000 FXUS62 KRAH 070121 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 921 PM EDT MON SEP 6 2010 .SYNOPSIS... A DRY RIDGE OF HIGH PRESSURE WILL EXTEND ACROSS NORTH CAROLINA THROUGH TUESDAY. A WEAK COLD FRONT WILL CROSS THE AREA LATE WEDNESDAY. BEHIND THE FRONT... HIGH PRESSURE WILL BUILD SOUTH INTO THE AREA FOR THE REMAINDER OF THE WORK WEEK. && .NEAR TERM /THROUGH TUESDAY NIGHT/... AS OF 920 PM MONDAY... TONIGHT: ONLY COSMETIC CHANGES REQUIRED TO PREVIOUS FORECAST. MID LEVEL H5 RIDGING CENTERED OVER NORTHERN AL HAS STRENGTHENED SOME 30 METERS PER 00Z RAOBS... AND WILL CONTINUE TO BUILD EASTWARD OVERNIGHT IN THE WAKE OF A DEPARTING L/W TROUGH AXIS SITUATED JUST OFFSHORE THE EAST COAST. MEANWHILE AT THE SURFACE... A 1023 MB RIDGE JUST OFFSHORE THE TIDEWATER OF VA SHOULD CONTINUE TO REMAIN WELL ENTRENCHED ACROSS THE CAROLINAS OWING TO THE SUBSIDENCE DOWNSTREAM OF THE MID LEVEL RIDGING. GENERALLY EASTERLY FLOW AROUND THE SURFACE RIDGE FOR THE PAST 12-18 HOURS HAS RESULTED IN A MODEST RECOVERY IN LOW LEVEL MOISTURE ACROSS CENTRAL NC DURING THAT TIME... WITH DEWPOINTS RANGING AT 9 PM FROM MAINLY UPPER 50S NORTHWEST TO MIDDLE 60S SOUTH. OVERNIGHT LOWS SHOULD NOT BE TOO FAR OFF OF THOSE OBSERVED DEWPOINT TEMPERATURES... WITH MOST LOCALES EXPECTED TO DIP TO WITHIN A FEW DEGREES OF 60. THE INCREASED LOW LEVEL MOISTURE IN CONJUNCTION WITH LIGHT-CALM WINDS WITHIN THE SURFACE RIDGE AXIS... AND AN INITIALLY MOSTLY CLEAR SKY (ASIDE FROM MAINLY SCT STRATOCUMULUS TRAPPED AT THE BASE OF A SUBSIDENCE ATTENDING THE INCOMING RIDGE ALOFT)... MAY PROMOTE THE DEVELOPMENT OF PATCHES OF STRATUS AND FOG BY DAYBREAK... PARTICULARLY IN THE HIGHER DEWPOINT AIR OVER THE SOUTHERN AND SOUTHWESTERN ZONES. THIS TREND WAS ALREADY REFLECTED IN THE GRIDDED FORECAST. TUESDAY AND TUESDAY NIGHT: THE AFOREMENTIONED NORTHERN PLAINS SHORTWAVE AND ITS ASSOCIATED SURFACE LOW WILL DRIVE A COLD FRONT SOUTHEAST INTO THE OHIO VALLEY REGION ON TUESDAY. THE AIR MASS SHOULD MODERATE A BUT MORE WITH A DEVELOPING SOUTHWESTERLY FLOW AND 1000 TO 850 MILLIBAR THICKNESSES FORECAST TO RISE INTO THE 1420 TO 1425 METER RANGE. THICKNESS AND 850 MILLIBAR MIXING BOTH YIELD VALUES CONSISTENT WITH STATISTICAL GUIDANCE...88 TO 92. CONTINUED WITH DRY FORECAST AS PW RECOVERY IS FORECAST TO BE RATHER WEAK DURING THE DAY TUESDAY AND MODELS ARE NOT SHOWING MUCH DEVELOPMENT OF A PREFRONTAL TROUGH WITH HIGH PRESSURE REMAINING DOMINANT. SHOULD SEE MAINLY CLEAR SKIES TUESDAY NIGHT WITH SOUTHWESTERLY FLOW AHEAD OF WHAT LOOKS TO BE A MAINLY DRY APPROACHING COLD FRONT. HAVE LOWS WARMER THAN GUIDANCE...65 TO 67. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... AS OF 235 PM MONDAY... FOR WEDNESDAY AND WEDNESDAY NIGHT: MID LEVEL VORTEX NOW PUSHING THROUGH THE DAKOTAS DEEPENS WHILE CROSSING THE UPPER GREAT LAKES TOWARD FAR SOUTHERN QUEBEC BY DAWN THURSDAY. A CORRESPONDING SURFACE FRONT PUSHES TO THE SOUTHERN APPALACHIANS BY WEDNESDAY AFTERNOON THEN DROPS SOUTHEAST THROUGH NORTH CAROLINA WEDNESDAY NIGHT... BUT WITH INCIPIENT DRY AIR (PRECIPITABLE WATER WELL UNDER ONE INCH) AND A WESTERLY DOWNSLOPE FLOW AT 850 MILLIBAR PRECEDING THE FRONT... PRECIPITATION CHANCES APPEAR LOW. WE WILL SEE PRECIPITABLE WATER VALUES INCREASE BRIEFLY TO NEAR 1.5 INCHES JUST AHEAD OF THE FRONT... AND THE NAM/GFS DO SHOW A WEAK WAVE DROPPING SOUTHEAST THROUGH THE STATE WITH THETA-E RIDGING NOSING IN AT 850 MILLIBAR. HOWEVER NO FORCING FOR ASCENT IS EVIDENT ELSEWHERE THROUGH THE TROPOSPHERE... INCLUDING AT JET LEVEL... AND THE PRECIPITATION AMOUNTS GENERATED BY THE MODELS ARE SLIM TO NONE. INSTABILITY IS ALSO LIMITED WITH THE GFS SHOWING SBCAPE PEAKING AT 500 J/KG RIGHT ALONG THE FRONT... WITH A DRY SUBCLOUD LAYER. WILL KEEP THE FORECAST DRY BUT INCLUDE A PERIOD OF ISOLATED SPRINKLES LATE AFTERNOON INTO MID EVENING AS LAPSE RATES ALOFT STEEPEN WITH BROAD MID LEVEL TROUGH PASSAGE. GIVEN THE DRY LOW LEVELS... A FEW ENHANCED GUSTS ARE POSSIBLE NEAR THESE SHOWERS. THICKNESSES CLIMB FURTHER WEDNESDAY TO AROUND 15 TO 20 METERS ABOVE NORMAL... AND WITH A DRY ADIABATIC LOW LEVEL PROFILE... HIGHS OF 89 TO 94 STILL LOOK FINE. GIVEN THE DELAY IN CLEARING OVERNIGHT ALONG WITH THE WEAK ADVECTION OF LOWER THICKNESSES AND STILL MILD AIR AT 925 TO 850 MILLIBAR... WILL BUMP LOWS UP JUST A BIT MAINLY ACROSS THE NORTH TO 64 TO 70. FOR THURSDAY AND THURSDAY NIGHT: THE FRONT IS EXPECTED TO SETTLE ALONG THE SOUTHERN BORDER OF NORTH CAROLINA THURSDAY. AN ELONGATED CANADIAN SOURCE HIGH ATTEMPTS TO BUILD IN BEHIND THE FRONT FROM THE NORTHWEST... HOWEVER THE COOLER AIR APPEARS TO BE HINDERED BY THE MOUNTAINS SUCH THAT LOW LEVEL THICKNESSES HOLD ABOVE NORMAL... AND A WEAK LEE TROUGH DEVELOPS OVER THE PIEDMONT IN THE AFTERNOON. THE COLUMN DRIES BACK OUT RATHER QUICKLY WITH NO INSTABILITY NOR LARGE SCALE LIFT... SO DO NOT ANTICIPATE ANY PRECIPITATION ALONG THE FRONTAL ZONE OR TROUGH ON THURSDAY... ALTHOUGH WE COULD SEE SOME MID OR HIGH LEVEL CLOUDINESS IN THE MORNING AS ANOTHER WEAK WAVE ALOFT DROPS THROUGH NORTH CAROLINA. HIGHS 87 TO 93. LOWS 58 TO 65 WITH A SECONDARY SURGE OF DRIER AIR DROPPING THROUGH THE STATE OVERNIGHT. -GIH && .LONG TERM /FRIDAY THROUGH MONDAY/... AS OF 235 PM MONDAY... FOR FRIDAY THROUGH SATURDAY: THE FRONT TO OUR SSW TRIES TO HEAD BACK NORTHEAST AS A WARM FRONT ON FRIDAY ACCORDING TO THE LATEST GFS/ECMWF. BUT AS THE MID LEVEL VORTEX SITS AND SPINS OVER NOVA SCOTIA WITH TROUGHING ENCOMPASSING MOST OF THE EASTERN SEABOARD AND WESTERN ATLANTIC... THE BLOCKING PATTERN SHOULD HELP KEEP THE SURFACE RIDGE DOMINANT THROUGH FRIDAY NIGHT. BUT THE ECMWF (AND TO A LESSER DEGREE THE GFS) INDICATES 850 MILLIBAR NOCTURNAL JETTING NOSING INTO SOUTHWESTERN NORTH CAROLINA ALONG THE FRONT LATE FRIDAY INTO FRIDAY NIGHT... WHICH COULD INDUCE ENOUGH FORCING AND LOW LEVEL MOISTURE POOLING TO KICK OFF SLOW MOVING SHOWERS OR STORMS. THE GFS/ECMWF BOTH SUGGEST THAT THIS MOISTURE IS ROOTED IN THE REMAINS OF NOW TROPICAL STORM HERMINE... QUITE REASONABLE CONSIDERING THE MEAN FLOW PATTERN AND GOOD MODEL AGREEMENT. WILL SPREAD IN CHANCE POPS FROM SOUTHWEST TO EAST VERY LATE FRIDAY THROUGH FRIDAY NIGHT INTO SATURDAY MORNING. THE GFS/ECMWF ALSO BOTH SHOW A LOW FORMING AND TRACKING ALONG THE FRONT JUST TO OUR SOUTH... WHICH RESULTS IN AN ENHANCED NORTHEAST LOW LEVEL FLOW FRIDAY NIGHT INTO SATURDAY...POTENTIALLY RESULTING IN A WEAK IN SITU DAMMING EVENT. WHILE IT IS TOO FAR OFF WITH TOO MUCH UNCERTAINTY TO PUT MUCH FAITH IN SUCH A SCENARIO... IT WILL BE SOMETHING TO WATCH IN THE COMING DAYS. FOR NOW WILL TAPER POPS DOWN SATURDAY AFTERNOON BUT KEEP A GOOD BIT OF CLOUD COVER OVER THE AREA. WILL HAVE HIGHS NEAR NORMAL... 82 TO 88... BUT CONFIDENCE IS BELOW NORMAL AND IF THE PRECIPITATION COVERAGE IS GREATER THAN EXPECTED FRIDAY NIGHT AND SATURDAY...HIGHS COULD BE BELOW NORMAL ON SATURDAY. LOWS IN THE 60S. FOR SATURDAY NIGHT THROUGH MONDAY: THE BLOCKING VORTEX OVER NOVA SCOTIA FINALLY WEAKENS AND GETS NUDGED EASTWARD BY A STRONG SHORT WAVE NOW APPROACHING THE PACIFIC NORTHWEST. THE MID LEVEL RIDGE BETWEEN THESE TWO FEATURES DEAMPLIFIES SO IT LOOKS LIKELY THAT BROAD TROUGHING WILL BECOME REESTABLISHED OVER THE EASTERN UNITED STATES BY MONDAY. THE OPERATIONAL MODELS DIVERGE HOWEVER WITH THE ECMWF MUCH MORE AMPLIFIED THAN THE GFS... RESULTING IN THE ECMWF LEANING TOWARD KEEPING THE SURFACE FRONTAL ZONE JUST SOUTH OF THE CWA WITH COOLER TEMPERATURES AND DRY WEATHER... WHILE THE GFS ALLOWS THE WARM SECTOR TO SURGE UP INTO THE CAROLINAS WITH MUCH WARMER TEMPERATURES AND BETTER RAIN CHANCES. THE LESS AMPLIFIED GFS HAS SUPPORT FROM THE ECMWF ENSEMBLE MEAN AND THUS WAS PREFERRED. WILL HAVE LOW CHANCES OF AFTERNOON THROUGH EVENING SHOWERS OR THUNDERSTORMS... WITH TEMPERATURES GENERALLY A CATEGORY ABOVE NORMAL. -GIH && .AVIATION /23Z MONDAY THROUGH SATURDAY/... AS OF 745 PM... VFR CONDITIONS THROUGH MUCH OF THE NIGHT. STRATUS SHOULD BE MORE WIDESPREAD TOWARD DAWN THAN MONDAY MORNING... ESPECIALLY SOUTH AND WEST. LIFR CONDITIONS POSSIBLE FROM ABOUT 5 AM TO 10 AM. CONDITIONS SHOULD MOSTLY REMAIN VFR THROUGH SATURDAY...BUT THE BEST CHANCE FOR MVFR/IFR CONDITIONS EXISTS WEDNESDAY AFTERNOON INTO THURSDAY MORNING WITH THE PASSAGE OF A WEAK FRONT. FOG AND/OR LOW STRATUS IS ALSO POSSIBLE OVERNIGHT TUESDAY NIGHT. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ NEAR TERM...MWS/SMITH SHORT TERM...HARTFIELD LONG TERM...HARTFIELD
NWS RAH Office Area Forecast Discussion