textproduct: NWS Wilmington

WHAT HAS CHANGED

Small Craft Advisory expanded to include nearshore waters from Little River to the South Santee River in South Carolina. Severe weather risk has increased for Monday. 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.

A warm front is expected to lift across the forecast area late this afternoon or evening. In its wake, influx of moist air, with widespread 60s dewpoints overspreading the FA tonight. Temps to remain steady in the 60s or may actually rise to 70 by daybreak Mon. Will have a developing southerly LLJ jet, with speeds 40 to possibly 50 kt just off the deck by sunrise Mon. Any Thunderstorm or just a strong shower with an elevated core, will have the potential to tap these winds and bring them to the sfc as a strong to severe gust. The 1 negative will be the strong onshore southerly winds affecting the area that will push the sfc based "stable" marine layer inland. The further inland one goes, the less depth this marine layer will encompass and therefore a better shot for those strong/severe thunderstorm wind gusts to be tapped and brought to the sfc.

Confidence continues to increase in the potential for severe weather on Monday. A strong cold front will lead an amplifying trough over the central US. Low level winds will strengthen late Sunday, continuing on Monday ahead of the late afternoon cold front. Southerly moisture transport and divergence aloft will support pre-frontal convection through the afternoon. The main squall line will arrive later in the afternoon or by early evening at the coast. Severe weather will be possible during both periods of convection.

Convection during the morning and early afternoon is likely to be isolated to widely scattered due to limited forcing and some weak subsidence ahead of the negatively-tilting trough. HREF probabilities favor showers and storms in coastal northeast SC and southeastern NC where stiff moisture transport taps into the warmer waters of the Gulf Stream. Despite these areas having the best chance of showers and storms, pre-frontal convection is possible for all areas.

Instability in pre-frontal convection will be limited compared to the increasingly impressive shear. Instability will naturally increase during the early afternoon and coverage of showers and storms should increase accordingly. Helicity values favor a few tornadoes in these developing showers and storms, especially where instability is greatest and storms can achieve brief supercellular structure. Given the strong winds just above the boundary layer, the damaging wind threat remains the primary concern. There is some uncertainty due to the marine layer which could generate poor lapse rates near the immediate coast and limit convective mixing.

The main squall line will arrive late Monday afternoon or early Monday evening. Steering flow being so strong, deterministic models are likely showing the arrival of the line slightly later than reality. Timing probs at the 10th percentile show the line could enter the area as early as 18Z (22Z at the coast) which would coincide with peak heating. Strong winds mixing to the surface and max instability bring high confidence in damaging wind gusts occurring across parts of the area. SPC reflects this potential with their latest outlook, advertising a 60% chance of damaging wind gusts within 25 miles a point for our I-95 counties. This coincides with a Moderate Risk (Level 4 of 5). Damaging wind probabilities are 45% elsewhere in our forecast area, so severe weather remains likely outside of the higher risk areas. Damaging wind gusts up to 70 mph are also possible across all areas. Tornadoes will be a concern, both ahead of the line and embedded within the QLCS. A few tornadoes could be strong (EF2+).

In addition to severe weather, clear-air wind gusts up to 45 mph are possible at times. Stronger winds are expected at the coast due to the unobstructed southerly fetch. A wind advisory may be needed for portions of the area, especially the Grand Strand and coastal Brunswick County.

KEY MESSAGE 2... Freezing temperatures are expected on Tuesday night and Wednesday morning. Subfreezing temperatures are possible on Monday night for parts of the area. Near freezing temperatures and frost are possible again on Wednesday night. Freezing temperatures will pose a threat to unprotected plants.

High pressure behind the cold front will build across the central and eastern US. Cold air advection on Monday night could bring temperatures to near freezing for areas along and west of I-95. Confidence is low due to mixing occurring throughout the night. Probabilities for temperatures at or below freezing are around 30%-40% for Darlington and Marlboro counties. Typically, deterministic models exaggerate the strength of the initial cold air, so we're holding off on a Freeze Watch for Monday night at this time.

There is much higher confidence in freezing temperatures on Tuesday night. Light winds and clear skies within a dry air mass will allow for nearly ideal radiative conditions. Forecast lows may not be cold enough at this time with widespread temperatures in the upper 20s to around 30. Probabilities have increased in sub-freezing temperatures for much of the area. Freeze warnings will likely be needed.

Northeasterly flow on Wednesday will reinforce cool air. Light winds and clear skies could bring temperatures close to freezing again through Thursday morning. Confidence is low at this time due to the existence of a weak pressure gradient feeding northeast winds (which are typically under-represented in our deterministic forecasts). Dew points increase slightly due to the easterly component of the wind and a full day of mid- March sun angles may moderate the existing high which may require us to nudge lows upward by a couple of degrees.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

VFR will only dominate thru mid-morning. There-after, looking at PROB30 groups for MVFR shower activity and ceilings in response to a northward moving warm front that should push thru from late this afternoon thru this evening. This WFP northward movement will be aided by the approach of a rather large amplifying/intensifying upper trof from the west. Have only included a tsra possibility late this evening thru the overnight, for initially the coastal terminals as the warm sector instability increases in conjunction with the passage of an embedded impulse in the mid-levels. Will observe a NE-E wind AOB 10 kt to start, veering to the ESE-SE 10-15 kt g20 kt as the warm front approaches. After the WFP, look for winds to become southerly 10-15 kt, with g20kt at the coastal terminals. MVFR ceilings to become VFR at all sites as the warm front lifts further north and away from the region. However, the threat for discrete showers/tstorms, especially inland, will occur during the pre-dawn Mon hrs. In addition, will need to monitor for any onshore movement of IFR vsby reduced sea fog and/or IFR low stratus ceilings.

Extended Forecast...An approaching strong cold front Mon will be well preceded by increasing surface winds gusting to 30 to 40kt and scattered showers and discrete strong to possibly severe thunderstorms. Associated with the front itself, Mon afternoon and early evening will be a line of severe thunderstorms with gusts to 50+ knots. After the CFP, VFR conditions should return late Mon night and prevail thru Thu. Much colder air and dry conditions will prevail.

MARINE

Through Tonight... SCA remains in effect for the NC Waters. SCA has been raised for the SC Waters, commencing at 9 pm this evening. Sfc pg to tighten across the local waters thruout this period as the center of a 1030+ mb high pulls off to the northeast and a 990 to 980 mb low reaches the Great Lakes. During today, a sfc warm front will lift northward, crossing the area late this afternoon and exiting to the north this evening. Looking at veering winds this period with increasing speeds as the sfc pg further tightens while LL winds increase to 40 to possibly 50 kt by daybreak Mon. The CWF will reflect with further increasing southerly winds by daybreak Mon. Seas will reflect the increasing winds with the hier winds and resulting seas offshore initially bleeding into the local waters prior to the WFP. After FROPA, seas will further increase with 6+ ft seas common across all waters tonight. The cool local SSTs will help keep the gustiness in the wind field aloft in check from reaching the ocean sfc. SCA winds, albeit strong, will manifest themselves late tonight, but moreso during Mon.

Monday through Thursday Night... Winds continue to strengthen on Monday ahead of a strong cold front. SCA conditions will be ongoing and a few gusts could approach gale force during the late morning and through the afternoon. Showers and thunderstorms will be developing over the nearshore waters and these storms could produce stronger wind gusts and a few waterspouts. Seas peak late Monday as the cold front begins to work its way offshore. The pressure gradient remains compressed on Monday night as winds turn northwesterly. Cold advection behind the cold front should keep winds and seas elevated into early Tuesday. Transient high pressure will move overhead and just offshore on Tuesday evening. Northerly flow will become northeasterly and should maintain some breeziness into Thursday as high pressure sits to our north and low pressure develops well to our south and east.

ILM WATCHES/WARNINGS/ADVISORIES

NC...None. SC...None. MARINE...Small Craft Advisory from noon today to 11 AM EDT Tuesday for AMZ250-252. Small Craft Advisory from 9 PM this evening to 11 AM EDT Tuesday for AMZ254-256.