685
FXUS61 KCTP 180823
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
323 AM EST SAT JAN 18 2025
SYNOPSIS
HIGH PRESSURE WILL DRIFT EAST OVERNIGHT. A COLD FRONT WILL
PUSH THROUGH THE STATE LATE SATURDAY, PRECEDED AND ACCOMPANIED
BY A PERIOD OF MAINLY SNOW (NORTH) AND RAIN AND WET SNOW
(SOUTH).
A DEEP UPPER LEVEL TROUGH WILL PIVOT EASTWARD INTO PENNSYLVANIA
SUNDAY, BRINGING ANOTHER CHANCE FOR SNOW ACROSS AT LEAST THE
EASTERN HALF OF PA, FOLLOWED BY DANGEROUS COLD FOR SEVERAL DAYS
NEXT WEEK.
NEAR TERM /UNTIL 8 AM THIS MORNING/
THIN HIGH CLOUDS ARE GOING TO THICKEN UP THROUGH THE NIGHT.
TIMING ON LATEST 00Z NAM IS SOLID WITH THE ARRIVAL OF PRECIP IN
THE FORECAST ALREADY, AND SIMILAR TO THE GFS. THUS, NO TWEAKS
ARE NECESSARY THERE. EXPECT JUST A FEW SNOWFLAKES WEST OF RTE
219 BEFORE DAYBREAK.
SHORT TERM /8 AM THIS MORNING THROUGH 6 PM SUNDAY/
CLOUD BASES WILL LOWER QUICKLY FROM THE WEST SAT MORNING WITH A
PERIOD OF LIGHT SNOW, THEN LIGHT RAIN SPREADING ACROSS THE
SOUTH AND PERIODS OF MAINLY SNOW ACROSS THE REGION NORTH OF
I-80.
THE PROGRESSIVE, SOUTHERN AND NORTHERN STREAM SHORTWAVES AHEAD
OF THE ARCTIC FRONT WILL WEAKLY PHASE AS THEY CROSS THE MID
ATLANTIC REGION ON SATURDAY, WHICH WILL KEEP PRECIP RATES AND
SNOWFALL GENERALLY QUITE LIGHT. LIQ EQUIV OF 0.15-0.25 OF AN
INCH NW AND JUST A FEW TO SEVERAL HUNDREDTHS ACROSS THE SE
ZONES.
THIS WILL EQUATE TO 1-2 INCHES OF SNOW ACCUM ACROSS THE NORTHERN
MTNS (ESP NEAR AND TO THE WEST OF KBFD) AND LESS THAN ONE INCH
FOR THE MOST PART ALONG THE I-80 CORRIDOR.
MIXED PRECIP ACROSS THE SOUTH WILL PREVENT ANYTHING MORE THAN A
COATING OF SNOW IN THE MORNING.
PRECIP PATTERN BECOMES LIGHT, MIXED, SHOWERY AND FRAGMENTED
LATE SAT AND SAT NIGHT AS BEST BEST SYNOPTIC FORCING AND LOW-
MID LEVEL WARM ADVECTION SCOOTS OFF TO THE NE LEAVING WET BULB
TEMPS HOVERING AROUND ZERO C THROUGH EARLY SAT NIGHT, BEFORE
TEMPS BEGIN TO MAKE A STEADY AND STEEPER DROP LATER SAT NIGHT.
MAX TEMPS SATURDAY WILL RANGE FROM THE LOW 30S NEAR THE PA/NY
BORDER TO THE LOW-MID 40S IN THE SCENT VALLEYS. LOWS AT DAYBREAK
SUNDAY WILL VARY FROM THE LOW TO MID TEENS IN THE NW TO THE MID
AND UPPER 20S IN THE SE.
LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/
SUNDAY: CONFIDENCE CONTINUES TO INCREASE FOR A PERIOD OF
ACCUMULATING SNOW ACROSS THE SOUTH AND EAST SUNDAY AS A WAVE OF
LOW PRESSURE DEVELOPS ALONG THE THERMAL BOUNDARY ALONG THE NORTH
CAROLINA COASTLINE. THERE IS HIGH CONFIDENCE IN TIMING (START
AROUND DAYBREAK SUNDAY AND OVER SHORTLY AFTER SUNSET SUNDAY
NIGHT). THERE IS MUCH LOWER CONFIDENCE IN WHERE AND HOW MUCH
SNOW WILL OCCUR IN THE HEAVIEST BAND. GIVEN HOW BROAD THE TROUGH
IS, THERE IS ABOVE NORMAL UNCERTAINTY IN THE EXACT PLACEMENT OF
THE AXIS OF HEAVIEST SNOW. HAVE ADOPTED A MIDDLE OF THE PACK
SOLUTION FOR THIS FORECAST CYCLE WITH THE HEAVIEST SNOW FALLING
ALONG AND SOUTHEAST OF THE I-81 CORRIDOR WITH A SWATH OF 6"+
EXPECTED AT THIS TIME.
THE REASONABLE WORST CASE SCENARIO (IN LINE WITH THE GFS/GEFS
AND CANADIAN) SHOWS A DEEPENING SURFACE LOW TRACKING CLOSE TO
THE COAST AND GENERATING AN EFFICIENT BAND OF FRONTOGENESIS
ALONG THE I-95 CORRIDOR WITH HEAVY BANDING JUST TO THE NORTHWEST
OF THERE. QPF VALUES APPROACHING 0.75" WITH RATIOS OF 13-15:1
WOULD EQUATE TO A NARROW STRIPE OF TOTALS EXCEEDING 8". A
REASONABLE BEST CASE SCENARIO (IN LINE WITH THE ECENS AND NAM)
WOULD BE A LESS AMPLIFIED LOW THAT CROSSES A BIT FARTHER FROM
THE COAST. SUCH A SCENARIO WOULD RESULT IN LOWER END TOTALS
(PERHAPS NO MORE THAN 3" IN ANY PORTION OF CENTRAL PA).
A CONSENSUS OF GUIDANCE AT THIS POINT LEADS TO A CONCLUSION THAT
THE MOST LIKELY PLACEMENT OF HEAVY SNOW WILL BE IN THE LOWER
SUSQUEHANNA VALLEY, WHICH PROMPTED ISSUANCE OF A WINTER STORM
WATCH FROM 7AM TO 9PM SUNDAY. AS THE SYSTEM COMES INTO FOCUS
OVER THE NEXT 12 TO 24 HOURS, IT IS NEARLY A GUARANTEE THAT
THERE WILL BE MODIFICATIONS - AND POTENTIALLY SIGNIFICANT ONES -
TO THE FORECAST. FRONTOGENESIS BANDING IS QUITE DIFFICULT TO
FORECAST AT THESE LEAD TIMES AND IT IS LIKELY THAT THERE WILL BE
A VERY SHARP GRADIENT IN SNOWFALL TOTALS WITH NORTHWARD EXTENT.
OUR CURRENT FORECAST BRINGS THE 4" CONTOUR UP TO THE I-99/I-80
CORRIDOR WITH LESSER AMOUNTS FARTHER NORTH, BUT WOULD EXPECT THE
GRADIENT OF TOTALS TO TIGHTEN UP IN THE COMING FORECAST CYCLES.
SUNDAY NIGHT INTO LATE WEEK.
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
-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 /08Z SATURDAY THROUGH WEDNESDAY/
VFR CONDITIONS WILL PREVAIL FOR SEVERAL MORE HOURS THIS MORNING
(ACROSS THE WESTERN HALF OF PA) AND LIKELY FOR ANOTHER 6-7 HOURS
(UNTIL AROUND 15Z) NEAR AND TO THE EAST OF A LINE FROM KIPT TO
KSEG AND KMDT.
THICKENING HIGH CLOUD SHIELD WILL LOWER TO BELOW 10 KFT AGL
DURING THE MID MORNING HOURS WITH MVFR THEN IFR CIGS AND VSBYS
SPREADING INTO THE WEST BETWEEN 13Z AND 16Z RESPECTIVELY.
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 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.
INCREASING SOUTH TO SOUTH-SOUTHWEST WINDS ALOFT, TOPPING RELATIVELY
LIGHT SURFACE WINDS WILL CREATE A FAVORABLE SETUP FOR FAIRLY
WIDESPREAD AND MODERATELY STRONG LOW-LEVEL WIND SHEAR.
CLOUDS WILL LOWER AND THICKEN DURING THE DAY TODAY FROM WEST TO
EAST. 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 ARE LIKELY IN
SNOWFALL, 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...
SAT...RAIN/SNOW MIX AND EVENING FOG POSSIBLE IN SOUTHEAST PA.
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...LAMBERT/DANGELO
NEAR TERM...LAMBERT/DANGELO
SHORT TERM...LAMBERT/DANGELO
LONG TERM...BANGHOFF
AVIATION...LAMBERT
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