FXUS62 KRAH 220615
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
114 AM EST TUE DEC 22 2009
.SYNOPSIS...
HIGH PRESSURE WILL EXTEND INTO NC THROUGH THURSDAY. A COLD FRONT
WILL CROSS THE REGION CHRISTMAS DAY.
&&
.NEAR TERM /THROUGH TUESDAY NIGHT/...
AS OF 8450 PM MONDAY...
A 1023-1024 MB SURFACE HIGH OVER THE OHIO AND TN VALLEYS WILL BUILD
EAST INTO THE AREA OVERNIGHT...THEN LINGER (WITH REINFORCEMENT
FROM A SOON-TO-BE STRENGTHENING INTO THE 1030S RIDGE FROM SOUTH
CENTRAL CANADA) THROUGH TUESDAY NIGHT.
CENTRAL NC WILL REMAIN IN A GENERAL CLOUD-FREE REGION TONIGHT...IN
THE NARROW VACATED ZONE BETWEEN THE NORTHERN AND SOUTHERN STREAM JET
(POLAR JET EXTENDING FROM THE GREAT LAKES SE THROUGH THE MID
ATLANTIC STATES...AND THE SUBTROPICAL EXTENDING FROM THE EASTERN
GULF TO BERMUDA). WINDS ARE ALREADY CALM AT MOST SITES...WITH TEMPS
CURRENTLY IN THE LOWER TO MID 30S...RUNNING IN LINE WITH CURRENT
FORECAST. ONLY MINOR/COSMETIC CHANGES TO FORECAST NEEDED WITH MIN
TEMPS IN THE MID TO UPPER 20S. -CBL
TUESDAY: AN UPSTREAM MID LEVEL JET IS FORECAST TO ACCELERATE ACROSS
VA TUESDAY... AND THIS MAY FEATURE MAY SPREAD A VEIL OF ALTOSTRATUS
THROUGH AN OTHERWISE CLEAR SKY OVER NC. PROJECTED AFTERNOON LOW
LEVEL THICKNESSES CLIMB 5-10 METERS FROM THOSE OF MONDAY... WHICH
EQUATES TO HIGH TEMPERATURES WITHIN A FEW DEGREES OF 50.
TUESDAY NIGHT: NOT TOO EXCITED ABOUT THE PROGGED INCREASING RH IN
THE LOWEST COUPLE OF THOUSAND FT OVER THE WESTERN PIEDMONT LATE
TUESDAY NIGHT INTO EARLY WEDNESDAY. IT APPEARS THE NWP MODELS
GENERATE THE MOIST LAYER AS A RESULT OF LIGHT SOUTHEASTERLY UPSLOPE
SURFACE FLOW AROUND THE AFOREMENTIONED SURFACE RIDGE (ORIGINATING
OVER SOUTH CENTRAL CANADA)... AND... ON THE EASTERN EXTENT OF A
DEFORMATION ZONE ATTENDANT AN APPROACHING 925 MB FRONTAL ZONE. WOULD
EXPECT ANY APPRECIABLE MOISTURE TRANSPORT AND LOW CEILINGS TO REMAIN
ALONG THE MS RIVER (IN CLOSER PROXIMITY TO THE PARENT CYCLONE)...
WITH A DOWNSTREAM DRYING DOWNSLOPE COMPONENT (ALONG THE AXIS OF
DILATATION OF THE AFOREMENTIONED DEFORMATION ZONE) INTO CENTRAL NC.
THUS... SCATTERED CLOUDS BETWEEN 1 AND 2 THOUSAND FT ARE POSSIBLE
INVOF AND WEST OF THE YADKIN RIVER... BUT NO WORSE THAN PARTLY
CLOUDY. MOSTLY CLEAR ELSEWHERE. LOWS AGAIN IN THE MIDDLE TO UPPER
20S. -MWS
&&
.SHORT TERM /WEDNESDAY THROUGH THURSDAY/...
AS OF 200 PM MONDAY...
FOR WEDNESDAY/WEDNESDAY NIGHT: DESPITE THE AREAS OF LOW CLOUDS IN
THE UPSLOPE REGION OF THE FAR NORTHWEST PIEDMONT WEDNESDAY
MORNING... QUIET WEATHER PERSISTS THROUGH MIDWEEK WITH FAIRLY FLAT
WEST-NORTHWEST MID LEVEL FLOW OVERHEAD. THE WEAK SURFACE HIGH OVER
THE SOUTHEAST MERGES WITH STRONGER POLAR RIDGE CENTERED OVER CENTRAL
ONTARIO. THE MID LEVEL VORTEX OVER NEW ENGLAND PROGRESSES OFF THE
COAST INTO THE NORTHWEST ATLANTIC ON WEDNESDAY... AND THE RESULTING
CONFLUENT FLOW ATOP THE SURFACE RIDGE HELPS DRIVE IT SOUTHWARD INTO
THE NC PIEDMONT WITH AN INCREASING NORTHEAST SURFACE WIND. FORECAST
SOUNDINGS HAVE CONSISTENTLY SHOWN INCREASING LOW LEVEL MOISTURE
PARTICULARLY OVER THE NORTHWEST CWA WHERE MOIST UPGLIDE IS NOTED IN
THE 275-280K LAYER... SO EXPECT CLOUDINESS TO BE MORE EXTENSIVE HERE
THROUGH THE DAY AS BOTH HORIZONTAL AND VERTICAL MIXING ARE INHIBITED
BY THE WEAK SURFACE WINDS AND STABLE SURFACE AIR. ADDITIONALLY...
THE APPROACH OF THE UPCOMING STORM SYSTEM -- A MID LEVEL VORTEX
SHIFTING OUT OF NM THROUGH OK/TX WEDNESDAY THROUGH WEDNESDAY NIGHT
-- SHOULD LEAD TO INCREASING HIGH CLOUDINESS AS A VEIL OF CIRRUS AND
CIRROSTRATUS SPREADS EASTWARD OVER THE CAROLINAS. BUT THE BUILDING
MID LEVEL RIDGING BETWEEN THE NEW ENGLAND AND NM LOWS SHOULD
SUPPRESS MOST MID-UPPER LEVEL LIFT. NEAR-NORMAL THICKNESSES SUPPORT
HIGHS WEDNESDAY OF 45-52 WITH PARTLY SUNNY SKIES NORTHWEST RANGING
TO MOSTLY SUNNY EAST. THE GFS DEPICTS INCREASING 850 MB THETA-E
ADVECTION AND A DEEPENING MOIST/STABLE SURFACE-BASED LAYER WEDNESDAY
NIGHT... AS THE LOW LEVEL WARM FRONT APPROACHES FROM THE SOUTHWEST.
WHILE THIS OVERRUNNING FLOW IS RATHER WEAK... GIVEN SUFFICIENT LOW
LEVEL MASS CONVERGENCE IT COULD GENERATE PATCHY DRIZZLE OR FREEZING
DRIZZLE IN THE TRIAD EARLY THURSDAY MORNING. DUE TO REDUCED
CONFIDENCE IN THE DEGREE OF LOW LEVEL FORCING... THIS WILL NOT BE
INCLUDED IN THE FORECAST NOW BUT MAY NEED TO BE CONSIDERED IN FUTURE
FORECASTS. OTHERWISE... EXPECT FAIR TO PARTLY CLOUDY SKIES ACROSS
THE REMAINDER OF THE FORECAST AREA AS SURFACE RIDGING HOLDS WITH
RISING HEIGHTS ALOFT. LOWS 28-35.
FOR THURSDAY: THE MID LEVEL VORTEX PIVOTS NORTHEAST OVER MO DURING
THURSDAY... AS ABUNDANT ENERGY DIVING INTO ITS WEST AND SOUTH HELP
TO SLOW IT DOWN. AT THE SURFACE... THE COLD FRONT IS EXPECTED TO
MAKE ITS WAY INTO THE OH VALLEY AND WESTERN TN/AL BY DAY'S END. THE
MODELS CONTINUE THEIR COLLECTIVE TREND OF A SLOWER ARRIVAL OF THE
PREFRONTAL PRECIP BAND INTO CENTRAL NC... AND TO MAKE A LONG STORY
SHORT... IT NOW LOOKS LIKE THE DAYLIGHT HOURS OF CHRISTMAS EVE WILL
BE LARGELY DRY AREAWIDE... WITH THE STACKED RIDGING HOLDING OFF
SIGNIFICANT LIFT MECHANISMS AND MOISTURE ADVECTION. THE GFS/ECMWF DO
INDICATE ENOUGH LOW LEVEL MOIST UPGLIDE FOR A LITTLE LIGHT RAIN OVER
THE EXTREME WESTERN CWA LATE IN THE DAY... BUT CHANCES ARE VERY
SMALL AND MUCH OF THE AREA WILL STAY DRY. LOW LEVEL THICKNESSES RISE
A BIT FROM WEDNESDAY... BUT MUCH OF THIS WARMTH IS REFLECTED IN WARM
ADVECTION A FEW THOUSAND FEET UP WITH FORECAST SOUNDINGS INDICATING
COOLER STABLE AIR LOCKED IN AT THE SURFACE WITH LIMITED HEATING.
HIGHS 41-52. -GIH
&&
.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
AS OF 235 PM MONDAY...
FOR CHRISTMAS EVE/CHRISTMAS DAY: DESPITE THE SLOWING TREND OF THE
MODELS WITH THE INCOMING FRONT AND PRECIP... WE'RE STILL EXPECTED
WET WEATHER HEADING INTO THE HOLIDAY. THE INTENSE MID LEVEL LOW
TAKES ITS TIME HEADING NORTHEASTWARD TOWARD CHICAGO ON CHRISTMAS DAY
WITH THE COLD FRONT SHIFTING NORTHEAST TOWARD SOUTHWEST NC. TIMING
DIFFERENCES EXIST WITH THE ECMWF A BIT SLOWER THAN THE GFS... BUT
BOTH SUGGEST ENOUGH COVERAGE IN THE WESTERN CWA TO RAMP POPS UP TO
LIKELY THERE DURING THE NIGHT... HOWEVER THE MOIST UPGLIDE IS NOT AS
DEEP NOR STRONG IN THE NORTHEAST CWA OVERNIGHT AND WILL NEED TO
DELAY THE HIGHER POPS HERE UNTIL AFTER SUNRISE CHRISTMAS DAY... WHEN
THE LOW LEVEL MOISTURE TRANSPORT IMPROVES. OVERALL THOUGH... THE
LARGE SCALE LIFT WITH THIS SYSTEM IS NOT VERY IMPRESSIVE. THE STRONG
UPPER JET (120-140 KTS) OVER TX/LA SHIFTS INTO AR/TN THROUGH FRIDAY
BUT THE BEST UPPER DIVERGENCE IN ITS LEFT EXIT REGION HOLDS TO OUR
NORTHWEST. MID LEVEL FLOW REMAINS CYCLONIC OVER NC BUT MODELS DO NOT
SHOW ANY TRACKABLE PERTURBATIONS PASSING OVERHEAD. AS SUCH... WILL
HOLD POPS TO NO BETTER THAN LIKELY THROUGH CHRISTMAS DAY. REGARDING
PRECIP TYPE... GFS FORECAST SOUNDINGS SHOW RESIDUAL WEDGING AND
NEAR-FREEZING TEMPS AT THE SURFACE THURSDAY NIGHT INTO EARLY FRIDAY
MORNING... CAPPED BY AN INCREASING AND DEEPENING WARM NOSE ALOFT.
THIS WOULD SUGGEST A POSSIBLE PERIOD OF FREEZING RAIN IN THE
NORTHWEST CORNER OF CENTRAL NC THURSDAY NIGHT. BUT GIVEN THAT THE
PARENT HIGH AND ITS UPPER SUPPORT WILL BE QUICKLY EXITING... EVEN IF
A SMALL STABLE POOL OF AIR LINGERS HERE... IT SHOULD RAPIDLY BE
OVERCOME BY THE RAIN DRAGGING DOWN WARMER AIR FROM ABOVE... ALONG
WITH THE SELF-LIMITING PROCESSES OF ANY FREEZING RAIN. THE LOW
PROBABILITIES PRECLUDE MENTIONING FREEZING RAIN IN THE TRIAD FOR
NOW... BUT WILL MONITOR MODEL TRENDS AND MAY INCLUDE IT IN LATER
FORECASTS IF THESE INDICATORS PERSIST. EITHER WAY... ANY FREEZING
PRECIP SHOULD BE VERY FLEETING AND CAUSE FEW DIFFICULTIES IF ANY.
REGARDING THE RISK OF THUNDER... BOTH THE 12Z ECMWF AND GFS DEPICT A
TRIPLE POINT LOW FORMING OVER GA AND RIDING UP ALONG HIGHWAY 1
THROUGH FRIDAY ATTENDING A 40-50 KT LOW LEVEL JET WITH 30-40 KT DEEP
LAYER SHEAR. HOWEVER... AS HAS BEEN THE CASE WITH MANY OF OUR RECENT
DYNAMIC SYSTEMS... THIS APPEARS TO BE A HIGH-SHEAR LOW-CAPE EVENT
WITH ONLY 400 J/KG OF MUCAPE OVER THE SOUTHEAST CWA ON THE GFS...
AND WIDESPREAD SEVERE WEATHER LOOKS VERY UNLIKELY. BUT WITH AT LEAST
THE EASTERN CWA SHOWN GETTING INTO THE WARM SECTOR WITH LIFTED
INDICES NEAR ZERO... WILL MENTION AN ISOLATED RISK OF THUNDER OVER
THE SANDHILLS AND COASTAL PLAIN. WITH THE RESIDUAL COOL POOL OVER
THE NORTHWEST CWA... HIGHS WILL VARY WIDELY ACROSS THE AREA... FROM
MID 40S NORTHWEST TO AROUND 60 SOUTHEAST... AFTER LOWS OF 33-41.
FOR FRIDAY NIGHT THROUGH MONDAY: EXPECT THE FRONT TO EXIT TO THE
NORTHEAST FRIDAY NIGHT WITH A GRADUAL TAPERING OF POPS SOUTHWEST TO
NORTHEAST OVERNIGHT. BRISK NORTHWEST LOW LEVEL FLOW TAKES OVER WITH
WESTERLY MID LEVEL FLOW AS THE GREAT LAKES VORTEX OPENS AND SHIFTS
NORTHEAST. SURFACE HIGH PRESSURE SLOWLY BUILDS IN FROM THE WEST OVER
THE WEEKEND. WITH GENERALLY DRY AND STABLE PROFILES... WE SHOULD SEE
PARTLY CLOUDY TO MOSTLY CLEAR SKIES AND BELOW NORMAL TEMPS THROUGH
MONDAY. -GIH
&&
.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
AS OF 100 AM TUESDAY...
A RIDGE OF HIGH PRESSURE AT THE SURFACE WILL EXTEND ACROSS CENTRAL
NC THROUGH THURSDAY. MEANWHILE ALOFT... A SERIES OF WEAK
PERTURBATIONS ALOFT WILL CROSS OVERHEAD. PATCHES OF MID/HIGH LEVEL
CLOUDINESS WILL PRECEDE EACH DISTURBANCE.
THURSDAY...VFR CONDITIONS EXPECTED TO LINGER WITH SURFACE RIDGE
STILL IN PLACE. MAY SEE INCREASING AND THICKENING MID LEVEL CLOUDS
LATE THU AS A FRONTAL SYSTEM ASSOCIATED WITH A STRONG LOW PRESSURE
SYSTEM OVER THE MIDDLE PART OF THE COUNTRY APPROACHES FROM THE WEST.
POTENTIAL EXIST LATE THU NIGHT INTO EARLY FRI FOR LOW LEVEL WIND
SHEAR CONDITIONS AS LLJ PROGGED TO INCREASE TO 30-40KTS BY CHRISTMAS
MORNING.
MAY SEE MVFR CEILINGS AND VISIBILITY FRIDAY INTO FRIDAY NIGHT WITH
PASSAGE OF FRONTAL SYSTEM. EXPECT A RETURN TO VFR CONDITIONS
SATURDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...WSS
NEAR TERM...CBL/MWS
SHORT TERM...HARTFIELD
LONG TERM...HARTFIELD
AVIATION...WSS
