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Area Forecast Discussion

625
FXUS62 KCHS 190126
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
926 PM EDT FRI APR 18 2014

...INCREASING POTENTIAL FOR A DANGEROUS FLASH FLOOD IN DOWNTOWN
CHARLESTON...

.SYNOPSIS...
LOW PRESSURE WILL TRACK FROM THE GULF OF MEXICO TO A POSITION OFF
THE GEORGIA COAST LATE SATURDAY...BEFORE ADVANCING FARTHER OFFSHORE
SUNDAY. THIS LOW SYSTEM WILL PRODUCE A SOAKING RAIN ACROSS THE
REGION. HIGH PRESSURE WILL BRIEFLY BUILD INTO THE REGION MONDAY. A
WEAKENING COLD FRONT WILL PUSH THROUGH THE AREA BY TUESDAY...
FOLLOWED BY HIGH PRESSURE AND WARMER TEMPERATURES THROUGH LATE
WEEK.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
THE MESOSCALE CONVECTIVE VORTEX /MCV/ IS NOW LOCATED OVER FAR
INTERIOR SOUTHEAST GEORGIA AND MOVING NORTH. COPIOUS AMOUNT OF
DEEP MOISTURE IS BEING DRAWN NORTHWARD AHEAD OF THIS FEATURE WHICH
COUPLED WITH STRONG UPPER DIVERGENCE AND A COASTAL FRONT JUST
OFFSHORE IS PRODUCING A LARGE AREA OF TORRENTIAL RAIN...
ESPECIALLY ALONG THE LOWER SOUTH CAROLINA COAST. MESOSCALE DATA
SUPPORT A BAND OF RAIN WITH VERY HIGH RAINFALL RATES AND EMBEDDED
TSTMS MOVING INTO THE CHARLESTON TRI-COUNTY AREA NEAR OR JUST
BEFORE HIGH TIDE. THIS IS VERY BAD NEWS WITH HIGH TIDE ALREADY
FORECAST TO REACH COASTAL FLOOD ADVISORY LEVELS. THERE IS A
POTENTIAL FOR A RATHER NASTY FLASH FLOOD EVENT TO UNFOLD ALONG THE
MIDDLE SOUTH CAROLINA COAST OVER THE NEXT FEW HOURS...POSSIBLY
CENTERED RIGHT OVER DOWNTOWN CHARLESTON. A FLASH FLOOD WARNING
REMAINS IN EFFECT UNTIL 1130 PM AND MAY NEED TO BE EXTENDED.

RAIN IS SLOWLY TAPERING OFF ACROSS SOUTHEAST GEORGIA AS THE MCV
MOVES NORTH WITH SOME SUBSIDENCE NOTED IN ITS WAY. THERE MAY STILL
BE SOME SHOWER ACTIVITY THAT REDEVELOPS THERE AFTER MIDNIGHT AS
THE UPPER TROUGH DRAWS CLOSER...BUT SUSPECT THE HEAVIEST RAINS ARE
PRETTY MUCH OVER THERE. HOWEVER...GIVEN THE VERY WET CONDITIONS
AND THE AMOUNT OF RAIN THAT HAS ALREADY FALLEN THERE...SO NO
REASON TO DROP THE FLASH FLOOD WATCH JUST YET IN THAT AREA.

ADJUSTED TEMPERATURES PER GOING TRENDS. THERE MAY VERY WELL BE A
SLIGHT RISE ALONG THE COASTAL COUNTIES AS LOW PRESSURE AND THE
COASTAL FRONT DRAW CLOSER SO HAVE TRIED TO ACCOUNT FOR THIS IN THE
GRIDDED FORECASTS.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
SATURDAY...THE CUT OFF LOW ALOFT WILL BECOME VERTICALLY STACKED OVER
THE SURFACE LOW LOCATED ALONG THE GEORGIA COASTLINE BY DAYBREAK.
MOST NUMERICAL MODEL SOLUTIONS INDICATE A DRY SLOT SLIPPING INTO THE
SOUTHERN COASTAL AREAS DURING THE MORNING HOURS...BEFORE FILLING IN
ON THE BACKSIDE OF THE COASTAL LOW AS IT SHIFTS SLIGHTLY EASTWARD BY
MIDDAY. AS A RESULT...THERE COULD BE A SLIGHT LULL IN PRECIPITATION
COVERAGE DURING THE MORNING...BEFORE RAIN INCREASES AGAIN BY THE
AFTERNOON. WILL INDICATE A GRADIENT OF LIKELY RAIN CHANCES IN THE
50 TO 60 PERCENT RANGE FOR THE NORTHERN HALF OF THE FORECAST AREA...
DECREASING FARTHER SOUTH TO THE 40 TO 50 PERCENT RANGE DURING THE
PEAK OF COVERAGE. LOW LEVEL INSTABILITY WILL BE CONFINED TO THE
MARINE ZONES AND IMMEDIATE COASTLINE...HOWEVER THERE ARE SOME HINTS
THAT MARGINAL CAPE VALUES COULD PUSH ONSHORE DURING THE AFTERNOON.
CONSIDERING IMPRESSIVE UPPER LEVEL SUPPORT FROM THE CUT OFF LOW
DIRECTLY OVER THE REGION...AND THE RESULTING STEEP MID LEVEL LAPSE
RATES...WILL CONTINUE TO MENTION A CHANCE OF THUNDER FOR THE
EASTERN HALF OF THE FORECAST AREA TO ADDRESS MAINLY ELEVATED
CONVECTION. NORTHEAST FLOW ON THE PERIPHERY OF THE COASTAL LOW AND
THICK CLOUD COVER WILL SUPPRESS HIGH TEMPERATURES IN THE LOW 60S
INLAND TO AROUND 70 DEGREES CLOSER TO THE COAST.

SATURDAY NIGHT...EXPECT A GRADUAL DECREASE IN RAIN COVERAGE FROM
SOUTHWEST TO NORTHEAST AS THE STACKED LOW SYSTEM SLOWLY SLIPS
EASTWARD AS THE NIGHT PROGRESSES. HOWEVER...RAIN CHANCES WILL
STILL BE AS HIGH AS 50 PERCENT CHARLESTON TRI COUNTY AREA TO 30
PERCENT SOUTHEAST GEORGIA AFTER MIDNIGHT. LOW TEMPERATURES WILL
FALL INTO THE LOW TO MID 50S...RIGHT AROUND SEASONAL NORMALS.

SUNDAY...THE PERSISTENCE OF WRAP AROUND PRECIPITATION BEYOND THE
MORNING HOURS WILL DEPEND GREATLY ON THE TRACK AND SPEED OF THE
DEPARTING COASTAL LOW SYSTEM AS IT DRIFTS EAST/NORTHEAST. THERE IS
DECENT MODEL AGREEMENT THAT SCATTERED SHOWERS COULD CONTINUE
THROUGH AT LEAST NOON ACROSS COASTAL AREAS AND THE CHARLESTON TRI
COUNTY REGION CLOSEST TO THE LOW SYSTEM...WITH ONLY ISOLATED
SHOWERS FARTHER WEST AND SOUTH. WILL MAINTAIN SOME MENTION OF RAIN
THROUGH THE DAYTIME HOURS...INTRODUCING DRIER CONDITIONS BY THE
EVENING TIME FRAME ONCE WEAK HIGH PRESSURE BUILDS IN FROM THE
WEST. THE PRESSURE GRADIENT WILL TIGHTEN SIGNIFICANTLY BETWEEN THE
BUILDING INLAND SURFACE HIGH AND THE DEPARTING ATLANTIC LOW...AND
EXPECT NORTHEAST WINDS UP TO 35 MPH ACROSS THE CHARLESTON COUNTY
COASTLINE SATURDAY NIGHT INTO SUNDAY AFTERNOON. LOW TEMPERATURES
SUNDAY NIGHT WILL ONCE AGAIN FALL INTO THE LOW TO MID 50S UNDER
GRADUALLY CLEARING SKIES.

MONDAY...SURFACE HIGH PRESSURE WILL BRIEFLY BUILD INTO THE
REGION...SUPPORTING MORE TRANQUIL CONDITIONS. A TRANSIENT UPPER
RIDGE WILL SHIFT OVER THE EAST COAST STATES...ASSISTING IN A
WARMING TREND AND HIGH TEMPERATURES IN THE MID TO UPPER 70S UNDER
PARTLY CLOUDY SKIES.

LAKE WINDS...GUSTY WINDS WILL LIKELY NECESSITATE A LAKE WIND
ADVISORY SATURDAY NIGHT THROUGH SUNDAY.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
A TROUGH AT THE SURFACE AND ALOFT AND AN ASSOCIATED BAND OF ENHANCED
MOISTURE WILL ADVANCE THROUGH THE REGION TUESDAY AFTERNOON INTO
TUESDAY NIGHT. MOISTURE IS NOT IMPRESSIVE...BUT AT LEAST SLIGHT
CHANCE POPS ARE JUSTIFIED. THEN...SURFACE HIGH PRESSURE WILL SUPPORT
RAIN-FREE CONDITIONS AND TEMPERATURES IN THE UPPER 70S/LOWER 80S
WEDNESDAY/THURSDAY.

&&

.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
HEAVY RAIN WILL AFFECT THE TERMINALS THIS EVENING. COULD SEE A
BRIEF REST AT KSAV AS A BIT OF BREAK MOVES THROUGH...BUT RAIN
SHOULD REDEVELOP AS A MESOSCALE CONVECTIVE VORTEX MOVES THROUGH.
THE WORST CONDITIONS LOOK TO OCCUR AT KCHS WHERE IFR CONDITIONS
ARE POSSIBLE. RAIN WILL GRADUALLY TAPER OFF AT KSAV PRIOR 04Z...
BUT WILL LINGER AT KCHS THROUGH ABOUT 11Z OR SO. RAIN WILL BECOME
MORE SHOWERY IN NATURE LATE TONIGHT WHICH IS TOUGH TO NAIL DOWN
IMPACTS THIS FAR OUT. REGARDLESS...MVFR CIGS WITH IFR CIGS AT
TIMES WILL CONTINUE.

EXTENDED AVIATION OUTLOOK...FLIGHT RESTRICTIONS INTO SUNDAY. VFR
THEREAFTER. GUSTY WINDS POSSIBLE SUNDAY.

&&

.MARINE...
SOLID SMALL CRAFT ADVISORY CONDITIONS WILL PERSIST INTO SATURDAY.
GALES ARE THEN POSSIBLE SATURDAY NIGHT INTO SUNDAY NIGHT AS THE
GRADIENT TIGHTENS BETWEEN OFFSHORE LOW PRESSURE AND INLAND HIGH
PRESSURE. CONDITIONS SHOULD THEN GRADUALLY IMPROVE MONDAY AND
MONDAY NIGHT...AND WIND/SEAS ARE THEN EXPECTED TO REMAIN BELOW
ADVISORY THRESHOLDS TUESDAY THROUGH MIDWEEK.

RIP CURRENTS...MODERATE TO HIGH RISK TODAY...HIGHEST ALONG THE
GEORGIA COAST. AN ENHANCED RISK FOR RIP CURRENTS WILL PERSIST
THROUGH THE WEEKEND AND PERHAPS INTO EARLY NEXT WEEK.

&&

.HYDROLOGY...
ANOMALOUS GULF AND ATLANTIC MOISTURE FEED INTO THE AREA AHEAD OF A
POWERFUL LOW PRESSURE SYSTEM WILL COMBINE WITH STRONG SYNOPTIC
FORCING TO PRODUCE WIDESPREAD MODERATE TO HEAVY RAINFALL THROUGH
TONIGHT. LATEST GUIDANCE SHOWS 3-4 INCHES OF RAINFALL WITH SOME
HIGHER AMOUNTS POSSIBLE IN EXCESS OF 5 INCHES. THE GREATEST FLOOD
THREAT WILL LIKELY OCCUR ACROSS COASTAL AREAS...PRIMARILY LATE
THIS EVENING AS WE HIT AN ABOVE NORMAL HIGH TIDE. WE ALREADY HAVE
A COASTAL FLOOD ADVISORY FOR MINOR SALT WATER FLOODING. WITH HEAVY
RAINFALL ON TOP OF THESE HIGH TIDES...DRAINAGE IN THE TYPICAL POOR
DRAINAGE COASTAL AREAS WILL BE LIMITED...POTENTIALLY LEADING TO
SIGNIFICANT FLASH FLOODING. A FLASH FLOOD WATCH IS IN EFFECT FOR
PORTIONS OF THE AREA.

MAIN STEM RIVERS AND ASSOCIATED TRIBUTARIES WILL LIKELY SEE
SIGNIFICANT RISES WITH THE POTENTIAL FOR MINOR FLOODING DEPENDING
ON OBSERVED RAINFALL AMOUNTS.

&&

.TIDES/COASTAL FLOODING...
ISSUED A COASTAL FLOOD ADVISORY FOR THE ENTIRE COAST FOR THE LATE
EVENING HIGH TIDE BASED ON CURRENT DEPARTURES OF 1-2 FT AND THE
FACT THAT WINDS WILL REMAIN NE OR ENE THROUGH MIDNIGHT.

TIDES WILL CONTINUE TO RUN HIGH THROUGH THE WEEKEND AND COASTAL
FLOOD ADVISORIES COULD BE NEEDED DURING THE NIGHTTIME HIGH TIDE
CYCLES.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...FLASH FLOOD WATCH UNTIL 6 AM EDT SATURDAY FOR GAZ087-088-
     099>101-114>119-137>141.
     COASTAL FLOOD ADVISORY UNTIL 2 AM EDT SATURDAY FOR GAZ117-119-
     139-141.
SC...FLASH FLOOD WATCH UNTIL 6 AM EDT SATURDAY FOR SCZ040-042>045-
     047>052.
     COASTAL FLOOD ADVISORY UNTIL 2 AM EDT SATURDAY FOR SCZ048>052.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 AM EDT MONDAY FOR AMZ350-352.
     SMALL CRAFT ADVISORY UNTIL 6 AM EDT MONDAY FOR AMZ330-354.
     SMALL CRAFT ADVISORY UNTIL 5 AM EDT TUESDAY FOR AMZ374.

&&

$$

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