Forecasting Outline

Science 123		Spring 2020
	Forecasting Guidelines

Development and/or strengthening of midlatitude cyclones (MLCs) and associated waves
- Surface lows develop (mostly) along the Polar Front in conjunction with upper air dynamics associated with the Polar Jet stream.
- A surface low will deepen if there is net loss of mass from the air column above the low. Mass loss can occur in many ways including:
  - regions of positive vorticity advection - predominantly at the 700 and 500 mb level
  - exit region on the left flank of a Jet Streak
  - baroclinic amplification of the upper-level wave due to warm/cold advection at lower levels - 850 mb
- A major shortwave trough moving into a longwave trough deepens the longwave trough and may transform a barotropic wave into a baroclinic wave.
- An upper trough with a "positive" axis orientation (axis slopes positively, as in a straight-line graph, from the SW to the NE) is generally in a development stage and the trough tends to deepen.
- An upper trough with a "negative" axis orientation (axis slopes negatively, as in a straight-line graph, from the NW to the southeast) is typically indicates the associated surface low has reached its maximum strength and will begin to weaken.

Movement of significant features
- Low centers at the surface move in a path determined by the leading warm front and the flow at the 500 mb level
- Low center speed of movement is approximately 70% of the 700 mb flow speed or 50% of the 500 mb flow speed above the surface low.
- Low centers and troughs will migrate southward as they cross the Rocky Mountains due to the conservation of potential vorticity.
- Closed upper air lows and/or stacked lows will move slowly.
- Cold front speed is approximately 85% of the 850 mb flow in the cold air behind the cold front.
- Warm front speed is approximately 70% of the 850 mb flow in the cold air ahead of the front.

Temperature

Temperature tendency - "tendency" means which way (higher or lower) will the 24-hour high temperature or 24-hour low temperature for the forecast point move. Recall some of the "controls" on temperature discussed in the chapter on this subject. They include cloudiness, atmospheric humidity, wind speed, and temperature advection. The table below summarizes these results.

Control	Tendency	24-hour Low Tendency	24-hour High Tendency
cloudiness*	$\uparrow$	$\uparrow$	$\downarrow$
cloudiness*	$\downarrow$	$\downarrow$	$\uparrow$
wind	$\uparrow$	$\uparrow$	$\downarrow$
wind	$\downarrow$	$\downarrow$	$\uparrow$
humidity	$\uparrow$	$\uparrow$	$\downarrow$
humidity	$\downarrow$	$\downarrow$	$\uparrow$
warm advection	$\uparrow$	$\uparrow$	$\uparrow$
cold advection	$\uparrow$	$\downarrow$	$\downarrow$

* - Follow this link to the Haby page on forecasting cloudiness.

Using the 850 mb Temperature to predict the 24-hour maximum:
- Winter : T_sfc = T₈₅₀ + 9 deg F
- Spring : T_sfc = T₈₅₀ + 12 deg F
- Summer : T_sfc = T₈₅₀ + 15 deg F
- Fall : T_sfc = T₈₅₀ + 12 deg F

The dew point temperature can be used as a lower limit on the 24-hour minimum temperature.

Here is a link to Haby's page on forecasting high and low temperatures.

Precipitation
- Precipitation requires moisture and cooling (rising motion).
- The presence of atmospheric moisture is determined by relative humidity, dew point, or dew point depression data at the lower levels (surface, 850 mb, and 700 mb)
- Rising motion is generated by convergence at the surface or divergence aloft.
  - Frontal boundaries are likely locations for surface convergence.
  - Upper air divergence is associated with positive vorticity advection (PVA)
  - jet streak left flank exit regions
- Precipitable Water Content - worksheet