FXUS61 KLWX 240853
AFDLWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
353 AM EST THU DEC 24 2009
.SYNOPSIS...
HIGH PRESSURE WILL REMAIN ACROSS THE MID ATLANTIC TODAY. A STORM
SYSTEM WILL AFFECT THE REGION LATE CHRISTMAS EVE NIGHT THROUGH
SATURDAY WITH A WINTRY MIX INITIALLY FOLLOWED BY RAIN. AN UPPER
LEVEL LOW WILL INFLUENCE THE REGION SUNDAY AND MONDAY...THEN COLD
HIGH PRESSURE BUILDS IN FOR MID WEEK.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
CONDS FINALLY CAME TOGETHER OVNGT FOR IDEAL RADIATIONAL COOLING.
TEMPS STARTING OFF THIS MRNG IN TEENS ACRS MAJORITY OF CWA...WITH
20S IN URBAN CENTERS AND ALONG CHESAPEAKE BAY. LOW LVLS SATURATED
ACRS SHENANDOAH VLY AND VA PIEDMONT...AS WELL AS ADJACENT VALLEY
LOCATIONS IN FOOTHILLS AND POTOMAC HIGHLANDS. PATCHY DNS FRZN FOG
HAS FORMED THERE...AND WILL CONT FOR AN HOUR OR SO AFTER SUNRISE.
LGT NLY FLOW AND MOSTLY CLEAR SKIES TDA WITH STRONG HIPRES CENTERED
OVR SE CANADA WEDGING DOWN E SIDE OF APLCNS. STAYED CLOSE TO MAV
MOS AS OPPOSED TO MET WHICH SEEMS TO BE OVER-AMPLIFYING EFFECTS OF
SNOWPACK. MAXIMA IN 30S...SIMILAR TO WED.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
HIPRES CONTS WEDGING DOWN THRU CWA TNGT...BUT WITH POWERFUL SFC
DRIFTING EWD ACRS MID MS VLY...FLOW BCMS MORE NE ORIENTED AT THE LOW
LEVELS...ALLOWING FOR INCRG MOISTURE. SHUD STILL HAVE MOSTLY CLEAR
SKIES AND LGT WINDS ERY IN NGT...SO TEMPS WILL RAPIDLY FALL INTO 20S.
WITH BOUNDARY LYR BCMG SATURATED AFTER MIDNIGHT...RISK FOR FRZG DRZL/
VERY LIGHT FRZG RAIN INCRS FROM SW TO NE. QPF IS VERY LOW...A COUPLE
HUNDREDTHS OF AN INCH AFTER MIDNIGHT...SO ICE ACCRETION WILL
MAINLY BE A TRACE UNTIL VERY LATE THU NGT WHEN MID/UPR LEVEL MSTR
FROM THE S/W BEGINS TO OVERRIDE COLD AIR AT SFC. WINTER STORM WATCH
ALONG/WEST OF BLUE RIDGE STARTING AT 6Z.
&&
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
SURFACE HIGH PRESSURE OVER EASTERN CANADA WILL MAINTAIN A WEDGE OF
COLD AIR NEAR THE SURFACE ACROSS THE MID ATLANTIC TO BEGIN CHRISTMAS
DAY. MEANWHILE...WARM ADVECTION AND ISENTROPIC LIFT WILL CONTINUE TO
INCREASE ACROSS THE APPALACHIANS WELL IN ADVANCE OF A LARGE POTENT
STORM SYSTEM OVER THE MIDWEST.
MODELS CONTINUE TO INDICATE THAT THE LARGE PRIMARY PLUME/BELT OF
MOISTURE AND LIFT WILL MAINLY AFFECT THE CWA 15Z-18Z AND BEYOND.
THIS WILL BE ENHANCED BY A WAVE OF SURFACE LOW PRESSURE
APPROACHING THE AREA FROM THE SOUTH IN THE AFTERNOON.
HOWEVER...EVEN AHEAD OF THE LARGE PLUME OF MOISTURE...THERE WILL
BE LIGHT PRECIPITATION THAT DEVELOPS DUE TO WARM ADVECTION ALOFT
AND A MOIST EASTERLY FLOW IN THE LOW LEVELS. AND WITH MUCH OF THE
CWA SUB-FREEZING TO START THE DAY...PRECIPITATION TYPE WILL BE
FREEZING DRIZZLE OR LIGHT FREEZING RAIN INITIALLY.
EXPECT SURFACE TEMPERATURES TO RISE DURING MID TO LATE
MORNING...WHERE FREEZING PRECIPITATION WILL CHANGE TO RAIN FOR MOST
AREAS EAST OF THE BLUE RIDGE. WEST OF THE BLUE RIDGE...ESPECIALLY
WITHIN THE DEEPER VALLEYS...SURFACE TEMPERATURES WILL BE SLOWER TO
RESPOND. THIS IS PROBLEMATIC IN THAT FREEZING RAIN WILL CONTINUE AS
THE MORE SIGNIFICANT QPF AFFECTS THE CWA. THIS MEANS THAT PORTIONS
WEST OF THE BLUE RIDGE WILL RECEIVE MORE PROLONGED ICING. WINTER
STORM WATCH WILL REMAIN IN PLACE WEST OF THE BLUE RIDGE...AND DAY
SHIFT WILL BE ABLE TO SPECIFY SPATIAL/TEMPORAL WARNING/ADVISORY
HEADLINES.
PRIMARY PLUME OF MOISTURE CONTINUES TO MOVE ACROSS THE CWA FRIDAY
NIGHT INTO SATURDAY MORNING...IN TANDEM WITH SURFACE LOW PRESSURE
MOVING ACROSS THE REGION. EVEN DEEPER VALLEYS IN THE WATCH AREA
SHOULD BE ABLE TO RISE JUST ABOVE FREEZING FRIDAY
EVENING...LEAVING RAIN AS THE GENERAL P-TYPE. TEMPERATURES WILL BE
STEADY/RISING FRIDAY NIGHT AND THIS TREND WAS REFLECTED IN GRIDS.
LIQUID AMOUNTS ACROSS THE CWA ARE EXPECTED TO BE AROUND ONE INCH.
CONSIDERING RECENT SNOWPACK/MELT...THESE AMOUNTS COULD RESULT IN
MINOR FLOODING PROBLEMS BEFORE RAIN ENDS FROM SOUTHWEST TO
NORTHEAST SATURDAY AS LOW PRESSURE DEPARTS.
ANOTHER ASPECT TO CONSIDER WILL BE WINDS/GUSTS WEST OF THE BLUE
RIDGE ESPECIALLY OVER HIGHEST ELEVATIONS FRIDAY/FRIDAY NIGHT.
STRONG WIND FIELDS MAY LEAD TO GUSTS UP TO 40 KNOTS DURING THIS
TIME FRAME AND A WIND ADVISORY MAY EVENTUALLY BE NECESSARY.
BIG MIDWEST STORM WHICH MOVES LITTLE FRIDAY/SATURDAY BEGINS TO SHIFT
EAST INTO THE GREAT LAKES SUNDAY AND THEN MOVES ACROSS THE
NORTHEAST AS AN OPEN WAVE ON MONDAY. RESULT WILL BE TO
PRODUCE/MAINTAIN UPSLOPE SNOW SHOWERS SUNDAY THROUGH MONDAY...AND
FLURRIES MAY EVEN PUSH EAST OF THE MTNS SUNDAY NIGHT INTO MONDAY
GIVEN THE STRONG VORT.
COLER AIR MOVES INTO THE MID ATLANTIC TUESDAY AND WEDNESDAY IN
NORTHWEST FLOW IN THE WAKE OF THE UPPER LOW. SNOW SHOWERS IN
FAVORED UPSLOPE AREAS MAY ALSO PERSIST THROUGH MID WEEK.
&&
.AVIATION /08Z THURSDAY THROUGH MONDAY/...
MVFR VSBY PSBL THRU 14Z AT KCHO...OTHERWISE SCT 25KFT CI THRU 6Z
TNGT WITH VFR VSBY FOR ALL AIRPORTS. LGT NLY WINDS GRADUALLY BCM
NELY BY TNGT...WITH MVFR/IFR STRATUS DVLPG AFTER MIDNIGHT.
IFR/LIFR CIGS/VSBYS ARE EXPECTED TO PREVAIL FRIDAY/FRIDAY NIGHT.
LIGHT PRECIPITATION DEVELOPS EARLY FRIDAY MORNING...MOST LIKELY
FREEZING DRIZZLE OR FREEZING RAIN. PRECIPITATION TYPE IS EXPECTED TO
QUICKLY CHANGE TO RAIN AT KBWI/KDCA FOLLOWED BY KIAD LATER FRIDAY
MORNING. CHANGEOVER WILL TAKE LONGER AT KCHO AND KMRB. RAIN WILL END
SATURDAY MORNING. IFR CONDITIONS EXPECTED THROUGH THEN...THEN
IMPROVEMENT TO MVFR SATURDAY AFTERNOON. AS LOW PRESSURE MOVES ACROSS
THE AREA...WIND SHIFT FROM EAST TO WEST/NORTHWEST WILL ALSO TAKE
PLACE SATURDAY AFTERNOON OR EVENING.
&&
.MARINE...
A BIT OF NLY CHANNELING WILL KEEP WIND GUSTS TO 15 KT PSBL THRU
TDA. TNGT PRESSURE GRADIENT INCRS WITH APPROACHING STORM...SO MORE
SYNOPTICALLY DRIVEN NE WINDS WILL AGAIN HAVE GUSTS TO 15 KT.
EASTERLY FLOW IS EXPECTED TO INTENSIFY ON FRIDAY...AND SMALL CRAFT
ADVISORY WILL BE ISSUED FRIDAY AFTERNOON/NIGHT. SMALL CRAFT ADVISORY
MAY NEED TO BE EXTENDED INTO SATURDAY/SATURDAY NIGHT AS LOW PRESSURE
MOVES ACROSS THE WATERS...USHERING IN A WIND SHIFT TO THE
WEST/NORTHWEST.
&&
.LWX WATCHES/WARNINGS/ADVISORIES...
DC...NONE.
MD...WINTER STORM WATCH FROM LATE TONIGHT THROUGH FRIDAY AFTERNOON
FOR MDZ003-501-502.
VA...WINTER STORM WATCH FROM LATE TONIGHT THROUGH FRIDAY AFTERNOON
FOR VAZ021-025>031.
WV...WINTER STORM WATCH FROM LATE TONIGHT THROUGH FRIDAY AFTERNOON
FOR WVZ050>055-501>504.
MARINE...SMALL CRAFT ADVISORY FROM NOON FRIDAY TO 6 AM EST SATURDAY FOR
ANZ530>543.
&&
$$
SYNOPSIS...SBK/PELOQUIN
NEAR TERM...SBK
SHORT TERM...SBK
LONG TERM...PELOQUIN
AVIATION...SBK/PELOQUIN
MARINE...SBK/PELOQUIN
