SURFACE THERMAL PATTERNS AS A FORECASTING AID

This method indicates that thermal tongues averaging 50 miles or less in width are favored locations  for tornado activity within the area of convective storm activity. These thermal tongues can be located quite readily on the surface synoptic chart. Use the following technique to supplement the existing tornado forecast. The procedures to locate areas of potentially severe convective activity with reference to the synoptic surface thermal pattern are as follows:

1. Draw isotherms for every 2C to locate thermal tongues.

2. Within the general area in which convective storms are forecast, locate the axis of all pronounced thermal tongues oriented nearly parallel to the gradient flow.

3. On this axis, locate the point with the greatest temperature gradient within 50 to 100 miles to the right, and normal to the flow.

4. From this reference point, a rectangle is constructed with its left side along the axis of the thermal tongue by locating comer points on the axis 25 miles upstream and 125 miles downstream from the reference point. The rectangle is 150 miles long and 50 miles wide.

5. This is the forecast area for possible tornado or funnel cloud development.

There are three distinct tornadic types over the United States. They are the Great Plains type, the Gulf Coast type, and West Coast type.

This type of tornado will generally form on the squall line in advance of a fast moving cold front, hence its prediction involves timely forecasting of the squall line formation along, or in advance of, a cold front, upper cold front, or trough. Conditions must favor a downrush of air from aloft.

In contrast to the air mass type (Great Plains type), tornadoes also form in equatorial type air masses that are moist to great heights. Such storms are most common on the coasts of the Gulf of Mexico and produce the waterspouts often reported over Florida. Tornadoes are triggered in this air mass primarily by lifting at the intersection of a thunderstorm line with a warm front, and less frequently by frontal and prefrontal squall lines.

Tornadoes also form in relatively cold moist air. This air mass tornado is the Pacific or West Coast type. It is responsible for waterspouts on the West Coast. Tornadoes in this type of air mass are normally in a rather extensive cloudy area with scattered rain showers and isolated thunderstorms. Clouds are mostly stratocumulus. Favorable situations for tornado development in this air mass type include the rear of Maritime Polar (mP) cold frontswell cooled air behind squall lines.

Waterspouts fall into two classes-tornadoes over water and fair weather waterspouts. The fair weather waterspout is comparable to a dust devil. It may rotate in either direction, whereas the other type of waterspout rotates cyclonically. In general, waterspouts are not as strong as tornadoes, in spite of the large moisture source and the reduced frictiion of the underlying surface. The water surface beneath a waterspout is either raised or lowered, depending on whether it is affected more by the atmospheric pressure reduction or the wind force. There is less inflow and upflow of air in a waterspout than in a tornado. The waterspout does not lift a significant amount of water from the surface. Ships passing through waterspouts have mostly encountered fresh water.

FORECASTING FOG AND STRATUS

LEARNING OBJECTIVES: Be familiar with the effects of air-mass stability on fog formation. Identify the procedures used in the forecasting of fog. Recognize conditions favorable for the formation of the various types of fog. Calculate fog parameters by using the Skew T Log P Diagram.

Fog and stratus clouds are hazardous conditions for both aircraft and ship operations. You will frequently be called upon to forecast formation, lifting, or dissipation of these phenomena. To provide the best information available, we will discuss the various factors that influence the formation and dissipation of fog and stratus.

EFFECT OF AIR MASS STABILITY ON FOG

Fog and stratus are typical phenomena of a warm air mass. Since a warm air mass is warmer than the underlying surface, it is stable, especially in the lower layers.

Through the use of upper air soundings, measurements can be made of temperature and relative humidity, from which stability characteristics can be determined. Refer to the publication, Use of the Skew T, Log P in Analysis and Forecasting, NAVAIR 50-IP-5, for complete information on analyzing upper air soundings.

GENERAL PROCEDURE FOR FORECASTING FOG

The synoptic situation, time of year, climatology of the station, air-mass stability, amount of cooling expected, strength of the wind, dewpoint-temperature spread, and trajectory of the air over favorable types of underlying surfaces are basic considerations you should take into account when forecasting fog.

This information is now available on CD in Adobe PDF Printable Format

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing