textproduct: NWS Wilmington

WHAT HAS CHANGED

Coastal Flood Advisory issued for the Lower Cape Fear River this evening between 8 PM EDT and 10 PM EDT. Gale Watch issued for all coastal waters on Saturday and Saturday night.

Slight forecast changes, primarily related to rainfall amounts and timing. Expanded on potential coastal hazards now that models provide some clarity on timing of the strongest gradient.

Aviation discussion updated for 18Z TAFs.

KEY MESSAGES

1) Heavy rain is likely late Saturday and Sunday. Isolated flash flooding and severe weather is possible, especially near the coast.

2) Hazardous surf conditions and tidal flooding are likely this weekend.

3) Hazardous marine conditions are likely this weekend.

DISCUSSION

KEY MESSAGE 1... Heavy rain is likely late Saturday and Sunday. Isolated flash flooding and severe weather is possible, especially near the coast.

Hurricane Isaias is currently located in the southern Gulf and is expected to make a northward turn imminently. The storm is still expected to strengthen over the Gulf through forecasted landfall on Friday evening. Across the Carolinas, a wedge of dry air will maintain cloudy, but dry weather through early Saturday. Tropical moisture lifting northward on Saturday will eventually work through a stubborn dry layer with heavy rain developing by Saturday evening from south to north.

The onset of rain continues to be a forecast challenge. Dry air in the low levels will slowly saturate throughout the day. Models are typically too bullish on precip onset, so the latest forecast pushes the start time a few hours later than the model consensus. WPC QPF cluster analysis even shows 2 of the 4 clusters with no QPF for the Wilmington area until 00Z Sunday. Rainfall is expected to begin across southern points of our area during the late morning with rain at all locations by late Saturday afternoon or Saturday evening. Rainfall will be light to begin, so Saturday plans should be minimally impacted. The heaviest rain arrives on Saturday night.

Rain amounts continue to carry limited confidence. In general, it appears coastal areas are favored for the highest totals. Latest ensemble probabilities show a mean of 2-4 inches near the coast, 2-3 inches in areas near and just east of I-95, and around 2 inches westward. Latest hi-res model ensembles are starting to show convective bands developing south of the warm front on Saturday night and early Sunday. Developing offshore, these bands are expected to move northwestward with heavy rain rates and enhanced totals. These convective elements are impacting model spread significantly and driving raw QPF model output to some extreme values at the coast. After collaboration with neighboring offices and WPC, we limited the QPF increase to slightly above the relative ensemble mean. This includes a mention of 4-6 inches at the coast (which takes into account the generalized potential for convection). Note that isolated flash flooding is possible, especially in our larger coastal communities and around a higher-than-normal high tide on Saturday night through early Sunday.

Onshore flow also brings the potential for severe weather, mainly Saturday night, less so into early Sunday. Weak instability in the low levels and effective helicity may support a few onshore-moving waterspouts on Saturday night. Confidence is low at this time due to poor instability and relative distance from the storm center. Winds turn southerly on Sunday, but activity becomes more isolated with the addition of dry air. This could slightly enhance instability near the end of the event, early Sunday.

Isaias will merge into the background 500 mb flow on Sunday. Dry air building across the southeast on Sunday afternoon may push a trough through the Carolinas with some additional development along this boundary. The latest forecast extends rain chances into Sunday evening, but rain chances should decrease rapidly thereafter. NBM is holding on to precip too long at this stage, likely due to timing differences in global models. A slight adjustment was made to account for this, but timing differences are still apparent in the forecast.

KEY MESSAGE 2... Hazardous surf conditions and tidal flooding are likely this weekend.

Increasing onshore flow this weekend will produce rough surf, strong rip currents, and lead to minor coastal flooding for all area beaches.

Dangerous rip currents are expected as onshore flow increases late Friday, peaking on Saturday and Sunday. Breaking waves in excess of 5 feet are possible and will produce dangerous surf in combination with strong longshore currents. A High Surf Advisory may be needed for Saturday. Lingering rough surf is also possible on Monday, even as winds turn westerly. Beach erosion is also possible during each high tide cycle on Saturday and Sunday.

Minor coastal flooding is expected to continue along the lower Cape Fear River through the remainder of this week due to a combination of onshore winds and the new moon. Water levels will increase this weekend, but should remain within minor flood levels. The most impactful high tides are expected Saturday evening, Sunday morning, and Sunday evening. Area beaches are also expected to experience minor coastal flooding with these tidal cycles. The Saturday evening high tide is likely to be the most impactful for these areas, but each high tide cycle has the potential to produce minor flooding.

KEY MESSAGE 3... Hazardous marine conditions are likely this weekend.

See marine section.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

VFR. Light winds will turn southeasterly near the coast due to a weak coastal influence. Ground fog is possible tonight due to light winds and near-surface moisture, mainly at the coast despite cloud cover. Lower confidence for inland areas due to good mixing today. Cirrus will maintain BKN to OVC skies.

Extended Outlook... VFR through at least Friday evening, outside of any patchy/shallow early morning fog. Flight restrictions due to low cigs are likely starting either Saturday or early Sunday as tropical moisture from Isaias works downward. Vis restrictions are possible at times in heavy rain, mainly for coastal terminals from late Saturday through Sunday. VFR should return as dry air filters in late Sunday night or on Monday.

MARINE

Quiet marine conditions are expected overnight. Northeast winds increase on Friday as an area of high pressure to the north builds a wedge across the Carolinas. The gradient will tighten late Friday as Isaias makes landfall along the Gulf coast. A Gale Watch has been issued, starting early Saturday morning, as a rapid increase in winds and seas is expected. SCA conditions develop prior to the beginning of the Gale Watch, but this period of time appears to be short enough to forgo the SCA for now. Confidence is lower in SC nearshore waters depending on the location of the frontal boundary.

Latest wind probs favor the potential for sustained gales and gusts to 40 knots in the offshore waters late Saturday and into early Sunday. Confidence is lower for the nearshore waters with wind gust probabilities dropping to just above 50% as you approach the coastline. Winds and seas progress quickly with the northward moving front and associated gradient. SCA conditions should return for the nearshore waters late Saturday and offshore gales ending early Sunday. Seas build rapidly on Saturday to 6-8 feet for nearshore areas and 8-10 feet offshore by late Saturday.

Southerly flow remains elevated through Monday as the surface low slowly lifts toward the Mid-Atlantic and a post-frontal gradient develops. Seas may continue to exceed 6 feet through early Monday morning.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Coastal Flood Advisory from 8 PM to 10 PM EDT this evening for NCZ107. SC...None. MARINE...Gale Watch from Saturday morning through late Saturday night for AMZ250-252-254-256. Gale Watch from Saturday morning through Sunday morning for AMZ280-284.