textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections were updated.

KEY MESSAGES

- 1) Dry conditions and seasonable temperatures expected this week. Colder conditions this weekend.

- 2) Frost/freeze conditions are possible Friday night through Sunday night.

DISCUSSION

KEY MESSAGE 1: Dry conditions and seasonable temperatures expected this week. Colder conditions this weekend.

Tonight: It will be another cool dry night for Southeast Georgia and the South Carolina Lowcountry as high pressure maintains its influence on the region. Similar to the past few nights, the boundary layer will decouple quickly after sunset yielding calm to light/variable winds. Model cross sections and simulated satellite imagery suggest some high clouds will traverse the area at times, but mostly clear skies should prevail in general. This will result in another night of modest to locally strong radiational cooling and 11/13z NBM lows were nudged down to the colder side of the guidance, blending in some of the higher resolution RRFS, H3R and RAP hourly temperature data to better align with known mesoscale influences. For example, the Francis Marion National Forest, the Santee-Cooper lakes, the open areas of Fort Stewart and areas adjacent to larger waterways such as Charleston Harbor, the Broad River and the Savannah River.

Another round of shallow ground is possible, especially over parts of the Charleston Tri-County. The fog could remain fairly shallow with 1000 hPa condensation pressure deficits remaining some unfavorable for dense fog develop. However, the fog could lift enough just before daybreak to yield about a 2-4 hour period of reduced visibilities, possibly impacting parts of the Charleston Metro Area per latest REFS and NBM visibility probabilities.

Tuesday through Friday: Conditions remain quiet Tuesday into Wednesday as broad high pressure gradually shifts offshore. Nights will be generally dominated by some degree of radiational influences within a calm/light wind regime, but there is uncertainty in the degree of high clouds that will spread out across the Southeast U.S. and Deep South ahead of the weakening upper low over northern Mexico as it shears out. This could have an impact on how cool overnight lows will get as radiative processes could be disrupted depending on cirrus thickness. Overall, seasonable temperatures are likely, highs in the mid- upper 70s with lows in the mid-upper 40s inland/near 60 at the beaches.

A potent northern stream shortwave will dig across the Northern Plains Tuesday and gradually phase with a broader upper-level cyclone over eastern Canada to produce a deep long-wave trough across the eastern CONUS into the weekend. A strong cold front is poised to cross the area late Thursday afternoon/evening bringing significantly cooler weather to the region. A westerly low-level flow will limit both Gulf and Atlantic moisture return ahead of the front which will curtail the rainfall potential despite modest UVVs associated with the favorable alignment of both the polar and subtropical jets and the passing of a shortwave embedded in the increasingly cyclonic flow aloft. NBM pops were limited to 20% or less which looks reasonable. Highs will warm well into the 70s Thursday then cool to into the lower 60s for Friday.

KEY MESSAGE 2: Frost/freeze conditions are possible Friday night through Sunday night.

It is becoming increasingly likely that the first frost/freeze event of the season will impact the South Carolina Lowcountry and Southeast Georgia beginning Friday night and continuing into Sunday night as cold high pressure builds into the region. Temperatures are expected to fall into the mid 30s inland Friday night with lingering post frontal cold air advection, but winds look to remain too elevated and the airmass too dry to support much in the way of frost. Strong radiational cooling is expected both Saturday and Sunday nights with high pressure overhead and this is when the risk for freezing temperatures and frost will be at its highest, especially inland away from the beaches.

11/13z NBM 10km neighborhood probabilities for emergency management warming shelter planning:

Friday Night/Saturday Morning: Inland Areas Coastal Corridor Temps 35 degrees or colder: 30-70% 5-10% Temps 32 degrees or colder: 10-30% <5% Temps 30 degrees or colder: 5-10% <5%

Saturday Night/Sunday Morning: Inland Areas Coastal Corridor Temps 35 degrees or colder: 60-80% 20-40% Temps 32 degrees or colder: 30-60% 5-10% Temps 30 degrees or colder: 10-30% <5%

Sunday Night/Monday Morning: Inland Areas Coastal Corridor Temps 35 degrees or colder: 20-40% 10-20% Temps 32 degrees or colder: 10-20% 5-10% Temps 30 degrees or colder: 5-10% <5%

Frost Advisories, Freeze Watches and Freeze Warnings may eventually be needed, especially for areas roughly inland of US-17 in the Lowcountry and I-95 in Southeast Georgia.

Detailed information on first freeze dates for the area can be found online at: https://www.weather.gov/media/chs/climate/AverageFirstLastFreezeDates.pdf

AVIATION /18Z MONDAY THROUGH SATURDAY/

09/18z TAFs: KCHS/KJZI: There is a risk for shallow ground fog early Tuesday. The fog could remain fairly shallow, but may yield some reductions, especially at KJZI 10-12z. The fog will quickly dissipate after daybreak. VFR otherwise.

KSAV: VFR. Low end risk for some brief shallow ground fog just before daybreak.

Extended Aviation Outlook: Moderate risk for vsbys reductions Tuesday night from shallow ground fog.

MARINE

Surface High pressure will continue across the Southeast U.S. through Wednesday, bringing a light to perhaps gentle breeze. Seas will be 2-3 ft within 20 NM and 3-4 ft for the 20-60 NM waters. Winds increase out of the southwest Thursday ahead of an approaching cold front, becoming gusty to near 30 knots. Likewise, wind-driven seas will build. Small Craft Advisories will likely be needed. Winds shift to become out of the west- northwest overnight into Friday, with gusts to near 30 knots continuing throughout the day. There is even a low-end risk for gales over the offshore legs, mainly Thursday night.

CHS WATCHES/WARNINGS/ADVISORIES

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