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 (M_HEIGHT, M_TEMP, M_WIND, and M_MOIST respectively.)

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

A brief summary of interpretting M_TOTAL (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, M_TOTAL can be interpretted rougly as how unusual the atmospheric conditions are across the eastern United States, Southeast Canada, and the adjacent waters.

PERSPECTIVE:

• There has yet to be an M_TOTAL of 4 or greater in the months of July or September since 1948. These remain the only two months when this has not happened.

Latest conditions and forecasts:

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:

• There have been no recent events to qualify.

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 M_TOTAL value slightly less than those analyzed by the higher-resolution NAM,and GFS grids. When comparing the forecast M_TOTAL values to historical cases, the 2.5 deg GFS forecasts (plotted above) should be used.

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