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\\Stormy AI\\

Agentic weather briefing

What you are reading

Though vibe coding is taking over the world, I still like to write my own code. I use Cursor and Codex as assistants, but I know every inch of the code that ships with my name on it. The How Stormy AI Works page takes you through the code, tools, model selection, and AWS infrastructure behind the briefing.

This briefing is a scheduled (4 times per day), AI-written weather analysis for my hometown of Atco, NJ. The briefing is built from current observations, National Weather Service products, radar, lightning data, and numerical model guidance. Stormy AI gathers those sources, runs deterministic meteorological diagnostics, and asks a language model to turn the evidence into the report below.

Weather Briefing — Atco, NJ 08004

  • Updated: Sunday, August 30, 2026 12:00 AM Eastern
  • Next update: Sunday, August 30, 2026 6:00 AM Eastern
  • Generated: 2026-08-30T04:04:16+00:00
  • Type: weather

Headline

A quiet, cool, and dry night is in place across Atco under high pressure, but a warm and increasingly humid pattern returns Sunday with daily chances for showers and thunderstorms from Sunday night through midweek — with Tuesday and Wednesday flagged as the days to watch for heavier rain and stronger storms.

Active Alerts

No active alerts. The National Weather Service has no watches, warnings, or advisories in effect for Atco or anywhere in New Jersey per the Mount Holly (KPHI) forecast office.

Current Weather

It is a calm, clear, and humid late-summer night in Atco.

NWS surface observation — Mount Holly, South Jersey Regional Airport (KVAY), 12.1 miles from Atco, observed at 2026-08-30 03:40 UTC (11:40 PM EDT Saturday): - Temperature: 61°F (16.0°C) - Dewpoint: 61°F (16.0°C) — relative humidity 100%, air is essentially saturated - Wind: Calm - Pressure: 30.20 inHg (1022.7 mb) - Visibility: 10 miles - Sky: Clear (CLR at 12,500 ft) - Present weather: None reported; no precipitation in the past 1, 3, or 6 hours

MRMS precipitation (analysis 03:58–04:00 UTC): No precipitation detected at Atco or anywhere within a 30 km radius — 0% echo coverage, no surface precipitation within 100 km. The deterministic diagnosis confirms dry conditions with high confidence (0.89).

NEXRAD Level II — The nearest radar is KPHL (Philadelphia), 20.3 km away. The latest available scan (20:35 UTC, ~7.5 hours old — a data-latency limitation worth noting) shows a mean low-level reflectivity of about −2 dBZ with an isolated maximum of 39.75 dBZ, but zero gates at or above 50 dBZ and no echoes ≥40 dBZ flagged — consistent with a dry, quiet radar scene rather than organized precipitation. Beam height over Atco is very low (mean ~159 m MSL), so sampling quality is excellent. Dual-pol fields (ZDR, ρHV, KDP) were not available in this scan.

Lightning — GOES-19 GLM shows zero flashes within 50 km over the past 10 minutes (03:51–04:01 UTC), with a steady trend. No electrically active storms are nearby.

Diagnosis summary: Dry, calm, and saturated near the ground — classic radiative-cooling conditions. With calm winds and 100% humidity, patchy fog is forming or will form toward dawn, exactly as the NWS forecast indicates (patchy fog after 2 AM).

NEXRAD KPHL reflectivity near Atco, NJ

Current Synoptic Setup

Per the Mount Holly (KPHI) Area Forecast Discussion (issued 6:59 PM EDT Saturday):

  • Surface: A high pressure system is in control through tonight, delivering a cooler and much drier airmass than recent days. Winds are very light to calm, allowing temperatures to radiate down to near-saturation — hence the patchy fog risk in sheltered areas, especially where residual ground moisture from recent rainfall exists. Increasing clouds late should keep widespread or dense fog from developing.
  • Upper air: The HRRR analysis shows a dry, benign profile — mid-level RH is very low (5–23% from 850–500 mb), with westerly flow aloft (500-mb winds ~27 kt from 264°, 300-mb ~34 kt from 305°). Broad upper-level ridging is building back into the eastern US for early next week, with surface high pressure across the East.
  • Transition: The key change is the return of warmer, more humid air Sunday — dewpoints climbing back into the upper 60s to low 70s — which sets the stage for daily convection chances into the middle of the week. The HRRR already shows surface dewpoint depression of only 3.1°C at the ground but a very dry layer aloft, a signature of the dry high-pressure airmass about to be replaced.

GFS Guidance

GFS cycle: 2026-08-30 00Z (all lead hours from this single cycle; model guidance, not an observation).

Day 1 — valid 2026-08-31 00Z (Sunday 8 PM EDT): - Surface: 23.5°C (74.5°F), dewpoint 20.0°C (68°F), MSLP 1018.4 mb, light 3-kt wind, negligible precip rate (0.009 mm/hr) - 500 mb: 588.8 dam heights — a warm ridge axis over the region; 30-kt flow - 850 mb: RH 72.8%, 17.5-kt flow — moisture beginning to return aloft - 300 mb: 39-kt jet — modest upper flow

Day 2 — valid 2026-09-01 00Z (Monday 8 PM EDT): - Surface: 25.2°C (77.4°F), dewpoint 18.5°C (65.3°F), MSLP 1017.6 mb, near-calm wind, dry (0.0 mm/hr) - 500 mb: 588.0 dam with increased vorticity (17.5×10⁻⁵ s⁻¹) and 38.6-kt winds — a subtle short-wave disturbance rounding the ridge, consistent with Monday's shower chance - 850 mb: RH 60.6%, 22.9-kt flow — slightly drier but windier aloft - 300 mb: 34.9 kt

Day 3 — valid 2026-09-02 00Z (Tuesday 8 PM EDT): - Surface: 28.5°C (83.3°F), dewpoint 21.4°C (70.5°F) — the most humid and warmest point guidance of the period, MSLP falling to 1014.7 mb, light precip rate 0.147 mm/hr - 500 mb: 590.3 dam heights (ridge at maximum), low vorticity (4.2×10⁻⁵ s⁻¹) - 850 mb: RH surges to 86.4% with 18-kt flow — deep moisture return - 300 mb: 54.1-kt jet — strengthening upper flow, increasing divergence/storm-organization potential

Synthesis: GFS depicts a classic summer ridge building through Tuesday with steadily increasing low-level moisture and a strengthening 300-mb jet by day 3 — an environment priming for the Tuesday/Wednesday convection the forecast office highlights.

Day 1 (Sun 8 PM EDT): GFS surface day 1 guidance GFS 500-mb day 1 guidance GFS 850-mb day 1 guidance GFS 300-mb day 1 guidance

Day 2 (Mon 8 PM EDT): GFS surface day 2 guidance GFS 500-mb day 2 guidance GFS 850-mb day 2 guidance GFS 300-mb day 2 guidance

Day 3 (Tue 8 PM EDT): GFS surface day 3 guidance GFS 500-mb day 3 guidance GFS 850-mb day 3 guidance GFS 300-mb day 3 guidance

HRRR Analysis

HRRR cycle 2026-08-30 03Z, forecast hour 0 (valid 03:00 UTC) — model guidance at the nearest grid point (0.36 km from Atco), not an observation.

Surface thermodynamics: 17.6°C (63.7°F) temperature, 14.5°C (58.1°F) dewpoint, surface pressure 1017.9 mb. Precipitation type: none (all categorical flags false), rate 0.0 mm/hr.

Vertical structure (HRRR-derived model sounding): - Freezing level: 4,614 m MSL (~4,570 m AGL) — a deep warm layer; any precipitation would be all rain. Wet-bulb zero at 2,581 m. - Instability: Zero CAPE and zero CIN at the surface, mixed-layer, and most-unstable levels — a completely stable, convection-suppressed environment right now. DCAPE of 824 J/kg reflects the dry mid-level air (potential for evaporatively cooled downdrafts if convection initiates later). - Lapse rates: Surface-to-3 km only 2.8°C/km (very stable, nocturnal inversion developing); 700–500 mb at 6.4°C/km (near-neutral mid-levels). - Moisture: Precipitable water just 13.7 mm (0.54 in) — very dry column, consistent with the high-pressure airmass. K-index of −19 and Total Totals of 25 are strongly non-convective. - Wind/shear: Light low-level flow (0–1 km shear 5.2 kt, southerly), but 27.1 kt of 0–6 km bulk shear from westerly flow aloft — decent deep-layer shear that would matter if storms develop in the coming days. - LCL: Low (403 m AGL), but with no LFC or EL (no parcel can reach free convection), confirming zero convective potential at this hour.

Bottom line from HRRR: A stable, dry, quiet nocturnal boundary layer with fog potential — but the dry mid-levels and moderate deep-layer shear are ingredients worth noting once humidity returns Sunday.

Outlook

Synthesizing the KPHI Area Forecast Discussion (6:59 PM EDT Sat, Aug 29, 2026):

  • What changed: Only an aviation discussion update; the overall forecast thinking is unchanged.
  • Key message 1 (tonight): High pressure holds through tonight with a cooler, much drier airmass. Lows in the mid-50s to low 60s, with patchy fog possible in sheltered areas due to calm winds and near-saturation — but increasing clouds late should prevent widespread or dense fog.
  • Key message 2 (Sunday onward): Warmer and more humid conditions return Sunday, with daily chances for showers and thunderstorms into the middle of the week. Broad upper-level ridging returns to the eastern US; highs climb to the mid-80s to low 90s (about 5°F above average for early September) with dewpoints in the upper 60s to low 70s.
  • Convective details: Storms starting Sunday should remain isolated to widely scattered, focused mainly along and NW of the I-95 corridor — Atco sits close to that favored zone, so it's on the fringe. Instability is the limiting factor; severe storms are unlikely, but gusty winds, small hail, and brief heavy downpours are possible with any stronger storms.
  • The bigger watch item: Thunderstorms rotating around the ridge periphery and the position of a warm front next week. The office explicitly flags Tuesday and Wednesday as the main days to monitor for heavy rain and severe weather potential, though exact timing, location, and intensity remain uncertain at this range.
  • Confidence: Moderate overall. High confidence in the warm-up and humidity return; lower confidence in the exact placement/timing of midweek convection.
  • Hazards: No watches/warnings/advisories for NJ. Marine: no prevailing hazards; low rip current risk at the Jersey Shore Sunday and Monday.

Forecast for Next 3 Days

Today (Sunday, Aug 30)

  • Overnight/early morning: Mostly clear then patchy fog after 2 AM, low around 62°F. Calm wind. RH 90–97% through 7 AM.
  • Daytime: Patchy fog before 8 AM, then mostly sunny. High near 86°F. Southwest wind 0–10 mph. Dewpoints climb from the low 60s into the upper 60s (68°F by late afternoon) — noticeably more humid than Saturday.
  • Precipitation: Only a 13% chance late (after 4 PM); hourly PoP stays ≤13% through 6 PM. Any afternoon storm would be isolated.
  • Evening transition: PoP climbs from 16% at 7 PM to 40% by 11 PM as a slight chance of rain showers (7–8 PM) becomes a chance of showers and thunderstorms (8 PM onward).

Sunday Night into Monday (Aug 30–31)

  • Sunday night: Chance of showers and thunderstorms 8 PM–5 AM, mostly cloudy, low around 69°F, SW wind 5–10 mph. Peak PoP ~47% around 3 AM. Rainfall amounts less than a tenth of an inch. Storms taper quickly after 5 AM.
  • Monday: Partly sunny, high near 87°F, light NW wind 0–5 mph. Chance of rain showers after 2 PM (31% PoP, <0.1"). Dewpoints hold around 70°F — muggy.
  • Monday night: Chance of rain showers continues (30–31% PoP through the evening, easing to 15% after 2 AM), mostly cloudy, low around 70°F, light S wind.

Tuesday (Sep 1)

  • Morning: Slight chance of rain showers before 8 AM (15% PoP), then partly sunny and warming — 74°F by 8 AM rising to 88°F by early afternoon. Dewpoints climb to 72–73°F — the most humid day of the period so far.
  • Afternoon/evening: Chance of showers and thunderstorms from 2 PM through the evening (31% PoP), partly sunny, high near 89°F, SW wind 0–5 mph. This is the first of the two days the forecast office flags for monitoring.
  • Tuesday night: Chance of showers and thunderstorms before 2 AM (22–31% PoP), mostly cloudy, low around 72°F.

Wednesday (Sep 2) — early look

  • Partly sunny, high near 90°F, light SE wind, slight chance of showers and thunderstorms after 2 PM (19% PoP). GFS day-3 guidance (valid Tue 8 PM) shows the ridge at maximum with 850-mb RH at 86% and a 54-kt 300-mb jet — the environment the discussion flags for heavy rain and severe weather potential Tuesday–Wednesday. Confidence in specifics is low at this range.

Bottom Line

Right now Atco is dry, calm, and clear with 61°F and 100% humidity — patchy fog is the only overnight concern, and no alerts are in effect. Sunday turns warmer and more humid (high near 86°F) under mostly sunny skies, with the first real storm chance arriving Sunday night (8 PM–5 AM, ~40–47% PoP, generally light amounts). Monday and Tuesday stay warm (87–89°F) and muggy with daily afternoon/evening shower and thunderstorm chances around 30%, and the Mount Holly office specifically flags Tuesday and Wednesday as the days to monitor for heavy rain and severe weather potential as storms ride the ridge periphery — though exact timing and coverage remain uncertain. No severe weather is expected before then; any stronger storms Sunday–Monday could produce gusty winds, small hail, or brief downpours.