949
FXUS61 KCTP 280821
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
421 AM EDT SUN JUN 28 2026
WHAT HAS CHANGED
INCREASING CONFIDENCE IN A LONG LASTING HEAT WAVE TUESDAY INTO
INDEPENDENCE DAY/4TH OF JULY
KEY MESSAGES
1) SIGNIFICANT, PROLONGED, AND DANGEROUS HEAT WAVE EXPECTED
FROM THE LAST DAY OF JUNE INTO INDEPENDENCE DAY/4TH OF JULY
HOLIDAY WEEKEND
DISCUSSION
KEY MESSAGE 1: SIGNIFICANT, PROLONGED, AND DANGEROUS HEAT WAVE
EXPECTED FROM THE LAST DAY OF JUNE INTO INDEPENDENCE DAY/4TH OF
JULY HOLIDAY WEEKEND
CONFIDENCE AND PREDICTABILITY REMAINS HIGH FOR A LARGE AND
ANOMALOUS 590+DM UPPER LEVEL RIDGE TO DELIVER A SIGNIFICANT,
MULTIDAY, AND POTENTIALLY DANGEROUS HEAT WAVE FROM THE LAST DAY
OF JUNE (TUESDAY 6/30) INTO INDEPENDENCE DAY/4TH OF JULY HOLIDAY
WEEKEND.
WIDESPREAD TEMPERATURES IN THE 90S TO 100F, COMBINED WITH HIGH
HUMIDITY, WILL RESULT IN FCST MAX HEAT INDICES IN THE 100-110F
RANGE WITH THE HIGHEST VALUES IN THE LOWER ELEVATIONS TO THE
EAST OF THE ALLEGHENY MOUNTAINS. THIS EQUATES TO WIDESPREAD
MAJOR TO EXTREME HEATRISK AND HIGH PROBABILITY OF HEAT WWA
HEADLINES FROM WEDNESDAY THROUGH FRIDAY OR SATURDAY.
(CONSIDERATION WAS GIVEN TO AN EXTREME HEAT WATCH WITH THIS
CYCLE PER COORDINATION WITH PHI). OVERNIGHT LOWS WILL ALSO BE
QUITE WARM, BRINGING LITTLE TO NO RELIEF FROM THE HEAT IN THE
NIGHTTIME HOURS.
THE GREATEST FORECAST CHALLENGE (HIGHEST UNCERTAINTY/LOWEST
CONFIDENCE) CENTERS AROUND RING OF FIRE CONVECTION THAT COULD
SPILL SOUTHEASTWARD AROUND THE NORTHERN PERIPHERY OF THE UPPER
RIDGE INTO THE AREA LATE IN THE WEEK.
---------------------------------------------------------------
IN THE SHORTER TERM, LOW STRATUS AND FOG EARLY THIS MORNING WILL
GIVE WAY TO MOSTLY CLOUDY CONDITIONS INTO THE AFTERNOON WITH
SPOTTY SHOWERS/THUNDERSHOWERS POSSIBLE. HREF/REFS ARE KEYING ON
SOUTHWEST PA INTO THE LAUREL HIGHLANDS WITH THE HIGHEST PRECIP
PROBS BETWEEN 8AM-8PM. SOME CLEARING EXPECTED OVERNIGHT WHICH
ALLOWS FOR AREAS OF FOG TO DEVELOP AGAIN INTO EARLY MONDAY
MORNING.
AVIATION /07Z SUNDAY THROUGH THURSDAY/
WINDS HAVE STARTED TO DECREASE SLIGHTLY ACROSS THE FORECAST
AREA THIS EVENING, WHICH WILL PROMOTE FAIRLY EXPANSIVE LOW CLOUD
AND/OR FOG FORMATION ACROSS CENTRAL PENNSYLVANIA. BASED ON THE
12Z HREF 25TH/50TH PERCENTILE MEMBERS OUTLINE ALL AIRFIELDS
OUTSIDE OF IPT WITH IFR-TO-LIFR CONDITIONS. MAIN REASON FOR
LEANING TOWARDS THESE MEMBERS REMAINS TO BE AMPLE LOW-LEVEL
MOISTURE CURRENTLY IN PLACE ACROSS THE ENTIRE REGION AND BASED
ON RAINFALL THAT HAS OCCURRED ACROSS THE AREA EARLIER. RECENT
GLAMP GUIDANCE HONES ONTO THIS TREND, THUS MODERATE-TO-HIGH
(60-80%) CONFIDENCE IN THESE CONDITIONS WITH TIMING BEING THE
MAIN UNCERTAINTY AT THIS TIME. GLAMP GUIDANCE REMAINS FAIRLY
SPARSE IN MENTIONS AT IPT; HOWEVER, HAVE INCLUDED LOWER CEILINGS
DUE TO CURRENT DEW POINT DEPRESSIONS ~6F AND HEAVIEST RAINFALL
ALONG THE I-80 CORRIDOR BUT SLIGHTLY LOWER (~40-60%) CONFIDENCE.
FOG/LOW CLOUD BEGIN TO MIX OUT FAIRLY QUICKLY AFTER SUNRISE ON
SUNDAY, WITH THE BULK OF IFR/LIFR MENTIONS OUT BY 15Z SUNDAY
WITH LIGHT WINDS BELOW 10 KNOTS. MAIN CONCERN IN THE LATER HALF
OF THE TAF PACKAGE WILL BE SOME POTENTIAL FOR SHRA/TSRA ACROSS
MAINLY THE SOUTHERN HALF OF THE FORECAST AREA AFTER 18Z. THE 00Z
TAFS GO AGAINST GLAMP GUIDANCE TO INCLUDE SOME MENTIONS AT ALL
AIRFIELDS OUTSIDE OF BFD/IPT WITH LOW-TO-MODERATE (30-50%)
CONFIDENCE GIVEN THE SCATTERED NATURE EXPECTED OF SHRA/TSRA. THE
MOST RECENT HREF 1-HR LIGHTNING GUIDANCE DOES INDICATE LOWER
(LESS THAN 30%) PROBABILITIES FOR TSRA, SO HAVE KEPT MENTIONS
OUT AT THIS TIME.
OUTLOOK...
MON-THU...AM FOG; OTHERWISE VFR. TRENDING HOTTER WITH ISOLD PM
T-STORMS PSBL. HIGH DENSITY ALTITUDE CONCERNS POSSIBLE.
CLIMATE
TRIPLE DIGIT MAX TEMPS ARE FORECAST IN THE LOWER SUSQ VALLEY
(LSV) ON THURSDAY AND FRIDAY BOOKENDED BY UPPER 90S WEDNESDAY
AND SATURDAY.
THE LAST TIME HARRISBURG OBSERVED CONSECUTIVE DAYS WITH MAXT
GREATER THAN OR EQUAL TO 100F WAS BACK IN 2011 ON JULY 21-22.
THE RECORD NUMBER OF CONSECUTIVE DAYS WITH MAXT >= 100F IS 3
AND HAS OCCURRED 5 TIMES: (ALL IN THE MONTH OF JULY)
1999-07-16 TO 1999-07-18
1999-07-04 TO 1999-07-06
1991-07-19 TO 1991-07-21
1966-07-02 TO 1966-07-04
1936-07-09 TO 1936-07-11
THE LAST TIME HARRISBURG HIT THE CENTURY MARK WAS JULY 16TH IN
2024. PRIOR TO THAT, IT WAS JULY 19TH IN 2020.
HARRISBURG AVERAGES 1 DAY ANNUALLY WITH A MAXT >100F. THE MOST
NUMBER OF DAYS OBSERVED IN A YEAR IS 6 IN 1999.
CTP WATCHES/WARNINGS/ADVISORIES
NONE.
WHAT HAS CHANGED...STEINBUGL
KEY MESSAGES...STEINBUGL
DISCUSSION...STEINBUGL
AVIATION...BEATY
CLIMATE...STEINBUGL
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