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Light to moderate snow will continue into Saturday over the Great Lakes, Central Appalachians, and Northeast. This weekend into next week, a series of atmospheric rivers will bring gusty winds, periods of heavy rain, and mountain snow to northern California and the Pacific Northwest. Colder temperatures are in store for the weekend from the Great Lakes to East Coast. Read More >

Polygon-Based River Flood Warnings

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On June 26th, 2014, the Philadelphia/Mount Holly National Weather Service Forecast Office began issuing polygon-based river flood warnings and statements for 48 river forecast points across the Hydrologic Service Area.

For text-based users, you will notice additional lat/lon pairs, which produce the warning polygons, after the last ampersands (&&). No changes to the VTEC (header) information will be made. The following is an example for the Schuylkill River at Pottstown, PA. The lat/lon coordinates are highlighted in yellow.

FLD OBSERVED FORECAST

LOCATION STG STG DAY TIME CREST

SCHUYKILL RIVER

POTTSTOWN 12.5 8.5 WED 1 PM 15.0 THU 9AM

&&

LAT...LON 4006 7542 4013 7535 4026 7562 4020 7569

$$

For graphical users, you will notice more specific flood warnings. Rather than entire counties being “painted” with a warning, only the areas near the rivers/streams/creeks with forecast points will be “painted”.

In addition, the Watch/Warning/Advisory Map on the front page of our website will now only display these more specific river flood warnings.

Please keep in mind that these new warning polygons are not inundation maps. Also keep in mind, the polygons will always be larger than the area actually affected. The width of these polygons range from 3.5 to 6.0 miles. This was done intentionally for visual purposes.

The following is a graphical example of county-based river flood warnings versus polygon-based warnings. The Schuylkill River is used. River coordinates and polygon coordinates were imported into Google Earth. In this example, river flood warnings are in place for Berne, Reading, and Pottstown. Legacy county-based warnings are in white and the new polygon-based warnings are in red. Using the new polygon warning method, which increases specificity, reduces the warned area across Berks, Montgomery, and Chester counties by nearly 90%.

The change in this warning methodology aligns river flood products with other types of National Weather Service warning products that are already polygon based.

The following graphic, again in Google Earth, displays all 48 warning polygons across the four states we serve.