FXUS61 KLWX 250902
AFDLWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
402 AM EST FRI DEC 25 2009
.SYNOPSIS...
PRECIPITATION WILL DEVELOP AHEAD OF A WARM FRONT AND LOW PRESSURE
SYSTEM TODAY. LOW PRESSURE MOVES NORTH AND EAST OF THE AREA ON
SATURDAY. AFTER WEAK HIGH PRESSURE PASSES OVERHEAD SATURDAY NIGHT
AND SUNDAY...AN UPPER LEVEL LOW PRESSURE AND SURFACE COLD FRONT
WILL MOVE ACROSS SUNDAY NIGHT AND MONDAY...WITH HIGH PRESSURE
BUILDING IN FOR THE MIDDLE OF NEXT WEEK.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
LARGE STORM SYSTEM WAS SPINNING ACROSS THE MID MISSISSIPPI VALLEY
EARLY THIS MORNING. ITS IMPRESSIVE HOW MUCH GEOGRAPHICAL AREA OF THE
CONUS HAS BEEN/IS BEING/WILL BE AFFECTED BY THIS STORM IN TERMS OF
PRECIPITATION AND WIND.
NO PRECIPITATION NOR WIND ASSOCIATED WITH THIS STORM SYSTEM HAVE
AFFECTED THE CWA YET...BUT THAT WILL CHANGE TODAY.
LOW CEILINGS HAVE BEEN DEVELOPING EAST OF THE MTNS OVER THE LAST
COUPLE HOURS DUE TO EASTERLY FLOW IN THE LOW LEVELS. ITS COLD...WITH
AIR TEMPERATURES BELOW FREEZING IN MOST AREAS EXCEPT AROUND OR JUST
ABOVE FREEZING ACROSS SOUTHERN MARYLAND. FORTUNATELY...AM THINKING
THAT THE DEPTH OF THE SATURATED LAYER WILL NOT BE ENOUGH TO PRODUCE
FREEZING DRIZZLE EARLY THIS MORNING. CWA WILL HAVE WAIT FOR THE
LEADING EDGE OF WARM ADVECTION/ISENTROPIC LIFT INDUCED PRECIPITATION
TO MOVE INTO THE AREA. THAT BELT OF MOISTURE WAS APPROACHING THE
POTOMAC HIGHLANDS EARLY THIS MORNING. AS ISENTROPIC LIFT CONTINUES
TO DEEPEN...THIS AREA WILL MORE THAN LIKELY EXPAND EAST THROUGH THE
MORNING.
RIGHT ALONG/EAST OF INTERSTATE 95...PRECIPITATION IS NOW EXPECTED TO
MOVE IN AROUND 15Z...AT A TIME WHEN SURFACE TEMPERATURES EXPECTED TO
HAVE RISEN ABOVE FREEZING. THEREFORE...THREAT OF FREEZING RAIN IS
SMALL IN DC/BALT AND POINTS SOUTHEAST AND ADVISORY WILL NOT BE
EXPANDED.
NORTHWEST OF INTERSTATE 95...INCLUDING THE NORTHWEST DC/BALT SUBURBS
AND ESPECIALLY WEST OF THE BLUE RIDGE...PRECIPITATION WILL ARRIVE AT
A TIME WHEN SURFACE TEMPERATURES WILL BE AT OR BELOW THE FREEZING
MARK. GIVEN THE WARM AIR ALOFT...LIQUID WILL MOST LIKELY FALL ONTO
SUB-FREEZING SURFACES /ALTHOUGH A LITTLE SLEET COULD MIX IN ACROSS
THE EXTREME NORTHWEST CWA INITIALLY/. A 2 TO 5 HOUR PERIOD OF MINOR
ICING MAY OCCUR IN THE ADVISORY COUNTIES EAST OF THE BLUE RIDGE
BEFORE SURFACE TEMPERATURES WARM ENOUGH TO CHANGE PRECIPITATION TO
RAIN. WEST OF THE BLUE RIDGE...SUB-FREEZING SURFACE AIR WILL LINGER
LONGER WHICH WILL MEAN A MORE PROLONGED PERIOD OF ICING. CURRENT
INDICATIONS ARE FOR A MID TO LATE AFTERNOON CHANGEOVER TO
RAIN...JUST BEFORE THE HEAVIEST QPF MOVES IN. THEREFORE...ICE
AMOUNTS MAY REMAIN UNDER WARNING CRITERIA /0.25"/. CAVEAT HERE IS
IF SUB-FREEZING TEMPERATURES LAST LONGER AND/OR HEAVIER QPF MOVES IN
SOONER...THEN PORTIONS OF THE HIGHLANDS/SHENANDOAH VALLEY COULD
TEASE WARNING CRITERIA.
SPEAKING OF QPF...HEAVY AXIS OF RAIN IS FORECAST TO DEVELOP ACROSS
NORTH CENTRAL VIRGINIA LATER IN THE AFTERNOON /SHIFTING NORTHEAST IN
THE EVENING/ AND GIVEN THE SNOWPACK/MELT AND POTENTIAL FOR AOA 1
INCH OF RAINFALL...FLOOD WATCH CONTINUES.
HAVE ALSO POSTED A WIND ADVISORY HEADLINE FOR THE HIGHLANDS GIVEN
NAM/GFS INDICATING 40 KT NEAR 2K FT.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
LARGE STORM SPINS A LITTLE NORTH INTO THE MIDWEST THIS EVENING.
MEANWHILE...WAVE OF LOW PRESSURE OVER NORTH CAROLINA LIFTS NORTHEAST
INTO THE MID ATLANTIC OVERNIGHT. RESULT WILL BE FOR RAIN ACROSS THE
CWA OVERNIGHT. HEAVIEST AXIS OF QPF MOVES THROUGH THE INTERSTATE 95
CORRIDOR DURING THE EVENING HOURS...WITH MINOR URBAN/SMALL STREAM
FLOOD PROBLEMS POSSIBLE.
TEMPERATURES WILL BE STEADY/SLOWLY RISING OVERNIGHT.
&&
.LONG TERM /SATURDAY THROUGH THURSDAY/...
COASTAL LOW WORKS NWD ACRS DELMARVA DRNG DAY SAT...WITH RAIN ENDING
FROM THE SW AS DRIER AIR PUNCHES IN FROM THE SW. WEAK HIGH PRESSURE
PASSES S OF THE AREA SAT NGT. UPR LOW WILL STILL BE SPINNING OVER
GREAT LAKES...WITH VORT MAXES PASSING ACRS THE AREA OVER THIS PD.
DOWNSLOPING FLOW SHUD KEEP MOST AREAS DRY...BUT LOW POPS ON SUN IN CASE
SOME SNOSHWR ACTIVITY CAN PASS THE APPALACHIANS...PSBLY BCMG RAIN SHOWERS
AS THEY WORK EAST.
UPR LOW FINALLY GETS CAUGHT IN POLAR JET AND MAKES A MOVE ACRS THE
NE CONUS ON SUN NGT/MON...WITH STRONG COLD FRONT MVG ACRS AND
SLIGHTLY BETTER CHANCE FOR PRECIP. BEHIND THE FRONT...UPSLOPE SNOW
LIKELY MON/TUE IN NW FLOW FOR THE WESTERN SLOPES OF ALLEGHENY FRONT.
HIGH PRESSURE PASSES THROUGH THE AREA FOR THE MIDDLE OF NEXT WEEK.
&&
.AVIATION /09Z FRIDAY THROUGH TUESDAY/...
IFR CIGS DEVELOP BETWEEN 09Z-12Z ACROSS THE TERMINALS THIS MORNING.
AS PRECIPITATION DEVELOPS BTWN 12Z-15Z AT KCHO/KMRB/KIAD...CIGS
LIKELY DROP TO LIFR. KDCA/KBWI FOLLOW SUIT 15Z-18Z. P-TYPE MOST
LIKELY TO BE -RA AT KDCA/KBWI...WITH A PERIOD OF -ZR AT
KIAD/KCHO/KMRB BEFORE CHANGING TO RAIN. AFT 18Z...HEAVIER
PRECIPITATION IS ALSO LIKELY TO PRODUCE IFR VSBYS...WHICH WILL LAST
AT LEAST TIL 12Z.
SUBVFR CONDS WITH RAIN CAUSING VSBY RESTRICTIONS AND LOW CIGS
THROUGH AT LEAST SAT MRNG. SKIES SHOULD CLEAR FIRST IN CHO THEN
WORKING NEWD WITH BWI/MTN BEING LATEST TO CLEAR...LIKELY LATE IN
AFTN. PATCHY VSBY RESTRICTIONS DUE TO FOG PSBL SAT NGT/SUN MRNG. VFR
CONDS ON SUN...WITH ANOTHER COLD FRONT AND CHANCE FOR PRECIP/SUBVFR
CONDS SUN NGT INTO MONDAY.
E WINDS 10 TO 15 MPH SAT BEFORE CDFNT PASSES ACRS MIDDAY...SHIFTING
WINDS TO NW AT SIMILAR SPEEDS THRU REMAINDER OF WEEKEND.
&&
.MARINE...
EASTERLY FLOW IS EXPECTED TO INTENSIFY TODAY...AND SMALL CRAFT
ADVISORY WILL CONTINUE TODAY/TONIGHT. MODELS INDICATE STRONG WINDS
ALOFT FROM THE SOUTHEAST...BUT SHOULD HAVE PROBLEM MIXING ANYTHING
MORE THAN 30 KNOTS DOWN SO NO GALE WARNING WILL BE ISSUED.
ELY FLOW IN ADVANCE OF CDFNT WILL BRING 15 TO 20 KT WINDS TO WATERS
THRU SAT MRNG...WITH GUSTS TO 25 KT PSBL. SCA CONT THRU THE MRNG.
CDFNT XPCD MIDDAY SAT...SWITCHING WINDS TO NW AT SPEEDS BELOW SCA
THRESHOLDS. CDFNT PASSES ACRS SUN NGT...WITH POTENTIAL FOR GUSTY
WINDS REACHING SCA CRITERIA ON MONDAY AND TUESDAY.
&&
.HYDROLOGY...
STRONG SLY FLOW AT MID LEVELS AND MOIST ELY FLOW AT LOW LEVELS WILL
RAISE PWATS TO OVER AN INCH ACRS AREAS S/E OF BLUE RIDGE. RAINFALL
AMOUNTS OF 1 TO 1.5 INCHES MAY BE ENOUGH...TO CAUSE MINOR
FLOODING...ESPECIALLY WHEN COMBINED WITH WARMER TEMPERATURES MELTING
THE SNOWPACK...WHICH CONTAINS NEARLY 1 INCH LIQUID EQUIVALENT IN
SOME LOCATIONS. URBAN AREAS/SMALL STREAMS WILL BE PARTICULARLY AT
RISK...WHERE SNOW MAY HINDER DRAINAGE. FLOOD WATCH WILL BE KEPT UP
THIS AFTERNOON AND TONIGHT.
&&
.LWX WATCHES/WARNINGS/ADVISORIES...
DC...FLOOD WATCH FROM 3 PM EST THIS AFTERNOON THROUGH LATE TONIGHT
FOR DCZ001.
MD...FLOOD WATCH FROM 3 PM EST THIS AFTERNOON THROUGH LATE TONIGHT
FOR MDZ004>007-009>011-013-014-016>018.
WINTER WEATHER ADVISORY UNTIL 10 AM EST THIS MORNING FOR
MDZ004>007-009-010.
WINTER WEATHER ADVISORY UNTIL 3 PM EST THIS AFTERNOON FOR
MDZ003-501-502.
WIND ADVISORY FROM 10 AM THIS MORNING TO 6 PM EST THIS EVENING
FOR MDZ501.
VA...FLOOD WATCH FROM 3 PM EST THIS AFTERNOON THROUGH LATE TONIGHT
FOR VAZ036>042-050>057.
WINTER WEATHER ADVISORY UNTIL 10 AM EST THIS MORNING FOR
VAZ036>042-050>053.
WINTER WEATHER ADVISORY UNTIL 3 PM EST THIS AFTERNOON FOR
VAZ021-025>031.
WIND ADVISORY FROM 10 AM THIS MORNING TO 6 PM EST THIS EVENING
FOR VAZ021.
WV...WINTER WEATHER ADVISORY UNTIL 3 PM EST THIS AFTERNOON FOR
WVZ050>055-501>504.
WIND ADVISORY FROM 10 AM THIS MORNING TO 6 PM EST THIS EVENING
FOR WVZ054-501>503.
MARINE...SMALL CRAFT ADVISORY FROM 9 AM THIS MORNING TO NOON EST
SATURDAY FOR ANZ530>534-537>543.
SMALL CRAFT ADVISORY FROM NOON TODAY TO 6 AM EST SATURDAY FOR
ANZ535-536.
&&
$$
SYNOPSIS...PELOQUIN/SBK
NEAR TERM...PELOQUIN
SHORT TERM...PELOQUIN
LONG TERM...SBK
AVIATION...PELOQUIN/SBK
MARINE...PELOQUIN/SBK
HYDROLOGY...SBK
