Diversion Airport Weather Planning: Complete Guide for Pilots

1. Weather Criteria for Alternate Airport Selection

Selecting appropriate diversion airports requires analyzing both current conditions and forecast weather patterns along your route. Unlike destination airport planning, alternate selection must account for deteriorating conditions that could force an unplanned diversion during any phase of flight.

The fundamental weather criteria for alternate airports include:

• Ceiling and visibility minimums: Must meet or exceed your aircraft's approach capabilities
• Wind conditions: Crosswind components within aircraft limitations
• Precipitation intensity: Avoiding moderate to heavy precipitation
• Convective activity: Clear of thunderstorms and associated hazards
• Icing conditions: Appropriate for your aircraft's anti-ice/de-ice capabilities

For IFR operations, alternates must forecast weather at or above specific minimums - typically 600-foot ceilings and 2 statute miles visibility for precision approaches, or 800 feet and 2 miles for non-precision approaches.

2. Analyzing METAR and TAF Data for Diversion Planning

Effective diversion planning requires systematic analysis of both current observations and forecast data. METAR reports provide current conditions, while TAF forecasts reveal expected changes during your flight window.

When evaluating potential alternates, focus on these METAR elements:

This example shows gusty winds, low visibility in heavy snow, and low ceilings - clearly unsuitable as an alternate. Compare with a more favorable option:

TAF analysis must extend beyond your planned arrival time. Look for:

• TEMPO and FM groups indicating short-term changes
• Probability forecasts (PROB30/40) for marginal conditions
• Trend indicators suggesting improving or deteriorating weather

3. Seasonal Weather Patterns and Regional Considerations

Diversion airport selection must account for seasonal weather patterns that affect entire regions. Understanding these patterns prevents selecting alternates that may be impacted by the same weather system affecting your destination.

Winter Considerations:

• Widespread icing conditions can eliminate multiple airports simultaneously
• Snowfall patterns often follow predictable geographic corridors
• Lake effect snow can create localized severe conditions near Great Lakes
• Temperature inversions may create persistent fog in valleys

Summer Considerations:

• Convective activity moves predictably with frontal systems
• Afternoon thunderstorms develop along convergence zones
• Heat-induced turbulence affects high density altitude airports
• Sea breeze effects create localized weather near coastlines

Regional Pattern Examples:

• Pacific Northwest: Marine layer intrusion affects coastal airports simultaneously
• Southwest: Monsoon flow creates widespread convection in summer afternoons
• Gulf Coast: Tropical systems impact hundreds of miles of coastline
• Great Plains: Squall lines move rapidly across multiple states

4. Distance and Fuel Considerations

Weather-suitable alternates must remain accessible given your fuel reserves and aircraft performance. This creates a complex balance between weather quality and operational feasibility.

Calculate alternate airport distances considering:

• Direct distance: Straight-line mileage to the alternate
• Routing constraints: Required airways or terrain avoidance
• Holding requirements: Potential delays for approach sequencing
• Approach type: Circling approaches require higher minimums

Fuel planning must account for:

• Flight to alternate at normal cruise power
• Descent and approach at destination before diverting
• Climb to cruise altitude for diversion
• Standard 45-minute reserve (IFR) or 30-minute reserve (VFR day)

Performance factors affecting alternate selection:

• Field elevation and density altitude effects on climb performance
• Runway length requirements with contaminated surfaces
• Navigation equipment required for available approaches
• Airport operating hours and fuel availability

5. In-Flight Weather Monitoring and Decision Points

Effective diversion planning continues throughout flight with real-time weather monitoring and predetermined decision points. Modern cockpit weather systems provide updated information that may require alternate airport reassessment.

Establish decision points based on:

• Time to destination: Begin alternate monitoring when 1 hour from arrival
• Fuel remaining: Specific fuel quantities triggering diversion decisions
• Weather deterioration rate: Rapid changes requiring immediate action
• Passenger considerations: Medical emergencies or severe turbulence

Monitor these information sources during flight:

• ATIS updates from destination and alternates
• Pilot reports (PIREPs) along your route
• ATC-provided weather updates
• Onboard weather radar and datalink weather
• Flight service station radio updates

Common decision-making errors include:

• Continuing toward deteriorating weather hoping for improvement
• Selecting the closest alternate without considering weather trends
• Failing to account for approach delays at busy alternate airports
• Overestimating aircraft capabilities in marginal conditions

6. Crosswind Assessment and Runway Selection

Wind conditions at potential alternates require careful evaluation, particularly crosswind components that may exceed aircraft limitations. Surface winds can vary significantly from forecast conditions, especially during frontal passages or convective activity.

Use the crosswind calculator to determine if runway options remain within your aircraft's demonstrated crosswind limits. Remember that these limits may be reduced in contaminated runway conditions.

Evaluate wind conditions considering:

• Multiple runway options: Airports with crossing runways provide flexibility
• Wind shear potential: Strong surface winds often indicate turbulent approaches
• Gusty conditions: Add half the gust factor to steady-state crosswind calculations
• Wind direction trends: Frontal passages cause predictable wind shifts

Airport-specific considerations:

• Terrain effects on wind patterns around the airport
• Seasonal wind patterns (prevailing directions)
• Time of day variations (land/sea breeze effects)
• Available approach types for each runway

Crosswind landing techniques require proficiency and practice. If you're not current on crosswind procedures, select alternates with runways aligned closer to forecast wind directions.

7. Comprehensive Weather Planning Checklist

Systematic diversion airport weather planning requires a methodical approach that considers all relevant factors. Use this checklist during preflight planning and in-flight reassessment.

Preflight Planning:

1. Obtain current weather briefing for route and alternates
2. Analyze forecast trends for flight time plus 2 hours
3. Verify alternate airport minimums meet regulatory requirements
4. Calculate fuel requirements including reserves
5. Confirm navigation equipment for available approaches
6. Check airport operating hours and services
7. Review NOTAMs for runway or approach restrictions

In-Flight Monitoring:

1. Monitor ATIS updates every 30 minutes
2. Request PIREPs from ATC near alternates
3. Update weather via datalink or flight service
4. Recalculate fuel requirements if routing changes
5. Establish firm decision points for diversion
6. Brief passengers on potential routing changes

Decision Criteria Matrix: