A dry weather screening program is the scheduled, proactive half of illicit discharge detection and elimination (IDDE). Field staff visit the outfalls their MS4 permit designates on dry days, document flow or no flow, trace any flow to its source, and close each investigation with evidence. It works when the record is built at the outfall, during the visit itself.
When you send someone to screen an outfall on a dry day in August, they are checking whether your community knows where its water goes. Most of the time nobody meant any harm. A lawn got overwatered, a restaurant rinsed a grease trap into the wrong drain, a car got washed in the driveway and the suds ran down to the curb.
The screenings get done. The record of them scatters across a spreadsheet, a printed map on the wall, assignments handed out at a Monday meeting, field notes in a truck, and photos on somebody’s phone, and when the annual report comes due you rebuild the year from memory.
Every program owes its regulator two answers: whether every screening the permit requires happened, and what you found at each one. Maggie Mathias (our Chief Product Officer) and I walked through five strategies for having both answers ready in our September 23 webinar, with 2NFORM running live.
What a Dry Weather Screening Program Has to Prove
Federal rules define an illicit discharge as any discharge to an MS4 that is not composed entirely of stormwater, with exceptions such as NPDES-permitted industrial sources and firefighting. The scale of the problem is older than most permits. EPA’s Phase II fact sheet on IDDE cites a 1987 Sacramento study that found almost half the water discharged from a local MS4 was not directly attributable to precipitation runoff.
The same fact sheet holds a number most vendors would leave out. Analyzing Montgomery County, Maryland, the Center for Watershed Protection and Pitt (2004) found staff identified and corrected about six inappropriate discharges a year through regular screening, against more than 185 corrected each year from citizen complaints and hotline calls.
Routine screening finds a small share of discharges. Its job is proving your program looked, on the schedule the permit sets, and it is one of the few times your staff stand face to face with the people whose habits reach the creek. The record of each visit is the work product.
Strategy 1: Put Every Required Screening on the Calendar Before the Season Starts
Your permit names the locations and sets the frequency. For most programs that information lives in a spreadsheet, a map on the wall, and a standing Monday meeting, and it gets rebuilt by hand every season.
In the Illicit Discharge module, designated screening locations sit on their own map, apart from the full outfall inventory. Most municipalities manage far more outfalls than land on the screening list, and a map of only the designated ones is the fastest way to show city leadership why those assets matter more. Each location carries its own frequency, so priority outfalls can run twice a year while the rest run annually, and the to-do list fills itself from that field. Blue means a screening is due. Red means a rescreen or follow-up is needed. When the permit reissues, you update the frequency one asset at a time or in bulk through a CSV export and re-import.
Assignment works the same way. Filter by receiving water, select the set, and assign it to a named person with a due date. Field staff filter the map to their own name and plan the route before they leave the yard, including whether another program’s inspection sits close enough to fit into the same trip.
The City of Chattanooga, Tennessee runs this way at scale. Field staff are assigned watershed by watershed with citywide coverage visibility, 29 city staff across five departments work in the same system, and the team completed 1,693 field inspections in FY2025.
Every required screening has an owner before the first truck rolls.
Strategy 2: Record It Once, at the Outfall
Your crew is standing at an outfall on a dry day. Most of the time there is nothing to find, and the screening still has to be documented, because a season of nothing found is exactly what you will be asked to show.
Two things break that record. The first is paper: a clipboard at the outfall becomes a stack on somebody’s desk, and the record does not exist until whoever filled it out finds time to type it in. The second is variation. When every asset type has its own form and every inspector has their own habits, what comes back depends on who went out that day.
The fix is one standardized form, opened on a phone or tablet at the asset, with the inspector’s location showing on the map. The 2NFORM screening form runs five tabs: general information, condition and maintenance, field measurements, sample collection, and submission. Date, staff, and inspection type arrive pre-filled. On a dry outfall in good shape, the inspector marks it dry, confirms condition, passes through the empty measurement and sampling tabs, and lands on a summary that reads no illicit discharge, in green.
Even when an outfall is dry as a bone, I add a photo. The inspection report generates on site with the photos attached, the icon turns green, and the outfall drops off the routine list with the visit saved to its history. Your newest inspector and the person who has done this job for 19 years submit the same record, and it is finished before they reach the truck.
The City of Oceanside, California saw what this does to a week. Before 2NFORM, its Industrial and Commercial inspectors spent the better part of their workweek on paperwork, data entry, and reporting. The program saved 566 hours per year in its first six months, and the team went back to being inspectors.
The documentation stops being a second job after the fieldwork and becomes a byproduct of the fieldwork itself.
Strategy 3: Hand Repairs to the Maintenance Crew From the Field
Sometimes there is no flow to investigate and the structure is failing anyway, with active erosion at the headwall or debris packed into the pipe. The screening crew did not bring repair tools, and that is fine, because repair is not their job that day. Their job is to flag the work. Today that usually means a note in a truck, a photo on a phone, and a conversation back at the yard, if everybody remembers.
In the same form, the inspector marks yes under structural condition and picks from the list, in this case debris and clogging needing a major repair, plus active erosion. The summary now shows two results, no flow in green and repairs needed in red. The outfall stays green on the screening map, because there is no illicit discharge, and the repair lands on the outfall routine maintenance list in the Municipal Maintenance module with a due date two weeks out, ready to assign.
Nobody carries it back to a supervisor. In Chattanooga, an automated weekly maintenance report lets the team manage by exception, so the flagged outfall surfaces without anyone hunting for it. Nothing falls through the cracks.
Strategy 4: Trace the Flow While It Is Still Running
The other outcome is flow, on a dry day, from a pipe that should be dry. Your clock starts. Intermittent flow is hard to track, and by the time you know to look for a source it may have stopped, so everything the crew records in the next hour does double duty: it helps them find the source now and it stands as evidence later.
Tracing the discharge upstream through the storm sewer is one of the source-finding methods EPA lists in its IDDE guidance, and it goes faster when the network is already on the map. The inspector marks active flow, records color and other observations, attaches a photo, and saves. The outfall turns red, an open illicit discharge and the start of source tracking.
From there the crew turns on storm drain lines, filters to the catchment draining to that outfall, and turns on manholes and catch basins. Each structure they open gets the same observation, active flow or dry, and the breadcrumb trail builds on the map as they walk. Active flow at a manhole and a dry catch basin above it narrows the source to the stretch between them.
When the source is not obvious, the rest of the program helps. Filter that catchment for regulated commercial facilities and one with open findings and a history of enforcement shows up, along with what colleagues documented on prior visits in the Industrial/Commercial module. It may not be the source. It tells you where to look next, and nobody assembled that layer for this investigation. It was there because the program had been doing the work.
Strategy 5: End Every Screening With a Decision You Can Defend
Every screening with flow ends one of three ways, and the crew answers two questions before leaving: does someone need to come back, and is the investigation closed.
The source is a resident. Say it turns out to be a teenager and their dad washing a car in the driveway, who had no idea the runoff reached the storm drain. The conversation does the work. Rescreen, no. Close the investigation, yes, with the justification, notes on the interaction, and a photo of the driveway. The outfall turns black, a closed illicit discharge.
The source is a facility with a history. Close the investigation, schedule a rescreen two weeks out, and leave notes on what the next inspector should look for. The rescreen appears on the follow-up list with a date and an owner.
The source is never found. The flow stopped before anyone could trace it. Rescreen, yes, on the timeline your permit sets (five business days in the example we ran). Close the investigation, no. The outfall stays red and the rescreen lands on the follow-up list.
Black and red on one map show at a glance which outfalls are settled and which still need an answer. When someone picks up an investigation they did not start, the asset fact sheet holds the full history, with notes, photos, and downloadable screening reports. EPA’s recommended plan ends on documenting actions taken, including the number of outfalls screened, for the MS4 annual report. In this workflow that record already exists, and the annual report stops being a panic month. The City of Salinas, California shows how far that goes. Its pre-2NFORM annual report ran 2,500 pages. It now runs 30.
What the City of Watsonville, CA Got Back
The time a program gets back on dry weather screening comes from the rebuild nobody has to do after the season ends. Jackie McCloud, Environmental Sustainability Division Manager at the City of Watsonville, California, put a number on it:
“We save cumulatively at least 80 hours over a quarter during the dry season.”
Almost nobody your crew meets on a dry day in August set out to pollute a creek. They left a hose running or washed a car in the driveway, and the inspector showing up is often the first time anyone connected that to the water downstream. Every hour not spent reconstructing data is an hour for that conversation, and it is the part of the job your inspector signed up for. They are environmentalists, not paper pushers.
When to Move Screening Off the Spreadsheet
These are the signs your screening program has outgrown its current setup:
- The screening schedule gets rebuilt by hand each season from a spreadsheet or wall map.
- Field notes and photos live in trucks and phones until someone types them in.
- The annual report means reconstructing which outfalls were screened and what happened at each.
An incomplete inventory is not a reason to wait. Most municipalities start with scattered records, and some start with no outfall features at all. Berkeley County, West Virginia built its program from no foundation into a fully funded MS4 operation, growing from 3 to 12 staff and securing $1.59M in grants. Bring us your messy data in whatever state it is in, and we will help clean and structure it. 2NFORM is the system of record for stormwater compliance and ArcGIS remains the system of record for spatial data, and 2NFORM works alongside asset management platforms like Cityworks, Cartegraph, and Lucity rather than replacing them.
See Your Screening Season in 2NFORM
Tell us how you run dry weather screening today and what you want to change, and we will walk you through the workflow against your own permit requirements.
Frequently Asked Questions
What is dry weather screening for stormwater?
Dry weather screening is the routine inspection of MS4 outfalls on dry days to find flow that should not be there. Because it has not rained, any flow points to a possible illicit discharge, such as wash water, a misconnected pipe, or improper disposal. It is the scheduled, proactive part of an IDDE program, as opposed to responding to public complaints.
How often do outfalls need dry weather screening?
Your MS4 permit sets the locations and frequency, and the frequency can differ by outfall. A program might screen priority outfalls twice a year and the rest annually. When a permit reissues with new requirements, the schedule should update from that change rather than being rebuilt by hand.
What should be documented when an outfall is dry?
Record the date, inspector, inspection type, flow status, structural condition, and at least one photo, even when nothing is found. A season of documented no-flow visits is the evidence that every required screening happened. Any structural problems observed should be flagged for maintenance at the same visit.
What happens when you find dry weather flow at an outfall?
The inspector records the flow and observations, then traces upstream through manholes and catch basins, marking each as active flow or dry to narrow the source. Each investigation closes one of three ways: source found and resolved, source found with a rescreen scheduled, or source not found with a rescreen on the timeline the permit sets.
Do we need a complete outfall inventory before starting a dry weather screening program in 2NFORM?
No. Most municipalities start with scattered or incomplete records, and some start with no outfall features at all. 2NDNATURE helps clean and structure inventory data in whatever state it arrives, so the screening program can run while the inventory improves.
Does 2NFORM replace our asset management system or ArcGIS?
No. 2NFORM is the system of record for stormwater compliance, and ArcGIS remains the system of record for spatial data. 2NFORM works alongside asset management platforms like Cityworks, Cartegraph, and Lucity as the MS4-specific compliance layer.
