Arctic Blast Set to Arrive on Thanksgiving; Dangerous Wind Chills, Lake Effect Snow, Severe Thunderstorms

A significant arctic outbreak will arrive in the northern Rockies and northern Plains on Thanksgiving into Friday and advance farther south and east through much of the Plains and Midwest this weekend. Dangerous wind chill temperatures are expected with a significant long duration lake effect snow event possible downwind of the Great Lakes. Severe thunderstorms may be possible in the Southeast. Read More >

Rises on Major Rivers (if applicable)

Biggest takeaway:
This type of flooding of major rivers is typically only a threat for those along major fresh water rivers such as the Potomac, Shenandoah, and Rappahannock – and their tributaries. The threat is categorized as minor, moderate, major, & record to help define the level of risk.

Warning/Trigger for Plan:
NWS Flood Warning for Rivers

Advance Notice/Time to Activate & Accomplish Your Planned Response:
20-60 minutes to as much as 4 days. The further downstream you are, the longer the advance notice.

Frequency:
~0-2/year for minor flooding of the lowest lying properties along the river.
Major flooding of a river is a rare event, but for those along large rivers one that needs to be planned for, given its catastrophic potential.

How Accurate Warnings?
NWS river flood warnings are very accurate, although a certain amount of leeway has to be given for the uncertainty of the exact height of the crest of a flooding river. The crest forecast accuracy is greatest once the bulk of the rain has already fallen, while the water is collecting in the headwaters. Note that rapid snowmelt upstream can also cause river flooding (Jan ‘96).

Determine if you are in an affected area
- The white shaded areas on this map define areas for which Flood Warnings for rivers are issued:
If your location is along the rivers in these highlighted areas:
- Use this webpage to find the nearest point to monitor your river height.
- Receive River Flood Warnings from the NWS.
- Know what river levels (height in feet read by the gauge) it would take to threaten your location to various degrees of severity.
- Have a plan for actions you will take if these various levels are expected/reached.
- Use USGS Water Alert service (or similar) to set up an alert for when critical levels are forecast or reached for your location.
- Know the various calibers of river flooding that will be included in the warning:
  - Minor: Nuisance flooding (non-structural, no significant roads)
  - Moderate: Starts to inundate the lowest homes/businesses and roads along the river.
  - Major: Significant flooding of riverside structures, roads, life & property
  - Record: Higher than ever recorded.
Safety References:
- Flood...The Awesome Power