FXUS62 KRAH 051920
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
220 PM EST SAT DEC 5 2009

.SYNOPSIS...

SURFACE LOW PRESSURE LOCATED OFF THE NORTH CAROLINA COAST WILL 
MOVE QUICKLY NORTHEAST ALONG THE MID ATLANTIC COAST THROUGH THIS
AFTERNOON. WIDESPREAD RAIN WILL CONTINUE ACROSS CENTRAL NORTH
CAROLINA...AND MAY MIX WITH WET SNOW OVER THE FAR NORTHERN
PIEDMONT OF NORTH CAROLINA...AND IN AREAS TO THE NORTH AND
WEST...BEFORE TAPERING OFF THIS EVENING... CHILLY HIGH PRESSURE
WILL BUILD IN FROM THE SOUTHWEST LATE TONIGHT THROUGH SUNDAY.

&&

.NEAR TERM /THROUGH SUNDAY NIGHT/... 
AS OF 1128 AM SATURDAY...

NO SIGNIFICANT FORECAST UPDATE MADE OR NECESSARY BASED ON THE 
LATEST OBSERVATIONAL AND MODEL DATA THIS MORNING. WE STILL EXPECT 
PERIODS OF MAINLY LIGHT RAIN AND DRIZZLE TO MIX WITH OR BRIEFLY 
CHANGE TO SNOW MAINLY WEST/NORTH OF INTERSTATE 85 LATE THIS 
AFTERNOON THEN EASTWARD INTO THE NORTHERN COASTAL PLAIN THIS 
EVENING. LITTLE (ON GRASSY AND ELEVATED SURFACES SUCH AS CAR TOPS) 
TO NO ACCUMULATION EXPECTED. 

THE H85 LOW WAS LOCATED NEAR HICKORY PER THE 12Z RAOB ANALYSIS. 
BENEATH DCVA FORCING FOR ASCENT ATTENDANT THE PRIMARY UPPER S/W 
TROUGH OVER THE TN VALLEY... THE H85 SOUTHEAST FLOW ALONG-AHEAD OF 
THE LOW TRACK WILL CONTINUE TO PROMOTE ISENTROPIC UPGLIDE/WAA AND 
ENHANCED FGEN OVER THE NORTHERN NC PIEDMONT NORTHWARD INTO CENTRAL 
VA TODAY. NOT SURPRISINGLY... THOSE LOCATIONS HAVE BEEN WHERE 
STRONGER RADAR RETURNS AND PRECIPITATION RATES (UPWARD OF FIVE 
HUNDREDTHS TO ONE TENTH OF AN INCH AT DANVILLE VA) HAVE BEEN FOUND 
THE LAST SEVERAL HOURS. ALSO OF INTEREST IS THAT THE RAIN HAS 
ALREADY CHANGED TO SNOW AS FAR SOUTHEAST AS LYNCHBURG AND LOUISA VA. 

WE STILL EXPECT THE MID LEVEL DEFORMATION BAND OVER AND JUST 
UPSTREAM OF THE BLUE RIDGE TO CONTINUE TO PIVOT EAST AND MERGE WITH 
THE AFOREMENTIONED LOWER LEVEL-FORCED PRECIPITATION. THE RESULT WILL 
BE CONTINUED GENERALLY LIGHT BUT STEADY RAIN AND DRIZZLE... WITH THE 
GREATEST LIQUID EQUIVALENT AMOUNTS OF ONE TO TWO TENTHS OF AN INCH 
OVER THE NORTHERN COUPLE OF TIER OF NC COUNTIES... TO ONLY A TRACE 
TO A FEW HUNDREDTHS WHERE ISENTROPIC DOWNGLIDE (SOUTH OF THE H85 
LOW) HAS ALREADY DEVELOPED OVER THE SOUTHERN PIEDMONT. THE FORECAST 
THERMAL PROFILE WOULD SUPPORT RAIN MIXED WITH AND BRIEFLY CHANGING 
TO SNOW -- MAINLY FROM THE TRIAD TO CHAPEL HILL TO ROANOKE RAPIDS 
AND POINTS NW) -- BUT CLOUD MICROPHYSICS AND MARKED DRYING ALOFT 
APPROACHING FROM THE UPSTATE OF SC WILL LIMIT SATURATION IN THE 
BERGERON/SNOW GROWTH LAYER. THAT FACTOR... IN ADDITION TO THE 
LIMITED LIQUID EQUIVALENT AMOUNTS BY THE TIME THE COLD AIR ARRIVES 
AND THE RELATIVELY WARM AND WET GROUND CONTINUE TO ARGUE FOR A 
RELATIVE SNOW NON-EVENT THIS TIME AROUND. 

WITH THE PRECIP... THICK CLOUD COVER... AND NEUTRAL THERMAL 
ADVECTION THROUGH MUCH OF THE DAY... TEMPS SHOULD MOVE MINIMALLY 
TODAY. EXPECT HIGHS OF 45-53. WITH RAPID CLEARING TAKING PLACE FROM 
03Z-07Z AND COLD ADVECTION... WE SHOULD SEE LOWS OF 27-33. -GIH

SUNDAY AND SUNDAY NIGHT:
A COLD DRY SURFACE HIGH IS EXPECTED TO SETTLE ACROSS CENTRAL NORTH 
CAROLINA ON SUNDAY... WITH A WEST-SOUTHWESTERLY FLOW ALOFT. THIS 
WILL ALLOW FOR MOSTLY SUNNY SKIES TO BECOME PARTLY SUNNY BY LATE IN 
THE DAY... AS HIGH LEVEL CLOUDINESS BEGINS TO FILTER INTO THE AREA. 
NW WINDS TO START THE DAY WILL BECOME LIGHT AND VARIABLE... AS THE 
HIGH SETTLES ACROSS THE AREA. AFTERNOON LOW LEVEL THICKNESS VALUES 
ARE EXPECTED TO GENERALLY RANGE FROM 1305 TO 1315 METERS... ROUGHLY 
15 TO 20 METERS BELOW NORMAL AND ABOUT 5 METERS COOLER THAN 
YESTERDAYS MODEL RUNS FOR SUNDAY. THUS... EXPECT HIGH TEMPS... EVEN 
WITH MOSTLY SUNNY SKIES FOR A GOOD PART OF THE DAY... TO BE 
GENERALLY IN THE MID TO UPPER 40S... WITH PERHAPS A FEW LOWER 50S IN 
THE SOUTHEAST.

THE SURFACE HIGH IS EXPECT TO SHIFT OFFSHORE ON SUNDAY NIGHT/MONDAY 
MORNING... AS A WEAK COLD FRONT ASSOCIATED WITH A NORTHERN STREAM 
TROUGH APPROACHES THE AREA. THIS SHOULD HELP TO INCREASE MOISTURE 
ACROSS THE REGION A BIT ALLOWING SKIES TO BECOME/REMAIN PARTLY 
CLOUDY... WHICH SHOULD HELP KEEP LOW TEMPS FROM FALLING OFF A CLIFF. 
THUS... WITH SKIES BECOMING PARTLY CLOUDY... AND LOW LEVEL THICKNESS 
VALUES INCREASING SOME 10 TO 15 METERS OVERNIGHT... EXPECT LOWS TO 
ONLY DROP INTO THE UPPER 20S TO NEAR 30 ACROSS THE NORTH TO THE 
LOWER TO MID 30S ELSEWHERE.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
AS OF 220 PM...

MONDAY A MIDWEST LOW PRESSURE SYSTEM WILL APPROACH AND MOVE THROUGH 
MICHIGAN WHILE SURFACE HIGH PRESSURE LINGERS OFF THE NORTH CAROLINA 
COAST. THE GFS IS STRONGEST WITH THE PRESSURE GRADIENT ACROSS NORTH 
CAROLINA MONDAY AND DEVELOPS A SIGNIFICANT 925 MILLIBAR TROUGH OVER 
THE WESTERN PIEDMONT AT MIDDAY... MOVING IT TO THE COASTAL PLAIN AT 
SUNSET WHILE RETAINING A LEE SIDE TROUGH IN THE FOOTHILLS. 
ISENTROPIC LIFT IS QUITE IMPRESSIVE ON THE GFS. HOWEVER THERE IS 
MINIMAL SUPPORT FROM THE ECMWF AND CANADIAN FOR THIS FEATURE. I HAVE 
FOUND THE GFS TO BE FREQUENTLY OVERDONE IN LOW LEVEL WINDS... BUT 
STILL SUPERIOR IN RELATION TO THE NAM MODEL. THUS WILL ADD A SLIGHT 
CHANCE FOR MEASURABLE RAIN TO MONDAY. SHOULD DRIZZLE OR LIGHT RAIN 
DEVELOP TEMPERATURES WILL BE DAMPENED A BIT AND HAVE DROPPED 
AFTERNOON HIGHS A FEW DEGREES... TO 49 TO 53.

NORTH FLOW MONDAY NIGHT AROUND SURFACE HIGH PRESSURE WILL BEGIN TO 
SHUNT LOW LEVEL MOISTURE TO THE SOUTH. NORTH PARTS WILL HAVE BEST 
RADIATIONAL COOLING CONDITIONS AND AS MUCH AS A TEN DEGREE GRADIENT  
IN TEMPERATURES NORTH TO SOUTH POSSIBLE... WITH MOST LOW 
TEMPERATURES IN THE 30S. 

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
AS OF 220 PM...

A STRONG UPPER LEVEL SHORT WAVE WILL EXIT THE ROCKIES TUESDAY... 
REACHING THE EASTERN PARTS OF THE OKLAHOMA PANHANDLE BY SUNSET. A 
STRONG SURFACE LOW IN CLOSE PROXIMITY TO THE UPPER TROUGH WILL BRING 
STRONG SOUTH RETURN FLOW AHEAD OF THE LOW INTO THE TENNESSEE VALLEY 
AND THE SOUTHEAST. RIDGING OVER THE EAST COAST HOLDS STRONG AND 
WINDS RELATIVELY LIGHT OVER NORTH CAROLINA TUESDAY. THUS MOISTURE 
ADVECTION AND DYNAMICS SHOULD BE RELATIVELY WEAK AND WILL CONTINUE 
TO LIMIT SLIGHT RAIN CHANCES TO THE WESTERN PIEDMONT TUESDAY. 
AFTERNOON HIGHS 50 TO 55. 

THE UPPER TROUGH IS FORECAST TO REACH MISSOURI MIDNIGHT TUESDAY NIGHT 
AND OVERRUNNING SOUTH FLOW OVER THE WEAKENING AND RETREATING SURFACE 
HIGH SHOULD GENERATE RAIN IN CENTRAL NORTH CAROLINA OVERNIGHT.
WARMING ALOFT MAY BE ABLE TO WORK TO THE SURFACE CAUSING LOW
TEMPERATURES TO OCCUR WELL BEFORE SUNRISE. LOW TEMPERATURES 38 TO
45. RAINFALL OF ONE QUARTER TO ONE HALF INCH POSSIBLE OVERNIGHT.
STRONG WARMING ALOFT DURING THIS EVENT AND AT THIS TIME
PRECIPITATION EXPECTED TO BE ALL RAIN. THE ONLY POSSIBLE RISK
WOULD BE FOR INITIAL RAINS TO COOL TEMPERATURES TO FREEZING FOR A
BRIEF PERIOD OF FREEZING RAIN OF LITTLE TO NO CONSEQUENCE. THIS IS
NOT EXPECTED... BUT NOT ENTIRELY IMPOSSIBLE.

THE GFS IS STRONGEST AT DEPICTING A WEAK MILLER B SECONDARY SURFACE 
LOW DEVELOPMENT OVER NORTH CAROLINA OR OFF THE COAST OF VIRGINIA 
LATE TUESDAY NIGHT OR WEDNESDAY MORNING... WITH THE INLAND LOW 
REMAINING DOMINATE. GIVEN THE LOCATION OF THE UPPER TROUGH THIS IS 
ACCEPTED. MODELS IN AGREEMENT IN PUSHING A STRONG DRY SLOT INTO 
NORTH CAROLINA BY NOON WEDNESDAY...SHUTTING DOWN PRECIPITATION IN 
SPITE OF THE SURFACE TROUGH STILL BEING OVER CENTRAL NORTH CAROLINA. 
WEDNESDAY HIGH TEMPERATURES NEAR 50 NORTHWEST TO MID 60S SOUTHEAST. 

SURFACE HIGH PRESSURE OVER THE MIDWEST WEDNESDAY NIGHT WILL ADVANCE 
EAST ON STRONG UPPER LEVEL WEST FLOW... REACHING THE EAST COAST 
FRIDAY. THE 12Z ECMWF AND THE LATEST GFS ARE IN AGREEMENT IN
BRINGING AN UPPER LEVEL SHORT WAVE INTO THE OHIO VALLEY
SATURDAY... BUT SURFACE FEATURES ARE SIGNIFICANTLY DIFFERENT...
THOUGH BOTH MANAGE TO GENERATE RAINFALL FRIDAY NIGHT AND SATURDAY.
ACCEPTING THIS SOLUTION FOR NOW... BUT WITH TIMING OF THE EVENT
SUSPECT THIS FAR OUT IN TIME IN A FAST FLOW PATTERN.

AFTERNOON HIGHS THURSDAY THROUGH FRIDAY IN THE UPPER 40S TO MID 50S. 
LOW TEMPERATURES 35 TO 45 WEDNESDAY NIGHT. SURFACE HIGH PRESSURE AND 
A DRY AIR MASS WITH THICKNESSES BELOW 1300 METERS WILL DROP LOW 
TEMPERATURES INTO THE UPPER 20S TO LOW 30S THURSDAY NIGHT. CLOUDS
AND SOUTH RETURN FLOW WILL WARM LOW TEMPERATURES FRIDAY NIGHT TO
THE MID 30S TO LOWER 40S.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
AS OF 100 PM SATURDAY...

THE PASSAGE OF AN UPPER LEVEL DISTURBANCE COMBINED WITH MOIST 
SOUTHEASTERLY FLOW FROM THE ATLANTIC WILL RESULT IN IFR/LIFR 
CONDITIONS AT ALL CENTRAL NORTH CAROLINA TERMINAL SITES THROUGH
THE EARLY TO MID AFTERNOON. AS COLD AIR RUSHES INTO THE REGION
THROUGH THE DEPTH OF THE ATMOSPHERE... THE RAIN MAY MIX WITH OR
CHANGE OVER TO SNOW (WITH ASSOCIATED REDUCED VISIBILITIES) FOR A
FEW HOURS (MAINLY AT TRIAD TERMINALS AND PERHAPS AS FAR EAST AS
RDU AND RWI). PRECIPITATION SHOULD END WITH A RETURN TO VFR FROM
SOUTHWEST TO NORTHEAST IN THE 22Z TO 04Z/5TH TIME FRAME... AS COLD
BUT DRY HIGH PRESSURE BUILDS INTO THE REGION FROM THE WEST TONIGHT
AND LINGERS THROUGH SUNDAY.

LOOKING BEYOND 18Z SUNDAY: THERE WILL BE ANOTHER CHANCE OF MVFR 
CONDITIONS AND A LITTLE LIGHT RAIN OR DRIZZLE... MAINLY AT EASTERN 
TERMINALS... AHEAD OF A WEAKER STORM SYSTEM ON MONDAY. AFTER A 
RETURN TO VFR FOR MONDAY NIGHT AND TUESDAY... EXPECT LIFR/IFR 
CONDITIONS AND A COLD RAIN ASSOCIATED WITH A STRONGER STORM SYSTEM 
TUESDAY NIGHT AND WEDNESDAY.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

AVIATION...MWS