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Area Forecast Discussion
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000
FXUS61 KPBZ 211722
AFDPBZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
1222 PM EST SUN DEC 21 2014

.SYNOPSIS...
HIGH PRESSURE WILL BE IN CONTROL THROUGH MONDAY. EXPECT
INCREASING PRECIPITATION CHANCES TUESDAY. STORM SYSTEM BRINGS RAIN
AND WIND CHRISTMAS EVE...AND SNOW SHOWERS CHRISTMAS DAY.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
NO MAJOR CHANGES NEEDED TO THE FORECAST FOR TODAY. A FEW BREAKS IN
THE STRATUS DECK ARE EVIDENT IN THE SAT LOOP. CLOUD DECK IS FAIRLY
SHALLOW...EVIDENCED BY VERY DRY AIR ABOVE H700 ON SOUNDING. NEW
NAM RUN SEEMS TO HAVE A GOOD HANDLE ON H850 MOISTURE
CURRENTLY...AND DRIER AIR AT THAT LEVEL IN OHIO IS SET TO ARRIVE
THIS AFTERNOON ON LIGHT W FLOW. THUS...STILL EXPECT A DECREASING
TREND IN CLOUD COVER WITH TIME. TEMPERATURES ON TRACK FOR HIGHS IN
THE LOWER TO MID 30S.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY/...
SURFACE HIGH WILL SLIDE EASTWARD TONIGHT...AS WILL THE UPPER LEVEL
RIDGE. WAA WILL GET STARTED MONDAY MORNING AND CONTINUE THROUGHOUT
THE DAY. THE WARMER AIR WILL ALSO PULL MOISTURE NORTHWARD...
INCREASING CLOUDS ONCE AGAIN. BIG QUESTION ON MONDAY IS THE OUTER
EDGE OF THE PRECIP SHIELD THAT IS ASSOCIATED WITH EASTERN SEABOARD
SYSTEM. IT APPEARS THAT SOME OF THIS PRECIP COULD CLIP THE
SOUTHEASTERN RIDGES ON MONDAY. THE PROBLEM WILL BE THE COLD AIR
TRAPPED IN THE RIDGES...THANKS TO THE LOW-LEVEL EASTERLY FLOW.
CONFIDENCE IS LOW HERE...AS MODELS HAVE BOUNCED BACK AND FORTH
WITH HOW FAR WEST TO SPREAD THE PRECIP. THERE IS VERY LITTLE UPPER
LEVEL SUPPORT THIS FAR WEST...BUT WITH THE EASTERLY FLOW...
SHOWERS MAY BE ABLE TO CLIP THE AFOREMENTIONED RIDGES. SURFACE
TEMPS WILL BE CRITICAL...BUT IT DOES APPEAR THAT AT LEAST THE
SHELTERED VALLEYS WILL BE ABLE TO HOLD ON TO TEMPS AT OR BELOW
FREEZING. WILL NEED TO INCLUDE THE MENTION OF FREEZING RAIN IN
THESE AREAS MONDAY.

LOW-LEVEL JET WILL GET STRONG WAA GOING MONDAY NIGHT AND TUESDAY.
STILL TOUGH TO GET A GOOD HANDLE ON HOW MUCH RAIN WILL REACH THE
REGION ON TUESDAY. THE UPPER LEVEL SUPPORT IS WEAK AND
UNORGANIZED...BUT THE LOW-LEVEL JET WILL PULL MOISTURE NORTHWARD
AS WELL. WILL INCREASE RAIN CHANCES FROM WEST TO EAST TUESDAY...BUT
KEEP EVERYTHING IN THE CHANCE CATEGORY.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
HIGHLIGHTS:
- A WET CHRISTMAS EVE 
- TEMPERATURES RETURN BACK TO NORMAL VALUES CHRISTMAS DAY
- SLIGHTLY ABOVE NORMAL TEMPERATURES RETURN BY FRIDAY

STRONG LOW PRESSURE SYSTEM THAT HAS BEEN MUCH ADVERTISED...GIVEN
THE TIME PERIOD WHICH IT AFFECTS OUR AREA...IS STILL ON TRACK.
LONG RANGE ENSEMBLE PREDICTION SYSTEMS AND OPERATIONAL MODELS ARE
CONVERGING ON A TRACK THROUGH IND / ILL THEN NORTH INTO CANADA.
THIS WILL MAKE FOR ALL LIQUID PRECIPITATION ON THE FRONT END.
TEMPERATURES WILL SOAR TO NEARLY 20 DEGREES ABOVE NORMAL WEDNESDAY
WITH 50S COMMON. COUPLED WITH INFLUX OF MOISTURE FROM THE GULF OF
MEXICO...SOUNDINGS ILLUSTRATE ELEVATED INSTABILITY WHICH PUSHES
TEMP DIFFERENTIAL BETWEEN H8 AND H5 TO NEARLY 0C. THIS COULD LEAD
TO ISOLATED THUNDERSTORMS AHEAD OF THE BAROCLINIC ZONE. FOR THE
TIME BEING...OPTED TO LEAVE THUNDER OUT OF THE FORECAST AND WAIT
FOR BETTER CONTINUITY GIVEN THIS STILL IS FIVE DAYS OUT. DESPITE
PWATS NEARLY TWO STANDARD DEVIATIONS ABOVE NORMAL...COMBINATION OF
KATABATIC WINDS AND DRY CONVEYOR BELT WILL LIMIT QPF POTENTIAL TO
NEGATE ANY FLOOD THREAT. LONG RANGE HYDROLOGIC MODELS SHOW VERY
LITTLE RISE IN AREA RIVERS...WHICH CORRELATES WELL TO THE
PROJECTED QPF OF 0.50 TO 1.00".

WITH THE TRACK OF THE CYCLONE DUE N...COLD AIR INJECTED INTO THE 
AREA WILL BE OF MIDWEST ORIGIN...SO TEMPS WILL RETREAT BACK TO 
SEASONAL BENCHMARKS CHRISTMAS DAY. FOR THOSE WHO ARE LOOKING FOR
SNOW ON CHRISTMAS...A CHANCE OF SNOW SHOWERS EXIST...BUT GIVEN
TRAJECTORY FROM THE SOUTHWEST ANY ACCUMULATION ASSOCIATED WITH THE
UPPER LEVEL SYSTEM WILL BE LIGHT AT BEST. HISTORICALLY...THERE IS
ONLY A 17 PERCENT CHANCE OF AT LEAST ONE INCH OF SNOW ON THE
GROUND AT PITTSBURGH CHRISTMAS MORNING.

H8 WINDS PUSH 50KTS WEDNESDAY NIGHT INTO THURSDAY AND HIGHEST 
ANOMALIES RESIDE ALONG THE LAKE SHORE...SO EXPECT HIGHEST GUSTS 
WOULD DO SO AS WELL.  NONETHELESS...PERIOD FROM WEDNESDAY NIGHT INTO 
THURSDAY STRONG ISALLOBARIC FLOW COULD YIELD GUSTS IN EXCESS OF 40 
MPH ESPECIALLY IN THE MOUNTAINS OF WRN MD AND NRN WV.  

BALANCE OF THE EXTENDED IS CHARACTERIZED BY UNCERTAINTY AS ONE WOULD 
EXPECT IN THE DAY 6 -7 TIME FRAME. ECMWF HAS A STRONGER SYSTEM
CROSSING FRIDAY NIGHT VERSUS GFS WHICH IS MUCH WEAKER. EITHER CASE
PTYPE SHOULD BE RAIN WITH SNOW SHOWERS ON THE BACKSIDE. NO PLUNGES
OF ARCTIC AIR ARE FORECAST IN THE NEXT 10 DAYS. MAX/MIN
TEMPERATURES WERE A BLEND OF THE FORECAST AND ECMWF BIAS CORRECTED
DATA SET.

&&

.AVIATION /17Z SUNDAY THROUGH THURSDAY/...
MVFR CLOUD DECK IS SLOWLY ERODING FROM THE SOUTH. EXPECT ALL SITES
TO ACHIEVE VFR BY 00Z...SAVE FOR POSSIBLY FKL/DUJ WHERE THE CLOUDS
WILL HANG ESPECIALLY TOUGH. VFR THROUGH MOST OF THE
NIGHT...ALTHOUGH SOME MVFR CIGS COULD REDEVELOP TOWARDS DAWN ALONG
THE RIDGES IN SE FLOW. HIGH CLOUDS WILL INCREASE AFTER SUNRISE
MONDAY...AND EXPECT A STRATOCU DECK TO FORM UP WITH TIME. ANY CIGS
SHOULD REMAIN LOW VFR THROUGH 18Z. SPOTTY FREEZING RAIN MAY BE
POSSIBLE IN A FEW VALLEYS IN NORTHEAST WV/WESTERN MD.

OUTLOOK.../SUNDAY NIGHT THROUGH THURSDAY/... NEXT STORM SYSTEM
WILL BEGIN TO IMPACT THE REGION ON TUESDAY AND RESULT IN
WIDESPREAD RAIN...CIG/VIS RESTRICTIONS...AND PERIODS OF LLWS INTO
THURSDAY.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...NONE.
OH...NONE.
PA...NONE.
WV...NONE.

&&

$$






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