Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 272226 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 526 PM EST THU NOV 27 2014 .SYNOPSIS... AN UPPER-LEVEL DISTURBANCE WILL PASS THROUGH THE AREA TONIGHT AND HIGH PRESSURE WILL BUILD OVERHEAD FRIDAY. THE HIGH WILL MOVE OFF THE COAST DURING THE WEEKEND. A COLD FRONT IS EXPECTED TO MOVE THROUGH MONDAY. && .NEAR TERM /THROUGH FRIDAY/... SURFACE HIGH PRESSURE WILL BUILD TOWARD THE REGION TONIGHT BEHIND THE COLD FRONT...BUT ANOTHER UPPER-LEVEL DISTURBANCE WILL SWING THROUGH THE AREA DURING THIS TIME. A PRESSURE SURGE WILL CAUSE BLUSTERY CONDITIONS AND IT WILL BE COLD DUE TO A NORTHWEST FLOW. AN UPSLOPE NORTHWESTERLY FLOW WILL BRING ACCUMULATING SNOW TO LOCATIONS ALONG AND WEST OF THE ALLEGHENY FRONT. MOISTURE WILL BE PARTIALLY LINED UP WITH THE MAXIMUM DENDRITIC SNOW GROWTH ZONE UNDERNEATH THE SUBSIDENCE INVERSION. THE SUBSIDENCE INVERSION WILL ALSO REMAIN RELATIVELY HIGH AND THERE IS PLENTY OF MOISTURE AROUND AS WELL. THEREFORE...A WINTER WEATHER ADVISORY HAS BEEN ISSUED FOR THE RIDGE TOPS IN WEST VIRGINIA AND VIRGINIA ALONG AND WEST OF THE ALLEGHENY FRONT. TWO TO FOUR INCHES OF SNOW IS EXPECTED ACROSS THESE AREAS. A FEW SNOW SHOWERS MAY SPILL EAST OF THE MOUNTAINS ACROSS NORTHERN MARYLAND...EXTREME NORTHERN VIRGINIA AND EASTERN WEST VIRGINIA...BUT LITTLE OR NO SNOW ACCUMULATION IS EXPECTED. FCST MIN TEMPS WILL RANGE FROM THE TEENS ALONG THE ALLEGHENY FRONT TO THE LOW 30S NEAR WASHINGTON AND BALTIMORE. THE UPPER-LEVEL DISTURBANCE WILL MOVE OFF TO THE EAST FRIDAY WHILE HIGH PRESSURE BUILDS OVERHEAD. ANY LEFTOVER SNOW SHOWERS ALONG THE ALLEGHENY FRONT WILL DIMINISH AND SUNSHINE IS EXPECTED. HOWEVER...IT WILL BE COLD DUE TO THE NORTHWEST FLOW AHEAD OF THE BUILDING HIGH. MAX TEMPS WILL BE BELOW CLIMO...RANGING FROM THE 20S IN THE MOUNTAINS TO THE LOWER 40S IN THE VIRGINIA PIEDMONT AND SOUTHERN MARYLAND. && .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... PATTERN SHIFT AS WRN CONUS RIDGE BREAKS DOWN AS AN UPR TROUGH SHIFTS SOUTH INTO THE PAC NW. AN ASSOCIATED WARM FRONT WILL MOVE EAST FROM THE MIDWEST STATES AND INTO THE NRN MID-ATLANTIC. 00Z ECMWF/12Z GFS SUGGEST LIFT FROM THIS FRONT WILL CAUSE PRECIP INTO THE CENTRAL APPALACHIANS FRIDAY NIGHT. IN SPITE OF WARM AIR ADVECTION...THERMAL PROFILES COOL ENOUGH FOR ALL SNOW. LIMITED POPS FOR NOW TO THE NWRN THREE ZONES (WEST OF THE ALLEGHENY FRONT). MID-LEVEL CLOUDS ELSEWHERE...THOUGH PLENTY OF COLD AIR AND LITTLE MIXING WILL ALLOW FOR MIN TEMPS IN THE LOW TO MID 20S INLAND...UPR 20S NEAR SHORE. WARM AIR ADVECTION CONTINUES THROUGH SATURDAY NIGHT IN SWLY FLOW WITH PLENTY OF CLOUD COVER. TEMPERATURES MODERATE THROUGH THE WEEKEND...REACHING NORMAL BY SUNDAY EVENING. TEMPS A BLEND OF MAV/MET AND SREF OVERNIGHT/MOSGUIDE DURING THE DAY. BY THE TIME PRECIP RETURNS TO WRN ZONES SATURDAY EVENING...IT SHOULD BE WARM ENOUGH FOR RAIN. NOT EXPECTING FREEZING RAIN AT THIS TIME DUE TO THE WARM NOSE DESCENDING...WILL HAVE TO LOOKOUT FOR THAT THOUGH. DRY EAST OF THE ALLEGHENY FRONT WITH SWLY/SLY FLOW 10 MPH OR SO MAINTAINED THROUGH SATURDAY NIGHT IN DECENT GRADIENT. && .LONG TERM /SUNDAY THROUGH THURSDAY/... HIGH PRESSURE SETS UP OFFSHORE FOR THE SECOND HALF OF THE WEEKEND INTO EARLY NEXT WEEK. THIS WILL SET UP A SOUTHERLY FLOW AND BRING IN WARMER TEMPERATURES...WITH HIGHS A FEW DEGREES ABOVE NORMAL EXPECTED SUNDAY AND MONDAY. A COLD FRONT WILL APPROACH FROM THE WEST LATE MONDAY...AND CROSS THE AREA THROUGH TUESDAY. THIS WILL BRING THE REGION ITS NEXT CHANCE OF PRECIPITATION...BUT TEMPERATURES SHOULD BE WARM ENOUGH FOR ALL RAIN. SOME WET SNOWFLAKES MAY MIX IN ONCE THE COLD FRONT PASSES WITH ANY LEFTOVER SHOWERS MONDAY EVENING ALONG THE MASON-DIXON LINE AND WEST OF THE BLUE RIDGE MOUNTAINS MONDAY EVENING. NORTHWEST FLOW BEHIND THE FRONT WILL LIKELY RESULT IN UPSLOPE SNOW SHOWERS OVERNIGHT MONDAY NIGHT ALONG THE WESTERN RIDGES OF THE ALLEGHENY FRONT ONCE TEMPERATURES DROP...BUT AIRMASS WILL BE MARGINAL SO ANY ACCUMULATION POTENTIAL IS QUESTIONABLE. A STRONG AREA OF HIGH PRESSURE /I.E. 1040 MB/ WILL BUILD TO THE NORTH TUESDAY INTO TUESDAY NIGHT. MEANWHILE...THE AFOREMENTIONED FRONT STALLS TO THE SOUTH...THEN SLOWLY RETURNS NORTH AS A WARM FRONT TUESDAY NIGHT. THERE IS A STRONG SIGNAL FOR COLD AIR DAMMING IN THE MODELS...WITH EVEN LOWER RESOLUTION GLOBAL MODELS SHOWING TEMPERATURES BELOW FREEZING AS PRECIPITATION BEGINS OVER MUCH OF THE SHENANDOAH VALLEY AND THE POTOMAC HIGHLANDS. ECMWF IS WARMER AT 850 MB AND DEEPER WITH LOW PRESSURE TO THE WEST RESULTING IN MORE PRECIPITATION AND SUGGESTING A HIGHER ICE POTENTIAL ESPECIALLY WEST OF THE METRO. GFS IS COLDER AT 850 MB AND WEAKER WITH LOW PRESSURE TO THE WEST SUGGESTING LIGHTER QPF AND A MORE GENERIC WINTRY MIX OF SNOW AND RAIN. FOR NOW...STICKING WITH A CHANCE OF RAIN/SNOW...BUT WILL DEFINITELY NEED TO MONITOR AND FINE-TUNE OVER THE COMING DAYS AS POTENTIAL IS THERE FOR OVERRUNNING PRECIP. OVER COLD AIR TO RESULT IN WINTRY PRECIP OVER AT LEAST PART OF THE AREA TUESDAY NIGHT INTO WEDNESDAY. LEFT OUT OF HWO FOR NOW BECAUSE 30 PERCENT CONFIDENCE IS NOT THERE THAT WE WILL BE ISSUING HAZARDS GIVEN LARGE SPREAD IN TIMING/TEMPERATURES. MODELS/ENSEMBLES BEGIN TO DIVERGE AFTER THIS TIME IN RESPONSE TO A PATTERN CHANGE ALOFT WITH MORE ZONAL FLOW OF PACIFIC ORIGIN TAKING HOLD. && .AVIATION /22Z THURSDAY THROUGH TUESDAY/... VFR CONDITIONS ARE EXPECTED THROUGH FRIDAY. BRIEF PERIOD OF CIGS AROUND 3000 FT PSBL WITH LIGHT PCPN SHOWERS THRU 00Z AS A TROUGH MOVES THROUGH. A PRESSURE SURGE BEHIND THIS BOUNDARY WILL CAUSE NORTHWEST TO GUST AROUND 20 KNOTS TONIGHT THROUGH MUCH OF FRIDAY. WINDS WILL DIMINISH TOWARD FRIDAY EVENING. VFR FRIDAY NIGHT THROUGH SATURDAY NIGHT. CALM FRIDAY NIGHT THEN SWLY/SLY FLOW INCREASES TO 10-15 KT SATURDAY...10 KT THROUGH SATURDAY NIGHT. VFR EXPECTED SUNDAY W/ S FLOW 10-15 KTS. SUB-VFR POSSIBLE IN SCTD SHWRS MON EVE W/ FROPA. WINDS BECOME W AND DIMINISH TO LESS THAN 10 KTS MON. FLOW BECOMES N THEN E BEHIND FROPA TUE W/ SUB-VFR POSSIBLE IN PRECIP LATE TUE. && .MARINE... WITH WIND GUSTS TO 20 KTS ALREADY BEING REPORTED ON MULTIPLE MESONET...HAVE GONE AHEAD AND BEGUN THE SCA ON ALL THE WATERS AS THE REINFORCING COLD FRONT MOVES THROUGH THE WATERS. THE SCA REMAINS IN EFFECT THROUGH FRIDAY. HIGH PRES OVERHEAD FRIDAY EVENING...WITH RETURN FLOW INCREASING SATURDAY. SLY CHANNELING EXPECTED WITH SCA LIKELY BY LATE MORNING SATURDAY CONTINUING WELL INTO SATURDAY NIGHT. SOUTHERLY FLOW WITH GUSTS EXCEEDING SMALL CRAFT ADVISORY CRITERIA IS LIKELY SUNDAY INTO SUNDAY EVENING AHEAD OF A COLD FRONT THAT WILL SLOWLY CROSS THE WATERS LATE MONDAY. WINDS DIMINISH AS THE FRONT PASSES...THEN STALLS NEAR THE WATERS THROUGH THE MIDDLE OF NEXT WEEK. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...WINTER WEATHER ADVISORY UNTIL 7 AM EST FRIDAY FOR VAZ503. WV...WINTER WEATHER ADVISORY UNTIL 7 AM EST FRIDAY FOR WVZ501-505. MARINE...SMALL CRAFT ADVISORY UNTIL 6 PM EST FRIDAY FOR ANZ530>543. && $$

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