008
FXUS61 KPBZ 181403
AFDPBZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
903 AM EST WED DEC 18 2024
SYNOPSIS
RAIN AND SNOW RETURNS TODAY, WITH COLDER TEMPERATURES AND SNOW
SHOWERS POSSIBLE FRIDAY AND THROUGH THE WEEKEND.
NEAR TERM /UNTIL 6 PM THIS EVENING/
KEY MESSAGES:
- RAIN CHANCES INCREASE THIS MORNING
- SNOW OR A WINTRY MIX IS POSSIBLE NORTH OF PITTSBURGH.
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LOW PRESSURE WILL QUICKLY MOVE FROM THE TENNESSEE VALLEY THIS
MORNING TO SOUTHEASTERN PA BY THIS EVENING. A SHORTWAVE TROUGH
WILL CROSS THE AREA THIS AFTERNOON, IN THE SOUTHWEST FLOW ALOFT,
WHICH WILL WORK TO KEEP THE SURFACE LOW MAINLY TO OUR SOUTH. A
PLUME OF DEEP MOISTURE WILL SPREAD ACROSS THE REGION AHEAD OF
THE UPPER LEVEL WAVE AS WELL. MORNING OBSERVATIONS AS THE
PRECIPITATION ARRIVES ARE SHOWING NEAR TO JUST ABOVE FREEZING
SURFACE TEMPERATURES ACROSS PARTS OF THE AREA, PRIMARILY EASTERN
OHIO AND WITHIN THE I-80 CORRIDOR IN PENNSYLVANIA. THAT SAID,
OUR MORNING SOUNDING SHOWS COPIOUS DRY AIR IN THE MID-LEVELS
THAT NEEDS TO SATURATE BEFORE PRECIPITATION REACHES THE GROUND.
850 MB TEMPERATURES SIT BETWEEN 1-5C, LOWEST NORTH AND HIGHEST
SOUTH. WITH SATURATION, THE PROFILES MAY WET-BULB DOWN TO NEAR
OR JUST BELOW FREEZING, ESPECIALLY FURTHER NORTH, WHERE SOME
BRIEF WET SNOW IS POSSIBLE BEFORE A CHANGEOVER TO RAIN.
ELSEWHERE, PLAIN RAIN IS EXPECTED AS SURFACE TEMPERATURES WARM
THIS MORNING.
WITH THE SURFACE LOW FURTHER SOUTH, THE HIGHER AMOUNTS OF RAIN
WILL STAY MAINLY SOUTH OF THE MASON-DIXON LINE. A FURTHER
SOUTHWARD TRACK OF THE LOW WILL ALSO ALLOW THE COLDER AIR TO THE
NORTH TO MOVE SOUTHEASTWARD FASTER. THIS PART OF THE FORECAST
IS HIGHLY UNCERTAIN AS IT WILL HAVE AN EFFECT ON A CHANGEOVER
BACK TO SNOW AS THE PRECIPITATION TYPE OVER THE NORTHERN
PORTION OF THE FORECAST AREA LATER TONIGHT. LOOKING CLOSELY AT
THE NBM PRECIPITATION TYPE OVER THE I-80 CORRIDOR, IT IS
DEFINITELY LEANING TOWARD SNOW WITH A 60% PROB OF THE WHITE
STUFF COMPARED TO A 27% PROB OF ALL RAIN. THIS WILL BE THE
TRICKY PART OF THE FORECAST TO PINPOINT WHERE THAT RAIN/SNOW
LINE WILL BE. ADDITIONALLY, AS THE EVENING APPROACHES, COLDER
AIR WILL BE DIVING SOUTHWARD AS THE SURFACE LOW EXITS TO THE
EAST WHICH WILL SUPPORT THE CHANGE OVER TO SNOW ON THE BACKSIDE
OF THE EXITING PRECIPITATION SHIELD. THE AREAS WITH THE HIGHEST
PROBABILITIES TO SEE SNOW ON THE BACKSIDE WOULD BE OVER THE
HIGHER ELEVATIONS TO THE SOUTH AND EAST OF PIT. SNOW RATIOS WILL
BE POOR WITH A LACK OF MOISTURE IN THE DGZ, SO ACCUMULATION IS
EXPECTED TO BE MINIMAL. IN EASTERN TUCKER COUNTY, PROFILES SHOW
SHALLOW MOISTURE EXTENDING TO BELOW ~-8C IN A CONDITIONALLY
UNSTABLE ENVIRONMENT WITH SUB-FREEZING SURFACE TEMPERATURES. THE
LACK OF ICE PRODUCTION IN THE CLOUD BEARING LAYER COULD POINT
TOWARD A PERIOD OF FREEZING RAIN AS THE PRECIPITATION TYPE
TONIGHT AND THE HREF PRECIP TYPE PROBS HINT AT THIS. WILL
EVALUATE THE 12Z GUIDANCE AS IT ARRIVES, BUT COULD SEE SOME
MINOR ICE ACCRETION IN THE OVERNIGHT HOURS DOWN THAT WAY.
THE TRACK OF THE SURFACE LOW WILL ALSO BE IMPORTANT TO HIGHS
TODAY. NBM IS INDICATING A VERY HIGH PROBABILITY (>90%) OF
HIGHS SURPASSING 35 DEGREES ACROSS THE ENTIRE FORECAST AREA.
HOWEVER THE HIGHEST PROBS OF HIGHS > 40 SHIFT TO THE SOUTH AND
ARE CONFINED MAINLY TO LOCATIONS SOUTH OF PIT. THESE
TEMPERATURES WILL GREATLY LIMIT THE THREAT FOR ACCUMULATIONS
OVER THE NORTH IF THEY DO REMAIN ALL SNOW.
SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/
KEY MESSAGES:
- SCATTERED LAKE ENHANCED SNOW SHOWERS TONIGHT.
- COLDER TEMPERATURES ON THURSDAY.
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NORTHWEST WINDS BEHIND THE FRONT WILL THEN SUPPORT SCATTERED
LIGHT LAKE-EFFECT SNOW SHOWERS OVERNIGHT INTO EARLY THURSDAY
MORNING, PRIMARILY NORTHEAST OF PITTSBURGH AND IN THE RIDGES.
LITTLE OVERALL ACCUMULATION IS EXPECTED, GENERALLY A DUSTING TO
PERHAPS A HALF INCH. ALL PRECIPITATION ENDS ON THURSDAY AS HIGH
PRESSURE BUILDS INTO THE AREA, THOUGH COLD ADVECTION BEHIND
WEDNESDAY'S FRONT WILL FAVOR MORE SEASONABLE TEMPERATURES AND
CONTINUED CLOUD COVER.
A DEEPER TROUGH WILL MOVE THROUGH THE GREAT LAKES ON FRIDAY WITH
A SURFACE LOW MOVING ALONG ITS SOUTHERN FLANK. THIS WILL
INCREASE THE RISK FOR SNOW SHOWERS ON FRIDAY. ACCUMULATIONS LOOK
TO BE LIGHT. NBM PROBS OF > 1 INCH ARE GENERALLY LESS THAN 20%
OUTSIDE OF THE RIDGES. PROBS > 1 INCH OVER THE HIGHER
ELEVATIONS ARE RUNNING 50 TO 70%.
LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/
KEY MESSAGES:
- HIGH PROBABILITY FOR TEMPERATURES AROUND 10 DEGREES BELOW
NORMAL FOR THE WEEKEND.
- SNOWFALL POTENTIAL CONTINUES TO APPEAR LOW END, WITH THE
THREAT FOR ANY IMPACTS LIKELY LIMITED TO NEAR LAKE ERIE.
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UPPER LEVEL TROUGH WILL LIKELY REMAIN OVER THE AREA THROUGH AT
LEAST SATURDAY. THIS WILL SUPPORT BELOW AVERAGE TEMPERATURES
THROUGH THE WEEKEND.
MODEL VARIATIONS REMAIN FAIRLY HIGH ON THE OVERALL DEPTH OF THE
TROUGH AS WELL AS THE SHORTWAVE MOVEMENT WITHIN IT; THESE
FACTORS WILL PLAY LARGER ROLES IN HOW ACTIVE LAKE EFFECT SNOW
DEVELOPS AND THE BOUNDARY LAYER FLOW ANGLE FOR THOSE BANDS.
DEEPER/COLDER LEADS TO MORE EFFECTIVE SNOW BANDS (MEANING
GREATER ACCUMULATION POTENTIAL FOR FAR NORTHWEST PA AND THE
HIGHER TERRAIN) WHILE LESS DEEP/SLIGHTLY WARMER KEEPS THE REGION
DRIER AND LEADS TO MORE INCONSEQUENTIAL SNOWFALL TOTALS. STAY
TUNED AS SMALL-SCALE VARIATIONS FOR THIS WEEKEND PERIOD WILL
DETERMINE IF IMPACTS MAY DEVELOP OR NOT.
AVIATION /14Z WEDNESDAY THROUGH SUNDAY/
VFR IS EXPECTED THROUGH MUCH OF THE MORNING BEFORE WIDESPREAD
MVFR/IFR RESTRICTIONS IN RAIN. ONSET TIME OF PRECIPITATION
CONTINUES TO TREND A BIT SLOWER THAN ORIGINALLY THOUGHT DUE TO
EVAPORATION IN A PRONOUNCED DRY SUB-CLOUD LAYER WHICH IS VISIBLE
IN THE PIT 12Z SOUNDING. LATEST ADJUSTMENT HAS RAIN BEGINNING
AT ZZV AROUND 13Z, AND MOVING INTO PIT AFTER 15Z. MVFR TO IFR
CIGS WILL FOLLOW RAIN ONSET AND PERSIST INTO THE OVERNIGHT
BEFORE GRADUALLY LIFTING NEAR 06Z.
COLD FRONTAL PASSAGE WILL SUPPORT A TRANSITION TO SNOW NORTH OF
PITTSBURGH DURING THE AFTERNOON. HOWEVER, LIMITED REMAINING
MOISTURE WILL CURTAIL ACCUMULATION TO A DUSTING OR PERHAPS A FEW
TENTHS OF AN INCH. STILL, FURTHER REDUCTION IN VISIBILITIES
WILL BE POSSIBLE DURING INSTANCES OF SNOW/RAIN MIX COMPARED TO
THOSE WITH JUST RAIN.
WIDESPREAD PRECIP DEPARTS AROUND 00Z,THOUGH LINGERING LAKE-
EFFECT SNOW SHOWERS MAY CONTINUE INTO THE OVERNIGHT HOURS FOR
AREAS AROUND AND NORTH OF PIT BEFORE ENDING COMPLETELY BY
DAYBREAK THURSDAY. ENSEMBLE MODELS REMAIN BEARISH FOR A REPRIEVE
OF MVFR CIGS (MOST TERMINALS WITH 50-80% PROBABILITY OF
PERSISTENCE) UNTIL THURSDAY EVENING.
OUTLOOK...
AN APPROACHING LOW PRESSURE SYSTEM OUT OF THE MIDWEST WILL
LIKELY KEEP RESTRICTION IN THE REGION THROUGH FRIDAY. NUMEROUS
ENSEMBLE MODELS SUGGEST THE PRECIP TYPE WITH THIS PASSING SYSTEM
WILL BE MOSTLY SNOW.
PBZ WATCHES/WARNINGS/ADVISORIES
PA...NONE.
OH...NONE.
WV...NONE.
SYNOPSIS...22
NEAR TERM...22/MLB
SHORT TERM...CERMAK/22/FRAZIER
LONG TERM...CERMAK/FRAZIER
AVIATION...CERMAK/RACKLEY
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