Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 210139
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
939 PM EDT MON APR 20 2015

.SYNOPSIS...
A COLD FRONT WILL CROSS THE AREA LATE TONIGHT. DRIER HIGH
PRESSURE WILL BUILD IN TUESDAY AND WEDNESDAY. A WEAK COLD FRONT
WILL DROP THROUGH THE AREA THURSDAY NIGHT...THEN WILL STALL TO
THE SOUTH. A STORM SYSTEM WILL AFFECT THE REGION FRIDAY NIGHT
THROUGH SATURDAY NIGHT. HIGH PRESSURE WILL BUILD IN FROM THE
NORTHWEST SUNDAY INTO MONDAY.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
21/01Z MESOANALYSIS PLACED THE COLD FRONT FROM THE WESTERN NORTH
CAROLINA PIEDMONT INTO THE SOUTH CAROLINA UPSTATE AND EXTENDING
SOUTHWEST TO NEAR COLUMBUS GEORGIA. ISOLATED STRONG TO SEVERE
TSTMS CONTINUE TO FIRE ALONG AND JUST AHEAD OF THE FRONT AS
INCREASING 0-6KM BULK SHEAR COUPLED WITH INTENSIFYING UPPER
FORCING ASSOCIATED WITH LEFT EXIT REGION OF A 140 JET JET STREAK
COMPENSATE FOR WANING SURFACE BASED INSTABILITY.

THE H3R HAS SHOWN REMARKABLE RUN-TO-RUN CONSISTENCY IN
REDEVELOPING ISOLATED TO PERHAPS LOW-END SCATTERED CONVECTION
ACROSS EAST-CENTRAL GEORGIA AND CSRA ALONG THE FRONT AS THE CORE
OF THE MOST INTENSE UPPER FORCING TRAVERSES THE AREA OVER THE NEXT
3-5 HOURS. THE 21/00Z KCHS RAOB SHOWED A STRONG CAPPING INVERSION
AT 872 HPA...WHILE WEST OF THE I-95 CORRIDOR RAP SOUNDINGS
DEPICTED CONSIDERABLY LESS CAPPING WITH MLCAPE HOLDING 500-700
J/KG THROUGH ABOUT 2 AM. SUSPECT THIS WILL BE ENOUGH TO SUSTAIN
ANY UPSTREAM FORCED CONVECTION FOR SEVERAL MORE HOURS... BEFORE
WEAKENING AS IT APPROACHES THE COAST. WILL HIGHLIGHT SLIGHT CHANCE
POPS ALONG AND JUST AHEAD OF THE FRONT FOR ALL AREAS. STILL CAN
NOT RULE OUT AN ISOLATED STRONG TO SEVERE TSTM GIVEN THE AMOUNT OF
BULK SHEAR AND DCAPE IN PLACE WITH DAMAGING WIND GUSTS BEING THE
PRIMARY HAZARD.

LOWS FROM THE UPPER 50S FAR INTERIOR SOUTHEAST GEORGIA TO THE MID
60S AT THE COAST LOOK REASONABLE.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
TUESDAY AND WEDNESDAY...SURFACE HIGH PRESSURE CENTERED OVER THE GULF
COAST STATES WILL EDGE EASTWARD AND EXPAND OVER THE AREA.
ALOFT...FLOW WILL REMAIN NEARLY ZONAL BETWEEN A LOW SPINNING OVER
EASTERN CANADIAN AND SUBTROPICAL RIDGE TO OUR SOUTH. COOLER AND
MUCH DRIER AIRMASS WILL TAKE HOLD...AND WITH SUBSIDENCE NOTED IN
MODEL SOUNDINGS BOTH DAYS SHOULD BE PRECIP-FREE. WEAK DOWNSLOPE
FLOW WILL HELP HIGHS REACH THE UPPER 70S TUESDAY...UPPER 70S/LOW
80S ON WEDNESDAY. LOWS TUESDAY NIGHT WILL BE IN THE LOW
50S...WHILE WEDNESDAY NIGHT IS MORE MILD WITH INCREASING CLOUD
COVER IN THE LOW 60S.

WEDNESDAY NIGHT INTO THURSDAY...SURFACE HIGH WILL SHIFT INTO THE
ATLANTIC AHEAD OF A WEAK COLD FRONT APPROACHING FROM THE NORTHWEST.
THE FRONT WILL CROSS THROUGH THE FORECAST ON THURSDAY EVENING.
DESPITE A PASSING SHORTWAVE...MOISTURE IS NOT OVERLY IMPRESSIVE.
GIVEN THAT MODELS ARE STILL PRETTY INCONSISTENT ON QPF DURING THIS
TIME FRAME WITH THE EURO NEARLY DRY/GFS THE WETTER SOLUTION...HAVE
OPTED TO CAP POPS BELOW 15 PERCENT FOR NOW. HIGHS WILL BE IN THE
UPPER 70S/LOW 80S.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
A BACK DOOR COLD FRONT WILL DROP THROUGH THE AREA THURSDAY NIGHT AND
FRIDAY...BRIEFLY STALLING TO THE SOUTH. FRIDAY TENTATIVELY LOOKS DRY
WITH HIGHS IN THE MID/UPPER 70S. INCREASING UPPER SHORTWAVE ENERGY
FRIDAY NIGHT INTO SATURDAY WILL ALLOW THE FRONT TO LIFT NORTH WITH A
LOW PRESSURE SYSTEM LIFTING INTO THE MID-ATLANTIC REGION. A COLD
FRONT WILL PUSH THROUGH SATURDAY OR SATURDAY NIGHT WITH WEAK HIGH
PRESSURE BUILDING IN SUNDAY. THERE ARE SIGNIFICANT MODEL DIFFERENCES
REGARDING PRECIPITATION POTENTIAL DURING THE PERIOD. WE MAINTAINED
THE HIGHEST POPS DURING THE DAY SATURDAY...DECREASING SATURDAY
NIGHT. BELOW-NORMAL TEMPS EXPECTED SUNDAY AND MONDAY AS HIGH
PRESSURE LINGERS TO THE NORTH.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
VFR. HIGH RESOLUTION MODELS SUGGEST A BROKEN LINE OF SHOWERS AND
TSTMS COULD MOVE ACROSS THE TERMINALS 04-07Z...BUT CONFIDENCE IS
QUITE LOW. WILL NOT INCLUDE A MENTION SHRA/TSRA ATTM.

EXTENDED AVIATION OUTLOOK...LOW STRATUS OR FOG POSSIBLE AROUND
DAYBREAK THURSDAY AND FRIDAY MORNINGS.

&&

.MARINE...
OVERNIGHT...A TIGHT PRESSURE GRADIENT BETWEEN A MESO-HIGH INDUCED
BY SEVERE CONVECTION OVER FLORDIA AND THE APPROACHING COLD FRONT
HAS BEEN SUPPORTING WIND GUSTS OVER 30 KT AT BOTH BUOY 41004 AND
41008 OVER THE PAST FEW HOURS. ELEVATED WINDS WILL PERSIST ACROSS
THE GEORGIA NEARSHORE WATERS FOR A FEW MORE HOURS...SO EXPANDED
THE SMALL CRAFT ADVISORY THERE UNTIL 2 AM. THE ADVISORY FOR THE
GEORGIA OFFSHORE WATERS REMAINS IN EFFECT UNTIL 8 AM. WINDS WILL
REMAIN 15-20 KT OVER THE SOUTH CAROLINA WATERS WITH 10 KT IN THE
CHARLESTON HARBOR. SEAS WILL AVERAGE 2-4 FT NEARSHORE WATERS AND
4-5 FT OFFSHORE WATERS.

TUESDAY THROUGH SATURDAY...CONDITIONS WILL REMAIN RATHER BENIGN OVER
THE MARINE ZONES. SURFACE HIGH PRESSURE WILL EXPAND OVER THE WATERS
TUESDAY BEHIND THE DEPARTING COLD FRONT...ALLOWING THE PRESSURE
GRADIENT TO RELAX. THE HIGH WILL THEN SHIFT EAST INTO THE ATLANTIC
WEDNESDAY INTO THURSDAY AHEAD OF A WEAK COLD FRONT APPROACHING FROM
THE NORTHWEST. THE FRONT IS EXPECTED TO CROSS THROUGH THE WATERS
THURSDAY NIGHT. ANOTHER STORM SYSTEM EXPECTED TO APPROACH SATURDAY.
WEAK PRESSURE GRADIENT WILL KEEP WINDS AT OR BELOW 15 KTS ON AVERAGE
WITH SEAS 2-4 FEET HIGHEST IN THE OUTER WATERS...WELL BELOW SMALL
CRAFT ADVISORY CRITERIA.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 8 AM EDT TUESDAY FOR AMZ374.
     SMALL CRAFT ADVISORY UNTIL 2 AM EDT TUESDAY FOR AMZ354.

&&

$$

ST



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