Official Data
📊 Reference Portal

Official Winter Weather
Classification Systems

A comprehensive reference for official NWS watch/warning criteria, NOAA's Regional Snowfall Index, wind chill dangers, and ice storm impacts — all sourced directly from federal meteorological agencies.

National Weather Service (NWS) NOAA / NCEI Weather Prediction Center
NOAA / NCEI Official Scale

Regional Snowfall Index (RSI)

The official NOAA scale for ranking snowstorm impacts, similar to Saffir-Simpson for hurricanes. Based on snowfall amount, spatial extent, and affected population.

0
Nuisance
RSI < 1
Minimal societal impact
1
Notable
RSI 1 – 3
Some disruption expected
2
Significant
RSI 3 – 6
Considerable disruption
3
Major
RSI 6 – 10
Major regional impact
4
Crippling
RSI 10 – 18
Extensive, prolonged impact
5
Extreme
RSI 18+
Historic, catastrophic event
ℹ️ How RSI Works: The Regional Snowfall Index uses regional-specific snowfall thresholds (e.g., 2"/5"/10"/15" for the Southeast vs. 4"/10"/20"/30" for the Northeast) combined with population data to normalize storm impacts across different climates.
National Weather Service

Official Winter Weather Products

NWS issues three tiers of alerts: Advisories (be aware), Watches (be prepared), and Warnings (take action). Each product has specific criteria.

Advisory

Winter Weather Advisory

Snow: 2–4" expected (varies by region)
Or: Any amount of freezing rain/sleet causing hazardous travel
Impact: Significant inconvenience, but not life-threatening

⚠️ Use caution when traveling
Watch

Winter Storm Watch

Timing: Potential for hazardous winter weather within 48 hours
Confidence: Conditions may warrant a warning
Action: Prepare now, monitor forecasts closely

📋 Prepare and stay informed
Warning

Winter Storm Warning

Snow: ≥5" in 12 hrs OR ≥7" in 24 hrs (varies by region)
Or: Dangerous ice/wind combination
Impact: Life-threatening conditions, travel extremely dangerous

🚫 Travel not recommended
Warning

Blizzard Warning

Wind: Sustained ≥35 mph or frequent gusts ≥35 mph
Visibility: ≤¼ mile from falling/blowing snow
Duration: Conditions persist for 3+ hours

🛑 DO NOT TRAVEL — Whiteout conditions
Warning

Ice Storm Warning

Ice: ≥0.25" of ice accumulation expected
Impact: Significant tree/power line damage likely
Hazard: Travel extremely dangerous or impossible

⚡ Expect power outages
Warning

Wind Chill Warning

Wind Chill: ≤-25°F expected (varies by region)
Danger: Frostbite possible in as little as 10-15 minutes
Risk: Hypothermia if outside for extended periods

🥶 Limit outdoor exposure
Official Criteria

Reference Data

Key thresholds and criteria from NWS and NOAA for winter hazards.

💨
Blizzard Criteria
NWS Official Definition
≥35
mph wind
Sustained or gusts
≤¼
mile visibility
Falling/blowing snow
3+
hours duration
Must persist
Note: There is NO temperature requirement. A "ground blizzard" involves existing snow blown by wind — no new snow falling.
🌡️
Frostbite Risk by Wind Chill
Time to Frostbite on Exposed Skin
0°F
Wind Chill
Low Risk
-10°F
Wind Chill
Caution
-20°F
Wind Chill
30 min exposure
-35°F
Wind Chill
10 min exposure
-50°F
Wind Chill
5 min exposure
-60°F
Wind Chill
Under 2 min
Frostbite times indicate how quickly exposed skin (face, fingers) can freeze. Seek shelter and cover skin when wind chill drops below -20°F.
🧊
Ice Accumulation Impacts
Infrastructure Damage Scale
Trace
Glaze
Slick spots on roads
0.25"
Advisory
Tree damage begins
0.50"
Warning
Widespread outages
1.0"+
CATASTROPHIC
Days/weeks recovery
Impact: 0.5" of ice adds ~500 lbs per power line span. Ice Storm Warnings are issued at ≥0.25" accumulation.
📊
RSI Regional Thresholds
NOAA Snowfall Categories

RSI uses different snowfall thresholds by region to normalize impact comparisons:

Region T1 T2 T3 T4
Southeast 2" 5" 10" 15"
South 2" 5" 10" 15"
Ohio Valley 4" 8" 16" 24"
Northeast 4" 10" 20" 30"
Upper Midwest 4" 8" 16" 24"
Northern Rockies/Plains 4" 8" 16" 24"
📊
NWS Wind Chill Chart
Official Wind Chill Temperature Index
Wind
(mph)		 40°F 35°F 30°F 25°F 20°F 15°F 10°F 5°F 0°F -5°F -10°F -15°F -20°F -25°F -30°F -35°F -40°F
5 363125191371-5-11-16-22-28-34-40-46-52-57
10 3427211593-4-10-16-22-28-35-41-47-53-59-66
15 3225191360-7-13-19-26-32-39-45-51-58-64-71
20 302417114-2-9-15-22-29-35-42-48-55-61-68-74
25 29231693-4-11-17-24-31-37-44-51-58-64-71-78
30 28221581-5-12-19-26-33-39-46-53-60-67-73-80
35 28211470-7-14-21-27-34-41-48-55-62-69-76-82
40 2720136-1-8-15-22-29-36-43-50-57-64-71-78-84
45 2619125-2-9-16-23-30-37-44-51-58-65-72-79-86
Frostbite possible in 30 minutes
Frostbite possible in 10 minutes
Frostbite possible in 5 minutes
NWS Wind Chill Formula: Wind Chill (°F) = 35.74 + 0.6215T − 35.75(V0.16) + 0.4275T(V0.16) Where T = Air Temperature (°F) and V = Wind Speed (mph)
🌨️
Snowfall Rate Categories
NWS Intensity Classifications
<0.5"
per hour
Light
0.5-1"
per hour
Moderate
1-2"
per hour
Heavy
2"+
per hour
Intense
Heavy snow warnings are typically issued for ≥1"/hr rates that persist for several hours.
❄️
Snow-to-Liquid Ratio
Snow Density by Temperature
10:1
Wet/Heavy
~32°F surface temp
Dense, packable snow
15:1
Average
20-28°F surface temp
Standard forecasting
20:1+
Dry/Fluffy
<20°F surface temp
Light, powdery snow
Ratio meaning: 10:1 = 10" snow per 1" liquid water. Higher ratios = fluffier snow.
🩺
Hypothermia Stages
Core Body Temperature Danger Levels
95-90°F Mild Shivering, numbness, confusion begins
90-82°F Moderate Shivering stops, drowsiness, slurred speech
82-70°F Severe Loss of consciousness, weak pulse
<70°F Critical Cardiac arrest, death imminent
⚠️ Seek emergency help if body temp drops below 95°F. Remove wet clothing and warm gradually.
👁️
Visibility Categories
NWS Travel Impact Scale
1+ mi
Good
Normal driving conditions
½-1 mi
Reduced
Slow down, use caution
¼-½ mi
Poor
Hazardous travel
<¼ mi
BLIZZARD
Do not travel
Blizzard conditions require visibility ≤¼ mile for 3+ hours with ≥35 mph winds.
NWS Regional Criteria

Winter Storm Warning Thresholds

NWS Weather Forecast Offices use regionally-calibrated thresholds. These are typical values — check your local WFO for exact criteria.

Region Advisory Warning (12-hr) Warning (24-hr)
SE Deep South (MS, LA)
 1-2" 2"+ 3"+
SE Southeast (NC, SC, GA)
 2-3" 3"+ 4"+
MA Mid-Atlantic (VA, MD, PA)
 2-4" 5"+ 7"+
OV Ohio Valley (OH, KY, IN)
 2-4" 5"+ 7"+
NE Northeast (NY, MA, ME)
 3-5" 6"+ 8"+
UM Upper Midwest (MN, WI)
 3-5" 6"+ 8"+
NP Northern Plains (ND, SD)
 4-6" 6"+ 10"+
RM Rocky Mountains (CO, WY)
 4-8" 8-12"+ 12-18"+
⚠️ Important: These are approximate regional values. Actual thresholds vary by individual NWS Weather Forecast Office and can be adjusted based on timing (rush hour), local conditions, and forecaster judgment.