Area Forecast Discussion

WHAT HAS CHANGED

The Aviation Section has been updated for the 00Z TAF issuance

KEY MESSAGES

- 1) Afternoon heat index values could climb into the 105 to 112 degree range across the I-95/coastal corridor on Saturday.

- 2) Daily coverage of showers and thunderstorms expected to increase through the weekend and into next week as a more typical summertime pattern returns to the region.

- 3) Minor coastal flooding possible along Charleston and Colleton County coasts during evening high tides this weekend into early next week.

DISCUSSION

KEY MESSAGE 1: Afternoon heat index values could climb into the 105 to 112 degree range across the I-95/coastal corridor on Saturday.

On Saturday, the day should begin will very warm temperatures in the upper 70s, possibly near 80 along the coast. Much like today, temperatures should warm into the 90s by mid-day, then peak near record values in the upper 90s. See Climate section below for temperature records. These temperatures combined with dewpoints well into the 70s could yield index values in the 108-112 degree range. It is possible that a Heat Advisory will be needed for the coastal counties tomorrow. However, temperatures should exceed the convective temperature in the low 90s around mid-day. Initially, cumulus cloud cover should rapidly develop across the forecast area, then showers and thunderstorms should begin to develop shortly afternoon. Given the hot and humid conditions, it is possible that SBCAPE will build in excess of 3000 J/kg along and ahead of the sea breeze. Thunderstorms should develop along and inland of the sea breeze, then track across the sea breeze towards the coast. 12Z HREF indicates a 70% of rainfall rates of 1"/3hr along the coast, with some pockets of 10% 3"/3hr. It is possible that a SVR warning or Flood Advisory will be needed on Sat. Rainfall could become excessive if heavy downpours overlap with the early evening high tide.

KEY MESSAGE 2: Daily coverage of showers and thunderstorms expected to increase through the weekend and into next week as a more typical summertime pattern returns to the region.

Through the weekend and into next week, the pattern across southeast GA and southeast SC will gradually shift toward a typical summertime pattern supportive of showers and thunderstorms each day. Aloft, the ridging that has been in place for the last several days will break down and be replaced by a more zonal flow through Sunday. Then as we head into the early and middle part of next week, large and broad troughing is expected to encompass the eastern half of the CONUS. At the surface, a weak front/trough is expected to drift into the vicinity Saturday and Sunday, followed by a more notable front for the early to middle part of next week. Overall thinking is that Saturday and Sunday will result in a modest increase in convective coverage. A more significant increase in coverage is likely in the early to middle part of next week due to the combination of the presence of the trough aloft and the aforementioned front.

For Saturday and Sunday, mid-level flow should be such that it only support storm motions on the order of 10-15 knots. Precipitable water values will be in the 1.8-2.0" range as well. So locally heavy rainfall is possible due to slow storm motion, but coverage should be limited to the point of keeping the threat localized. As we move into next week, mid-level flow will increase into the 20-30 knot range and the threat for heavy rainfall will come primarily from training storms and the presence of precipitable water values in excess of 2 inches. Overall, the severe risk for each day is relatively low due to the lack of organization. Also, the freezing level (14.5-15.0 kft) and -20C (25.0+ kft) height will be quite high each day which means it will take a very tall storm to produce a damaging wind gust threat. The best chance for a strong to severe storm will come where boundary interactions occur and boost updraft strength. This could change a bit into next week as the mid-level flow improves.

KEY MESSAGE 3: Minor coastal flooding possible along Charleston and Colleton County coasts during evening high tides this weekend into early next week.

Breezy south-southeasterly winds post sea breeze each day along with the upcoming lunar Perigee (June 14th) and New Moon (June 15th) should result in elevated tidal departures above 1/2 ft this weekend into early next week, setting the stage for minor coastal flooding along the Charleston and Colleton County coasts during evening high tide cycles. The latest TWL forecast calls for 7.1 - 7.3 ft MLLW in the Charleston Harbor Saturday evening, and similar water levels are possible during evening high tides into early next week as astronomical influences peak and breezy onshore winds persist. Coastal Flood Advisories could eventually be needed.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

13/00Z TAFs: Showers and thunderstorms slowly dwindle across the region, however VCTS was maintained at KSAV through 03Z as some lingering showers and thunderstorms persist nearby. Heading into the overnight, conditions should remain VFR with light southwesterly winds. Expect similar conditions on Saturday as showers and thunderstorms could be triggered by the afternoon seabreeze again. Thus, a VCSH has been noted at KCHS/KJZI/KSAV for 17-18Z Saturday.

Extended Aviation Outlook: VFR conditions should prevail at CHS/JZI/SAV terminals through the weekend, although TEMPO flight restrictions are possible with afternoon showers/thunderstorms throughout the weekend. Chances for flight restrictions increase early next week with shower/thunderstorm coverage expanding locally ahead of an arriving cold front.

MARINE

Tonight through Saturday, the pressure pattern will support southwest winds between 15 to 20 kts. Wave heights are forecast to remain between 2 to 3 ft.

Sunday through Wednesday: The gradient across the local waters will get enhanced a bit as a surface trough takes hold inland. This should drive southwest flow into the 15-20 knot range across all the waters, with potential for 25 knot gusts across the SC waters and the outer GA waters. Similar wind speeds will be possible each day into the middle of the week, primarily in the afternoon and evening time period. Conditions could become marginally supportive of Small Craft Advisories at times for portions of the waters.

CLIMATE

Record High Temperatures:

June 12: KCHS: 97/2016 *New record of 98 degrees set. KCXM: 97/1998 KSAV: 100/1977

June 13: KCHS: 99/2011 KCXM: 96/1998

June 14: KCHS: 99/2010 KCXM: 97/1981

Record High Minimum Temperatures:

June 12: KCHS: 77/1986 KCXM: 80/1998 KSAV: 78/1899

June 13: KCHS: 80/2013 KCXM: 82/1998 KSAV: 80/1880

June 14: KCHS: 79/2010 KCXM: 81/2010

CHS WATCHES / WARNINGS / ADVISORIES

GA...None. SC...None. MARINE...None.