Special Weather Statement, NWS Charleston, SC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
338 PM EDT SUN MAR 18 2012

.SYNOPSIS...
A WEAK COLD FRONT WILL LINGER ACROSS SOUTH CAROLINA TONIGHT BEFORE
DISSIPATING MONDAY. ATLANTIC HIGH PRESSURE WILL THEN REGAIN CONTROL
TUESDAY THROUGH MUCH OF THE UPCOMING WEEK. THE NEXT FRONTAL SYSTEM MAY
IMPACT THE AREA SOMETIME NEXT WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
MAINLY SCATTERED SHOWERS AND TSTMS TO DEVELOP AND MOVE SOUTHEAST
ACROSS THE AREA THIS EVENING AS A SURFACE TROUGH OF LOW PRESSURE
REMAINS INLAND AND THE SEA BREEZE PUSHES INLAND. A FEW STORMS
COULD BECOME MARGINALLY SEVERE WITH LARGE HAIL AN INCH OR MORE IN
DIAMETER AND/OR WIND GUSTS NEAR 60 MPH. DIURNAL STABILIZATION
WILL REDUCE RAIN CHANCES OVERNIGHT ALTHOUGH A WEAK BACKDOOR COLD
FRONT WILL LINGER ACROSS SOUTH CAROLINA AND COULD KEEP A FEW
SHOWERS GOING MUCH OF THE NIGHT...MAINLY NORTHERN AND WESTERN
AREAS. DIMINISHING SKY COVER AND SUFFICIENT LOW LEVEL MOISTURE
WILL CAUSE SOME FOG TO DEVELOP TOWARD DAYBREAK BUT IT SHOULD BE
LIGHT AND MORE PATCHY IN NATURE AND THUS HAVE NOT INCLUDED MENTION
IN THE FORECAST AT THIS TIME. LOWS WILL RANGE FROM THE MID/UPPER
50S INLAND TO THE LOWER 60S AT THE BEACHES.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
MONDAY...THE AXIS OF A HIGHLY AMPLIFIED MID/UPPER RIDGE WILL LIE
OVER OR JUST WEST OF THE FORECAST AREA...WHILE A BROAD AND DEEP
TROUGH IS FOUND OVER MUCH OF THE WESTERN STATES. ONE WEAK IMPULSE
WITHIN THE NORTHERLY FLOW ALOFT WILL MOVE THROUGH NC IN THE LATE
MORNING...AND INTO OUR NE ZONES DURING THE AFTERNOON. MEANWHILE...A
WEAK PRESSURE PATTERN WILL PREVAIL AS A DECAYING COLD FRONT IS FOUND
OVER OR NEAR THE AREA AND DISSIPATES BY DAYS END.

THE SYNOPTIC SITUATION WILL BE MORE DIFFERENT THAN IT HAS BEEN
DURING RECENT DAYS...WITH LESS COVERAGE OF CONVECTION. WITH THE AXIS
OF THE RIDGE ALOFT TO BE IN CLOSER PROXIMITY TO THE REGION THERE IS
AN INCREASE IN SUBSIDENCE ALOFT. PLUS THE MAIN INITIATORS FOR
CONVECTION WILL BE THE SEA BREEZE AND THE DIFFUSE FRONT. SO WE/LL
LIMIT SHOWER/T-STORM PROBABILITIES TO ONLY 20 PERCENT. WHILE THIS
MIGHT BE A CONSERVATIVE FORECAST IF THERE IS GREATER MESOSCALE
BOUNDARY INTERACTIONS IN THE AFTERNOON...THIS DOES MESH WELL WITH
SURROUNDING WFO/S AND IS EVEN A LITTLE HIGHER THAN MOST OF THE
TABULAR GUIDANCE. A NE STEERING FLOW LOOKS TO KEEP THE ACTIVITY
INLAND FROM THE COAST...AND MAINLY WEST OF US-17. WITH LIFTED
INDICES NEAR -3/-4C...WET BULBS NEAR 9500-10000 FT AND MID LEVELS
TEMPS STILL DOWN NEAR -14C...THERE IS THE RISK OF MARGINALLY SEVERE
HAIL IN A FEW OF THE STRONGEST STORMS THAT DEVELOP. ISOLATED
DOWNBURSTS MAY ALSO OCCUR WHERE BOUNDARY COLLISIONS OCCUR...AS
DCAPES ARE NEAR 600 J/KG.

TEMPS AT 850 MILLIBARS ARE CLOSE TO 12-13C...AND THAT ALONG WITH
DECENT INSOLATION WILL ALLOW FOR MAX TEMPS LIKELY REACH 80 OR
GREATER YET AGAIN. HIGHEST VALUES WILL BE ACROSS INTERIOR SE
GA...WHERE MID 80S ARE EXPECTED. A PREVAILING ONSHORE
BREEZE...ALBEIT LESS THAN 10-15 MPH WILL HOLD SHORELINE COMMUNITIES
DOWN CLOSE TO 70.

MONDAY NIGHT...WITH THE LOSS OF DAYTIME HEATING AND WEAKENING
INSTABILITY...DIURNAL CONVECTION FAR INLAND WILL END QUICKLY AROUND
SUNSET OR SHORTLY THEREAFTER. LATE NIGHT FOG MAY BECOME A CONCERN AS
WE APPROACH OUR CROSS-OVER TEMPS LATE AND CLOUD COVER DIMINISHES.
LOWEST TEMPS WILL BE IN THE UPPER 50S INLAND AND LOWER 60S ALONG THE
COAST.

TUESDAY THROUGH WEDNESDAY...THE TROUGH OUT IN THE WESTERN STATES
WILL CLOSE OFF TUESDAY AND MOVE INTO THE SOUTHERN PLAINS ON
WEDNESDAY. DOWNSTREAM...THE RIDGE OVER THE EASTERN STATES BECOMES
MORE NARROW AS IT BECOMES SQUISHED BETWEEN THE LOW AND A MID LEVEL
TROUGH OVER THE WESTERN ATLANTIC. WHILE THIS TRANSPIRES...A SURFACE
TROUGH OVER THE NEARBY ATLANTIC ON TUESDAY WILL MOVE ONSHORE OR
DISSIPATE WEDNESDAY...AS STRENGTHENING HIGH PRESSURE BUILDS OVER THE
WEST AND NW ATLANTIC. AS A RESULT OUR SYNOPTIC FLOW BECOMES EAST AND
SE...AND DEEPENS WITH TIME. WHILE THERE STILL COULD WEAK IMPULSES
ALOFT...THE MAIN FOCUS FOR CONVECTION WOULD BE THE SEA BREEZE AND
THE CLOSE PROXIMITY TO THE TROUGH. WITH A DEEPER ONSHORE
FLOW...CONVECTION WILL GET AN EARLIER START EACH DAY AND MOVE MORE
QUICKLY INLAND. BUT GIVEN ONLY MARGINAL LAPSE RATES AND LESS
INSTABILITY WE WILL CAP DIURNALLY DRIVEN POPS AT ONLY 20 PERCENT
BOTH DAYS. WITH SLIGHTLY LOWER 850 MB TEMPS AND 1000-850 MB
THICKNESS VALUES...PLUS THE ONSHORE FLOW...MAX TEMPS WILL NOT BE
QUITE AS WARM. 77-83 WILL SUFFICE FROM NE TO SW TUESDAY...AND ONLY
74-81 NE TO SW FOR WEDNESDAY. THE BEACHES WILL BE CONSIDERABLY
COOLER BOTH DAYS.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
NUMERICAL MODEL TIMING DIFFERENCES ABOUND REGARDING THE EVOLUTION
AND PROGRESSION OF SIGNIFICANT SYNOPTIC SCALE FEATURES THROUGH THE
EXTENDED FORECAST PERIOD. THERE IS GENERAL AGREEMENT THAT THE
EXPANSIVE CUT OFF LOW ACROSS THE CENTRAL UNITED STATES WILL
STEADILY PROGRESS EASTWARD THURSDAY AND FRIDAY...DRIVING A SURFACE
FRONT TOWARD THE REGION AND INCREASING RAIN CHANCES THROUGH THE
WEEKEND. CONFIDENCE IN FORECAST DETAILS HOWEVER IS QUITE LOW DUE
TO CONTINUED CONSIDERABLE TIMING DIFFERENCES BETWEEN MODEL
SOLUTIONS. HAVE LEANED TOWARD A SLOWER PROGRESSION OF THE SYSTEM
AT THIS TIME...INCREASING RAIN CHANCES FROM SOUTHWEST TO NORTHEAST
BEGINNING LATE THURSDAY...WITH HIGHEST POPS IN THE 30 PERCENT
RANGE ACROSS ALL AREAS BY SATURDAY. STILL PREFER TO REMAIN
CONSERVATIVE UNTIL MODEL CONSENSUS IMPROVES. HAVE ALSO CONTINUED
TO ADVERTISE ABOVE NORMAL TEMPERATURES...REACHING AT LEAST THE MID
TO UPPER 70S INTO THE WEEKEND AWAY FROM THE LOCALLY COOLER
COASTLINE. BUT EVEN THESE TEMPS ARE MUCH WARMER THAN WE TYPICALLY
EXPERIENCE IN MARCH.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
THE PRIMARY CONCERN FOR THE 18Z TAFS WILL BE SH/TSTMS...MAINLY
THROUGH EARLY EVENING DUE TO AN INLAND LOW PRESSURE TROUGH AND
INLAND-MOVING SEA BREEZE. WITH SH/TSTMS HAVING ALREADY FIRED AT
KCHS HAVE AMENDED THE 18Z TAF ALREADY THROUGH 20Z AND MAY HAVE TO
AGAIN BASED ON UPSTREAM CONVECTION. KSAV IS IN THE CLEAR FOR NOW
WITH RAIN NORTH AND SOUTH OF THERE. OTHERWISE...LOW PROB FOR MVFR
FOG AROUND 12Z SO DID NOT INCLUDE ATTM.

EXTENDED AVIATION OUTLOOK...THERE ARE NO SIGNIFICANT CONCERNS AT
THIS TIME WITH A CONFIDENCE OF 30 PERCENT OR GREATER.

&&

.MARINE...
TONIGHT...A WEAK BACKDOOR COLD FRONT WILL PUSH INTO THE CHARLESTON
TRI-COUNTY WATERS LATE BUT THERE ARE NO SIGNIFICANT MARINE CONCERNS
OTHER THAN STRONG STORMS PUSHING OFF THE COAST THIS EVENING WITH
HAIL...GUSTY WINDS AND CLOUD-TO- WATER LIGHTNING.

MONDAY THROUGH FRIDAY...A VERY WEAK COLD FRONT WILL LIE ACROSS THE
WATERS MONDAY...BEFORE DISSIPATING. A WEAK TROUGH WILL THEN FORM
NEAR THE GULF STREAM MONDAY NIGHT AND WILL STRUGGLE TO CROSS THE
COOLER SHELF WATERS TUESDAY...BEFORE DISSIPATING OR MOVING ONSHORE
DURING WEDNESDAY. WHILE THIS EVOLVES...A WEAK AREA OF HIGH
PRESSURE FROM NEAR BERMUDA TO NORTH FLORIDA MONDAY WILL STRENGTHEN
TUESDAY AND WEDNESDAY...AS IT BECOMES ABSORBED BY A
LARGER/STRONGER HIGH FROM THE NW ATLANTIC.

A WEAK PRESSURE PATTERN MONDAY WILL GRADUALLY GIVE WAY TO AN EAST
AND SE SYNOPTIC FLOW AROUND THE BUILDING ATLANTIC RIDGE TUESDAY
THROUGH WEDNESDAY. EVEN WITH AFTERNOON/EVENING ENHANCEMENTS NEAR THE
COAST AND IN CHARLESTON HARBOR...WINDS WILL HOLD AT NO MORE THAN 10
OR 15 KT. SEAS WILL BE LIMITED TO 3 FT OR LESS MONDAY AND
TUESDAY...REACHING ANOTHER FOOT OR TWO HIGHER FOR WEDNESDAY AS A
LONG ONSHORE FETCH ALLOWS FOR HIGHER SEAS TO BUILD.

THE ATLANTIC RIDGE WILL DROP SOUTH AND ORIENT ITSELF EAST-WEST
ACROSS THE MARINE AREA LATE IN THE WEEK. WINDS AND SEAS WILL HOLD
UNDER ANY SCA CONDITIONS DURING THIS TIME.

&&

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

&&

RJB/33

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