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/coastal trough 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. Along and just behind the warm front, the precip will likely become more banded and contain some embedded convective elements with very heavy rain. It's these bands that will pose the greatest risk of flooding. The time period of most concern for these heavy rates and flooding potential are overnight Sat night into early Sun morning. 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 with convection nearing the coast, whether some of the moisture transport will be cutoff by strong convection over the Gulf Stream is in question. If this happens, amounts would be quite a bit less for areas outside of the coastal waters. 09/12z REFS LPPM has come up with amounts however, suggesting amounts in the 4-6+" range possible for the immediate coast and OBX zones. For this reason, have gone ahead and issued a Flood Watch for all but the interior ENC zones effective later Sat afternoon through early Sun afternoon.
KEY MESSAGE 2, Possibility of convective elements reaching the coast would warrant a tornado risk later Sat evening as the aforementioned warm front lifts through ENC. Plenty of low level helicity in place with the inc serly flow in place and strong veering wind profile downstairs. The greatest tor threat would likely be focused near the srn/sern coast where better instability advects onshore. For Sunday, flow becomes more unidirectional with swrly low level winds ensuing during the afternoon. While helicity wanes, better instability will be in place due to daytime highs expected in the low 80s and TD's in the 70s. Will have to watch for continued severe threat Sunday afternoon ahead of the cold front, esp if there are some breaks in the cloud cover.
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 drying conditions developing Sun night into Mon.
KEY MESSAGE 3, Strengthening onshore winds, building seas, and higher than normal 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.
00z Saturday through Wednesday Forecast confidence is moderate to high over the next 24 hours.
Widespread mid-level clouds will steadily lower through the night and into Saturday as low-mid level moisture advection increases. The TAFs reflect this expectation, with CIGs eventually falling to MVFR levels late tonight or Saturday morning. Lifr/ifr CIGs are then possible by the end of the 00z TAF cycle. However, it should be noted that guidance differs quite a bit regarding the timing of when IFR/LIFR CIGs will occur. Because of this, I held off on introducing those flight categories just yet, and instead opted to introduce a SCT IFR level cloud base to show a rough idea of when these conditions may be realized. Some guidance is much quicker with IFR/LIFR conditions, though. The gradient will begin to strengthen as a frontal zone strengthens to the south of ENC. It's expected that this will lead to increasing east winds on Saturday, with gusts of 20-30kt likely.
Outlook (Sat night through Wed): Widespread rain, heavy at times, and gusty nerly to erly winds Sat night along with likely IFR conditions. Potential for embedded TSRA Sat night, esp coastal terminals. Clearing conditions expected by Monday but with gusty wrly to nwerly winds behind it.
Good boating conditions are expected to continue through tonight with nerly winds gradually inc overnight to 10-20 kt. Nerly gradient strengthens through the day Sat, with winds ramping up to gales later in the afternoon from S to N across the coastal waters. For the sounds and rivers, SCA winds develop with these nerly pinched gradient winds.
Outlook (Sat night through Wed): Strong nerly winds turn to the east and gradually weaken below gales later Sat night as winds turn further to the s and SW. Gales remain for the outer waters with this swrly flow on Sunday. For the rivers and sounds, winds should remain in the strong SCA range with speeds of 20-30 kt expected. In addition to the winds, nearshore seas are expected to peak in the 6-10ft range at 7-8s. For the outer coastal waters, seas may reach 8-11ft.
Thunderstorms capable of strong winds and waterspouts will be possible as well, mainly Saturday night into Sunday. Conditions gradually improve by Sun evening with clearing returning by Monday morning as high pressure begins to build in.
Nc, flood watch from Saturday afternoon through Sunday afternoon for ncz047-081-092-094-194>196-198-199-203>205. High surf advisory from midnight Saturday night to 8 pm EDT Sunday for ncz204-205. Marine, small craft advisory from 2 pm Saturday to midnight EDT Sunday night for amz135. Small craft advisory from 2 pm Saturday to 2 am EDT Sunday for amz136-137. Gale watch from late Saturday night through Sunday morning for amz150-180. Gale warning from 6 pm Saturday to 2 am EDT Sunday for amz152- 154. Gale warning from 2 pm Saturday to midnight EDT Saturday night for amz156-158-186-188. Gale warning from 6 pm Saturday to 2 pm EDT Sunday for amz182- 184. Small craft advisory from 6 pm Saturday to 8 pm EDT Sunday for amz230-231.
