799
FXUS61 KRNK 210742
AFDRNK
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
242 AM EST SAT NOV 21 2009
SYNOPSIS
HIGH PRESSURE WILL REMAIN IN THE AREA TODAY INTO EARLY TONIGHT. LOW
PRESSURE DEVELOPING ACROSS THE GULF COAST STATES WILL IMPACT OUR
WEATHER SUNDAY WHILE HIGH PRESSURE WEDGES DOWN FROM THE NORTHEAST.
NEAR TERM /THROUGH TONIGHT/
THE 00Z MODELS HAVE CONTINUED TO TREND DRIER THIS RUN WITH THE
SYSTEM COMING OUT OF THE GULF COASTAL REGION. THE 00Z GFS ATTM...IS
DOING A BETTER JOB HANDLING THE CS POURING OVER THE REGION AHEAD OF
THE NRN STREAM TROUGH ACROSS THE MIDWEST. THESE HIGH CLOUDS HAVE
LIMITED FOG DEVELOPMENT...BUT STILL SHOULD SEE SOME THINNING AROSS
THE RIVER VALLEYS SO KEPT SOME PATCHY FOG IN THIS MORNING.
FOR TODAY...THE MAIN FORECAST CHALLENGE WILL BE EXTENT OF CLOUDINESS
AND EFFECT ON TEMPS. DURING THE LATE FALL MONTHS THE GFS SEEMS TO DO
A BETTER JOB WITH HIGH CLOUDS IN TERMS OF ITS RH DEPICTION IN THE
UPPER LVLS...AND THE AMOUNT OF OPAQUENESS IS SOMETIMES
UNDERESTIMATED. FOR NOW...WITH THE NRN STREAM SHORTWAVE MOVING
ACROSS THE OHIO VALLEY TODAY THINK THE HIGH CLOUDS WILL GET SHUNTED
EAST...WITH SOME LOWER CLOUDS ARRIVING IN THE NWRN CWA...ALTHOUGH
NOT IMPRESSED WITH THAT ATTM. WITH THIS IN MIND...HAVE RUN WITH A
MIDDLE OF THE ROAD APPROACH FOR SKY COVER WITH 40-60 PERCENT
COVERAGE. HIGHS AS SUCH WILL REMAIN CLOSE TO THE PREVIOUS FORECAST
OF 50S IN THE MTNS TO LOWER 60S OUT EAST...CLOSE TO A MOS BLEND.
TONIGHT...THE NRN STREAM SHORTWAVE SLIDES SLOWLY INTO THE MID
ATLANTIC STATES WITH SHORTWAVE RIDGING OCCURRING OVER OUR
CWA....WHILE THE TROUGH OVER TX SLIDES INTO THE GULF COAST STATES.
HAVE BACKED OFF ON POPS THINKING THAT THE RAIN SHIELD STAYS SOUTH OF
OUR CWA UNTIL AFTER 12Z SUNDAY. HOWEVER...KEPT THE CHANCE IN THERE
FOR POTENTIAL OF NRN STREAM SHORTWAVE SLIDING FURTHER EAST. THE E-SE
FLOW LATE TONIGHT MAY HELP SATURATE THE LOW LVLS OVER THE NC MTNS.
ALL IN ALL...PRETTY DRY TONIGHT...WITH CLOUDS DECREASING IN THE
NORTH BUT INCREASING AFTER MIDNIGHT OVER THE SOUTH. LOWS TONIGHT
WILL REMAIN CLOSE TO MOS...DROPPED A COUPLE DEGREES FROM PREVIOUS
FORECAST DUE TO SLOWER ARRIVAL OF CLOUDS AND SOME EVAP COOLING
POTENTIAL WITH VIRGA IN THE SOUTH.
SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
SIGNIFICANT PLACEMENT AND TIMING ISSUES EXIST BETWEEN THE MID-
RANGE MODELS IN HANDLING THE EJECTION OF THE SOUTHERN STREAM
CLOSED LOW...WHICH WILL EVENTUALLY LIFT AND FILL AS IT MOVES
NORTHEAST OUT OF THE GULF COAST STATES.
NAM AND GEM SLOWEST OF THE MODELS BY ALLOWING NORTHERN STREAM ENERGY
TO BYPASS BLACKSBURG AREA WELL TO THE NORTH. PASSAGE OF NORTHERN
STREAM TROF AND SUBSEQUENT BUILDING HEIGHTS IN ITS WAKE SUPPORTS
AMPLIFICATION OF NEW ENGLAND SURFACE HIGH PRESSURE WHICH WEDGES
SOUTHWEST INTO MID ATLANTIC REGION...AND RETARDS NORTHEASTWARD PUSH
OF SOUTHERN STREAM ENERGY AND ADVANCE OF DEEPER MOISTURE PROFILES.
LATEST RUN OF GFS AND PREVIOUS RUN OF ECMWF A LITTLE MORE
PROGRESSIVE WITH ADVANCE OF SOUTHERN STREAM ENERGY...BUT PERHAPS A
BIT SLOWER THAN EARLIER RUNS.
FINAL PLACEMENT AND TIMING OF THIS SYSTEM WILL HAVE A HUGE IMPACT ON
TEMPERATURES AND THREAT OF PRECIPITATION DURING THE MID RANGE
PERIOD. CONSIDERING THE EXTENT OF SURFACE RIDGING EXPECTED TO OCCUR
SOUTHWEST INTO THE MID ATLANTIC REGION IN THE WAKE OF NORTHERN
STREAM SHORT WAVE...EXPECT TO SEE AMPLE MID AND HIGH LEVEL CLOUD
CANOPY ON SATURDAY NIGHT...BUT WITH ATMOSPHERE BELOW 600 MILLIBARS
REMAINING QUITE DRY...NORTHERN EDGE OF PRECIPITATION SHIELD IN
ADVANCE OF SOUTHERN STREAM WAVE WILL LIKELY BE HARD PRESSED TO REACH
THE SURFACE WITH THE EXCEPTION OF THE EXTREME SOUTHWESTERN MOUNTAINS
BY LATE AT NIGHT. AN EARLY DROP OF TEMPERATURE IS EXPECTED DUE TO
RADIATIVE COOLING...ESPECIALLY ACROSS THE NORTHERN PORTION OF THE
FORECAST AREA WHERE CLOUD COVER WILL BE INITIALLY QUITE THIN...TO BE
FOLLOWED BY A FLATTER FALL AS CLOUDS LOWER AND THICKEN. EXCEPTION
WILL BE ACROSS THE EXTREME SOUTHERN AREAS...WHERE THICKER CLOUD
COVER WILL PREVENT RAPID INITIAL DROP IN TEMPERATURE...WITH
REDUCTION OCCURRING LATE AT NIGHT IF PRECIPITATION ARRIVES AND
DRIVES TEMPERATURES TOWARD WET BULB VALUES.
TENACIOUS SURFACE WEDGE WILL LIKELY HOLD OVER THE BLACKSBURG
FORECAST AREA THROUGHOUT SUNDAY...AND WILL LIKELY STRENGTHEN AS THE
DAY PROGRESSES AS WEAK UPSLOPING AND EVAPORATIVE COOLING DUE TO
PRECIPITATION BEGINS OVER AT LEAST PORTIONS OF THE FORECAST AREA.
WITH STRONG WEDGE IN PLACE...BEST BAROCLINIC ZONE AND ISENTROPIC
LIFT WILL REMAIN SOUTH AND EAST OF THE BLACKSBURG AREA...ALTHOUGH
UPPER DIFFLUENCE RESULTING FROM RIGHT ENTRANCE REGION OF NORTHERN
STREAM AND LEFT EXIT REGION OF SOUTHERN STREAM WILL BE IN/NEAR THE
BLACKSBURG AREA OF CONCERN.
ABUNDANT CLOUD COVER...NORTHEASTERLY SURFACE SLOW...AND INCREASING
THREAT OF PRECIPITATION SHOULD HOLD TEMPERATURES DOWN TO BELOW THAT
FORECASTED BY MOS GUIDANCE...AND THIS ALREADY WELL DEPICTED IN
CURRENT SET OF GRIDS. WITH DEEPEST MOISTURE AND BEST UPWARD VERTICAL
MOTION FIELDS THOUGH INCREASINGLY SATURATED COLUMN OF ATMOSPHERE
DISPLACED TO THE SOUTH AND EAST...WILL CONTINUE TO RUN HIGHEST
THREAT OF PRECIPITATION ACROSS SOUTHERN AND EASTERN ZONES...
TRENDING DOWNWARDS FURTHER NORTHWEST TOWARD OUR WV COUNTIES.
SOUTHERN STREAM ENERGY AND RESULTING DEVELOPMENT OF COASTAL LOW
PRESSURE SHOULD LIFT NORTHEAST OF THE BLACKSBURG FORECAST AREA
SUNDAY NIGHT INTO MONDAY...TO BE FOLLOWED BY SLOWLY CLEARING SKIES
FROM THE SOUTHWEST MONDAY AFTERNOON INTO MONDAY NIGHT AS WEAK HIGH
PRESSURE BEGINS TO BUILD IN FROM THE WEST.
AS WOULD BE EXPECTED UNDER A STRONG WEDGE SITUATION...MOS GUIDANCE
LIKELY TOO WARM BY DAY AND CLOSE TO OR A BIT COOL BY NIGHT
CONSIDERING THE VERY COMPRESSED DIURNAL TEMPERATURE CURVE THAT WILL
BE REALIZED THROUGH THIS PERIOD.
LONG TERM /TUESDAY THROUGH FRIDAY/
ECMWF INDICATES UPPER LEVEL RIDGING APPROACHING THE AREA TUESDAY
AFTERNOON INTO EARLY WEDNESDAY MORNING...WHILE THE SURFACE WEDGE
HANGS ON THROUGH THE END OF THE WEEK. IT ALSO SUGGESTS A SURFACE AND
UPPER LOW PRESSURE SYSTEM DEVELOPING OVER THE MIDWEST AND PUSHING
TOWARD THE AREA BY MID TO LATE WEEK. HAVE AT LEAST CHANCE TO SLIGHT
CHANCE OF POPS ACROSS THE COUNTY WARNING AREA THROUGH THE EXTENDED
PERIOD. WITH THICKNESSES 1000-850MB AND 850-700MB THICKNESSES
LOWERING BY MID WEEK...THERE COULD BE SOME LIGHT SNOW MIXED WITH
RAIN IN THE HIGHER ELEVATIONS OF SOUTHEAST WEST VIRGINIA. IN THESE
SAME AREAS MAINLY ALONG THE RIDGES...SURFACE LOW TEMPERATURES COULD
ALSO REACH FREEZING OR A FEW DEGREES BELOW.
GFS HAS BEEN HAVING A HARD TIME WITH RUN TO RUN CONSISTENCY IN THE
EXTENDED. IT HAS BEEN FASTER AND MORE PROGRESSIVE WITH PUSHING THE
SYSTEMS THROUGH THE FORECAST AREA. DUE TO THIS HAVE LEANED MORE
TOWARD THE ECMWF AS IT SEEMS TO BE A BIT MORE CONSISTENT IN EACH RUN
THE PAST FEW DAYS.
AVIATION /08Z SATURDAY THROUGH WEDNESDAY/
HIGH PRESSURE WILL REMAIN OVER THE REGION THROUGH THIS EVENING.
KEPT THE LIFR FOG IN AT LWB AT THE 10Z TIME FRAME...BUT THE DENSER
HIGH CLOUDS MAY KEEP THAT FROM HAPPENING. DAN WILL FLUCTUATE
BETWEEN MVFR AND VFR VSBY THROUGH 12Z SO KEPT THEM PREDOMINANTLY
MVFR AT 5SM. OTHERWISE VFR CONDITIONS AREA EXPECTED THROUGH
SATURDAY NIGHT.
THE NEXT THREAT OF WIDESPREAD IFR/MVFR CONDITIONS WILL BE
ENCOUNTERED SUNDAY INTO MONDAY AS A SLOW MOVING AREA OF LOW
PRESSURE MOVES NORTHEAST FROM THE GULF OF MEXICO. CLOUD BASES
SATURDAY AND SATURDAY NIGHT SHOULD REMAIN ABOVE 10KFT.
SUNDAY...CLOUD BASES WILL LOWER WITH CIGS FALLING BLO 3KFT SUNDAY
AFTERNOON AND INTO SUNDAY NIGHT. A 12-24 HOUR PERIOD OF -RA OR
DRIZZLE WILL ACCOMPANY THE IFR/MVFR CIGS...AND VSBYS MAY OBSCURE
THE MTN RIDGES.
TUESDAY WILL FEATURE IMPROVING CONDITIONS AS THE SFC LOW TRACKS
NORTHEAST UP THE EASTERN SEABOARD WITH HIGH PRESSURE BUILDING OVER
THE MID ATLANTIC FOR MID WEEK.
RNK WATCHES/WARNINGS/ADVISORIES
VA...NONE.
NC...NONE.
WV...NONE.
SYNOPSIS...PM/WP
NEAR TERM...WP
SHORT TERM...PM/WERT
LONG TERM...REB
AVIATION...AMS/PM/WP
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