North Central Research Station

Thaw-Freeze Events

We used the National Climate Data Center's (NCDC) Summary of the Day daily maximum and minimum air temperature data for the 1950 to 1998 period, inclusive, to create a measure of extreme stress - the winter (December-March) incidence of thaw-freeze events.  Thaw-freeze events were defined as follows:

• Scenario 1:  maximum daily air temperatures ≥ 1°C for at least one day followed by daily minimum air temperatures ≤ -10°C for at least one day within the succeeding ten days since the last day of thaw.
• Scenario 2:  maximum daily air temperatures ≥ 5°C for at least two consecutive days followed by daily minimum air temperatures ≤ -15°C for at least one day within the succeeding six days since the last day of thaw.
• Scenario 3:  maximum daily air temperatures ≥ 10°C for at least three consecutive days followed by daily minimum air temperatures ≤ -20°C for at least one day within the succeeding three days since the last day of thaw.
• Scenario 4:  maximum daily air temperatures ≥ 10°C for at least three consecutive days followed by daily minimum air temperatures ≤ -10°C for at least one day within the succeeding seven days since the last day of thaw.

Thaw-freeze events were mapped across USDA Forest Service Region 9 for each year, and as an average of the 1950-1998 period.

Ten or more Scenario 1 events tend to occur each winter season over much of the North-central and Northeast US based on the 1950-1998 average.  The exception is over the coastal areas of the Mid-Atlantic and over southern Illinois and much of Kentucky.  Notably high incidence (20+ events/yr) is apparent along the Appalachian Mountains in parts of West Virginia, Pennsylvania, New York, Vermont, New Hampshire, and Maine.

Scenario 2 events also occurred region-wide (2+ events/yr), with 4 or more events/yr typically occurring over areas of West Virginia, New York, Vermont, and New Hampshire.  More than two events per year are also common over southwestern Iowa and northwestern Missouri.

Scenario 3 events were almost non-existent.  With only a few isolated occurrences, we were unable to create maps illustrating this scenario.

Scenario 4 events were typically most common in the southern half of the region.  In the northern tier of states, these events, although rare, have been especially damaging.  Marked thaws characterized by a meltdown of the snowpack, and sap ascent in specific cases, have resulted in extensive damage to fine roots and sapwood when conditions suddenly turned cold.  Interesting, the major episodes of dieback have occurred mainly in the northern half of the region.  Further south, where these events are fairly common, the risk of damage is less since trees seldom initiate sap ascent under a brief +10°C warming and there is rarely an accumulated snowpack (which paradoxically, serves as insulation but also limits frost hardening of fine roots in more northern areas).

Maps of individual years for each scenario can be viewed below:

Publications can be obtained from your local library, the appropriate journal, or the authors, if supplies remain.

• Monitoring the effects of extreme climate disturbances on forest health in the Northeast US  Auclair A, Heilman W, Busalacchi P. 2003. American Meteorological Society. 6(12):146-151.

For more information:

• Warren E Heilman  Project Leader, Research Meteorologist