WFO RAH Logo

WFO RAH Text Data

FXUS62 KRAH 190239
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
939 PM EST FRI JAN 18 2008

.SYNOPSIS...

LOW PRESSURE DEVELOPING OVER THE WESTERN GULF OF MEXICO WILL 
APPROACH THE AREA FROM THE SOUTHWEST TONIGHT...AND MOVE NORTHEAST 
ALONG THE GEORGIA AND SOUTH CAROLINA COAST SATURDAY. WINTRY 
PRECIPITATION IS A POSSIBILITY FOR SATURDAY. A VERY COLD AIR MASS 
WILL OVERTAKE THE REGION SATURDAY NIGHT AND SUNDAY. 

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
AS OF 900 PM FRIDAY...
LOW STRATUS OVER MUCH OF CENTRAL NORTH CAROLINA. THERE ARE
INDICATIONS IN THE MODELS THAT CLOUDS MAY BECOME LESS WIDESPREAD
TOWARD DAWN AS SURFACE WINDS SHIFT TOWARD THE NORTH TO NORTHEAST.
THUS MANY LOCATIONS WILL BE VERY SLOW TO FALL...WITH TEMPERATURES
PERHAPS TAKING A DIP TOWARD SUNRISE. HAVE MINIMALLY RAISED LOWS...
MOSTLY IN THE NORTHWEST.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY/...
AS OF 900 PM FRIDAY... 
ADVISORIES AND WARNINGS WILL CONTINUE AS POSTED. 
AS USUAL...A DIFFICULT FORECAST SUBJECT TO CHANGE IN AN INSTANT. 
HAVE RISKED LOWERING SNOW ACCUMULATIONS SLIGHTLY AND LIQUID 
EQUIVALENT AND AM IN AGREEMENT WITH HPC AND THE ECMWF AND GFS MODELS.

SEVERAL CONCERNS WITH THIS SYSTEM. OF MOST CONCERN IS THE ALMOST
TOTAL LACK OF DYNAMICS BELOW 850 MILLIBARS...IN SPITE OF THE
FORECAST COASTAL LOW DEVELOPMENT. STRONGEST LIFT IS CENTERED NEAR
600 MILLIBARS FOR THE EVENT WHICH IS ABNORMALLY HIGH ABOVE THE
GROUND. ISENTROPIC LIFT IS SLIM TO NONE BELOW 850 MILLIBARS...AS
IS ANY SIGNIFICANT CONVERGENCE. IT IS DIFFICULT TO GET COPIOUS
AMOUNTS OF PRECIPITATION WITH SO LITTLE LIFT AT LOW LEVELS. 850
MILLIBAR WIND SPEEDS AND TRAJECTORIES SHOW MOISTURE INFLUX IS
LARGELY CUT OFF FOR THE EVENT...WITH THE POSSIBLE EXCEPTION OF
THE COASTAL PLAIN. ALL MODELS SHEAR THE STRONG VORTICITY CENTER IN
MEXICO TO ALMOST NOTHING AS IT APPROACHES NORTH CAROLINA. THERE IS
AN EXCELLENT 250 MILLIBAR JET...BUT IT STILL NEEDS SOMETHING TO
WORK WITH NEAR THE SURFACE.

WITH LOW TEMPERATURES AT SUNRISE SATURDAY NO LOWER THAN THE MID
30S THE GROUND WILL START WARM. WITH SLEET OR FREEZING RAIN EITHER
EXTREMELY BRIEF OR NONEXISTENT...EVEN MODERATE SNOWS WILL WILL
MELT FOR A TIME UNTIL HEAT IS SUFFICIENTLY EXTRACTED FROM THE
GROUND. IT LOOKS LIKE ALL SNOW IN THE NORTHWEST PIEDMONT... BUT
MOISTURE COULD BE LACKING...AND A PORTION OF WHAT FALLS WILL GO
TOWARD MELTING AND COOLING THE GROUND. FURTHER EAST...MOISTURE IS
BETTER BUT PRECIPITATION MAY START OUT AS RAIN OR A RAIN SNOW MIX
BEFORE TRANSITIONING TO SNOW. BELIEVE THESE PITFALLS WILL CUT INTO
SNOW ACCUMULATIONS. ANOTHER FACTOR IS THE RELATIVE WARMTH OF THE
EVENT... WITH SNOW RATIOS PERHAPS AS LOW AS SIX TO ONE...AND
CERTAINLY NOT LIKELY TO BE MORE THAN EIGHT TO ONE DURING THE
MAJORITY OF THE EVENT.

BRIDGES AND OVERPASSES WILL BECOME HAZARDOUS FIRST SATURDAY...WITH
SHADY AND NORTH FACING ASPECTS TO FOLLOW.

IN FAVOR OF MORE SNOW WOULD BE THE COASTAL LOW TRACKING CLOSER TO
THE COAST...OR DYNAMICS BEING STRONGER...AND GENERATING MORE
PRECIPITATION. IT IS LIQUID EQUIVALENT AMOUNTS THAT WILL DRIVE
THE SEVERITY OF THIS EVENT. THE LATER IN THE DAY THE EVENT
BEGINS...THE LESS LIKELY ANY RAIN WILL BE.

IN THE SOUTHEAST...MOISTURE IS MOST ABUNDANT...BUT THE NECESSARY
COLD AIR IS SLOW TO ARRIVE. THIS IS THE AREA THAT WILL HAVE TO BE
WATCHED CLOSELY AS DIABATIC COOLING WITH MODERATE TO HEAVY
PRECIPITATION SHOULD SEND THE LOW LEVEL SOUNDING TO ISOTHERMAL
AROUND FREEZING. IF THE PRECIPITATION IS HEAVY ENOUGH...
SNOW ACCUMULATIONS MAY BE CURRENTLY UNDERDONE.

ANY SIGNIFICANT PRECIPITATION APPEARS TO BE OVER BEFORE OR SHORTLY
AFTER MIDNIGHT... ALTHOUGH SOME MINOR SNOW SHOWERS MAY LINGER IN
THE NORTHEAST.

WITH TEMPERATURES BELOW FREEZING FOR MUCH OF SATURDAY NIGHT...
THIS WILL PERHAPS BE OF MORE CONCERN THAN THE PRECIPITATION
SATURDAY. THINGS SHOULD FREEZE SOLID IN MOST LOCATIONS AS LOW
TEMPERATURES REACH THE LOW TO MID 20S. TRAVEL COULD BE QUITE
HAZARDOUS. 

HAVE SLIGHTLY LOWERED HIGH TEMPERATURES SATURDAY AND SHOWN MORE OF
A FALL WITH THE BEGINNING OF PRECIPITATION AND INTO THE AFTERNOON.
SUNDAY HIGH TEMPERATURES ACCEPTED.


AS OF 400 PM FRIDAY...
STRONGLY DYNAMIC SYSTEM CONTINUES TO DIG INTO THE WESTERN GULF OF
MEXICO ...IMPRESSIVE WATER VAPOR LOOP... WITH DEEP SOUTHWEST FLOW 
DEVELOPING ACROSS THE SOUTHEASTERN UNITED STATES. PRECIP WILL SPREAD 
VERY RAPIDLY UP THE ATLANTIC COAST OVERNIGHT AS THE GULF SYSTEM 
INTERACTS WITH A NORTHERN STREAM TROUGH...INDUCING CYCLOGENESIS. THE 
ENSUING LOW WILL MOVE UP THE COAST...PASSING OFF THE NC COAST 
TOMORROW AFTERNOON.

EXPECT TO SEE RAIN INTO THE SOUTHERN TIER OF THE CWFA BY SUNRISE... 
SPREADING ALL THE WAY TO THE VA BORDER BY LATE MORNING. INITIALLY... 
THERMAL PROFILES WILL SUPPORT SNOW...PERHAPS MIXED WITH SOME 
RAIN...IN THE FAR NORTHWEST (TRIAD AREA)... AND RAIN ELSEWHERE.

COLD AIR WILL BE EDGING IN FROM THE NORTHWEST AS THE DAY 
PROGRESSES...AIDED BY EVAPORATIVE COOLING. WILL SEE A CHANGEOVER 
FROM RAIN TO A MIX OF RAIN AND SNOW ...THEN FINALLY TO ALL 
SNOW...EASTWARD IN THE AFTERNOON OVER THE PIEDMONT. ALONG INTERSTATE 
95 AND EASTWARD...CHANGEOVER WILL BE IN THE MID TO LATE AFTERNOON.

PRECIPITATION AXIS LOOKS TO LIE GENERALLY ALONG AND EAST OF 
INTERSTATE 95...BUT THIS IS SUBJECT TO ERRORS IN THE TRACK AND 
STRENGTH OF THE LOW...WHICH HAS YET TO GEN UP. STRONG DYNAMICS AND 
UPPER DIFFLUENCE IN THE ENTRANCE OF A 170KT JET WOULD POINT TOWARDS 
ERRORS BEING ON THE LOW SIDE...WITH STRONGER CYCLOGENESIS AND MORE 
PRECIPITATION. AT THIS TIME...EXPECT 1 TO 3 INCHES IN THE NORTHWEST 
WHICH WILL HAVE PREDOMINANT SNOW...BUT LESSER AMOUNTS DUE TO 
DISTANCE FROM THE STORM. SNOW ACROSS THE REMAINDER OF THE PIEDMONT 
WILL RANGE FROM 2 TO 4 INCHES IN THE TRIANGLE AREA...WHERE THE SNOW 
WILL TAPER OFF AFTER MIDNIGHT. THREE TO FIVE INCHES ARE FORECAST 
FOR THE NORTHERN COASTAL PLAIN...ROCKY MOUNT...TARBORO...ROANOKE 
RAPIDS AREAS...WHERE SNOW WILL LINGER THROUGHOUT THE NIGHT...ALBEIT 
LIGHTER TOWARDS MORNING. 

THE CENTRAL AND SOUTHERN COASTAL PLAIN ARE MORE PROBLEMATIC...AS 
THEY WILL BE INITIALLY WARMER AND THE RAIN CHANGEOVER TO SNOW WILL 
OCCUR LATER IN THE DAY. AT THIS TIME WILL ISSUE A WINTER STORM WATCH 
FROM 4 PM THROUGH TOMORROW NIGHT...THERE IS UNCERTAINTY AS TO 
EXACTLY WHEN THE CHANGEOVER WILL OCCUR AND HOW MUCH PRECIPITATION 
CAN BE EXPECTED AFTERWARD.

WILL ISSUE A WINTER STORM WARNING FOR SNOWFALL OF 3 TO 5 INCHES FROM 
ALAMANCE COUNTY AND AREAS  EAST...ACROSS THE NORTHERN 
TIER...INCLUDING THE CENTRAL AND NORTHERN PIEDMONT AND NORTHERN 
COASTAL PLAIN COUNTIES. 

ELSEWHERE ACROSS THE FORECAST AREA...A WINTER STORM ADVISORY WILL BE 
ISSUED FOR SNOWFALL ACCUMULATIONS OF 1 TO 3 INCHES...BEGINNING AS 
EARLY AS THE MORNING HOURS IN THE TRIAD...AND AS LATE AS THE EVENING 
HOURS IN THE SOUTHERN COASTAL PLAIN AND SANDHILLS. SEE RDUWSWRAH FOR 
FULL DETAILS. 

SLOWER TIMING OF THE NAM WITH NEGATIVELY TILTING VORTICITY MAXIMUM 
SWINGING ACROSS THE AREA COMBINED WITH STRONG COLD HORIZONTAL AND 
VERTICAL THERMAL ADVECTION...RESULTS IN RE-GENERATION OF MOISTURE IN 
DENDRITIC GROWTH ZONE...BELOW 700 MB. ADDITIONALLY THE NAM SHOWS 
STRONG MID FRONTOGENETIC FORCING...WITH THE POTENTIAL FOR SLANTWISE 
CONVECTIVE BURSTS OF SNOW OR FLURRIES AS SYSTEM EXITS THE AREA. THUS 
WILL WILL INTRODUCE A SLIGHT CHANCE OF SNOW OR FLURRIES SUNDAY 
MORNING ACROSS THE NORTHERN COASTAL PLAIN THROUGH 15Z. 

HIGH TEMPERATURES SUNDAY WILL GREATLY DEPEND ON PENDING SNOW 
COVER...AS FULL SUN THICKNESSES WOULD SUPPORT HIGHS IN THE LOWER 30S 
NORTH TO MID 30S SOUTH.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
AS OF 300 PM FRIDAY...
SUNDAY NIGHT...LOOK FOR COLDEST NIGHT DURING THIS ARCTIC OUTBREAK TO 
OCCUR SUNDAY NIGHT/MONDAY MORNING...WITH MODEL CONSENSUS INDICATING 
OPTIMAL RADIATIONAL CONDITIONS AS THE STRONG 1039 TO 1042 MB SURFACE 
HIGH BUILDS DIRECTLY OVER THE AREA......BEFORE PUSHING OFFSHORE 
DURING THE DAY ON MONDAY. MINIMUM TEMPERATURES SUNDAY NIGHT WILL
PLUNGE EASILY INTO THE LOWER TO MID TEENS...OR EVEN COLDER...WITH
THE POTENTIAL FOR RESIDUAL SNOW COVER. OBVIOUSLY...BLACK ICE WILL
ONCE AGAIN POSE A MAJOR THREAT ACROSS ALL OF CENTRAL NC AS ANY
SNOW MELT OR RESIDUAL MOISTURE ON ROADS WILL FORM A THIN GLAZE OF
ICE. WILL HIGHLIGHT THE THREAT IN THE HAZARDOUS WEATHER OUTLOOK.

THEN THE MODERATION OF THE FRIGID CONDITIONS BEGIN DURING THE DAY ON 
MONDAY AS SOUTHWEST LOW-LEVEL WARM AIR ADVECTION DEVELOPS IN
RESPONSE TO HIGH PRESSURE MOVING OFFSHORE AND RISING HEIGHTS
ALOFT. HIGHS IN THE UPPER 30S NW TO MID 40S SE.

TUESDAY THROUGH FRIDAY...LOW PREDICTABILITY INVOLVING SHORTWAVE WAVE 
IMPULSES IN A PROGRESSIVE ZONAL FLOW RESULTS IN LOW CONFIDENCE 
FORECAST OF RAIN CHANCES AND MOISTURE AVAILABILITY THROUGH THE 
PERIOD. WILL KEEP SLIGHT CHANCE POPS IN FOR TUESDAY THROUGH 
WEDNESDAY WITH NORTHERN STREAM CLIPPER PASSES THROUGH THE GREAT 
LAKES. BEST CHANCE FOR RAIN LOOKS TO BE TOWARDS THE END OF THE WEEK 
WITH APPROACHING SOUTHERN STREAM INVERTED SURFACE TROUGH.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
AS OF 900 PM FRIDAY...LOW IFR/LIFR STRATUS CONTINUES ACROSS MUCH OF 
THE WESTERN PIEDMONT...SANDHILLS AND SOUTHERN COASTAL PLAIN THIS 
EVENING. MODEL GUIDANCE IS OBLIVIOUS TO THIS LOW CLOUD COVER ACROSS 
THE AREA...AND THEREFORE NOT MUCH HELP IN DETERMINING WHEN THIS 
STRATUS LAYER MAY LIFT. BASED ON TRENDS THE PAST SEVERAL HOURS...IN 
ADDITION TO LIGHT WINDS AND A NOCTURNAL INVERSION PREVENTING ANY 
MIXING...EXPECT THAT THIS IFR/LIFR STRATUS WILL PREVAIL THROUGH 
08-13Z ACROSS MOST TAF SITES...WITH EROSION BEGINNING SOMETIME 
DURING THAT TIME FRAME AS THE MSLP GRADIENT PICKS UP AND NORTHERLY 
LOW LEVEL WINDS INCREASE. MVFR VIS IN FOG POSSIBLE ACROSS THE AREA 
THROUGH THE NIGHT.

ACCORDING TO THE LATEST MODEL GUIDANCE...PRECIP SHOULD BEGIN TO 
ARRIVE ACROSS THE AREA BY 15Z AS ALL SNOW OR A RAIN/SNOW 
MIX...TRANSITIONING QUICKLY TO SNOW IN MOST AREAS BY 18Z (EXCEPT SE 
COASTAL PLAIN WHERE THE TRANSITION TO SNOW WILL OCCUR LATER IN THE 
AFTERNOON). IFR/LIFR CONDITIONS IN LOW CEILINGS AND VISBYS ARE 
EXPECTED FROM MID/LATE MORNING THROUGH ROUGHLY 00Z BEFORE PRECIP IS 
EXPECTED TO QUICKLY WIND DOWN FROM WEST TO EAST.

HIGH PRESSURE WITH AN ARCTIC AIRMASS WILL ARRIVE LATE SAT NIGHT TO 
IMPROVE CONDITIONS TO VFR BY DAYBREAK SUN. VERY COLD AIR IS EXPECTED 
FOR LATE SAT NIGHT-MON NIGHT.  OTHERWISE THE DRY AIRMASS SHOULD 
MAINTAIN VFR CONDITIONS.  

FOR TUE...MOIST SWLY FLOW AND INCREASING MOISTURE MAY LEAD TO 
LOWERING STRATOCUMULUS CLOUD BASES AND POSSIBLE MVFR CONDITIONS WITH 
SCATTERED SHOWERS. -VINCENT/RFG

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 4 PM SATURDAY TO 6 AM EST SUNDAY 
FOR NCZ077-078-085-086-088-089.
WINTER WEATHER ADVISORY FROM 12 PM SATURDAY TO MIDNIGHT EST 
SATURDAY NIGHT FOR NCZ073>075-083-084.
WINTER STORM WARNING FROM 12 PM SATURDAY TO MIDNIGHT EST 
SATURDAY NIGHT FOR NCZ007>010-023>026-040-041-076.
WINTER WEATHER ADVISORY FROM 6 AM TO 9 PM EST SATURDAY FOR 
NCZ021-022-038-039.
WINTER STORM WARNING FROM 4 PM SATURDAY TO 6 AM EST SUNDAY FOR 
NCZ011-027-028-042-043.

&&

$$

LONG TERM...CBL
AVIATION...VINCENT/RFG