Andy WoodUniv. of Washington

Dept. of Civil & Envir. Engr.

Statistics related to the merging of short and long lead precipitation predictions in the continental U.S.

BACKGROUNDThe use of medium-range forecasts (out to 15-days) of precipitation to augment precipitation predictions at longer lead times is becoming common both in climate forecast groups and applications sectors.

A preliminary step in understanding the relative merits of combining forecast information at different lead times is to explore statistics relating precipitation from an initial short forecast period (e.g., from 3 to 15 day) to precipitation during a longer period continuing from (and including) the short period (e.g., out to 3 or 6 months).

OBJECTIVETo explore variations in the importance of the short initial period precipitation for estimating precipitation over longer periods, depending on time of year, region and aggregation period length.

APPLICATION IN 3-MONTH OUTLOOKS

References / Acknowledgements

The author gratefully acknowledges the support of NOAA CDEP and CPPA programs.The author also recognizes the NOAA ACIS network for providing the station data records used in this work.

APPLICATION IN WATER SUPPLY FORECASTING

The value of short-to-medium range precipitation forecasts for longer lead predictions varies by time of year, location and lead time, hence a blanket approach for considering the former in creating the latter is unsuitable.

The CPC 3-month Drought Outlooks may benefit most from medium range predictions in the West Coast in winter & spring, and the Plains / Colorado and Upper Midwest regions in fall.

Western water supply forecasts may gain the most from initial period forecasts in California in late winter, and PNW in early winter. Suggestions for further analysis are encouraged!

Summary Observations

APPROACHDATA

Daily COOP precipitation records from 2131 stations having more than 40 years of record across the continental US were aggregated into periods of:

1, 2, 3, 4, 7, 10, and 15 days 1, 2, 3, 6, 9, and 12 months, and the period until the End Of the Water Year (EOWY)

for start dates in approximately each week of the year -- on the 1st , 8th, 16th and 24th of each month.

ANALYSISFor each station, aggregation period and start date, the median precipitation was calculated and used to categorize the aggregated records as wet or dry. The probabilities of transitioning from each category for a short initial period to each category for longer periods having the same start date were then calculated.

CONTEXT2 week and shorter forecasts are considered by US Drought Outlook authors in the preparation of the 3-month Outlook for the Continental US. The relationship between precipitation in coming 1 and 2 week periods to the coming 3 month period is illustrated using the probability that the category (wet/dry) of the short initial period will persist for the longer period.

CONTEXT2 week and shorter precipitation forecasts are considered by NWS River Forecast Center and NRCS National Water and Climate Center forecasters in preparing long lead streamflow forecasts for summer water supply in the western US. Typical forecast periods are April-July and April-September. As part of this process, the medium range forecasts may also used to estimate fraction of normal precipitation for the coming month.

The relationship between precipitation in coming 1 and 2 week periods to the coming 1 month period (at right) and to the remainder of the water year (EOWY, see below) is illustrated using the probability that the category (wet/dry) of the short initial period will persist for the longer period.

7 day lead in to 91 day period

Areas/times of stronger persistence:

California – Feb to JuneUpper Midwest – NovNorthern Plains – Oct, NovPacific Northwest -- Dec

15 day lead in to 91 day period

Areas/times of stronger persistence:

California – Feb to JuneUpper Midwest – Sep, Nov, DecNortheast – Nov to Jan Northern Plains – Oct, NovPacific Northwest – Dec, JanGulf – Oct to JanLower Mississippi – Apr to JunMid-Atlantic – Oct to Jan

Probabilities were averagedover the three regionsshown at bottom right. Theirbehavior during the wateryear is plotted at right, with atypical summer forecastperiod shown in gray.

WestCoast

Plains / Colorado Upper

Midwest

Mid-Atlantic

4 day lead in to 30 day period

Areas/times of stronger persistence:

N. California – Nov, DecCalifornia – Jun, Jul, Aug (dry season)Upper Midwest – NovNortheast – Nov, JanPacific Northwest – Dec, JulSouthwest – Mar to May

Probabilities were averagedover the four regions shownat bottom right. Theirbehavior during the wateryear is plotted at right.

Cali-fornia

ColoradoR. Basin

PacificNorthwest

7 day lead in to EOWY period

Areas/times of stronger persistence:

California – Feb, Mar, AprPacific Northwest – DecMany locations – Sep (for obvious reasons)

15 day lead in to EOWY period

Areas/times of stronger persistence:

California – Feb to JunPacific Northwest – Dec, Jan, May, JunSoutheast – JunEverywhere – Sep (for

obvious reasons)