Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 041821 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 221 PM EDT TUE AUG 4 2015 .SYNOPSIS...
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A SURFACE TROUGH WILL CONTINUE TO GRADUALLY SHIFT SOUTHEAST THROUGH THE AREA TODAY. HIGH PRESSURE WILL BUILD INTO THE AREA LATER TONIGHT THROUGH WEDNESDAY. THE BOUNDARY WILL RETURN NORTH AS A WARM FRONT AND STALL OUT OVER THE AREA FOR THURSDAY AND FRIDAY. LOW PRESSURE WILL RIDE ALONG THE BOUNDARY...PASSING THROUGH THE REGION THURSDAY NIGHT AND FRIDAY. HIGH PRESSURE WILL GRADUALLY BUILD IN BEHIND THE LOW NEXT WEEKEND.
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&& .NEAR TERM /THROUGH WEDNESDAY/...
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AN UPPER LOW IS LOCATED NEAR HUDSON BAY WHILE THE JET STREAM ROUNDS THE TROUGH ACROSS THE GREAT LAKES INTO THE MID- ATLANTIC THIS AFTERNOON. A SURFACE LOW IS ON THE EAST SIDE OF HUDSON BAY WHILE A WEAK COLD FRONT...MORE REPRESENTATIVE OF A SURFACE TROUGH IS DRAPED ACROSS THE NORTHEAST AND ALONG THE I-95 CORRIDOR THIS AFTERNOON. A COASTAL LOW OFF MYRTLE BEACH WILL MOVE NE AND OUT TO SEA. THE SURFACE TROUGH HAS DIVIDED THE MID-ATLANTIC WITH DEWPTS DROPPING ACROSS THE POTOMAC HIGHLANDS AND TEMPS IN THE MID 80S AND WARM AND HUMID CONDITIONS EAST OF THE BLUE RIDGE WITH TEMPS IN THE UPPER 80S ALONG I-95 AND DEWPTS IN THE UPPER 60S TO LOW 70S IN THE SOUTHERN MD. A CU FIELD HAS FORMED ACROSS MUCH OF THE REGION AS MIXING HAS OCCURRED. THE CU HAS A FORWARD TILT TO THEM SHOWING SHEAR IN THE MID- LEVELS. SHOWERS AND THUNDERSTORMS ARE EXPECTED TO FORM NEAR THE SURFACE TROUGH AXIS AND EAST WITH COVERAGE AND INTENSITY INCREASING NEAR AND EAST OF I-95 WHERE TEMPS AND DEWPTS ARE HIGH AND INSTABILITY IS GREATER THAN 1500 J/KG. EFFECTIVE SHEAR RANGES FROM 30-40KTS ACROSS THE REGION. A STABLE LAYER IS APPARENT ON RAP FCST SOUNDINGS FOR S MD LEADING TO LITTLE CU FIELD ACROSS AREA OF CONCERN...PIEDMONT AND S MD. THIS SHOULD BE OVERTAKEN THIS AFTERNOON. THE CONVECTION FROM THE COASTAL LOW NEAR THE MYRTLE BEACH HAS PRODUCED NORTHWARD MOVING OUTFLOW BOUNDARIES THAT MAY REACH THE MID-ATLANTIC. THE BEST CHANCE FOR STRONG TO SEVERE STORMS APPEARS TO BE I-95 AND EAST INCLUDING SOUTHERN MD...TIDEWATER REGION AND SURROUNDING WATERS. SEVERE THREATS INCLUDE DMG WINDS...LARGE HAIL AND ISO TORNADOES. THE SFC TROUGH MOVES OFF THE COAST THIS EVENING AND HIGH PRESSURE MOVES INTO THE REGION. FCST MIN TEMPS RANGE FROM THE LOW 60S IN THE HIGHLANDS TO THE LOW 70S IN COASTAL AREAS INCLUDING THE METROS. THE GRADIENT BETWEEN HIGH PRESSURE MOVING IN FROM THE WEST AND THE SFC TROUGH NEAR THE EAST COAST WILL CAUSE NW WINDS 10-15MPH WITH GUSTS UP TO 25 MPH.
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&& .SHORT TERM /WEDNESDAY NIGHT/...
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HIGH PRESSURE CONTINUES TO MOVE INTO THE REGION WEDNESDAY NIGHT. CLOUDS WILL BEGIN TO INCREASE FROM THE SW INTO THURSDAY MORNING AS THE NEXT SYSTEM APPROACHES. A STRONG SHORTWAVE TROUGH ALOFT WILL INDUCE A WAVE OF LOW PRESSURE ALONG A STALLED FRONTAL BOUNDARY OVER THE OHIO VALLEY THURSDAY...THEN TRACK BASICALLY DIRECTLY OVER THE CWA THURSDAY NIGHT INTO FRIDAY. DEEP-LAYER SOUTHERLY FLOW ACROSS THE GULF COAST INTO THE TENNESSEE/OHIO VALLEYS WILL INFUSE PLENTY OF MOISTURE INTO THE SYSTEM AS IT APPROACHES THE AREA...WITH PRECIPITABLE WATER VALUES OF 1.75 TO 2.00 INCHES NEAR AND SOUTH OF THE LOW TRACK. EXPECTING WIDESPREAD SHOWERS WITH A FEW EMBEDDED CONVECTIVE ELEMENTS AND POSSIBLY SOME ISOLATED THUNDER GIVEN POTENTIAL MODEST CAPE/STRONG FORCING. HEAVY DOWNPOURS POSSIBLE...WITH A SOAKING RAIN EXPECTED OVERALL. 12Z NAM OFFERS WORST CASE WITH 3 TO 5 INCHES OF RAIN BUT THINK THIS IS OVERDONE GIVEN PROGRESSIVE NATURE OF THE SYSTEM. LOW PRESSURE PUSHES EAST INTO THE WESTERN ATLANTIC LATER FRIDAY WITH SHOWERS COMING TO AN END FROM WEST TO EAST. WITH ABUNDANT CLOUDS AND HIGH HUMIDITY...EXPECT NIGHTTIME TEMPERATURES TO BE AROUND NORMAL...BUT DAYTIME HIGHS TO BE A FEW DEGREES ON THE COOL SIDE OF NORMAL.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/...
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HIGH PRESSURE/MID- TO UPPER-LEVEL RIDGING WILL MOVE ACROSS OVER THE WEEKEND. DRY WEATHER AND SEASONABLY MILD WEATHER IS EXPECTED DURING THIS TIME. AS RIDGING BEGINS RE-BUILDING OVER THE SOUTH-CENTRAL CONUS BY EARLY NEXT WEEK...HEIGHTS WILL BEGIN TO FALL OVER THE GREAT LAKES AND NORTHEAST. AS THIS HAPPENS...ANOTHER FRONT WILL APPROACH THE AREA AND INCREASE THE CHANCE FOR SHOWERS AND THUNDERSTORMS NEXT MONDAY AND TUESDAY.
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&& .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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SHRA/VCTS POSSIBLE AT CHO-IAD THROUGH THE AFTERNOON AND CONTINUES FOR DCA-BWI-MTN THROUGH 00Z. SOME TS MAY PRODUCE GUSTY WINDS AND SMALL HAIL. AT THIS TIME COVERAGE SEEMS LOW TO PUT TSRA IN TAF...MAY HAVE TO AMD THIS AFTERNOON AND EARLY AFTERNOON. VFR CONDITIONS EXPECTED TONIGHT AND WEDNESDAY. WIDESPREAD SUB-VFR EXPECTED BY THU EVE...LASTING INTO FRI AM IN -RA. WIND VARIABLE AOB 10 KT AS LOW PRES PASSES OVERHEAD. VFR LIKELY RETURNS BY SAT AM...W/ WINDS BECOMING NWLY AROUND 10 KT.
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&& .MARINE...
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SW-W WINDS EXPECTED ON THE WATERS THIS AFTERNOON. SHOWERS AND THUNDERSTORMS EXPECTED ON THE WATERS THIS AFTERNOON AND EARLY THIS EVENING. SOME THUNDERSTORMS MAY PRODUCE GUSTY WINDS AND WATERSPOUTS AND SMW MAY BE WARRENTED. THE BEST CHANCE FOR STRONG TO SVR STORMS IS ALONG THE TIDAL POTOMAC AND SOUTHER OF BALTIMORE HARBOR THIS AFTERNOON AND EARLY EVENING. HIGH PRESSURE WILL MOVE EASTWARD BUT A GRADIENT WILL PERSIST ACROSS THE WATERS THROUGH WEDNESDAY. A SCA IS IN EFFECT FOR THE WATERS WEDNESDAY AFTERNOON. LOW PRESSURE WILL RIDE ALONG A STALLED FRONT AND CROSS THE WATERS THURSDAY NIGHT INTO FRIDAY. DEPENDING ON THE EXACT TRACK OF THE LOW...A SMALL CRAFT ADVISORY COULD BE REQUIRED FOR PARTS OF THE WATERS DURING THIS TIME.
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&& .CLIMATE...
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REAGAN NATIONAL AIRPORT /DCA/ HIT AT LEAST 90 DEGREES TODAY FOR THE 12TH CONSECUTIVE DAY. THIS IS THE LONGEST SUCH STRETCH SINCE JULY OF 2010...AND THE 15TH LONGEST STRETCH ON RECORD AT THE SITE.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...SMALL CRAFT ADVISORY FROM NOON TO 8 PM EDT WEDNESDAY FOR ANZ530>532-538>540. SMALL CRAFT ADVISORY UNTIL MIDNIGHT EDT TONIGHT FOR ANZ534-537- 543.
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&& $$ SYNOPSIS...HAS NEAR TERM...HAS SHORT TERM...HAS/DFH LONG TERM...DFH AVIATION...HAS/DFH MARINE...HAS/DFH CLIMATE...DFH

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