FXUS62 KILM 201151 AFDILM AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE WILMINGTON NC 751 AM EDT SAT SEP 20 2008 .SYNOPSIS... HIGH PRESSURE ACROSS THE NORTHEAST WILL MOVE RAPIDLY EAST AND OUT TO SEA THROUGH TONIGHT...AS LOW PRESSURE WELL OFFSHORE DEVELOPS ALONG A STALLED FRONT. NEW HIGH PRESSURE WILL BUILD IN NEXT WEEK ACROSS SOUTHEASTERN CANADA...MAINTAINING COOL NORTHEAST WINDS OVER THE CAROLINAS. TEMPERATURES SHOULD REMAIN BELOW NORMAL WITH LITTLE IF ANY RAINFALL. CLOUDS MAY INCREASE SUNDAY INTO MONDAY AS AN UPPER LEVEL TROUGH MOVES ACROSS. LOW PRESSURE SHOULD DEVELOP OFF THE CAROLINA COAST LATE IN THE COMING WEEK. && .NEAR TERM /THROUGH TONIGHT/... AS OF 3 AM SATURDAY...LATEST NAM/GFS/CANADIAN ALL IN GOOD AGREEMENT WITH RESPECT TO LOCATION OF SFC LOW OFF THE CAROLINA COAST. GUIDANCE HAD BEEN BRINGING THE LOW AND ITS ASSOCIATED MOISTURE CLOSER TO THE COAST BUT NOW SHOW THE LOW A LITTLE FARTHER N AND FARTHER E. STRONGER SFC RIDGING AND DRIER MID LEVEL AIR...SEEN IN LATEST WV IMAGERY...WILL PREVENT ANY SIGNIFICANT PRECIP FROM FALLING BUT SOUNDINGS DO SUGGEST A LOW LEVEL STRATUS DECK BETWEEN 3K AND 4K FT. SHOULD SEE A LITTLE MORE CLOUD TODAY THAN YESTERDAY THOUGH WINDS WILL NOT BE A STRONG. HIGHS WILL END UP A COUPLE DEGREE COOLER THAN YESTERDAY...UPPER 70S IN MOST PLACES. CLOUD DECK WILL LINGER INTO THE FIRST PART OF THE OVERNIGHT BUT WILL DISSIPATE AS MID LEVELS DRY OUT AND SFC LOW OFF HATTERAS CONTINUES MOVING NE. 850 TEMPS WOULD SUGGEST LOWS IN THE LOW TO MID 50S BUT LINGERING CLOUD AND WELL MIXED BOUNDARY LAYER WILL KEEP TEMPS A FEW DEGREES WARMER...LOW 60S AT THE COAST WITH UPPER 50S INLAND. TYPICALLY COLD SPOTS COULD DIP INTO THE MID 50S. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... AS OF 3 AM SATURDAY...A STALLED FRONT WILL REMAIN OVER THE WESTERN ATLANTIC A COUPLE HUNDRED MILES OFF THE CAROLINA COAST. 1030+ MB HIGH PRESSURE OVER SOUTHEASTERN CANADA WILL RIDGE DOWN THE EAST COAST AND MAINTAIN A FLOW OF COOL DRY AIR OVER THE REGION. THE SURFACE PATTERN BY ITSELF IS FAIRLY STRAIGHTFORWARD...HOWEVER THE UPPER AIR PATTERN IS CONSIDERABLY MORE COMPLEX: A SPLIT FLOW REGIME SUNDAY MORNING OVER THE MISSISSIPPI VALLEY WILL SHIFT TO THE EAST COAST BY MONDAY. STRONG RIDGING OVER THE GREAT LAKES REPRESENTS THE NORTHERN BRANCH OF THE JET...WHILE WEAK TROUGHING OVER THE CAROLINAS REPRESENTS THE SOUTHERN BRANCH. THERE WILL BE ENOUGH DEEP-LAYER MOISTURE IN THIS TROUGH LATE SUNDAY INTO MONDAY THAT LIGHT SHOWERS CANNOT BE RULED OUT...ALTHOUGH THE LACK OF ANY ASSOCIATED SURFACE FEATURES WILL LIMIT LOW-LEVEL MOISTURE CONVERGENCE. WITH THE NORTHEAST FLOW STRAIGHT FROM EASTERN CANADA...TEMPERATURES WILL REMAIN A DEGREE OR TWO BELOW NORMAL THROUGH THE PERIOD. && .LONG TERM /TUESDAY THROUGH FRIDAY/... AS OF 3 AM SATURDAY...CONSIDERABLE MODEL DISAGREEMENT LEAVES THE LONG TERM PORTION OF THE FORECAST WITH BELOW NORMAL CONFIDENCE. THE DIFFERENCES HINGE ON WHAT HAPPENS WITH A STRONG PACKET OF NORTHERN STREAM VORTICITY THAT DIVES INTO NEW ENGLAND ON TUESDAY. THE 12Z (SEP 19TH) ECMWF BRINGS THIS ENERGY SOUTH AND CUTS OFF A RATHER IMPRESSIVE UPPER LOW ALONG THE CAROLINA COAST TOWARD THE END OF THE WEEK. THE 00Z GFS IS MORE RELUCTANT TO CUT OFF THE LOW...BUT MAINTAINS A VERY POSITIVELY TILTED TROUGH A FEW HUNDRED MILES OFF THE COAST WEDNESDAY THROUGH THE WEEKEND. SINCE THERE WILL STILL BE A BAROCLINIC ZONE (THE STALLED FRONT) OFFSHORE THROUGHOUT THE PERIOD SURFACE LOW PRESSURE IS GOING TO DEVELOP. THE QUESTIONS ARE: HOW CLOSE TO THE COAST...HOW STRONG...AND WHETHER ANY SUBTROPICAL/ HYBRID CHARACTERISTICS ARE INVOLVED. 700 MB FORECAST RELATIVE HUMIDITY FIELDS FROM BOTH THE GFS AND ECMWF SHOW PLENTY OF DRY AIR SHOULD BE IN PLACE THROUGH THE PERIOD. IN FACT THE ONLY MOISTURE THAT APPEARS AT 700 MB IS DIRECTLY ASSOCIATED WITH THE SURFACE LOW. THIS DRY AIR MAY TEND TO REDUCE THE SIZE OF ANY PRECIPITATION SHIELD...ESPECIALLY NORTH AND WEST OF THE EVENTUAL TRACK OF THE SURFACE LOW. WILL MAKE FEW CHANGES TO THE PRIOR PRECIPITATION FORECAST WHICH MAINTAINS "SILENT" 20% POPS FOR MOST LOCATIONS...WITH 30% POPS IN THE CAPE FEAR REGION THURSDAY THROUGH FRIDAY. THIS IS ONLY 10% OR SO ABOVE CLIMATOLOGY FOR 12-HOUR POPS AND INDICATES OUR LOW CONFIDENCE. TEMPERATURES SHOULD CONTINUE TO RUN ON THE COOL SIDE OF CLIMATOLOGY...GENERALLY UPPER 70S TO AROUND 80 FOR DAYTIME HIGHS...AND UPPER 50S TO LOWER 60S FOR OVERNIGHT LOWS. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... SOME STRATUS THIS MORNING AT KMYR AND KLBT SHOULD MIX OUT WITHIN THE NEXT FEW HOURS. SURFACE LOW OFF THE COAST WILL KEEP A PERSISTENT NORTH-NORTHEAST WIND THROUGH TONIGHT AT ALL TERMINALS. WHILE A FEW GUSTS TO 15 KTS ARE POSSIBLE THROUGH MIDDAY...OVERALL THE WINDS WON'T BE AS GUSTY AS YESTERDAY. A DECK OF STRATA CU BETWEEN 2K AND 3K WILL DEVELOP AT ALL TERMINALS BETWEEN 16Z AND 18Z. ASIDE FROM KFLO...THIS DECK WILL REMAIN WITH US THROUGH 06Z TONIGHT BEFORE DRIER AIR MOVES IN. WITH A SIMILAR WIND DIRECTION TO LAST NIGHT MORE STRATUS MAY REDEVELOP AT KMYR EARLY SUNDAY MORNING. OUTLOOK THROUGH MONDAY...MODELS ARE TRENDING MORE TO AN OFFSHORE LOW SCENARIO WHERE THE MOISTURE IS FAR ENOUGH OFFSHORE TO KEEP TERMINALS VFR THROUGH SUNDAY. SOME RETURN FLOW IS POSSIBLE LATE SUNDAY INTO TUESDAY WITH POSSIBLE PRECIP AND MVFR CEILINGS ALONG THE COAST. && .MARINE... NEAR TERM /THROUGH TONIGHT/... AS OF 3 AM SATURDAY...STRONG NE FLOW WILL CONTINUE TODAY THOUGH SPEEDS WILL GRADUALLY BEGIN TO DECREASE TODAY AS GRADIENT RELAXES IN RESPONSE TO SFC LOW MOVING AWAY FROM THE WATERS. DESPITE WIND SPEEDS DECREASING THIS MORNING ALL HEADLINES WILL NEED TO CONTINUE OVERNIGHT AS PROLONGED PERIOD OF STRONG NE FLOW WILL KEEP SEAS AWAY FROM SHORE ABOVE 6 FT FOR AN EXTENDED PERIOD OF TIME. SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... AS OF 3 AM SATURDAY...STRONG HIGH PRESSURE OVER SOUTHEASTERN CANADA WILL RIDGE SOUTH INTO THE AREA SUNDAY INTO MONDAY...WHILE A STALLED FRONT REMAINS A FEW HUNDRED MILES OFF THE SOUTHEAST COAST. MUCH OF THIS FORECAST KEYS ON HOW LOW SURFACE PRESSURES FALL OFF THE GEORGIA AND NORTH FLORIDA COASTLINE MONDAY IN ADVANCE OF AN APPROACHING UPPER TROUGH. STRONGER PRESSURE FALLS THERE WILL EQUATE TO A STRONGER PRESSURE GRADIENT HERE AND WORSE WIND/SEA CONDITIONS. OUR FORECAST AT THIS POINT CALLS FOR SMALL CRAFT ADVISORY CONDITIONS REDEVELOPING BY LATE MONDAY AFTERNOON OR MONDAY EVENING. LONG TERM /TUESDAY THROUGH WEDNESDAY/... AS OF 3 AM SATURDAY...THE VARIOUS LONG RANGE FORECAST MODELS ARE AT ODDS WITH ONE ANOTHER FOR THE MIDDLE OF THE COMING WEEK. DIFFERENCES IN THE POSITION AND STRENGTH OF SURFACE LOW PRESSURE OFF THE SOUTHEAST COAST COULD MEAN SUBSTANTIAL DIFFERENCES IN WIND...SEA...AND WEATHER CONDITIONS EXPERIENCED LOCALLY. IT APPEARS RATHER STRONG NORTHEAST WINDS OF AT LEAST 20-25 KT WILL CONTINUE ACROSS THE AREA REGARDLESS. THE LONG SUSTAINED PERIOD OF NORTHEAST WINDS WILL BUILD SEAS TO 6-8 FT ALONG EXPOSED PORTIONS OF THE COASTAL WATERS...WITH EVEN HIGHER SEAS EXPECTED FARTHER OFFSHORE. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...NONE. NC...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 8 AM EDT SUNDAY FOR AMZ250-252-254-256. && $$ SYNOPSIS...TRA NEAR TERM...III SHORT TERM...TRA LONG TERM...TRA AVIATION...HEDEN