074
FXUS61 KCTP 181221
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
721 AM EST SAT JAN 18 2025
SYNOPSIS
*CLOUDY AND NOT AS COLD TODAY WITH PERIODS OF LIGHT SNOW/RAIN
*QUICK-HITTING, PLOWABLE TO LOCALLY HEAVY SNOWFALL ON SUNDAY
*ARCTIC BLAST AND DANGEROUSLY COLD WIND CHILLS MONDAY-WEDNESDAY
NEAR TERM /THROUGH TONIGHT/
LOW AMPLITUDE/PROGRESSIVE LEAD SHORTWAVE ENERGY AND STRONG LLVL
WARM ADVECTION (MILDER SSW FLOW) WILL DRIVE A PERIOD OF LIGHT
MIXED PRECIPITATION ACROSS CENTRAL PA TODAY. HIRES MODEL DATA
SHOWS RAIN/SNOW MIX SPREADING ACROSS THE NORTHWEST 1/2 TO 3/4 OF
THE CWA BY LATE THIS AFTERNOON PRIMARILY TO THE NORTHWEST OF
THE I-81 CORRIDOR. THERE WILL BE SOME DRY AIR (10+ DEGREE
DEWPOINT DEPRESSIONS) TO OVERCOME INITIALLY WITH SOME WET BULB
EFFECTS ANTICIPATED.
TEMPERATURE PROFILES ARE RATHER MARGINAL WITH SLR>10:1 AND MAXES
RISING ABOVE THE FREEZING MARK INTO THE MID/UPPER 30S TO AROUND
40F THIS AFTERNOON IN MOST LOCATIONS. SNOW RATES ARE EXPECTED
TO REMAIN LIGHT (<0.5 IN/HR) WITH TOTAL ACCUMULATION REDUCED TO
SOME EXTENT BY THE RELATIVELY MILDER AIR THAT WILL MAKE A VERY
BRIEF VISIT TODAY AHEAD OF AN APPROACHING ARCTIC FRONTAL BLAST.
THE HEAVIEST SNOW IS EXPECTED TO FALL PRIOR TO 18Z/1PM WITH A
C-1" FOCUSED OVER THE LAUREL HIGHLANDS AND ACROSS THE NORTHWEST
ALLEGHENIES WHERE RATES/LIFT WILL BE ENHANCED A BIT VIA WEAK
UPSLOPE FLOW. LITTLE TO NO SNOW IS FORECAST FROM THE CENTRAL
RIDGES THROUGH THE MIDDLE AND LOWER SUSQUEHANNA VALLEY.
AN INCREASINGLY DRIER NORTHERLY FLOW WILL RESULT IN DIMINISHING
PRECIP TONIGHT WITH LINGERING LIGHT SNOW OR SNIZZLE (PATCHY
FZDZ) POSSIBLE OVER THE NORTHERN AND WESTERN MTNS. FCST
SOUNDINGS SHOW MOISTURE PROFILES GETTING VERY SHALLOW WITH NO
ICE ALOFT. TEMPS WILL DROP OFF MORE NOTICEABLY OVERNIGHT
FALLING INTO THE LOWER TEENS TO UPPER 20S FROM NW TO SE BY
DAYBREAK SUNDAY.
SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/
*QUICK-HITTING, PLOWABLE TO LOCALLY HEAVY SNOWFALL REMAINS
INCREASINGLY LIKELY ON SUNDAY FOR PORTIONS OF SOUTHERN &
EASTERN PA
CONFIDENCE CONTINUES TO INCREASE FOR A PERIOD OF MODERATE
(PLOWABLE) TO LOCALLY HEAVY SNOW (5-6" OR SO) FOCUSED ACROSS
THE SOUTHERN AND EASTERN PORTIONS OF THE FCST AREA TOMORROW.
THE SNOWFALL WILL BE IN RESPONSE TO A FAVORABLE UPPER JET
STRUCTURE (DIFFLUENT LEFT-EXIT REGION DYNAMICS) AND TIGHTENING
THERMAL AND MOISTURE GRADIENTS ALONG THE LEADING EDGE OF ARCTIC
AIR-MASS. MODELS SHOW 850-700MB FGEN INCREASING THROUGH THE DAY
WHICH EVENTUALLY SETS UP A BAND OF MODERATE TO HEAVY SNOW
PIVOTING OVER SOUTHEAST PA WITH RATES INCREASING TOWARD 1"/HR
IN THE EARLY TO MID AFTERNOON.
WHILE WE CONSIDERED A EXPANSION TO THE WATCH FOR ANOTHER LAYER
OF COUNTIES FARTHER TO THE NORTH & WEST, WE ULTIMATELY DECIDED
NOT TO MAKE ANY CHANGES FOR THIS CYCLE. THE HIGHEST PROBS TO
REACH WARNING CRITERIA REMAIN WELL POSITIONED AND ALIGNED WITH
THE CURRENT WATCH AREA WITH MAX SNOW AMOUNTS PROJECTED IN THE
MIDDLE TO LOWER SUSQUEHANNA VALLEY. WE WILL NEED TO REMAIN
MINDFUL OF THE POTENTIAL FOR A HIGHER INITIAL SNOW "THUMP"
ACROSS THE LAUREL HIGHLANDS AND SOUTH CENTRAL ALLEGHENIES, BUT
THE HREF DATA SHOWS THE BEST RATES OVER THE SOUTHEAST ZONES.
ONE MORE HIRES MODEL CYCLE WILL COINCIDE WITH OPTIMAL LEAD TIME
FOR THE DAYSHIFT TO MAKE FINAL ADVISORY AND POTENTIAL WARNING
UPGRADES LATER TODAY. SLIGHT SHIFTS IN THE SNOW AXIS IS STILL
POSSIBLE WITH A SHARP GRADIENT ON THE NORTHWEST SIDE OF THE
SNOW SWATH LINING UP NEAR THE I80 TO US220 CORRIDOR.
SNOW QUICKLY EXITS BY SUNDAY NIGHT WITH AN ACCUM PERIOD LARGELY
CONTAINED WITHIN A 12HR WINDOW BETWEEN 7AM-7PM. BITTERLY COLD
ARCTIC AIR WILL SPILL INTO THE AREA SUNDAY NIGHT WITH LOW TEMPS
IN THE POSITIVE SINGLE DIGITS TO MID TEENS. WIND CHILLS WILL BE
COLDER IN THE NEGATIVE AND POSITIVE SINGLE DIGITS BY DAYBREAK
MONDAY.
LONG TERM /MONDAY THROUGH FRIDAY/
HIGH CONFIDENCE CONTINUES IN AN IMPRESSIVE STRETCH OF FRIGID
AIR. A REINFORCING SHOT OF BITTERLY COLD AIR ARRIVES SUNDAY
NIGHT INTO MONDAY. TEMPERATURES WILL BOTTOM OUT TUESDAY, WITH
HIGHS RUNNING 20 TO 30 DEGREE BELOW AVERAGE. CONSIDERING THAT
LATE JANUARY IS THE CLIMATOLOGICAL COLDEST TIME OF THE YEAR,
HIGHS THAT FAR BELOW AVERAGE MEANS THAT WE ARE IN FOR A
SIGNIFICANT BOUT OF ARCTIC AIR. TUESDAY AND WEDNESDAY MORNING
WILL BE THE COLDEST OF THE STRETCH WITH WIND CHILLS AS LOW AS
MINUS 15F TO 25F ACROSS THE LAURELS AND NORTHERN MOUNTAINS.
HAVE ISSUED AN EXTREME COLD WATCH FROM 4AM MONDAY THROUGH 12PM
WEDNESDAY FOR LOCATIONS NORTH AND WEST OF THE ALLEGHENY FRONT.
TO PUT THE UPCOMING COLD IN HISTORICAL PERSPECTIVE, IF THE
FORECAST HOLDS, ABOUT 80 HOURS WITH TEMPERATURES IN THE MID
TEENS OR COLDER AT HARRISBURG AND WILLIAMSPORT RIVALS THE COLD
SNAP OF FEB 1996 IN TERMS OF DURATION. SEVERAL YEARS IN THE
1970S FEATURED LONG DURATION COLD AS WELL.
THE UPPER-LEVEL TROUGH WILL BEGIN TO LIFT OUT OF THE AREA BY
LATE NEXT WEEK, WHICH SHOULD RESULT IN SLOWLY MODERATING
TEMPERATURES.
AVIATION /12Z SATURDAY THROUGH WEDNESDAY/
VFR TRANSITIONING TO MVFR ACROSS THE WRN MTN TAF SITES KBFD
SOUTH TO KJST BETWEEN 13-14Z WHILE VFR CONDITIONS PREVAIL
UNTIL 15-16Z FROM KAOO AND KUNV EAST TO THE SUSQUEHANNA VALLEY.
PERIODS OF IFR TO LIFR WILL DEVELOP LATE THIS MORNING ACROSS
THE WESTERN PA MTN AIRFIELDS OF KBFD AND KJST AND WILL STAY IN
THAT LOWER CATEGORY FOR MUCH OF THE AFTERNOON AND EVENING HOURS
SATURDAY.
LLWS (WIDESPREAD AND MODERATELY STRONG) IS EXPECTED (90-100
PERCENT CONFIDENCE) UNTIL AROUND THE TIME THE SNOW OR WINTRY MIX
OF PRECIP STARTS FROM WEST TO EAST RESPECTIVELY DURING THE MID
TO LATE MORNING HOURS.
MAINLY MVFR CEILINGS WILL PERSIST THROUGH THE MID AFTERNOON
HOURS BEFORE CIGS DIP INTO THE UPPER IFR RANGE LATE THIS
AFTERNOON/EVENING. PERIODS OF IFR VISIBILITY EXPECTED IN SNOW
OF LIGHT TO MODERATE INTENSITY, WHICH IS MOST LIKELY FROM THE
BFD VICINITY EAST ACROSS THE NORTHERN MTNS OF PA.
PRECIPITATION TYPE AND DURATION IS A BIT MORE UNCERTAIN AT THE
REST OF THE AIRFIELDS, THUS IMPACTING THE POTENTIAL FOR IFR
VISIBILITY. A COUPLE HOURS OF SNOW FOLLOWED BY A TRANSITION TO
RAIN IS FAVORED AT JST, AOO, UNV AND IPT WITH PRIMARILY RAIN AT
MDT, AND LNS.
OUTLOOK...
TONIGHT...LIGHT RAIN/SNOW MIX AND EVENING FOG POSSIBLE IN
SOUTHEAST PA. LOW STRATUS LAYER AND FOG DEVELOPING TONIGHT AT
KBFD AND KJST AS SHALLOW COLDER AIR PUSHES INTO THE REGION
BEHIND AN ARCTIC FRONTAL BOUNDARY AND THE LLVL FLOW BECOME
ORTHOGONAL WITH THE RIDGES.
SUN...AM LOW CIGS W MTNS AND SOUTHEAST PA. SNOW/IFR CONDITIONS
INCREASINGLY LIKELY FOR AT LEAST THE SOUTHERN AND SOUTHEASTERN
1/2 TO 1/3 OF THE AREA SUNDAY PM.
MON...MVFR TO IFR IN SNOW SHOWERS ACROSS WESTERN PA; GUSTY WEST
WINDS.
TUE-WED...NO SIG WX EXPECTED; VERY COLD.
CTP WATCHES/WARNINGS/ADVISORIES
EXTREME COLD WATCH FROM LATE SUNDAY NIGHT THROUGH WEDNESDAY
MORNING FOR PAZ004>006-010>012-017-018-024-033-037-041-042.
WINTER STORM WATCH FROM SUNDAY MORNING THROUGH SUNDAY EVENING
FOR PAZ036-056>059-063>066.
SYNOPSIS...STEINBUGL
NEAR TERM...STEINBUGL
SHORT TERM...STEINBUGL
LONG TERM...BANGHOFF
AVIATION...LAMBERT
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