Current and Forecast Meteorological Rarity

Robert Hart and Richard Grumm

Updated 16 April 2003 to note non-record 10 April 2003 cut-off cyclone.


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Description

The rarity of observed and forecast conditions across the eastern half of the U.S. and SE Canada is depicted here.

The magnitude of rarity is defined as the maximum absolute number of standard deviations fields depart from daily norms (the anomaly magnitude).

This is calculated for each of four variables: height, temperature, wind, and moisture.

Example current and forecast plan-view anomaly fields are available for output from the Ensemble webpage , ENSEMBLES, and the deterministic webpage Deterministic.

The vertical mass-weighted mean anomaly is calculated for each of the above four variables (MHEIGHT, MTEMP, MWIND, and MMOIST respectively.)

The total anomaly (measure of rarity or extreme) is the mean of the four anomalies:

MTOTAL = (MHEIGHT + MTEMP + MWIND + MMOIST) / 4

A brief summary of interpretting MTOTAL (assuming a 2.50 grid resolution):

MTOTAL value
(Standard Deviations)
Generic Description
Approximat return period
based on 52-year database
5.00
Not observed since 1948
?
4.75
Historic activity
50 years
4.50
Historic activity
15-20 years
4.25
Extreme activity
5-10 years
4.00
Rare activity
3 years
3.50
Unusual activity
5-10 months
2.75
Above average activity
2 weeks
2.0-2.5
Near normal activity
daily
1.75
Below average activity
2 weeks
1.50
Unusual inactivity
2-4 months
1.40
Rare inactivity
6-8 months
1.30
Extreme inactivity
3-5 years
1.20
Historic Inactivity
50 years
1.10
Not observed since 1948
?

 

Therefore, MTOTAL can be interpretted rougly as how unusual the atmospheric conditions are across the eastern United States, Southeast Canada, and the adjacent waters.


PERSPECTIVE:


Latest conditions and forecasts:

Latest 2.50x2.50 GFS Analyses and Forecast:

Important NOTE: Model-forecast tropical cyclones are not intended to be included as part of the rarity index shown here.
Bogus (falsely predicted) tropical cyclones can produce erroneously large forecast anomalies, especially beyond 72hrs.

Other current model forecasts: [ NAM |

Archive of model forecasts.

To compare the current and forecast climate departures above to past events, check the historical rankings analysis from 1948-current.


Notes on recent events:

Important Note: These preliminary updates to the "official" rankings are not included until the departures are recalculated using the NCEP 2.5 degree reanalysis fields. These typically aren't available for 1-3 months after the event and tend to have an MTOTAL value slightly less than those analyzed by the higher-resolution NAM,and GFS grids. When comparing the forecast MTOTAL values to historical cases, the 2.5 deg GFS forecasts (plotted above) should be used.


Last Updated: 3 October 2005

Robert Hart, FSU Meteorology,Tallahasse, FL
Rich Grumm, NWS State College