FXUS62 KRAH 101819
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
115 PM EST THU DEC 10 2009
.SYNOPSIS...
COOL HIGH PRESSURE WILL CONTINUE TO BUILD INTO THE REGION THROUGH
FRIDAY. A LOW PRESSURE SYSTEM MOVING NORTH FROM THE GULF OF MEXICO
WILL AFFECT THE SOUTHEAST STATES OVER THE WEEKEND.
&&
.NEAR TERM /THROUGH FRIDAY NIGHT/...
AS OF 115 PM THURSDAY...
TONIGHT...
WITH PINNACLE OF COLD AIR IN PLACE BY 12Z FRIDAY(LOW-LEVEL
THICKNESSES BOTTOMING OUT AROUND 1265 TO 1270M)...AND WINDS DROPPING
OFF THE LESS THAN 05 KTS...THE MID TO UPPER 20 FORECAST BY MOS
STATISTICAL GUIDANCE LOOKS VERY REASONABLE...WITH SOME OF THE
TYPICAL CLIMATOLOGICAL COLDER SPOTS DIPPING DOWN INTO THE LOWER 20S.
FRIDAY AND FRIDAY NIGHT...
MODERATING SURFACE HIGH OVER THE LOWER MS VALLEY STRENGTHENS AS IT
BUILDS DIRECTLY OVER THE REGION...UNDER THE CONFLUENT FLOW ALOFT AND
DOWNSTREAM OF THE APPROACHING LOW AMPLITUDE S/W RIDGE. THICKNESSES
RISE SLOWLY THROUGH THE PERIOD...BUT REMAIN 30 TO 40 METERS BELOW
NORMAL. UNDER THE ACTIVE SOUTHERN STREAM JET...MODEL TIME HEIGHT
CROSS SECTIONS INDICATE INCREASING HIGH CLOUDS FRIDAY AFTERNOON AND
INTO FRIDAY NIGHT. WILL CONTINUE FAVOR COOLER MET STATISTICAL
GUIDANCE. HIGHS FRIDAY INTO THE LOWER TO MID 40S. DEPENDING ON
OPAQUENESS OF HIGH CLOUDS...LOWS FRIDAY NIGHT COULD POTENTIALLY
DIP DOWN INTO THE UPPER TEENS TO MID 20S (18-24 DEGREE RANGE)GIVEN
THE CALM WINDS AND THE EXPECTED DRY SINGLE DIGIT SFC DEWPOINTS.
NOT QUITE READY TO GO THAT COLD...BUT WILL KNOCK PREVIOUS MIN T
FORECAST DOWN A CATEGORY. LOWS IN THE LOWER TO MID 20S.
&&
.SHORT TERM /SATURDAY/...
AS OF 325 AM THURSDAY...
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.
&&
.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
AS OF 330 AM THURSDAY...
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 /18Z THURSDAY THROUGH TUESDAY/...
AS OF 100 PM THURSDAY...
VFR CONDITIONS THROUGH THE TAF PERIOD AS A COLD AND DRY ARCTIC
AIRMASS BUILDS OVER THE AREA. GUSTS OF 20 TO 25 MPH
WILL SUBSIDE THIS EVENING...WITH NO SIGNIFICANT GUSTS
EXPECTED ON FRIDAY.
A DEVELOPING LOW PRESSURE CENTER NEAR THE SOUTHEAST COAST
WILL BRING MVFR TO IFR CEILINGS OR VISIBILITIES TO THE AREA SATURDAY
EVENING THROUGH SUNDAY. VFR CONDITIONS WILL RETURN LATE SUNDAY NIGHT
INTO MONDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...RE
NEAR TERM...CBL
SHORT TERM...RLH
LONG TERM...SMITH
AVIATION...CBL
