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Area Forecast Discussion
Issued by NWS State College, PA
Issued by NWS State College, PA
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349 FXUS61 KCTP 081851 AFDCTP Area Forecast Discussion National Weather Service State College PA 251 PM EDT Mon Jul 8 2024 .SYNOPSIS... * Dog days of summer continue as heat and humidity persists * Maximum heat index values >100F are likely in parts of south Central PA and the Susquehanna Valley again today through Wednesday afternoons * The best chance of showers and thunderstorms will be later Wednesday as deep, tropical moisture gets funneled north between the remnants of Beryl to the west and a strengthening Bermuda High to the east. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A Heat Advisory remains in effect for the Lower Susq Valley today through Tuesday evening, where afternoon heat index values are expected to exceed 100F today and tomorrow. Diurnally driven, isolated to scattered showers and thunderstorms are possible this afternoon into early this evening as weak troughing develops after the departure of the ridge of high pressure that`s been over the region the past 24 hours. The easterly/southeasterly flow on the back side of the departing high will bring an increase of low level moisture/humidity back to Central Pennsylvania. A compact, moderately strong mid/upper level swrly wind max (and weak 700 mb thermal trough) crossing NW PA and Southern New York later today should increase the chc for at least SCT showers/TSRA as much of the CWA will experience a several hour period of MESO-B scale UVVEL associated with the passage of the thermally direct, right entrance region at the time of (and just after) max heating. Highs today will range from the mid 80s in the northern tier to middle or upper 90s in the south. Dewpoints in the mid to upper 60s will make it feel a bit muggy, but will remain tolerable relative to building humidity later in the week. && .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... Increasing clouds are likely tonight as a result of the combination of wind flow and positive moisture flux. As a result, lows on at daybreak Tuesday will be a bit milder in the 65-75F range (+10-15F above climo), making it feel quite muggy once again as dewpoints remain in the same range. Fog will be possible in locations that get rain Monday afternoon and evening, but clouds and southeast flow should limit radiational cooling and the extent/thickness of the fog. Lowered shower/storm chances for Tuesday given neutral to positive height tendencies as the remnants of Beryl and a mid- latitude trough slowly phase in the Mississippi Valley. Will still see some terrain-induced storms pop along with additional convection along any remaining/subtle outflow boundaries, but large scale forcing looks negligible (compared to Monday afternoon) at this time. Isolated showers/storms seem like the most likely scenario. Most of the area south/east of the I-80/I-99 corridor will join the Heat Advisory on Tuesday. Tuesday looks like the hottest day of the week for most of Central PA as temperatures approach the century mark in parts of south central PA. Dewpoints again in the 60s will combine with these hot temperatures to push heat index values near or above 100F Tuesday afternoon. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... There is a MRGL risk for severe weather and a SLGT risk for excessive rainfall on Wed as the remnants of Tropical Cyclone Beryl pass to our west. The risk for river flooding is low, given favorably low antecedent streamflows and expected rainfall amounts of less than 2 inches on average. The latest forecast cone from NHC brings the center of Beryl through the Ohio Valley as it weakens to a post-tropical depression and gets absorbed by a mid-latitude trough. The eastern edge of the forecast cone does clip Warren and McKean counties as the circulation makes its closest approach late Wed. Even if the remnant circulation passes well to our west, the gradient between the low pressure system and strengthening high pressure over Bermuda will result in a conveyor belt of deep tropical moisture with PWAT progged to exceed 2.00 inches over much of central PA between Wed afternoon and Thursday. Also of note, the GFS is showing the freezing level between 13-15kft Wed-Thu, suggesting efficient warm rain processes. The highest PoPs look to be Wednesday afternoon into Wed night, although we will likely see continued chc/slight chc PoPs right into next weekend (peaking with the daytime heating each aftn/eve). A reasonable worst case scenario for 24 hr rainfall is 2 to 3 inches from Wed-Thu afternoon, as per the NBM 90th percentile. The most likely scenario brings 0.75-1.25 inch of rain across much of central PA, with locally higher amounts possible in training convection. SPC mentions a risk of both damaging winds and isolated tornadoes in their discussion for Wed. The combination of 30 kts of 0-3km shear, MLCAPE > 1000 K/kg, and LCLs between 500-1000m is favorable for severe weather. The amount of low level SRH, or veering of the flow in low levels of the atmosphere, will depend on the exact track of Beryl`s remnants. WPC has expanded the SLGT risk of excessive rainfall to cover the northern 2/3 of PA. Farther south, where conditions over the past few months have been much drier, any rainfall coming this week will largely be beneficial. In terms of heat and humidity, the extended period looks to feature a continuation of the warm and humid weather we`ve been seeing recently. Heat index values will have a shot at the low 100s again Wednesday in the lower Susq valley, especially if clouds/precip hold off until later Wed afternoon or Wed night. Although Wednesday will not get quite as hot as earlier in the week, dewpoints surging into the low to middle 70s will make for a sultry afternoon. This humidity will also help keep low temperatures extremely mild during the overnight hours through the end of the week. See the climate section for more details about potential record overnight warmth (and a notable streak of temperatures at or above 70F at Harrisburg). A shot of somewhat less humid air (upper 50s to low 60s dewpoints) appears possible behind the remnants of Beryl late Thursday into Friday across western zones, but the tropical moisture will be slow to move out of the east. A chance of at least scattered diurnal convection will continue through at least Saturday, maximized over the eastern half of PA, with PoPs decreasing to just a slight chance for most of the CWA Sun-Mon. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... The rest of today looks VFR with sct to bkn cu forming with bases at or above 5000 ft. There is a slight chance for a pop up thunderstorm or two later this afternoon into this evening, but coverage of these storms will remain limited. Tonight will look very similar to last night with patchy valley fog developing across portions of Pennsylvania, mainly in the north. A few early morning visibility restrictions are possible for Tuesday morning with this fog. Much of Tue will remain VFR, with scattered showers/storms in the afternoon and evening. Outlook... Wed-Thu...More numerous SHRA/TSRA causing restrictions Fri-Sat...Scattered mainly afternoon showers/storms, otherwise VFR && .CLIMATE... A prolonged stretch of very mild nights is in store thanks to anomalous moisture in Central PA. The following minimum temperatures records are in jeopardy of being tied or broken at Harrisburg and Williamsport this week. HARRISBURG WILLIAMSPORT DATE HIGHEST MIN T YEAR HIGHEST MIN T YEAR July 9 76 1994 July 10 77 1993 74 1925 July 11 76 1989 72 1910, 1921 July 12 75 1911, 2011 The duration of warmth may also end up being notable. Harrisburg has not dropped below 70 degrees since July 3rd. Here are the longest stretches with temperatures at or above 70 on record at Harrisburg. # OF DAYS DATES YEAR 17 7/19-8/04 2020 13 8/16-8/28 2021 13 7/13-7/25 2010 12 7/18-7/29 2022 12 7/17-7/28 1999 12 7/20-7/31 1940 11 8/04-8/14 1977 10 8/10-8/19 2016 10 7/14-7/23 2013 10 7/21-7/30 1898 9 12 times, most recently 7/7-7/11/2020 ................................................. 4+ 7/04-?? 2024 && .CTP WATCHES/WARNINGS/ADVISORIES... Heat Advisory from noon to 8 PM EDT Tuesday for PAZ026>028-035- 046-049>053-058. Heat Advisory until 8 PM EDT Tuesday for PAZ036-056-057-059- 063>066. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert/Banghoff SHORT TERM...Guseman/Lambert/Banghoff LONG TERM...Evanego/Colbert AVIATION...Colbert/Bowen CLIMATE...Banghoff