FXUS61 KLWX 240232 AAA
AFDLWX
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
932 PM EST WED DEC 23 2009
.SYNOPSIS...
HIGH PRESSURE WILL REMAIN ACROSS THE MID ATLANTIC THROUGH THURSDAY.
A STORM SYSTEM WILL AFFECT THE REGION LATE CHRISTMAS EVE NIGHT
THROUGH SATURDAY WITH A WINTRY MIX INITIALLY FOLLOWED BY RAIN. HIGH
PRESSURE RETURNS SUNDAY INTO EARLY NEXT WEEK.
&&
.NEAR TERM /UNTIL 7 AM THURSDAY MORNING/...
INSULATING CLOUD COVER NOT AN ISSUE TONIGHT. TEMPS ACROSS THE
REGION WILL CONTINUE TO DROP AT A NEAR-NORMAL RATE OVERNIGHT...
DOWN INTO THE M-U10S ACROSS MUCH OF THE NRN HALF OF THE CWA AND
L20S ACROSS THE SOUTH. LIGHT WINDS AS THE SRN EDGE OF A SFC HIGH
SINKS DOWN INTO THE MID ATLC AND NEW ENGLAND. THE PREVIOUS 2
NIGHTS WERE RIDDLED W/ LARGE GUIDANCE DISCREPANCIES W/ THE SNOW
COVER AND INTERMITTENT BKN STRATUS MOVING THRU THE AREA.
SHELTERED VALLEYS ACROSS THE NRN VA PIEDMONT-FOOTHILLS AND
PORTIONS OF THE SHENANDOAH WILL SEE PATCHY FOG DEVELOPMENT...
MAINLY A 3-5SM VARIETY. DENSE LOW CLOUDS WILL CONTINUE TO TRUDGE
DOWN ACROSS CNTRL PA...W/ QUICKLY STREAMING UPPER DECKS PASSING
WELL TO THE SOUTH OF AREA. IN THE MIDDLE LIES A FAIRLY CLEAR MID
ATLC REGION...THOUGH CONDITIONS WILL CHANGE INTO THE PRE-DAWN HRS.
THE LARGE-SCALE STORM SYSTEM STRENGTHENING OVER THE CNTRL PLAINS
AND THE LENGTH OF THE MS RVR VLY WILL CONTINUE TO INCH EWD...W/
THE LEADING EDGE OF WARM-SECTOR MOISTURE SLIDING UP THE WINDWARD
SIDE OF THE APLCNS INTO THU MRNG.
&&
.SHORT TERM /7 AM THURSDAY MORNING THROUGH SATURDAY NIGHT/...
THURSDAY...
FOR MUCH OF THE DAY THU...JUST A GRADUAL OVER-TAKING OF MID-
HIGH CLOUD DECKS INTO THE REGION FROM THE SW...ASSOCIATED W/ THE
CNTRL PLAINS STORM SYSTEM. TEMPS THAT BOTTOMED-OUT IN THE UPPER
TEENS OVERNIGHT WILL GRADUALLY RISE TO NEAR FREEZING...BUT NOT
UNTIL EARLY AFTN. LOCALES ACROSS THE NRN SHENANDOAH VLY AND THE
POTOMAC HIGHLANDS MAY EVEN REMAIN JUST BELOW FREEZING FOR HIGHS
TOMORROW...SETTING-UP FOR A POSSIBLY ICY SCENARIO INTO CHRISTMAS
MORNING.
THURSDAY NIGHT...
SOUTHERN STREAM LOW PHASES WITH NORTHERN STREAM TROUGH AND STALLS
OVER THE N-CNTRL CONUS THURSDAY NIGHT INTO THE WEEKEND. AN
ASSOCIATED OCCLUDED FRONTAL SYSTEM APPROACHES THE MID ATLANTIC
STATES THURSDAY NIGHT...WITH ASSOCIATED PRECIP CONTINUING THROUGH
SATURDAY AS AN EMBEDDED LOW DEVELOPS ALONG THE NC PIEDMONT FRIDAY
AND TRACKS TO THE DELMARVA THROUGH SATURDAY.
PRECIP CHANCES START WITH LOW LEVEL SATURATION LATE THURSDAY NIGHT
ACROSS THE AREA AFTER THE RIDGE PUSHES OFF THE COAST AND THE SYSTEM
APPROACHES FROM THE SW THURSDAY EVENING. VALLEYS WEST OF THE BLUE
RIDGE...COLD LAYER AT SFC PERHAPS DEEP ENOUGH TO START AS SLEET LATE
THURSDAY NIGHT...BUT AFTER WARMER EASTERLY FLOW ABOVE ENTERS IN BY
SUNRISE ONLY FREEZING RAIN IS EXPECTED.
EARLY FRIDAY...
EAST OF BLUE RIDGE...WITH WARM SHALLOW SATURATED LAYER ABOVE
SUB-FREEZING SFC...PRECIP WOULD START AS FREEZING DRIZZLE...EVEN A
BRIEF GLAZE DOWN IN SOUTHERN MARYLAND. AS FRONTAL FORCING APPROACHES
FROM THE SW FRIDAY MORNING... MOISTURE DEEPENS WITH A THICKENING
WARM LAYER ALOFT. EXPECT A RAPID TRANSITION TO WARMER RAIN TO MOVE
FROM SE TO NW ACROSS THE I-95 CORRIDOR WITH SOME LIGHT ICE ACCRETION
POSSIBLE IN THE EARLY MORNING.
FRIDAY...
EXPECT A QUICK NWARD AND NOT AS QUICK WESTWARD EXPANSION OF THE
WARMER AIR THROUGH MIDDAY FRIDAY. GUIDANCE SAYS PROGRESSION WEST OF
THE BLUE RIDGE TO BE EARLY AFTERNOON...THINK THE MODELS ARE
OVERDOING THE WESTWARD SPEED ATTM. EXPECTING EXTENDED ACCRETING
FREEZING RAIN THREAT TO BE FOR VALLEYS WEST OF THE BLUE
RIDGE...INCLUDING THE SHENANDOAH VALLEY. EXPECTING VALLEY
TEMPERATURES TO REMAIN COLD ENOUGH FOR AROUND ONE QUARTER INCH OF
ICE TO ACCRETE. SEE WSWLWX FOR FURTHER INFORMATION.
RAINFALL/FLOODING THREAT...12Z GUIDANCE PROGS 50F DEWPOINT LINE TO
EXTEND THROUGH THE I-95 CORRIDOR FOR SEVERAL HOURS FRIDAY NIGHT.
THIS WARM AIR...COMBINED WITH RAINFALL AROUND AN INCH WOULD MELT A
MAJORITY /IF NOT ALL/ OF REMAINING SNOWPACK EAST OF THE BLUE
RIDGE...LEADING TO BOTH LOCAL/SMALL STREAM...AND POSSIBLY RIVER
FLOODING. A CHANGEOVER FROM FREEZING RAIN TO RAIN WEST OF THE BLUE
RIDGE IS EXPECTED TO BEGIN FRIDAY AFTERNOON...SO MELT INDUCED
FLOODING A POSSIBILITY FOR THE SHENANDOAH VALLEY BEGINS BY FRIDAY
EVENING.
FRIDAY NIGHT INTO SATURDAY...
SHOULD THE LOW PRESSURE SYSTEM LINGER OVER THE MID WEST/NORTHERN
PLAINS AS CURRENTLY PROGGED...THE FRONTAL SYSTEM WOULD BE SLOW TO
MOVE THROUGH THE CWA. FURTHER COMPLICATING THE FROPA IS A MESO LOW
DEVELOPING AND CROSSING THE AREA. CURRENTLY EXPECTING PRECIP
/MAINLY RAIN/ TO BEGIN TO TAPER OFF SW TO NE THROUGH SATURDAY.
&&
.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
SUNDAY THE UPPER TROF WILL BE MOVING IN...AND BY LATE SUN NIGHT AND
EARLY MON MORN THE GRADIENT WINDS WILL KICK IN FROM THE NW. THE NW
FLOW WILL ALSO RESULT IN UPSLOPE MOUNTAIN SNOWS SUN AND MON. TUE
AND WED HIGH PRES MOVES IN.
&&
.AVIATION /01Z THURSDAY THROUGH MONDAY/...
VFR CONDS WL PREVAIL THRU VALID TAF CYCLE. W/ HIPRES OHD...SKIES
SHUD BE MOCLR-CLR AND WNDS WL BE LGT NW...AND DVLPG FM THE NE
TMRW.
VFR CONTINUES INTO THURSDAY NIGHT WHEN ONSHORE FLOW SATURATES THE
LOW LEVELS...BRINGING LIGHT WINTRY PRECIP /MOST LIKELY FREEZING
DRIZZLE - THEN WARMER RAIN/ LATE THURSDAY NIGHT THROUGH FRIDAY.
BEST ACCRETION CHANCES IN THE VALLEYS WEST OF THE BLUE RIDGE.
RAIN/IFR CONDITIONS THEN CONTINUE THROUGH SATURDAY AS THE FRONTAL
SYSTEM IS SLOW TO MOVE THROUGH. UPPER TROUGH THEN OVER THE REGION
INTO EARLY NEXT WEEK IN DOWNSLOPING FLOW.
&&
.MARINE...
HIPRES BLDS ACRS WATERS TNGT-TMRW. WNDS 10-15 KT TNGT...5-10 KT TMRW.
ONSHORE FLOW BEGINS LATER THURSDAY...INCREASING TO SCA LEVELS BY
FRIDAY AFTERNOON. BRIEF FREEZING DRIZZLE POSSIBLE EARLY
FRIDAY...THEN RAIN INTO THE WEEKEND.
SMALL CRAFT ADVISORIES ARE EXPECTED ON MON.
&&
.LWX WATCHES/WARNINGS/ADVISORIES...
DC...NONE.
MD...WINTER STORM WATCH FROM LATE THURSDAY NIGHT THROUGH FRIDAY
AFTERNOON FOR MDZ003-501-502.
VA...WINTER STORM WATCH FROM LATE THURSDAY NIGHT THROUGH FRIDAY
AFTERNOON FOR VAZ021-025>031.
WV...WINTER STORM WATCH FROM LATE THURSDAY NIGHT THROUGH FRIDAY
AFTERNOON FOR WVZ050>055-501>504.
MARINE...NONE.
&&
$$
UPDATE...GMS
PREV DISCUSSION...HTS/BAJ
