FXUS62 KRAH 100832
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
330 AM EST THU DEC 10 2009
.SYNOPSIS...
A COLD FRONT WILL MOVE EAST THROUGH THE REGION EARLY THIS EVENING...
USHERING IN A MUCH COLDER AIR MASS FOR THURSDAY THROUGH FRIDAY.
ANOTHER STORM SYSTEM WILL AFFECT THE SOUTHEAST STATES OVER THE
WEEKEND.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 310 AM...
INCREASED GUSTINESS FOR MID MORNING INTO EARLY EVENING... OTHERWISE
NO SIGNIFICANT CHANGES. MIXED SURFACE LAYER FORECAST TO EXTEND TO
THREE TO FOUR THOUSAND FEET WHERE WEST WINDS OF 25 TO 30 KNOTS
FORECAST. EXPECT SOME MINOR GUSTINESS... TO PERHAPS 25 MPH.
CURRENT BAND OF HIGH CLOUDINESS WILL SHIFT EAST LATER THIS MORNING
ALLOWING FOR MAXIMUM SUNSHINE. CURRENT FORECAST TEMPERATURES ALOFT
IN A WELL MIXED BOUNDARY LAYER SUPPORT TEMPERATURES MUCH COOLER
THAN CURRENTLY FORECAST. HAVE SEEN MODELS BE UNDERDONE IN MIXING
HEIGHTS IN SIMILAR CASES AND THEREFORE NOT HAVE ENOUGH WARMTH
PORTED TO THE SURFACE FROM ALOFT. WILL STAY NEAR THE COOLER NAM
GUIDANCE BUT ABOVE THAT DERIVED FROM 900 MILLIBAR TEMPERATURES BY
THREE TO FOUR DEGREES.
MODELS IN AGREEMENT IN DROPPING LOW LEVEL THICKNESS AT GREENSBORO
BELOW 1270 METERS BY SUNRISE FRIDAY. EXPECT BRIEF TO NO CALM WINDS
OVERNIGHT WITH SOME MOISTURE IN THE AIR COLUMN ABOVE 500 MILLIBARS
ONLY MINIMALLY IMPACTING TEMPERATURES. THIS FITS VERY WELL WITH
MID 20S FOR LOW TEMPERATURES IN THE NORTH AND UPPER 20S SOUTH.
&&
.SHORT TERM /FRIDAY THROUGH SATURDAY/...
AS OF 325 AM...
SURFACE HIGH PRESSURE OVER TENNESSEE FRIDAY MORNING WILL BE OVER
THE VIRGINIAS AND NORTH CAROLINA FRIDAY NIGHT. STRONG LOW LEVEL
FLOW OFF THE ATLANTIC AND GULF OF MEXICO SATURDAY BEGIN TO
GENERATE RAIN OVER THE GULF COAST STATES AND INTO SOUTH CAROLINA
FRIDAY. ACCEPT THE CURRENT THINKING OF HIGH PRESSURE HOLDING ON
OVER NORTH CAROLINA SATURDAY AND RETURN FLOW BEING WEAK AT TWENTY
KNOTS OR LESS. THE AIR COLUMN WILL SATURATE FROM THE TOP DOWN AND
DO NOT EXPECT ANY MEASURABLE PRECIPITATION TO SURVIVE FALLING INTO
THE NEAR SURFACE DRY AIR MASS... SO NO MENTION OF PRECIPITATION
SATURDAY.
WILL CONTINUE TO STAY ON THE COOL SIDE OF GUIDANCE... WITH
AFTERNOON HIGH TEMPERATURES MOSTLY IN THE MID 40S. WITH HIGH
PRESSURE OVERHEAD FRIDAY NIGHT AND A DRY AIR MASS... LOW
TEMPERATURES WILL AGAIN RETURN TO THE MID 20 AT MOST LOCATIONS.
&&
.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
AS OF 330 AM...
TRICKIEST PART OF THE FORECAST CONTINUES TO BE SATURDAY NIGHT INTO
SUNDAY...AS A WEAK PERTURBATION (OR PERTURBATIONS) ORIGINATING OVER
SO CALIFORNIA IS FORECAST TO LIFT NE TO THE OHIO VALLEY REGION BY
SATURDAY NIGHT. THE INDUCED BROAD...WEAK ISENTROPIC LIFT ATOP THE
LINGERING SURFACE RIDGE FROM THE HIGH SHIFTING OFF THE MID ATLANTIC
WILL SPREAD PRECIP TOWARD CENTRAL NC FROM THE SW. GIVEN THE VERY
DRY ANTECEDENT AIRMASS EXPECTED WITH THE ARCTIC HIGH...MOISTURE
RECOVERY AHEAD OF THE PRECIP WILL STILL ONLY PRODUCE DEWPOINTS IN
THE UPPER TEENS TO LOWER 20S IN THE NW PIEDMONT BASED ON GUIDANCE.
WITH FORECAST HIGHS IN THE LOWER 40S ON SAT...SURFACE WETBULB TEMPS
WILL LIKELY BE JUST BELOW FREEZING FROM THE TRIAD TO MAYBE PERSON
COUNTY. WARM ADVECTION AT MID LEVELS WOULD PUT H85-H7 THICKNESSES
ABOVE 1550M....WITH H10-H85 THICKNESSES AT OR JUST ABOVE 1300M FROM
THE TRIAD NWWRD. THERE IS FAIRLY GOOD AGREEMENT AMONG MODELS ON
THIS ASPECT...SUPPORTING A SMALL CHANCE AT WINTRY MIX AT
ONSET...LEANING MORE TOWARDS FREEZING RAIN OR DRIZZLE AND QUICKLY
CHANGING OVER THE RAIN. THE QUESTION THAT MODELS HAVE YET TO COME
INTO AGREEMENT ON IS WHEN AND HOW MUCH PRECIP WILL FALL. THE TREND
HAS BEEN SLOWER ON THE ONSET TIMING...TOWARDS THE 06-12Z SUN TIME
FRAME..WITH THE GFS BEING A FAST OUTLIER. THE NAM/GFS KEEP BETTER
PRECIP FURTHER SOUTH AND EAST...CLOSER TO A WEAK SURFACE LOW THAT
DEVELOPS OFF THE SE COAST. THE ECMWF/CANADIAN ARE A LITTLE MORE
ROBUST WITH THE PRECIP...BRINGING 0.25-0.5" ACROSS THE
CWA...POSSIBLY DUE TO A STRONGER SURFACE RIDGE AND GREATER
ISENTROPIC LIFT. SREF MEMBERS AT 72HR (00Z SUN) ARE CLUSTERED
TOWARDS A MORE SOUTHERN PLACEMENT OF PRECIP. CONFIDENCE REMAINS LOW
AT THIS POINT GIVEN MODEL DISAGREEMENT AND THE FACT THAT WE ARE MORE
THAN 24 HOURS FROM EVEN BEGINNING TO SEE THE FORCING EMERGE OVER THE
WEST COAST. WITH LACK OF REAL SIGNIFICANT UPPER FORCING...WOULD
TEND TO FAVOR THE GFS SOLUTION WITH BETTER PRECIP SE CLOSER LOW
LEVEL FORCING. A SMALL CHANCE REMAINS OF FROZEN PRECIP IN THE FAR
NW AT ONSET IF THERE IS SUFFICIENT QPF TO OVERCOME THE INITIALLY DRY
AIRMASS.
RAIN CHANCES LINGER THROUGH THE DAY ON SUNDAY...DECREASING SUNDAY
NIGHT AS THE COASTAL LOW BEGINS TO PULL AWAY FROM THE COAST. A COOL
AIRMASS...REINFORCED BY POTENTIAL PRECIP AND THE NRLY FLOW BEHIND
THE SURFACE LOW... REMAINS OVER THE AREA THROUGH EARLY MON...WHEN
LOW LEVEL S-SWRLY KICKS IN AHEAD OF A COLD FRONT BARRELING THROUGH
THE MIDWEST ON MON AND TUES. HIGHS ON SUNDAY MAY TO GET OUT OF THE
LOWER 40S (OR EVEN UPPER 30S) IN THE NW....BUT SHOULD MODERATE BACK
TO NORMAL OR WARMER ON MONDAY. PRECIP CHANCES RETURN BY LATE TUES AS
THE COLD FRONT APPROACHES...ALTHOUGH MODEL DISAGREEMENT CONTINUES
THROUGH THIS PORTION OF THE FORECAST ALSO. THE COMBINATION OF THE
COLD FRONT AND THE DEVELOPMENT OF A LONGWAVE TROF OVER THE EASTERN
US WILL BRING BELOW NORMAL READINGS FOR THE LATTER HALF OF THE WEEK.
&&
.AVIATION /08Z THURSDAY THROUGH MONDAY/...
AS OF 125 AM...
VFR CONDITIONS THROUGH THE TAF PERIOD. GUSTY WINDS NEAR AND EAST
OF HIGHWAY ONE AT THIS TIME. WEST SOUTHWEST WINDS GRADUALLY
INCREASING WITH HEIGHT TO AS MUCH AS 40 KNOTS IN THE EAST...
PERHAPS 25 KNOTS WEST. LOW LEVEL WIND SHEAR CRITERIA NOT MET.
GUSTS TO 25 MPH FROM MID MORNING INTO EARLY EVENING ACROSS THE
AREA.
VFR CONDITIONS THURSDAY NIGHT INTO SATURDAY MORNING. A DEVELOPING
LOW PRESSURE CENTER TO THE SOUTH MAY BRING MVFR TO IFR CEILINGS OR
VISIBILITIES TO THE AREA SATURDAY EVENING INTO SUNDAY. VFR LATE
SUNDAY NIGHT INTO MONDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...
NEAR TERM...
SHORT TERM...
LONG TERM...SMITH
AVIATION...
