FXUS61 KLWX 232002
AFDLWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
302 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 /THROUGH THURSDAY/...
HIPRES ABT TO CREST OVR CWFA...AND WL BE OHD TNGT. THAT LEAVES AREA 
CLR AND CLAM...OR AT LEAST CLOSE TO IT. THOSE ARE TWO VARIABLES THAT 
HVNT BEEN TRUE PRVS NGTS. THUS...RADL COOLING SHUD BE BETTER OVR A 
WIDER PORTION OF REAL ESTATE...AND FEEL OK GOING ALNG W/ THE COOLER 
MIN-T BEING OFFERED BY MOS.

MAXT TMRW WL BE A BIGGER CHALLENGE...AS A SPREAD IN MOS DVLPS. RDG 
AXIS SFC-H5 ALL REMAIN OVR CWFA...SO DONT XPCT SGFNT WARMING TO 
OCCUR. DUE TO THE COLD START AND LOW SUN ANGLE...LWR-MID 30S MAY BE 
ALL WE/RE ABLE TO ACHIEVE. MOREOVER...XPCTG HI CLDS TO OVRSPRD CWFA 
IN ADVC OF NXT STORM...SO INSOLATION WONT BE ALL THAT GREAT EITHER. 
&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY 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. 

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.

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 /20Z WEDNESDAY THROUGH MONDAY/...
VFR CONDS WL PREVAIL THRU VALID TAF CYCLE. W/ HIPRES OHD...SKIES 
SHUD BE MOCLR-CLR AND WNDS WL BE LGT NW...DIMINISHING TNGT...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.

&&

$$
SYNOPSIS...BAJ
NEAR TERM...HTS
SHORT TERM...BAJ
LONG TERM...STRONG
AVIATION...HTS/BAJ
MARINE...HTS/BAJ/STRONG