textproduct: Charleston

SYNOPSIS

A strong cold front will push offshore this afternoon. High pressure will then prevail for the remainder of the week. Another cold front could affect the area next weekend.

NEAR TERM /THROUGH TONIGHT/

Update: Near term guidance has been consistent in keeping the thin line of showers with the front pretty well in tact as it crosses the area. Pops were increased to 40% for all areas and the QPF was nudged up slightly based on these trends and upstream radar observations. The rest of the forecast is on track.

Today: Strong cold air advection (CAA) in the wake of a powerful shortwave rotating around the base of the upper low north of the Great Lakes will push a cold front across the Southeast U.S. today with the front progged to clear the coast during the mid-afternoon hours. Pre-frontal warm air advection (WAA) aided by a moderate low-level jet will allow for one more warm day across the Lowcountry and Southeast Georgia despite increased cloud cover. Highs are poised to warm into the upper 60s/lower 70s before temperatures level out and begin to fall after FROPA with the onset of strong CAA. The corridor of deep layered forcing associated with the shortwave is forecast to pass by well to the north, but modest 850 hPa theta-e advection ahead of the front coupled with a ribbon of PWATs rising to 1.25-1.35" and low-level forcing right along the leading edge of the frontal surface itself could support a narrow band of showers as the front propagates east across the area. This is scenario is represented rather well by the various simulated reflectivity products off the CAMs. Pops around 40% were highlighted today, mainly confined to the timing of the front. Any rainfall should be very light given the rapid progression of the line, likely only averaging a trace to a few hundredths. Conditions will become rather breezy after daybreak with gusts nearing 30 mph at times. Gusts should remain below Wind Advisory criteria. See the Lake Wind section below for expected conditions on Lake Moultrie.

Tonight: The cold front will be offshore by sunset with strong CAA dominating through the evening hours. The intensity of the CAA will weaken after midnight as high pressure over the Plains builds east. The gradient will remain somewhat enhanced through the night, so winds will remain elevated. Lows are expected to bottom out in the upper 20s well inland to the mid 30s at the beaches. Wind chill values could briefly touch 20 degrees across far interior Southeast Georgia just before daybreak Wednesday. This is right at Cold Weather Advisory criteria. The situation looks too brief and marginal to support the issuance of a Cold Weather Advisory at this time, but the 29/00z HREF did show probabilities 40-60% of reaching advisory thresholds. The need for an advisory will be reassessed with the noon forecast package. Wind chills will otherwise drop into the lower-mid 20s across the remainder of the Lowcountry and Southeast Georgia.

Lake Winds: RAP soundings across Lake Moultrie show a 200-300 ft marine layer holding over the open lake waters this morning into the early afternoon hours ahead of the approaching cold front. Despite adjacent land areas becoming gusty, the marine layer should subdue winds a bit with conditions largely holding below Lake Wind Advisory criteria until FROPA at mid-afternoon. The marine-layer will quickly mix out after FROPA with soundings showing thermal profiles becoming increasing favorable for vigorous mixing with the onset of post-frontal CAA. Expect winds to reach 15-20 kt with gusts to 25 kt by mid-late afternoon then peaking 20-25 kt with gusts to 30 kt this evening as CAA maxes out. Winds will then quickly diminish after midnight as CAA begins to wane. A Lake Wind Advisory is in effect from 3 PM until 1 AM. Waves will build 2-3 ft with the highest winds and strongest wave action occurring over the central and southeast portions of the lake.

SHORT TERM /TUESDAY THROUGH THURSDAY/

Much cooler conditions expected for Tuesday and Wednesday as the cold air mass lingers across the region. Highs on Tuesday will barely get up into the 50s as cold air advection (CAA) continues. Overnight into Wednesday CAA ceases as winds weaken, allowing for radiational cooling to become the primary driver of how much we cool. Areas inland are expected to dip down into the mid 20s, and with the winds remaining light wind chills are not expected to reach into cold weather advisory criteria. Areas along the coast will be a bit warmer in the upper 20s to lower 30s. With weak northwesterly flow persisting on Wednesday, ample sunshine will begin to bring a little closer to normal in the mid 50s across the region.

Overnight, lows dip down into the mid 30s across inland areas while areas near the beaches are a bit warmer in the upper 30s to lower 40s. A weak front, noted mostly by a shift in winds, moves through on Thursday with highs near normal in the lower 60s are expected.

LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/

Temperatures continue to moderate on Friday as a warm front begins to move into the region, though it looks to stall somewhere across the forecast area. Areas behind the front will likely make it up into the mid 60s, while areas ahead of the front a couple degrees cooler in the upper 50s to lower 60s. A weak shortwave looks to move across the area on Saturday, accompanied by a surface low pressure system, bringing a chance for light to moderate rain showers along with temperatures in the lower to upper 60s.

AVIATION /09Z MONDAY THROUGH FRIDAY/

29/12z TAF Discussion: KCHS/KJZI/KSAV: Gusty winds will impact all terminals today through this evening as a cold front crosses the area. The degree of gustiness will diminish overnight. A thin line of frontal showers will cross all terminals roughly 18-20z. A TEMPO group for a period of reduced vsbys was introduced. Southwest winds will turn west and eventually northwest with the passage of the cold front. A large area of stratus moving northeast out of the Gulf early this morning is progged to reach all terminals near or just after daybreak. Cloud bases should gradually rise as low-level wind fields increase and temperatures rise. Do not anticipate IFR cigs reaching KCHS or KJZI at this time (MVFR cigs more likely), but a period is possible at KSAV, mainly through 14z.

Extended Aviation Outlook: VFR conditions will prevail through the week.

MARINE

Through Tonight: Small Craft Advisories remain in effect for all waters. Southwest winds will increase to advisory levels later this morning into the afternoon as low-level wind fields increase ahead of an approaching cold front. Cold nearshore waters may limit mixing somewhat in the WAA regime ahead of the front, but frequent gusts to 25 kt still seems likely. Mixing profiles will become increasingly favorable for robust mixing late this afternoon into tonight as the cold front clears the coastal waters and post frontal CAA peaks. Northwest winds will peak 20-25 kt with gusts to 30 kt with 20 kt with gusts to 30 kt in the Charleston Harbor. There is about a 40% chance for frequent gusts to 35 kt gales to occur in the far eastern portions of the South Santee-Edisto Beach nearshore leg, but the impacted area looks too small to justify an upgrade to a Gale Warning at this time. Winds will begin to diminish late as CAA begins to wane. Peak seas will build to 3-5 ft nearshore and 5-7 ft over the Georgia offshore waters. Waves in the Charleston Harbor will reach as high as 2 ft.

Blow Out Tides: Strong offshore winds could result in tides dropping to -1.0 ft MLLW at both Fort Pulaski and Charleston Harbor at low tide this evening. Levels should remain above critical low water thresholds of -1.5 ft MLLW in the Charleston Harbor and -2.0 ft MLLW at Fort Pulaski. Mariners should still exercise caution as some structures that are typically underwater could become exposed and pose a risk to navigation.

Tuesday through Friday: Conditions continue to improve throughout the day as northwesterly winds weaken and seas calm, allowing for the small craft advisories to expire. Winds remain out of the northwest overnight and throughout most of Wednesday as surface high pressure builds across the southeast. Winds pickup overnight into Thursday as a front moves towards and then through the area, bringing the waters from Savannah to Altamaha Sound 20-60nm out to Small Craft Criteria as winds gusts near 25 knots for a few hours Thursday morning. Westerly winds remain gusty throughout the day, weakening overnight into Friday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...Lake Wind Advisory from 3 PM this afternoon to 1 AM EST Tuesday for SCZ045. MARINE...Small Craft Advisory from 10 AM this morning to 1 AM EST Tuesday for AMZ330. Small Craft Advisory until 5 AM EST Tuesday for AMZ350. Small Craft Advisory from 10 AM this morning to 5 AM EST Tuesday for AMZ352-354. Small Craft Advisory until 1 PM EST Tuesday for AMZ374.