Key MESSAGE 1, A plume of tropical moisture characterized by 2"+ PWATs and dewpoints in the 70s is forecast to get pulled north through the southeastern CONUS over the weekend to the east of the northward-advancing remnants of TC Isaias. At the surface, a warm front will trudge north along the southeastern coastline, likely with a weak, non-tropical area of low pressure developing along it. Along and ahead of the warm front, a strong southeasterly LLJ will develop, supporting increasing WAA and moisture transport into the area with a shield of moderate to heavy rain developing across the coastal Carolinas. It's expected that this initial swath of precip should support 1-3" of rainfall for much of ENC. Along and just behind the warm front, the precip will likely become more banded and contain some embedded convective elements. It's these bands that will pose the most intense rainfall rates, as well as the greatest risk of flooding. Convective guidance suggests rainfall amounts as high as 4-6"+ are possible if those bands manage to reach the coast. As is often the case in these scenarios, it's unclear how far inland the instability axis and, consequently, the heaviest rates will get.
Rainfall exceedance probabilities from the NBM have stabilized from last night, still suggesting a 20-30% of seeing more than 4" rain focused along the southern coastline roughly along the NC24 corridor including Morehead City and Jacksonville. The 00z suite of guidance has also stayed consistent with only minor fluctuations in overall totals. Wpc continues to keep much of our area in a Slight (Level 2 of 4) risk of excessive rainfall and flash flooding. After collaboration with neighboring offices, consideration of a Flood Watch has been deferred to the next shift with heaviest rainfall likely not until Sunday.
KEY MESSAGE 2, Steady increase in moisture through the weekend peaks on Sunday contributing to an increase in instability across portions of ENC, greatest south of Highway 264. Factors like cloud cover will determine how much we end up getting, but ensemble averages suggest CAPEs as high as 1000+ J/kg in some spots. While low- level forcing is in question, it's a setup that bears watching for a conditional severe thunderstorm risk. If a stronger storm develops, the environment would be favorable for rotating cells and a tornado threat given elevated SRH along the front.
The remnants of Isaias are forecast to round the upper level ridge and move off the Northern Mid-Atlantic Coast early next week. Drier air then moves in behind it with a lower chance of showers and thunderstorms by Tuesday.
KEY MESSAGE 3, Strengthening onshore winds, building seas, and higher astronomical tides may support some coastal impacts this weekend. There remains uncertainty on the magnitude of impacts, especially given the shorter-duration of the stronger winds, as well as what should be a weaker swell compared to the recent Nor'easter. With that in mind, there will be an increased risk of rip currents and large waves, at a minimum. High Surf Advisory remains in effect between Ocracoke and Oregon Inlets focused on Sunday. Additionally, some of the most vulnerable locations along the Outer Banks may see at least some minor issues, especially with Sunday morning's high tide (7-9am). There may also be some minor coastal flooding around sounds and tidal waterways around the Crystal Coast due to the higher astronomical tides.
12z Friday through Tuesday Highly transient and shallow fog will quickly ebb this morning giving way to VFR conditions with overcast skies as cirri stream off of TC Isaias in the central Gulf. Light SW winds are expected to become northeasterly today as front clears the area. This will be most felt across NOBX where winds will jump to 10-15 kt with a few higher gusts. As moisture deepens today cloud bases will lower, and southern terminals will begin to flirt with MVFR cigs by sunrise Saturday.
Outlook: Conditions change over the weekend as tropical moisture gets pulled north off the Gulf and into the Carolinas. This will support periods of low CIGs, widespread RA, and potentially a TSRA risk. Gusty southeast to south winds are likely as well, especially Saturday into Sunday along the Outer Banks where gusts could near 35 kt. Clearing conditions expected by Monday but with gusty northwesterly winds.
Good boating conditions are expected to continue through today as high pressure remains overhead and winds are generally at, or below, 10-15kt. Dry front pushes across area waters later today ushering in northeasterly winds by morning with the potential for a stronger surge of 10-15 kt, mainly north of Cape Hatteras.
Outlook: A strong pressure gradient develops in advance of a warm front and a weak, non-tropical, low that is forecast to lift north along the southeast coastline this weekend. It's likely that this will support a period of moderate to strong east to southeast winds Saturday into Saturday night, followed by a period of moderate to strong south to southwest winds Sunday into Sunday night. Most waters will almost certainly reach Small Craft Advisory criteria (25kt gusts), with increasing risk for Gales at least across all outer waters beyond 20nm. Gale Watches were expanded across these zones this morning, but held off on nearshore zones for now given some lingering uncertainty. Additional headlines will likely be needed this afternoon. In addition to the winds, seas are expected to peak in the 6-9ft range at 7-8s. For the outer coastal waters, seas may reach 8-10ft.
Thunderstorms capable of strong winds and waterspouts will be possible as well, mainly Saturday night into Sunday. Conditions gradually improve early next week as high pressure builds in behind the weekend system.
Nc, high surf advisory from midnight Saturday night to 8 pm EDT Sunday for ncz204-205. Marine, gale watch from Saturday morning through Sunday morning for amz158-188. Gale watch from late Saturday night through Monday morning for amz180. Gale watch from Saturday evening through Monday morning for amz182. Gale watch from Saturday afternoon through Monday morning for amz184-186.
