Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 170907 AFDCTP Area Forecast Discussion National Weather Service State College PA 507 AM EDT Thu Aug 17 2017 .SYNOPSIS... Warm and increasingly humid air will surge into our region from the southwest this afternoon and tonight as a warm front draped across the Ohio River Valley lifts across the state. Expect showers and thunderstorms, some with locally heavy rain and frequent lightning to become more frequent tonight into early Friday morning. An approaching cold front will spark more thunderstorms during the day Friday, especially across the eastern half of the state where some may become strong to severe. Cooler and drier should improve our weekend weather. A ridge builds in early next week and we should warm up before another northern stream cold front triggers showers again and ushers in cooler drier around mid- week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... GOES-16 10.3-3.9um fog product shows dendritic valley fog taking shape across NRN PA along with the Susq Valley. The fog will become locally dense for several hours during the mid morning as a result of a high pressure ridge right over Central PA providing mainly clear skies, light wind, and quite cool temps in the lower to mid 50s across the nrn mtns (where air/stream temp/dwpt spreads are 20 to 25 deg F. The fog will lift into a thin bkn-ovc stratus deck between 13-15Z before dissolving and partly to mostly sunny skies follow for the midday and afternoon hours as a warm front lift north into the region and some mid and high clouds from upstream TSRA spill to the ESE. Will maintain 05-07Z National Blend of Models POPS for late today and this evening which are in the CHC to low Likely range over the approx western half of the CWA. If anything, these pops may be slightly overdone across the Central Mtns and Susq Valley. Temps will be warmer than Wednesday with more humidity...especially late in the day as the warm front approaches from the south. Highs this afternoon will be mainly in the 80s, though the northern and southwestern mountains could hold in the U70s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... The chance of rain and thunderstorms increases this evening across the SW half of the CWA, and overnight elsewhere as a deep cyclone moves into the western Great Lakes. Several distinct LLJ features will stream north across the CWA during this period ahead of an approaching sfc cold front, and will interact with the Warm Front and approx 2 inch PWATs to support locally very heavy rain in training TSRA. Some of the TSRA will contain frequent lightning and possibly some brief strong wind gusts. Cape actually climbs slightly across southern and central PA late today and through the first half of tonight. This increase in instability to these levels (of several hundred to just over 1000 j/kg during the overnight hours only occurs about once a year, if that here in Central and Ern PA, and is notorious for waking many from a sound sleep during the late night/predawn hours with frequent vivid lightning and booming thunder. with PWAT of 2 inches (or more across the SW part of the CWA) leading to extremely heavy hourly and three hourly rainfl rates across the Central and SW part of our CWA, will have to closely monitor for training TSRA. Hourly FFG is still around 1 inch or less across parts of the Lower Susq Valley, while much of the rest of the region has hourly values of 2 inches or more. Therefore, unless we can foresee a specific target area comprised of both time and duration of potentially repeating TSRA (via later version of the HRRR or other CAMS/mesoscale models), then we probably won`t be posting a FFA with a tremendous amount of lead time in excess of 6-8 hours. Due to high PW and widespread/thick clouds, it will stay warm overnight with min temps only in the low to mid 60s across nrn and wrn PA, and L70s in the southeast. Sfc cold front pushes East to the Susq Valley or Central Mtns between 20-22Z (The prime hours to fire up some locally strong to severe TSRA. A few discrete TSRA/supercells (with some with rotating updrafts) are likely during this period. 0-1KM EHIs climb to between 1-2m2/s2 across parts of central and scentral PA between 02-06Z. Although updrafts may not be rooted right at the sfc, the presence of the aforementioned LLJ S-SSW wind maxes, very high pwat (Low LCLs) and the close presence of the warm front which will add a source for LLVL shear and helicity, there is a threat for rotating updrafts/supercells and even a small chance for a few weak tornadoes. Locally heavy rain, and frequent lightning maybe the biggest threats. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... The GEFS and CMCEFS suggest our PW values drop during the day Friday back to normal as the front is forecast to push to our east. This means improving weather from morning to afternoon. Lowering POPS over time. We should remain relatively dry as the PW values stay near normal until perhaps Tuesday. This said there are hints of a weak wave and scattered showers perhaps Sunday. Tuesday could be rather warm as there is a surge of above normal 850 hPa temperatures and PW values above normal. Not surprisingly the GEFS shows higher POPS Tuesday afternoon into Wednesday with the wave and front. We should enter the second half of the weak in a cooler and drier air mass as currently forecast. And thus in the grids. Bottom line in this period the NCEP guidance suggests low probability of significant QPF. && .AVIATION /08Z THURSDAY THROUGH MONDAY/... Some fog possible late. 06Z TAFS sent. Earlier discussion below. VFR continues late this evening and into the overnight hours with high pressure and large-scale subsidence overhead. Fog impacts are again expected to develop late and continue into Thursday mid-morning - after which much of the day on Thursday should be VFR. Scattered showers will pop up Thursday afternoon in the west and south and move northeastward. The coverage of these showers before 18Z is too low to even mention PROB30s for any site in this package. However, Thursday afternoon into Friday evening hold a much more widespread chance of thunderstorm impacts as a warm front lifts across the area and a cold front follows 24 hours later. .OUTLOOK... Thu night-Fri...SCT-NMRS TSRA impacts likely. Sat-Mon...No sig wx. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Grumm/Lambert LONG TERM...Grumm/Lambert AVIATION...Martin/RXR is the U.S. government's official web portal to all federal, state and local government web resources and services.