Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 211858 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 258 PM EDT SUN SEP 21 2014 .SYNOPSIS... LOW PRESSURE OFF THE THE MID-ATLANTIC COAST WILL CONTINUE TO MOVE NORTHEAST. A COLD FRONT WILL CROSS THE REGION THIS EVENING INTO THE OVERNIGHT. HIGH PRESSURE WILL BUILD OVER THE AREA MONDAY INTO TUESDAY...THEN SHIFT NORTHEAST THROUGH THE LATE WEEK. && .NEAR TERM /THROUGH TONIGHT/... AS OF 18Z...COASTAL LOW IS CENTERED OFF THE NC/VA BORDER PER REGIONAL IR/WV SATELLITE IMAGERY. A COLD FRONT IS OVER OHIO AND SHIFTING EAST (THERE ARE SEVERAL WIND REPORTS OF 30 TO 35 MPH IN IN/OH). A PREFRONTAL LINE OF SHOWERS IS ENTERING THE POTOMAC HIGHLANDS. THIS IS THE SECOND WAVE OF THE DAY...WITH ANOTHER WAVE OR TWO EXPECTED WITH THE FRONT ITSELF. THE INVERSION AT 700MB FROM THE 12Z KIAD RAOB IS ONLY SLOWLY BEING ERODED PER CLEAR SKIES FROM THE BLUE RIDGE TO INTERSTATE 95. MLCAPE 500 J/KG CURRENTLY OVER THE SHENANDOAH VALLEY AND WEST PER SPC ANALYSIS. MID AFTERNOON THUNDER THREAT IS ISOLATED AT BEST (AND WEST OF THE BLUE RIDGE)...WITH MORE SCATTERED COVERAGE WITH THE FRONT THIS EVENING WHEN MLCAPE SHOULD EXCEED 1000 J/KG. ISOLATED THUNDER WORDING EXTENDS EAST TO THE INTERSTATE 95 CORRIDOR FROM 5 TO 8PM. SHOULD THE CAP BE COMPLETELY BROKEN...AN 85/62 INITIAL PARCEL WOULD ALLOW FOR AROUND 1000 J/KG SBCAPE. IN SPITE OF FROPA TIMING AFTER SUNSET EAST OF I-95...PRESENCE OF INSTABILITY SHOULD ALLOW ISOLATED THUNDER. NWLY FLOW SPREADS IN BEHIND THE FRONT WITH GUSTS GENERALLY 25 TO 30 MPH. MIN TEMPS ELEVATED IN MIXING FLOW IN SPITE OF COOLER AIR COMING IN. MID TO UPR 50S INLAND...LOW TO MID 60S NEARSHORE. UPSLOPE RAIN SHOWERS CONTINUE THROUGH THE NIGHT WEST FROM THE ALLEGHENY FRONT. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... A FEW UPSLOPE SPRINKLES WEST FROM THE ALLEGHENY FRONT MONDAY MORNING. OTHERWISE DRY WITH NWLY GUSTS 25 MPH DECREASING THROUGH THE AFTERNOON. GOOD MIXING AND 850MB TEMPS OF 4-6C SUGGEST MAX TEMPS IN THE LOW TO MID 70S. HIGH PRESSURE CONTINUES TO BUILD MONDAY NIGHT WITH THE GRADIENT SLACKING PARTICULARLY WEST OF THE BLUE RIDGE BY DAYBREAK TUESDAY. WITH DRY AIR IN PLACE...MIN TEMPS COULD BE IN THE MID 30S IN WRN VALLEYS AND THE POTOMAC HIGHLANDS. SINCE THIS IS THE SHELTER TEMPERATURE (ABOUT SIX FEET OFF THE GROUND) GROUND TEMPERATURES COULD HIT FREEZING AND THUS CAUSE FROST. A MENTION FOR FROST WILL BE PUT IN THE HWO FOR OUT THERE. OTHERWISE...SUB NORMAL MIN TEMPS WITH LOW TO MID 40S PIEDMONT AND WEST...UPR 40S INLAND COASTAL LOW LANDS AND LOW TO MID 50S URBAN/NEARSHORE. HIGH PRESSURE SHIFTS TO THE NORTHERN MID-ATLANTIC TUESDAY AND TUESDAY NIGHT. CONTINUED DRY CONDITIONS WITH NLY FLOW AND BELOW NORMAL TEMPS. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... HIGH PRESSURE OVER NEW ENGLAND WILL SLOWLY MOVE OUT INTO THE NORTHWESTERN ATLANTIC AS A BROAD AREA OF LOW PRESSURE DRIFTS WESTWARD INTO THE SOUTHEASTERN CONUS. A MOIST EASTERLY FLOW IN A TIGHTENING GRADIENT BETWEEN THESE TWO FEATURES WILL RESULT IN INCREASING CLOUDS THROUGHOUT THE DAY WEDNESDAY...WITH ABUNDANT CLOUD COVER IN A MARITIME AIRMASS EXPECTED TO PERSIST INTO THE END OF THE WEEK. THE OPERATIONAL 00Z ECMWF MODEL WAS DEFINITELY THE MOST AGGRESSIVE WITH THE NORTHWARD EXTENT OF PRECIP ASSOCIATED WITH LOW PRESSURE TO THE SOUTH IN THE THURSDAY-FRIDAY TIMEFRAME...BUT DOES NOT HAVE OVERWHELMING SUPPORT FROM ITS OWN ENSEMBLE MEMBERS. THERE IS A HIGH CHANCE FOR PERIODS OF DRIZZLE DURING THIS PERIOD GIVEN THE PERSISTENT EASTERLY FLOW...BUT MEASURABLE PRECIPITATION ASSOCIATED WITH THE LOW WELL TO THE SOUTH APPEARS RELATIVELY LOW AT THE MOMENT. HAVE KEPT POPS VERY LOW FOR THIS REASON. AS THE LOW PRESSURE OVER THE SOUTHEAST WEAKENS AND THE HIGH PRESSURE MOVES FURTHER OFFSHORE...UPPER-LEVEL RIDGING WILL RE-STRENGTHEN AND RESULT IN SOUTHERLY FLOW AT THE SURFACE WITH A WARMING TREND EXPECTED BY NEXT WEEKEND. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/... SWLY FLOW AHEAD OF A COLD FRONT. SCATTERED SHOWERS WITH ISOLATED THUNDERSTORMS SPREAD EAST ACROSS THE DC METROS LATE THIS AFTERNOON/EARLY EVENING. HIGHEST THUNDER THREAT IS KMRB WHERE A VCTS 20Z TO 00Z IS IN THE TAF. NWLY FLOW GUSTS 20 TO 25 KT BEHIND THE COLD FRONT TONIGHT THROUGH MONDAY. VFR AND NLY FLOW THEN TUESDAY WITH HIGH PRESSURE SPREADING NORTH OF THE AREA. SUB-VFR CONDITIONS APPEAR INCREASINGLY LIKELY WITH LOW CLOUDS/FOG MAKING A VISIT TO THE AREA WED-THU NIGHT. WINDS GENERALLY EASTERLY 10-15 KNOTS WITH A FEW GUSTS TO 20 KNOTS POSSIBLE. && .MARINE... LOW PRESSURE MOVES NORTHEAST FROM OFF THE DELMARVA. NELY FLOW WILL BECOME SWLY AHEAD OF A COLD FRONT THAT WILL CROSS TONIGHT. NWLY FLOW GUSTS 20 TO 25 KT STARTING AROUND MIDNIGHT...SO THAT IS WHEN THE SCA BEGINS FOR ALL WATERS. SCA THROUGH MONDAY WITH GUSTS IN THE MORNING AROUND 25 KT...SLOWLY EASING THROUGH THE AFTERNOON. NLY THEN NELY FLOW AS HIGH PRESSURE SHIFTS NORTH TUESDAY. WINDS INCREASE TO NEAR SCA LEVELS DURING THE MIDDLE PART OF THE WEEK IN A TIGHTENING PRESSURE GRADIENT BETWEEN A DEPARTING HIGH TO THE NORTH AND LOW PRESSURE DRIFTING INTO THE SOUTHEAST CONUS FROM THE ATLANTIC. WINDS/WAVES SUBSIDE AS THE TWO PRESSURE SYSTEMS WEAKEN AND MOVE AWAY BY NEXT WEEKEND. && .TIDES/COASTAL FLOODING... NLY FLOW HAS LOWERED THE DEPARTURE BELOW HALF A FOOT. FLOW IS COMING SWLY BEFORE A COLD FRONT THAT WILL CROSS THIS EVENING. HOWEVER...THAT IS NOT FAVORABLE FOR COASTAL FLOODING. NWLY FLOW WILL RETURN LEVELS NEAR NORMAL MONDAY. TIDAL ANOMALIES WILL INCREASE IN PERSISTENT MODERATE EASTERLY FLOW WEDNESDAY NIGHT-FRIDAY WHICH ALONG WITH A NEW MOON MAY CAUSE COASTAL FLOODING AROUND TIMES OF HIGH TIDE STARTING LATE WEDNESDAY. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM EDT MONDAY FOR ANZ530>543. && $$ SYNOPSIS...BAJ NEAR TERM...BAJ SHORT TERM...BAJ LONG TERM...DFH AVIATION...BAJ/DFH MARINE...BAJ/DFH TIDES/COASTAL FLOODING...BAJ/DFH

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