Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 172004
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
404 PM EDT SUN AUG 17 2014

.SYNOPSIS...
A WEAK COLD FRONT WILL MOVE THROUGH THE APPALACHIANS FROM THE WEST
ON MONDAY. THIS WEAK BOUNDARY WILL GENERALLY LINGER OVER OUR AREA
THROUGH MUCH OF THE WEEK KEEPING A GOOD CHANCE FOR SHOWERS AND
THUNDERSTORMS EVERY DAY.

&&

.NEAR TERM /THROUGH MONDAY/...
AS OF 245 PM EDT SUNDAY...

CONVECTION REMAINS AT A MINIMUM LATE THIS AFTERNOON GIVEN LACK OF DEEP
INSTABILITY PER SPC ANALYSIS AND RESIDUAL CLOUD COVER. APPEARS COULD
STILL POP A SHOWER IN THE NEAR TERM ACROSS THE MOUNTAINS DUE TO
HEATING...OTRW MAIN CONCERN WITH THE LEADING EDGE PRECIP OVER EASTERN
KY/TN WELL AHEAD OF THE UPSTREAM WAVE BACK OVER SOUTHERN INDIANA.
MODELS NOT HANDLING THIS WELL BUT AGAIN WILL BE MOVING INTO AN AREA
OVER THE FAR WEST WHERE CAPE VALUES ARE LOW...AND LOW LEVEL THETA-E
LACKING AROUND THE TIME OF LOSS OF INSOLATION. THUS WILL ONLY KEEP LOW
POPS MAINLY I-77 CORRIDOR INTO THIS EVENING AND SHOW A DECREASE BY
MIDNIGHT WITH COMBO OF THIS INITIAL AXIS FADING AND LOSS OF ANY
RESIDUAL INSTABILITY. POTENT WAVE ALOFT MOVES EAST ALONG THE FRONT
OOZING IN FROM THE NORTH OVERNIGHT TO ACROSS THE OHIO RIVER BY MORNING.
GUIDANCE BRINGING THE NEXT ROUND OF LIFT INTO THE WEST BY DAYBREAK IF
NOT A BIT SOONER BASED ON PAST EVENTS IN WESTERLY FLOW ALOFT. THUS
PUSHED CAT/LIKELY POPS INTO THE FAR WEST LATE...WITH DECENT CHANCE POPS
OUT TO THE BLUE RIDGE AND SLIGHTS ACROSS THE PIEDMONT BY 12Z. OTRW
PARTLY TO BECOMING MOSTLY CLOUDY WITH MILDER LOWS MOSTLY IN THE 60S PER
MOISTENING DEWPOINTS.

STRONG VORT CENTER PUNCHES EAST ACROSS THE AREA MONDAY WITH MODEL
DIFFERENCES ON WHERE THE SURFACE  GOES AND WHETHER OR NOT A
SECONDARY WAVE FORMS ALONG THE TRAILING TROUGH AXIS TO THE SOUTH.
SOLUTIONS STILL RANGE FROM A FLATTER ECMWF WITH HEAVIER RAINFALL GOING
OVER SOUTHERN VA/NW NC TO THE SLOWER NAM WHICH TAKES THE BEST LIFT UP
ALONG THE NW SLOPES AND THEN EAST NORTH OF HIGHWAY 460 LATER MONDAY.
LATEST ENSEMBLE RUN REMAINS IN BETWEEN WITH AN AXIS OF HEAVIER SHOWERS
NW THEN DIPPING SOUTH ALONG THE BLUE RIDGE INTO NW NC AS PERHAPS A
SECOND WAVE DEVELOPS SO FOLLOWED THIS SCENARIO A BIT CLOSER. COMBO OF
DECENT LIFT...HIGH PWATS OF NEARLY 2 INCHES AND VEERING ALOFT WOULD
SUGGEST A PERIOD OF MODERATE TO HEAVY RAIN ACROSS MUCH OF THE AREA
MONDAY...WITH EMBEDDED CONVECTION LIKELY ESPCLY NW AND OVER THE EAST
WHERE MAY SQUEEZE OUT A LITTLE MORE INSTABILITY.

GIVEN A FEW DAYS OF DRYING SINCE THE LAST HEAVY RAIN EVENT WILL TAKE
HIGHER RAINFALL RATES OR PROLONGED BANDS OF SHRA/TSRA TO PRODUCE
FLOODING EXCEPT OVER PARTS OF SE WEST VA WHERE FFG REMAINS RATHER LOW.
SINCE IFFY AS TO RATES...WILL HOLD OFF ON A WATCH FOR NOW BUT A GOOD 1-
2 INCHES POSSIBLE WHERE THE CORE OF THE WAVE TRACKS ON MONDAY SO
WIDESPREAD CAT/LIKELY POPS. HIGH TEMPS REMAIN QUITE TRICKY PENDING
EXPANSE OF HEAVY RAIN AS MAY SEE A WEAK WEDGE FORM IF RAIN ARRIVES
EARLIER SO TRIMMED BACK HIGHS TO MOSTLY 70S...EXCEPT LOW/MID 80S FAR
SOUTH WHICH MAY BE TOO WARM.
&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
AS OF 345 PM EDT SUNDAY...

TIMING OF WAVE EXITING FORECAST AREA IS TRICKY FOR MONDAY
EVENING...BUT LEANING TOWARD THE SLOWER NAM SOLUTION AT THIS POINT
PARTLY DUE TO CURRENT POSITION OF SURFACE AND UPPER LOW STILL BACK
OVER SRN ILLINOIS WHICH IS MORE IN LINE WITH 18Z POSITION IN NAM
VS FASTER GFS. THUS LEFT SOME HIGH LIKELY TO LOW CATEGORICAL POPS
IN FOR THE NORTHEAST FOR MONDAY EVENING...BEFORE EXITING
LATE...BUT STILL SOME LINGERING UPSLOPE SHOWERS IN FAR WEST.
BEHIND THIS WAVE THERE APPEARS TO BE SOME SUBSIDENCE AND ALSO THE
OVERNIGHT SHOWERS COULD LEAVE ATMOS FAIRLY STABLE FOR TUES...SO
THINK THIS WILL BE A BIT OF A DOWN DAY FOR PRECIP CHANCES BUT LEFT
SOME LOW CHANCE POPS IN MAINLY BLUE RIDGE AND WEST. MAY NEED TO
LOWER THESE EVEN MORE BUT FOR NOW TRENDED DOWNWARD. UNCERTAINLY AS
FAR AS HOW MUCH CLOUD COVER WILL REMAIN BEHIND THIS WAVE...BUT
EVEN WITH SOME PARTIAL CLEARING ODDS ARE THAT CU WILL REFORM
QUICKLY SO NOT A GREAT DEAL OF SUN. STILL...MUCH WARMER THAN
MONDAY WITH LACK OF PRECIP...PERHAPS BRIEF SUN...AND DOWNSLOPE
INTO PIEDMONT. STILL GOING JUST A TAD BELOW GUIDANCE THOUGH WHICH
NOW HAS LOWER 90S FOR DAN. STICKING WITH UPPER 80S FOR WARMEST
TEMPS.

TIMING OF SHOWERS AND ANY THUNDERSTORMS THEN BECOMES CHALLENGING
WITH MODELS VERY DIFFERENT ON ARRIVAL OF CONVECTIVELY DRIVEN
DISTURBANCES FROM UPSTREAM...AS WELL AS THEIR TRACKS. SEEMS NOW
MOST LIKELY TIMING IS THE NEXT SIGNIFICANT WAVE COMING BY TUES
NIGHT OR EARLY WED MORNING...SO THIS MIGHT MEAN A BETTER CHANCE
FOR SHOWERS...PERHAPS THUNDER...IN THE WEST OVERNIGHT TUES NIGHT
AND LESS OF A CHANCE DURING THE DAY WED. NEW 12Z ECMWF A LITTLE
SLOWER AND SUGGEST THIS NEXT WAVE MAYBE EARLY WED MORNING.
STILL..DAYTIME HEATING MAY BE ENOUGH TO KICK OF ADDITIONAL
ACTIVITY DURING THE DAY WED BUT THIS MAY BE MAINLY IN THE
MOUNTAINS. GIVEN THE COMPLEX AND HIGHLY UNCERTAIN NATURE OF THESE
WAVES AND LINGERING FRONTAL BOUNDARY...WE REALLY WILL NOT GET A
DECENT HANDLE ON THE TIMING AND LOCATION OF WHERE TO PUT HIGH
CHANCES OR LIKELY POPS VERSUS LOW CHANCES OR SLIGHT CHANCE UNTIL
MAYBE 24 HRS IN ADVANCE...SO AT THIS POINT AM STICKING WITH
SHOWING THE BEST CHANCES IN MOUNTAINS...BUT LINGERING CHANCES
OVERNIGHT AND EVEN WELL AFTER MIDNIGHT...GIVEN THE FORCING FROM
THESE DISTURBANCES AND SURFACE BOUNDARY...ALTHOUGH VERY HARD TO
SAY REALLY HOW MUCH INFLUENCE THAT WILL HAVE. 850 WESTERLY FLOW
WILL HELP AID CHANCES IN MOUNTAINS VS PIEDMONT AS WELL. BEST
CHANCES FOR THUNDER DURING THE AFTERNOON AND EVENING...BUT LEFT A
SLIGHT CHANCE LATE AT NIGHT AND EARLY MORNING.

DESPITE ONGOING PRECIP CHANCES THROUGH THE PERIOD...INCLUDING
OVERNIGHT...DO NOT SEE A HIGH THREAT FOR ESPECIALLY HEAVY
RAINFALL RATES OR THE SAME AREAS GETTING HIT DAY AFTER DAY AS THE
WAVES AND BOUNDARY ARE LIKELY TO SHIFT AROUND EACH DAY AND PRECIP
WATER VALUES ACTUALLY A LITTLE BELOW CLIMATOLOGY...SO NO MAJOR
CONCERNS FOR FLASH FLOODING...BUT WORTH KEEPING AN EYE ON.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
AS OF 100 PM EDT SUNDAY...

FORECAST FOR LATE IN THE WEEK INTO NEXT WEEKEND REMAINS HIGHLY
UNCERTAIN STARTING WITH A SOMEWHAT PROGRESSIVE PATTERN AND LOW
AMPLITUDE TROUGH MOVING ACROSS NE US...THEN TRYING TO BECOME HIGHER
AMPLITUDE WITH BUILDING RIDGE INTO GREAT LAKES REGION AND THE TROUGH
OFF NEW ENGLAND COAST DEEPENING AND SLOWING DOWN.  THIS GENERAL
PATTERN WOULD INITIALLY HAVE OUR REGION IN WESTERLY TO NW FLOW WITH
A WEAK FRONTAL BOUNDARY SHIFTING ACROSS THE REGION THE REMAINING
NEARLY STATIONARY...BEFORE EVENTUALLY GETTING PUSHED SOUTH OF THE
AREA BY A BACK DOOR PUSH AS NEARLY STATIONARY TROUGH OFF NEW ENGLAND
DEEPENS. LAST NIGHTS GFS RUNS SHOWED HIGHER AMPLITUDE PATTERN THEN
ECMWF AND GFS ENSEMBLES WERE IN DECENT AGREEMENT.  THIS WOULD ARGUE
FOR STRONGER PUSH OF THE BACK DOOR FRONT AND THUS COOLING OFF BY AT
LEAST LATE IN THE WEEKEND WITH DEEPER MOISTURE AND HEAVIER RAINFALL
RATES DROPPING TO OUR SOUTH...OR AT LEAST FAR SW PART OF FORECAST
AREA.  NOW THE NEW 12Z GFS IS EVEN WEAKER AMPLITUDE THAN 00Z ECMWF
AND ALLOWS MORE RIDGING INTO WESTERN PART OF FORECAST AREA. THIS
WOULD LEAVE A NEARLY STATIONARY BOUNDARY MORE OVER THE HEART OF THE
FORECAST AREA. AND KEEP CHANCES FOR CONTINUED SHOWERS AND STORMS
FAIRLY HIGH. ALSO WARMER ACROSS AT LEAST THE SOUTHWEST PART OF AREA.

GIVEN ALL THIS UNCERTAINTY...BEST APPROACH IS TO LEAN TOWARD
CLIMATOLOGY AND KEEP AT LEAST LOW CHANCE POPS UP THROUGH THIS ENTIRE
PERIOD...AND WHILE A DIURNAL TREND IS STILL LIKELY...NEED TO KEEP
CHANCE POPS UP OVERNIGHT WITH BOUNDARY HANGING AROUND. USUALLY WHEN
HIGHER AMPLITUDE FLOW BEGINS TO DEVELOP...THE MODELS DONT BRING BACK
DOOR FRONT IN STRONGLY ENOUGH AND GIVEN THE KIND OF SUMMER WE HAVE
ALREADY HAD...WILL STILL PUT SOME WEIGHT TOWARD THIS HIGHER
AMPLITUDE IDEA LIKE PREVIOUS GFS RUN. THUS A BIT COOLER OVER THE
WEEKEND...MORE SO SUNDAY...AND A BIT OF SHIFT IN HIGHER POPS TOWARD
SOUTHWEST VA AND NW NC MOUNTAINS.  BUT OVERALL WILL HAVE GENERALLY
HIGHER POPS IN MTNS AND LOW CHANCE TO SLIGHT CHANCE POPS IN PIEDMONT.

BOTTOM LINE GIVEN ALL THIS UNCERTAINTY IN PATTERN...AND ALSO
POSSIBILITY THAT NOCTURNAL CONVECTION COULD CONTINUE WITH AID OF
FRONTAL BOUNDARY AND UPSTREAM WAVES FROM MIDWEST CONVECTION...JUST
NEED TO KEEP POPS UP THROUGH THIS ENTIRE PERIOD AND DO NOT FEEL
CONFIDENT TO GO LIKELY POPS ANYWHERE YET...NOT NOT AS LOW AS SLIGHT
CHANCE IN TOO MANY PLACES EITHER.  IF FRONTAL BOUNDARY HANGS AROUND
ALL WEEKEND...WE MAY BE DEALING WITH SOME SIGNIFICANT PRECIP AMOUNTS
IN SOME AREAS...BUT HOPEFULLY THIS STRONGER BACKDOOR PUSH OF THE
BOUNDARY WITH HIGHER AMPLITUDE PATTERN WILL PUSH IT OFF TO THE SOUTH
AS THE WEEKEND PROGRESSES.  EXPECT QUITE A FEW CHANGES AS THAT TIME
FRAME DRAWS NEAR.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
AS OF 100 PM EDT SUNDAY...

GOOD CONFIDENCE THROUGH FIRST PORTION OF TAF PERIOD...DECREASING
THEREAFTER.

WEAK DYNAMIC FORCING ASSOCIATED WITH A WEAK FRONTAL BOUNDARY OVER
THE AREA WILL COMBINE WITH SOME LOW LEVEL INSTABILITY TODAY TO
GET SOME ISOLATED SHOWERS GOING ACROSS THE AREA. HOWEVER...THE
ISOLATED COVERAGE AND LACK OF A SPECIFIC FOCUS INDICATES GOING
WITH VCSH FOR THE EARLY PART OF THE TAF PERIOD WOULD BE MOST
REPRESENTATIVE FOR THE TAF SITES. AS TEMPS CLIMB INTO THE
80S...CONVECTIVE LIFTING SHOULD DEVELOP A VFR CIG OF CU GOING TO
SC WITH SOME AC ABOVE...THEN LOSING THE LOWER CLOUD DECK AS WE
HEAD INTO THE EVENING.

UPPER DYNAMICS WHICH WERE HELPING TO DRIVE CONVECTION LAST NIGHT
WILL BE LESSENING A BIT AS THEY APPROACH TAF SITES TONIGHT. BELIEVE
THE OVERALL FLAVOR OF THE FORECAST WILL BE FOR CONDITIONS TO BECOME
INCREASINGLY MORE UNSETTLED AS WE GET PAST MIDNIGHT. THIS WILL
BRING AN INCREASING CHANCE OF SHOWERS ALONG WITH GRADUAL LOWERING
OF CIGS TO THE EDGE OF VFR AS WE GET TOWARD DAYBREAK. BELIEVE
THERE WILL BE ENOUGH HIGHER AC CLOUDS AROUND TO KEEP FOG FORMATION
FROM BOTTOMING OUT VISIBILITIES SO WILL KEEP THEM IN THE MVFR
RANGE FOR PREVAILING AND TEMPO GROUPS. THUNDER BECOMES MORE LIKELY
AS INSTABILITY INCREASES LATE IN THE TAF PERIOD.

WINDS WILL BE ON THE LIGHT SIDE AND WILL NOT POSE A SIGNIFICANT
PROBLEM FOR AVIATION THROUGH THE TAF PERIOD.

     EXTENDED AVIATION...

FRONT IS EXPECTED TO STALL NEAR OUR AREA THROUGH THE MIDDLE
OF THE WEEK AS A SERIES OF UPPER-LEVEL DISTURBANCES TRACK FROM
WNW-ESE ACROSS THE REGION. MAIN ACTIVITY EXITS THE EAST BY TUESDAY
MORNING...WITH MAINLY DIURNALLY DRIVEN SHOWERS AND THUNDERSTORMS
ANTICIPATED FOR THE REMAINDER OF THE WEEK. INCREASED MOISTURE AND
RAIN CHANCES WILL RESULT IN A DAILY THREAT FOR LATE NIGHT/EARLY
IFR- LIFR CONDITIONS IN FOG AND LOW CLOUDS AT THE USUAL
SITES...LWB/BCB/LYH...WITH MVFR POSSIBLE AT OTHER LOCATIONS.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...NONE.
NC...NONE.
WV...NONE.

&&

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


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