FXUS62 KRAH 120317
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1017 PM EST FRI DEC 11 2009
.SYNOPSIS...
COLD HIGH PRESSURE WILL BUILD INTO THE REGION THROUGH TONIGHT...
MOVING OFF THE MID-ATLANTIC COAST ON SATURDAY. A LOW PRESSURE
SYSTEM WILL MOVE UP THE SOUTHEAST COAST SATURDAY NIGHT AND SUNDAY.
&&
.NEAR TERM /TONIGHT/...
AS OF 1000 PM FRIDAY...
SFC ANALYSIS THIS EVENING SHOWED 1033 MB ARCTIC HIGH PRESSURE
CENTERED OVER THE CAROLINAS. ALOFT...00Z RAOB ANALYSIS SHOWED
WEST/WSW FLOW ALOFT OVER THE CAROLINAS. AN INTENSE UPPER LEVEL JET
STREAK WAS NOTED OVER THE TN VALLEY AND MID-ATLANTIC. WALLOPS ISLAND
VIRGINIA (KWAL) REPORTED THE STRONGEST WIND...WITH 190 KNOTS
(ROUGHLY 218 MPH) AT 250 MB...PRETTY IMPRESSIVE!
TONIGHT:
WITH AN UNSEASONABLY COLD/DRY ARCTIC AIRMASS IN PLACE TONIGHT...THE
ONLY VARIABLE (ASIDE FROM A SNOWPACK) PREVENTING US FROM ACHIEVING
MAXIMUM RADIATIONAL COOLING IS THE STEADY STREAM OF UPPER LEVEL
CLOUD COVER MOVING INTO THE CAROLINAS FROM THE SOUTHWEST. WATER
VAPOR IMAGERY SHOWS THIS RELATIVELY NARROW BAND OF UPPER LEVEL
MOISTURE EMANATING FROM THE SUBTROPICAL PACIFIC...HITCHING A RIDE ON
THE SOUTHERN STREAM JET INTO THE DEEP SOUTH AND CAROLINAS. WITH
PLENTY OF UPPER LEVEL CLOUD COVER UPSTREAM OF THE AREA...AND A
100-150 KNOT SOUTHERN STREAM JET MOVING IT TOWARD THE CAROLINAS...
SEE NO REASON WHY THIS SHOULD NOT CONTINUE THROUGH SUNRISE. AS A
RESULT...WILL BE MENTIONING MOSTLY CLOUDY SKIES OVERNIGHT. ALTHOUGH
THE CLOUD COVER IS GENERALLY AT 20-25 KFT...IT SHOULD BE
SUFFICIENTLY OPAQUE TO HAVE SOME EFFECT ON MINIMUM TEMPERATURES.
HOWEVER...GIVEN THAT THERE WILL BE OCCASIONAL BREAKS...IT IS LIKELY
THAT SOME LOCATIONS WILL SEE TEMPS DROP INTO THE LOWER 20S (PERHAPS
CLOSE TO 20F)...WHILE OTHER LOCATIONS MAY NOT DROP LOWER THAN THE
MID 20S. HAVE MADE ONLY MINOR ADJUSTMENTS TO THE PREVIOUS LOW
TEMPERATURE FORECAST. -VINCENT
&&
.SHORT TERM/SATURDAY AND SATURDAY NIGHT/....
AS OF 200 PM FRIDAY...
A SERIES OF WEAK UPPER S/W TROUGHS WILL MOVE ONTO TO THE CALIFORNIA
COAST TODAY AND TONIGHT...AND WILL CONTINUE EAST IN THE FAST LOW
AMPLITUDE FLOW ALOFT...REACHING THE LOWER MS AND TN VALLEYS BY
SATURDAY EVENING. THE RESULTANT LIFT ACROSS THE SOUTHERN CONUS
AND WITHIN THE SUBTROPICAL JET WILL INDUCE CYCLOGENESIS
ALONG THE GULF COAST REGION. SOUTHERLY LOW-LEVEL WIND FLOW WILL
STRENGTHEN IN RESPONSE TO THE LOWERING OF THE PRESSURE FIELDS...
FEEDING HIGH GULF PWATS NORTHWARD INTO THE CAROLINAS. ISENTROPIC
WARM ASCENT WILL SPREAD PRECIPITATION INTO CENTRAL NC STARTING
SATURDAY EVENING AS A SECONDARY LOW DEVELOPS OFF THE SOUTH CAROLINA
COAST...TRACKING NORTHEAST UP THE NORTH CAROLINA COAST ON SUNDAY.
THERE IS A DEFINITE WESTWARD TREND WITH THE LATEST MODEL ITERATIONS
REGARDING THE LOCATION AND TRACK OF THE COASTAL LOW...LIKELY DUE TO
BETTER OBSERVATION SAMPLING AS THE SUBTLE WAVES MOVE CLOSER ASHORE.
THIS WESTWARD SHIFT RESULTS IN A STRONGER SOUTHERLY LOW-LEVEL
COMPONENT THAT WILL SERVE TO BOTH PROVIDE 1)HIGHER RAINFALL AMOUNTS
AND 2)AND A WARMER/MORE PROMINENT 850 WARM NOSE. THIS TREND IS
READILY EVIDENT IN MODEL SOUNDINGS...THICKNESSES...AND NOMOGRAM
TRENDS...SIGNALING A LIQUID/RAIN EVENT. THE ONLY EXCEPTION WILL BE
THE FAR NW TIER(ESSENTIALLY THE TRIAD)...WHERE THE RESIDUAL LOW
DEWPOINTS MAY BRIEFLY SUPPORT A TRANSITIONAL SLEET MIXED WITH RAIN
INITIALLY AT THE ONSET ...AS EVAPORATIONAL COOLING WILL LIKELY
SUPPORT A DEEP NEAR-ISOTHERMAL LAYER. THE STRONG LOW-LEVEL WAA KICKS
IN QUICKLY AND WARMS THE ENTIRE NEAR SURFACE LAYER TO ABOVE FREEZING
WITH THE HORIZONTAL CAA HAVING LONG HAVE CEASED SATURDAY WITH THE
HIGH WELL OFF THE SOUTHERN MID-ATLANTIC COAST. WILL NOT MENTION ANY
FROZEN PRECIP IN FORECAST AS TRANSITION WILL BE SO QUICK TO OCCUR.
THE TIMING OF THE ONSET OF PRECIP REMAINS PRETTY MUCH
UNCHANGED...SPREADING INTO THE AREA BETWEEN 03 TO 06Z...WITH THE
FIRST SEVERAL HUNDREDTHS GOING TO MOISTENING THE COLUMN. ONLY
COSMETIC/MINOR CHANGES TO POPS WITH CATEGORICAL POPS BY DAYBREAK
SUNDAY.
HIGH TEMPS SATURDAY WILL BE DEPENDENT ON TIMING OF MID AND HIGH
LEVEL CLOUDS SPREADING BACK INTO THE AREA...WITH CURRENT MODEL TIMING
SHOWING A DECENT AMOUNT OF INSOLATION THROUGH 18Z. SO UNLIKE
TODAY...LATE MORNING/EARLY AFTERNOON SUN SHOULD WARM TEMPERATURES
IN THE LOWER 40S NORTH TO UPPER 40S SOUTH(ANOTHER REASON WHY NO
IMPACTS/ICE ACCUMULATIONS ARE EXPECTED). LOWS SATURDAY NIGHT
ARE EXPECTED TO OCCUR DURING THE EVENING...THEN GRADUALLY WARM
OVERNIGHT. LOWS IN THE LOWER TO MID 30S NORTHWEST TO LOWER 40S
SOUTHEAST.
&&
.LONG TERM /SUNDAY THROUGH FRIDAY/...
AS OF 225 PM FRIDAY...
DEEP MOISTURE IS IN PLACE EARLY SUNDAY MORNING...WITH STRONG
ISENTROPIC LIFT ON THE 300K SURFACE WHICH WEAKENS THROUGH 18Z AND
BECOMES ISENTROPIC DESCENT BY 00Z MONDAY. CURRENTLY LEANING MORE
TOWARD THE GFS TRACK OF THE SURFACE LOW A LITTLE FARTHER EAST THAN
THE NAM...WHICH SEEMS CLIMATOLOGICALLY FAVORED AND KEEPS THE TRIAD
IN THE WEDGE FOR MOST OF THE DAY. IF THE GFS VERIFIES...WITH ITS
MODERATELY STRONG 850MB DIVERGENCE BY 18Z AS THE SURFACE LOW LIFTS
NORTHEAST...AND DIMINISHING UPPER-LEVEL DIVERGENCE WITH STRONGER
MID-LEVEL SHORTWAVES EXITING CENTRAL NORTH CAROLINA TO THE NORTHEAST
AS WELL...WIDESPREAD RAIN SHOULD BECOME MORE SCATTERED WITH AREAS OF
DRIZZLE BY SUNDAY AFTERNOON. INCREASING SUBSIDENCE BY 00Z...AND
DRYING...WITH K INDICES FALLING INTO NEGATIVE VALUES SUNDAY
EVENING...SHOULD END THE THREAT OF MEASURABLE PRECIPITATION BY 00Z
MONDAY. IF SKIES CLEAR QUICKLY ENOUGH...LACK OF STRONG BOUNDARY
LAYER DRYING AND RECENT RAIN COULD RESULT IN AT LEAST PATCHY FOG
EVEN BEFORE MIDNIGHT MONDAY MORNING. PROGRESSIVE AND WEAK HIGH
PRESSURE MOVES IN FOR OVERNIGHT SUNDAY AND MONDAY. GUIDANCE HIGH
TEMPERATURES ARE QUITE DIVERGENT SUNDAY BETWEEN THE GFS AND
NAM...BUT WILL LEAN TOWARD THE MAINLY COOLER GFS FOR HIGHS SUNDAY
AND LOWS SUNDAY NIGHT. THICKNESSES ALONE WOULD SUPPORT HIGHS IN THE
LOWER TO MID 60S MONDAY...BUT MODEL SOUNDINGS DO NOT SUPPORT STRONG
MIXING SO HAVE OPTED FOR A CATEGORY LOWER ACROSS CENTRAL NORTH
CAROLINA FOR MONDAY.
THE BRIEF SUMMARY OF WEATHER FOR THE PERIOD MONDAY NIGHT THROUGH
SUNDAY IS A CHANCE OF RAIN RETURNING LATE MONDAY NIGHT CONTINUING
THROUGH TUESDAY...WITH BELOW NORMAL TEMPERATURES AND DRY CONDITIONS
EXPECTED FOR THE REST OF THE WEEK. FOR THE PERIOD MONDAY NIGHT
THROUGH TUESDAY NIGHT...THE GFS IS THE FASTEST MODEL AND HAS BEEN
FAIRLY CONSISTENT WITH ITSELF...BEING MORE PROGRESSIVE WITH THE
UPPER TROUGH MOVING ACROSS THE SOUTHERN TIER OF STATES. THE
DGEX...ECMWF...AND GEM ARE SLOWER...PARTICULARLY THE DGEX...IN THE
TIMING OF THE FRONT AND DIMINISHING THE PRECIPITATION LATE TUESDAY
NIGHT...AND INTO EARLY WEDNESDAY IN THE EAST. THE ECMWF HAS BEEN
CONSISTENT WITH ITSELF AS WELL OVER THE LAST TWO OR THREE MODEL
RUNS...SHOWING A BETTER CHANCE FOR PRECIPITATION BEGINNING INTO
EARLY TUESDAY AS THE UPPER TROUGH DIGS A LITTLE FARTHER SOUTH AND IS
SLOWER IN ITS PROGRESS EAST COMPARED TO THE GFS.
THICKNESSES...SOUNDINGS...AND SURFACE TEMPERATURES ALL SHOW RAIN FOR
MONDAY NIGHT INTO TUESDAY...AND WILL CONTINUE WITH A BASIC CHANCE
FOR SHOWERS WEST LATE MONDAY NIGHT WITH A SLIGHT CHANCE IN THE
NORTHEAST. BETTER CHANCES FOR SHOWERS WILL COME TUESDAY AS A COLD
FRONT MOVES THROUGH CENTRAL NORTH CAROLINA DURING THE DAY INTO THE
EARLY EVENING. WINDS ALOFT ARE NOT AS STRONG WITH THIS SYSTEM AS
OTHERS MORE RECENTLY EXPERIENCED...AND THE THREAT FOR ANY THUNDER IS
MARGINAL AT THIS POINT BASED ON MODEL SOUNDINGS IN VICINITY OF 100
HOURS. SOME ELEVATED INSTABILITY IS NOTED EARLY TUESDAY IN THE
NORTHWEST PIEDMONT ON GFS MODEL SOUNDINGS WHERE WINDS JUST BELOW
850MB INCREASE TO 40KTS...BUT OTHERWISE THE MID LEVELS APPEAR WARM
ENOUGH TO KEEP THUNDER OUT OF THE FORECAST DESPITE SOMEWHAT
FAVORABLE FRONTAL TIMING PARTICULARLY CENTRAL AND EAST. GIVEN THE
UNCERTAINTY IN THE SPEED OF THE FRONTAL PASSAGE AND DRY AIR MOVING
INTO CENTRAL NORTH CAROLINA...WILL MAINTAIN A CHANCE OF SHOWERS
TUESDAY EVENING...LEAST TOWARD THE TRIAD...WITH ALL SHOWERS TO THE
EAST OF THE AREA BY 12Z WEDNESDAY. IF THE SLOWER MODELS VERIFY
COMPLETELY...AN ALTERNATIVE COULD BE A FEW SHOWERS LINGERING INTO
WEDNESDAY MORNING IN THE COASTAL PLAIN.
FOR THE REST OF THE WORK WEEK...COOL HIGH PRESSURE AND DRY AIR BUILD
INTO CENTRAL NORTH CAROLINA...WHICH SHOULD RESULT IN HIGHS AND LOWS
A CATEGORY...OR MORE LIKELY TWO...BELOW NORMAL. SOME MORNING
THICKNESSES ESPECIALLY ON THE ECMWF PLUNGE WELL DOWN INTO THE 1200S
WHICH WOULD SUGGEST LOWS IN THE TEENS FOR WEDNESDAY NIGHT...BUT THE
GFS THICKNESSES ARE WARMER AND WILL TREND JUST A CATEGORY DOWN FOR
THE LOW THEN FOR NOW...GIVEN A GENERAL COLD BIAS TO SOME GUIDANCE
THAT FAR OUT IN TIME IN WINTER. MODERATE MID-LEVEL WAVES IN THE FAST
FLOW ALOFT OVER THE GREAT LAKES INTO THE NORTHEAST COULD EXTEND SOME
MAINLY HIGH CLOUDS OVER A PART OF THE AREA LATE THURSDAY INTO
FRIDAY...BUT THE THREAT OF PRECIPITATION FOR NOW REMAINS WELL NORTH
NEAR AND OVER THE GREAT LAKES...WITH HIGH PRESSURE OVER THE AREA AND
THE GULF...STRONGLY KEEPING DEEP MOISTURE WELL AWAY FROM THE REGION.
&&
.AVIATION /00Z SATURDAY THROUGH TUESDAY/...
AS OF 705 PM FRIDAY...
A COLD DRY ARCTIC HIGH PRESSURE WILL SETTLE OVER THE REGION
TONIGHT...LEADING TO CALM WINDS WITH ONLY A STREAM OF HIGH CLOUDS
MOVING NE ACROSS CENTRAL NC. A SURFACE LOW WILL DEVELOP ALONG THE
SE COAST AND TRACK ALONG THE CAROLINA COAST SATURDAY NIGHT.
CEILINGS WILL BEGIN TO LOWER BY THE END OF THE CURRENT TAF PRECIP AS
MOISTURE INCREASES FROM THE SOUTH...BUT VFR CONDITIONS SHOULD
PREVAIL THROUGH 00Z AS SURFACE WINDS BEGIN TO TURN TO EASTERLY.
CEILINGS AND VISIBILITIES WILL LOWER TO IFR/LIFR OVERNIGHT SATURDAY
NIGHT AS PRECIPITATION OVERSPREADS THE AREA. RAIN WILL PERSIST
DURING THE DAY ON SUNDAY....BEFORE COMING TO AN END SUNDAY NIGHT.
LIGHT WINDS AND CLEARING SKIES MAY PROMOTE THE DEVELOPMENT OF FOG BY
EARLY MONDAY MORNING...WITH VFR CONDITIONS OTHERWISE EXPECTED TO
RETURN FOR THE BEGINNING OF THE WEEK.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...CBL
NEAR TERM...VINCENT
SHORT TERM...CBL
LONG TERM...DJF
AVIATION...SMITH
