JetStream Glossary: Q's


Like the thermal wind, these are not physical realities (they do not exist). However, they arise mathematically from the Omega Equation and they help explain the results of physical processes in the atmosphere.

Thus, they are useful diagnostic tools to forecasters. The divergence of the Q-Vectors through a layer depicts the synoptic scale vertical motions. Moreover, Q-vectors themselves, when they point from cold to warm air (when overlaying the thermal field on the Q-Vector field), it infers a tightening of the thermal gradient.

This causes frontogensis. If this frontogensis is strong enough, a low pressure may form.

QPF (Quantitative Precipitation Forecast)
A spatial and temporal precipitation forecast that will predict the potential amount of future precipitation for a specified region, or area.
QPF Discussion (PFD)

This forecast discussion is directed completely to explaining manual forecasts of areas in the contiguous 48 states expected to receive 1/4" (6 mm) or more precipitation during a 24-hour period.

The manual forecasts are explained in terms of initial conditions and differences and/or similarities in the numerical model forecasts. General confidence in the manual forecast is expressed where it is appropriate and possible alternatives may be offered. This product is issued 3 times a day.

NCEP Excessive Rainfall Discussion.
NCEP Heavy Snow Discussion.
NCEP Precipitation Forecast Discussion.
Describes a low or high pressure area or a front that is nearly stationary.