This freezing degree day plot provides a crude guideline for potential frazil ice problems on the 
lower Kenai River.  Phase 1 freezing degree days (FDD) are accumulated for Soldotna.  Phase 2 FDD 
are calculated only when daily minimum temperatures drop below 0 deg. F.  While daily Tmin < 0 F, 
Phase 2 FDD accumulate and Phase 1 FDD remain constant.  Once the Tmin > 0 F, the accumulated 
Phase 2 FDD are added to Phase 1 FDD.  Phase 2 FDD are then reset to zero.  

Soft guidance thresholds are provided to alert for the possibility of frazil ice jams.  Sufficient 
cooling of the river (Phase 1) is required before a cold outbreak (Phase 2) can cause significant 
frazil ice development.  

For more information review the following presentation:

Forecasting Frazil Ice Jams on the Kenai River