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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KRAH 292009
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
ISSUED BY NATIONAL WEATHER SERVICE BLACKSBURG VA
309 PM EST THU JAN 29 2015

.SYNOPSIS...
-- Changed Discussion --
A COLD FRONT WILL MOVE EAST ACROSS THE AREA TONIGHT AND OFFSHORE
FRIDAY MORNING. HIGH PRESSURE WILL BUILD IN FROM THE WEST ON
FRIDAY...WHICH WILL HELP TO KEEP CONDITIONS DRY INTO THE WEEKEND.
LOW PRESSURE WILL APPROACH FROM THE SOUTHWEST SUNDAY BRINGING THE
NEXT CHANCE OF RAIN LATER SUNDAY INTO SUNDAY NIGHT OR EARLY
MONDAY.
-- End Changed Discussion --
&&

.NEAR TERM /THROUGH FRIDAY/...
-- Changed Discussion --
AS OF 245 PM EST THURSDAY...

INITIAL WAVE AXIS ALOFT IN ADVANCE OF THE FRONTAL SYSTEM TO THE WEST
CONTINUES TO PUSH A BAND OF MAINLY -RA EAST WHICH MAY PUSH ACROSS
MUCH OF THE CWA BEFORE SUNSET. THUS INCLUDING SOME LOW POPS WITH
HIGHER CHANCE CENTRAL/WEST TO INIT AND THEN MORE SLIGHT CHANCE
NATURE TOWARD THE SANDHILLS AND COASTAL PLAIN WHERE CURRENT BAND MAY
TEND TO FADE GIVEN DRY AIR IN PLACE.

SHOULD SEE A BREAK IN MOISTURE BETWEEN THIS LEAD SHORTWAVE AND THE
ACTUAL SURFACE COLD FRONT THAT WILL CROSS THE MOUNTAINS THIS EVENING
AND MOST OF CENTRAL NORTH CAROLINA SHORTLY AFTER MIDNIGHT. MODELS
SHOW LITTLE ADDED DEEP MOISTURE ALONG THE BOUNDARY ESPCLY WITH THE
FLOW TURNING MORE WESTERLY BEHIND THE FIRST IMPULSE. MAY SEE ANOTHER
RIBBON OF LOWER CLOUDS MOVE EAST WITH THE BOUNDARY BUT EXPECTING ANY
RESIDUAL MOISTURE TO STAY WELL TO THE WEST SO GRADUALLY TRIMMING
CLOUDS BACK TO PC OVERNIGHT. BEST 85H COLD ADVECTION WILL LAG BACK
OVER THE MOUNTAINS EARLY ON BUT THEN SURGE EAST LATE AS A TRAILING
5H VORT SWINGS THROUGH THE BASE OF THE UPPER TROUGH. THIS SHOULD
BRING ENOUGH COLD AIR TO PUSH LOWS MOSTLY IN THE 30S BUT PERHAPS
ONLY AROUND 40 SANDHILLS AND SE SECTIONS GIVEN MIXING AS THE COLD
AIR ARRIVES.

A BLUSTERY/WINDY DAY UNDER CHILLY TEMPS IN STORE ON FRIDAY AS STRONG
HIGH PRESSURE SLIDES EAST WHILE THE PRESSURE GRADIENT REMAINS QUITE
TIGHT BETWEEN THE RIDGE AND DEEPENING LOW PRESSURE OFF NEW ENGLAND.
APPEARS GUSTS TOPPING 30-35 MPH AT TIMES MAY APPROACH ADVISORY
LEVELS FOR A BRIEF PERIOD GIVEN DEEP MIXING AND AIDED VIA A
FINAL PASSING MID LEVEL WAVE TO THE NORTH. OTHERWISE STRONG
SUBSIDENCE SHOULD LEAD TO MAINLY SUNNY SKIES WITH HIGHS MOSTLY IN
THE 40S GIVEN HEATING OF VERY DRY AIR.
-- End Changed Discussion --
&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
-- Changed Discussion --
AS OF 235 PM EST THURSDAY...

COLD ADVECTION CONTINUES AS SURFACE HIGH APPROACHES FROM WEST BUT
WITH SURFACE HIGH STILL WEST OF APPALACHIANS BY 12Z SAT DO NOT THINK
CENTRAL NORTH CAROLINA WILL COMPLETELY DECOUPLE...DESPITE MAINLY
CLEAR SKIES. CHANCE OF SOME CIRRUS MOVING IN LATE FRIDAY NIGHT TOO
THOUGH. SO WILL NOT GO ANY COLDER THAN GUIDANCE FOR LOWS...WHICH IS
PRETTY CONSISTENT IN THE LOW 20S MOST OF THE AREA.

DRY HIGH PRESSURE DOMINATES THEN THROUGH SATURDAY NIGHT...AND MAY
CONTINUE ITS INFLUENCE INTO EARLY SUNDAY BEFORE MOISTURE INCREASES
FROM THE WEST AHEAD OF NEXT CLIPPER TYPE SYSTEM. WITH DRY LOW LEVELS
INITIALLY IT COULD TAKE PRECIP A WHILE TO MAKE IT TO THE GROUND ON
SUNDAY...STARTING IN WEST BY MIDDAY AND SPREADING TO THE EAST BY
LATE IN DAY OR EARLY EVENING. GIVEN DAYTIME START TO PRECIP AND WARM
ADVECTION AHEAD OF DEVELOPING BUT RATHER WEAK SURFACE LOW OVER
TENNESSEE VALLEY...NO CONCERNS ABOUT ANY LIGHT WINTRY PRECIP DESPITE
EVAPORATIVE COOLING INTO DRY AIRMASS.

MODEL CONSENSUS DOES AGREE THAT PRECIP CHANCES WILL BE HIGHEST
SUNDAY NIGHT AND QPF VALUES OF A QUARTER INCH OR HIGHER ARE
POSSIBLE...GFS WITH ALMOST A HALF AN INCH...BUT SEE NO CONCERNS AT
ALL FOR BOUNDARY LAYER OF SURFACE TEMPS TO SUPPORT ANY WINTRY PRECIP
EVEN OVERNIGHT AHEAD OF THE FRONT. IN FACT IT MAY BE WARMING
OVERNIGHT SUNDAY NIGHT...BUT AS YET HAVE NOT REFLECTED THAT ON
HOURLY TEMPS...BUT DO NOT HAVE IT DROPPING VERY MUCH EITHER SUNDAY
NIGHT. LATEST MEX GUIDANCE FROM GFS MUCH MILDER AND TRENDED UP A BIT
BUT NOT THAT HIGH AS OF YET.  LOWS SUNDAY NIGHT IN UPPER 30S
NORTHWEST TO MID 40S SOUTHEAST...BUT SEE POTENTIAL FOR IT TO BE EVEN
MILDER.
-- End Changed Discussion --
&&

.LONG TERM /MONDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
AS OF 300 PM EST THURSDAY...

MONDAY: MODEL TRENDS...AND NOW A LITTLE BETTER CONSENSUS...TO DEEPEN
AND SLOW THE EXITING LOW PRESSURE SYSTEM AND FRONT AS IT MOVES OFF
THE MID ATLANTIC COAST COULD NOW MEAN SOME LINGERING SHOWERS ACROSS
CENTRAL NORTH CAROLINA FOR AT LEAST EARLY IN THE DAY MONDAY...BUT
SHOULD PUSH EAST BY AFTERNOON. STILL GOING TO KEEP A SLIGHT CHANCE
EAST OF RALEIGH FOR SECOND HALF OF DAY IN CASE ADDITIONAL SLOWING
OCCURS. COLDER AIR THEN ALSO A LITTLE SLOWER TO FILTER IN BEHIND
SYSTEM...SO BUMPED UP HIGHS JUST A COUPLE DEGREES.  MAY END UP BEING
NEARLY STEADY MUCH OF DAY AND DROPPING QUICKLY BY EARLY AFTERNOON
BUT GIVEN THE UNCERTAINTY IN TIMING BEING FOUR DAYS OUT...DECIDED
NOT TO SHOW THIS NON DIURNAL TREND JUST YET.

MONDAY NIGHT THROUGH TUESDAY NIGHT: CHILLY HIGH PRESSURE TO BUILD IN
BEHIND THE FRONT...1025 TO 1030 MB STRENGTH. HIGH CONFIDENCE THAT
ANY PRECIP WILL BE WELL EAST OF THE AREA BY MONDAY NIGHT...WITH THE
COLD AND DRY HIGH PRESSURE IN CONTROL THROUGH TUESDAY AND INTO
TUESDAY NIGHT....BUT MILDER TUES NIGHT WITH WARM ADVECTION AND CLOUD
COVER ALREADY INCREASING AHEAD OF NEXT SYSTEM IN THIS PROGRESSIVE
PATTERN.

MORE UNCERTAINTY COMES INTO PLAY BY MID WEEK AS NORTHER AND SOUTHERN
BRANCHES OF THE JET STREAM TRY TO PHASE...AND A COLD FRONT
APPROACHES FROM THE WEST BY LATE WED...WHILE SHORT WAVE ENERGY IN
THE SOUTHERN STREAM TAPS SOME MOISTURE FROM THE GULF WHICH HEADS
NORTHEAST TOWARD NORTH CAROLINA.  CURRENTLY THERE ARE FAIRLY
SIGNIFICANT DIFFERENCES IN THE TIMING AND AMOUNT OF MOISTURE WITH
THIS SYSTEM...WITH 00Z ECMWF FASTER AND INCREASING THE CHANCES FOR
PRECIP AS EARLY AS LATE TUESDAY NIGHT INTO WED MORNING...WITH THE
12Z GFS NOT UNTIL DURING THE DAY WED. MOST GFS ENSEMBLES SEEM TO BE
IN THIS CAMP AS WELL. SO AM NOT INDICATING ANY CHANCE POPS UNTIL
DURING THE DAY WED AT THIS POINT.  GFS MODEL SOUNDINGS SUGGEST IF
THIS COMES IN EARLY ENOUGH THERE COULD BE SOME LIGHT WINTRY MIX AT
THE ONSET BUT THINK THIS WILL BE VERY LIGHT AND HAVE NO IMPACT. MOST
LIKELY PRECIP WILL HOLD OFF UNTIL LATER WHEN BOUNDARY LAYER TEMPS
ARE WARM ENOUGH TO SUPPORT RAIN ONLY. DO HAVE SOME CHANCE RAIN AND
SNOW IN THE GRIDS FOR WHEN TEMPS ARE AT OR BELOW FREEZING EARLY
WED...AND AGAIN LATE WED NIGHT BEHIND COLD FRONT...BUT THIS MAY
AMOUNT TO NO MORE THAN FLURRIES.

THEN BETTER CONFIDENCE IN BLUSTERY AND COLDER FOR THURSDAY BEHIND
FRONT...AND MAY EVENTUALLY NEED TO GO COLDER THAN CURRENT FCST BUT
HEDGING SOMEWHAT FOR NOW SINCE GFS IN PARTICULAR TENDS TO HAVE A
COLD BIAS BEHIND FRONTS THAT FAR OUT.
-- End Changed Discussion --
&&

.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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AS OF 1202 PM EST THURSDAY...

24 HOUR TAF PERIOD: BANDS OF HIGH AND MID CLOUDS WILL CONTINUE TO
SWEEP EAST THIS AFTERNOON AHEAD OF AN APPROACHING COLD FRONT JUST
WEST OF THE MOUNTAINS. CEILINGS MAY LOWER BELOW 8K FEET ACROSS
THE WESTERN SITES THIS AFTERNOON AS A DEEPER BAND OF MOISTURE
SWINGS EAST AND WEAKENS. THIS COULD RESULT IN A PERIOD OF VERY
LIGHT RAIN ACROSS THE REGION. HOWEVER EXPECT CEILINGS/VSBYS TO
REMAIN VFR THROUGH THE TAF PERIOD AS THIS FRONT SLIDES ACROSS THE
FROM THE WEST TONIGHT. THERE IS A SLIGHT CHANCE THAT CEILINGS
COULD APPROACH MVFR LEVELS BETWEEN 03Z-08Z /10PM-3AM BUT IFFY AND
MAINLY WEST OF KRDU AT THIS POINT. SOUTH TO SOUTHWEST WINDS MAY
ALSO GUST TO BETWEEN 15-20 KTS THROUGH LATE THIS AFTERNOON BEFORE
RELAXING AGAIN AFTER SUNSET.

THE PASSING COLD FRONT WILL BRING A SHIFT IN WINDS TO THE
WEST/NORTHWEST AFTER MIDNIGHT FOLLOWED BY GUSTY NORTHWEST WINDS
FOR MUCH OF THE DAY ON FRIDAY. APPEARS SUSTAINED NORTHWEST WINDS
OF 15-22 KNOTS LIKELY WITH GUSTS TO 25-30 KNOTS OR PERHAPS HIGHER
NEAR THE END OF THE VALID TAF PERIOD.

LONG TERM: SUB-VFR CONDITIONS WILL BE POSSIBLE WITH A LOW PRESSURE
SYSTEM THAT WILL CROSS THE AREA LATE SUNDAY INTO MONDAY. VFR
CONDITIONS WILL PREVAIL ON TUESDAY AS HIGH PRESSURE BUILDS EAST
BEHIND THIS SYSTEM.
-- End Changed Discussion --

&&

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

&&

$$
SYNOPSIS...JH
NEAR TERM...JH
SHORT TERM...SK
LONG TERM...SK
AVIATION...JH