Winter Weather in Virginia, December 25, 2009

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
902 PM EST THU DEC 24 2009

.SYNOPSIS...
HIGH PRESSURE WILL MOVE OFF THE COAST TONIGHT. A STORM SYSTEM WILL
TRACK THROUGH THE MIDWEST LATE TONIGHT INTO FRIDAY. THE WARM FRONT
ASSOCIATED WITH THIS SYSTEM WILL BRING RAIN AND FREEZING RAIN FRIDAY
MORNING BEFORE ENOUGH WARM AIR WORKS IN FOR PRECIPITATION TO TURN TO
ALL RAIN BY FRIDAY EVENING. THE COLD FRONT ASSOCIATED WITH THIS
SYSTEM WILL PASS THROUGH SATURDAY AND HIGH PRESSURE WILL BUILD
OVERHEAD SUNDAY. AN UPPER-LEVEL LOW WILL PASS THROUGH SUNDAY NIGHT
INTO MONDAY...THEN COLD HIGH PRESSURE BUILDS IN FOR MID WEEK.

&&

.NEAR TERM /UNTIL 7 AM FRIDAY MORNING/...
TEMPS WILL LIKELY BOTTOM-OUT LATE TONIGHT...W/ THE APPROACHING
SOLID SHIELD OF LOW-MID CLOUDS FROM THE SW. THE LEADING EDGE
ALREADY PUSHING INTO THE CNTRL SHENANDOAH VLY...MOVING NE AS AN
EVEN LOWER SHIELD OF LOW MARINE STRATUS NOW CRAWLING OVER DELMARVA
AND TOWARD THE CHESAPEAKE BAY. THE LINGERING INFLUENCES OF A
1040MB SFC HIGH OVER CNTRL QUEBEC WILL KEEP A NELY COMPONENT TO
THE SFC WINDS INTO EARLY FRI. THE STRENGTH OF THE HIGH WILL ALLOW
CAD TO TAKE PLACE EVEN AS THE INCOMING WARM SECTOR OVERTAKES THE
ERN SEABOARD THROUGH THE NEXT SEVERAL HRS.

PRECIP ASSOCIATED W/ THE INCOMING AND VERY FAR-REACHING LOW
PRESSURE SYSTEM STILL BACK ACROSS THE SRN APLCNS...NOW BEGINNING
TO PUSH INTO WRN NC/SWRN VA. TEMPS ACROSS THESE AREAS WELL ABOVE
FREEZING...BUT INTO THE LEE OF THE APLCNS NEAR RNK...THE
SUBFREEZING TEMPS BECOME MORE WIDESPREAD. ACROSS THE MID ATLC AT
THIS HR...MANY AREAS HAVE DROPPED INTO THE U10S-L20S...CLOSER TO
FREEZING NEAR THE BAY.

AS THE INSULATING CLOUD SHIELD CONTINUES TO OVERTAKE THE REGION
LATE TONIGHT...TEMPS WILL THEN BE MODERATED ONLY A FEW DEGREES
/WARMER/ THEN HOLD FAIRLY STEADY THROUGHOUT THE OVERNIGHT HRS AND
EVEN INTO TOMORROW MORNING AFTER SUNRISE. AREAS OF DRIZZLE AND
LIGHT RAIN WILL BEGIN TO STREAM INTO THE REGION DURING THE PRE-
DAWN FROM /FROM SW-TO-NE/ ACROSS THE SHENANDOAH VLY AND NEARBY
SLOPES OF THE ALLEGHENY MTNS.

VERY DRY AIR ALOFT AND AN INCOMING LOW LEVEL SECTOR OF WAA WILL
COMBINE TO PUSH A WARM-NOSE /ABOVE FREEZING/ INTO THE MID MORNING
HRS...SO HIGHER ELEVATION AREAS WILL ACTUALLY BE LESS-SUSCEPTIBLE
FOR FROZEN PRECIP AS TIME GOES ON...AND W/ STRONGER WINDS AT THIS
LEVEL THEY WILL ALSO SEE GUSTS IN THE 20-30MPH RANGE WHILE THE
VALLEYS EXPERIENCING WINDS ONLY IN THE 5-10MPH RANGE...MAYBE SOME
GUSTS TO 20MPH.

PREV DISC...
HV A VERY FORGIVING ADVY CRIT FOR ICING /IE...A TRACE/.
THEREFORE...WL PROVIDE A LTL BUFFER ROOM IN TERMS OF WINT WX
ADVY/WRNG DECISION. SINCE MOST OF THE QPF WL BE IN THE 2ND PD...WL
DISCUSS LOGIC BELOW. BUT...THE BTTM LN WL BE AN ADVY WIBIS W/ THIS
PKG FOR PARTS OF CWFA...STARTING BTWN MIDNGT-SUNRISE. W OF BLURDG
WL BE 1ST SINCE IT/LL BE COLDER...THEN BTWN BLURDG AND I95 TWD
DAWN. USING WINT WX VS FRRA ADVY IN CASE SOME PL MIXED IN AT ONSET
DUE TO LOW DEWPTS.

&&

.SHORT TERM /7 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
AT START OF PD...PCPN WL BE OVRSPREADING CWFA FM SW TO NE. DO NOT HV
STRONG CONFIDENCE ON EXACTLY WHERE THE LEADING EDGE WL BE DUE TO QPF
GNLY HOLDING OFF. THAT GIVES ERN SXNS AN OPPORTUNITY TO WARM PRIOR
TO PCPN ONSET. THE LACK OF CONFIDENCE OF A 2ND PD EVENT PRECLUDES
ISSUING AN ADVY FOR I-95 EWD...INCL DC AND BALT CITY.

W OF THERE...THE PCPN WL OVRSPREAD WHILE TEMPS BLO FRZG. NAM SOLN
THE SLOEST...AND SEEMS TOO SLOW OVRALL FOR ONSET OF A WAA PCPN PTTN.
BUT...GFS SNDGS DONT HV THE PROPER RESOLUTION TO CAPTURE COLD AIR
DAMMING...AND WARM WAY TOO QUICKLY. HV FOUND MIDDLE GROUND...ROOTED
IN CLIMO. MAXT GRIDS REPRESENTS A BLEND BARELY UNDERCUTTING THE LOW
END OF THE SOLN ENVELOPE.

BTWN I-95 AND BLURDG...THE ABV FRZG LN SHUD BE PROGRESSING WWD WHILE
PCPN OVRSPREADS...AND SHUD CHG OVR DURING THE MRNG. ICING SHUD BE
MINIMAL...BUT HV ENUF CONFIDENCE FOR ADVY. AM MORE CONCERNED FOR
MEASUREABLE ICING W OF MTNS...AS THE INITIALLY COLD TEMPS AND LOW
DEWPTS WL LEAD TO A RESILIENT ALBEIT SHALLOW COLD LYR NEAR THE GRND.

WHILE AMPLE QPF WL BE AVBL...THINK ITLL HOLD OFF TIL AFTN. BY
THEN... COLD AIR MAY BE SCOURED OUT IN ALL BUT THE DEEPEST VLYS.
OVRALL...DO NOT HV THE CONFIDENCE IN REACHING WRNG ICE CRIT...BUT A
TENTH OR TWO W/IN REASON W OF SHEN VLY.

ALSO OF CONCERN WL BE THE WNDS...WHICH WL INCR ABV SFC INVSN.
HWVR...AM CONCERNED THAT IT WONT MIX TO SFC XCPT FOR THE RDG TOPS
WHERE THE ICING WONT BE A CONCERN. THE FAR WRN FRINGE /THE APPLCHNS/
WL SEE HIER WNDS TWD DAWN...OTRW IT/LL BE A 2ND PD HAZARD. SO...WL
CONT W/ MNTNS IN HWO AND A SENTENCE IN THE WSW.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
ALL PRECIP WILL BE IN THE FORM OF RAIN FRIDAY NIGHT. RAIN WILL
TAPER OFF FROM SOUTHWEST TO NORTHEAST SATURDAY BEHIND THE COLD
FRONT THAT IS ASSOCIATED WITH THIS STORM SYSTEM. A FLOOD WATCH HAS
BEEN ISSUED...DISCUSSED IN DEPTH BELOW.

HIGH PRESSURE WILL BUILD IN BEHIND THE SYSTEM SATURDAY NIGHT THROUGH
SUNDAY. AN UPPER-LEVEL LOW WILL BRING THE CHANCE FOR SNOW SHOWERS
SUNDAY NIGHT INTO MONDAY...ESPECIALLY ACROSS THE NORTHERN AND
WESTERN CWA. ACCUMULATING SNOW WILL BE MOST LIKELY ACROSS THE
POTOMAC HIGHLANDS. HIGH PRESSURE WILL RETURN FOR THE MIDDLE PORTION
OF THE WEEK BRINGING COLD AND DRY CONDITIONS.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
STILL BUT IMPACTS ARE QUICKLY APPROACHING FROM THE SW /AND THE
SE/. VFR CONDS ATTM...BUT A SOLID SHIELD OF 3-4KFT STRATUS MOVING
IN FROM THE SW...AS WELL AS MARNIE-LAYER CIGS 1-2KFT FROM THE SE
DRIFTING TOWARD THE BAY REGION...MEETING IN THE MIDDLE OVER THE
I-95 CORRIDOR OVERNIGHT. A GRADUAL STREAM OF LIGHT DRIZZLE WILL
DEVELOP OVER THE REGION INTO THE PRE-DAWN HRS AND LOCALES W OF
I-95 WILL LIKELY STILL BE BELOW FREEZING...SO A LIGHT ICING EFFECT
MAY OCCUR INTO THE MID MORNING HRS FRI.

CHO WL EXPERIENCE A LONGER PD OF ICING DUE TO ERLR ONSET AND SITE
PLACEMENT IN SHADOW OF BLURDG. THE LATEST CHGOVR WL BE MRB...
WHICH MAY BE STUBBORN AND LKLY WONT HPPN TIL AFTN.

IFR/SUBIFR CIGS AND VSBYS ARE EXPECTED FRIDAY NIGHT INTO SATURDAY
ALONG WITH RAIN. IFR/SUBIFR CONDITIONS WILL CONTINUE INTO SATURDAY
AFTERNOON ACROSS KBWI AND KMTN. AN EAST-NORTHEAST FLOW AROUND 10
KNOTS IS EXPECTED AT THE SURFACE FRIDAY NIGHT...BUT EAST-SOUTHEAST
WINDS AROUND 40-50 KNOTS ARE EXPECTED JUST A COUPLE THOUSAND FEET
FROM THE SURFACE.

HIGH PRESSURE WILL BUILD IN BEHIND THE COLD FRONT SATURDAY AFTERNOON
AND EVENING. HIGH PRESSURE WILL REMAIN OVERHEAD SUNDAY. THERE MAY BE
ENOUGH LOW-LEVEL MOISTURE TRAPPED UNDERNEATH THE INVERSION FOR
SUBVFR CIGS AND VSBYS SATURDAY NIGHT INTO SUNDAY MORNING. AN
UPPER-LEVEL LOW WILL PASS THROUGH SUNDAY NIGHT INTO MONDAY.

&&

.MARINE...
LGT WNDS ATTM UNDER HIPRES. ELY WNDS INCR TMRW IN ADVC OF CYCLONE.
WL KEEP SCA AS IS.

A STRONG EASTERLY FLOW IS EXPECTED AHEAD OF A COLD FRONT FRIDAY
NIGHT INTO SATURDAY. SOLID SCA WINDS ARE EXPECTED WITH GUSTS AROUND
30 KNOTS OVER THE BAY AND LOWER TIDAL POTOMAC. A GALE WARNING MAY BE
WARRANTED FRIDAY NIGHT ACROSS THE CHESAPEAKE BAY AND LOWER TIDAL
POTOMAC. WILL HAVE NEXT COUPLE SHIFTS ANALYZE THAT WITH MORE
GUIDANCE. HAVE EXTENDED THE SCA THROUGH SATURDAY MORNING ACROSS THE
BAY AND LOWER TIDAL POTOMAC.

HIGH PRESSURE WILL BUILD OVER THE WATERS SUNDAY. AN UPPER-LEVEL
LOW WILL PASS THROUGH MONDAY. SCA WINDS ARE POSSIBLE MONDAY INTO
TUESDAY NIGHT ACROSS THE WATERS.

&&

.HYDROLOGY...
A POTENT STORM SYSTEM AND ITS ASSOCIATED COLD FRONT WILL BRING
MODERATE TO HEAVY RAIN FRIDAY AFTERNOON THROUGH FRIDAY NIGHT.
RAINFALL TOTALS AROUND 1.00 TO 1.50 INCHES ARE EXPECTED. RAIN
COMBINED WITH MELTING SNOW WILL BRING A THREAT FOR
FLOODING...ESPECIALLY IN URBAN AREAS AS WELL AS SMALL STREAMS AND
AREAS OF POOR DRAINAGE. A FLOOD WATCH HAS BEEN ISSUED FOR LOCATIONS
EAST OF THE BLUE RIDGE MOUNTAINS WHERE WARMER TEMPERATURES WILL
RESULT IN A MORE RAPID SNOW MELT.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...FLOOD WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY NIGHT
     FOR DCZ001.
MD...FLOOD WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY NIGHT
     FOR MDZ004>007-009>011-013-014-016>018.
     WINTER WEATHER ADVISORY FROM 4 AM TO 10 AM EST FRIDAY FOR
     MDZ004>007-009-010.
     WINTER WEATHER ADVISORY FROM 1 AM TO 3 PM EST FRIDAY FOR
     MDZ003-501-502.
VA...FLOOD WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY NIGHT
     FOR VAZ036>042-050>057.
     WINTER WEATHER ADVISORY FROM 4 AM TO 10 AM EST FRIDAY FOR
     VAZ036>042-050>053.
     WINTER WEATHER ADVISORY FROM 1 AM TO 3 PM EST FRIDAY FOR
     VAZ021-025>031.
WV...WINTER WEATHER ADVISORY FROM 1 AM TO 3 PM EST FRIDAY FOR
     WVZ050>055-501>504.
MARINE...SMALL CRAFT ADVISORY FROM NOON FRIDAY TO NOON EST SATURDAY FOR
     ANZ530>534-537>543.
     SMALL CRAFT ADVISORY FROM NOON FRIDAY TO 6 AM EST SATURDAY FOR
     ANZ535-536.

&&

UPDATE...GMS
PREV DISCUSSION...HTS/BJL

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
918 PM EST THU DEC 24 2009

ANZ530>543-DCZ001-MDZ006-007-011-013-014-016>018-VAZ052>057-
251000-
CHESAPEAKE BAY NORTH OF POOLES ISLAND MD-
CHESAPEAKE BAY FROM POOLES ISLAND TO SANDY POINT MD-
CHESAPEAKE BAY FROM SANDY POINT TO NORTH BEACH MD-
CHESAPEAKE BAY FROM NORTH BEACH TO DRUM POINT MD-
CHESAPEAKE BAY FROM DRUM POINT MD TO SMITH POINT VA-
TIDAL POTOMAC FROM KEY BRIDGE TO INDIAN HEAD MD-
TIDAL POTOMAC FROM INDIAN HEAD TO COBB ISLAND MD-
TIDAL POTOMAC FROM COBB ISLAND MD TO SMITH POINT VA-
PATAPSCO RIVER INCLUDING BALTIMORE HARBOR-
CHESTER RIVER TO QUEENSTOWN MD-EASTERN BAY-
CHOPTANK RIVER TO CAMBRIDGE MD AND THE LITTLE CHOPTANK RIVER-
PATUXENT RIVER TO BROOMES ISLAND MD-
TANGIER SOUND AND THE INLAND WATERS SURROUNDING BLOODSWORTH
ISLAND-DISTRICT OF COLUMBIA-NORTHERN BALTIMORE-HARFORD-
SOUTHERN BALTIMORE-PRINCE GEORGES-ANNE ARUNDEL-CHARLES-ST. MARYS-
CALVERT-PRINCE WILLIAM/MANASSAS/MANASSAS PARK-FAIRFAX-
ARLINGTON/FALLS CHURCH/ALEXANDRIA-STAFFORD-SPOTSYLVANIA-
KING GEORGE-
918 PM EST THU DEC 24 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE MARYLAND PORTION OF THE
CHESAPEAKE BAY...TIDAL POTOMAC RIVER...AND ADJACENT COUNTIES IN
CENTRAL MARYLAND AND NORTHERN VIRGINIA AS WELL AS THE DISTRICT OF
COLUMBIA.

.DAY ONE...TONIGHT

A WINTER WEATHER ADVISORY IS IN EFFECT FOR THE NORTHERN AND
WESTERN SUBURBS OF WASHINGTON AND BALTIMORE BEGINNING AT 4 AM.
LIGHT AMOUNTS OF FREEZING RAIN ARE EXPECTED. PLEASE REFER TO
THE LATEST WINTER WEATHER MESSAGE FOR FURTHER DETAILS.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

A WINTER WEATHER ADVISORY REMAINS IN EFFECT UNTIL 10 AM FOR THE
NORTHERN AND WESTERN SUBURBS OF WASHINGTON AND BALTIMORE. PLEASE
REFER TO THE LATEST WINTER WEATHER MESSAGE FOR FURTHER DETAILS.

A FLOOD WATCH IS IN EFFECT FOR LATER FRIDAY AFTERNOON THROUGH
FRIDAY NIGHT. PLEASE REFER TO FFALWX FOR FURTHER DETAILS.

A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE UPPER TIDAL POTOMAC
FRIDAY AFTERNOON THROUGH FRIDAY NIGHT...AND FOR THE REST OF THE
WATERS FRIDAY AFTERNOON THROUGH SATURDAY MORNING.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL NOT BE NEEDED THROUGH TONIGHT.

MDZ003>005-009-010-501-502-VAZ021-025>031-036>042-050-051-
WVZ050>055-501>504-251000-
WASHINGTON-FREDERICK MD-CARROLL-MONTGOMERY-HOWARD-
EXTREME WESTERN ALLEGANY-CENTRAL AND EASTERN ALLEGANY-HIGHLAND-
AUGUSTA-ROCKINGHAM-SHENANDOAH-FREDERICK VA-PAGE-WARREN-CLARKE-
NELSON-ALBEMARLE-GREENE-MADISON-RAPPAHANNOCK-FAUQUIER-LOUDOUN-
ORANGE-CULPEPER-HAMPSHIRE-MORGAN-BERKELEY-JEFFERSON-PENDLETON-
HARDY-WESTERN GRANT-EASTERN GRANT-WESTERN MINERAL-EASTERN MINERAL-
918 PM EST THU DEC 24 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR EASTERN WEST VIRGINIA...
NORTHERN AND CENTRAL VIRGINIA...AND CENTRAL AND WESTERN MARYLAND
WITH THE EXCEPTION OF GARRETT COUNTY.

.DAY ONE...TONIGHT

A WINTER WEATHER ADVISORY IS IN EFFECT BEGINNING AFTER MIDNIGHT.
PLEASE REFER TO THE LATEST WINTER WEATHER MESSAGE FOR FURTHER
DETAILS.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

A WINTER WEATHER ADVISORY REMAINS IN EFFECT THROUGH MIDDAY FRIDAY.
PLEASE REFER TO THE LATEST WINTER WEATHER MESSAGE FOR FURTHER
DETAILS.

A FLOOD WATCH IS IN EFFECT FOR LATER FRIDAY AFTERNOON THROUGH
FRIDAY NIGHT FOR LOCATIONS EAST OF THE BLUE RIDGE MOUNTAINS.
PLEASE REFER TO FFALWX FOR FURTHER DETAILS.

WINDS WILL INCREASE FRIDAY...WITH STRONG WIND GUSTS POSSIBLE OVER
RIDGE TOPS ABOVE 2000 FEET FRIDAY AND FRIDAY NIGHT.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL NOT BE NEEDED THROUGH TONIGHT.