Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 212236
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
636 PM EDT SUN SEP 21 2014

.SYNOPSIS...
A COLD FRONT WILL APPROACH TONIGHT AND THEN ADVANCE THROUGH THE
REGION LATE MONDAY...STALLING SOUTH OF THE AREA THROUGH NEXT
WEEKEND. A BROAD AREA OF HIGH PRESSURE WILL BUILD FROM THE NORTH
TUESDAY AND WILL PREVAIL INTO NEXT WEEKEND...WHILE A TROUGH OF LOW
PRESSURE DEVELOPS AND STRENGTHENS NEAR THE SOUTHEAST COAST.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
A THIN VEIL OF CIRRUS IS MOVING RAPIDLY NORTHEAST THROUGH THE
FORECAST AREA EARLY THIS EVENING AND WILL EXIT THE COAST OVER THE
NEXT 1-2 HOURS. EXPECT CLEAR SKIES TO PREVAIL FOR MUCH OF THE
NIGHT BEFORE CLOUDS BEGIN TO INCREASE INLAND AHEAD OF AN
APPROACHING COLD FRONT. MOST OF THE MODELS KEEP ANY MEASURABLE
PRECIPITATION WITH THE FRONT WELL INLAND...POSSIBLY REACHING THE
FAR INTERIOR ZONES JUST AFTER SUNRISE. WILL KEEP POPS 5-10 PERCENT
OR LESS THERE BETWEEN 09-12Z WITH ZERO POPS ELSEWHERE.

SUSPECT THE BOUNDARY LAYER WILL MOST DECOUPLE...ESPECIALLY
INLAND...LATER THIS EVENING. THIS COMBINED WITH LOW DEWPOINTS AND
CLEAR SKIES SHOULD PROMOTE MODEST RADIATIONAL COOLING. OPTED TO
NUDGE SOME OF THE NORMALLY COOLER AREAS DOWN A FEW DEGREES PER 18Z
COOP GUIDANCE. LOWS WILL RANGE FROM THE MID 60S INLAND TO THE
LOWER 70S AT THE BEACHES...WITH A FEW LOWER 60S WELL INLAND AND IN
SHELTERED AREAS.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
MONDAY AFTERNOON...A SHARP UPPER TROUGH WILL DRIVE A COLD FRONT
INTO THE REGION. ONCE DEEP LAYER FORCING FOR ASCENT ERASES THE
CAPPING INVERSION EVIDENT WITHIN MODEL SOUNDINGS THROUGH EARLY
AFTERNOON...MODELS REMAIN QUITE AGGRESSIVE WITH REGARDS TO RAPID
DEVELOPMENT AND COVERAGE OF CONVECTION BY MID TO LATE AFTERNOON.
PER LATEST TRENDS...RAISED AFTERNOON/EVENING POPS MOST NOTABLY
TO LIKELY IN A BAND PRECEDING THE SURFACE FRONT/CLOSER TO THE
COAST.

21/12Z NAM/GFS SOLUTIONS DIFFER REGARDING THE DEGREE OF INSTABILITY
WHICH WILL POOL AHEAD OF THE FRONT...THUS UNCERTAINTY PERSISTS
REGARDING THE POTENTIAL FOR SEVERE WEATHER. AT LEAST...MULTICELL
THUNDERSTORMS AND PWATS INCREASING TO 1.75-2 INCHES/ASSOCIATED HEAVY
RAIN WILL SUPPORT A THREAT FOR ISOLATED/DAMAGING WET MICROBURSTS. IF
STRONGER INSTABILITY DEVELOPS AS DEPICTED BY THE NAM...A QLCS AND AN
ASSOCIATED GREATER THREAT FOR SEVERE WEATHER COULD DEVELOP. WILL
MENTION THE POTENTIAL WITHIN THE HAZARDOUS WEATHER OUTLOOK MONDAY
AFTERNOON INTO MONDAY EVENING.

MONDAY NIGHT...THE SURFACE COLD FRONT WILL CONTINUE TO PRESS
SOUTH...AND COVERAGE OF SHOWERS/THUNDERSTORMS SHOULD AT LEAST
DIMINISH WITH TIME.

LATE MONDAY NIGHT INTO WEDNESDAY...FORECAST SCENARIOS ESPECIALLY
REGARDING THE EVOLUTION OF THE UPPER TROUGH DIVERGE AND FORECAST
CONFIDENCE DECREASES SIGNIFICANTLY. THE 21/12Z GFS MAINTAINS A
PROGRESSIVE SCENARIO PUSHING THE UPPER TROUGH OUT OF THE REGION AND
MAINTAINING THE CHANCE FOR SHOWERS MAINLY NEAR THE COAST/CLOSEST TO
A TROUGH DEVELOPING OFFSHORE. HOWEVER...THE 21/12Z NAM/EUROPEAN
MODELS DEPICT THE UPPER LOW CUTTING OFF OVER THE REGION THROUGH THE
PERIOD. COMPLICATING THE FORECAST ASSESSMENT...DESPITE THE PRESENCE
OF THE UPPER LOW THE NAM ALSO LIMITS SHOWERS CLOSE TO THE
COAST...MAINLY ASSOCIATED WITH A SURFACE TROUGH DEVELOPING JUST OFF
THE COAST TUESDAY THROUGH WEDNESDAY. HOWEVER...THE EUROPEAN AGAIN
MAINTAINS DEEP MOISTURE AND A MUCH WETTER/COOLER SCENARIO
TUESDAY/WEDNESDAY.

GIVEN MODEL DIVERGENCE AND ASSOCIATED FORECAST UNCERTAINTY...MAJOR
FORECAST SHIFTS ARE NOT YET JUSTIFIED FOR TUESDAY-WEDNESDAY.
MAINTAINED THE ONGOING FORECAST OF POPS LIMITED TO COASTAL/FAR
SOUTHERN AREAS TUESDAY AND SPREADING INLAND WEDNESDAY AS THE SURFACE
TROUGH PUSHES TOWARD THE COAST. HOWEVER...SIGNIFICANT FORECAST
ADJUSTMENTS COULD BE FORTHCOMING WITHIN SUBSEQUENT FORECAST
PACKAGES.

COLD ADVECTION WITH HIGH PRESSURE BUILDING IN FROM THE NORTH AND
INCREASING CLOUD COVER WILL DROP LIMIT HIGH TEMPS TO THE UPPER
70S/AROUND 80F NORTH AND IN THE LOWER TO MID 80S FAR SOUTH
TUESDAY...AND BELOW-NORMAL HIGHS IN THE MID/UPPER 70S ARE EXPECTED
WEDNESDAY. OF COURSE...TEMPERATURES WILL REMAIN HIGHLY DEPENDENT ON
PRECIPITATION TRENDS.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
THE LONG TERM PERIODS LOOK RATHER WET AS BROAD UPPER LEVEL LOW
PRESSURE LINGERS OVER THE SOUTHEAST UNITED STATES AND AN EXPANSIVE
COASTAL TROUGH JUST OFF THE COAST MAINTAINS MOISTURE AND LIFT ACROSS
THE AREA. A COOL WEDGE OF HIGH PRESSURE WILL REMAIN TO THE NORTH
THROUGH SATURDAY BEFORE SHIFTING EAST INTO THE ATLANTIC SUNDAY. WE
EXPECT FAIRLY GOOD COVERAGE OF SHOWERS AND ISOLATED TSTMS WEDNESDAY
NIGHT THROUGH FRIDAY WITH DECREASING RAIN CHANCES OVER THE WEEKEND
AS AN UPPER RIDGE BUILDS OVER THE WESTERN ATLANTIC. TEMPS WILL
REMAIN AT OR BELOW NORMAL.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
VFR. RISK FOR TSTMS WILL INCREASE LATE IN THE PERIOD...GENERALLY
AFTER 21Z AT BOTH TERMINALS. GIVEN ITS SO FAR OUT AND TIMING IS
STILL SOMEWHAT UNCERTAIN...NO MENTION OF TSRA WILL BE INCLUDED
JUST YET. THIS WILL BE REEVALUATED WITH THE 06Z TAF CYCLE.

EXTENDED AVIATION OUTLOOK...BRIEF FLIGHT RESTRICTIONS POSSIBLE IN
THUNDERSTORMS MONDAY NIGHT. LOW PROBABILITY FOR OCCASIONAL FLIGHT
RESTRICTIONS ESPECIALLY IN SHOWERS THEREAFTER.

&&

.MARINE...
TONIGHT...A COLD FRONT WILL BEGIN TO APPROACH THE LOCAL WATERS FROM
THE NORTHWEST LATE. AS IT DOES...WINDS WILL TURN MORE SOUTHWESTERLY
WITH TIME WITH A BIT OF NOCTURNAL SURGING NOTED. HOWEVER...CONDITIONS
ARE EXPECTED TO REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS. WINDS
MAY REACH 15-20 KT NEAR 20 NM IN THE CHARLESTON COUNTY WATERS...BUT
MOST LOCATIONS WILL BE 10-15 KT OR A SOLID 15 KT. SEAS WILL
GENERALLY BE 2 FT WITHIN 20 NM AND 2-3 BEYOND.

A FAIRLY WEAK GRADIENT WILL EXIST MONDAY/MONDAY EVENING BEFORE
A COLD FRONT DROPS THROUGH THE WATERS. HIGH PRESSURE BUILDING FROM
THE NORTH WILL TIGHTEN THE SURFACE PRESSURE GRADIENT AND WILL
ENHANCE MARINE BOUNDARY LAYER MIXING LATE MONDAY NIGHT THROUGH LATE
WEEK. THIS WILL SUPPORT HAZARDOUS WINDS/SEAS...ESPECIALLY
TUESDAY-WEDNESDAY AS A COASTAL TROUGH ENHANCES THE GRADIENT. SMALL
CRAFT ADVISORY CONDITIONS ARE ALL BUT CERTAIN ACROSS ALL OR PORTIONS
OF THE WATERS TUESDAY AND WEDNESDAY...AND SCA CONDITIONS COULD
PERSIST THROUGH LATE WEEK.

RIP CURRENTS...AN ELEVATED RISK FOR RIP CURRENTS COULD DEVELOP ON
AREA BEACHES TUESDAY THROUGH LATE WEEK.

&&

.TIDES/COASTAL FLOODING...
INCREASING NORTHEAST FLOW TUESDAY THROUGH NEXT WEEKEND WILL RESULT
IN MODERATE CHANCES FOR SHALLOW COASTAL FLOODING ALONG THE COAST
ESPECIALLY THE SC COAST INCLUDING DOWNTOWN CHARLESTON AROUND THE
TIMES OF HIGH TIDE.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...NONE.

&&

$$

ST



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