A series of Pacific weather systems will bring several rounds of precipitation and wind to the western U.S. this week. Several inches of rain could fall in portions of WA, OR and CA, especially in the coastal ranges and foothills. Snow is forecast in the higher mountains. In addition, winds will be strong, especially in mountainous areas, which could pose a hazard to high-profile vehicles. Read More >
Polygon-Based River Flood Warnings
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On June 26th, 2014, the Philadelphia/Mount Holly National Weather Service Forecast Office will begin 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
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.
For additional information, please contact the office between the hours of 8:00 am and 4:00 pm at 609-261-6615.