WFO RAH Logo



FXUS62 KRAH 151905
 AFDRAH
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE RALEIGH NC
 305 PM EDT FRI APR 15 2011
 
 .SYNOPSIS...
 HIGH PRESSURE WILL DRIFT OFFSHORE TONIGHT. A POWERFUL STORM SYSTEM 
 WILL CROSS OUR REGION ON SATURDAY...FOLLOWED BY HIGH PRESSURE FOR 
 SATURDAY NIGHT AND SUNDAY.
 
 &&
 
 .NEAR TERM /THROUGH SATURDAY NIGHT/...
 AS OF 200 PM FRIDAY...
 
 ...SEVERE WEATHER OUTBREAK WITH A MODERATE RISK FOR SEVERE STORMS
 SATURDAY...
 
 1730Z SPC DAY 2 OUTLOOK INCREASED SEVERE WEATHER THREAT TO MODERATE
 FOR SATURDAY. FROM THE SPC DISCUSSION...WIDESPREAD DAMAGING WINDS AND
 A FEW STRONG TORNADOES WILL BE POSSIBLE ACROSS THE CENTRAL AND ERN
 CAROLINAS ON SATURDAY. SATURDAY IS SETTING UP TO BE A TEXT BOOK
 LARGE SCALE SEVERE WEATHER OUTBREAK WITH THE POTENTIAL FOR
 WIDESPREAD DAMAGE.
 
 TONIGHT: CURRENT BAND OF SHOWERS AND STORMS OVER ERN TENN SHOULD
 SLOW ITS FORWARD PROGRESSION AND HANG UP ALONG THE MOUNTAINS
 SINCE IT HAS RACED OUT AHEAD OF THE PRIMARY FORCING AND UPPER LOW.
 FOR TONIGHT INCREASING SLW FLOW WITH THE DEVELOP OF THE LOW LEVEL
 JET OVERNIGHT WILL START TO SURGE MOISTURE NORTH INTO THE REGION
 AFTER MIDNIGHT. WITH THE INCREASE IN MOISTURE AND LOW LEVEL
 CONFLUENCE SOME RAPIDLY MOVING SHOWERS MOVING SOUTH TO NORTH COULD
 DEVELOP FROM 09-15Z AND THIS IS SUPPORTING ON A NUMBER OF THE WRF
 RUNS. A CHANCE FOR SHOWERS AFTER MIDNIGHT WEST OF HIGHWAY
 ONE...WITH PERHAPS A TENTH INCH WEST OF HIGHWAY 220 BEFORE
 SUNRISE...AND CHANCES FOR MEASURABLE RAIN SPREADING EAST BEFORE
 SUNRISE. WITH INCREASING SLY FLOW...CLOUD COVER AND LOW LEVEL
 WINDS TEMPS OVERNIGHT WILL BE UNIFORM AND RATHER MILD IN THE
 MIDDLE 50S. SKIES WILL BECOME OVERCAST AFTER MIDNIGHT.
 
 SATURDAY...MAIN ITEM TO NOTE IS THE MODELS HAVE INCREASED
 INSTABILITY FROM 11 AM THROUGH 5 PM WITH MLCAPE 1600 J/KG ACROSS
 CENTRAL NC. THIS COMPARED TO MODELS YESTERDAY PREDICTING WEAKER
 MLCAPE FROM 500-1000 J/KG. SHEAR WILL SUPPORT DISCRETE CELLS
 DEVELOPING AHEAD OF A DANGEROUS SQUALL LINE. 0-3 KM SHEAR IS
 FORECAST AROUND 40 M/S WITH HELICITY IN EXCESS OF 450 M2/S2. IN
 FACT 0-1 KM SHEAR WILL BE IN EXCESS OF 20 M/S WITH HELICITY OVER
 300 M2/S2 SUPPORTIVE OF TORNADOES. ALONG WITH THE INSTABILITY AND
 SHEAR LCL VALUES FALL TO LESS THAN 1500 FEET. THE DANGEROUS
 COMBINATION OF HIGH SHEAR...MODERATE CAPE AND LOW LCLS WILL BE
 SUPPORTIVE OF DISCRETE SUPERCELLS CAPABLE OF PRODUCING TORNADOES
 AHEAD OF A MAIN SQUALL LINE CAPABLE OF WIDESPREAD DAMAGING WIND
 AND ADDITIONAL TORNADOES. A FEW INTENSELY ROTATING SUPERCELLS
 WITH A THREAT OF TORNADOES WILL LIKELY EXIST.
 
 TIMING: THE NAM IS A LITTLE SLOWER WITH THE SYSTEM THAN THE GFS BY
 ABOUT 2-3 HOURS. WILL TREND TOWARDS THE FASTER WRF SOLUTIONS
 WHICH BRING THE LINE OF STORMS TO THE TRIAD AROUND NOON...TO THE
 TRIANGLE AROUND 2 PM AND EAST OF I-95 AROUND 3 PM. THE EXACT
 TIMING OF THE SQUALL LINE IS DIFFICULT AND IT COULD COME IN A
 LITTLE LATER IF THE NAM IS CORRECT. THAT SAID EXPECT DISCRETE
 STORMS SOME OF WHICH COULD BE SEVERE OR TORNADIC WELL AHEAD OF THE
 MAIN LINE ESPECIALLY AS WE MOVE INTO THE AFTERNOON AND INSTABILITY
 INCREASES.
 
 HIGHS SATURDAY IN THE MID 70S WITH DEW POINTS IN THE LOWER 60S. 
 WITH THE TIGHT PRESSURE GRADIENT EXPECT GUSTY WINDS OF 30 TO 40
 MPH WITH THE STRONG SLY FLOW OUTSIDE OF THE STORMS. IF THIS
 INCREASES A WIND ADVISORY MAY BE NEEDED FOR SATURDAY AFTERNOON. 
 
 SATURDAY NIGHT, A SMALL BAND OF SHOWERS MAY BE POSSIBLE WITH THE 
 UPPER LOW SATURDAY EVENING DUE TO THE LARGE SCALE ASCENT AND SURFACE 
 FRONT. AS OF NOW MOISTURE LOOKS TOO LIMITED TO PRODUCE SHOWERS, BUT 
 WILL HAVE TO KEEP AN EYE ON THIS WITH LATER FORECASTS. LOWS SATURDAY 
 NIGHT IN THE MID 40S. 
 
 &&
 
 .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... 
 AS OF 230 PM FRIDAY...
 
 SURFACE HIGH PRESSURE WILL BE IN CONTROL OF THE WEATHER ACROSS 
 CENTRAL NC AS IT PROGRESSES FAIRLY QUICKLY EASTWARD ACROSS THE 
 SOUTHEASTERN US. THIS WILL RESULT IN FAIR WEATHER AND THEN AS RETURN 
 FLOW SETS BY SUNDAY AFTERNOON...A WARMING TREND THROUGH THE PERIOD. 
 THICKNESS VALUES WILL REBOUND TO NEAR 1380M BY MONDAY AFTERNOON 
 WHICH SHOULD TRANSLATE TO HIGH TEMPS IN THE MID TO UPPER 70S (A GOOD 
 FIVE DEGREES WARMER THAN WHAT IS EXPECTED ON SUNDAY). OVERNIGHT LOWS 
 WILL ALSO BE ON A MODERATING TREND...WITH UPPER 40S TO NEAR 50 
 DEGREES EXPECTED SUNDAY NIGHT AND MID 50S EXPECTED BY MONDAY 
 NIGHT.      
 
 &&
 
 .LONG TERM /TUESDAY THROUGH FRIDAY/... 
 AS OF 255 PM FRIDAY...
 
 DURING THE BEGINNING OF THE LONG TERM PERIOD...THE SUBTROPICAL RIDGE 
 IS PROGGED TO STRENGTHEN OVER THE SOUTHWESTERN ATLANTIC AND THE 
 SOUTHEASTERN US. THIS FEATURE IS EXPECTED TO PUSH THE NEXT STORM 
 SYSTEM MOVING OUT OF THE PLAINS NEXT WEEK NORTH OF THE FORECAST AREA 
 AS IT TRACKS INTO THE UPPER GREAT LAKES REGION. THEREFORE THE 
 TRAILING FRONTAL ZONE AND ASSOCIATED PRECIP IS EXPECTED TO ALSO 
 REMAIN NORTH OF THE REGION THROUGH AT LEAST THURSDAY...RESULTING IN 
 DRY CONDITIONS AND A CONTINUED WARMING TREND. HIGH TEMPS ARE 
 EXPECTED TO RISE FROM THE LOW 80S ON TUESDAY TO THE MID TO UPPER 80S 
 BY THURSDAY. THE AFOREMENTIONED SUBTROPICAL RIDGE IS THEN PROGGED TO 
 BECOME SUPPRESSED TO THE SOUTH WHICH SHOULD ALLOW THE FRONTAL ZONE 
 TO DRIFT SOUTHWARD INTO THE REGION LATE THURSDAY INTO FRIDAY. 
 THEREFORE WILL SHOW LOW END CHANCE POPS FOR THE THURSDAY NIGHT INTO 
 FRIDAY...WITH SLIGHTLY COOLER TEMPS ON FRIDAY. 
 
 &&
 
 .AVIATION /17Z FRIDAY THROUGH TUESDAY/...
 AS OF 200 PM FRIDAY...
 
 VFR CONDITIONS WILL PERSIST THIS AFTERNOON AND EVENING...BUT CLOUD
 COVER WILL INCREASE WITH A STRENGTHENING AND INCREASINGLY MOIST
 SOUTHEASTERLY FLOW AND ASSOCIATED. EXPECT IFR-MVFR CONDITIONS
 BETWEEN 04-08Z SATURDAY. INCREASING SOUTHERLY WINDS (TO
 AROUND 40 KTS) AND ASSOCIATED SHEAR JUST ABOVE THE SURFACE WILL
 ALSO PRODUCE MECHANICAL TURBULENCE OVERNIGHT. A SOUTHEASTERLY
 SURFACE WIND WILL INCREASE TO 15 KT RANGE WITH OCCASIONAL
 GUSTINESS TO AROUND 25 KTS BY SATURDAY MORNING. A PERIOD OF
 SCATTERED SHOWERS ARE POSSIBLE EARLY SATURDAY FROM 09-14Z WITH THE
 STRONG SLY LOW LEVEL FLOW.
 
 CONDITIONS MAY BRIEFLY IMPROVE BY A CATEGORY MID TO LATE SATURDAY 
 MORNING...TO LOW MVFR (PARTICULARLY AT EASTERN TERMINALS 
 (KRWI/KFAY)...BEFORE LOWERING AGAIN IN WIDESPREAD SHOWERS AND 
 THUNDERSTORMS. EXPECT WINDY CONDITIONS IN SOUTH SOUTHEASTERLY 
 SURFACE FLOW...MIDDAY THROUGH SATURDAY AFTERNOON WITH GUSTS TO 35 
 KT. 
 
 A DANGEROUS SQUALL LINE IS FORECAST TO MOVE FROM WEST TO EAST
 ACROSS ALL TERMINAL FROM 17Z-22Z WITH A THREAT OF WINDS IN EXCESS
 OF 52KTS AND ISOLATED TORNADOES. 
 
 VFR CONDITIONS ARE EXPECTED TO RETURN AS HIGH PRESSURE BUILDS IN 
 SATURDAY NIGHT THROUGH EARLY NEXT WEEK.
 
 &&
 
 .RAH WATCHES/WARNINGS/ADVISORIES...
 NONE.
 
 &&
 
 $$
 SYNOPSIS...JO
 NEAR TERM...JO
 SHORT TERM...KRR
 LONG TERM...KRR
 AVIATION...JO