Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 240124 AAA AFDLWX AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 924 PM EDT MON MAR 23 2015 .SYNOPSIS...
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AN UPPER LEVEL DISTURBANCE WILL PASS THROUGH THE AREA LATE TONIGHT AND HIGH PRESSURE WILL RETURN FOR TUESDAY. THE HIGH WILL MOVE OFF THE COAST WEDNESDAY AND A COLD FRONT IS EXPECTED TO PASS THROUGH LATE THURSDAY INTO THURSDAY NIGHT. CANADIAN HIGH PRESSURE WILL RETURN FOR FRIDAY INTO THE WEEKEND.
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&& .NEAR TERM /THROUGH TUESDAY/...
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FAST-APPROACHING IS AN UPPER LEVEL DISTURBANCE...LOCALIZED TO A RELATIVELY SMALL ZONE OF PRECIP NOW SPREADING FROM ERN OH INTO WRN PA/WV. A DECENT AMOUNT OF LOCALIZED QPF AND FORCING FOR SOME MODERATE SNOW SHOWERS ACROSS PORTIONS OF THE UPPER MIDWEST EARLIER THIS AFTN/EVE. SOME DIVERGENCE HOWEVER IN VARIOUS GUIDANCE MEMBERS W/ HOW THIS SYSTEM WILL HOLD TOGETHER ONCE IT REACHES THE CNTRL APLCNS IN THE NEXT FEW HRS. 18Z RUNS OF THE GFS/NAM ARE QPF LIGHT AND FAIRLY DISJOINTED IN TERMS OF HOW THEY SPREAD PRECIP ACROSS THE REGION...WHILE THE 12Z EURO HOLDS W/ THE VERY LIGHT QPF PLACED MAINLY OVER THE WRN HALF OF THE CWA. REGARDLESS OF EXPECTED MODEL QPF...PLENTY OF LOW LEVEL DRY AIR WILL NEED TO BE OVERCOME IN ORDER TO HAVE SNOW REACH THE SFC - ONCE LOW LEVEL TEMPS SUPPORT SNOW. THE LOWEST KM ABOVE THE SFC STILL ABOVE FREEZING WHICH SHOULD ONLY TAKE A MORE HRS TO DROP BACK INTO THE 30S. THE LOWEST 2.5 KM HOWEVER SHOWS THAT THE TEMP AND DEWPOINT ARE VERY FAR APART FROM SATURATION...SO DESPITE THE FAST-APPROACHING RADAR RETURNS - MODERATE TO HEAVY PRECIP WILL BE NEEDED TO REDUCE THE INCREDIBLE DRY AIR IN THIS LAYER BEFORE WE SEE SNOW FALLING OR EVEN ACCUMULATING. HEADING INTO THE OVERNIGHT HRS...ONCE THE LOW LEVEL HAVE NEARED SATURATION...SOME OF THE HIGHER TERRAIN LOCALES WILL SEE LIGHT SNOW AND HEAVIER BATCHES OVER THE POTOMAC HIGHLANDS AS WELL. THE MASON-DIXON LINE WILL ALSO INITIALLY BE A FOCUS FOR SOME POTENTIAL LIGHT SNOW AS A PSEUDO-WARM FRONTAL BOUNDARY SWINGS ACROSS THIS AREA W/ WEAK FORCING. SLIGHTLY REDUCED THE SNOW ACCUMNS FOR NRN MD AND INCREASED THEM FOR THE BLUE RIDGE AND SOME OF THE POTOMAC HIGHLANDS REGION OF THE WV PANHANDLE/WRN MD. HI-RES HOURLY MODELS ARE WELL INTO VIEW NOW FOR VARIOUS WRF VERSIONS AND THE HRRR WHICH SHOW MUCH OF THE INCOMING PRECIP DISSIPATING OVER THE CNTRL APLCNS AND ONLY A FRACTION MAKING IT FURTHER EAST. KEPT A LOW-MED CHANCE OF SNOW OVERNIGHT AND INTO EARLY TUE...W/ BRIEF LIKELIES FOR THE NRN/WRN HALVES OF THE CWA FOR A LIGHT DUSTING/COATING OF SNOW TOWARD THE PREDAWN HRS OR EARLIER. MUCH OF THIS ACTIVITY SHOULD DISSIPATE AND MOVE EAST OF THE AREA POST DAWN TUE...W/ A LINGERING LOW CLOUD DECK HANGING AROUND THRU THE REMAINDER OF THE DAY AND A COOL ONSHORE SFC BREEZE. MAX TEMPS WILL REMAIN WELL BELOW CLIMO...RANGING FROM THE 30S IN THE MOUNTAINS TO THE MID AND UPPER 40S IN CENTRAL VIRGINIA.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
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THE NEXT WEAK UPPER DISTURBANCE WILL CROSS THE REGION LATE TUESDAY NIGHT INTO WEDNESDAY MORNING. HIGH PRESSURE WILL BE TO THE NORTH AND PROVIDE JUST ENOUGH COLD AIR TO INTRODUCE THE POTENTIAL FOR SOME LIGHT IP OR ZR FOR THE FAR N AND W SUBURBS. ELSEWHERE...IT APPEARS TEMPS WILL BE WARM ENOUGH FOR RAIN. HIGH PRESSURE WILL EXIT OFF THE MIDDLE ATLANTIC COAST BY WEDNESDAY AFTERNOON AND WINDS WILL TURN SOUTHERLY. WARMER TEMPERATURES AND HIGHER MOISTURE WILL STREAM NORTH INTO THE FORECAST AREA DURING THE AFTERNOON AND INTO THE NIGHT. WARM AIR ADVECTION PATTERN WILL YIELD A CHANCE FOR RAIN DURING THIS TIME FRAME. TEMPERATURES WEDNESDAY NIGHT MAY NOT FALL OFF THAT MUCH WITH SOUTHERLY FLOW. MOSTLY CLOUDY AND WARM CONDITIONS ARE EXPECTED FOR THE FIRST PART OF THURSDAY. TEMPERATURES ARE EXPECTED TO RIDGE THROUGH THE 60S IN MANY LOCATIONS...EVEN A FEW 70S OVER CENTRAL VA AND SOUTHERN MD. MOISTURE AND INSTABILITY WILL BE IN PLACE AHEAD OF A COLD FRONT THAT WILL SWEEP ACROSS THE REGION THURSDAY NIGHT. SHOWERS AND A FEW TSTMS WILL BE POSSIBLE. SOME OF THE STORMS COULD CONTAIN GUSTY WINDS. THE SEVERE THREAT APPEARS MINIMAL AT THIS TIME BUT THE SITUATION WILL NEED TO BE MONITORED.
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&& .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
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A COLD FRONT WILL SWEEP ACROSS THE AREA THURSDAY EVENING INTO THE NIGHT. RAIN SHOWERS ARE EXPECTED AS THE FRONT APPROACHES...AND HAVE KEPT A SLGT CHANCE OF THNDRSTMS FOR THE FIRST HALF OF THE NIGHT. KEPT SLIGHT CHANCE FOR MORE PRECIP POST-FRONTAL. UPPER THROUGH SWEEPS THROUGH THE AREA FRI NIGHT INTO EARLY SAT...SO KEEPING SLIGHT CHNC OF POPS FOR THAT. HIGH PRESSURE BUILDS LATE ON SATURDAY BRINGING COOLER AND DRIER CONDITIONS BEFORE ANOTHER COLD FRONT APPROACHES SUNDAY NIGHT INTO MONDAY. BELOW NORMAL TEMPS ARE EXPECTED FOR FRI NIGHT INTO SAT NIGHT.
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&& .AVIATION /01Z TUESDAY THROUGH SATURDAY/...
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AN UPPER-LEVEL DISTURBANCE WILL PASS THROUGH THE AREA LATE TONIGHT INTO TUESDAY MORNING. A PERIOD OF LIGHT SNOW OR FLURRIES IS POSSIBLE...ESPECIALLY ACROSS THE NORTHERN TERMINALS. SUBVFR CIGS AND VSBYS ARE EXPECTED IN ANY SNOW THAT DEVELOPS. VFR TO START TUESDAY NIGHT FOLLOWED BY SUB-VFR CIGS AND VSBYS ALONG WITH A CHANCE FOR MOSTLY RAIN WEDNESDAY MORNING INTO THE NIGHT. SOME LIGHT IP OR ZR MAY BE POSSIBLE FOR AREAS TO THE NORTH AND WEST OF KDCA AND KBWI FOR A SHORT PERIOD WEDNESDAY AM. CIGS WILL LIKELY STAY BKN-OVC MUCH OF WEDNESDAY INTO THURSDAY. A COLD FRONT WILL BRING SHOWERS AND A FEW THUNDERSTORMS THURSDAY AFTN AND EVENING. SOME OF THE STORMS COULD CONTAIN GUSTY WINDS. THE UPPER-LEVEL DISTURBANCE WILL MOVE AWAY FROM THE AREA LATER TUESDAY MORNING...BUT A STRATO CU DECK MAY HANG AROUND FOR MUCH OF THE DAY WITH MVFR CIGS POSSIBLE. A COLD FRONT WILL MOVE THROUGH THURSDAY EVENING/NIGHT...WITH SHRA LIKELY AND A PSBL TSTM INTO FRI. S-SW WINDS WITH SOME PSBL GUSTS 15-20KTS TURNING NW THUR NIGHT. VFR LIKELY FRIDAY WITH NW WINDS FRI. GUSTY CONDITIONS ON SATURDAY.
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&& .MARINE...
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SURFACE HIGH PRESSURE WILL EXTEND OVER THE WATERS THROUGH TUESDAY. WINDS WILL REMAIN BELOW SCA CRITERIA DURING THIS TIME. SCA CONDITIONS MAY BE POSSIBLE ON THE WATERS WEDNESDAY AND THURSDAY AHEAD OF A COLD FRONT. SOUTHERLY WINDS WILL SHIFT TO WEST LATE THURSDAY NIGHT INTO FRIDAY. EXPECT PERIODS OF RAIN...SHOWERS AND POSSIBLY A FEW THUNDERSTORMS WEDNESDAY INTO THURSDAY. SOME OF THE STORMS COULD CONTAIN GUSTY WINDS THURSDAY AFTERNOON AND EVENING. BETTER CHANCE OF SCA THURSDAY WITH STRONGER S-SW WINDS AHEAD OF A COLD FRONT. SHOWERS AND A PSBL TSTM THURSDAY EVENING INTO THE NIGHT. WINDS SHIFT TO THE NW THUR NIGHT BEHIND THE FRONT. POSS SCA ON SAT WITH INCREASING WINDS.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...NONE. && $$ UPDATE...GMS PREV DISCUSSION...BJL/KRW/IR

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