Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 301049 RRA
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
645 AM EDT THU JUL 20 2015

.SYNOPSIS...A COLD FRONT WILL APPROACH FROM THE WEST TODAY...THEN
TRACK SOUTHEAST ACROSS THE CAROLINAS TONIGHT AND SETTLE SOUTH OF THE
AREA ON FRIDAY. MEANWHILE...A WEAK LOW PRESSURE SYSTEM OFF THE
SOUTHEAST COAST WILL TRACK NORTH NORTHEAST OFFSHORE THE CAROLINA
COAST.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 405 AM THURSDAY...

A COLD FRONT...WHICH CURRENTLY STRETCHES FROM EASTERN CANADA THROUGH
THE OHIO VALLEY TO THE MID-MISSISSIPPI VALLEY REGION...WILL APPROACH
FROM THE NORTHWEST TODAY AS A SHORTWAVE TRAVERSES THE GREAT LAKES ON
THE SOUTHERN PERIPHERY OF AN INTENSE VORTEX NEAR THE HUDSON.  THE
FRONT IS GENERALLY EXPECTED TO MOVE INTO THE WESTERN PIEDMONT BY
LATE AFTERNOON...REACHING THE TRIANGLE AREA PRIOR TO MIDNIGHT AND
THEN MOVE INTO THE COASTAL PLAIN EARLY FRIDAY MORNING. AHEAD OF THE
FRONT...CENTRAL NC LIES IN A RELATIVE MIN IN PW...WITH AREAS NEAR 2
INCHES TO OUR NORTH AND SOUTH.  STRONG HEATING AND 850MB TEMPS
APPROACHING 20C WILL PUSH TEMPS IN TO THE LOW TO MID 90S TODAY...BUT
INSTABILITY IS PROJECTED TO BE WEAK TODAY...AROUND 500-1000 J/KG
WITH A SLIGHT UPTICK IMMEDIATELY AHEAD OF THE FRONT OWING TO A
NARROW RIBBON OF PREFRONTAL MOISTURE CONVERGENCE.  MOST OF THE CAMS
AND LARGER SCALE NWP ARE IN GOOD AGREEMENT ON JUST A BAND OF
SCATTERED CONVECTION REACHING THE TRIAD AROUND 20Z OR SO...THEN
STRUGGLING TO MAINTAIN INTENSITY AND COVERAGE AS IT MOVES EAST IN TO
A LESS SUPPORTIVE ENVIRONMENT AFTER SUNSET.  SHEAR PARAMETERS WILL
BE WEAK THROUGHOUT AND WIDESPREAD SEVERE WEATHER IS NOT ANTICIPATED.

NOTICEABLY DRIER AIR BEHIND THE FRONT AND CLEARING WILL HELP ALLOW
TEMPS TO FALL INTO THE UPPER 60S IN THE NORTHWEST PIEDMONT..BUT
AREAS TO THE EAST SHOULD STILL SEE LOWS IN THE LOWER 70S AS THE
FRONT SLOWLY PUSHES EAST AND PRECIP CHANCES DWINDLING TOWARD 12Z
FRIDAY.

NOTE...THE WEAK LOW OFF THE GEORGIA COAST ...NOTED BY NHC AS HAVING
ONLY A 10 PERCENT CHANCE OF FURTHER DEVELOPMENT...WILL DRIFT NORTH
TODAY AND BECOME ABSORBED BY THE FRONT OFF THE OBX LATER
TONIGHT...WITH NO DIRECT SENSIBLE IMPACT INLAND.

&&

.SHORT TERM /FRIDAY AND FRIDAY NIGHT/...
AS OF 405 AM THURSDAY...

THE COLD FRONT WILL PUSH TOWARD THE COAST AND THEN STALL ON FRIDAY
AS A BROAD UPPER TROUGH SLOWLY BEGINS TO DEVELOP OVER THE EASTERN
US.  THE NAM AND GFS ARE FAR ENOUGH EAST WITH THE FRONT AND MORE
UNSTABLE AIRMASS TO BE COMPLETELY DRY ON FRIDAY AFTERNOON.  THE
ECMWF HANGS THE FRONT UP JUST A TOUCH FURTHER WEST AND DEVELOPS SOME
QPF EAST OF I-95.  WILL KEEP A SLIGHT CHANCE OF THE COASTAL
PLAIN...BUT MOST OF THE AREA WILL BE DRY AND MUCH LESS HUMID AS
DEWPOINTS HAVE THE POTENTIAL TO MIX OUT INTO THE UPPER 50S AND LOWER
60S BEHIND THE FRONT. ONLY A SMALL DROP IN THICKNESSES IS
FORECAST...SO HIGHS WILL STILL BE AROUND NORMAL...89-94.  LOWS
FRIDAY NIGHT IN THE MID 60S TO AROUND 70.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
AS OF 405 AM THURSDAY...

THE MODELS GENERALLY AGREE THAT A RELATIVELY AMPLIFIED FLOW
PATTERN...ONE CHARACTERIZED BY A STRONG RIDGE CENTERED OVER THE
SOUTHWESTERN US AND DOWNSTREAM A BROAD TROUGH EAST OF THE
ROCKIES...WILL PERSIST THROUGH THE MEDIUM RANGE...THOUGH WITH
INDICATION FROM THE ENSEMBLE MEANS OF SOME RELAXATION OF THE
FLOW/TOWARD LESS AMPLIFIED-MORE ZONAL LATE NEXT WEEK.

DESPITE THE PRESENCE OF A TROUGH AND CYCLONIC FLOW ALOFT OVER THE
EASTERN US...UNSEASONABLY DRY AIR WILL HAVE SPREAD IN THE LEE OF THE
APPALACHIANS...BEHIND A LEAD FRONTAL ZONE FORECAST TO CROSS THE RAH
FORECAST AREA LATER TONIGHT. THIS BOUNDARY WILL EFFECTIVELY PIN RICH
MOISTURE ALONG COASTAL AREAS...WITH MUCH DRIER AIR...PARTICULARLY
WITH DIURNAL HEATING/BL MIXING WESTWARD ACROSS CENTRAL NC. THIS
DRIER AIR WILL SIGNIFICANTLY HINDER THE DEVELOPMENT OF ANY
APPRECIABLE CAPE AND PRECIPITATION CHANCES THROUGHOUT THE FORECAST
PERIOD...WITH THE RELATIVELY HIGHER CHANCES/~25 PERCENT ALONG AND
EAST OF A PIEDMONT TROUGH EACH LATE AFTERNOON-EARLY EVENING.

ANY EVEN WEAK STORM WRT REFLECTIVITY...HOWEVER...WOULD POSE A RISK OF
STRONG...EVAPORATIVELY-COOLED DOWNBURST WINDS IN AN ENVIRONMENT
CHARACTERIZED BY STEEP/NEAR DRY ADIABATIC LOW LEVEL LAPSE RATES AND
LARGE/~30-35 DEGREE SURFACE DEWPOINT DEPRESSIONS...DRIVEN BY A
COMBINATION OF THE AFOREMENTIONED MIXING OUT OF AN ALREADY
CONTINENTAL AIR MASS...AND HOT SURFACE TEMPERATURES IN THE LOWER TO
MIDDLE 90S.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
AS OF 645 AM THURSDAY...

A COLD FRONT WILL APPROACH FROM THE NORTHWEST TODAY AND CROSS THE
AREA LATE THIS EVENING AND TONIGHT.  INSTABILITY IS EXPECTED TO BE
LIMITED WHEN THE FRONT ARRIVES...ESPECIALLY TONIGHT AS THE FRONT
MOVES EAST ACROSS CENTRAL NC.  SCATTERED SHOWERS AND A FEW ISOLATED
STORMS WILL BE POSSIBLE ALONG THE FRONT...FIRST AROUND KINT/KGSO
AFTER 21Z AND THEN KRDU/KRWI/KFAY IN THE 00Z TO 06Z TIME FRAME.
WHILE WIDESPREAD ADVERSE AVIATION CONDITIONS ARE NOT EXPECTED...A
HEAVY DOWNPOUR AND BRIEF PERIOD OF SUB-VFR CONDITIONS IS POSSIBLE AT
AT TAF SITE. LIGHT WINDS WILL SHIFT TO NORTHWESTERLY BEHIND THE
FRONT AS IT SETTLES SOUTH AND EAST OF THE AREA.

LOOKING AHEAD: THE COLD FRONT WILL STALL NEAR THE COAST ON
FRIDAY...WITH PREDOMINATELY VFR CONDITIONS EXPECTED THIS WEEKEND.

&&

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

&&

$$

SYNOPSIS...22
NEAR TERM...22
SHORT TERM...22
LONG TERM...26
AVIATION...22



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