Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 251611 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1105 AM EST WED FEB 25 2015 .SYNOPSIS...AN AREA OF LOW PRESSURE WILL FORM OVER THE NORTHERN GULF OF MEXICO TODAY AND WILL THEN TRACK QUICKLY NORTHEAST...OFF THE CAROLINA COAST OVERNIGHT THROUGH EARLY THURSDAY. COLD CANADIAN HIGH PRESSURE WILL THEN BUILD INTO THE REGION FRIDAY AND INTO THE WEEKEND. && .NEAR TERM /TODAY/... AS OF 1105 AM WEDNESDAY... ...WINTER WEATHER ADVISORY FOR BLACK ICE THROUGH NOON TODAY... WILL CANCEL THE WINTER WEATHER ADVISORY SHORTLY. MORNING UPPER AIR ANALYSIS SHOWS THE SHEARING SHORT WAVE TROUGH OVER TEXAS THIS MORNING WITH SHORT WAVE RIDGING OUT AHEAD OF IT ACROSS THE SOUTHEAST. HIGH CLOUDS HAVE BEEN ADVANCING NORTHEAST AHEAD OF THE SYSTEM AND MOST OF CENTRAL NC IS ALREADY UNDER AT LEAST A THIN VEIL OF CIRRUS. REGIONAL RADAR IMAGERY SHOWS PRECIPITATION EXPANDING ACROSS THE DEEP SOUTH WITH MEASURABLE PRECIP NOW REPORTED ACROSS CENTRAL ALABAMA. NO PRECIPITATION IS EXPECTED IN CENTRAL NC DURING THE TODAY PERIOD BUT TODAYS WEATHER COULD STILL PLAY A ROLE IN THE IMPACT OF TONIGHTS WINTER STORM. THE TEMPERATURES FORECAST FOR TODAY IS PROBLEMATIC AS YESTERDAYS SNOWFALL WILL DAMPEN THE TEMPERATURE RECOVERY THIS AFTERNOON AND ENERGY IS DIVERTED INTO MELTING AND EVAPORATING SNOW AND MELT WATER. EVEN SO...SOME GUIDANCE INCLUDING THE HRRR SUGGEST HIGHS IN THE MID 40S TO AROUND 50. NOT READY TO JUMP THAT HIGH BUT CURRENT TEMP AT OUR OFFICE IS 37 AT 1045 AND HAVE MADE A SLIGHT UPWARD ADJUSTMENT IN TEMPERATURES FOR TODAY. THIS UPWARD TREND IN BOUNDARY LAYER TEMPERATURES WOULD DELAY THE CHANGEOVER FROM RAIN OR MIXED RAIN AND SNOW THIS EVENING TO SNOW AND POTENTIALLY LIMIT ACCUMULATIONS A BIT...PRIMARILY ACROSS THE FAR SOUTHERN TIER IN THE SANDHILLS AND SOUTHERN COASTAL PLAIN. -BLAES && .SHORT TERM /TONIGHT THROUGH THURSDAY/... AS OF 1100 AM WEDNESDAY... *** QUICK 1100 AM UPDATE. QUICK LOOK AT 12Z GUIDANCE NOTES NO IMMEDIATE OR SIGNIFICANT FORECAST ADJUSTMENTS ARE NEEDED BUT NAM CONTINUES TO TREND NORTH A BIT AND WITH THE GFS SHOWING A SLIGHT NORTHWARD ADJUSTMENT. ...WINTER STORM WARNING HAS BEEN ISSUED FOR TONIGHT AND THURSDAY FOR ALL THE PIEDMONT...NORTHERN/CENTRAL COASTAL PLAIN AND SANDHILLS... ...WINTER WEATHER ADVISORY HAS BEEN ISSUED FOR TONIGHT AND THURSDAY FOR THE SOUTHERN COASTAL PLAIN... THERE HAS BEEN A CONTINUED SLIGHT NORTHWARD SHIFT IN GUIDANCE SINCE YESTERDAY...WHICH EQUATES TO A WETTER AND MARGINALLY WARMER SOLUTION FOR CENTRAL NC...THOUGH THE OVERALL PATTERN REMAINS UNCHANGED WITH THE DEAMPLIFICATION/ENE SHEARING OF A SHORTWAVE TROUGH FROM NEAR THE RIO GRANDE THIS MORNING TO THE MIDDLE ATLANTIC COAST BY THU MORNING. THE ASSOCIATED FORCING FOR ASCENT ATOP A PAIR OF CLOSELY-SPACED AND STRONG FRONTAL ZONES DRAPED ACROSS THE NORTHERN GOM TO A SFC LOW ABOUT MIDWAY BETWEEN CAPE HATTERAS AND BERMUDA...WILL RESULT IN THE CONTINUED DEEPENING OF A SFC LOW OVER THE NW GOM THIS MORNING. GUIDANCE IS THEN FAIRLY TIGHTLY CLUSTERED WITH THE TRACK OF A MILLER "A" SFC LOW ACROSS THE NORTHERN FL PENINSULA THIS EVENING AND ROUGHLY ALONG THE GULF STREAM TO JUST EAST OF CAPE HATTERAS BY 12Z THU. AS HAS BEEN NOTED IN PREVIOUS DISCUSSIONS...THE LACK OF A PARENT COLD HIGH IS ADMITTEDLY SOMEWHAT CONCERNING. AN ARCTIC SFC HIGH AT 1045 MB WILL BE CENTERED OVER SOUTHERN ALBERTA/SASKATCHEWAN AND WILL CONSEQUENTLY HAVE LITTLE INFLUENCE AT OUR LATITUDE DURING THE HEIGHT OF THE STORM TONIGHT. HOWEVER...A VERY FAVORABLE MILLER "A" CYCLONE TRACK ALONG AND JUST OFFSHORE THE SE COAST...IN CONJUNCTION WITH A PRECEDING ARCTIC HIGH THAT APPEARS TO BE JUST COLD ENOUGH DESPITE BEING IN A DRAMATICALLY WEAKENED STATE FROM A COUPLE OF DAYS AGO...BUT IS STILL CHARACTERIZED BY SFC TEMPERATURES AND DEWPOINTS MOSTLY IN THE TEENS TO 20S OVER NC THIS MORNING...WILL SUPPORT A SIGNIFICANT WINTER STORM **WITH A NARROW P-TYPE (SNOW/RAIN) TRANSITION ZONE AND DEEP NEAR FREEZING ISOTHERMAL PROFILES CHARACTERISTIC OF MILLER "A` SFC PATTERNS.** PRECIPITATION RATES...OCCASIONALLY MODERATE TO HEAVY...WILL ONLY SERVE TO ENHANCE THE DEEP NEAR FREEZING ISOTHERMAL PROFILES RIGHT ALONG THE SNOW/COLD RAIN INTERFACE...OWING TO MELTING. A LITTLE MIX OF SLEET AND FREEZING RAIN ARE HARD TO AVOID IN CENTRAL NC WINTER STORMS...AND IN MILLER "A" STORMS INCLUDING THIS ONE...SUCH OCCURRENCE IS MOST LIKELY NEAR THE NARROW TRANSITION SNOW/RAIN ZONE. NO APPRECIABLE ICING IS ANTICIPATED...HOWEVER...IN PART SINCE SURFACE TEMPERATURES ARE EXPECTED TO HOLD NEARLY STEADY IN THE 29 TO 33 DEGREE RANGE OVER THE WARNING AREA...AND 32 TO 34 DEGREES OVER THE ADVISORY AREA WHERE SUCH ICING OCCURRENCE WOULD BE MOST PROBABLE. DESPITE SOME MINIMAL OVERALL RISE IN THICKNESS VALUES VERSUS THE SLIGHTLY COLDER AND DRIER SOLUTIONS OF YESTERDAY...PROJECTED PARTIAL THICKNESS VALUES OVER MOST OF THE PIEDMONT AND NORTHERN COASTAL PLAIN REMAIN SQUARELY IN THE RANGE TYPICAL OF SIGNIFICANT SNOWSTORMS IN CENTRAL NC. WITH A SLIGHTLY MILDER SOLUTION COMES A SLIGHTLY WETTER ONE...LED BY A BLEND OF THE VERY SIMILAR 00Z ECMWF AND GFS...SO QPF HAS BEEN RAISED TO AROUND ONE HALF INCH OVER THE NW PIEDMONT TO AS MUCH AS ONE INCH OVER THE SOUTHERN COASTAL PLAIN. PARTIAL THICKNESS VALUES HEAVILY WEIGHTED TOWARD THE FAVORED 00Z ECMWF AND GFS...FAVORS MOSTLY SNOW OVER THE PIEDMONT AND NORTHERN COASTAL PLAIN...WITH A MIX OF SNOW AND A COLD RAIN (OR FREEZING RAIN WITH LITTLE ACCRUAL WHERE SFC TEMPERATURES OSCILLATE ABOUT THE FREEZING MARK) OVER THE ADVISORY AREA. SNOW RATIOS BETWEEN 00-06Z THU OF 10-14:1 PER WPC WWD SLR GUIDANCE AND SUPPORTED BY THERMAL PROFILES THAT RANGE FROM DEEP NEAR FREEZING ISOTHERMAL IN THER NARROW TRANSITION ZONE TO WELL BELOW FREEZING... EXCEPT RIGHT AT THE SURFACE...ALONG AND NORTH OF THE I-40/I-85 CORRIDOR...ARE EXPECTED TO DECREASE PARTICULARLY NEAR NARROW TRANSITION ZONE TO CLOSER TO 8-10:1 PER WHAT IS TYPICAL OF EVENTS CHARACTERIZED BY PARTIAL THICKNESS VALUES IN THE ~1285/~1535 METER RANGE. SLIGHTLY LOWER RATIOS...CLOSER TO 4-6:1 WITHIN THE SNOW/RAIN TRANSITION ZONE - OR ROUGHLY ALONG THE BOUNDARY OF WINTER STORM WARNING AND WINTER WEATHER ADVISORY - WHERE THE MIXING WITH A COLD RAIN WILL RESULT IN A MORE SLUSHY ACCUMULATION VERSUS POINTS NORTHWARD. BASED ON THESE RATIOS...AND THE QPF NOTED ABOVE...WIDESPREAD SNOWFALL ACCUMULATIONS OF 3 TO 6 INCHES ARE LIKELY OVER THE WARNING AREA...WITH A STRIPE OF 6 TO 8 INCHES ROUGHLY INVOF OR JUST NORTH OF THE US HIGHWAY 64 CORRIDOR...WITH LOCALLY HIGHER AMOUNTS FROM THE TRIANGLE TO ROANOKE RAPIDS AND ROCKY MOUNT. MEANWHILE...LESSER TOTALS OF 1 TO 3 INCHES ARE EXPECTED OVER THE ADVISORY AREA...WHERE AGAIN...PARTIAL THICKNESS AND SFC TEMPERATURE VALUES MARGINALLY ABOVE FREEZING WILL SUPPORT A MIXING WITH OR CHANGEOVER TO A COLD RAIN BETWEEN MIDNIGHT AND 5 AM. EVEN THERE...HOWEVER...THE NORTHERN PORTIONS OF THE ADVISORY COUNTIES MAY EXCEED WARNING CRITERIA OF 3 INCHES. THE APPROACH OF A WEAK CLIPPER LOW AND ATTENDING SHORTWAVE TROUGH WILL CAUSE THE LINGERING STRATUS LAYER AND ASSOCIATED MOSTLY CLOUDY SKIES TO PRODUCE OCCASIONAL DRIZZLE OR FREEZING DRIZZLE...AFTER LINGERING LIGHT SNOW...OWING TO A LOSS OF SATURATION ALOFT/IN THE ICE NUCLEATION ZONE - BUT WITH MINIMAL ADDITIONAL IMPACT. CHILLY TEMPERATURES IN THE LOWER TO MIDDLE 30S. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... AS OF 340 AM WEDNESDAY... SHORTWAVE DISTURBANCES EMBEDDED IN THE CONTINUED SWLY FLOW ALOFT AND LINGERING LOW-LEVEL MOISTURE OWING TO WHAT WILL LIKELY BE A MOIST SNOW COVER GROUND(AT LEAST FOR THE NORTHERN TWO-THIRDS OF THE FORECAST AREA)...WILL RESULT IN RATHER EXTENSIVE CLOUDINESS INTO SATURDAY. COMBINATION OF THIS CLOUD COVER...SNOW COVER AND THE ARRIVAL OF ANOTHER FRESH SUPPLY OF ARCTIC AIR INTO THE REGION LATE THURSDAY NIGHT AND INTO FRIDAY...AS A STRONG +1040MB CANADIAN SURFACE HIGH BUILDS OVER THE REGION...WILL SUPPORT TEMPERATURES WELL BELOW NORMAL(15-20 DEGREES FRIDAY AND SATURDAY. DRY WITH HIGHS IN THE MID 30S NORTH TO POSSIBLY LOWER 40S SOUTH...WITH SATURDAY BEING THE COLDEST DAY OF THE TWO. LOWS IN THE 20S...WITH SOME TEENS SATURDAY NIGHT. FLOW ALOFT FLATTENS OUT TO ZONAL BY SUNDAY WHILE MODERATING CANADIAN HIGH STARTS TO SHIFT OFF THE MID-ATLANTIC COAST AS A SW-NE ORIENTED FRONTAL ZONE STRETCHING FROM THE SOUTHERN PLAINS INTO THE OHIO VALLEY MOVES SLOWLY EASTWARD. MODELS HAVE SLOWED DOWN THE ONSET OF WAA/SOUTHERLY RETURN FLOW AND RESULTANT PRECIP INTO THE AREA LATE SUNDAY AFTERNOON/EVENING...BY WHICH TIME AIRMASS LOOKS TO HAVE MODERATED ENOUGH TO SUPPORT ALL RAIN. IT THEN LOOKS LIKE A WET PERIOD SUNDAY NIGHT THROUGH EARLY NEXT WEEK AS THE AFOREMENTIONED FRONT SETTLES SOUTHWARD INTO AREA...EVENTUALLY RETURNING NORTHWARD AS A WARM FRONT WITH CAD IMPLICATIONS TUESDAY/TUESDAY NIGHT AS HIGH PRESSURE OVER THE GREAT LAKES/NEW ENGLANDS NOSES SOUTH INTO THE AREA. PARTIAL THICKNESSES CURRENTLY SUGGEST ALL RAIN FOR NOW...BUT WILL CONTINUE TO MONITOR. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... AS OF 642 AM WEDNESDAY... 24-HR TAF PERIOD: IFR STRATUS DECK HAS STEADILY SHIFTED SOUTHEASTWARD OVER THE PAST HOURS AS WEAK SURFACE HIGH BUILDS INTO THE AREA FROM THE NORTHWEST. SOUNDINGS INDICATE THAT THIS SHRINKING AREA OF LOW CLOUDS SHOULD MIX OUT RATHER QUICKLY...AOA 14Z WITH VFR CONDITIONS EXPECTED UNDERNEATH A THICKENING CIRRUS SHIELD. CONDITIONS WILL DETERIORATE RAPIDLY BETWEEN 00Z TO 03Z AS A LOW PRESSURE SYSTEM TRACKS FROM THE NORTHERN GULF OF MEXICO TO OFF THE CAROLINA COAST OVERNIGHT THROUGH EARLY THURSDAY MORNING...SPREADING WINTRY PRECIP INTO THE AREA. MODERATE TO HEAVY SNOW WILL OVERSPREAD ALL OF THE TERMINALS IN THE 00Z-03Z TIME FRAME....POSSIBLY MIXING WITH RAIN AT KFAY. PRECIP WILL GRADUALLY TAPER OFF BETWEEN 12 TO 15Z THU...ENDING AS FREEZING RAIN/DRIZZLE. LOOKING AHEAD: SUB-VFR CEILINGS WILL LIKELY PERSIST IN THE WAKE OF THE COASTAL LOW PRESSURE SYSTEM DURING THE DAY THU. VFR CONDITIONS MAY RETURN FOR A PERIOD ON FRI...THOUGH INTERMITTENT SUB-VFR CONDITIONS WILL BE POSSIBLE FRI INTO THE WEEKEND IN ASSOC/W THE UNSETTLED PROGRESSIVE SYNOPTIC PATTERN IN PLACE. && .RAH WATCHES/WARNINGS/ADVISORIES... WINTER WEATHER ADVISORY UNTIL NOON WEDNESDAY FOR NCZ007>011-024>028- 040>043-076>078-085-086-088-089. WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO NOON EST THURSDAY FOR NCZ078-085-086-088-089. WINTER STORM WARNING FROM 6 PM THIS EVENING TO NOON EST THURSDAY FOR NCZ007>011-021>028-038>043-073>077-083-084. && $$ SYNOPSIS...CBL NEAR TERM...BLAES SHORT TERM...BLAES/RAH LONG TERM...CBL AVIATION...CBL/BSD/VINCENT

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