Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 190244 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1044 PM EDT SAT APR 18 2015 .SYNOPSIS...HIGH PRESSURE WILL RIDGE ACROSS THE MIDDLE ATLANTIC STATES...WHILE A NEARLY STATIONARY FRONT OVER NORTH-CENTRAL NC REMAINS SO...THROUGH SUN. THE FRONT WILL RETREAT NORTH THROUGH THE MIDDLE ATLANTIC STATES...IN ADVANCE OF LOW PRESSURE FORECAST TO TRACK THROUGH THE UPPER MIDWEST AND GREAT LAKES...SUN NIGHT AND MON. A TRAILING COLD FRONT WILL SWEEP ACROSS OUR REGION LATE MON AND MON NIGHT. && .NEAR TERM /THROUGH TONIGHT/... AS OF 1040 PM SATURDAY... BENEATH MID-UPPER LEVEL SHORTWAVE RIDGING...A W-E ORIENTED NEARLY STATIONARY FRONT - ONE THAT SEPARATED SURFACE DEWPOINTS IN THE MIDDLE TO UPPER 60S OVER MOST OF THE SE US FROM 40S AND 50S OVER VA - WAS ANALYZED ACROSS CENTRAL NC AT 02Z...WHILE A STRONGER COLD FRONT WAS ANALYZED OVER SOUTHERN PA. NUMEROUS OUTFLOW BOUNDARIES FROM CONVECTION THAT INITIATED OVER THE FRONT OVER CENTRAL NC SAT AFTERNOON REMAIN ACROSS OUR AREA...THE MOST NOTABLE OF WHICH HAS MAINTAINED SCATTERED...WESTWARD-PROPAGATING SHOWERS AND STORMS FROM NEAR KRWI TO NEAR FIVE COUNTY STADIUM. THE MAIN THREAT FROM THIS CONVECTION WILL BE HEAVY RAIN OWING TO THE RELATIVELY SLOW MOVEMENT/OUTFLOW PROPAGATION AMIDST WEAK INSTABILITY SAMPLED BY THE 00Z KGSO AND KMHX RAOBS...AND SUPPORTED BY MORE RECENT SPC MESO- ANALYSIS DATA. A SEPARATE AREA OF WEAKER SHOWERS HAVE BEEN MAINTAINED OVER THE SOUTHERN HALF OF THE FORECAST AREA...IN ASSOCIATION WITH A NEARLY STATIONARY 850-700 MB TROUGH/SHEAR AXIS EVIDENT IN BOTH THE 12Z/18TH AND 00Z/19TH RAOB DATA. NWP GUIDANCE SUGGESTS THIS TROUGH AXIS WILL RETREAT NORTH INTO THE WESTERN CAROLINAS AND BECOME ORIENTED INCREASINGLY NW-SE WITH TIME. WHILE SCATTERED SHOWERS OR AN ISOLATED THUNDERSTORM OR TWO WILL CONTINUE TO PERCOLATE INVOF THE E-W SFC BOUNDARY...GUIDANCE SUGGESTS IT WILL BE THE 850-700 MB TROUGH AXIS - IN A ZONE OF INCREASINGLY CONFLUENT FLOW IN THAT LAYER - THAT WILL FOCUS MORE NUMEROUS SHOWERS OVERNIGHT INTO EARLY SUN...WHICH WILL INCLUDE THE SOUTHERN AND WESTERN PIEDMONT AND SANDHILLS OF NC. LOWS MOSTLY IN THE 60S...THOUGH SOME UPPER 50S WILL BE POSSIBLE OVER THE NORTHERN COASTAL PLAIN AND FAR NE PIEDMONT...WHERE SOME OF THE DRIER LOW LEVEL AIR OVER VA WILL LIKELY FILTER INTO OVERNIGHT. && .SHORT TERM /SUNDAY AND SUNDAY NIGHT/... AS OF 330 PM SATURDAY... SUNDAY...AN INCREASINGLY MOIST AIR MASS (PRECIP WATER VALUES 1.5-1.7 INCHES PROBABLE BY AFTERNOON) COUPLED WITH DECENT CONVERGENCE IN THE LOWER LEVELS AND INCREASING DIVERGENCE ALOFT (DUE TO THE NOSE OF 300MB JET CROSSING THE NORTHERN GULF) WILL CAUSE SHOWERS TO OVERSPREAD CENTRAL NC FROM THE SW. LOW LEVEL WIND WILL HAVE AN UPSLOPE COMPONENT OVER THE WESTERN PIEDMONT...SUGGESTING A SUSTAINED PERIOD OF SHOWERS WITH MODERATE TO BRIEFLY HEAVY RAINFALL RATES LIKELY. WHILE NOT ANTICIPATING WIDESPREAD FLOODING ISSUES AT THIS TIME...PROLONGED MODERATE RAINFALL IN THE URBANIZED AREAS OF THE TRIAD COULD LEAD TO STREET FLOODING ALONG WITH SIGNIFICANT RISES ALONG AREAS CREEKS AND STREAMS. WILL REFRAIN FROM ISSUING A FLASH FLOOD WATCH AT THIS TIME AS BEST LIFT PROJECTED TO OCCUR OVER THE FOOTHILLS/HIGHER TERRAIN OF WESTERN NC. LATEST MODEL GUIDANCE HAS TRENDED MORE UNSTABLE LATE SUNDAY-SUNDAY EVENING ACROSS THE SOUTHERN HALF OF CENTRAL NC WITH BULK SHEAR INCREASING 35-40KTS AND MLCAPE 500-1000J/KG(NAM). WHILE CANNOT RULE OUT THE POTENTIAL FOR AN ISOLATED STRONG/SEVERE STORM SUNDAY NIGHT OVER THE SOUTHERN COUNTIES...FEEL THAT SEVERE POTENTIAL BETTER FARTHER SOUTH ACROSS GA/SOUTHERN SC WHERE ATMOSPHERE WILL HAVE A BETTER CHANCE TO DESTABILIZE. WIDESPREAD SHOWERS AND OVERCAST SKIES SHOULD LIMIT TEMP RECOVERY SUNDAY. HIGH TEMPS SHOULD VARY FROM THE MID 60S-LOWER 70S FAR SOUTH- SOUTHEAST OTHERWISE...SHOULD SEE SHOWER DECREASING IN COVERAGE AND INTENSITY SUNDAY NIGHT FROM THER SW AS MID LEVEL S/W LIFTS NEWD INTO THE NORTHERN MID ATLANTIC. THE APPROACH OF YET ANOTHER S/W (VERY POTENT) TOWARD DAYBREAK MONDAY WILL CONTINUE A THREAT SHOWERS OVERNIGHT. MIN TEMPS 60-65. && .LONG TERM /MONDAY THROUGH SATURDAY/... AS OF 255 PM SATURDAY... MON/MON NIGHT: WILL HOLD ONTO JUST A FEW HOURS OF SHOWERS IN THE FAR NE EARLY MON MORNING... CONSIDERING THAT THE MODELS HAVE BEEN TRENDING A BIT FASTER TO MOVE PRECIP OUT OF CENTRAL NC... AND WILL FOLLOW WITH A PERIOD OF NO POPS FOR MID TO LATE MORNING... UNTIL THE ONSET OF CONVECTION. STILL LOOKS LIKE A GOOD PROBABILITY OF STRONG TO SEVERE STORMS MON AFTERNOON INTO EARLY EVENING WITH A FEW SUPERCELLS POSSIBLE. WE`LL BE FULLY IN THE WARM SECTOR WITH WARM MOIST AIR NEAR THE GROUND AND A GOOD PUNCH OF MID LEVEL DRY AIR COMING IN FROM THE SW. LATEST MODEL RUNS ARE SHOWING A LITTLE BETTER INSTABILITY THAN YESTERDAY`S RUNS FOR MON AFTERNOON... WITH MUCAPE OF 1000-2000 J/KG ON THE GFS AND 1500-3000 J/KG ON THE NAM... AND SREF MEAN MLCAPE VALUES PEAKING AT 1200-1500 J/KG... ALTHOUGH THE DEEP LAYER SHEAR STILL LOOKS MARGINAL AT AROUND 30 KTS. NAM/GFS BUFR SOUNDINGS SHOW A WEAK CAP LOCATED BETWEEN 1 AND 2 KM AGL IN THE MORNING... BREAKING EARLY IN THE AFTERNOON... AND ONCE IT DOES... CONVECTION SHOULD DEVELOP AND BECOME VIGOROUS QUICKLY WITH BROAD AND DEEP CAPE IN THE MID-UPPER LEVELS. DYNAMIC FORCING FOR ASCENT WILL BE PRESENT AS WELL WITH STRONG UPPER DIVERGENCE IN THE LEFT EXIT REGION OF A 140 KT SUBTROPICAL JET... AND A SHOT OF MID LEVEL DPVA AS THE TROUGH AXIS ROTATES UP THROUGH THE AREA. WILL MAINTAIN A MENTION IN THE HAZARDOUS WEATHER OUTLOOK OF DAMAGING WIND AS THE PRIMARY THREAT GIVEN THE ABUNDANCE OF DRY AIR 700-300 MB... HOWEVER THE EXPECTED CAPE IN THE -10C TO -30C LAYER APPEARS SUFFICIENT TO SUPPORT LARGE HAIL IF WE CAN GET ENOUGH MOISTURE ALOFT. THICKNESSES ARE PROJECTED TO START THE DAY AROUND 20 M ABOVE NORMAL... SO HIGHS IN THE UPPER 70S TO MID 80S (NW TO SE) LOOK ATTAINABLE WITHIN GOOD WARM AIR ADVECTION. WILL TREND OUT SHOWERS/STORMS IN THE MID-LATE EVENING FROM WEST TO EAST... WITH COLD FRONTAL PASSAGE EXPECTED LATE OVERNIGHT. LOWS FROM THE LOWER 50S NW TO AROUND 60 SE. TUE-SAT: THE COLD FRONT WILL HAVE PUSHED OFF THE COAST BY DAYBREAK TUE... AS MILD HIGH PRESSURE BUILDS IN FROM THE WEST. A LARGE VORTEX IS EXPECTED TO SIT AND SPIN OVER THE NRN GREAT LAKES THROUGH MIDWEEK... BEFORE WOBBLING EASTWARD TO NEW ENGLAND BY LATE SAT... ALL THE WHILE ENCOMPASSING A LARGE CHUNK OF CENTRAL AND EASTERN NOAM INCLUDING THE MIDATLANTIC REGION. AS THE MODIFIED SURFACE HIGH BUILDS IN... EXPECT TEMPS TO BE A BIT BELOW NORMAL TUE/WED... BUT READINGS WILL REBOUND ONCE THE SURFACE HIGH DRIFTS TO OUR SOUTHEAST AND OFFSHORE LATE WED... SO EXPECT TEMPS BACK UP NEAR NORMAL THU. ONE PARTICULARLY POTENT WAVE ROTATING THROUGH THE BROAD BASE OF THE LOW LATE IN THE WEEK IS LINKED TO A SURFACE COLD FRONT WHICH IS EXPECTED TO PUSH SOUTHEAST THROUGH NC LATE THU OR THU EVENING. IMPROVING MOISTURE STREAMING INTO NC AHEAD OF THIS FRONT SHOULD YIELD A FEW SHOWERS MAINLY WED NIGHT AND THU... HOWEVER THE LOW LEVEL FLOW WILL REMAIN LIGHT WITH MINISCULE TAPPING OF ATLANTIC OR GULF MOISTURE... AND LITTLE IN THE WAY OF FORCING FOR ASCENT GIVEN THE FAST WNW GENTLY CYCLONIC FLOW ALOFT... SO EXPECT POPS TO HOLD UNDER CLIMATOLOGY. FRONTAL PASSAGE LATE THU WILL INTRODUCE A COOLER AIR MASS IN THE FORM OF A SPRAWLING CANADIAN HIGH EXPANDING SSE FROM CENTRAL CANADA FOR FRI/SAT... SO WILL TREND BACK TO DRY WEATHER WITH BELOW NORMAL TEMPS. -GIH && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... AS OF 806 PM SATURDAY... 24 HOUR PERIOD: SCATTERED SHOWERS WITH A FEW THUNDERSTORMS MAY PRODUCE LOCALIZED AREAS OF SUB-VFR CONDITIONS OVER THE NEXT FEW HOURS BEFORE THIS ACTIVITY BEGINS TO SUBSIDE. NONE ARE CURRENTLY NEAR ANY OF THE AREA TERMINALS... AND VFR CONDITIONS SHOULD PREVAIL FOR THE NEXT 6-12 HOURS. CIGS WILL DROP TO MVFR... THEN LIKELY IFR... AT KINT AND KGSO BETWEEN 06Z AND 12Z... AND PROGRESS EASTWARD THROUGH THE DAY AS SHOWERS AND AREAS OF HEAVY RAIN OVERSPREAD THE AREA AHEAD OF A WARM FRONT THAT WILL CROSS THE REGION LATE SUNDAY. THE RAIN WILL DIMINISH FROM THE SW AFTER 00Z MONDAY WITH SHOWERS VERY SPOTTY AFTER 06Z. RESIDUAL MOISTURE WILL LIKELY CAUSE WIDESPREAD IFR/MVFR CEILINGS UNTIL EARLY MONDAY MORNING. A STRONG LOW PRESSURE SYSTEM WILL TRAVERSE CLOSE TO CENTRAL NC MONDAY. THIS SYSTEM INTERACTING WITH AVAILABLE MOISTURE AND INSTABILITY WILL BRING A GOOD CHANCE FOR SCATTERED STRONG TO SEVERE THUNDERSTORMS MONDAY AFTERNOON-MONDAY EVENING. IMPROVING AVIATION CONDITIONS WITH WIDESPREAD VFR CONDITIONS ANTICIPATED FOR TUESDAY THROUGH THURSDAY. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...RAH NEAR TERM...RAH SHORT TERM...WSS LONG TERM...HARTFIELD AVIATION...RAH/WSS

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