172
FXUS61 KPBZ 181802
AFDPBZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
102 PM 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 /THROUGH TONIGHT/
KEY MESSAGES:
- RAIN CHANCES INCREASE THIS MORNING
- SNOW OR A WINTRY MIX IS POSSIBLE NORTH OF PITTSBURGH.
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PRECIPITATION SHIELD CONTINUES TO SPREAD ACROSS THE REGION EARLY
THIS AFTERNOON DOWNSTREAM OF AN UPPER TROUGH ACROSS THE MIDWEST.
RECENT OBSERVATIONS HAVE CONTINUED WITH THE THEME OF RAIN FOR
MOST, SAVE THE I-80 CORRIDOR WHERE WET-BULBING OF 0 TO 1C 850 MB
TEMPERATURES HAS RESULTED IN A COUPLE HOURS OF SNOW. VISIBILITY
HAS COME DOWN TO HALF A MILE AT TIMES IN SNOW, BUT WEBCAMS ARE
NOT SHOWING MUCH, IF ANY, ACCUMULATION OUTSIDE OF GRASSY AND
ELEVATED SURFACES. ROAD TEMPERATURES ARE IN THE MID TO UPPER
30S, SO WITHOUT OVERWHELMING SNOWFALL RATES, EXPECT MINIMAL
IMPACTS. ELSEWHERE, PLAIN RAIN WILL CONTINUE.
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 /THURSDAY 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 WEDNESDAY/
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 /18Z WEDNESDAY THROUGH MONDAY/
CEILINGS AND VISIBILITIES ARE BEGINNING TO TANK AS LIGHT TO AT
TIMES MODERATE RAIN/SNOW CONTINUE ACROSS THE REGION. EXPECT
PREVAILING MVFR/IFR WITH PERIODS OF LIFR IN HEAVIER INSTANCES OF
PRECIPITATION THROUGHOUT THE REMAINDER OF THIS AFTERNOON. PRECIP
DEPARTS TO THE EAST BY 00Z SAVE FOR MGW WHERE IT MAY LINGER AN
HOUR OR TWO LONGER. WITH THAT DEPARTURE, AREA TERMINALS SHOULD
BEGIN TO SEE SLIGHT IMPROVEMENT IN CEILINGS AND VISIBILITIES AS
DRIER POST-FRONTAL AIR MAKES ITS WAY INTO THE AREA, MAINLY AFTER
06Z. THERE IS STILL A CHANCE FOR A FEW SCATTERED LAKE-EFFECT
SNOW SHOWERS TO LINGER NORTH OF PIT, WHICH IS REFLECTED VIA
PROB30S IN THE FKL AND DUJ TAFS UNTIL CLOSER TO 09Z. LOW
CONFIDENCE AND LIMITED SPATIAL COVERAGE PRECLUDE A MENTION IN
PREVAILING GROUPS AT THIS TIME.
HI-RES ENSEMBLE GUIDANCE CONTINUES TO INDICATE AT LEAST 50-80%
PROBABILITIES (CLOSER TO 80-90% ALONG THE RIDGES) FOR MVFR/IFR
CEILINGS TO STICK AROUND AFTER PRECIPITATION ENDS THROUGH THE
REMAINDER OF THIS 24-HR TAF PERIOD.
OUTLOOK...
A BRIEF IMPROVEMENT TO VFR CEILINGS WILL BE POSSIBLE THURSDAY
EVENING, SAVE FOR FKL/DUJ WHERE PROBABILITIES FOR MVFR REMAIN
AROUND 30-50%. THE NEXT APPROACHING LOW PRESSURE SYSTEM BRINGS
RESTRICTIONS BACK TO THE REGION THURSDAY NIGHT AND PRECIPITATION
CHANCES ON FRIDAY. RESTRICTIONS MAY LINGER THROUGH AT LEAST
SATURDAY AS COLD ADVECTION UNDER NORTHWEST FLOW PROMOTES THE
CONTINUATION OF LAKE-EFFECT CLOUDS AND POSSIBLY A FEW SCATTERED
LIGHT SNOW SHOWERS. THE NEXT CHANCE FOR VFR DOESN'T LOOK TO
RETURN UNTIL SATURDAY NIGHT OR SUNDAY.
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
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