Synoptic Test 1

Prefrontal compressional warming

Air heats up right before a cold front because the advancing cool air squeezes and compresses the warm air in front of it.

Golden rule of weather forecasting

Consult weather observations first, create weather forecasts last.

Model hierarchy progression

Globally -> Nationally -> Regionally -> Locally

Model continuity

How closely successive runs of the same model agree for the same valid forecast time.

Model consensus

How well solutions from different models agree for the same valid forecast time. Yields the best forecasts.

Deterministic forecasting type

Using a single, latest model run.

Probabilistic/Ensemble forecasting type

Running multiple model iterations with slightly altered initial conditions to determine a range of possibilities, standard derivation, and ensemble mean.

Model output statistics (MOS)

Correlates NWP model outputs with historical observations and climatology to auto generate point forecasts.

Diagnosis before prognosis

Do not jump straight to computer model plots, you must diagnose the current atmospheric state before predicting its future state.

Meridional flow

Fluid motion that usually moves north-south.

Zonal flow

east-west flow.

Advection

Moving a body of air from one place to another. Can be vertical or horizontal.

Persistent forecasting approcach

Works best in subtle air masses away from fronts. Adjust slightly day to day for air mass modification.

Upstream/steady state frontal forecasting approach

Estimate temperature changes behind an approaching front by evaluating the 24 hour temperature change at an upstream station that was in the same relative position behind the front the previous day.

Afternoon maximum (Tmax) forecasting rule

Take the 850mb temperature on the 12z morning sounding and mix it dry adiabatically down to surface elevation. Works best during warm seasons and sunny conditions.

Overnight minimum (Tmin) forecasting rule

Find the dew point at the time of maximum daytime temperature. This value is a strong estimate for the overnight low. Best during clear sky conditions.

Divergence of wind

Spreading out. Horizontal divergence at the surface causes sinking motion.

Convergence of wind

Coming together. Horizontal convergence at the surface causes rising motion.

Winter weather top down technique

Modern sounding based approach using Skew T diagrams to analyze warm layers, moisture, and low level freezing boundaries to determine surface precipitation types.

Consensus

A group of average forecasters almost always outperforms a single expert; always discuss forecasts with peers or NWS area forecast discussions.

Continutiy

Avoid drastic forecast shifts without a clear technical reason.

1 degree rule

Never make a 1 degree temperature change to previous forecasts. Preserve continuity by retaining overlapping target numbers.

Synoptic meteorology

The study of the structure and evolution of weather systems on spatial scales on order of 1000 + km. Data is collected at regular places and regular times.

Asynoptic meteorology

Data collected at irregular places and times.

Scales of motion

Planetary scale -> Synoptic scale -> Mesoscale -> Microscale

Geostrophic

Relating to the fact that the earth is rotating. PGF and CF.

Katafront

Active weather along or just ahead of the surface front (Most cold fronts).

Anafront

Active weather along or just behind the surface front (Quiet weather).

Isopleths

A curved line on a map joining points where a scalar quantity is equal in value.

Isobar

Equal pressure (Only at the surface).

Isotherm

Equal temperature.

Isohypse

Equal elevation.

Trough

Open, broad area of low pressure that merges into the general poleward decrease of pressure in the middle latitude westerlies.

Col

Saddle in the pressure distribution with low pressure north and south and high pressure east and west.

The atmosphere wants to be...

-Barotropic

-Geostrophic

-Hydrostatic

Geopotential height

approximates the actual height of a pressure surface above mean sea level.

Geostrophic wind

The wind flow that occurs in the middle latitudes aloft in the troposphere.

Real wind equation

V = Vag + Vg

What to look for at 850 mb

-Temperature forecasting

-Cloud forecasting

-Moisture return/advection

-LLJ

Warm air advection (WAA)

Wind blows from a region of warm air to a region of cooler air. Occurs ahead of the low. Produces upward motion.

Cold Air Advection (CAA)

Wind blows from a region of cool air to a region of warmer air. Occurs on the backside of a low. Produces sinking motion.

What to look for at 700 mb

Moisture

Vertical Velocity (Omega)

The speed at which air is rising or sinking. Rising air is negative omega, sinking air is positive omega.

What to look for at 500 mb

Vorticity.

Vorticity

Rising and spinning air.

Divergence

When a stronger wind moves away from a weaker wind or when air streams move in opposite directions.

Diffluence

The spreading of wind vectors. Height contours become further spaced from each other over distance.

Convergence

When a stronger wind moves towards a weaker wind or when air streams move in the same directions.

Confluence

The coming together of wind vectors. Height contours become closer spaced to each other over distance.

What to look for at 300 mb

Jet stream.

Jet streak

Segment of the jet stream that has relatively high velocity winds.

Entrance regions of jet streaks

Where winds are accelerating into the back/upstream side of the streak.

Straight lined jet streak

Typical motion/pattern.

Anticyclonically curved jet streak

Enhanced convergence/divergence on the poleward side.

Cyclonically curved jet streak

Enhanced convergence/divergence on the equatorward side.

Positively tilted trough

A sign that a weather system will weaken soon.

Negatively titled trough

A sign of a strengthening weather system.

Non tilted troughs

Vertically stacked. Not good for a developing cyclone.

Adiabatic process

One in which when air rises or sinks there is no exchange of mass, moisture, and momentum between the parcel and the environment.

Stable environment

A parcel, when lifted, returns to its original position. Air parcel is cooler than the environment.

Unstable environment

A parcel, when lifted, accelerates away from its original position. Air parcel is warmer then its environment.

Neutral environment

No acceleration but will not return to its original position (Lofting).

Environmental lapse rate

How temperature changes with height outside of air parcels.

Dry adiabatic lapse rate

The rate unsaturated parcels cool with height (10 C/Km).

Moist adiabatic lapse rate

The rate saturated parcels cool with height (6 C/Km).

Inversions

A deviation from the normal change of an atmospheric property with altitude.

Radiation inversion

When lowest layers of the atmosphere are cooled by contact.

Subsidence inversion

Descent and compressional warming of layers.

Frontal inversion

The transition from one mass to another in the vertical.

lifted condensation level (LCL)

Level where a lifted parcel first saturates (Base of a cloud).

How to find LCL

From the dewpoint curve, follow a saturation mixing ratio line upward. From the temperature curve follow a dry adiabat line upward. Where these lines intersect is the ...

Level of free convection (LFC)

Level where a lifted air parcel is first warmer than its environment, where a parcel becomes buoyant (Unstable).

How to find LFC

From the LCL proceed upward along a saturation adiabat until you intersect the temperature curve for the last time in the lower levels.

Equilibrium Level (EL)

The point at which a positively buoyant parcel becomes negatively buoyant. Can predict the height of a thunderstorm.

How to find the EL

From the LFC proceed upward along a saturation adiabat until it intersects the temperature curve.

Convective condensation level (CCL)

The height to which a parcel of air if heated sufficiently from below will rise adiabatically until it is just saturated. The base of a convective cloud.

How to find the CCL

Follow the saturation mixing ratio line from the dewpoint, until you intersect with temperature.

Convective temperature (Tc)

The surface temperature that must be reached to start formation of convective clouds by surface heating.

How to find Tc

From the CCL on the temperature curve, follow a dry line down to the surface.

CAPE

The maximum amount of energy available on an ascending air parcel.

Convective inhibition (CIN)

The amount of energy that will prevent an air parcel from rising from the surface to the level of free convection.

S.L.I.M.

Shear. Lift. Instability. Moisture.

Mixing ratio

Ratio of the mass of water vapor to the mass of dry air in a sample of moist air.

Saturation mixing ratio

The mixing ratio that a parcel of air would have if it were saturated.

Saturation vapor pressure

The partial pressure that water would add to the total atmospheric pressure if the air was saturated.

Virtual temperature

The temperature at which dry air at the same pressure would have the same density as the moist air.

Vapor pressure

The part of the total atmospheric pressure that is contributed by water vapor.

Potential temperature

The temperature a parcel would have if it were brought dry adiabatically to 1000 mb.

Equivalent potential temperature

After the moisture has been condensed out of the parcel, the parcel is brought down dry adiabatically to 1000 mb.

Lifted Index

The difference between the temperature of a parcel lifted from the lower atmosphere to the 500 mb level and the environmental temperature of that level.

>0 Deep convection not expected

K index

Thunderstorm potential as a function of vertical temperature lapse rate, low level moisture content, and depth of the moist layer. <15 No real potential. >40 100% chance for development

Showalter index

The Celsius degree difference in temperature between 850 mb parcel and the environmental air temperature at 500 mb.

+3 to +1 -RA, TRSA poss

Total totals

Computed using the temperature and dewpoint at 850 mb and the temperature at 500 mb.

44 TRSA poss

55+ SVR TRSA likely, poss TOR

Severe weather threat index

Takes into account the wind profile in assessing severe weather potential.

150 to 300 Slight severe

400+ Tornado possible