textproduct: Charleston

WHAT HAS CHANGED

The Marine and Aviation sections were updated.

KEY MESSAGES

- 1) Below normal temperatures this week as high pressure prevails and a powerful nor'easter meanders off the Mid- Atlantic coast.

- 2) Risk for high surf will end this morning along much of the Georgia coast. High risk for rip currents for all beaches through this evening.

DISCUSSION

KEY MESSAGE 1: Below normal temperatures this week as high pressure prevails and a powerful nor'easter meanders off the Mid- Atlantic coast.

Through Tonight: The synoptic pattern has changed little in the past 24 hours characterized by a slowly deepening nor'easter centered about 300 miles southeast of Cape Hatteras and expansive high pressure building south over the entire Southeast U.S. and Deep South. The surface cyclone will gradually become more vertically stacked with its upper-level counterpart over the next 12-24 hours and gradually occlude based on the evolution of the 1000-500 hPa thickness ridging pattern and related gradients.

The nor'easter is forecast to gradually meander offshore of the North Carolina Outer Banks through tonight. Locally, high pressure will prevail keeping the dry, fall weather in place across the Lowcountry and Southeast Georgia. Some low-level cloudiness spreading in from off the Atlantic this morning will bring some cloud cover for areas south of I-16 and possibly along parts of the lower South Carolina coast, but as the main steering flow gradually backs with time, these clouds should mix out as even drier air to the north advects into the region. Sunny to mostly sunny skies are expected with highs warming into the 67-72 degree range (values right at the Third Quartile of the 19/01z NBM interquartile range/IQR), warmest along/south of the I-16 corridor and nudging up into the US-17 corridor into parts of the Charleston Metro Area. Gusty north winds that have plagued the region for the past few days will gradually wane this afternoon as the surface high builds even farther to the south and the tightest pressure gradient between it and the nor'easter shifts farther offshore. Much lighter winds are expected overnight and the boundary layer may even fully decouple in some locations resulting in light/variable to calm winds. This opens up the possibility for some modest radiational cooling to occur given the dry airmass in place and ongoing clear skies. Lows were adjusted down a bit off the 19/01z NBM as a result, ranging from the lower 40s well inland and up into the Francis Marion National Forest to the mid-upper 40s along the coastal corridor with lower-mid 50s at the beaches and Downtown Charleston.

Tuesday through Sunday: A nor'easter will be located well off the coast of NC Tuesday morning. Meanwhile, strong High pressure will be located north of the Great Lakes region, with it's periphery stretching well down into the Southeast U.S. The nor'easter will very slowly shift away from our region through Thursday, then accelerate away Friday into the weekend. Though, the periphery of the distant High will persist across the Southeast U.S. This will bring our area dry conditions with sunny skies. MOS, 850 mb temperatures, and low-level thickness values, support high temperatures in the middle to maybe upper 70s Tuesday and Wednesday, and the lower to middle 70s Thursday into the weekend. However, the dry conditions, clear skies, and light to calm winds at night will cause decent radiational cooling. This will lead to low temperatures that are several degrees below normal this week.

KEY MESSAGE 2: Risk for high surf will end this morning along much of the Georgia coast. High risk for rip currents for all beaches through this evening.

High Surf: The risk for 5+ ft breakers will linger along the middle and upper Georgia coast through early afternoon. After that, the risk for high surf wane a bit as seas offshore gradually diminish. A High Surf Advisory is in effect for the beaches along the middle and upper Georgia coast through 2 PM.

The last several NWPS runs have shown larger breaking wave heights (4-7 ft) occurring along the Charleston County beaches later this afternoon into tonight with the arrival of a more powerful, higher period (~12 sec) back-swell. This is even as seas offshore slowly subside. Surf shop reports out of Folly Beach Sunday reported messy breakers of 3-4 ft, so it would not take much of an increase in swell period and/or swell power to breach the 5 ft High Surf Advisory criteria. If these model trends persist, a High Surf Advisory may need to be issued for the Charleston County beaches to cover the mid afternoon into the Tuesday morning time period. Guidance does show breakers closer to 4-5 ft occurring farther down along the remainder of the lower South Carolina and Georgia coasts as the long period swell train works to the southwest, so the need for an advisory there looks a bit more marginal.

Rip Currents: Today: The combination of gusty winds and elevated, lingering back- swell will support a high risk for rip currents at all beaches through this evening.

Tuesday: Strong and gusty N winds at daybreak will ease during the morning and into the afternoon. Wave heights will be subsiding during this time period. However, the distant nor'easter will be generating a 4-5 ft NE wave at 12-13 seconds just offshore. We have a Moderate Risk of rip currents for all of our beaches. However, if the trajectory of these waves change to more directly impact our beaches, then that may raise the risk higher.

Wednesday: The risk should lower for our SC beaches as the waves subside. However, it may remain elevated for our GA beaches, possibly through the end of the week.

AVIATION /10Z MONDAY THROUGH FRIDAY/

19/12z TAFs: KCHS/KJZI/KSAV: VFR through 20/12z. Satellite shows an area of stratocumulus just offshore the Charleston County coast. This may brush KJZI through daybreak. A TEMPO for MVFR cigs may be needed pending last minute satellite trends. Gustiness will diminish during the mid- late afternoon hours.

Extended Aviation Outlook: There are no concerns.

MARINE

Today and Tonight: Elevated winds and seas will impact the coastal waters through tonight even as the tightest pressure gradient between the high building across the Southeast U.S. and the nor'easter off the North Carolina Outer Banks gradually shifts farther to the east and northeast. North/northeast winds should diminish to 15-20 kt with gusts to 25 kt nearshore and 20-25 kt over the offshore waters during the afternoon and overnight hours. Seas 4-9 ft nearshore, 9-13 ft offshore will slowly subside to 4-8 ft nearshore, 7-10 ft offshore by late afternoon into tonight. Small Craft Advisories will be in place for all nearshore waters through tonight, including the Georgia offshore waters.

Tuesday through Friday: A nor'easter will be located well off the coast of NC Tuesday morning. Meanwhile, strong High pressure will be located north of the Great Lakes region, with it's periphery stretching well down into the Southeast U.S. The nor'easter will very slowly shift away from our region through Thursday, then accelerate away Friday into the weekend. The interaction between these two synoptic features will be generating strong and gusty N winds Tuesday morning. Winds are expected to ease into Tuesday afternoon, followed by a brief surge Tuesday evening into Wednesday morning. Winds become noticeably lower later Wednesday into Thursday.

Seas will be their highest Tuesday morning, then gradually subsiding into Thursday. Though, the highest waves will remain in the waters beyond 20 nm. We have Small Craft Advisories in effect for the waters within 20 nm through Wednesday. Though, the end time may need to be adjusted based on the future forecasts of the sea state, especially longer-period NE waves from the distant nor'easter.

There appears to be another surge of winds and higher seas on Friday, with the potential for more gales.

CHS WATCHES/WARNINGS/ADVISORIES

GA...High Risk for Rip Currents through this evening for GAZ217-219-239- 241. High Surf Advisory until 2 PM EDT this afternoon for GAZ217- 219-239-241. SC...High Risk for Rip Currents through this evening for SCZ148>151. MARINE...Small Craft Advisory until 2 PM EDT Wednesday for AMZ360-362. Small Craft Advisory until 6 PM EDT Wednesday for AMZ364. Small Craft Advisory until 6 AM EDT Saturday for AMZ384.