FXUS61 KLWX 241938
AFDLWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
238 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 /THROUGH TONIGHT/...
HIPRES OHD ATTM...AND ITS STAYING POWER WL LAST THRU THE NGT. W/ LOW 
DEWPTS...AND ONLY SCT CI FOR SKYCOVER...THINK TEMPS WL BE ABLE TO 
DROP RATHER EFFICIENTLY THIS EVNG BEFORE HI CLDS OVRSPREAD AREA. 
TOOK AN ENSEMBLE BLEND FOR MIN-T...WHICH PAINTS LOWS IN THE 20S ACRS 
MOST OF CWFA...AND MAYBE EVEN SOME TEENS IN THE MTNS. MEANWHILE...H8 
WNDS INCR...WHICH WL RSLT IN A WARM NOSE ON SNDGS. THE COOL LYR WL 
BE QUITE THIN /AOB 2K FT/ BUT WUD BE STUBBORN ONCE RA FALLS INTO 
THIS LYR.

GDNC CONTS TO PUSH BACK ONSET OF RAFL. HENCE...MOS POPS LWR THAN 
PRVS CYCLE. HWVR...THE OVNGT SETUP W/ ITS SHALLOW MOIST LYR CUD BE 
CONDUCIVE FOR DZ...WHICH CONSIDERING TEMPS WUD BE FZDZ. SO...HV 
BACKED OFF FM PRVS FCST...BUT WONT BE DRY AREAWIDE. HV PAINTED SMALL 
POPS ERN SLOPES OF BLURDG /WHICH WUD EXPERIENCE UPSLP/ WWD TO THE 
MTNS /CLOSER TO FORCING/ FOR THE TNGT PD. E OF THERE /INCL I-95 
CRRDR/ ANY MEASUREABLE PCPN WUD BE A 2ND PD EVENT. 

BUT...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 /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 /19Z THURSDAY THROUGH TUESDAY/...
VFR CONDS ATTM...BUT WL DETERIORATE LT TNGT AS PCPN NEARS. WL BE 
DROPPING TO IFR AFTR MIDNGT...AND BLO THAT BEFORE DAYBRK AS FZDZ 
NEARS. LGT ICING PSBL IN DC-BALT CRRDR BEFROE CHGOVR TO RA MIDDAY 
TMRW. 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.

&&

$$
SYNOPSIS...BJL
NEAR TERM...HTS
SHORT TERM...HTS
LONG TERM...BJL
AVIATION...HTS/BJL
MARINE...HTS/BJL
HYDROLOGY...BJL