in the strongest storms. When the cold front arrives early Monday night, this will bring one final round of strong storms and wind gusts may reach gale force around the time of its passage. Breezy west winds will take over in its wake, with winds gradually subsiding through Tuesday to around 10 kts. Winds rebound on Tuesday night as a shot of cold air advection arrives on north winds before subsiding again on Wednesday and veering to northeasterly.

Seas will already be 4-6ft in the NC zones mainly east of Cape Fear by Sunday evening, necessitating a Small Craft Advisory for those zones. This will be driven by a SE swell with a period of 8 sec. Seas will increase across all waters during Sunday night, with SCA conditions for the SC zones anticipated mainly after midnight. After sunrise on Monday, increasing south winds are expected to cause the swell direction to veer to southerly. Seas will continue to rise until they peak in the 6-9 ft range late Monday afternoon just prior to the front's arrival, with the long fetch of south winds resulting in a period around 7 sec. Behind the cold front, seas will subside, eventually falling below 6 ft by late Monday night and holding around 2-4 ft for Tuesday and Wednesday. However, they will be a mix of lingering southerly swells with a period around 8 sec and westerly wind waves with a period around 3 sec, creating confused seas.

ILM WATCHES/WARNINGS/ADVISORIES

NC...None. SC...None. MARINE...Small Craft Advisory from noon Sunday to 11 AM EDT Tuesday for AMZ250-252.