textproduct: Charleston

WHAT HAS CHANGED

Key Message 1 was updated. An equipment section was introduced.

KEY MESSAGES

- 1) Unsettled weather with a risk for minor/flash flooding this morning.

- 2) Cooler temperatures along with breezy to windy conditions are expected into the middle of next week.

- 3) Coastal flooding and significant coastal hazards are expected during the middle of the week.

DISCUSSION

KEY MESSAGE 1: Unsettled weather with a risk for minor/flash flooding this morning.

No major changes were made for the update this morning other than to lower temperatures a degree or two for most locations. The front looks to be south of the area with cool high pressure slowly building south into the region.

Today: The aforementioned front will likely be on the move during morning hours, shifting south/offshore during the day and remaining the primary focus for rain/shower activity with the potential for flooding issues into mid-late morning hours. Latest forecast soundings indicate drier air beginning to infiltrate from the north and inland by mid-morning as high pressure wedges south into the area. This will put an end to a heavy rainfall and/or high rainfall rate concerns locally with time, starting across northern and inland areas early morning, followed by southern/coastal areas in Southeast Georgia late morning. Given the expected trend, the greatest risk for minor and/or perhaps limited flash flooding should occur south of I-16 in Southeast Georgia, but mainly closer to the Altamaha River/coastal Georgia areas where deep moisture (PWATs 2.25-2.5 inches) and low CAPE values persist prior to fropa. The latest WPC Excessive Rainfall Outlook follows this trend, marking parts of Southeast Georgia near the Altamaha River/coastal Georgia areas in a Marginal Risk for excessive rainfall. Once fropa occurs, drier air begins to settle into the area, limiting light shower activity to far southern zones during afternoon hours, prior to the bulk of activity shifting offshore late afternoon into early evening.

KEY MESSAGE 2: Cooler temperatures along with breezy to windy conditions are expected into the middle of next week.

Today: A cold front is currently moving through southeast South Carolina, pushing into and through southeast Georgia later this morning into the afternoon hours. Cooler air begins to filter in throughout the day, and with the mostly cloudy skies daytime highs will struggle to get higher than the mid 70s. Northeasterly winds start to get breezy during the afternoon hours, though gusts should remain below 20 mph.

Tuesday: Overnight into Tuesday, a strong 1025mb surface high pressure moves out of the Great Lakes region and towards the northeast coast, while a surface low pressure develops along the front south of the region over the Atlantic during the morning. This will result in increasingly breezy/windy conditions on Tuesday, peaking in the afternoon and decreasing into the overnight hours. As noted from the previous discussion, the trend of slightly weaker winds continues as the pressure gradient tightens over the Atlantic. Now areas well away from the coast may see some gusts to near 20 mph, while those along the coast are in the 30 to 40 mph range, highest across southeast Georgia. Elevated and/or more exposed areas along the coast may exceed 40 mph, which may necessitate the need for a wind advisory along the southeast Georgia coastline, with a Lake Wind advisory possible as well for Lake Moultrie where wind gusts to 30 knots are possible.

As weak shortwaves make their way around the base of the trough, light to moderate rain is possible mostly across portions of southeast Georgia, though rainfall chances may extend north into Beaufort/Hilton Head area. As dry air filters in from the north, there will likely be a sharp gradient where rain falls, with precipitable water values as low as 0.6-0.7" near Lake Moultrie and as high as 1.7-1.8" near Darien. With h925 temps dropping into the 16-18 deg C range, high temperatures remain in the lower to mid 70s

Wednesday: The surface high pressure weakens as it slides down the east coast to near 1020 mb by daybreak, with the aforementioned surface low remaining well offshore of GA/FL keeping the surface pressure gradient elevated across the region before decreasing throughout the afternoon hours. This will result in areas far from the coast likely experiencing gusts below 20 mph while those along the coast will be closer to 25 to 35 mph during the morning hours, decreasing into the afternoon and evening hours. Chances for rainfall remain low as dry low-mid levels overspread the area. Temperatures on Wednesday remain uncertain as models tend to underestimate how quickly the CAD erodes, though the increasing sunshine should bring afternoon temperatures back up into the upper 70s.

Thursday-Saturday: As weak ridging occurs aloft, surface high pressure looks to linger across the southeast region with the area of low pressure well offshore of GA/FL on Thursday. On Friday, the surface high pressure strengthens as it moves towards the northeast coast, potentially bringing another round of cooler and wetter conditions on Saturday as h500 shortwaves move overhead.

KEY MESSAGE 3: Coastal flooding and significant coastal hazards are expected during the middle of the week.

Coastal Flooding: Strong northeast winds associated with a wedge of high pressure inland and low pressure offshore in combination with high astronomical tide levels will likely support coastal flooding issues this week. The pattern suggests a minor/moderate saltwater inundation setup Tuesday and Wednesday during late afternoon/early evening high tide cycles, with the greatest risk for moderate coastal flooding along Charleston and Coastal Colleton Counties. There is a more limited risk for minor coastal flooding issues into Thursday along the SC coast. Coastal Flood Advisories will likely be needed mid-week.

Rip Currents/High Surf/Beach Erosion: The setup looks highly favorable for strong and dangerous rip currents Tuesday into Wednesday with breezy/gusty northeast winds expected. In addition, there will be an increased risk for high surf along the beaches with the latest guidance still showing a potential for 4-7 ft breakers developing in the surf zone (largest for coastal Georgia) by Tuesday afternoon and lingering into Wednesday. A High Surf Advisory should be needed for some beaches. Significant beach erosion is also likely, particularly around times of high tide. Guidance suggests beach erosion chances are highest around the Tuesday afternoon high tide.

AVIATION /13Z MONDAY THROUGH FRIDAY/

CHS/JZI: Periods of IFR conditions are expected through ~18Z today, mainly in the form of low cigs, but also during light to moderate rain/showers through morning hours. Conditions should improve to MVFR by 21Z today as drier air filters into the region, followed by VFR by around 01Z Tuesday. However, low-end VFR cigs should persist through 12Z Tuesday.

SAV: MVFR conditions could prevail for an hour to start out this morning, but expect IFR conditions to occur with light to moderate showers through much of the morning and into early afternoon. IFR cigs should then prevail through much of the afternoon, then slowly improve improve to MVFR by around 01Z Tuesday. MVFR conditions should then prevail for much of the night, although there are some signals of low-end VFR cigs occurring between 06Z-12Z Tuesday.

Extended Aviation Outlook: Gusty winds and MVFR (or lower) stratus are likely on Tuesday, with breezy winds continuing on Wednesday.

MARINE

Today: A cold front will be pushing through and south of the region today, resulting in increasingly breezy northeasterly winds developing. A wedge of strong high pressure builds into the region overnight into Tuesday, further strengthening the winds with gusts of 25 to 30 knots by daybreak. There remains a possibility for Small Craft Advisories to be needed, but given how quickly Gales may develop Tuesday morning we held off for now as the watch starts at 5 AM. For the Charleston Harbor, a Small Craft Advisory goes into affect at 5 AM Tuesday.

Tuesday: Marine conditions are expected to quickly deteriorate throughout the morning hours as wind gusts of 35 to 40 knots build, strongest across both zones from Savannah to Altamaha Sound, GA as a surface low pressure develops along a front offshore of GA/FL. 6 foot waves build quickly build with the windy conditions, with 4-7 ft waves along the coast by the afternoon and 10-11 ft waves in all zones from 20-60nm offshore. Wind strength decreases into the overnight hours as the low pressure moves further out to sea, bringing an end to the Gale conditions during the early overnight hours on Wednesday. For the Charleston Harbor, the Small Craft Advisory expires at 2 AM.

Wednesday: The pinched surface pressure gradient continues into Wednesday, though wind gusts will have lowered into the 25 to 34 knot range which will necessitate the need for a Small Craft Advisory. Wave heights are expected to slowly subside on Wednesday, with 4 to 5 foot breaker waves along the coast by the evening hours. In the 20-60nm offshore zones expect 6 to 9 foot waves by the evening hours, highest in the Savannah to Altamaha Sound zone.

Thurday-Friday: Conditions are expected to improve on Thursday, though rough seas will likely persist into early Friday especially for the 20 to 60nm offshore zones. Another surface high pressure may setup along the northeast coast on Friday, potentially increasing wind speeds and wave heights across local waters.

EQUIPMENT

The Metter, GA, NOAA Weather Radio Station (WWH-25) will be offline from mid-morning Wednesday, October 7, through the afternoon of Thursday, October 8. The transmitter is being relocated.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from 5 AM Tuesday to 2 AM EDT Wednesday for AMZ340. Gale Watch from late tonight through late Tuesday night for AMZ360-362-380. Gale Watch from late tonight through late Tuesday night for AMZ364-382-384.