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FXUS62 KRAH 161934
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
230 PM EST WED JAN 16 2008

.SYNOPSIS...

COLD HIGH PRESSURE CENTERED OVER NORTH CAROLINA...WILL SHIFT 
OFFSHORE THIS EVENING.  MEANWHILE... A LOW PRESSURE SYSTEM WILL 
DEVELOP OVER THE GULF OF MEXICO...AND TRACK NORTHEAST ALONG MID 
ATLANTIC COAST ON THURSDAY.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
AS OF 925 AM WED...
MOISTURE IS INCREASING ALOFT AT CIRRUS LEVEL AND WITH RELATIVELY 
UNIDIRECTIONAL WNW FLOW AND MID LEVEL STABILITY...WE MAY SEE SOME 
OROGRAPHIC ENHANCEMENT ADDED TO THE WEAK ISENTROPIC LIFT TO THICKEN 
CIRRUS ACROSS THE AREA TODAY.  LOWER LEVEL THICKNESS INDICATES MAX 
NEAR-FULL SUN TEMPERATURE READINGS IN THE 42-46 RANGE.  CURRENT 
READINGS ARE AVERAGING 3-4 DEG BELOW YDA AT THIS TIME AND ABOUT 1 
DEG BELOW FORECAST.  FORECAST TEMPS FOR TODAY WILL BE SHADED 1-2 DEG 
FROM PREVIOUS FORECASTS.  DEW POINTS ARE 4-6 DEG DRIER THAN YDA AND 
CLOSE TO FORECAST.

UA ANALYSES FROM 12Z SHOW SOME HIGHLY AGEOSTROPHIC FLOW AT MID 
LEVELS UPSTREAM AT KLCH AND KJAX...PERHAPS INDICATIVE OF 
CONVECTIVELY INDUCED SUBSIDENCE TO THE NORTH OF DEEP CONVECTION IN 
THE DELTA REGION NOW.  DEEP CONVECTION ALSO APPEARS SLIGHTLY NORTH 
OF HEAVIEST CONVECTIVE PCPN SHOWN BY THE MODELS.  ASSOCIATED LATENT 
HEATING MAY BE CONTRIBUTING TO THE MID LEVEL WARMING SEEN AT KLCH 
AND KJAX AND MAY CONTRIBUTE POTENTIAL VORTICITY TO FURTHER ENHANCE 
850-700 MB SELY FLOW...OR LOW LEVEL INFLOW JET... AHEAD OF THE 
CONVECTION.  THE IMPACT OF DEEP CONVECTION WILL NEED TO BE FACTORED 
INTO THE DOWNSTREAM QPF OVER INTERIOR SECTIONS. OVERALL PCPN
SHIELD IS CLOSE TO MODEL PROJECTIONS. -RFG

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/...
AS OF 230 AM WEDNESDAY...THE WINTER STORM WATCH AREA (VALID
09-21Z THURSDAY) HAS BEEN UPGRADED TO A WINTER STORM WARNING.
A WINTER WEATHER ADVISORY HAS BEEN ISSUED ELSEWHERE FOR LOCATIONS
ALONG AND WEST OF US HIGHWAY 1. LOCATIONS THAT WILL BE AFFECTED BY
THE WINTER WEATHER ADVISORY INCLUDE (BUT ARE NOT LIMITED TO) THE
TRIANGLE... SANFORD...SOUTHERN PINES...ROCKINGHAM...WADESBORO...
ALBEMARLE...OXFORD AND WARRENTON.

TONIGHT AND THURSDAY...
1030 MB HIGH PRESSURE IS PROGGED TO BE CENTERED OVER NC/VA BY SUNSET 
THIS EVENING. THE LOW PRESSURE SYSTEM CURRENTLY LOCATED IN THE 
WESTERN GOMEX WILL MOVE NORTHEAST ACROSS THE FLORIDA PANHANDLE 
TONIGHT...EMERGING ALONG OR JUST OFF THE GA/SC COAST BY SUNRISE 
THURSDAY. THE LOW PRESSURE SYSTEM WILL SUBSEQUENTLY MOVE ALONG OR 
JUST OFFSHORE THE SC/NC COAST DURING THE DAY THURSDAY. THE GFS...NAM 
AND ECMWF ARE IN VERY GOOD AGREEMENT ON THE GENERAL TRACK/EVOLUTION 
OF THE LOW PRESSURE SYSTEM DURING THIS TIME FRAME.

THE LATEST 00Z NAM/GFS RUNS ARE GENERALLY CONSISTENT WITH THE 
PREVIOUS RUNS W/REGARDS TO THE LOW TRACK/EVOLUTION...HOWEVER...THEY 
ARE BOTH SLIGHTLY COLDER AND WETTER THAN PREVIOUS RUNS. THIS WAS A 
MAJOR REASON FOR UPGRADING THE WINTER STORM WATCH TO A WARNING...AND 
ISSUING A WINTER WEATHER ADVISORY ELSEWHERE ALONG/WEST OF US HIGHWAY 
1. ALTHOUGH THIS IS NOT A CLASSIC WINTER STORM SETUP SINCE NO 
CLASSIC CAD EVENT WILL BE PRESENT...STRONG HYBRID DAMMING WILL 
NONETHELESS ALLOW FOR FROZEN PRECIPITATION ACROSS THE ADVISORY/ 
WARNING AREA.

MORE SPECIFICALLY...A 1030 MB HIGH WILL INITIALLY BE IN PLACE ACROSS 
CENTRAL NC/VA ON THURSDAY MORNING...ALONG WITH A VERY COOL/DRY 
AIRMASS. ALTHOUGH THE SFC HIGH IS TRANSIENT AND MOVES NORTHEAST WELL 
AWAY FROM THE MID-ATLANTIC DURING THE DAY THURSDAY...THE VERY DRY 
AIR (DEWPOINTS BETWEEN 5-15F) IN PLACE DURING THE MORNING HOURS WILL 
SET THE STAGE FOR STRONG EVAPORATIVE COOLING ONCE PRECIP BEGINS TO 
OVERSPREAD THE AREA FROM S/SW TO N/NE BETWEEN 06-13Z (1-8 AM). 
SURFACE TEMPS ARE EXPECTED TO BE STEADY IN THE UPPER 20S TO LOWER 
30S IN MOST OF THE AREA DURING THE PREDAWN HOURS BEFORE PRECIP 
BEGINS...FALLING SEVERAL DEGREES ONCE PRECIP DOES BEGIN. BASED ON 
THICKNESSES OFF THE LATEST GFS/NAM RUNS...PRECIP IS INITIALLY 
EXPECTED TO BEGIN AS A MIXTURE OF SNOW/SLEET.

HOWEVER...STRONG EASTERLY AND SOUTHEASTERLY FLOW ALOFT WILL ALLOW 
RAPID WARMING IN THE 850-700 MB LAYER FROM SE TO NW ACROSS CENTRAL 
NC BETWEEN 06-15Z...CONTINUING THROUGH LATE MORNING INTO THE 
AFTERNOON HOURS. THIS WILL ALLOW FOR A CHANGEOVER TO FREEZING RAIN 
AND THEN RAIN ACROSS THE ENTIRE AREA FROM SE TO NW THROUGHOUT THE 
DAY. AS A RESULT...ASIDE FROM A VERY BRIEF PERIOD OF SNOW/SLEET AT 
THE ONSET...ALL RAIN IS EXPECTED IN THE COASTAL PLAIN. FURTHER TO 
THE NORTH/WEST...THE CHANGEOVER TO ALL RAIN WILL TAKE LONGER...AND 
AS A RESULT...A BRIEF PERIOD OF ACCUMULATING SNOW/SLEET FOLLOWED BY 
A PERIOD OF ACCUMULATING FREEZING RAIN IS LIKELY. THE TRIAD / NW 
PIEDMONT REGION WILL EXPERIENCE FROZEN PRECIP THE LONGEST BEFORE THE 
CHANGEOVER TO RAIN OCCURS...WITH 0.10-0.25" OF ICE ACCRUAL ASSOC/W 
FREEZING RAIN POSSIBLE GIVEN THE LATEST QPF AMOUNTS OFF THE NAM/GFS. 
MINOR ACCUMULATIONS OF ICE ASSOC/W FREEZING RAIN ARE LIKELY AS FAR 
EAST AS THE TRIANGLE. PLEASE SEE WSWRAH FOR MORE DETAILS.

THERE ARE A FEW LIMITING FACTORS FOR THIS UPCOMING WINTER STORM 
THREAT. ONE IS THAT THE COLD SURFACE HIGH IS FORECAST TO BE 
PROGRESSIVE. OTHER FACTORS INCLUDE...THE MID LEVELS ARE FORECAST TO 
RAPIDLY WARM AS THE PRECIPITATION BEGINS TO FALL...AND THE EVENTUAL 
EXPECTED PREDOMINATE P-TYPE OF FREEZING RAIN IN THE W/NW PART OF 
CENTRAL NC IS EXPECTED TO BE A SELF LIMITING PROCESS DUE TO THE 
ABSENCE OF LOW LEVEL COOL/DRY AIR ADVECTION.

ANOTHER BIG FACTOR WILL BE THE TIMING OF THE ONSET OF PRECIP ACROSS 
THE AREA THURSDAY MORNING. THE EARLIER THAT PRECIP BEGINS TO 
FALL...THE MORE FROZEN PRECIP WILL OCCUR BEFORE THE CHANGEOVER TO 
RAIN.

OUTSIDE THE WARNING/ADVISORY AREA...A MIX OF SNOW AND SLEET WILL BE 
POSSIBLE AT THE VERY ONSET OF PRECIP...CHANGING QUICKLY TO ALL RAIN 
BY OR SHORTLY AFTER SUNRISE. PREDOMINATELY RAIN IS EXPECTED IN THE 
SANDHILLS AND COASTAL PLAIN. HIGHS 35-45 NW TO SE. -VINCENT

THURSDAY NIGHT AND FRIDAY...
AS OF 2PM WEDNESDAY...PRECIPITATION/DRIZZLE WILL TAPER OFF FROM SW 
TO NE THURSDAY EVENING AS FLOW IN THE 850-700MB LAYER VEERS AROUND 
TO THE WSW...WITH LOW LIFTING NORTHEAST OF THE AREA. FORECAST 
SOUNDINGS KEEP LOWEST 1KFT SATURATED THROUGH 12Z FRI...AS WELL AS 
INCREASE IN HIGH CLOUDS...FAVORING MOSTLY CLOUDY SKIES OVERNIGHT. 
THE CLOUD COVER SHOULD GREATLY INHIBIT RADIATIONAL COOLING...KEEPING 
TEMPERATURES NEARLY STEADY OVERNIGHT...AND LIMITING THE BLACK ICE 
THREAT FROM THE RESIDUAL RUN-OFF TO LOCATIONS NORTH AND WEST OF 
TRIANGLE AND PREDOMINATELY IN RURAL/OUTLYING AREAS.  

THE HIGH CLOUDINESS IS ALSO EXPECTED TO LIMIT THE TEMPERATURE 
RISE...PREVENT THE FULL REALIZATION OF FULL-SUN THICKNESSES ON 
FRIDAY. HIGHS IN THE UPPER 40S NW TO LOWER SE. -CBL 

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
AS OF 200 PM WEDNESDAY...THERE IS HIGH AGREEMENT OF THE LONG WAVE 
PATTERN ACROSS NORTH AMERICA THIS PERIOD...AS THE POSITIVE PHASE OF 
THE PNA IS FORECAST TO CLIMAX THIS WEEKEND...RESULTING IN AN 
ANOMALOUSLY DEEP FULL LATITUDE TROUGH ACROSS THE EASTERN TWO-THIRDS 
OF THE CONUS. GFS ENSEMBLE INDICATES NEGATIVE HEIGHT ANOMALIES ON 
THE MAGNITUDE OF 180 TO 240 METERS OVER THE MID ATLANTIC 
REGION...ALLOWING A FULL BLAST OF ARCTIC AIR INTO THE REGION 
SATURDAY NIGHT THROUGH MONDAY....VERY LIKELY... REPRESENTING THE 
COLDEST OUTBREAK OVER THE CAROLINAS IN A VERY LONG TIME. 

SHORTWAVE ENERGY DIGGING SOUTH OF THE RIO GRANDE AND THROUGH THE 
BASE OF THE LONG WAVE TROUGH...RESULTS IN SFC CYCLOGENESIS IN THE 
GULF OF MEXICO LATE FRIDAY...THAT THEN TRACKS NE ALONG THE SOUTHEAST 
COAST ON SATURDAY. CURRENT MODEL PROJECTIONS INDICATE THE LOW WILL 
TRACK FARTHER EAST AND OFFSHORE THAN TOMORROW'S WINTER 
SYSTEM...SUGGESTING LITTLE TO NO MEASURABLE PRECIPITATION WEST OF 
INTERSTATE 95. WILL KEEP SLIGHT CHANCE OF COLD RAIN WEST TO SMALL 
CHANCE EAST WITH HIGHS SATURDAY IN THE LOWER TO MID 40S. 

THE LATEST GFS AND ECMWF INDICATE 1000-850 THICKNESSES IN THE  
1250-1256M RANGE SUNDAY AFTERNOON. A PERFECT PROG OF THESE 
THICKNESSES AND USING THE IN-HOUSE TEMPERATURE SCHEME WOULD SUPPORT 
HIGHS IN THE LOWER TO MID 20S! GFS STATISTICAL GUIDANCE IS MUCH 
WARMER...WITH HIGHS IN THE MID TO UPPER 30S...OBVIOUSLY ADJUSTED 
TOWARDS WARMER CLIMATOLOGY. PREVIOUS FORECAST WAS ALREADY WELL BELOW 
GUIDANCE...AND WILL CONTINUE THE TREND AS MODELS HAVE NOW 
CONSISTENTLY SHOWN THE ANOMALOUS COLD THICKNESSES FOR SEVERAL RUNS 
NOW. MINS TEMPERATURES SATURDAY NIGHT AND SUNDAY IN IN THE LOWER TO 
MID TEENS. SUNDAY'S HIGHS IN THE UPPER 20 NW TO MID 30S SE. HIGHS
MONDAY IN THE MID 30S NW TO LOWER 40S SE. 

MEAN MID AND UPPER FLOW DAMPENS TO ZONAL FLOW BY MID WEEK. A FLAT 
WAVE APPROACHES THE AREA ON TUESDAY...HOWEVER MOISTURE SHOULD BE 
LIMITED WITH WESTERLY FLOW ALOFT. WILL KEEP POPS BELOW 20 PERCENT 
WITH TEMPERATURES RETURNING TO NEAR NORMAL. 

&&

.AVIATION /17Z WEDNESDAY THROUGH MONDAY/...
AS OF 1205 PM WEDNESDAY...

MODELS ARE REASONABLY CONSISTENT IN SHOWING A COASTAL LOW PRESSURE 
SYSTEM OVER THE NRN GULF OF MEXICO THIS EVENING TRACKING NE UP THE 
COAST OF THE CAROLINAS BY THU EVENING. MOIST FLOW ALOFT VEERING
FROM EAST TO SOUTH WITH HEIGHT AHEAD OF THE LOW WILL OVERRIDE
COLDER AIR AT THE SURFACE OVER CENTRAL NC TO PRODUCE GRADUALLY
LOWERING CLOUD BASES FROM CIRRUS LEVEL TO AROUND 10 THSD FT BY
MIDNIGHT. THEN BETWEEN MIDNIGHT AND DAYBREAK WITH THE ONSET OF
PRECIPITATION FROM THE SOUTH...EXPECT A MORE RAPID DECLINE IN
CLOUD BASES TO IFR AND LIFR CONDITIONS AROUND DAYBREAK.
PRECIPITATION WILL LIKELY TAKE THE FORM OF BRIEF PERIODS OF SNOW
OR SLEET IN THE PREDAWN HOURS...TURNING TO RAIN AROUND DAYBREAK...
WITH VISIBILITY LOWERING BELOW 3 MI. SURFACE TEMPERATURES OF
FREEZING TO SLIGHTLY BELOW FREEZING IN NW SECTIONS OF CENTRAL NC
WILL LIKELY CAUSE FREEZING RAIN AT KINT AND KGSO THROUGH THU
MORNING AND POSSIBLY 1 TO 2 HOURS OF FREEZING RAIN AROUND DAYBREAK
AT KRDU.

RAIN SHOULD TAPER OFF TO DRIZZLE THEN END ACROSS CENTRAL NC THU 
EVENING.  LOW CLOUDINESS WILL LINGER OVER THE AREA IN THE WAKE OF 
THE DEPARTING LOW PRESSURE CENTER...POSSIBLY KEEPING IFR AND LIFR 
CIGS TO NEAR DAYBREAK FRI.  CONDITIONS SHOULD IMPROVE TO VFR WITH 
WEAK HIGH PRESSURE EXPANDING INTO THE AREA FROM THE WEST FRI.

ANOTHER LOW PRESSURE CENTER IS EXPECTED TO TRACK NE FROM THE GULF OF 
MEXICO FRI NIGHT AND SAT...BUT SHOULD REMAIN FAR ENOUGH SOUTH TO 
AVOID LOW CLOUDINESS AND PCPN OVER CENTRAL NC.  THEN HIGH PRESSURE 
AND VERY COLD AIR WILL SPREAD SOUTHEAST INTO THE AREA TO MAINTAIN 
VFR CONDITIONS THROUGH MON.  -RFG


&&

.RAH WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FROM 4 AM TO 4 PM EST THURSDAY FOR NCZ007-
021>023-038-039.
WINTER WEATHER ADVISORY FROM 4 AM TO 12 PM EST THURSDAY FOR 
NCZ008>010-024>026-040-041-073>076-083-084.

&&

$$
SYNOPSIS...RFG
NEAR TERM...RFG
SHORT TERM...VINCENT/CBL
LONG TERM...CBL
AVIATION...RFG