Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 200216
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
916 PM EST FRI DEC 19 2014

.SYNOPSIS...
WEAK HIGH PRESSURE WILL PREVAIL THROUGH THIS EVENING. UNSETTLED
WEATHER IS EXPECTED LATER TONIGHT THROUGH MONDAY NIGHT AS SEVERAL
DISTURBANCES MOVE OVER THE REGION. A STRONG COLD FRONT WILL
APPROACH TUESDAY NIGHT AND SHIFT THROUGH THE AREA WEDNESDAY...WITH
DRIER CONDITIONS TO FOLLOW THROUGH LATE NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
WEAK CYCLOGENESIS IS ONGOING OFF THE SOUTHWEST LOUISIANA COAST
THIS EVENING WITH SUPPORT FROM THE RIGHT ENTRANCE REGION OF THE
SUBTROPICAL JET AND SUBTLE SHORTWAVE ENERGY MOVING ACROSS
NORTHEAST TEXAS. THESE FEATURES WILL GRADUALLY TRANSLATE EAST
TONIGHT WITH SECONDARY CYCLOGENESIS INITIATING OFF THE SOUTHEAST
U.S. COAST NEAR THE WESTERN WALL OF THE GULF STREAM BY SUNRISE
SATURDAY. FORCING WITH THIS SYSTEM DOES NOT LOOK OVERLY IMPRESSIVE
IN THE VARIOUS MODEL MASS FIELDS WITH ONLY MODEST ISENTROPIC
ASSENT NOTED ALONG THE 290-305K SURFACES. SOUTHWEST FLOW AHEAD OF
THIS SYSTEM WILL ADVECT GULF MOISTURE NORTHEAST INTO SOUTHEAST
SOUTH CAROLINA AND SOUTHEAST GEORGIA OVERNIGHT WITH MODEL PWAT
PROGS SHOWING PWAT VALUES REACHING AS HIGH AS 1 TO 1.10 INCHES BY
MID-MORNING SATURDAY. LIGHT RAIN IS THEREFORE EXPECTED TO
GRADUALLY OVERSPREAD THE AREA WELL AFTER MIDNIGHT AS NOSE OF
STRONGEST ISENTROPIC ASSENT NUDGES IN FROM THE SOUTHWEST...BUT IT
WILL TAKE SOME TIME FOR THE SUB-CLOUD LAYER TO MOISTEN UP ENOUGH
TO SUPPORT ANYTHING MORE THAN SPRINKLES OR VERY LIGHT RAIN.

HAVE ADJUSTED THE TIMING OF HIGHEST POPS ONCE AGAIN WITH THE LATE
EVENING UPDATE...MAINLY TO DELAY THE ONSET OF CATEGORICAL POPS
UNTIL AFTER 4 AM BASED ON THE LATEST HIGH RESOLUTION DATA. THE
HIGHEST PROBABILITIES FOR MEASURABLE RAINFALL WILL REMAIN CONFINED
TO A CORRIDOR ROUGHLY BOUNDED BY MILLEN-REIDSVILLE-NORTH
CHARLESTON-HARLEYVILLE BOX THROUGH SUNRISE. LOWS WILL RANGE FROM
THE LOWER 40S NORTH AND FAR WEST TO THE MID- UPPER 40S
CENTRAL...SOUTH AND COASTAL AREAS.

CHANGE SUMMARY FOR THE EARLY EVENING UPDATE...
* MODIFIED HOURLY POPS TO BETTER FIX THE EXPECTED EVOLUTION OF
  RAIN OVERNIGHT INTO EARLY SATURDAY MORNING.
* ADJUSTED HOURLY TEMPERATURE AND DEWPOINT GRIDS THROUGH 3 AM
  BASED ON CURRENT OBSERVATIONS AND A 50/50 BLEND OF GLAMP AND RAP
  DATA.
* LOWERED QPF THROUGH 20/12Z SLIGHTLY.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
SATURDAY...CONDITIONS WILL BE UNSETTLED AND COOLER THAN TODAY IN
RESPONSE TO ISENTROPIC ASCENT THAT LIFTS MOISTURE OVER A SHALLOW
HIGH PRESSURE WEDGE AND A STALLED FRONT OVER THE ATLANTIC TO THE SE.
THAT ALONG WITH AN EMBEDDED SHORT WAVE WITHIN THE FAST MOVING
WESTERLY FLOW ALOFT THAT WILL MOVE THROUGH THE TN VALLEY
TONIGHT...AND AN ELONGATED 125 KT UPPER JET THAT STRETCHES FROM OFF
THE MID ATLANTIC COAST TO TEXAS WILL PROVIDE ADDITIONAL FORCING FOR
ASCENT. THERE IS STILL A LITTLE DRY AIR IN THE LOWER LEVELS AT THE
START OF THE PERIOD...BUT IT WON/T MATTER AS THE VERTICAL COLUMN
BECOMES QUICKLY SATURATED INTO THE MID LEVELS. LIGHT RAINS WILL BE
ONGOING ACROSS MUCH OF THE FORECAST AREA AT DAYBREAK...AND WILL
BECOME MODERATE AT TIMES ACROSS OUR CENTRAL AND NORTHERN ZONES IN
CLOSER PROXIMITY TO THE DYNAMICS ALOFT. WE MIGHT FIND AN EVEN
SHARPER GRADIENT IN REGARDS TO PRECIP THAN WE ARE NOT
FORECASTING...BUT FIGURE ON NEAR 100 PERCENT POPS OVER MOST OF SC
AND IN MOST COMMUNITIES NORTH OF I-16 IN SE GA. POPS THEN TAPER TO
40-50 PERCENT OVER MCINTOSH COUNTY. AS THE SHORT WAVE MOVES EAST
TOWARD THE ATLANTIC BY LATE IN THE DAY OUR RAIN CHANCES WILL DWINDLE
FROM SW/WEST TO EAST/NE...WITH THE BULK OF THE RAIN OFFSHORE BY
NIGHTFALL. DUE TO THE PROGRESSIVE NATURE OF THE RAIN AND THE BULK OF
THE INFLUX OF MOISTURE FROM OFF THE ATLANTIC...OUR QPF IS NO MORE
THAN ABOUT 1/10 TO 2/10 OF AN INCH. COOL AIR WITHIN THE INLAND WEDGE
WILL LIMIT TEMPS TO THE LOWER AND MIDDLE 50S NORTH OF I-16 IN
GA...WITH UPPER 50S TO MAYBE NEAR 60 CLOSER TO THE ALTAMAHA RIVER
BASIN.

SATURDAY NIGHT...THE HIGH PRESSURE WEDGE HOLDS FIRMLY IN PLACE WITH
A WAVE OF LOW PRESSURE ON A STATIONARY FRONT TO THE EAST/SE OF THE
GULF STREAM. HE LACK OF FORCING UPSTAIRS WILL ALLOW FOR MUCH OF THE
RAIN TO HAVE ENDED...BUT SOUNDINGS SHOW A SHARP INVERSION OFF THE
DECK THAT WILL HOLD IN PLACE. THIS WILL RESULT IN AT LEAST A LITTLE
LIGHT RAIN OR MORE LIKELY DRIZZLE. DEPENDING UPON HOW LONG THE BUILD
DOWN OF STRATUS TRANSPIRES WE COULD SEE SOME FOG CONCERNS. LIMITED
COOL ADVECTION AND WIDESPREAD CLOUD COVER WILL PREVENT MUCH OF A
DROP IN TEMPS...AT BEST ABOUT A 10 DEGREE DIURNAL SWING.

SUNDAY AND SUNDAY NIGHT...ANOTHER SOUTHERN SHORT WAVE FEATURE WILL
ARRIVE FROM THE SW LATE IN THE DAY OR AT NIGHT. THIS WILL TRIGGER
ANOTHER WAVE OF LOW PRESSURE ALONG THE BAROCLINIC ZONE TO OUR
EAST/SE...WHILE THE INLAND WEDGE REMAINS ANCHORED IN PLACE. BUT AS
THE FLOW ALOFT BACKS MORE SW IT PULLS THIS FEATURE A LITTLE CLOSER
TO THE COST AND ALSO OPENS UP THE GULF ALOFT FOR IT/S FEED OF
MOISTURE. WE DON/T HAVE TOO MUCH COVERAGE OF RAIN UNTIL THE ARRIVAL
OF THIS SHORT WAVE...SHOWING MAINLY 20-30 POPS SUNDAY...BUT RAMPING
UP QUICKLY FROM SW TO NE SUNDAY NIGHT INTO THE CATEGORICAL RANGE. WE
DON/T YET HAVE 100 POPS GIVEN SOME MINOR TIMING ISSUES...BUT FIGURE
THAT EVERYONE WILL GET A STEADY RAIN DEVELOPING AT SOME POINT SUNDAY
NIGHT WITH THE LARGE SCALE FORCING FROM THE SHORT WAVE AND THE RIGHT
ENTRANCE REGION OF THE UPPER JET. WITH PWATS TO CLIMB TO AROUND 1.4
TO 1.5 INCHES WE ANTICIPATE HIGHER PRECIP AMOUNTS IN THE 1/2 TO 3/4
INCH RANGE. TEMPS WILL BE BELOW NORMAL DURING THE DAY...THEN FALL
ONLY ABOUT 6-10 DEGREES AT NIGHT.

MONDAY...WE/RE STILL STUCK WITHIN THE COOL AIR WEDGE REGIME WITH ANY
COASTAL WARM FRONT TO REMAIN OUT NEAR THE GULF STREAM. THERE IS
STILL ENOUGH FORCING IN RESPONSE TO THE PROXIMITY TO THE WARM
FRONT...THE TAIL OF THE MID LEVEL SHORT WAVE AND NEARBY UPPER
JET...PLUS ISENTROPIC UPGLIDE PERSISTS. PWATS ACTUALLY MAKE A RUN
FOR ALMOST 2 STANDARD DEVIATIONS ABOVE NORMAL...SO WE/RE STILL
SHOWING 80-90 PERCENT RAIN CHANCES. THERE IS AT LEAST A LITTLE
ELEVATED INSTABILITY...WITH DECENT MID LEVEL LAPSE RATES AND
SHOWALTER INDICES APPROACHING ZERO. BUT CERTAINLY NOT ENOUGH TO
JUSTIFY ADDING MENTION OF THUNDER TO THE GRIDDED ELEMENTS.
ADDITIONAL QPF OF 1/2 TO 2/3 INCHES. MY CONFIDENCE ON TEMPS IS NOT
THE GREATEST...BUT WITH COOL AIR DAMMING STILL IN PLACE WE
CONTINUE TO SHOW BELOW NORMAL TEMPS.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
MONDAY NIGHT INTO TUESDAY NIGHT...THE PATTERN WILL REMAIN UNSETTLED
AS AHEAD OF COLD FRONT THAT EVENTUALLY PUSHES THROUGH THE SOUTHEAST
EARLY WEDNESDAY. AT THIS TIME...MODELS SUGGEST CHANCES OF PRECIP
INCREASING MONDAY NIGHT INTO TUESDAY AS A WARM FRONT TRIES TO LIFT
NORTH INTO A WEAKENING WEDGE OF HIGH PRESSURE AT THE SFC. A PERIOD
OF WARMING SHOULD ALSO OCCUR WITHIN A SOUTHERLY FLOW AHEAD OF THE
APPROACHING COLD FRONT TO THE WEST. HAVE THEREFORE INTRODUCED
SHOWERS FOR ALL AREAS...RATHER THAN RAIN. WE COULD SEE ENOUGH
INSTABILITY TO INCLUDE A FEW THUNDERSTORMS IN THE FORECAST SHOULD
TRENDS PERSIST...MAINLY IN SOUTHEAST GEORGIA TUESDAY AFTERNOON AND
EVENING. OVERALL HIGH TEMPS SHOULD RANGE IN THE LOW/MID 60S FOR MOST
AREAS ON TUESDAY...EVEN WITH CLOUD COVER PRESENT. UPPER 60S ARE
EXPECTED OVER SOUTHEAST GEORGIA. CHANCES OF PRECIP SHOULD QUICKLY
DIMINISH AS THE COLD FRONT BEGINS TO SHIFT THROUGH THE AREA LATE
TUESDAY NIGHT. TUESDAY NIGHT LOWS SHOULD RANGE IN THE UPPER 40S TO
AROUND 50 OVER MOST AREAS.

WEDNESDAY THROUGH FRIDAY...A MUCH QUIETER/DRY PERIOD OF WEATHER IS
ANTICIPATED AS HIGH PRESSURE BUILDS OVER THE REGION BEHIND A COLD
FRONT SHIFTING OFFSHORE EARLY WEDNESDAY. TEMPS WILL BE SLOW TO WARM
THROUGH THURSDAY...UNTIL HIGH PRESSURE BECOMES MORE CENTERED OVER
THE WESTERN ATLANTIC. OVERALL HIGH TEMPS SHOULD RANGE IN THE
MID/UPPER 50S WEDNESDAY AND THURSDAY...BEFORE SOUTHERLY FLOW RETURNS
TO THE AREA ALONG THE WESTERN EDGE OF HIGH PRESSURE ON FRIDAY. TEMPS
SHOULD PEAK INTO THE LOW/MID 60S BY LATE WEEK.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
HIGH CONFIDENCE IN VFR CONDITIONS THROUGH THIS EVENING...AND GIVEN
LOW-LEVEL DRY AIR INITIALLY IN PLACE ODDS FAVOR VFR CONDITIONS
PERSISTING THROUGH 12Z. HOWEVER...AS RAIN OVERSPREADS THE
AREA...THE POTENTIAL FOR AT LEAST MVFR CEILINGS WILL INCREASE
AFTER AFTER 09Z. ONCE CEILINGS LOWER SATURDAY MORNING...MVFR
CEILINGS SHOULD PREVAIL AT KCHS FOR THE BALANCE OF THE 18Z TAF
PERIOD WITH CIGS IMPROVING TO LOW-END VFR AT KSAV. IFR VSBYS/CIGS CAN
NOT BE COMPLETELY RULED OUT WHERE POCKETS OF HEAVIER RAINS
OCCUR...BUT PROBABILITIES ARE TOO LOW TO INCLUDE ANY MENTION ATTM.

EXTENDED AVIATION OUTLOOK...PERIODS OF MVFR OR LOWER CONDITIONS
APPEAR LIKELY WITH RAIN/SHOWERS AND LOW STRATUS SATURDAY AFTERNOON
THROUGH MONDAY NIGHT WHILE A WEDGE OF HIGH PRESSURE PERSISTS INLAND
AND WAVES OF LOW PRESSURE LIFT NE ACROSS THE NEARBY ATLANTIC.
ADDITIONAL FLIGHT RESTRICTIONS LIKELY TUESDAY INTO EARLY WEDNESDAY
AS A STRONG COLD FRONT APPROACHES FROM THE WEST...ACCOMPANIED BY
SHRA AND POSSIBLE TSRA. WINDY WEATHER BEHIND THE FRONT WEDNESDAY.

&&

.MARINE...
TONIGHT...A TIGHTENING PRESSURE GRADIENT SHOULD TRANSLATE TO MORE
UNIFORM N/NE WINDS WHICH ARE EXPECTED TO INCREASE TO 10-15 KT
OVERNIGHT AS WEAK LOW PRESSURE DEVELOPS OFFSHORE. ACCORDINGLY...
SEAS SHOULD BUILD TO 2-3 FT LATE.

SATURDAY AND SATURDAY NIGHT...A WAVE OF LOW PRESSURE WILL FORM ALONG
A STALLED OUT FRONT TO THE EAST/SE OF THE LOCAL WATERS...WHILE AN
INLAND WEDGE OF HIGH PRESSURE HOLDS IN PLACE. THERE IS AT LEAST SOME
PINCHING OF THE PRESSURE GRADIENT TO GENERATE NORTH OR NE WINDS AS
HIGH AS 15 OR 20 KT. THE RESULTING SEAS WILL PEAK AT 3 OR 4 FT. SO
FOR NOW WE DON/T ANTICIPATE ANY SMALL CRAFT ADVISORY HEADLINES.

SUNDAY THROUGH MONDAY...THE FIRST WAVE OF LOW PRESSURE PULLS AWAY OR
DISSIPATES...BUT A SECOND WAVE OF WILL TAKE SHAPE IN SIMILAR
POSITION TO THE EAST/SE OF THE MARINE AREA. AS THE FLOW ALOFT BACKS
TO THE SW WILL ATTEMPT TO DRAW THE SURFACE WAVE AND ASSOCIATED WARM
FRONT BACK CLOSER TO THE COAST BY LATE IN THE PERIOD. BUT INLAND
WEDGE IS RELUCTANT TO RELAX ITS GRIP ACROSS THE SE...SO ANY VEERING
OF WINDS FROM THE NE AROUND TO THE SE OR SOUTH WILL TAKE SOME
TIME...IF AT ALL DURING THIS PERIOD. WINDS WILL AGAIN PEAK AT 15 OR
20 KT PENDING THE EXACT EVOLUTION OF THE PATTERN...WITH SEAS AGAIN
NO MORE THAN 3 OR 4 FT. THERE COULD BE A LITTLE THUNDER AND
LIGHTNING OUT NEAR THE GULF STREAM SUNDAY NIGHT INTO MONDAY...IN
CLOSE PROXIMITY TO THE COASTAL WARM FRONT.

TUESDAY AND WEDNESDAY...THE COASTAL WARM FRONT LIFTS NORTH OF THE
AREA TUESDAY AND GIVES WAY TO A STRONG COLD FRONT EARLY WEDNESDAY.
MARINE LAYERING EFFECTS WITHIN THE WARM ADVECTION REGIME WILL LIKELY
HOLD WINDS BELOW ANY SMALL CRAFT THRESHOLDS...BUT STEADY ISALLOBARIC
PRESSURE RISES AND COLD ADVECTION BEHIND THE FRONT WILL LEAD TO AT
LEAST SMALL CRAFT ADVISORIES OR MAYBE GALES. NOT YET SURE HOW THE
SITUATION WILL UNFOLD...BUT THERE IS AT LEAST A LOW END RISK OF SOME
STRONG OR SEVERE T-STORMS.

SEA FOG...MARINERS ARE ADVISED THAT LOW STRATUS DECKS AND PERIODS OF
RAIN OR DRIZZLE COULD EQUATE TO REDUCED VISIBILITIES SATURDAY INTO
MONDAY. THEN AS SLIGHTLY WARMER AIR ARRIVES ATOP THE MID AND UPPER
50S SHELF WATERS WITHIN A SOUTH AND SW FLOW LATE MONDAY NIGHT AND
TUESDAY...THIS MAY ALLOW FOR THE DEVELOPMENT OF SEA FOG PRIOR TO
WINDS INCREASING ENOUGH TO SCOUR IT OUT LATER TUESDAY AND TUESDAY
NIGHT.

&&

.TIDES/COASTAL FLOODING...
ALTHOUGH THERE ARE NO IMMEDIATE CONCERNS IN REGARDS TO TIDES...
GIVEN THE PERIGEAN SPRING TIDES EARLY NEXT WEEK...THERE IS AT
LEAST A MODERATE RISK OF SHALLOW COASTAL FLOODING CONCERNS MONDAY
AND TUESDAY AROUND THE TIME OF THE MORNING HIGH TIDE.

&&

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

&&

$$

ST




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