textproduct: Charleston

WHAT HAS CHANGED

The aviation section was updated for the 18z TAF issuance.

KEY MESSAGES

- 1) Confidence continues to increase in an accumulating snowfall event across a portion of the SC Lowcountry and SE Georgia Saturday morning through Saturday night.

- 2) Uncommonly cold temperatures are expected to impact the area this weekend into early next week with dangerously cold conditions expected Saturday night and Sunday morning.

DISCUSSION

KEY MESSAGE 1: Confidence continues to increase in an accumulating snowfall event across a portion of the SC Lowcountry and SE Georgia Saturday morning through Saturday night.

On Saturday, a coastal low pressure system is expected to develop off the southeast coast out ahead of a strong upper level trough. While the southward extent of the precipitation shield remains uncertain somewhere near the Altamaha River Valley, scattered to areas of drizzle/rain are expected to develop throughout the day given weak isentropic lift in a well saturated atmospheric column. As the aforementioned low increases in strength, cooler air is filtered down into the region, with no warm air aloft causing any precipitation type headaches, bringing a switch from rain to snow throughout the afternoon hours on Saturday. The dendritic growth zone remains well saturated for a few hours Saturday afternoon into early Sunday morning, and when combined with the strong forcing from the upper level trough swinging through, would not be surprised to see snowfall rates reach up into the 0.5 to 1 inch per hour range. This is especially true given the atmospheric columns well below freezing across the region, leading to rather "dry" snowfall as snow-to-liquid ratios near the 20:1 mark. In addition, seeing some signals that instability-driven frontal banding may further result in higher snowfall totals, especially across portions of southeastern South Carolina.

With numerous hours where the environment is capable of producing and sustaining snowfall accumulations, light to moderate snowfall continues to look likely Saturday afternoon into early Sunday morning. While overall QPF amounts are looking to remain light at maybe a tenth of an inch across southern southeastern Georgia and just over a quarter inch across northern Berkeley and Charleston counties, the cold temperatures and thus high snow-to-liquid ratio has potential to create some impressive snowfall totals. Areas across northern Berkeley have a decent chance (30-40%) for seeing 4 inches of snow, while the rest of the tri-county region looks to remain in the 1 to 3 inch range. Elsewhere across southeast South Carolina, between half an inch to 2 inches of snow can be expected. Across southeast Georgia, between half an inch to an inch is expected along and south of the I-16 corridor, rising into the 1 to 3 inch range heading further north of I-16 into the Jenkins/Screven county area. Regardless of where you are, the "light and dry" nature of the snow combined with some breezy winds will likely lead to lowered visibilities in tandem with the accumulating snow, so be on the lookout for difficult travel conditions.

KEY MESSAGE 2: Uncommonly cold temperatures are expected to impact the area this weekend into early next week with dangerously cold conditions expected Saturday night and Sunday morning.

Confidence continues to increase that an extended period of uncommonly cold temperatures will impact southeast GA and southeast SC this weekend and into early next week. There continues to be excellent model agreement concerning the cold air, including indications of temperatures on the order of 5 standard deviations below normal in the column across the region for Saturday and Sunday. A shot of very cold air will push through the area Saturday and Saturday night as arctic high pressure pushes in from the west and an area of low pressure develops off the Southeast coast. The coldest night is expected to occur Saturday night into Sunday morning when widespread teens are expected across the forecast area. In fact, we cooled the forecast low from the deterministic NBM by blending in the even colder NBM50 as the deterministic values continue to run outside the high end of the IQR. Also, the presence of snow on the ground should provide support for even lower temperatures. The forecast now advertises low to mid teens inland and upper teens along the coast. When combined with persistent northwest winds gusting into the 20-25 mph range, wind chills are forecast to plunge into the single digits for Sunday morning. With this in mind, we have issued an Extreme Cold Watch for all of southeast GA and southeast SC from Saturday evening through midday Sunday.

For Sunday night and Monday morning, wind chills are expected to fall well into the teens. Temperatures will only be a couple of degrees higher than the night before, but winds will be significantly less which will yield higher wind chill values. A Cold Weather Advisory will almost certainly be needed. Tuesday morning will again be cold, but wind chills will be even higher. There could be a few isolated areas with wind chills down to around 20 degrees, but a Cold Weather Advisory looks less likely.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

VFR conditions will prevail at KCHS, KJZI, and KSAV through 18z Friday.

Extended Aviation Outlook: VFR conditions are expected to prevail into Friday night. The probability of widespread flight restrictions will increase for Saturday and Saturday night as an area of low pressure develops off the coast. There is also increasing potential for snow and gusty winds at the terminals.

MARINE

Tonight through Friday: Overall, quiet conditions are expected to prevail across the local waters through Friday. Winds should remain 10 knots or less with seas 1-2 feet.

The main time period of concern for marine conditions will start early Saturday morning as an area of low pressure develops and strengthens off the Southeast coast. Northwest winds will strengthen Saturday and likely peak Saturday night into Sunday. Conditions should then improve Sunday night into Monday as the low pulls away and the gradient weakens.

Confidence is increasing that widespread gales will impact the local waters starting Saturday afternoon and potentially continuing into early Sunday afternoon. Therefore, a Gale Watch has been issued for all waters (not including Charleston Harbor) for gusts up to around 40 knots. Gusts could be close to gale force for Charleston Harbor as well, but confidence isn't quite high enough for a watch there. If a Gale Warning is eventually issued, a period of Small Craft Advisories will be needed later Sunday and through Sunday night once the gales come to an end Sunday afternoon.

CLIMATE

Record Low Temperatures:

January 31: KCHS: 15/1966 KCXM: 19/1966 KSAV: 16/1966

February 1: KCHS: 21/1977 KCXM: 23/1900 KSAV: 23/1977

February 2: KCHS: 19/1980 KCXM: 17/1917 KSAV: 18/1917

Record Low Maximum Temperatures:

January 31: KCHS: 36/1948 KCXM: 34/1936 KSAV: 37/1909

February 1: KCHS: 38/1980 KCXM: 36/1900 KSAV: 38/1900

February 2: KCHS: 38/1980 KCXM: 38/1898 KSAV: 38/1951

Record Snowfall:

January 31: KCHS: 0.6/1977 KSAV: 1.3/1977

February 1: KCHS: no record established KSAV: no record established

EQUIPMENT

The KCLX radar remains out of service. We hope to have the radar restored by Saturday. Users should use adjacent WSR-88D sites, including KCAE, KLTX, KJAX, KVAX and KJGX.

CHS WATCHES/WARNINGS/ADVISORIES

GA...Winter Storm Watch from Saturday morning through Sunday afternoon for GAZ087-088-099>101. Extreme Cold Watch from Saturday evening through Sunday morning for GAZ087-088-099>101-114>119-137>141. SC...Winter Storm Watch from Saturday morning through Sunday afternoon for SCZ040-042>045-047>052. Extreme Cold Watch from Saturday evening through Sunday morning for SCZ040-042>045-047>052. MARINE...Gale Watch from Saturday afternoon through Sunday afternoon for AMZ350-352-354-374.