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Area Forecast Discussion

924
FXUS62 KCHS 202208
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
608 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/...
MESOSCALE UPDATE...20/21Z MESOANALYSIS PLACED THE COLD FRONT
ACROSS THE MOUNTAINS OF NORTH CAROLINA AND EXTENDING SOUTHWEST TO
NEAR THE ATLANTA METRO AREA INTO SOUTH-CENTRAL ALABAMA. A LINE OF
STRONG TO SEVERE TSTMS IS ONGOING ACROSS THE SOUTH CAROLINA
UPSTATE AND TRAILING SOUTHWEST INTO CENTRAL GEORGIA ALONG A
PRONOUNCED INSTABILITY GRADIENT. HIGH RESOLUTION MODELS ARE
SIMILAR IN SHOWING THIS CONVECTIVE LINE MOVING SLOWLY EAST AND
POTENTIALLY EXPANDING IN AREA OVER THE NEXT FEW HOURS AS THE FRONT
DRAWS CLOSER. THIS CONVECTIVE GROWTH APPEARS TO BE TIED INTO
INCREASING UPPER FORCING ASSOCIATED WITH THE LEFT EXIT REGION
REGION OF A 140 KT JET STREAK CURRENTLY MOVING ACROSS SOUTHERN
ALABAMA.

ALTHOUGH INSTABILITY WILL BE MARGINAL THROUGH THE REMAINDER OF
THE EVENING HOURS...THE COMBINATION OF UPPER FORCING AND
STRENGTHENING 0-6KM BULK SHEAR MAY BE ENOUGH TO ALLOW A FEW
LOOSELY ORGANIZED CONVECTIVE LINES TO DEVELOP...ESPECIALLY IF
POCKETS OF HIGHER INSTABILITY FARTHER WEST ARE REALIZED...WHICH
COULD POSE A MARGINAL RISK FOR DAMAGING WINDS. CONFIDENCE IN THIS
SCENARIO REMAINS LOW AS IT REMAINS UNCLEAR WHETHER THE MARGINAL
NATURE OF THE THERMODYNAMIC ENVIRONMENT CURRENTLY IN PLACE ACROSS
SOUTHEAST SOUTH CAROLINA AND SOUTHEAST GEORGIA CAN BE OVERCOME BY
INCREASED SYNOPTIC FORCING. AT THIS POINT...SEE NO REASON TO
DEVIATE FROM THE CURRENT MENTION OF ISOLATED SEVERE THUNDERSTORMS
IN THE HAZARDOUS WEATHER OUTLOOK...BUT CONVECTIVE TRENDS CONTINUE
TO BE MONITORED.

FOR THE MESOSCALE UPDATE...
* INCREASED POPS SLIGHTLY TO 20-30 PERCENT FOR MOST AREAS.
* ADJUSTED SKY COVER SLIGHTLY PER CURRENT SATELLITE TRENDS AND
  RAP/CANADIAN CLOUD PRODUCTS.
* ADJUSTED HOURLY TEMPERATURES TO BETTER MATCH EXPECTED THERMAL
  FALLS THROUGH MIDNIGHT.

&&

.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 /22Z MONDAY THROUGH SATURDAY/...
VFR THROUGH THE 18Z TAF PERIOD. LOW PROBABILITY FOR THUNDERSTORMS
IN THE VICINITY LATER THIS EVENING AT TERMINALS...TOO LOW TO
JUSTIFY ANY MENTION WITHIN 18Z TAFS. LOW PROBABILITY FOR BRIEF
MVFR CEILINGS AS COLD FRONT PASSES LATE TONIGHT/EARLY TUESDAY...
MAINLY 09Z- 12Z.

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

&&

.MARINE...
ELEVATED SW WINDS AND SEAS AS HIGH AS 3-4 FT WILL PERSIST UNTIL A
COLD FRONT PASSES LATE TONIGHT OR EARLY TUESDAY. THEN...WINDS WILL
SHIFT TOWARD THE W/NW...AND AFTER A BRIEF/INITIAL SURGE WINDS
SHOULD RELAX FAIRLY QUICKLY. CANCELLED THE ONGOING SCA FOR
CHARLESTON HARBOR...BUT RAISED AN SCA FOR AMZ374 FOR THE
LIKELIHOOD OF 25 KNOTS GUSTS.

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.

&&

$$

ST



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