WFO RAH Text Data |
FXUS62 KRAH 192023 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 320 PM EST SAT JAN 19 2008 .SYNOPSIS... LOW PRESSURE OVER THE GULF OF MEXICO WILL SHIFT RAPIDLY NORTHEAST OFF OF THE NORTH CAROLINA COAST THIS EVENING...MOVING RAPIDLY AWAY INTO THE ATLANTIC OVERNIGHT. COLD HIGH PRESSURE WILL BUILD IN TONIGHT AND PROVIDE FOR CHILLY TEMPERATURES SUNDAY INTO MONDAY. THE HIGH MOVES QUICKLY TO THE EAST TUESDAY AHEAD OF A COLD FRONT...WHICH SWEEPS THROUGH THE AREA TUESDAY NIGHT. && .NEAR TERM /UNTIL MIDNIGHT TONIGHT/... AS OF 200 PM...PRECIPITATION RAPIDLY BRAKING OUT ACROSS THE PIEDMONT OF NC. THIS PRECIPITATION IS IN RESPONSE TO MID LEVEL FRONTOGENETICAL FORCING AND LIFT IN THE RIGHT REAR OF THE UPPER JET. COOLING AT THE LOW LEVELS ALONG WITH COOLING FROM MELTING SNOW ALOFT WILL ALLOW PRECIPITATION TO CHANGE OVER TO SNOW THIS AFTN AND EVE. SNOW HAS BEEN FALLING IN THE TRIAD AREA FOR AROUND AN HOUR TO 90 MINUTES AGO. TEMPERATURES HAVE FALLEN DRAMATICALLY SINCE THE PRECIP STARTED WITH GSO FALLING 7 DEGREES OR SO IN THE HOUR SINCE THE SNOW STARTED. TEMPS SHOULD FALL TO AROUND FREEZING BY AROUND 400 PM ACROSS MOST OF THE TRIAD. THE 18Z RAOB FROM GSO WAS DRY IN THE LOWER LEVELS BUT BECOMING COLD AND SUFFICIENT FOR SNOW. THE WARM AIR TEMPERATURES AND THE SHORT DURATION OF SNOW SHOULD KEEP ACCUMULATIONS IN THE TRIAD AREA DOWN...GENERALLY RANGING AROUND AN INCH WITH UP TO 2 INCHES IN LOCATIONS EAST OF GSO AND HBI. GETTING REPORTS OF SNOW MIXING WITH THE RAIN IN WESTERN PARTS OF THE NORTHEAST PIEDMONT AND TRIANGLE AREA (SANFORD...CARY AND DURHAM). LATEST AMDAR AIRCRAFT SOUNDING AT RDU FROM 1724Z HAD THE FREEZING LEVEL AT AROUND 2600 FEET. SHOULD SEE THE CHANGE OVER BEGIN IN THE TRIANGLE AREA OCCUR BETWEEN 200 AND 300 PM. LOOKS LIKE THE PRECIP IN THE TRIANGLE AREA WILL BE SUFFICIENT TO COOL TEMPS TO THE LOWER 30S BY AROUND 500 OR 600 PM. ACCUMULATIONS WILL BE DIFFICULT TO PINPOINT BUT THEY SHOULD RANGE BETWEEN 1 TO 2 INCHES ON GRASSY SURFACES WITH SOME SPOTS POTENTIALLY GETTING UP TO 3 INCHES MAINLY IN THE SWEET SPOT WHERE THE COLD AIR ARRIVES EARLY ENOUGH AND THE QPF WILL BE SUFFICIENT. THE SWEET SPOT SHOULD SETUP EAST OF TTA...RDU AND LHZ. ACROSS THE COASTAL PLAIN AND SANDHILLS IT WILL STILL BE ANOTHER FEW HOURS FOR THE CHANGEOVER TO ARRIVE BUT FOR SNOW LOVERS THE PRECIP WILL PERSIST THE LONGEST SO THESE LOCATIONS COULD BE THE BIG WINNERS IN THE RAH CWA. WILL NOT MAKE MAJOR CHANGES TO FORECAST IN THIS AREA WHILE MONITORING THE LARGE AREA OF LIGHT TO MODERATE PRECIP UPSTREAM ACROSS SC. ACCUMULATION TOTALS SHOULD RANGE IN THE 1 TO 2 INCH RANGE WITH THE HIGHER END OF THOSE AMOUNTS NORTH AND WEST OF FAY AND GSB. NOTE THAT SNOW IS STILL FALLING IN ATLANTA WITH VSBYS AROUND 1 MILE AFTER HAVING SEVERAL INCHES OF SNOW...A POTENTIALLY TROUBLING SIGN FOR OUR SE COUNTIES. PRECIP WILL TAPER OFF QUICKLY FROM WEST TO EAST THIS EVENING. SNOW SHOULD END IN THE TRIAD BETWEEN 5 AND 7 PM IN THE TRIANGLE BETWEEN 6 AND 9 PM AND IN THE COASTAL PLAIN TOWARD MIDNIGHT. WILL KEEP ADVISORIES AS THEY ARE. SEE LITTLE VALUE IN POTENTIALLY ALTERING THEIR STRUCTURE WITH UNCERTAINTY REMAINING IN THE FORECAST. ACCUMULATION AMOUNTS WILL KEEP IN MIND THAT MOST ACCUMULATION WILL OCCUR ON GRASSY SURFACES... CARS... AND ROOFTOPS GIVEN THE ABOVE FREEZING NEAR SURFACE TEMPERATURES AND ALREADY WET GROUND. THUS ROADWAYS.. SHOULD MAINLY BE WET OR SLUSHY THROUGH SUNSET... WITH A HARD FREEZE OVER WHEN TEMPERATURES DROP INTO THE 20S AFTER SUNSET. -BLAES && .SHORT TERM /MIDNIGHT TONIGHT THROUGH TUESDAY NIGHT/... AS OF 200 PM SATURDAY... LOW LEVEL DOWNSLOPE FLOW BEGINS IN EARNEST OVERNIGHT... ALTHOUGH THE ARRIVAL OF THE SURFACE COLD FRONT... NOW STRETCHING FROM THE EASTERN GREAT LAKES THROUGH TN... WILL LAG THE 850 MB FRONT AND SHOULD LEAD TO LINGERING LOW CLOUDS IN THE EAST OVERNIGHT. OTHERWISE SKIES SHOULD CLEAR SWIFTLY BY MORNING WITH STRONG SUBSIDENCE AND DRYING THROUGH THE COLUMN AS THE MID LEVEL SHORTWAVE TROUGH AXIS SWINGS THROUGH. LOWS OF 19-27 TONIGHT ARE A SHADE BELOW MOS GUIDANCE NUMBERS IN THE WEST BUT MATCH WELL WITH UPSTREAM LOWS LAST NIGHT. THE POLAR HIGH NOW CENTERED OVER EAST KS IS STILL EXPECTED TO BRING FRIGID TEMPERATURES FOR SUNDAY... WITH FORECAST 850 MB TEMPERATURES DOWN TO -12 TO -13C AND ANOMALOUSLY LOW 1000-850 MB THICKNESSES IN THE 1240S. AS THE SURFACE FRONT WILL HOLD OFF PASSING THROUGH CENTRAL NC UNTIL LATE TONIGHT... THE INCREASE IN WINDS WILL FORTUNATELY NOT OCCUR UNTIL NEAR OR SHORTLY AFTER DAYBREAK... BUT NEVERTHELESS THE WIND CHILL VALUES ARE LIKELY TO DROP WELL DOWN INTO THE TEENS THROUGH SUNDAY MORNING AND EARLY AFTERNOON. CLEAR SKIES ARE STILL ANTICIPATED WITH FORECAST SOUNDINGS INDICATING A DRY COLUMN. LOCAL HIGH TEMPERATURE TECHNIQUE WITH A NEARLY DRY ADIABATIC PROFILE FROM 900 MB DOWN INDICATES HIGHS OF 30-37... A SLIGHT DOWNWARD BUMP FROM THE PREVIOUS FORECAST. THE 1040 MB HIGH DRIFTS OVERHEAD SUNDAY NIGHT AND WILL HAVE LOWS OF 10-18 DEGREES UNDER CLEAR SKIES. THIS WILL BE THE COLDEST NIGHT OF THIS WINTER SEASON... AND BASED ON UPSTREAM LOWS LAST NIGHT... EVEN A FEW SINGLE DIGIT READINGS ARE LIKELY. FOR MONDAY THROUGH TUESDAY NIGHT... DRY AND STABLE CONDITIONS PERSIST THROUGH MONDAY. THICKNESSES REBOUND SLIGHTLY BUT THE AIR MASS CHARACTER AND COLD START TO THE DAY STILL FAVOR MUCH BELOW NORMAL HIGHS OF 37-44. THE SURFACE HIGH SHIFTS OFF TO THE ENE MONDAY NIGHT INTO TUESDAY... PROPELLED BY A POSITIVELY TILTED MID LEVEL TROUGH DROPPING FROM CENTRAL CANADA INTO THE UPPER MIDWEST... AND ITS ATTENDANT COLD FRONT ALONG WHICH A SURFACE LOW TRACKS ACROSS THE PLAINS INTO THE GREAT LAKES. LOW LEVEL RETURN FLOW AROUND THE SURFACE HIGH SHOULD BRING AN INCREASE IN CLOUDS MONDAY NIGHT... PRIMARILY IN WESTERN NC WHERE ISENTROPIC UPGLIDE BEGINS EARLY TUESDAY AND IN COASTAL SECTIONS ALONG A MODEL-INDICATED INVERTED TROUGH. THIS UPGLIDE STRENGTHENS TUESDAY... AND WHILE THE MID LEVEL FLOW REMAINS QUITE FLAT WITH MINOR PERTURBATIONS KICKING OUT OF THE DESERT SOUTHWEST DISSIPATING BEFORE REACHING NC... THE STRENGTHENING UPPER JET TO OUR WEST MAY PROVIDE LARGE SCALE SUPPORT FOR LIFT AND ENHANCE THE EFFECTS OF THE ISENTROPIC UPGLIDE. WILL BRING IN CHANCE POPS TUESDAY THROUGH TUESDAY NIGHT... UNTIL THE COLD FRONT DROPS THROUGH CENTRAL NC IN THE PREDAWN HOURS WEDNESDAY. THE DIFFUSE NATURE OF THE FORCING AND THE LACK OF A DISCERNABLE FOCUS SHOULD LEAD TO MODEST PRECIP AMOUNTS. FORECAST SOUNDINGS AND MSLP PROGS DEPICT AN IN SITU COOL POOL OVER THE NORTHWEST CWA... WHICH MAY SUPPORT A LITTLE FREEZING RAIN AT THE ONSET IN THE TRIAD AREA EARLY TUESDAY... BUT WHILE SUCH A RESIDUAL COOL STABLE POCKET IS NOT OUT OF THE QUESTION GIVEN THE VERY DRY NATURE OF THE EXITING AIR MASS... IT IS FAR TO EARLY TO BE ABLE TO PIN THIS DOWN. WILL MENTION LIQUID PRECIP ONLY FOR NOW. WILL HAVE LOWS OF 24-29 MONDAY NIGHT... AND HIGHS OF 43-50 TUESDAY. -GIH && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... AS OF 300 PM SATURDAY... PRECIP SHOULD QUICKLY EXIT THE COASTAL PLAIN AROUND DAYBREAK WEDNESDAY AS THE COLD FRONT SHOULD BE TRACKING OFF THE COAST. HIGH PRESSURE BUILDS IN WEAKLY FROM THE WEST AT THE SURFACE AND THE ECMWF INDICATES DOWNSLOPE LOW LEVEL FLOW AT 12Z WEDNESDAY FAVORING A MORE RAPID CLEARING TREND THAN THE 12Z OP GFS INDICATES... AND WHILE THIS CLEARER SOLUTION IS PREFERRED... THE STRENGTHENING 250 MB JET SHOULD BRING A WIDESPREAD SHIELD OF HIGH CLOUDS WEDNESDAY WITH OROGRAPHIC ENHANCEMENT POSSIBLE. 1000-850 MB THICKNESSES DROP BACK DOWN INTO THE 1290S ALTHOUGH THIS LAYER IS CAPTURING A PART OF THE STABLE LAYER ALOFT... AND BASED ON FORECAST SOUNDINGS... HIGHS MAY STRUGGLE TO GET PAST THE LOWER 40S IN THE PIEDMONT ESPECIALLY IF THE SHROUD OF HIGH CLOUDINESS IS THICK. ANOTHER POTENTIAL SYSTEM OF CONCERN ARRIVES THURSDAY AS ANOTHER SHORTWAVE TROUGH DROPS THROUGH THE MIDATLANTIC AND NORTHEAST... BUT THE ECMWF AND GFS BEGIN TO DIFFER SIGNIFICANTLY. THE ECMWF IS MORE AMPLIFIED WITH THIS WAVE AND THUS SUPPRESSES THE SURFACE HIGH SOUTHWARD OVER THE CAROLINAS AND SOUTHEAST AND HOLDS A DEVELOPING SURFACE LOW WELL DOWN OVER THE GULF... WHEREAS THE GFS IS LESS AMPLIFIED WITH A MORE NORTHERLY SURFACE HIGH AND LOW CLOSER TO THE GULF COAST... BRINGING MORE PRECIP INTO THE SOUTHEAST STATES. THE GFS HAS WAVERED IN THE LAST FEW RUNS AND THE MORE STABLE AND DRIER ECMWF IS PREFERRED. CONSIDERABLE CLOUD COVER IS STILL EXPECTED HOWEVER AS THE WAVE THAT DROPS THROUGH IS RATHER POTENT BUT SHOULD BE OPERATING ON A LOW-PW COLUMN. HIGHS THURSDAY SHOULD REMAIN WELL BELOW NORMAL... AROUND 40 TO 46. ANOTHER SURFACE HIGH PASSES OVERHEAD THROUGH FRIDAY WITH BRIEF LOW AMPLITUDE RIDGING IN THE MID LEVELS... AND CONTINUED COOL CONDITIONS ARE EXPECTED. THE GFS/ECMWF BRING IN ANOTHER CHANCE OF PRECIPITATION OVER THE WEEKEND AS BROAD TROUGHING KICKS INTO THE PLAINS... HOWEVER THEY DO NOT AGREE ON TIMING OR THERMAL STRUCTURE... AND PREFER TO KEEP ANY POPS ON THE LOW SIDE WITH BELOW NORMAL TEMPERATURES LIKELY TO PERSIST UNDER CONSIDERABLE CLOUD COVER. -GIH && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... AS OF 1255 PM SATURDAY... PRECIPITATION RAPIDLY BRAKING OUT ACROSS THE PIEDMONT OF NC. THIS PRECIPITATION IS IN REPSONSE TO MID LEVEL FRONTOGENETICAL FORCING AND LIFT IN THE RIGHT REAR OF THE UPPER JET. HAD A FEW REPORTS OF SNOW AND GRAUPEL IN THE TRIAD AREA. COLD AND ESPECIALLY DRY AIR IS MAKING ITS WAY INTO THE AREA WITH DEWPOINTS DOWN TO NEAR 20 IN THE TRIAD AND LOW 30S ACROSS THE SOUTHERN AND NORTHAST PIEDMONT. SFC TEMPS ARE STILL MILD...BUT EVAPORATIONAL COOLING SHOULD INITIALLY DROP TEMPS SEVERAL DEGREES AND THEN MELTING SHOULD COOL THE REAMINDER OF THE COLUMN TO NEAR FREEZING. THIS WILL ALLOW THE ADVERTISED TRANSITION FROM RAIN TO SNOW TO OCCUR THIS AFTERNOON. PRECIP SHOULD START OUT AS SNOW IN THE TRIAD (INT AND GSO). RAIN AT RDU SHOULD CHANGEOVER BETWEEN 18 AND 20Z WITH THE CHANGEOVER OCCURING AT RWI AND FAY BETWEEN 20 AND 22Z. CURRENTLY VFR CONDITIONS ARE REPORTED IN THE PIEDMONT (RDU...GSO AND INT) BUT THIS SHOULD BE SHORT LIVED AS PRECIP WILL BRING VSBYS AND CIGS DOWN TO MVFR TEMPO IFR FOR MUCH OF THIS AFTERNOON AND EARLY EVE. ACROSS THE COASTAL PLAIN LIGHT RAIN HAS REDUCED VSBYS TO 3 TO 4 MILES AT FAY AND RWI WITH VFR CIGS. AS PRECIP BECOMES STEADIER SHOULD SEE BOTH CIGS AND VSBYS DROP TO MVFR AND THEN IFR CONDITIONS THIS AFTERNOON. ALL OF THE PRECIP SHOULD TAPER OFF CLOSE TO SUNSET IN THE TRIAD... AROUND 9PM NEAR RDU AND TOWARD MIDNIGHT IN FAY. RAPID DRYING WILL MOVE OVERNIGHT AND SUNDAY SHOULD FEATURE VFR CONDITIONS WITH A NW FLOW. AS THE HIGH PRESSURE AREA MOVES OFF THE NORTHEAST COAST TUESDAY... A MOIST SOUTHWEST FLOW WILL LEAD TO INCREASING MOISTURE AND A LOWERING OF CLOUD BASES. MVFR CONDITIONS WITH SCATTERED SHOWERS WILL BE POSSIBLE TUESDAY AND TUESDAY NIGHT. HIGH PRESSURE WILL BUILD BACK OVER THE AREA WEDNESDAY. -BLAES && .RAH WATCHES/WARNINGS/ADVISORIES... WINTER WEATHER ADVISORY UNTIL 6 AM EST SUNDAY FOR NCZ077-078- 085-086-088-089. WINTER WEATHER ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR NCZ073>075-083-084. WINTER STORM WARNING UNTIL MIDNIGHT EST TONIGHT FOR NCZ007>010- 023>026-040-041-076. WINTER WEATHER ADVISORY UNTIL 9 PM EST THIS EVENING FOR NCZ021- 022-038-039. WINTER STORM WARNING UNTIL 6 AM EST SUNDAY FOR NCZ011-027-028- 042-043. && $$ SYNOPSIS...DJF/MWS NEAR TERM...BLAES/MWS SHORT TERM...HARTFIELD LONG TERM...HARTFIELD AVIATION...BLAES