FXUS61 KRNK 231101
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
601 AM EST WED DEC 23 2009

.SYNOPSIS...
HIGH PRESSURE ACROSS THE GREAT LAKES WILL DRIFT SLOWLY EAST AND
GRADUALLY WEDGE SOUTH ALONG THE BLUE RIDGE TODAY INTO THURSDAY.
STRONG LOW PRESSURE WILL PASS WELL TO THE WEST THURSDAY NIGHT AND
FRIDAY...PUSHING A TRAILING FRONT EAST ACROSS THE REGION FRIDAY
NIGHT INTO EARLY SATURDAY. THIS WILL RESULT IN A MIXTURE OF
RAIN...SLEET...AND FREEZING RAIN OVER THE MID APPALACHIAN REGION
THURSDAY NIGHT...GRADUALLY CHANGING TO RAIN IN MOST LOCATIONS ON
CHRISTMAS DAY AS TEMPERATURES SLOWLY RISE ABOVE FREEZING.

&&

.NEAR TERM /THROUGH TONIGHT/...
BASICALLY A CLOUD/TEMP FORECAST OVER THE NEXT 24 HOURS AS 5H
HEIGHTS BUILD EAST AHEAD OF THE COMPLEX LOW PRESSURE SYSTEM
EJECTING OUT OF THE SW. WELL AHEAD OF THIS TROFFINESS WILL SEE
LOTS OF MID/HIGH CLOUDS FAN EAST TOWARD THE AREA TODAY IN AHEAD
OF THE FRONT ALOFT BEFORE THE SYSTEM DEEPENS/COMPACTS A BIT MORE
OVER THE SOUTHERN PLAINS LATER TONIGHT. COULD EVEN SEE A FEW MID
DECK SPRINKLES MAKE A RUN AT THE WEST LATER TODAY BEFORE DRIER
SURFACE HIGH PRESSURE WEDGES IN MORE ALONG THE BLUE RIDGE
OVERNIGHT. THUS WILL GO A BIT CLOUDIER OVER THE SW TODAY WHERE
GUIDANCE SHOWS A BETTER AXIS OF MID/HIGH LEVEL RH OTRW BASICALLY A
PARTLY CLOUDY FLAVOR OVERALL GIVEN CONFLUENCE ALOFT. CLOUDS MAY
ACTUALLY THIN SOME SPCLY EAST OVERNIGHT AS BETTER RIDGING
SURFACE/ALOFT TAKES SHAPE SO PC TO MOSTLY CLEAR EAST. BEST WARMING
SHOULD TAKE PLACE OVER THE SW TODAY WHERE 85H TEMPS RISE SLIGHTLY
WHILE LIGHT LOW LEVEL EASTERLY FLOW MAY KEEP THE NE COOLER. OFFSET
OF TEMPS VIA CLOUDS A BIT TRICKY BUT OVERALL KEPT TREND OF GOING
CLOSER TO THE MAV MOS EAST AND WARMER MET WEST OUTSIDE OF THE
DEEPER SNOW PACK. THINK WITH BETTER RAD CONDITIONS WILL SEE QUITE
A RANGE IN LOWS TONIGHT FROM THE TEENS TO LOW 20S VALLEYS TO ONLY
LOW 30S WESTERN RIDGES WHERE DEVELOPING SE FLOW SHOULD KEEP THINGS
MORE MIXED AFTER MIDNIGHT.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
LARGE WEDGE OF HIGH PRESSURE WILL REMAIN ANCHORED FROM SOUTHEASTERN
CANADA SOUTH INTO THE MID ATLANTIC REGION THROUGH THURSDAY. MEANWHILE...
ABUNDANT MID AND HIGH LEVEL MOISTURE WILL BE STREAMING NORTHEAST
IN ADVANCE OF A STRONG LOW PRESSURE SYSTEM THAT WILL SLUGGISHLY
MOVE NORTHEAST UP THE MISSISSIPPI VALLEY. LOW SUN ANGLE AND LIMITED
INSOLATION WILL HINDER EXTENT OF DAYTIME HEATING...ESPECIALLY AS
LOW LEVEL WINDS INCREASINGLY VEER INTO MORE OF AN UPSLOPE COMPONENT
EAST OF THE BLUE RIDGE. MOS GUIDANCE QUITE SIMILAR FOR DAYTIME
HIGHS...AND GENERALLY ACCEPTED.

WEATHER BECOMES QUITE INTERESTING THURSDAY NIGHT INTO CHRISTMAS
DAY AS MOISTURE IN ADVANCE OF WESTERN SYSTEM APPROACHES THE AREA
FROM THE SOUTHWEST. STRONG AND DEEP ISENTROPIC LIFT IN 850-600
MILLIBAR LAYER COMBINING WITH INCREASING DIVERGENCE ALOFT WITH
APPROACH OF RIGHT ENTRANCE REGION OF UPPER JET...AND STRONG LOW
LEVEL UPSLOPING WILL RESULT IN THE DEVELOPMENT OF WIDESPREAD
PRECIPITATION OVER THE REGION...MAINLY AFTER 06Z. WITH INITIALLY
DRY AIR MASS IN PLACE OVER AREA...TEMPERATURES SHOULD WET-BULB DOWN
TO OR BELOW FREEZING IN MOST LOCATIONS WITH THE ONSET OF PRECIPITATION.

FORECAST SOUNDINGS INDICATE THAT ANY RAIN AT THE ONSET OF THE EVENT
SHOULD TRANSITION OVER TO MORE OF A SLEET/FREEZING RAIN MIX BEFORE
TRANSITIONING BACK TO RAIN LATER CHRISTMAS MORNING INTO CHRISTMAS
AFTERNOON AS SURFACE TEMPERATURES SLOWLY RISE.

EXCEPTION WILL LIKELY BE ACROSS...AND IMMEDIATELY EAST OF THE
SPINE OF THE BLUE RIDGE...WHERE COOLING FROM STRONG EAST-SOUTHEASTERLY
FORCED ASCENT MAY PREVENT TEMPERATURES FROM INCHING ABOVE FREEZING
UNTIL LATER IN THE DAY...AFTER MUCH OF THE HEAVIER PRECIPITATION
HAS ENDED WITH ARRIVAL OF DRIER AIR ALOFT.

CURRENT THINKING IS THAT GENERAL AMOUNTS OF UP TO ONE TENTH OF AN
INCH OF GLAZE IS LIKELY IN MANY LOCATIONS...WITH AROUND ONE QUARTER
INCH ACROSS HIGHER EAST FACING SLOPES OF THE BLUE RIDGE. SINCE
INITIAL ICING WOULD COME IN 4TH PERIOD OF FORECAST...WILL NOT
HOIST ANY WINTER STORM WATCH AT THE PRESENT TIME...BUT MAY HAVE TO
DO SO LATER TODAY IF CURRENT REASONING STILL HOLDS. 
LOWER/NEGLIGIBLE AMOUNTS OF GLAZING ARE EXPECTED FROM BLUEFIELD
SOUTH INTO TAZEWELL COUNTY...WHERE STRONG DOWNSLOPING WINDS WILL
HOLD TEMPERATURES UP SOMEWHAT...AND ALSO ACROSS THE EXTREME
SOUTHEASTERN COUNTIES...WHICH WILL NOT BE QUITE AS ENTRENCHED IN
COOL WEDGE.

IN ADDITION TO A POSSIBLE ADVISORY/WARNING EVENT FOR ICING...WILL
ALSO HAVE TO WATCH FOR THE NEED FOR A WIND ADVISORY/WARNING ACROSS
THE SOUTHWESTERN MOUNTAINS AND DOWNSLOPE AREAS OF EXTREME
SOUTHWESTERN VIRGINIA INTO EXTREME SOUTHEAST WEST VIRGINIA AS 850
MILLIBAR WINDS INCREASE TO 65-75 KNOTS LATER THURSDAY NIGHT INTO
CHRISTMAS MORNING IN ADVANCE OF APPROACHING OCCLUDED FRONT.

ONE FINAL WEATHER CONCERN IS CENTERED AROUND THE POTENTIAL FOR
FLOODING ON CHRISTMAS DAY IF RAINFALL AND SNOW MELT ARE MAXIMIZED.
FORTUNATELY...THE MAXIMUM AMOUNTS OF PRECIPITATION FROM THE STORM
SHOULD OCCUR WEST OF THE BLACKSBURG FORECAST AREA...CLOSER TO
WHERE UPPER DYNAMICS WILL BE MAXIMIZED...AND TO OUR SOUTHEAST...WHERE
STRONG CYCLOGENESIS WILL BE OCCURRING IN BAROCLINIC ZONE ALONG THE
SOUTHEAST COAST. CONVECTIVE ACTIVITY TO OUR SOUTHEAST MAY HELP TO
DISRUPT/DIVERT ATLANTIC MOISTURE/WARM CONVEYOR BELT INTO MID
APPALACHIANS...THEREBY MINIMIZING OVERALL QPF. IN ADDITION...DEVELOPMENT
OF LOW PRESSURE ALONG SOUTHEAST WILL LIKELY REINFORCE COOL WEDGE
OVER BLACKSBURG FORECAST AREA...SUCH THAT DEW POINT TEMPERATURES
REMAIN COOL ENOUGH TO PREVENT SIGNIFICANT SNOW MELT. ALTHOUGH WARM
INTRUSION MAY BRIEFLY PUSH INTO SOUTHEASTERN AREAS...SNOW PACK IN
THAT AREA IS LIMITED...THEREBY MINIMIZING OVERALL POTENTIAL
RUNOFF.

AS WOULD BE EXPECTED UNDER STRONG WEDGE SITUATION...WENT WITH
COMPRESSED DIURNAL TEMPERATURE CURVE IN MOST LOCATIONS THURSDAY
NIGHT INTO FRIDAY...GENERALLY COLDER THAN INDICATED BY MOS
GUIDANCE.

PASSAGE OF OCCLUDED FRONT ON FRIDAY EVENING/NIGHT WILL END ANY
LINGERING THREAT OF LIGHT PRECIPITATION/DRIZZLE...TO BE FOLLOWED
BY DOWNSLOPE DRYING/CLEARING EAST OF THE BLUE RIDGE. WEST OF THE
BLUE RIDGE...UPSLOPE CLOUDS AND SOME SCATTERED MOUNTAIN 
SHOWERS/FLURRIES ARE POSSIBLE LATER FRIDAY NIGHT INTO SATURDAY...BUT
SOUTHWESTERLY POST- FRONTAL TRAJECTORY NOT PARTICULARLY FAVORABLE
FOR STRONG UPSLOPE PRECIPITATION EVENT. MAV/MEX MOS GENERALLY
ACCEPTED DURING THE POST-FRONTAL PERIOD.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
LARGE VORTEX OVER THE WESTERN GREAT LAKES WILL BE THE MAIN WEATHER
FEATURE DURING THE LONG TERM FORECAST. GENERALLY USED THE 12Z/22
ECMWF FOR THIS FORECAST...WITH BROADER EASTERN US TROF SOLUTION
WHICH IS IN AGREEMENT WITH NCEP ENSEMBLES. THIS SOLUTION KEEPS OUR
FORECAST AREA IN A ZONAL FLOW MONDAY NIGHT INTO TUESDAY.

CAA AND SHALLOW RESIDUAL LOW LEVEL MOISTURE WILL BRING SOME UPSLOPE
SNOW SHOWERS THROUGH SUNDAY...BUT WITH WEST/SOUTHWEST FLOW WILL
LEAVE POPS IN THE CHANCE CATEGORY.

UPSLOPE SNOW SHOWERS MAY BE SLIGHTLY ENHANCED MONDAY AS A SHORTWAVE 
PIVOTING AROUND THE UPPER LOW SWEEPS THROUGH THE REGION WHICH 
ENHANCES CAA AND STRENGTHENS LOW LEVEL WESTERLY WINDS. BY MONDAY 
NIGHT AND TUESDAY...SFC RIDGE BUILDS IN FROM THE WEST SHUTTING DOWN 
UPSLOPE SNOW.

TEMPERATURES SHOULD BE CLOSE TO OR A BIT BELOW AVERAGE THROUGH
THE PERIOD.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
PATCHY EARLY MORNING FOG WILL RESULT IN LOCALIZED IFR/LIFR ACROSS KLWB
AND OTHER MOUNTAIN VALLEYS EARLY THIS MORNING AND PERHAPS AGAIN
LATER TONIGHT. OTRW HIGH PRESSURE WILL BUILD SOUTHEAST INTO THE
REGION TODAY WITH BLACKSBURG TERMINAL FORECAST POINTS
(KROA...KLYH...KDAN...KBLF...KLWB) LIKELY TO REMAIN VFR UNDER MID
AND HIGH CLOUDS THROUGH MOST OF THE PERIOD.

AS SURFACE HIGH PRESSURE BUILDS FURTHER SOUTHEAST INTO AREA ON
WEDNESDAY NIGHT...LOW LEVEL WINDS WILL BECOME SOUTHEAST. MVFR
CEILINGS MAY DEVELOP IN SOUTHWEST VIRGINIA AND NORTHWEST NORTH
CAROLINA ON THE EASTERN UPSLOPE SIDE OF THE BLUE RIDGE. HOWEVER
SINCE MOST GUIDANCE SHOWING LESS LOW LEVEL MOISTURE THAN EARLIER
WILL OPT TO LEAVE OUT ANY MVFR CIGS AT THE TAF SITES FOR NOW.

AN APPROACHING STORM SYSTEM WILL LIKELY BRING ANOTHER LENGTHY PERIOD
OF WIDESPREAD IFR TO LIFR WEATHER THURSDAY NIGHT THROUGH CHRISTMAS
DAY...WITH FREEZING RAIN AND/OR RAIN THE MOST PROBABLE PRECIPITATION
THREAT. STRONG SOUTHEAST WINDS ARE POSSIBLE THURSDAY NIGHT INTO
FRIDAY/CHRISTMAS DAY AT BLUEFIELD.

THE ARRIVAL OF STRONG COLD WESTERLY WINDS ON THE BACKSIDE OF THIS
SYSTEM SHOULD ALLOW FOR RETURN TO VFR WEATHER EAST OF THE BLUE
RIDGE SATURDAY INTO SUNDAY...BUT WITH TYPICAL MVFR TO LOCALIZED
IFR RESTRICTIONS CONTINUING IN THE UPSLOPE AREAS WEST OF THE BLUE
RIDGE (KBLF/KLWB) DUE TO SCATTERED SNOW SHOWERS.

&&

.HYDROLOGY...
WILL BE MONITORING POTENTIAL OF LOCALIZED FLOODING FOR CHRISTMAS
DAY AS NEXT WEATHER SYSTEM AND ASSOCIATED RAINS COMBINE WITH
MELTING SNOW PACK TO PRODUCE NOTABLE RUNOFF.

&&

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

&&

$$
SYNOPSIS...AMS/JH/WERT
NEAR TERM...JH
SHORT TERM...WERT
LONG TERM...PH
AVIATION...AMS/JJ/WERT/JH
HYDROLOGY...