textproduct: Charleston

WHAT HAS CHANGED

The aviation section has been updated for the 18z TAF issuance. Minor changes were made to the forecast and late weekend trending more for wetter solution.

KEY MESSAGES

- 1) Dry and breezy conditions Thursday, with dry conditions persisting into Friday.

- 2) Very cold temperatures will occur Friday morning, with lows in the upper teens to lower 20s away from the beaches.

- 3) Arctic cold front arriving on Sunday will usher below normal temperatures, along with the possibility of flurries Saturday night into Sunday morning.

DISCUSSION

KEY MESSAGE 1: Dry and breezy conditions Thursday, with dry conditions persisting into Friday.

A strong cold front is timed to sweep across the Coastal Plain of GA and SC this evening. In the wake of the front, winds should veer from the northwest and develop gusts between 20-25 mph. The air mass will be further dried by downslope flow in the lee of the southern Appalachians and deep mixing. As a result, sfc dewpoints will fall through the day, pushing well into the teens by Thursday afternoon. RH values between 20 to 25 percent are expected away from the immediate coast. The combination of gusty northwest winds, low RH, and mostly sunny conditions should rapidly dry fine fuels. Depending on fuel conditions reported from fire weather partners, a Fire Danger Statement could be needed for portions of our area on Thursday.

The center of the dry air mass will build across the Southeast U.S. on Friday. Dewpoints could be as low as the single digits at a few locations Friday morning, with most areas between 10 to 15 degrees. RH values should fall to the upper teens to around 20 percent very far inland and the mid 20s closer to the coast. Fortunately, the weak pressure gradient should only support winds between 5 to 10 mph, so no Fire Danger Statements are expected.

KEY MESSAGE 2: Very cold temperatures will occur Friday morning, with lows in the upper teens to lower 20s away from the beaches.

Surface high pressure will be approaching from the west. Clear skies and dry conditions will yield lots of radiational cooling. Expect low temperatures in the upper teens west of I-95 and in the typical colder spots. Along and east of the I-95 corridor to about US Highway 17, expect low temperatures in the lower to mid 20s. Along the beaches, expect low temperatures in the upper 20s (including the Charleston Peninsula). Temperatures will be below freezing for 8 to 12 hours. Though, winds will ease early Friday morning. The end result will be temperatures or wind chills in the teens to around 20 degrees in the coldest spots, so a Cold Weather Advisory may be needed.

KEY MESSAGE 3: Arctic cold front arriving on Sunday will usher below normal temperatures, along with the possibility of flurries Saturday night into Sunday morning.

A Rex Block will gradually loose cohesion as a wave situated over the eastern Pacific approaches from the west. In response to this downstream effect, a substantial shortwave trough will dive southeast out of Manitoba and approach the region on Sunday morning. This will drive another cold front through South Carolina and Georgia Sunday. Latest deterministic guidance (along with Ensembles) have nudged this trough further south favoring stronger lift and moisture return over the Southeast. There remains considerable uncertainty with this system, as it hinges on how far south the trough shifts and how rapidly it develops a negative tilt. It's important to note the most recent 12Z GFS hints at the potential for a coastal low developing offshore of the Carolinas ahead of the approaching shortwave, and this setup would favor higher precip. amounts. However, temperatures will be borderline along the coastline on Saturday night into Sunday morning, thus it'll be hard to support pure snow and it's more likely a rain/snow mix will apparent near the coastline. The probabilistic Winter Storm Severity Index (WSSI-P) continues to show a 5-10% chance for a trace of snowfall from the colder solutions. Again, caution should be exercised here as the forecast can change, and will likely change over the next couple days. It's too far out in the forecast to talk about exact snow accumulations and any small change in the forecast could move the precip. off the coast and keep the region dry.

Behind the cold front, temperatures will again be seasonably cold Monday with low temperatures in the mid 20s. However, there is a much larger spread in the interquartile range (IQR) range (around 8 to 10 degrees) compared to Friday morning of this week and that makes sense. Ahead of the approaching high pressure, winds may remain just strong enough (near 5 knots) where wind chills may dip into the 15 to 20 degrees range, with highest probabilities again for the furthest inland areas. A Cold Weather Advisory may be needed. Highs on Monday remain below normal as temps. only reach into the upper 40s to low 50s.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Predominantly VFR conditions through 18Z Thursday at CHS/JZI/SAV terminals. Expect bands of light showers to continue to track across the terminals through the afternoon ahead of an approaching cold front. These showers have been highlighted with a TEMPO in each terminal, generally between 18-22Z. Winds should favor a west- southwest direction around 10 kts, with gusts nearing 20 kts late tonight. Expect the front to push through Thursday early morning, with skies clearing and winds shifting more westerly. Additional breeziness is expected behind the front and gusts near 20 kts were maintained at all terminals through Thursday mid-day.

Extended Aviation Outlook: VFR conditions will continue for KCHS, KJZI and KSAV through late week. Another system could impact the region late this weekend.

MARINE

Through the rest of the afternoon, expect the marine zones to remain between a developing low pressure system over the Gulf Stream and a cold front over the western Carolinas. A weak gradient on the west side of the low keeps benign marine conditions in place. Expect a band of light to moderate showers to sweep across the nearshore waters late this afternoon.

A strong cold front will cross the Atlantic waters overnight, with west-northwest winds developing in the wake of the front. It'll become quite breezy with FROPA with wind speeds ranging from 15 to 20 with gusts up to 25 kts until Thursday night. Also, expect seas to build tonight through Thursday, peaking between 3 to 5 ft across the nearshore waters with 6 footers beyond 30 NM Thursday evening. Small Craft Advisories have been issued for all marine zones outside the CHS Harbor from this evening through Thursday night. The center of a large area of high pressure will arrive on Friday and may linger into Saturday, supporting winds around 10 kts with 1-2 ft seas. Winds should gradually strengthen as a dry cold front sweeps across the region on Sunday, with gusty winds and building seas, possibly reaching Small Craft Conditions across a portion of the waters.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from 7 PM this evening to midnight EST Thursday night for AMZ350. Small Craft Advisory from midnight tonight to midnight EST Thursday night for AMZ352-354. Small Craft Advisory from midnight tonight to 4 AM EST Friday for AMZ374.