Area Forecast Discussion
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000 FXUS61 KPBZ 282322 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 622 PM EST SAT FEB 28 2015 .SYNOPSIS... LOW PRESSURE WILL SPREAD SNOW AND RAIN OVER THE UPPER OHIO REGION ON SUNDAY. WARMER...MORE SEASONABLE...WEATHER CAN BE EXPECTED FOR THE COMING WEEK. && .NEAR TERM /THROUGH SUNDAY NIGHT/... NO CHANGES WERE NEEDED FOR THE EARLY EVE UPDATE. PREVIOUS... BROAD WARM ADVECTION WILL BE THE MAIN DRIVER OF PRECIPITATION THROUGH THE PERIOD...UNTIL FROPA SUNDAY NIGHT. A FRONTAL BOUNDARY WILL START TO ORGANIZE TO THE NORTHWEST OF THE CWA ON SUNDAY... WITH MOIST SW FLOW OVER OUR AREA. THIS WILL ALSO PUSH WARMER AIR NORTHWARD. SHORTWAVE CROSSES THE GREAT LAKES SUNDAY NIGHT AND ACTS AS THE IMPETUS TO GET THE FRONT MOVING...AND EXPECT IT TO CROSS BY 06Z MONDAY. COLDER AIR THEN SPILLS BACK IN BEHIND. MODELS ARE IN GENERAL AGREEMENT WITH THIS PATTERN. EXPECT FIRST FLAKES TO ARRIVE NEAR ZZV AROUND MIDNIGHT...AND SPREAD EAST FROM THERE. EXPECT MOST POINTS TO SEE SNOW BY 12Z AS LIFT FED BY WARM ADVECTION AND A MODEST ISENTROPIC CONTRIBUTION BECOMES MORE WIDESPREAD. EXPECTING RAIN TO MIX IN LATER IN THE MORNING AS THE TRUE LOW-LEVEL WARM PUNCH ARRIVES. ICING THREAT IS FAIRLY LOW OVERALL WITH NOT MUCH OF A WARM LAYER ALOFT...BUT COULD SEE A TRACE OF ICING FROM FZRA/IP ACROSS SOME OF THE SOUTHERN COUNTIES AT THE BEGINNING OF THE CHANGEOVER. EXPECT THE RAIN/SNOW MIX TO REACH AS FAR NORTH AS BVI/IDI...WITH A PERIOD OF ALL RAIN AT SOUTHERN SITES LIKE MGW...AND MOSTLY SNOW TO THE NORTH. CHANGEOVER BACK TO SNOW WILL OCCUR AFTER 00Z AS COLD ADVECTION RETURNS BEHIND FRONT. REGARDING SNOW AMOUNTS...AM STILL NOT IMPRESSED BY SNOW RATIOS GIVEN THE WARM INTRUSION. DESPITE EXPECTING MOSTLY SNOW ACROSS THE NORTH...THE EXPECTED LOW RATIOS...DROPPING TO AROUND 12 TO 1 SUNDAY ACROSS THE NORTH AND LOWER TO THE SOUTH... PLUS THE 24-30 HOUR DURATION OF THE STORM... POINT TO THE NEED FOR A HIGH-END ADVISORY RATHER THAN A WARNING. SURROUNDING OFFICES AGREED TO THIS...AND WE ENDED UP WITH A BROAD WINTER WEATHER ADVISORY. TOTALS RANGE FROM 6-8 INCHES IN 24-30 HOURS ACROSS THE FAR NORTH...TO 3-6 IN A RIBBON FROM ZZV THROUGH PIT TO LBE...AND 3 OR LESS TO THE SOUTHEAST. KEEP IN MIND THAT A MIX WITH OR CHANGE TO RAIN WILL OCCUR FOR MUCH OF THE AREA...SO SOME OF THE INITIAL SNOWFALL WILL MELT ON THE GROUND IN RAIN. KEPT SEVERAL SOUTHERN COUNTIES OUT OF THE ADVISORY GIVEN LESSER SNOW TOTALS AND NO SIGNIFICANT ICING. EXPECT PRECIPITATION TO START TO WIND DOWN LATER SUNDAY NIGHT AS DEEPER MOISTURE PULLS OFF TO THE EAST AND SUBSIDENCE TAKES OVER. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... SNOW WILL BE WINDING DOWN ACROSS THE AREA ON MONDAY MORNING AS HIGH PRESSURE BUILDS INTO THE REGION FROM THE WEST. SNOW SHOWERS ALONG THE RIDGES WILL PERSIST UNTIL INVERSION HEIGHT LOWERS SUFFICIENTLY...LIKELY BY LATE MONDAY AFTERNOON. AGGRESSIVE SOUTH-SOUTHWESTERLY 850MB JET WILL STRENGTHEN MONDAY NIGHT...PROMOTING ADVECTION OF ANOMALOUSLY POTENT WARM AND MOIST AIR FROM THE GULF OF MEXICO. LIGHT PRECIPITATION IN THIS WARM AIR ADVECTION REGIME WILL BEGIN TO ENCROACH ON THE FORECAST AREA LATE MONDAY NIGHT AND RAPIDLY ACROSS THE REGION TUESDAY MORNING. PRECIPITATION LIKELY WILL BEGIN AS LIGHT SNOW AS OVERNIGHT TEMPERATURES WILL DROP INTO THE MID-UPPER TEENS. IN RESPONSE TO DEEPENING LOW PRESSURE IN ILLINOIS...WIND WILL SHIFT TO THE EAST-SOUTHEAST LATE MONDAY NIGHT...WHICH WILL DO LITTLE TO VACATE THE VERY COLD LOW-LEVEL AIRMASS DESPITE THE STRONG WARM AIR ADVECTION ALOFT. HISTORICALLY...SOUTHERLY WIND DIRECTION IS NEEDED TO ERADICATE LOW-LEVEL COLD AIR...WHICH WILL NOT OCCUR UNTIL LATER IN THE DAY TUESDAY WHEN SURFACE LOW PRESSURE NEARS. AS A RESULT...THE ATMOSPHERE WILL BE PRIMED FOR THE EARLY LIGHT SNOW TO TRANSITION TO A PERIOD OF FREEZING RAIN/SLEET ON TUESDAY MORNING. WEATHER AND ICE GRIDS WERE ADJUSTED TO REFLECT THIS EXPECTATION. SURFACE TROUGH AND COLD FRONT WILL ENCROACH UPON THE AREA TUESDAY LATE AFTERNOON AND EVE...WITH SURFACE LOW PRESSURE PASSING TO THE NORTHWEST OF THE FORECAST AREA. THE OHIO VALLEY WILL BE SOLIDLY IN THE WARM SECTOR AS WIND VEERS TO SOUTHWESTERLY...HELPING TO ERODE THE LOW-LEVEL COLD AIR AND CHANGE ALL PRECIPITATION TO RAIN. ALL MODELS AND MOS GUIDANCE OFFER STRONG EVIDENCE OF MEASURABLE PRECIPITATION OCCURRING TUESDAY NIGHT...FOCUSED IN PART BY THE APPROACHING BOUNDARY. POPS WERE INCREASED TO 100 PERCENT DURING THIS TIME PERIOD TO REFLECT THIS EXPECTATION...TIMED WITH THE PASSAGE OF THE BOUNDARY. THERE IS A GOOD SYNOPTIC SIGNAL FOR AN ANOMALOUS RAINFALL EVENT WITH THIS SYSTEM...AMID STRONG MOISTURE FLUX /PWATS APPROACHING MAXIMUM CLIMATOLOGICAL VALUES/ AND PERSISTENT SOUTHWESTERLY LOW- LEVEL FLOW. THAT SAID...MODELS ALSO TEND TO OVERDO QPF IN WARM AIR ADVECTION PATTERNS. GIVEN THAT THIS SYSTEM IS AN OPEN WAVE AND PRECIPITATION IS DRIVEN IN PART BY WARM ADVECTION...QPF ON TUESDAY WAS TONED DOWN SLIGHTLY FROM THE MORE ASSERTIVE GFS VALUES. TEMPERATURES TUESDAY NIGHT LIKELY WILL CLIMB OR REMAIN STEADY IN THE 40S...PERHAPS WARMER THAN THEIR TUESDAY DAYTIME VALUES. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... COLD AIR ADVECTION IN THE WAKE OF THE TUESDAY NIGHT COLD FRONTAL PASSAGE WILL RETURN THE REGION TO NEAR- OR SUB-FREEZING TEMPERATURES FOR THE REMAINDER OF THE WEEK. UPSLOPE SNOW SHOWERS LIKELY WILL PERSIST THROUGH WEDNESDAY EVENING...BUT SHOULD SEE A MARKED DECREASE IN COVERAGE BY LATE WEDNESDAY NIGHT. A QUICK-MOVING SHORTWAVE TROUGH COULD AFFECT THE REGION LATE IN THE WEEK WITH LITTLE FANFARE ANTICIPATED. THE DRY AIRMASS AT THE SURFACE PRECLUDES SIGNIFICANT IMPACTS AT THE MOMENT SAVE MODEST LAKE-ENHANCED SNOW. && .AVIATION /23Z SATURDAY THROUGH THURSDAY/... SNOW IS EXPECTED TO DEVELOP LATE TONIGHT...AROUND 6Z NEAR ZZV TO AROUND 9Z NEAR LBE/DUJ. ONCE SNOW COMMENCES...CONDITIONS WILL DETERIORATE TO IFR BY MORNING. SNOW WILL MIX WITH AND CHANGE TO RAIN AT MOST TERMINALS /EXCEPT FKL AND DUJ/ MID-DAY...WITH MGW SEEING MOSTLY RAIN THRU THE DAY. A WIND SHIFT IS EXPECTED SUNDAY EVE ASSOCIATED WITH A COLD FRONTAL PASSAGE. PRECIPITATION WILL CHANGE BACK TO SNOW IN THE WAKE OF THE COLD FRONT. .OUTLOOK.../SUNDAY NIGHT THROUGH THURSDAY/... RESTRICTIONS LIKELY WILL PERSIST THROUGH MUCH OF MONDAY. LOW PRESSURE CROSSING THE GREAT LAKES IS LIKELY TO SPAWN WIDESPREAD RESTRICTIONS ONCE AGAIN BY LATE TUESDAY. && .HYDROLOGY... MANY OF THE AREA RIVERS AND STREAMS ARE ICE COVERED. AN INCREASE IN FLOW AND STAGE IS EXPECTED TO CAUSE ICE MOVEMENT STARTING TUESDAY...BEGINNING ON THE SOUTHERNMOST RIVER BASINS. THE CURRENT FORECAST CONTINUES TO HAVE A HIGH LEVEL OF UNCERTAINTY ABOUT PRECIPITATION PHASE AND AMOUNTS AS WELL AS THE DEGREE OF SNOW MELT. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...WINTER WEATHER ADVISORY FROM 4 AM SUNDAY TO 7 AM EST MONDAY FOR MDZ001. OH...WINTER WEATHER ADVISORY FROM 4 AM SUNDAY TO 7 AM EST MONDAY FOR OHZ039>041-048>050-057>059. PA...WINTER WEATHER ADVISORY FROM 4 AM SUNDAY TO 7 AM EST MONDAY FOR PAZ007>009-013>016-020>023-029-073>076. WV...WINTER WEATHER ADVISORY FROM 4 AM SUNDAY TO 7 AM EST MONDAY FOR WVZ001>003-023-041. && $$ 15

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