What Weather Forecasts Should I Use For My Destination And Alternate?

by CarlValeri on January 4, 2011

Are Forecast Region MapMany do the majority of their flying to airports with Terminal Aerodrome Forecasts (TAF) and can easily determine if they can make it to the destination and if an alternate is needed. A recent conversation with a pilot reminded me of the necessity for a review of the requirements for both weather reporting and when we should file an alternate.

As pilot in command for a flight under instrument flight rules we must become familiar with all available information concerning our flight which includes weather reports and forecasts (FAR 91.103 Preflight Action). While preparing for our flight all available information should include both commercial and government publications including TV weather forecasts.

Destination Weather

Larger airports normally have terminal aerodrome forecasts (TAF). The TAF is a report established to include a five statute mile radius around an airport. TAFs are issued by National Weather Service (NWS) Weather Forecast Offices (WFO) four times a day, every six hours beginning at 0000 UTC and are issued for a specified period, normally 24 hours.

If a TAF is unavailable for your destination you will need to use the aviation area forecast (FA). The FA gives a forecast over a large area encompassing several states and is what is used as forecast data at smaller airports that do not have terminal forecasts. Area forecasts are issued three times a day and are valid for 18 hours.

If there is no weather reporting at our destination airport and we want to determine if we need an alternate then we should first look towards the VFR clouds and weather section of the area forecast (FA). The forecast weather is for the next 12 hours with an outlook for an additional 6 hours.

Review the precautionary statements section to determine if your destination will be IFR and if you will need to file an alternate. All statements in area forecasts regarding heights are given in MSL or will be stated as above ground level (AGL) or ceiling (CIG).

AIRMET Sierra within the precautionary statements should be referenced for IFR conditions and mountain obscuration. Again all heights are in MSL unless otherwise noted as AGL or CIG.

Alternate Weather

To determine the need for an alternate airport we need to look at our destination and apply a simple rule per 91.169 most pilots call the 1-2-3 rule. For at least 1 hour before and for 1 hour after the estimated time of arrival at the destination, the ceiling must be at least 2,000 and the visibility at least 3 statute miles or you will need to file an alternate.

So how do we determine our alternate weather and if the alternate is appropriate for filing as an alternate. CFR Part 91.169 describes the minimum weather requirements at our estimated time of arrival at the alternate and the eligibility of an airport to be including as an alternate on your flight plan.

For an alternate airport with a precision approach the minimum ceiling must be 600 feet and visibility 2 statute miles. For an alternate airport with a non-precision approach a minimum ceiling of 800 feet and visibility 2 statute miles is required. If you want to file a flight plan with an alternate that does not have an instrument approach procedure; the ceiling and visibility must allow for a descent from the minimum enroute altitude (MEA), fly the approach, and land under basic VFR.

To determine if the alternate airport has other than standard alternate minimums look at the notes section of the approach plate. A dark triangle with a capital A in the center indicates standard alternate minimums are not applicable to this airport. You must now look at the alternate minimums page to determine the published alternate minimums.

If your chosen alternate does not have an instrument approach procedure or alternate minimums are not authorized you have only one choice. You must be able to descend from the minimum en route altitude (MEA), fly the approach, and land in VFR conditions.

The symbol NA next to the bold A designates that alternate minimums are not authorized due to unmonitored facility or absence of a weather reporting facility. Many times this can lead to pilots interpreting that we can never use the airport as an alternate. Again, as long as we can descend in VFR conditions from the MEA to the airport we can file this airport as an alternate in our IFR flight plan.

Example 1:

We are planning a flight from Ft Lauderdale to Tampa International and we need an alternate because Tampa is forecasting a ceiling of 1000 feet. We decide to use Lakeland, FL as our alternate with an estimated arrival time in Lakeland of 1300 local.

Note: Lakeland has non standard alternate minimums and no source of weather information is available when the tower is closed. The tower at Lakeland is open from 0600 to 2200.

Example 2:

We are planning a flight from Ft Lauderdale to Tampa International and we need an alternate because Tampa is forecasting a ceiling of 1000 feet. We decide to use Lakeland, FL as our alternate with an estimated arrival time in Lakeland but at 2300 local. Since we are arriving after the tower is closed we will need to be able to descend VFR from the MEA to the airport and land in VFR conditions to file Lakeland as our alternate.

Example 3:

We are planning an IFR flight from Tampa international to Lakeland, FL (KLAL) and the ceiling is forecast to be 1000 feet at our time of arrival. We decide to use Wauchula, FL (KCHN) because the weather south of Lakeland is sky clear and unrestricted visibility and Wauchula is far south of Lakeland.

If you look at an approach plate for Wauchula we notice the Bold A in a triangle and the NA next to it. This means that Standard Alternate minimums are not authorized and we will need to be able to descend from the MEA to the airport and land in VFR conditions. Since the weather is clear we can use KCHN as an alternate.

Notes on approach plate for Wauchula:

Conclusion:

When determining the weather when planing an IFR flight we must use all available information. Remember safety is our first priority and even though we can legally fly to a destination and are not required to file an alternate the most prudent choice many times is file an alternate even if not required by regulations.

If the weather at our destination is forecast to have a ceiling of 2000 feet and 3 miles visibility we don’t legally need an alternate but prudence tells us we should file one. Furthermore, when choosing an alternate we should look towards those airports with the best approach facilities and ability to leave after the weather gets better.

In example 3 above I would prefer Sarasota KSRQ as the alternate even though Wauchula is a legal alternate in this case. Sarasota has an ILS and weather reporting. If you have ever been in rural Florida around midnight you know not many 24 hour diners are open and most FBOs close early.


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{ 1 comment… read it below or add one }

Greg Williams March 22, 2014 at 12:51 pm

I like your article on alternates; however, what about when the weather is reporting occasional ceilings that are lower than the 2000 and 3 at the destination?
Let’s say we are traveling into an airport that is calling 2500 overcast and 3 miles visibility at our arrival time, but the destination airport is also calling for occasional 1800 and 1 3/4 mile visibility. Do you have to file an alternate with the consideration of the occasional? If so, where can I find that information?

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