Area Forecast Discussion
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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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