A moisture tongue is a region of relatively high dewpoints. Often a moisture tongue coincides with a region of high Theta-E air. The area a moisture tongue covers is often the size of several states but can be confined to a narrower region in some circumstances. Theta-E is a region of relatively high moisture and/or temperature, usually analyzed in the PBL. The location of a moisture tongue is strongly influenced by warm air advection. Stronger wind speeds from a warm moist moisture source will advect this air from its source region. Good sources to observe moisture tongues are hour by hour surface maps of dewpoints and hour by hour surface wind speed charts. Higher dewpoints being advected from the south (usually a southerly component) identifies a moisture tongue. The majority of well-defined moisture tongues in the U.S. originate from the Gulf of Mexico and are therefore east of the Rocky Mountains. Often, a developing mid-latitude cyclone will draw warm moist air from the Gulf of Mexico. A low-level jet forms out ahead of the cold front. Therefore, a moisture tongue is found in the warm sector of a mid-latitude cyclone and out ahead of the cold front. Since a large temperature gradient is produced from the cold front, wind speeds are enhanced ahead of the cold front boundary.
Why is a moisture tongue important?
1. They are associated with instability. Warm and moist PBL air enhances a large thermodynamic instability.
2. The high speed air associated with a moisture tongue fuels developing thunderstorms by producing a large storm relative inflow and helicity (speed and directional shear in the PBL).
3. Warm air advection and moisture advection are a dynamic lifting mechanism (warm moist air is less dense which causes a stretching and subsequent dynamic lifting of the low levels of the troposphere).
4. Since a moisture tongue is a region of instability, thunderstorms often develop and become strongest within the moisture tongue region.
5. Precipitable Water values (PWs) will be higher in the moisture tongue.
6. Isentropic lifting of a moisture tongue can cause widespread precipitation north of a warm front.