textproduct: Charleston

WHAT HAS CHANGED

Added the mention of this weekend's winter weather potential and updated the aviation section to reflect the 18Z TAF issuance. Also, a Small Craft Advisory was issued for the Charleston Harbor until this evening. A climate section was also added to highlight the potential for record breaking low temperatures this weekend.

KEY MESSAGES

- 1) An arctic air mass will yield very cold overnight temperatures tonight through the end of the week. A Cold Weather Advisory is in effect for SE SC from 10 PM tonight through 9 AM Tuesday morning and additional Cold Weather Advisories are likely through the week.

- 2) Very cold temperatures and the possibility of winter weather could impact the region this weekend.

DISCUSSION

KEY MESSAGE 1: An arctic air mass will yield very cold overnight temperatures tonight through the end of the week. A Cold Weather Advisory is in effect for SE SC from 10 PM tonight through 9 AM Tuesday morning and additional Cold Weather Advisories are likely through the week.

As strong arctic high pressure (~1040 mb) ushers cold, dry air into the region in the midst of post-FROPA conditions, temperatures are forecast to be well stay below normal for extended period of time. The ESAT NAEFS highlights this well with values ranging from -2 to -4 for minimum temperatures up through early next week. Also, the ECMWF MSLP anomalies showcase values from +2 to +5 across the Southeast. It's also fairly high model confidence as IQR shows a 1 to 2 degree spread for minimum temperature values tonight and Tuesday night.

Expect minimum temperatures forecast to drop into the mid-teens and low 20s along and west of I-95 (also including the Francis Marion Forest) with temps. a tad warmer near the coastline tonight. Mixed the NBM/NBM25 into the overnight temperatures as the NBM appeared to be struggling to realize the strength of the arctic air funneling into the region. Even taking a look at the 850mb temps. overnight, they dip below 0C across the entire region. These conditions combined with light west-northwesterly winds will likely result in widespread wind chill values in the low to mid teens from interior counties all the way down to the coastline. 12Z HREF probabilities indicate a 75- 80% chance of temperatures dropping below 20 degrees between 3AM and 7AM. Some guidance has been indicating that minimum wind chill values could drop into the single digits for counties like Bulloch, Evans, Tattnall, and Candler, however low confidence remains with this and any potential dip into the single digits will be short lived. A Cold Weather Advisory is now in effect from 10 PM this evening until 9 AM Tuesday morning across the SC Lowcountry and SE Georgia.

Forecast remains a bit warmer as temperatures range in the low to mid 20s across the interior counties with upper 20s to low 30s along the coastline on Tuesday night into Wednesday morning. 12Z HREF probabilities for temperatures to drop below 20 degrees will be highest across inland Georgia (however only a 35% chance), thus it would see more likely we would need a Cold Weather Advisory across those portions of the forecast region. At the moment, conditions do not scream the need to issue a Cold Weather Advisory and this will continue to be monitored.

This very cold airmass will likely stick around through late this week as a reinforcing cold front moves through on Thursday morning and pushes another shot of cold air into the region from the northwest. It's reasonable to say that we could have a few hours of wind chill values in the upper teens to low 20s each night. Additional Cold Weather Advisories will likely be required for at least portions of the forecast area through the week.

KEY MESSAGE 2: Very cold temperatures and the possibility of winter weather could impact the region this weekend.

Beginning Friday, there is good model agreement (deterministic and ensemble) that a sharp trough and developing closed low will dive southwards out of the Great Lakes region. This upper low is expected to steadily deepen as it pushes to the south, settling somewhere across the Carolinas Saturday night into Sunday. At the surface, arctic high pressure is expected to drop southward out of central Canada and across the central CONUS. As this occurs, the aforementioned upper low should drive surface low development somewhere along or just off the Southeast coast Saturday and Saturday night. The proximity of this developing low to the coast would then have significant implications for the inland extent of any precipitation development, as well as the potential for any winter weather across southeast GA and southeast SC.

To be clear, this setup looks completely different compared to the winter weather event across the region this past weekend. The upper low and arctic surface front look to usher in an anomalously and deeply cold airmass. In fact, the 26/00z NAEFS suggests the potential for temperatures through the column on the order of 3 to 4 standard deviations below normal. There remains significant uncertainty surrounding the surface low including the timing of its development and proximity to the Southeast coast, all of which will have crucial implications regarding the potential for winter weather and its extent across the forecast area. However, there is enough model support and consistency to warrant the inclusion of precipitation chances across the entire forecast areas as well as the mention of snow this weekend.

While the above forecast items remain low confidence, there is relatively high confidence in a period of very cold temperatures this weekend. Highs only in the 30s look increasingly likely Saturday and Sunday, with Saturday night/Sunday morning lows dipping well into the teens. Such values would be on the order of 20-25 degrees below normal for late January and early February. See the Climate section below for a list of record low temperatures and record low max temperatures.

AVIATION /18Z MONDAY THROUGH SATURDAY/

26/18Z TAFs: Recent observations continue to indicate MVFR ceilings (~1500ft) across KCHS/KJZI as a cold front located offshore continues to push away yielding mighty CAA across the region. As of 17Z, satellite indicates horizontal convection rolls (aka cloud streets) shifting overhead at KCHS/KJZI and BKN015 has been notated until ~19Z. Thereafter, skies should become clear and VFR will prevail for the rest of the 18Z TAF period. It'll be fairly breezy through the evening with gusts of 25-27kt noted between 18-23Z at KCHS/KJZI and 18-01Z at KSAV as post-FROPA conditions hold steady. After the sun goes down, winds should taper back and become light through Tuesday morning.

Extended Aviation Forecast: Expect VFR conditions prevail as an arctic high pressure dominates the forecast.

MARINE

Tonight through Tuesday: Northwest flow will continue to surge across the local waters this evening and through the overnight. Solid high end Small Craft Advisory (SCA) conditions are expected with frequent gusts up to around 30 knots, especially in the GA waters. Seas will also be highest in the GA waters with up to 6 ft out to 20 nm and 6-8 ft in the outer waters. Conditions will then improve significantly on Tuesday as the gradient weakens allowing winds and seas to both come down. SCA's continue for all waters through the night, then come to an end by midday Tuesday.

Tuesday night through Friday: The main time periods for concern for the rest of the week include Tuesday night/early Wednesday morning, then Wednesday night. Expect a period of surging west-northwest flow during these times, with conditions possibly being near marginal SCA thresholds.

Friday night and Saturday: There is growing concern for the arrival of an arctic cold front and the development of an offshore surface low. This would bring the possibility for an enhanced gradient across the local waters with hazardous marine conditions. Some guidance sources even suggest the potential for gales across all waters. Confidence is low in details at this point, but certainly something worth watching throughout the week.

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

EQUIPMENT

The KCLX radar will remain out of service. Critical parts are on order and should arrive on Monday. Users should use adjacent WSR-88D sites, including KCAE, KLTX, KJAX, KVAX and KJGX.

CHS WATCHES/WARNINGS/ADVISORIES

GA...Cold Weather Advisory from 10 PM this evening to 9 AM EST Tuesday for GAZ087-088-099>101-114>119-137>141. SC...Cold Weather Advisory from 10 PM this evening to 9 AM EST Tuesday for SCZ040-042>045-047>052. MARINE...Small Craft Advisory until 7 PM EST this evening for AMZ330. Small Craft Advisory until 6 AM EST Tuesday for AMZ350-352-354. Small Craft Advisory until 10 AM EST Tuesday for AMZ374.