172
FXUS61 KPBZ 211232
AFDPBZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
732 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.
--------------------------------------------------------------------
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.
--------------------------------------------------------------------
BY SUNDAY, HIGH PRESSURE IS EXPECTED TO BE JUST TO THE NORTH OF
THE FORECAST AREA AS THE CORE OF THE COLDEST AIR FINALLY
DESCENDS ON 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.
--------------------------------------------------------------------
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 /12Z SATURDAY THROUGH WEDNESDAY/
SHALLOW MIXING OVERNIGHT WITH STEEPENING LOW LEVEL LAPSE RATES
ALLOWED FOR A SCATTERING OF THE CLOUD DECKS TO MORE OF A LINEAR
CONVECTIVE NATURE WITH LIGHT SNOW SHOWERS. LATEST TRENDS ARE
POINTING TOWARD A MORE PREDOMINANT VFR SOLUTION TODAY OUTSIDE OF
SNOW SHOWERS IN WHICH RESTRICTIONS ARE LIKELY. THERE IS HIGH
CONFIDENCE IN A MORE INTENSE NORTH/SOUTH ORIENTED SNOW BAND SETTING
UP SOMEWHERE IN EASTERN OH, THE WV PANHANDLE, AND/OR INTO FAR
WESTERN PA. PLACEMENT OF THIS BAND STILL REMAINS LOW CONFIDENCE WITH
SPREAD EXHIBITED IN THE HIGH-RESOLUTION MODEL GUIDANCE. WITHIN THE
BAND, SHOULD IT HAVE DIRECT IMPACT ON A TERMINAL, AT LEAST TEMPORARY
IFR IS LIKELY IN SNOW AS HI RES ENSEMBLE PROBABILITY FOR VIS <1SM AT
SOME POINT WITHIN A SIX HOUR PERIOD THIS MORNING IS +80%; THIS EXACT
TIMING UNCERTAINTY IS REPRESENTATIVE OF THE GUIDANCE STRUGGLING TO
HONE IN ON A MOST LIKELY SOLUTION. THE HIGHEST CHANCES 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 TIMING AND LOCATION OF IMPACTS.
NORTHERLY WIND WILL REMAIN ELEVATED TODAY WITH GUSTS TO 20 KNOTS AT
MAJORITY OF TERMINALS, ESPECIALLY NEAR THE APPALACHIANS. SNOW
CHANCES AND SURFACE WIND DROP OFF TONIGHT WITH HIGH PRESSURE
SETTLING IN DIRECTLY OVERHEAD.
OUTLOOK...
VFR UNDER THE INFLUENCE OF HIGH PRESSURE IS FAVORED
SUNDAY INTO MONDAY BEFORE THE NEXT UPPER LEVEL SYSTEM CROSSES INTO
THE 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...SHALLENBERGER
AVIATION...MLB/LUPO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page