Area Forecast Discussion
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000 FXUS61 KPBZ 272115 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 415 PM EST FRI FEB 27 2015 .SYNOPSIS... 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. && .NEAR TERM /THROUGH SATURDAY/... ANY LINGERING 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. THE ONLY FLY IN THE PROVERBIAL OINTMENT WILL BE INCREASING CIRRUS DEPICTED IN THE MODELS ASSOCIATED WITH A 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. MINIMA TONIGHT MAY REACH NEAR-RECORD LOW FOR 28 FEB. SEE CLIMATE SECTION BELOW. 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. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... MODEL GUIDANCE CONTINUES TO BE IN GOOD AGREEMENT IN THE LOW PRESSURE SYSTEM THAT IS FORECAST TO MAKE ITS APPROACH LATE SATURDAY NIGHT/SUNDAY MORNING. THIS LOW WILL MOVE NORTHEASTWARD THROUGH THE UPPER OHIO VALLEY WITH THE SUPPORT OF A DIGGING UPPER LEVEL TROUGH DURING THE DAY ON SUNDAY. WITH THE LOWS PLACEMENT JUST A BIT TO THE NORTH...WARM ADVECTION LOOKS TO SUPPORT A MIX OF RAIN AND SNOW FOR PART OF THE REGION. ALTHOUGH CONFIDENCE ON THE EXACT PLACEMENT OF THE RAIN-SNOW LINE AND IT`S MOVEMENT ON SUNDAY STILL REMAINS IN QUESTION...THE LAST SEVERAL MODEL RUNS HAVE TRENDED WARMER WITH NEAR/ABOVE FREEZING SURFACE TEMPERATURES AND HAVE PUSHED CRITICAL THICKNESS VALUES FURTHER NORTH. THUS...WHILE ALL OF THE DETERMINISTIC RUNS SUGGEST 0.75 INCHES OF QPF AREA- WIDE...WARM AIR ALOFT AND AT THE SURFACE SHOULD CUT DOWN ON SNOWFALL ACCUMULATIONS FOR THE LOCATIONS THAT REMAIN IN THE AREA WITH GREATEST UNCERTAINTY. AT THIS POINT...THE HIGHEST CONFIDENCE IN ACCUMULATING SNOW REMAINS FOR POINTS NORTH OF I-80 AND A WINTER STORM WATCH HAS BEEN ISSUED FOR THOSE COUNTIES. WITH THESE LOCATIONS REMAINING COLDEST THROUGH THE ENTIRE DAY ON FRIDAY...EVEN MODEST SNOW RATIOS SHOULD BRING VALUES CLOSE TO 6 OR MORE INCHES. FURTHER SOUTH...THE CHANGEOVER TO RAIN SHOULD CUT DOWN ON ACCUMULATION AND POINTS SOUTH OF I-70 ARE MOST LIKELY TO SEE THE LONGEST PERIOD OF RAIN AT THIS POINT. A CHANGEOVER BACK TO SNOW IS EXPECTED EVERYWHERE OVERNIGHT SUNDAY AS COLD ADVECTION RETURNS WITH THE DEPARTURE OF THE LOW. SUBSIDENCE WILL QUICKLY END SNOWFALL FOR MOST OF THE AREA BY MONDAY MORNING...WITH LINGERING SNOW POSSIBLE IN THE RIDGES. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... 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. && .AVIATION /21Z FRIDAY THROUGH WEDNESDAY/... 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. && .HYDROLOGY... ...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. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...WINTER STORM WATCH FROM LATE SATURDAY NIGHT THROUGH MONDAY MORNING FOR PAZ007>009-013>016. WV...NONE. && $$

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