Area Forecast Discussion
Issued by NWS

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000 FXUS61 KPBZ 271727 AAB AFDPBZ AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE PITTSBURGH PA 1227 PM EST FRI FEB 27 2015 .SYNOPSIS...
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DRY CONDITIONS AND A SLIGHT WARMING TREND IN TEMPERATURES CAN BE EXPECTED AS WE HEAD INTO THE WEEKEND BEFORE LOW PRESSURE PROVIDES RAIN AND SNOW SUNDAY THROUGH MONDAY MORNING. A SIGNIFICANT WARMUP IS POSSIBLE TUESDAY INTO WEDNESDAY BEFORE COLD AIR RETURNS.
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&& .NEAR TERM /THROUGH SATURDAY NIGHT/...
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SCATTERED STRATOCU ARE DEVELOPING IN SHALLOW BUT WELL-MIXED BOUNDARY LAYER. WITH DENDRITIC GROWTH ZONE SOLIDLY IN THIS LAYER...SOME OF THESE CLOUDS HAVE GENERATED BRIEF SNOW SHOWERS... ESPECIALLY IN THE VICINITY OF LATROBE AND MORGANTOWN. SNOW SHOWERS WILL DISSIPATE QUICKLY THIS AFTERNOON AS HIGH PRESSURE BUILDS INTO THE REGION. SKY WILL CLEAR CONSIDERABLY AMID SUBSIDENCE...SNOW COVER AND LIGHT WIND...PROMOTING CONDITIONS THAT WILL BE FAVORABLE FOR EFFECTIVE RADIATIONAL COOLING TONIGHT. ONLY FLY IN THE PROVERBIAL OINTMENT WILL BE INCREASING CIRRUS DEPICTED IN THE MODELS ASSOCIATED WITH MID-LEVEL JET STREAK. THE CIRRUS SHOULD NOT BE THICK ENOUGH TO INHIBIT COOLING BY ANY RADICAL MARGIN...SO SHARP COOLING CONTINUES TO BE EXPECTED. THE DAY WILL START EXCEPTIONALLY COLD ON SATURDAY. HIGH PRESSURE WILL DOMINATE THE WEATHER ON SATURDAY. SKY WILL BE MOSTLY CLEAR THROUGH EARLY AFTERNOON AS VERY DRY AIR INVADES...OFFERING FULL SUN AND A SHARP TEMP INCREASE INTO THE 20S BY AFTERNOON. CIRRUS AHEAD OF AN APPROACHING TROUGH WILL BEGIN TO INFILTRATE LATE IN THE DAY ON SATURDAY. MODEL TRENDS HAVE BEEN TOWARD A FASTER ONSET OF PRECIPITATION WITH A SYSTEM ON SUNDAY...WITH LIGHT SNOW EXPECTED TO DEVELOP LATE SATURDAY NIGHT AS A WARM FRONT SHARPENS FROM WESTERN PA THRU OH TO AN ORGANIZING LOW PRESSURE SYSTEM IN EASTERN INDIANA. POPS WERE INCREASED SLIGHTLY FROM PREV SHIFT...WHICH ALREADY HAD A WELL- TIMED RAPID ONSET OF SNOW.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY/...
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LATEST MODEL RUNS SHOW RELATIVELY GOOD AGREEMENT FOR THE NEXT LOW PRESSURE SYSTEM TO IMPACT THE REGION...AND OVERNIGHT CHANGES TO THE FORECAST WERE BASED MAINLY ON A BLEND OF EC...GFS...AND ENSEMBLE GUIDANCE AS THE HPC PREFERRED SOLUTIONS...WITH LESS WEIGHT PUT ON THE SREF AND NAM. PRECIPITATION WILL START OUT AS ALL SNOW...BUT BEGIN CHANGING OVER TO RAIN OVER THE COURSE OF THE MORNING AND NOW ANTICIPATE SNOW SLIGHTLY FARTHER TO THE SOUTH...AND A RAIN AND/OR RAIN/SNOW MIX MAINLY LIMITED TO LOCATIONS SOUTH OF INTERSTATE 80. LOOKING AT CRITICAL THICKNESSES THERE IS STILL SOME UNCERTAINTY...LARGELY SOUTH OF INTERSTATE 80 AS LOW-LEVEL WARM AIR ADVECTION WILL BE STRONG ON SUNDAY...WHICH IS REFLECTED IN THE 850-700MB VALUES. THEREFORE...HAVE HELD BACK FORECAST SNOW AMOUNTS SOUTH OF INTERSTATE 80 FOR NOW AS A CHANGEOVER TO RAIN ON SUNDAY AFTERNOON OR MIXED PRECIPITATION WILL IMPACT ACCUMULATIONS. NORTH OF I-80 SNOW AMOUNTS WERE INTRODUCED OR INCREASED AS FORECAST CONFIDENCE IN ALL SNOW IS HIGHER HERE. HEADLINES MAY BE INTRODUCED SOON...AT LEAST FOR NORTHERN LOCATIONS. BY MONDAY MORNING...ANY ONGOING PRECIPITATION WILL HAVE CHANGED BACK OVER TO SNOW WITH COLD AIR ADVECTION ON THE BACKSIDE OF THE SYSTEM.
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&& .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
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A WARM UP IN TEMPERATURES EXPECTED EARLY ON IN THE EXTENDED AS TROUGHING DEEPENS OVER THE WESTERN US AND SOUTHWESTERLY FLOW SETS UP ACROSS THE EASTERN STATES. A CLOSED-OFF AREA OF LOW PRESSURE EJECTING OUT OF THE DESERT SOUTHWEST EARLY IN THE WEEK WILL START TO TRANSITION EAST MONDAY AND HELP DRAG IN MOISTURE OFF THE SOCAL COAST INTO THE STRONG SOUTHWESTERLY FLOW...WITH ADDITIONAL MOISTURE POOLING IN FROM THE GULF. ENSEMBLE MEMBERS ARE SHOWING MINIMAL SPREAD IN THIS SETUP...WHICH MAY RESULT IN A SIGNIFICANT AMOUNT OF PRECIPITATION THROUGH THE MIDDLE OF THE WEEK ACROSS THE REGION. FORECAST PWAT VALUES ARE AROUND THE 99TH PERCENTILE BY WEDNESDAY OVER THE AREA AND IVT VALUES ARE SHOWING A BULLS EYE OVER THE REGION. THIS...COMBINED WITH THE ADDITIONAL SNOW ACROSS THE NORTH AND A RIPER SNOWPACK TO THE SOUTH RAISES FLOODING CONCERNS. WE WILL BE FOLLOWING THIS CLOSELY IN UPCOMING MODEL RUNS. SEE HYDROLOGY SECTION BELOW.
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&& .AVIATION /16Z FRIDAY THROUGH SUNDAY NIGHT/...
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SCATTERED SNOW SHOWERS VCTY KLBE AND KMGW WILL DISSIPATE QUICKLY AS SFC HIGH PRESSURE BUILDS INTO THE REGION. VFR CONDITIONS WILL PERSIST OVERNIGHT AMID LIGHT WIND...AND THROUGH SATURDAY AS THE CENTER OF HIGH PRESSURE MOVES ACROSS THE AREA. CIRRUS WILL INCREASE SATURDAY EVENING AHEAD OF AN APPROACHING SYSTEM. LIGHT SOUTHERLY WIND AND A RAPID ONSET OF LIGHT SNOW ARE EXPECTED LATE SATURDAY NIGHT...WITH LIGHT SNOW DEVELOPING AT ALL TERMINALS BY MID-MORNING SUNDAY. RESTRICTIONS IN SNOW/RAIN CAN BE EXPECTED AT ALL TERMINALS ON SUNDAY. A SHARP WIND SHIFT TO THE NORTHWEST IS EXPECTED IN THE WAKE OF THE SYSTEM AS A COLD FRONT TRAVERSES THE OHIO VALLEY. OUTLOOK.../MONDAY THROUGH WEDNESDAY/... RESTRICTIONS ARE LIKELY AS LOW PRESSURE YIELDS WIDESPREAD RAIN TUESDAY NIGHT AND WEDNESDAY.
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&& .HYDROLOGY...
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...FIRST SIGNIFICANT RIVER RISE OF THE SEASON EXPECTED TUESDAY AND WEDNESDAY MAR 3 AND 4... GIVEN THE FACT THAT MANY OF THE AREA RIVERS... TRIBUTARIES AND SMALLER STREAMS AND CREEKS ARE COVERED OVER WITH ICE... A DRAMATIC INCREASE IN FLOW AND STAGE LEVELS WILL CAUSE ICE MOVEMENT STARTING TUESDAY MAR 3. CURRENT FORECAST HAS A HIGH LEVEL OF UNCERTAINTY ABOUT MANY FACTORS...INCLUDING RAIN AMOUNT...TEMPERATURE...SNOW MELT AND ICE MOVEMENT. SMALL STREAMS AND TRIBUTARIES WILL BE FIRST AFFECTED TUESDAY MAR 3. RISES ON THE MAJOR RIVERS ARE PROJECTED TO OCCUR STARTING TUESDAY NIGHT AND CONTINUE THROUGH WEDNESDAY MAR 4.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$ UPDATE...KRAMAR

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