Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 031908 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 308 PM EDT FRI JUL 3 2015 .SYNOPSIS...
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LOW PRESSURE WILL ADVANCE EAST-NORTHEAST UP THE OHIO VALLEY TONIGHT. A SECONDARY LOW WILL DEVELOP OVER VIRGINIA AND HEAD EAST-NORTHEAST INTO THE ATLANTIC OCEAN ON SATURDAY. HIGH PRESSURE WILL BUILD ACROSS THE AREA ON SUNDAY. ANOTHER LOW PRESSURE WILL MOVE INTO THE AREA MONDAY AND LINGER IN THE AREA FOR A FEW DAYS.
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&& .NEAR TERM /THROUGH SATURDAY/...
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AS OF 18Z...A STATIONARY FRONT WAS LOCATED ALONG THE BORDERS OF NORTH CAROLINA AND VIRGINIA...STRETCHING WEST ALONG THE BORDERS OF KENTUCKY AND TENNESSEE...AND THE BORDERS OF MISSOURI AND ARKANSAS...FINALLY TERMINATING IN OKLAHOMA. A WAVE OF LOW PRESSURE WAS LOCATED ALONG THIS FRONT NEAR THE KENTUCKY/TENNESSEE BORDER...WHILE HIGH PRESSURE DOMINATED THE GREAT LAKES. MOST GUIDANCE DEPICTS THIS LOW AS MOVING EAST-NORTHEAST UP THE OHIO RIVER VALLEY TOWARDS SOUTHWESTERN PENNSYLVANIA TONIGHT...WITH A SECONDARY LOW DEVELOPING OVER VIRGINIA AND HEADING EAST-NORTHEAST INTO THE ATLANTIC ON SATURDAY. THIS IS CERTAINLY MORE OF A WINTER-LIKE SYNOPTIC PATTERN THAN WHAT YOU WOULD EXPECT TO SEE IN JULY...BUT THE RESULTS WILL BE FAIRLY SIMILAR...WITH SOME WARM ADVECTION/ISENTROPIC LIFT INDUCED RAIN MOVING INTO THE REGION TONIGHT...THEN PERHAPS DRY SLOT WITH LINGERING DRIZZLE DEVELOPING OVER THE AREA ON SATURDAY. AS THE UPPER TROUGH ASSOCIATED WITH THIS SYSTEM MOVES EAST ACROSS THE AREA LATER SATURDAY...FORCING WILL LIKELY ALLOW SHOWERS TO POP BACK UP ACROSS THE AREA. GIVEN THE MUCH MORE HUMID AIR MASS COMPARED TO WINTER...AND THE UNUSUAL DYNAMICS FOR JULY...ISOLATED THUNDERSTORMS ARE A POSSIBILITY...BUT ENOUGH STABILITY APPEARS PRESENT TO PREVENT ANY SORT OF STRONG TO SEVERE STORMS FROM DEVELOPING. THERE ARE HINTS IN SOME GUIDANCE OF A SIGNIFICANT UPSLOPE FLOW PRODUCING EXCESSIVE RAIN ACROSS OUR NORTHWESTERN ZONES...BUT CONFIDENCE IS NOT HIGH AND SOME MODELING SOLUTIONS IN FACT INCLUDE THESE AREAS IN THE DRY SLOT LATER ON CAUSING THEM TO MISS OUT. THEREFORE...DECIDED TO HOLD OFF ON ANY FLOOD WATCHES UNTIL THERE IS A BETTER SIGNAL FOR EXCESSIVE RAIN. LOWS TONIGHT WILL DROP BACK INTO THE 60S IN MOST AREAS...WHILE HIGHS ON SATURDAY WILL STAY MOSTLY IN THE 70S THANKS TO CLOUDS...DRIZZLE...AND SHOWERS.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
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LOW PRESSURE MOVES AWAY FROM THE AREA SATURDAY NIGHT...BUT UPPER TROUGH MAY STILL KICK OFF A FEW SHOWERS. OTHERWISE...CONDITIONS SHOULD GENERALLY BE IMPROVING...BUT IF THERE IS CLEARING LATE SOME PATCHY FOG MAY DEVELOP. LOWS WILL AGAIN BE IN THE 60S. NORTHWARD PROGRESSION OF THE FRONTAL BOUNDARY HOVERING NEAR THE VA/NC BORDER WILL BE MINIMAL ON SUNDAY AS HIGH PRESSURE BUILDS IN AT THE SFC UNDER A DEVELOPING RIDGE ALOFT. LOW PRESSURE RIDING THE FRONT SHOULD EXIT THE AREA BY 12Z SUN...WITH DRY CONDITIONS THEN EXPECTED THE REMAINDER OF THE MORNING. DEPENDING ON WHERE EXACTLY THE FRONT ALIGNS...A STRAY SHOWER OR TSTM COULD MOVE INTO THE SOUTHERN PORTION OF THE CWA...SO WILL JUST CONTINUE WITH THE CHC POPS FOR NOW. BY SUNDAY EVENING...12Z GFS AND ECMWF BOTH INDICATING RIDGE AXIS ALIGNS FAR ENOUGH EAST TO ALLOW PROGRESSION OF A SOUTHERN STREAM SHORTWAVE TO PROGRESS ALONG THE APPALACHIANS. STILL UNSURE HOW FAR NORTH THE PCPN WILL PROGRESS DURING THE NIGHT WITH ANY SFC BOUNDARIES LOOKING TO LINGER FURTHER SOUTH...AND NOT PUSHING NORTH TILL THE BEGINNING OF NEXT WORK WEEK. WARM AND MOIST AIRMASS IN PLACE DURING THE DAY SUNDAY. 850MB TEMPS RIGHT AROUND 13-14C...WITH MOST OF THE AREA PUSHING INTO THE 80S. DEW PTS GENERALLY HOVERING IN THE MID TO UPPER 60S. ENOUGH INSTABILITY SHOULD BE GENERATED TO RESULT IN THUNDER...WHEREVER ANY CONVECTION DOES DEVELOP. HOWEVER...LACK OF SHEAR SHOULD KEEP TSTMS BELOW SVR LVLS.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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UPPER LEVEL CUT-OFF LOW OPENING UP AND FLOW BECOMING ZONAL OVER MID ATLANTIC REGION AS TROUGH ON NORTHERN TRACK MOVES EASTWARD. NEXT TROUGH TO AFFECT REGION FRIDAY. AREAS OF WEAK TO MODERATE PVA PRESENT THROUGH THE FORECAST PERIOD. SURFACE LOW PRESSURE TROUGH OVER REGION WILL SLOWLY FILL THROUGH TUESDAY NIGHT AND BE REPLACED BY WEAK HIGH PRESSURE RIDGING. DYNAMICS SUPPORT CONVECTIVE DEVELOPMENT DURING THE DAY MONDAY WITH DRYING OVERNIGHT. MORE DEVELOPMENT DUE AS A FRONT FROM THE WEST CROSSES THE REGION WEDNESDAY THROUGH THURSDAY NIGHT.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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STORM SYSTEM MOVING IN WILL RESULT IN WIDESPREAD RAIN SHOWERS WITH CIGS AND VIS LOWERING TO IFR AT TIMES LATE TONIGHT AND SATURDAY MORNING. CONDITIONS SHOULD IMPROVE DURING THE DAY SATURDAY AS THE LOW BEGINS TO MOVE AWAY. THUNDERSTORMS ARE POSSIBLE THIS EVENING AND AGAIN SATURDAY...BUT COVERAGE IS EXPECTED TO BE NO HIGHER THAN ISOLATED. WINDS SHOULD GENERALLY REMAIN BELOW 10 KNOTS THROUGH THE NEXT 36 HOURS. SOME POTENTIAL FOR REDUCED CIGS/VIS IF PATCHY FOG FORMS IN THE WAKE OF THE RAIN SATURDAY NIGHT. MVFR CIGS PSBL INITIALLY SUN MORNING...WITH CONDITIONS IMPROVING TO VFR AT ALL TAF SITES DURING THE DAY. KCHO HAS THE BEST CHC OF SEEING ANY PCPN SUN-SUN NIGHT. WINDS WILL BE LIGHT...LESS THAN 8 KTS. VFR CONDITIONS SHOULD PREVAIL THRU NEXT WEEK. SHOWERS AND THUNDERSTORMS WILL REMAIN A CONCERN.
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&& .MARINE...
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LOW PRESSURE PASSING TO THE SOUTH WILL INTENSIFY WINDS UP TO LOW- END SCA LEVELS TONIGHT INTO SATURDAY MORNING. OTHERWISE...MAIN CONCERN IS LOWERED VISIBILITY IN RAIN SHOWERS TONIGHT INTO SATURDAY. THUNDER IS POSSIBLE BUT SHOULD BE ISOLATED. WINDS SHOULD DIMINISH SATURDAY NIGHT. WEAK PRESSURE GRADIENT WILL RESULT IN A LIGHT FLOW SUN-SUN NIGHT...KEEPING SUB-SCA CONDITIONS IN PLACE. SLIGHT CHC OF SHOWERS SUN NIGHT...THOUGH WITH MINIMAL IMPACT ON VSBY. SUB-SCA CONDITIONS SHOULD PREVAIL THRU NEXT WEEK. SHOWERS AND THUNDERSTORMS WILL REMAIN A CONCERN.
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&& .HYDROLOGY...
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SOME GUIDANCE IS SHOWING SOME SIGNIFICANT RAINFALL ACROSS THE APPALACHIANS. THOUGHT WAS GIVEN TO THE POTENTIAL TO ISSUE A FLOOD WATCH...BUT AT THIS TIME FEEL PRECIPITATION WILL FALL SLOWLY ENOUGH AND BE LIGHT ENOUGH TO NOT CAUSE SIGNIFICANT FLOOD CONCERNS. WILL CONTINUE TO MONITOR.
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&& .TIDES/COASTAL FLOODING...
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EASTERLY FLOW AROUND A PASSING LOW PRESSURE SYSTEM WILL HELP ELEVATE WATER LEVELS ALONG TIDAL PORTIONS OF THE POTOMAC AND THE CHESAPEAKE BAY SHORE. AT THIS TIME THINK LEVELS WILL STAY JUST BELOW MINOR FLOOD...BUT COASTAL FLOOD ADVISORIES WOULD BE NECESSARY WITH ONLY A MODEST INCREASE IN PROJECTED WATER LEVELS. WILL CONTINUE TO MONITOR.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...SMALL CRAFT ADVISORY FROM 6 PM THIS EVENING TO 6 AM EDT SATURDAY FOR ANZ534-536-537-543. && $$ SYNOPSIS...SEARS NEAR TERM...RCM SHORT TERM...SEARS/RCM LONG TERM...CEM AVIATION...CEM/SEARS/RCM MARINE...CEM/SEARS/RCM HYDROLOGY...RCM TIDES/COASTAL FLOODING...RCM

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