Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 290840 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 340 AM EST MON DEC 29 2014 .SYNOPSIS...A COLD FRONT WILL MOVE SOUTH THROUGH THE REGION THIS MORNING. A WAVE OF LOW PRESSURE WILL DEVELOP ALONG THE FRONT AND MOVE ACROSS THE REGION THIS AFTERNOON. THE FRONT WILL THEN MOVE OFFSHORE TONIGHT BEFORE HIGH PRESSURE EXTENDS INTO THE REGION ON TUESDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 320 AM MONDAY... RAIN IS SPREADING INTO THE AREA FROM THE SOUTHWEST AS THE EFFECTS OF A DEAMPLIFYING SHORTWAVE LIFTING FROM TN/AL/MS ALONG THE APPALACHIANS ARE BEGINNING TO BE REALIZED. A COLD FRONT STILL LIES JUST TO OUR NORTH IN VIRGINIA...WHILE A PREFRONTAL TROUGH IS STARTING TO MAKE ITS WAY INTO CENTRAL NC. AS THE ANAFRONTAL RAIN EXPANDS EASTWARD...MODELS SHOW A ZONE OF STRONG FGEN...SLOPED UPWARD TOWARD THE NORTHWEST...DEVELOPING BETWEEN 900-750MB BETWEEN 09Z AND 15Z. WITH A STRONG 160KT UPPER JET TO OUR NORTH...MODELS ALSO SHOW THE POTENTIAL FOR SOME CONDITIONAL SYMMETRIC INSTABILITY AND A SOUTHWEST TO NORTHEAST ORIENTED BAND OF HEAVIER RAIN FROM THE SOUTHERN PIEDMONT THROUGH THE CENTRAL PIEDMONT. SREF PROBABILITIES OF CSI INGREDIENTS EXCEED 80 PERCENT AND SOME CONVECTION ALLOWING MODELS ALSO HINT AT BANDING. IF THIS COMES TO PASS...QPF AMOUNTS AREN`T TREMENDOUSLY HIGH BUT COULD RESULT IN SOME LOCATIONS RECEIVING OVER ONE HALF INCH...WHICH IS HIGHER THAN THE GENERAL THIRD OF AN INCH THAN MOST MODELS PAINT ACROSS THE AREA TODAY. CATEGORICAL RAIN WILL MOVE OFF TO THE NORTHEAST THIS AFTERNOON AS THE SHORTWAVE/JET SHIFT SHIFT NORTHEAST AWAY FROM THE AREA AND A PUNCH OF DRIER MID-LEVEL AIR SURGES IN FROM THE SOUTHWEST. SATURATED LOW-LEVELS AND SOME WEAK ISENTROPIC UPGLIDE AROUND 290K SHOULD MAINTAIN SOME PERIODS OF LIGHT RAIN AND DRIZZLE THROUGH THE EVENING AND INTO THE OVERNIGHT PERIOD. ANOTHER LOW AMPLITUDE SHORTWAVE CURRENTLY OVER CROSSING THE INNER-MOUNTAIN WEST WILL RACE EAST TODAY AND GIVE ANOTHER GLANCING BLOW OF WEAK HEIGHT FALLS LATE TONIGHT...WHICH SHOULD RESULT IN SOME UPWARD MOISTENING AND RENEWED MEASURABLE RAIN OVER MAINLY THE NORTHWEST PIEDMONT. WILL KEEP POPS IN THE CHANCE RANGE OVERNIGHT GIVEN THAN THE FORCING IS QUICK AND MAINLY NORTHWEST OF THE CWA. HIGHS FOR TODAY ARE PRETTY MUCH OCCURRING THE MORNING...AS THE LOW TO MID 50 OBSERVED TEMPS SHOULD NOT MOVE VERY MUCH WITH THE IMPENDING RAINFALL AND WILL LIKELY FALL THIS AFTERNOON AS BETTER POST-FRONTAL COLD ADVECTION FINALLY ARRIVES FROM THE NORTH. TEMPS WILL THEN FALL INTO THE 40S THIS EVENING AND END UP IN THE UPPER 30S NORTH TO LOWER 40S SOUTH BY TUESDAY MORNING. && .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... AS OF 335 AM MONDAY... A STRONG MID-UPPER JET OVER THE SIERRAS THIS MORNING (OBSERVED AT 100 KTS AT 500 MB OVER THE PAC NW PER 00Z RAOB DATA) WILL CAUSE A LEAD SHORTWAVE TROUGH TO DEAMPLIFY/SHEAR EAST FROM THE CENTRAL PLAINS TO THE MIDDLE ATLANTIC COAST TUE THROUGH TUE NIGHT. THE PASSAGE OF THIS FEATURE AND ASSOCIATED GLANCING BLOW OF THE SOUTHERN PERIPHERY OF HEIGHT FALLS ALOFT (AND ASSOCIATED FORCING FOR ASCENT) WILL PRODUCED RENEWED LIFT ATOP A STILL-SATURATED LAYER IN ROUGHLY THE LOWEST SEVERAL THOUSAND FT --DEEPEST NEAR THE VA BORDER/IN CLOSEST PROXIMITY TO THE STRONGEST HEIGHT FALLS ALOFT OVER THE MIDDLE ATLANTIC-- SUCH THAT PERIODS OF DRIZZLE THROUGH EARLY TUE MAY BRIEFLY BECOME MEASURABLE LIGHT RAIN DURING THE MORNING TO MIDDAY HOURS. BEHIND THE PASSING SHORTWAVE TROUGH...CAA IN THE 925-850 MB LAYER...INCLUDING A WNW DOWNSLOPE TRAJECTORY AT 850 MB...WILL PROMOTE A DISSIPATION OF THE CAD INVERSION - BOTH MECHANISMS THAT SHOULD SERVE TO SCOUR THE CAD REGIME AND PROMOTE GENERALLY NORTH TO SOUTH CLEARING LATE AFTERNOON OVER THE NORTHERN PIEDMONT AND DURING THE EVENING ELSEWHERE. TEMPERATURES WILL CONSEQUENTLY REMAIN COOL OWING TO THE CAD INFLUENCE THROUGH MOST OF THE DAY. BY THE TIME CLEARING MATERIALIZES...TEMPS WILL LIKELY NOT HAVE TIME TO RESPOND TO THE SETTING AFTERNOON SUN...PARTICULARLY SINCE LOW LEVEL CAA WILL HAVE REDUCED ALREADY MEAGER WARMING POTENTIAL - HIGHS MAINLY IN THE LOWER TO MIDDLE 40S. STRONG...ARCTIC HIGH PRESSURE WILL OTHERWISE SPRAWL EAST ACROSS THE MIDDLE ATLANTIC AND SOUTHEAST STATES...WITH ASSOCIATED MUCH COLDER TEMPERATURES BY WEDNESDAY MORNING - IN THE MIDDLE TO UPPER 20S. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... AS OF 305 PM SUNDAY... WEDNESDAY AND THURSDAY...S/W IN THE NORTHERN STREAM WILL CROSS THE GREAT LAKES AND NORTHERN MID ATLANTIC LATE WEDNESDAY AND WEDNESDAY NIGHT. THIS FEATURE WILL PROPEL A DRY SURFACE COLD FRONT ACROSS CENTRAL NC WEDNESDAY NIGHT. THIS PASSAGE WILL OCCUR PRETTY UNEVENTFUL WITH JUST A DECK OF MID-HIGH LEVEL CLOUDS EXPECTED. THUS PARTIAL CLEARING EARLY WEDNESDAY EXPECTED TO BE SHORT-LIVED AS APPROACH OF S/W AND ASSOCIATED MID-HIGH LEVEL CLOUDINESS WILL CROSS CENTRAL NC LATE WEDNESDAY AND WEDNESDAY EVENING. NLY WINDS BEHIND THIS PASSAGE WILL REINFORCE THE COOL AIR MASS ALREADY IN PLACE BUT ADVECT A BATCH OF DRIER AIR AS DEWPOINTS WILL LOWER INTO THE TEENS/LOWER 20S. LOW LEVEL THICKNESSES BY EARLY THURSDAY ARE IN THE 1290-1295M RANGE. HISTORICALLY...THIS SUPPORTS MIN TEMPS IN THE MID-UPPER 20S. THE COOL DRY AIR MASS FOR THURSDAY WILL SUPPORT MOSTLY SUNNY SKIES FOR THE FIRST DAY OF 2015 WITH AFTERNOON TEMPS WILL AVERAGE 5-6 DEGREES BELOW NORMAL. HIGH TEMPS THURSDAY MID 40S NORTH TO NEAR 50 SOUTH. FRIDAY THROUGH SUNDAY... WEATHER PATTERN OVER CENTRAL NC BECOMES UNSETTLED AS AN UPPER LEVEL LOW WILL SLUGGISHLY LIFT EAST-NE ACROSS THE MID MS AND OH VALLEYS. MODEL GUIDANCE OFFER DIFFERING SOLUTIONS WITH TEH GFS MORE PROGRESSIVE THAN THE ECMWF. USUALLY...BETTER TO LEAN TOWARD THE SLOWER SOLUTION THIS FAR OUT. EXPECT AIR MASS TO MODIFY FRIDAY AS AS FLOW ASSUMES A RETURN FLOW OFF THE ATLANTIC. PRECIP CHANCES WILL LIKELY BEGIN TO INCREASE LATE FRIDAY...AND MORE SO SATURDAY PERSISTING INTO SUNDAY. PRESENCE OF A MODIFIED ARCTIC AIR MASS IN PLACE OVER CENTRAL NC PRIOR TO PRECIP ARRIVAL SIGNAL POTENTIAL FOR HYBRID OR INSITU COLD AIR DAMMING SATURDAY INTO THIS SUNDAY. IF THIS MATERIALIZES...THIS WOULD CREATE A WIDE VARIANCE IN TEMPS ACROSS CENTRAL NC (CHILLY NW TO RELATIVELY MILD SE). FOR NOW...WILL KEEP MAX TEMPS CLOSE TO AVERAGE ACROSS THE PIEDMONT...AND A CATEGORY OR TWO ABOVE NORMAL IN THE SE. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... AS OF 100 AM MONDAY... SOME TERMINALS ARE SEEING DENSE FOG EARLY THIS MORNING WITH LIGHT WINDS UNDER SOME BREAKS IN HIGH CLOUDS. EXPECT THE FOG TO PERSIST FROM KRDU TO KFAY AND KRWI FOR A FEW MORE HOURS BEFORE A COLD FRONT CURRENTLY MAKING ITS WAY EAST OF THE APPALACHIANS PUSHES SOUTHEAST THROUGH CENTRAL NC IN THE 08-10Z TIME FRAME. AT THE SAME TIME...HOWEVER...RAIN WILL SPREAD IN FROM THE SOUTHWEST AND CAUSE CEILINGS TO LOWER TO IFR MOST EVERYWHERE BY 09-12Z. RAIN WILL CONTINUE FOR MOST OF THE DAY...EVENTUALLY ENDING FROM WEST TO EAST LATER THIS AFTERNOON...BUT THE LOW CLOUDS WILL BE SET IN WITH A NORTHEASTERLY WIND AND NO WAY FOR THE MOISTURE TO SCOUR. THUS...THERE IS HIGH CONFIDENCE THAT AT LEAST IFR CONDITIONS WILL PERSIST THROUGH THE END OF THE TAF PERIOD AND INTO TUESDAY. LOOKING AHEAD...LINGERING LOW CLOUDS ALONG WITH SOME LIGHT RAIN AND DRIZZLE WILL CONTINUE INTO TUESDAY AND WILL BEGIN TO IMPROVE LATE TUESDAY AND WEDNESDAY. FAIR WEATHER AND NO ADVERSE AVIATION CONDITIONS ARE EXPECTED WEDNESDAY THROUGH EARLY FRIDAY BEFORE THE NEXT STORM SYSTEM APPROACHES LATE FRIDAY. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...BLAES NEAR TERM...BLS SHORT TERM....MWS LONG TERM...WSS AVIATION...BLS/CBL

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