Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 290121 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 921 PM EDT TUE JUL 28 2015 .SYNOPSIS... WEAK STATIONARY FRONT WILL DISSIPATE TONIGHT...WITH HIGH PRESSURE RESIDING JUST OFF SHORE. A COLD FRONT MOVES THROUGH THE REGION THURSDAY...WITH HIGH PRESSURE BUILDING IN THE WAKE. ANOTHER QUICK MOVING COLD FRONT BRUSHES THE MID ATLANTIC OVER THE WEEKEND...WITH HIGH PRESSURE BUILDING BACK IN HEADING INTO NEXT WEEK. && .NEAR TERM /THROUGH WEDNESDAY/... AN ACTIVE EVENING ACROSS PORTIONS OF THE MID-ATLANTIC. SHOWERS AND THUNDERSTORMS KEPT REGENERATING ON OUTFLOW BOUNDARIES ACROSS THE REGION IN A VERY WARM AND HUMID AIRMASS. HEAVY RAIN WAS REPORTED WITH AMTS OF 2-3 INCHES IN THE HEAVIEST STORMS. THE IAD RAOB WAS RELEASED LATE DUE TO THUNDERSTORMS OVER THE OFFICE THIS EVENING. USING 00Z RNK RAOB AND MESOANALYSIS...INSTABILITY IS PRESENT ACROSS THE REGION WITH WEAK STEERING FLOW. SHOWERS AND THUNDERSTORMS HAVE BEEN MOVING SLOW TODAY AND WILL CONTINUE TONIGHT. THE OVERALL TREND IS THAT INTENSITY IS DECREASING WITH THE LOSS OF DAYTIME HEATING. SHOWERS HAVE BECOME MORE STRAITFORM ACROSS THE CENTRAL SHENANDOAH VALLEY AND CENTRAL FOOTHILLS WITH MODERATE RAINFALL RATES. SIMILAR TO LAST EVENING THE NEXT STORY IS FOG. LIGHT FLOW AND CLOUDS CLEARING OVER A VERY SATUATED GROUND WILL ALLOW FOG...DENSE AT TIMES TO FORM MAINLY ACROSS THE CENTRAL FOOTHILLS...PIEDMONT AND INTO NORTHERN VA AND WASHINGTON DC SUBURBS. HEAVY RAIN WEST OF THE BLUE RIDGE WILL ALSO LEAD TO AREAS OF FOG MAINLY ACROSS THE SHENANDOAH VALLEY INTO THE EASTERN PANHANDLE OF WV. WARM CONDITIONS TONIGHT...TEMPS IN THE UPPER 60S AND LOW 70S...IN THE UPPER 70S IN THE METROS. FLOW BECOMES SLIGHTLY STRONGER TOMORROW AND MORE SRLY BRINGING DEWPOINTS UP WITH HEAT INDICES AROUND 100F. CHANCE OF T-STORMS CREEPS UP TO ABOUT I-70 IN NCNTRL MD. && .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/... INCRSG CONFIDENCE OF CONVECTION OCCURRING THURS AS A COLD FRONT APPROACHES FROM THE WEST. FALLING HEIGHTS ON THURSDAY AS UPPER LVL RIDGE WEAKENS/PUSHES EAST AHEAD OF THE APPROACHING TROUGH. NEAR ZONAL FLOW TAKES SHAPE DURING THE DAY THURSDAY WITH THE A MORE PROGRESSIVE SHORTWAVE ROUNDING THE CLOSED LOW OVER CANADA DIGGING SOUTH INTO THE GREAT LAKES...PROVIDING THE EXTRA PUSH TO FINALLY PUSH THE COLD FRONT THROUGH AND TO THE SOUTHEAST BY FRIDAY MORNING. 12Z MODEL SUITE COMING INTO BETTER ALIGNMENT WITH THE FRONTAL PASSAGE TIME...PLACING IT SOMETIME IN THE AFTN IN THE WEST...AND THEN OFFSHORE BY 12Z FRI MORNING. AHEAD OF THE FRONT THURS...WAA PLACES 850MB TEMPS NEAR 20 DEGREES CELSIUS. DAYTIME HIGHS IN THE LOW TO MID 90S. MOISTURE ADVECTION ALSO WILL RESULT IN DEW PTS HOVERING AROUND TO RIGHT ABV 70. HEAT INDICES NEARING THE CENTURY MARK...WITH PSBL 100-105 RANGE IF TEMPS WARM SLIGHTLY MORE OR IF DEW PTS INCREASE SLIGHTLY. WILL CONTINUE TO WATCH THE TREND. THE COMBO OF HIGHER TEMPS AND DEW PTS SHOULD RESULT IN A FAIRLY UNSTABLE AIRMASS...WITH SBCAPE VALUES 1000-2000 J/KG BY THE AFTN. WITH 0-6 KM SHEAR ABOUT 20 KTS AND STEEPENING MID LVL LAPSE RATES ALSO BY THE AFTN...SUFFICIENT INGREDIENTS EXIST FOR THE DEVELOPMENT OF SOME STRONGER STORMS. WHILE THE SFC FORCING EXISTS WITH THE FRONTAL BOUNDARY...LACK OF SHORTWAVE ENERGY REACHING THE MID ATLANTIC SUGGESTS MOST OF THE CONVECTION...PARTICULARLY ANYTHING STRONGER TO SVR WOULD BE RIGHT ALONG THE FRONT LATE AFTN INTO THE EVENING. PWATS INCRSG TO AROUND 1.8 INCHES WOULD ALSO SUGGEST THE THREAT OF HEAVY RAINFALL. FRONT PUSHES SOUTH AND EAST SLOWLY THURS NIGHT...WITH THE PCPN TAPERING OFF AS DRIER AIR MOVES IN BEHIND THE FRONT...DCRSG DEW PTS BACK INTO THE 60S...AND THE 50S FOR ISOLATED SPOTS. THE COLDER AIR DOESNT QUITE MAKE IT ALL THE WAY INTO THE CWA. SO WHILE TEMPS WILL BE SLIGHTLY COOLER ON FRIDAY...STILL EXPECTING A WARM...YET SLIGHTLY DRIER DAY. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... UPPER LEVEL TROUGH QUASI-STATIONARY OVER REGION THROUGH PERIOD. WEAK PVA AFFECTING ARE THROUGH WEEKEND. MODERATE PVA ARRIVING MONDAY. MUCH DRIER AIR ASSOCIATED WITH SURFACE HIGH BUILDING ACROSS REGION FRIDAY WILL GIVE WAY TO A WEAK COLD FRONT SATURDAY. AFTERNOON HEATING AND A SLIGHT INCREASE OF MOISTURE MAY BE ENOUGH TO TRIGGER AN ISOLATED SHOWER OR THUNDERSTORM OVER SOUTHERN MD DURING THE WEEKEND. INCREASING MOISTURE...STRONGER UPPER LEVEL SUPPORT AND LEE-SIDE TROUGH LENDS GREATER CONFIDENCE IN THE DEVELOPMENT OF ISOLATED SHOWERS/THUNDERSTORMS SUNDAY NIGHT AND MONDAY ESPECIALLY ALONG SOUTHERN AND SOUTHEAST FRINGES OF AREA. && .AVIATION /01Z WEDNESDAY THROUGH SUNDAY/... SHRA/TSRA DIMINISHING THIS EVENING. VCTS STILL EXPECTED AT CHO THIS EVENING. FOG DEVELOPMENT TONIGHT WILL LIKELY IMPACT CHO-MRB- IAD WITH IFR CONDITIONS. AT THIS TIME... KEPT IAD AT MVFR BUT THE RECENT HEAVY RAIN WILL LIKELY CAUSE THEM TO GET TO IFR BY WED MORNING. SUB-VFR CONDITIONS POSSIBLE AT DCA INTO WED MORNING. VSBYS WILL IMPROVE WED MORNING. ISO-SCT SHRA/TSRA POSSIBLE WED AFTERNOON. AT THIS TIME...COVERAGE SEEMS LIMITED AND THEFORE KEPT OUT OF TAFS. PSBL MVFR CIGS THURS MORNING FROM ON NIGHT PCPN. EXPECTING CONDITIONS TO IMPROVE TO VFR BY MIDDAY. OUTSIDE OF CONVECTION...VFR CONDITIONS THEN THURS AFTN-FRI. INCRSG CHC OF SHOWERS AND TSTMS THURS AFTN/EVENING WITH A COLD FRONT MVG THRU. ANY STORM COULD BRING CONDITIONS DOWN TO IFR...WITH HEAVY RAIN...AND GUSTY WINDS. && .MARINE... LIGHT AND VARIABLE WINDS THROUGH TONIGHT. FLOW BECOMES SLIGHTLY STRONGER AND BETTER DEFINED FROM THE SOUTH. GUSTS UP TO 17 KT LOWER BAY. SUB-SCA CONDITIONS ON ALL WATERS THUR MORNING. INCRSG SLY FLOW AHEAD OF THE APPROACHING COLD FRONT COULD BRING GUSTS BACK TO AROUND 18-20 KTS THURS AFTN AND NIGHT ON THE CHESAPEAKE BAY. && .TIDES/COASTAL FLOODING... ANOMALIES RUNNING AROUND A HALF FOOT ABOVE NORMAL. MAIN CONCERN IS ANNAPOLIS LATE TONIGHT. AT THIS TIME...WATER LEVELS SHOULD REMAIN JUST UNDER MINOR FLOOD. WILL NEED TO MONITOR OVERNIGHT. ELSEWHERE...WATER LEVELS SHOULD STAY UNDER FLOOD THRESHOLDS. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...NONE. && $$ SYNOPSIS...LFR NEAR TERM...HAS/LFR SHORT TERM...SEARS LONG TERM...SEARS AVIATION...CEM/SEARS/HAS/LFR MARINE...CEM/SEARS/LFR TIDES/COASTAL FLOODING...HAS

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