A moist low-level air mass north of an approaching warm front resulted in widespread dense fog across portions of northern Delaware starting during the evening of May 15th and continuing into the early morning of May 16th. The fog dissipated after sunrise.
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The episode as NWS PHI wrote it up.
A moist low-level air mass north of an approaching warm front resulted in widespread dense fog across portions of northern Delaware starting during the evening of May 15th and continuing into the early morning of May 16th. The fog dissipated after sunrise.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at May 15, 4:52 PM EST, 2 hours 8 minutes earlier.
Area Forecast Discussion National Weather Service Mount Holly NJ 552 PM EDT Thu May 15 2025 .SYNOPSIS... Low pressure will dissipate through the evening. Brief period of ridging comes in tonight, with a warm front coming through on Friday morning. Unsettled weather with periods of showers, storms, and high humidity continues through Saturday until a strong cold front comes through on Saturday Night. Drier high pressure will settle into the region by Sunday into early next week. && .NEAR TERM /THROUGH FRIDAY/... 552 PM update... No actual changes at this moment, just noting we have resumed local operations following the AWIPS update. 439 PM update... The low clouds and fog from this morning continue to mix out and improve this afternoon. With the mid level trough axis remaining in the vicinity and the weak surface low nearby as well, diurnal instability has resulted in some isolated to scattered showers/storms this afternoon. PoPs were increased slightly across the board for this afternoon. With PWats around 1.25-1.50", heavy downpours will be possible as well as gusty winds and small hail. PoPs are generally around 40-60%, but those that see something could see some urban and poor drainage flooding. A Marginal ERO (1/4) remains across most of the area given potentially slow storm motion and a saturated surface with some areas seeing 2-3" yesterday. Severe thunderstorms are unlikely as shear is pretty weak, but can't rule it out completely. High temperatures will be mainly in the 70s and a bit muggy. A brief period of quiet weather comes tonight as a brief period of ridging moves overhead. Marine stratus is expected to push inland again tonight. Could see some areas of fog yet again, though not expecting it to be as widespread or as dense as this morning. Mainly dry through tonight following the diminishing diurnal convection, but still mild and humid, with lows in the upper 50s/low 60s. For Friday, a decaying MCS from upstream across the Midwest this evening is anticipated to impact the region. The threat of severe hazards will be conditional based on the timing of the MCS. If it comes through very fast and early in the morning, the severe threat will likely be low. However, a slower propagation will yield increasing instability ahead of it, which would support strengthening. These systems do tend to trend faster and further south of what guidance suggests. If a slower evolution comes to fruition, all severe hazards would be possible, mainly focused on locally damaging wind gusts, but some hail or a brief tornado could be possible if all the stars align properly. Brief heavy downpours possible as well. This MCS and the environment ahead of it will need to be monitored closely overnight. The SPC has introduced a Slight Risk for portions of our area to highlight this threat. Depending on how quickly the MCS passes offshore, we could potential for diurnally driven convection developing in the afternoon or early evening as well. A faster/earlier departure of the MCS will yield a greater threat for severe thunderstorms in the afternoon, but a slower/later departure will mean a much lower threat for severe thunderstorms in the afternoon/evening. Something else to keep an eye on as well. In summary, Friday's weather and forecast will be highly mesoscale driven and have low predictability beyond about 6 hours or so. High temperatures should be mostly in the mid 70s to mid 80s, and muggy with depends in the mid to upper 60s. It will be mostly cloudy with light south to southwest winds. Remain alert for additional forecast updates regarding the potential convective threats. && .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... Forcing strengthens with an incoming potent shortwave digging from the Great Lakes Friday night into Saturday. At the surface, an area of low pressure slides by to the north Saturday into Saturday Night, with a cold front advancing towards and eventually through the region. Another decaying MCS may impact the area again Friday night or early Saturday. Any convection Friday night and Saturday could be impacted by this system. Temperatures will climb into the 80s and it will feel quite muggy ahead of the front. Instability looks to be on the order of around 1000 J/kg with relatively favorable shear. Thinking any showers and thunderstorms will come in the afternoon and evening as the front arrives. Main concern would be damaging wind gusts and heavy downpours, but still a good amount of uncertainty being a couple days out, especially given the potential for another early MCS. Coverage in showers and thunderstorms diminishes into the night with the loss of daytime heating and passage of the front. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Sunday through about Tuesday actually looks pretty nice for now. With the front coming through, it should result in dry conditions and seasonable temperatures to close out the weekend and start next week. Perhaps a little gusty on Sunday, with wind gusts near 30 MPH possible, but nothing significant. Next chance for rain comes in the middle of the week with longer range guidance hinting at a potential coastal storm. Still widespread timing and track differences but another spring rainmaker could potentially be on the horizon. Stuck close to NBM given the uncertainty in the ensembles and deterministic guidance. && .AVIATION /22Z THURSDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 00Z...Mostly VFR, but some MVFR ceilings possible at times. Isolated SHRA/TSRA, but have maintained VCSH given isolated coverage. Winds 5 kts or less favoring an E to S direction. Medium confidence overall. Tonight...Most likely VFR to start, but restrictions likely return overnight with low clouds and areas of fog. Low confidence on timing and details. Winds light and variable to calm. Friday...Restrictions likely to start, then improvement to VFR expected between 15-18Z. A complex of SHRA/TSRA is anticipated to impact the area early in the day, but confidence on timing and details is low. Opted for an SHRA with PROB30 for TSRA for now. Details should become more clear in future updates tonight. Winds mainly 5 kts or less, settling out of the SW by 18Z. Additional SHRA/TSRA possible by 21Z. Low confidence overall. Outlook... Conditions expected improve for Friday night and Saturday with prevailing VFR likely during the daytime, aside from any showers and thunderstorms move in. VFR conditions look to come Saturday Night and beyond. A few gusts out of the west/northwest 20-25 kt possible but other than that, no significant weather beyond Saturday Night. && .MARINE... The Marine Dense Fog Advisory and Small Craft Advisory were allowed to expire at 4 PM. Some isolated showers/storms possible this afternoon. For tonight, winds and seas remain below advisory criteria. However, areas of fog will remain and some locally dense fog is possible. Another Dense Fog Advisory may need to be considered in a future update. Outlook... Sub-SCA conditions expected Friday through Monday. However, showers and spotty thunderstorms may continue to be a risk through Saturday, especially during the afternoon and evening hours. && .TIDES/COASTAL FLOODING... A Coastal Flood Advisory was issued for the upper portions of the tidal Delaware River for minor flooding forecast at Burlington. Philadelphia should stay just shy of minor flood thresholds through the remainder of the week. Minor coastal flooding is forecast with the early morning high tides through Sunday morning as well, especially along the upper reaches of the tidal Delaware River. Additional advisories may be needed. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Coastal Flood Advisory from 2 AM to 8 AM EDT Friday for PAZ106. NJ...Coastal Flood Advisory from 2 AM to 8 AM EDT Friday for NJZ019. DE...None. MD...None. MARINE...None. && SYNOPSIS...Hoeflich/Staarmann NEAR TERM...RCM/Staarmann SHORT TERM...Hoeflich/Staarmann LONG TERM...Hoeflich/Staarmann AVIATION...Hoeflich/Staarmann MARINE...Hoeflich/Staarmann TIDES/COASTAL FLOODING...Staarmann
The national radar mosaic for the moment the first event began.
1 dense fog. Local standard time, as filed.
Dense fog with visibility 1/4 mile or less was observed for several hours at Wilmington Airport.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Delaware in 2025Every state in 2025This day in weather historyWarnings in Delaware, May 2025
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 1258005.