615
FXUS61 KPBZ 210904
AFDPBZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
404 AM EST SAT DEC 21 2024
SYNOPSIS
HIGH PRESSURE WILL SLOWLY ENCROACH FROM THE WEST AS NORTHWEST
FLOW WILL BRING LAKE EFFECT SNOW SHOWERS TO THE REGION THROUGH
TONIGHT. AFTERWARD COLDER TEMPERATURES WILL SETTLE IN FOR THE
START OF THE NEW WEEK.
NEAR TERM /THROUGH TONIGHT/
KEY MESSAGES:
- BEST CHANCE OF ACCUMULATING SNOW THROUGH THE MORNING AND INTO
THE COMING DAY LIES IN THE RIDGES WITH DEVELOPING NORTHWEST
FLOW AND PLACEMENT OF BANDED SNOWFALL.
- LAKE EFFECT/UPSLOPE SNOW SHOWERS EXPECTED INTO THIS EVENING.
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HIGH PRESSURE WILL REMAIN OVER THE MIDWEST AS IT SLOWLY TRACKS
EAST WITH TIME. UNTIL THEN, THE EAST SIDE OF THE RIDGE WILL MAKE
FOR FAVORABLE WIND DIRECTIONS THROUGH THIS MORNING AND THE
COMING DAY. THE OTHER FACTOR IN PLACE IS WITH THE COLDER AIR
MASS ON ITS WAY FROM THE NORTH, THUS, THE 41 TO 44 DEGREE WATER
TEMPERATURES OFF OF LAKE ERIE COMPARED TO THE AIR TEMPERATURES
THIS MORNING AND INTO TODAY WITH HIGHS ONLY REACHING THE UPPER
20S TO NEAR 30. CONSEQUENTLY, TO DETERMINE HOW WELL THE BAND OR
THE SNOW SHOWERS HOLD TOGETHER TODAY WILL DEPEND ON THE
DIFFERENCE BETWEEN THE 850MB TEMP AND THE LAKE TEMP. WHICH THE
DIFFERENCE BETWEEN THE LAKE TEMP AND THE 850MB TEMP NEEDS TO BE
MINUS 3 DEGREE CELSIUS OR GREATER FOR EFFICIENT DEVELOPMENT AND
LAKE SNOW CONVECTION. AT 12Z THIS MORNING THE DIFFERENCE IS
AVERAGED TO BE MINUS 10 DEGREES BUT WITH THE COLDER AIR ARRIVING
TODAY, THE AVERAGE DIFFERENCE WILL INCREASE TO MINUS 12 DEGREES
WHICH IS NEAR THE MINUS 13 DEGREE CELSIUS OR LESS THRESHOLD FOR
THE BEST LAKE CONVECTIVE SNOWFALL.
ALL OF THIS SAID, THE BAND WILL BEGIN TO DEVELOP JUST BEFORE
DAWN AND ALONG OR WEST OF THE PA/OH BORDER. HI-RES MODELS
SUGGEST A POSSIBLE 2 BAND SET UP WITH AN EARLY ONE IN THE
CENTRAL OH COUNTIES AND ALONG OR JUST EAST OF THE PA/OH BORDER.
GIVEN THE INSTABILITY IN PLACE, THERE ARE SOME LOCALIZED AREAS
THAT COULD REACH 3 INCHES. THE NATIONAL BLEND OF MODELS GIVES A
10% TO 20% CHANCE OF 3 INCHES OR MORE BUT LIKELY IS NOT HANDLING
THE HI RES DATA VERY WELL COMPARED TO THE HRRR WHERE AMOUNTS ARE
MORE CLOSER TO 1 TO 2.5 INCHES. OF COURSE THIS IS ALL OUT THE
WINDOW IF THE BAND OVER PERFORMS. THUS, TO COVER THE MORNING AND
INTO THE COMING DAY, WILL HIGHLIGHT THE BAND WITH AN SPS UNTIL
ONSET OF HEAVIER SNOW AND LEAVE FURTHER SPS'S OR PERHAPS A SNOW
SQUALL WARNING FOR WHAT TRANSPIRES THROUGH THE DAY.
HEADING INTO TONIGHT, NORTHWEST FLOW WILL CONTINUE THROUGH 00Z
AND THIS WILL KEEP A FEW SNOW SHOWERS CONTINUING THROUGH THE
OVERNIGHT. AS THE NIGHT PROGRESSES, WINDS WILL SLOWLY DECOUPLE
AND BECOME LIGHT AND VARIABLE AS THE CENTER OF THE HIGH
APPROACHES FROM THE WEST. THUS, BY 12Z SUNDAY MORNING, THE LAKE
EFFECT PROCESS SHOULD COME TO AN END.
SHORT TERM /SUNDAY THROUGH MONDAY/
KEY MESSAGES:
- COLD AND DRY SUNDAY AND MONDAY MORNING.
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BY SUNDAY, HIGH PRESSURE IS EXPECTED TO BE JUST TO THE NORTH OF
THE FORECAST AREA AS THE CORE OF THE COLDEST AIR FINALLY
DESCENDSON THE UPPER OH VALLEY. TEMPERATURES IN OUR FAR
NORTHEASTERN COUNTIES ADVERTISE A 60% TO 80% CHANCE OF LESS THAN
10 DEGREES FOR A LOW SUNDAY MORNING. ITS EVEN COLDER FOR MONDAY
MORNING AS THE PROBS IN THE NORTHEAST (FOREST COUNTY) NOW
INCREASES TO AROUND 90% BUT ALSO SHOWING AROUND 40% FOR THE
RIDGES AND SOUTH INTO THE WV COUNTIES. HIGHS ARE EXPECTED TO BE
BELOW NORMAL FOR SUNDAY BUT THE AIR MASS WILL MODIFY
SIGNIFICANTLY AFTER A COLD MORNING ON MONDAY AS HIGHS SOAR INTO
THE LOW 40S. THE NBM ENSEMBLE SUGGESTS A 40% TO 60% PROBABILITY
OF HIGH TEMPERATURES ABOVE 40 DEGREES FOR MONDAY. THIS PROVIDES
QUITE A SHIFT IN TEMPERATURES JUST BEFORE CHRISTMAS.
ENSEMBLE GUIDANCE CONTINUES TO INDICATE FLAT MID-LEVEL RIDGING
MOVING ACROSS THE UPPER OHIO VALLEY ON MONDAY, WITH CONTINUED DRY
WEATHER LIKELY. WARM ADVECTION AND RISING HEIGHTS SHOULD LEAD TO
THE PREVIOUSLY DISCUSSED WARMER TEMPERATURES, WITH MAX VALUES
PERHAPS AROUND 10 DEGREES HIGHER THAN SUNDAY, DEPENDING ON HOW
QUICKLY CLOUDS THICKEN AHEAD OF THE NEXT SYSTEM.
LONG TERM /MONDAY NIGHT THROUGH FRIDAY/
KEY MESSAGES:
- GENERALLY LIGHT PRECIPITATION APPEARS LIKELY MONDAY NIGHT INTO
CHRISTMAS EVE DAY, WITH A MINOR SNOW ACCUMULATION POSSIBLE
NORTH OF PITTSBURGH.
- A WARMING TREND AND LOWER PRECIPITATION CHANCES MAY FOLLOW FOR
CHRISTMAS DAY THROUGH THE END OF THE WEEK.
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A FAIRLY WEAK SHORTWAVE APPEARS MOST LIKELY TO CROSS THE AREA MONDAY
NIGHT INTO CHRISTMAS EVE DAY. MODEL CLUSTERING SHOWS GENERALLY MINOR
DIFFERENCES IN STRENGTH AND TIMING, BUT MOST AGREE THAT QPF WILL
GENERALLY BE LIGHT AND FOCUSED NORTH OF PITTSBURGH CLOSER TO THE
WAVE TRACK. PROBABILITIES OF ACCUMULATING SNOW ARE BEING MODULATED
BY TEMPERATURE, WITH HIGHS LIKELY IN THE MID TO UPPER 30S FROM
PITTSBURGH NORTH AND IN THE RIDGES, AND IN THE LOWER 40S TO THE
SOUTH. THESE LIKELIHOODS RANGE FROM NEAR 10 PERCENT AT ZANESVILLE,
TO 40 PERCENT AT PITTSBURGH, AND 90 PERCENT AT DUBOIS.
PROBABILITIES OF AN INCH OR MORE REMAIN CONFINED TO AREAS NORTH
OF I-80 AND MAX OUT AT AROUND 40-50%. THIS WOULD BE THE AREA
THAT HAS THE BEST CHANCE OF HAVING AN INCH OF SNOW ON THE GROUND
COME CHRISTMAS MORNING, BUT THE ABOVE-FREEZING TEMPERATURE
COULD PLAY A ROLE IN LIMITING SNOW DEPTH.
THEREAFTER, ENSEMBLES CONTINUE TO SHOW GOOD PROBABILITIES OF RIDGING
DURING THE MIDWEEK PERIOD, WHICH WILL CONTINUE THE WARMING TREND
THROUGH CHRISTMAS DAY AND THE REMAINDER OF THE WEEK. TEMPERATURES
COULD PERHAPS RISE TO AROUND 10 DEGREES ABOVE NORMAL BY FRIDAY. A
MINORITY OF ENSEMBLE MEMBERS ARE SHOWING LOWER HEIGHTS/FLATTER
RIDGING, PERHAPS LEADING TO LESS OF A WARMUP. ALONG WITH THE
WARMING, THE MAJORITY MODEL OPINION WOULD LEAD TO FAIRLY LOW CHANCES
OF LIGHT PRECIPITATION AT BEST FOR THE SECOND HALF OF THE HOLIDAY
WEEK.
AVIATION /08Z SATURDAY THROUGH WEDNESDAY/
MODERATE TO HIGH CONFIDENCE REMAINS IN THE MAINTENANCE OF MVFR
DECKS WITH LOW LEVEL COLD ADVECTION IN NORTHERLY FLOW THROUGH AT
LEAST SATURDAY MORNING BEFORE SOME LIFTING IS POSSIBLE IN
MIXING AND SUBSIDENCE COME EARLY AFTERNOON WHEN ENSEMBLE
PROBABILITIES FOR MVFR TAKE A TUMBLE TO <50%. THERE IS HIGH
CONFIDENCE IN A NORTH/SOUTH ORIENTED SNOW BAND SETTING UP
SOMEWHERE IN EASTERN OH, THE WV PANHANDLE, AND/OR FAR WESTERN
PA. PLACEMENT OF THIS BAND STILL REMAINS LOW CONFIDENCE TONIGHT
WITH SPREAD STILL EXHIBITED IN THE HI RES MODEL GUIDANCE. THE
HIGHEST CHANCES OF ANY PORT TO SEE RESTRICTIONS FROM THIS
FEATURE WILL BE BVI WHERE A TEMPO GROUP DOWN TO IFR HAS BEEN
MAINTAINED. OTHERWISE, PROB30S ARE NOTED AT PIT, AGC, AND HLG
FOR NOW. PREDICTABILITY OF THIS BAND WILL BE RATHER SHORT-FUSED
AND UNTIL WE SEE IT TAKE SHAPE, IT WILL BE TOUGH TO PIN DOWN
EXACT IMPACTS. WITHIN THE BAND, AT LEAST TEMPORARY IFR IS
POSSIBLE IN SNOW, BUT ELSEWHERE ESPECIALLY AFTER 18Z, VFR SEEMS
MOST LIKELY WITH THE CONTINUATION OF MIXING AND UPPER
SUBSIDENCE. NORTHERLY WIND WILL REMAIN ELEVATED OVERNIGHT AND
TOMORROW WITH GUSTS TO 20 KNOTS AT 40-60% LIKELIHOOD. SNOW
CHANCES AND SURFACE WINDS DROP OFF TOMORROW NIGHT WITH HIGH
PRESSURE DIRECTLY OVERHEAD.
OUTLOOK...
VFR UNDER THE INFLUENCE OF HIGH PRESSURE IS FAVORED
SUNDAY INTO MONDAY BEFORE THE NEXT UPPER LEVEL SYSTEM CROSSES
THEN REGION LATE MONDAY INTO TUESDAY, BRINGING ANOTHER ROUND OF
LIGHT SNOW AND AREAWIDE RESTRICTIONS.
PBZ WATCHES/WARNINGS/ADVISORIES
PA...NONE.
OH...NONE.
WV...NONE.
SYNOPSIS...SHALLENBERGER
NEAR TERM...SHALLENBERGER
SHORT TERM...SHALLENBERGER
LONG TERM...CL
AVIATION...MLB/MILCAREK
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