Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 260246 AAA AFDLWX AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 946 PM EST TUE NOV 25 2014 .SYNOPSIS... A LOW PRESSURE SYSTEM WILL CONTINUE TO STRENGTHEN OFF THE SOUTHEAST COAST TONIGHT...TRACK UP THE ATLANTIC COAST INTO WEDNESDAY. THE LOW WILL QUICKLY MOVE OFF TO OUR NORTHEAST WEDNESDAY NIGHT. AN UPPER-LEVEL DISTURBANCE WILL PASS THROUGH THE AREA THURSDAY AND HIGH PRESSURE WILL BUILD OVERHEAD FRIDAY. THE HIGH WILL MOVE OFF THE COAST DURING THE WEEKEND. && .NEAR TERM /THROUGH WEDNESDAY/... LATE EVENING UPDATE... NO HEADLINE CHANGES AND MINIMAL GRID/FORECAST MODIFICATIONS FOR THE INCOMING LOW PRES SYSTEM. SINCE THIS TIME YESTERDAY EVENING...A RELATIVELY NARROW RAIN BAND HAS BEEN SET-UP FROM THE GULF COAST NEWD TOWARD THE VA TIDEWATER. OVER THE PAST SEVERAL HRS...MORE RAIN HAS BEEN FILLING THE REGIONAL SCOPE - ACROSS MORE OF THE SOUTHEAST. A DYNAMIC SCENARIO IS TAKING SHAPE AS LARGE- SCALE FEATURES FROM THE PAC NW TO THE SOUTHEAST WILL BEGIN TO INFLUENCE ONE LARGE LOW PRES SYSTEM. THE MOIST CORRIDOR OVER THE DEEP SOUTH IS GROWING AND PUSHING MID DECKS AND CIRRUS DEBRIS FURTHER UP THE ERN SEABOARD AND INTO OUR REGION. CLEAR SKIES OVER THE WRN HALF OF THE CWA EARLIER TODAY HAS ALLOWED TEMPS TO DROP MUCH FURTHER THAN AREAS TO THE EAST. MANY SHEN VLY OBS ALREADY IN THE M-U30S OR BARELY STILL INTO THE 40S. THE RELATIVELY LOWER DEWPOINTS ALSO IMPORTANT FOR KEEPING TEMPS DOWN...EVEN AS INSULATING CIRRUS DEBRIS BUILDS OVERHEAD. OVERNIGHT... A POTENT VORT MAX SCRAPING THE GULF COAST STATES WILL BE FORCED NEWD...TURNING THE UPPER SHORT SHORTWAVE TO A NEGATIVE TILT OVERNIGHT. THIS TRANSITION WILL OCCUR BECAUSE OF A SECONDARY VORT BEHIND IT - TO THE NW - THAT WILL HELP DIG THE UPPER TROUGH EVEN FURTHER SOUTH. THE ALREADY MOIST AND DYNAMIC WARM SECTOR STATIONED OVER THE WARMER GULF STREAM WATERS WILL STRENGTHEN THE LARGE SYSTEM FURTHER AS IT MOVES NE IN THE COMING HRS. A SFC HIGH RETREATING TO THE NE WILL ALSO FILTER COLDER/DRIER AIR INTO THE NRN PORTIONS OF THE AREA...ESPEC THE SHEN VLY HEADING EARLY MRNG HRS. TEMPS ACROSS THE AREA WILL LARGE REMAIN ABOVE FREEZING WELL INTO THE OVERNIGHT AND ONLY BEGIN TO DROP TOWARD THE PREDAWN HRS. LIGHT PRECIP IS EXPECTED TO OVERSPREAD THE AREA GRADUALLY INTO THE EARLY MRNG HRS/PREDAWN...AS THE COASTAL LOW TRACKS NORTHEAST TOWARD CAPE HATTERAS. POST-DAWN WED MRNG... THE SFC LOW WILL INTENSIFY AS IT TRACKS NORTHEAST...AND THIS WILL ALLOW FOR MOISTURE FROM THE ATLANTIC AND GULF OF MEXICO TO GET WRAPPED INTO THE STORM. THEREFORE...PRECIP WILL BE MODERATE TO LOCALLY HEAVY AT TIMES DURING THE DAY WEDNESDAY. THE TRACK OF THE LOW AND POSITION OF THE SURFACE HIGH SUGGESTS THAT THERE WOULD BE ENOUGH COLD AIR FOR SNOW. THERE IS LITTLE COLD AIR ASSOCIATED WITH THIS STORM SINCE THE SOURCE REGION FROM THE HIGH WAS ORIGINALLY OVER THE CENTRAL AND SOUTHERN PLAINS INSTEAD OF CANADA. THERE WILL BE LOW-LEVEL DRY AIR THAT FILTERS INTO THIS STORM FROM THE NORTH AND THERE WILL BE FRONTOGENETICAL FORCING THAT COULD LEAD TO LOCALLY HEAVY PRECIPITATION...CAUSING DYNAMICAL COOLING. THESE COOLING MECHANISMS SHOULD BE ENOUGH FOR ANY MIXED PRECIPITATION TO CHANGE TO SNOW FOR LOCATIONS NEAR THE MASON-DIXON LINE AND ALSO NEAR AND WEST OF THE BLUE RIDGE MOUNTAINS. THE HIGHER ELEVATIONS ARE EXPECTED TO GET THE MOST SNOW. THE HEAVIEST SNOW IS EXPECTED WEDNESDAY MORNING THROUGH THE MIDDLE AFTERNOON HOURS BEFORE PRECIPITATION TAPERS OFF AS THE LOW DEPARTS. THERE SHOULD BE DECENT MOISTURE AND LIFT WITH THIS SYSTEM...SO SEVERAL INCHES OF SNOW ARE EXPECTED. A WINTER STORM WARNING IS IN EFFECT FOR THESE AREAS. FOR THE NORTHERN AND WESTERN SUBURBS OF WASHINGTON AND BALTIMORE... THERE WILL BE ENOUGH WARM AIR FOR PRECIPITATION TO START OUT AS RAIN LATE TONIGHT INTO WEDNESDAY MORNING. HOWEVER...COLDER AIR WILL WORK ITS WAY IN CAUSING PRECIPITATION TO MIX WITH AND PERHAPS EVEN CHANGE TO ALL SNOW FOR A PERIOD BEFORE ENDING LATER WEDNESDAY AFTERNOON. A WINTER WEATHER ADVISORY IS IN EFFECT FOR THESE AREAS...BUT AS OF NOW IT APPEARS THAT MOST OF THE SNOW ACCUMULATION WOULD BE ON GRASSY SURFACES WITH SURFACE TEMPS JUST ABOVE FREEZING. FOR LOCATIONS NEAR AND EAST OF INTERSTATE 95 MOSTLY RAIN IS EXPECTED. SNOW MAY MIX IN FOR A PERIOD LATE WEDNESDAY MORNING INTO WEDNESDAY AFTERNOON BEFORE PRECIPITATION ENDS...BUT TEMPERATURES WILL REMAIN ABOVE FREEZING AND THIS WILL CAUSE LITTLE OR NO SNOW ACCUMULATION. DESPITE GENERAL MODEL AGREEMENT IN THE TIMING/POSITION OF THE SURFACE LOW...THE BOTTOM LINE IS THAT THE TEMPERATURE PROFILES IN THE LOWEST THOUSAND FEET OF THE ATMOSPHERE WILL BE VERY SENSITIVE AS TO DETERMINING THE TYPE AND AMOUNT OF PRECIPITATION THAT FALLS. THERE IS A LITTLE MORE MODEL SPREAD IN THIS AREA...AND OF COURSE THESE VALUES WILL BE MARGINAL FOR SNOW WITH THIS SYSTEM. WPC HAS NOTED THAT THE NAM HAS BOTH A COLD BIAS AND A BIAS FOR GENERATING TOO MUCH PRECIPITATION IN THE COLD AIR. BEING THAT MODELS STRUGGLE IN THE BOUNDARY LAYER...CONFIDENCE IS NOT HIGH ON THE EXACT TIMING OF CHANGEOVER...ALTHOUGH AS NOTED...LOWER ELEVATIONS ARE LOOKING LIKE MUCH MORE RAIN THAN SNOW. IT IS CONCEIVABLE THE MID ELEVATIONS MAY SWITCH BACK AND FORTH DEPENDING ON PRECIP INTENSITY...MAKING FOR POTENTIAL VARIABILITY IN ACCUMULATIONS ACROSS SHORT DISTANCES IN THESE LOCATIONS. PRECIPITATION WILL END FROM SOUTHWEST TO NORTHEAST LATER WEDNESDAY AFTERNOON INTO WEDNESDAY EVENING AS THE LOW MOVES AWAY FROM THE AREA. && .SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/... TEMPERATURES WILL DIP BELOW FREEZING ACROSS MOST AREAS SO ANY WET OR SLUSHY ROADS MAY BECOME ICY. AN UPPER-LEVEL DISTURBANCE WILL MOVE INTO THE REGION AFTER MIDNIGHT AND THIS MAY TRIGGER MORE SNOW SHOWERS...ESPECIALLY ACROSS THE ALLEGHENY HIGHLANDS INTO CENTRAL VIRGINIA. LIGHT SNOW ACCUMULATIONS ARE POSSIBLE ACROSS THESE AREAS WITH TEMPERATURES CLOSE TO FREEZING. UPPER-LEVEL TROUGH/CLIPPER SYSTEM SWINGS THROUGH THE AREA THURSDAY AND COULD BE ACCOMPANIED BY SCATTERED SNOW SHOWERS OR FLURRIES. LITTLE TO NO ACCUMULATION IS EXPECTED AT THIS TIME...EXCEPT PERHAPS IN A FEW SPOTS OVER THE WESTERN RIDGES. && .LONG TERM /FRIDAY THROUGH TUESDAY/... HIGH PRESSURE BUILDS IN FOR THE END OF THE WEEK ACCOMPANIED BY BELOW NORMAL TEMPERATURES. AS THE HIGH MOVES OFFSHORE OVER THE WEEKEND...SOUTHERLY FLOW SETS UP BRINGING TEMPERATURES BACK TO NEAR NORMAL. A FRONT WILL APPROACH BY LATE IN THE WEEKEND...BRINGING ANOTHER CHANCE OF PRECIPITATION EARLY NEXT WEEK. BUT THE MILDER AIRMASS THAT WILL HAVE BUILT IN OVER THE WEEKEND WILL STILL BE IN PLACE EARLY NEXT WEEK SO WINTRY PRECIPITATION CHANCES APPEAR NEGLIGIBLE AT THIS TIME. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... VFR CONDITIONS WILL CONTINUE THROUGH THIS EVENING WITH W TO NW WINDS GENERALLY UNDER 10 KT. LOW PRESSURE WILL MOVE UP THE EASTERN SEABOARD FROM LATE TONIGHT THROUGH WEDNESDAY. CIGS WILL CONTINUALLY LOWER...REACHING IFR LEVELS (AND BELOW) AFTER DAYBREAK. PRECIPITATION WILL SPREAD FROM SOUTH TO NORTH BETWEEN 08-11Z. AT THE TERMINALS...PRECIPITATION WILL INITIALLY BE MOSTLY RAIN...EXCEPT SNOW COULD MIX IN AT MRB. CHANGEOVER TO SNOW WILL OCCUR EARLY MORNING AT MRB. MIX ARRIVES AT IAD...BWI AND CHO DURING MID TO LATE MORNING. CHANGEOVER FULLY TO SNOW QUESTIONABLE AT THESE SITES...BUT IF SO...WOULD HAPPEN MIDDAY TO EARLY AFTERNOON. DCA AND MTN WILL BE MOSTLY RAIN FOR A MAJORITY OF THE EVENT...BUT SNOW COULD MIX IN PAST MIDDAY. VSBY IN TAFS WOULD BE CONSERVATIVE IF HEAVIER SNOW BANDS DEVELOP...WHICH WOULD BE MOST LIKELY AT MRB. WINDS WILL BE NORTHERLY...WITH SLIGHTLY HIGHER MAGNITUDES NEAR 15 KT FOR THE EASTERN SITES. CONDITIONS IMPROVE WEDNESDAY EVENING...BUT SNOW SHOWERS MAY AFFECT CHO DURING THE LATE NIGHT HOURS. SUB-VFR POSSIBLE IN SCTD SNOW SHWRS THU W/ W FLOW 10-20 KTS. MAINLY VFR LIKELY THU NIGHT- SUN...W WINDS 10-15 KTS BECOMING S FRI NIGHT 10 KTS OR LESS THROUGH THE WEEKEND. && .MARINE... LIGHT NW TO N WINDS THROUGH THIS EVENING. WINDS WILL INCREASE AFTER MIDNIGHT AS LOW PRESSURE MOVES UP THE COAST FROM THE SOUTH. WINDS WILL FIRST REACH SMALL CRAFT LEVELS FOR THE CENTRAL AND SOUTHERN MD BAY...AND CONTINUE INCREASING NORTHWARD THROUGH THE MORNING. CURRENT WIND PROJECTS REMAIN BELOW GALE LEVELS...BUT WILL BE CLOSE OVER OPEN AREAS OF THE BAY. WINDS ARE STILL FORECAST TO REMAIN BELOW SCA LEVELS FOR THE UPPER TIDAL POTOMAC. PRECIPITATION WITH THIS SYSTEM WILL BE MAINLY RAIN OVER THE WATERS...BUT COULD MIX WITH SNOW OVER THE UPPER PART OF THE BAY BY MIDDAY WEDNESDAY. THE SCA HAS BEEN EXTENDED UNTIL 10 PM FOR THE MID/LOWER MD BAY AND LOWER TIDAL POTOMAC...BUT WINDS WILL OVERALL SHOW A DIMINISHING TREND OVERNIGHT. A COUPLE OF SNOW SHOWERS OR FLURRIES COULD MEANDER OVER THE WATERS IN ASSOCIATION WITH A CLIPPER SYSTEM THURSDAY. WESTERLY FLOW GUSTING TO SMALL CRAFT ADVISORY LEVELS IS EXPECTED LATE THURSDAY INTO FRIDAY...THEN SOUTHERLY WINDS UP TO SMALL CRAFT ADVISORY LEVELS IS FORECAST FOR SATURDAY. && .TIDES/COASTAL FLOODING... WATER LEVELS ARE SLIGHTLY ABOVE ASTRO NORMS BUT HAVE BEEN SHOWING A SLIGHT DECREASING TREND TODAY. LOW PRESSURE TRACKING UP THE COAST WEDNESDAY MAY CAUSE A RISE IN WATER LEVELS...BUT THE QUICK SPEED OF SYSTEM AND NORTHERLY WINDS SUGGEST THAT TIDES WILL REMAIN BELOW MINOR FLOODING LEVELS. && .CLIMATE... CLIMATE... RECORDS FOR EARLIEST MEASURABLE SNOW ALREADY PASSED A FEW WEEKS AGO...BUT BELOW IS A LIST OF DAILY MAXIMUM SNOWFALL RECORDS SPECIFIC TO THE DATE OF NOVEMBER 26TH. DCA 2.2" (1898) BWI 4.5" (1898) IAD 1.1" (1978) && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...WINTER STORM WARNING FROM 3 AM TO 6 PM EST WEDNESDAY FOR MDZ003-501-502. WINTER STORM WARNING FROM 6 AM TO 6 PM EST WEDNESDAY FOR MDZ004>006. WINTER WEATHER ADVISORY FROM 8 AM TO 6 PM EST WEDNESDAY FOR MDZ007-009>011. VA...WINTER STORM WARNING FROM 3 AM TO 6 PM EST WEDNESDAY FOR VAZ025>031-036>040-503-504. WINTER STORM WARNING FROM 6 AM TO 6 PM EST WEDNESDAY FOR VAZ042-501. WINTER WEATHER ADVISORY FROM 8 AM TO 6 PM EST WEDNESDAY FOR VAZ050>053-502. WV...WINTER STORM WARNING FROM 3 AM TO 6 PM EST WEDNESDAY FOR WVZ050>053-055-501>506. MARINE...SMALL CRAFT ADVISORY FROM 1 AM TO 10 PM EST WEDNESDAY FOR ANZ532>534-537-540>543. SMALL CRAFT ADVISORY FROM 6 AM TO 6 PM EST WEDNESDAY FOR ANZ530-531-538-539. && $$ UPDATE...GMS PREV DISCUSSION...ADS/BJL/DFH

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