Starting today (Wednesday, December 3rd, 2014), several counties in the northern and western Baltimore/Washington metropolitan area, along with the Blue Ridge Mountains of central Virginia will be subdivided for NWS Watches, Warnings, and Advisories for long duration hazards (such as winter weather).

For example, prior to this change, the National Weather Service recognized Montgomery County Maryland as a single geographic region with common climatological characteristics.  Therefore, residents of eastern Montgomery County would get the same Winter Storm Warning as the residents of western Montgomery County.  People familiar with the weather in Montgomery County understand that there is usually quite a variation in snow accumulations between downtown Silver Spring in southeastern Montgomery County, and the typically larger snow accumulations in Damascus in northern Montgomery County. 

With today's change, residents of Silver Spring will get a separate warning than the residents in Damascus, with both warnings being more precise and accurate for their respective areas. Moreover, in some situations if the differences were great enough, southeastern Montgomery County might not need a warning at all – reducing false alarms.

Similar changes are being made in Loudoun County Virginia; as well as Howard, Baltimore, and Harford Counties in Maryland.  Additionally, the higher elevations along the northern and central Blue Ridge Mountains in Virginia will also be segregated into two new forecast areas.

This will not affect:

• the forecast for any point
• watches or warnings for "short-fused" threats: Such as tornado, severe thunderstorm, or flash flooding.
• NOAA Weather Radio alerts: These are based on county level (or independent city) activation.
• FIPS codes / SAME codes: These "county-based" codes drive weather radio alerts and will remain unchanged.

This will only affect where watch, warnings, or advisoires are located for longer duration weather hazards. The counties impacted in the Baltimore/Washington metro are and around the Blue Ridge of Virginia are:

Metro Baltimore/Washington Counties:  Click here for map of changes

Loudoun VA

Montgomery MD

Howard MD

Baltimore MD (existing split reoriented)

Harford MD

These counties will be divided between their northern and western higher elevation areas, and their more heavily populated southern and eastern lower elevation areas.

• Loudoun VA (currently NWS VA forecast zone 042 - VAZ042)
  ...will become Western Loudoun (VAZ505), and Eastern Loudoun (VAZ506)
   
• Montgomery MD (currently NWS MD forecast zone 009 - MDZ009)
  ...will become Central and Southeastern Montgomery (MDZ504), and Northwest Montgomery (MDZ503)
   
• Howard MD (currently NWS MD forecast zone 010 - MDZ010)
  ...will become Central and Southeast Howard (MDZ506), and Northwest Howard (MDZ505)
   
• Southern and Northern Baltimore County MD will remain the same, except the dividing line will tilt more from southwest to northeast to match the break line  that will now extend through central and northeast MD.
   
• Harford MD (currently NWS MD forecast zone 007 - MDZ007)
  ...will become Southeast Harford (MDZ508), and Northwest Harford (MDZ507).

VA Blue Ridge:  Click here for map of changes

Higher elevations along the Blue Ridge will be cut from their respective counties from Warren and Rappahannock Counties in the north, to Nelson and Augusta Counties in the south. This Blue Ridge forecast zone will be divided in two - a northern and southern portion.

Northern VA Blue Ridge (new NWS VA forecast zone 507 - VAZ507)
This will consist of the higher elevations along Blue Ridge from Warren and Rappahannock Counties in the north, south to the Greene/Albemarle County line.

Central VA Blue Ridge (new NWS VA forecast zone 508 - VAZ508)
This will consist of the higher elevations along the Blue Ridge from the Greene/Albemarle County line, south through Augusta and Nelson Counties.

The counties that these new zones are cut from will remain the same in coding, but will not include these higher elevation areas along the Blue Ridge.

Full details (as well as updated GIS shapefiles for download) are in the NWS Service Change Notice that was issued on Sep 18th, 2014; click here: 
http://www.nws.noaa.gov/om/notification/scn14-47lwx_zones.htm