Transcript Statistics related to the merging of short and long lead precipitation

Statistics related to the merging of short and long lead
precipitation predictions in the continental U.S.
APPROACH
BACKGROUND
The 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).
To 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.
Univ. of Washington
Dept. of Civil & Envir. Engr.
APPLICATION IN WATER SUPPLY FORECASTING
DATA
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.
ANALYSIS
OBJECTIVE
Andy Wood
For 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.
CONTEXT
2 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.
4 day lead in to 30 day period
Areas/times of stronger persistence:
 N. California – Nov, Dec
 California – Jun, Jul, Aug (dry
season)
 Upper Midwest – Nov
 Northeast – Nov, Jan
 Pacific Northwest – Dec, Jul
 Southwest – Mar to May
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.
APPLICATION IN 3-MONTH OUTLOOKS
CONTEXT
7 day lead in to EOWY period
2 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.
Areas/times of stronger persistence:
 California – Feb, Mar, Apr
 Pacific Northwest – Dec
 Many locations – Sep (for obvious reasons)
7 day lead in to 91 day period
Probabilities were averaged
over the three regions
shown at bottom right. Their
behavior during the water
year is plotted at right, with a
typical summer forecast
period shown in gray.
Areas/times of stronger persistence:




California – Feb to June
Upper Midwest – Nov
Northern Plains – Oct, Nov
Pacific Northwest -- Dec
Probabilities were averaged
over the four regions shown
at bottom right. Their
behavior during the water
year is plotted at right.
15 day lead in to EOWY period
Areas/times of stronger persistence:
Pacific
Northwest
 California – Feb to Jun
 Pacific Northwest – Dec, Jan,
May, Jun
 Southeast – Jun
 Everywhere – Sep (for obvious
reasons)
California
15 day lead in to 91 day period
Colorado
R. Basin
Areas/times of stronger persistence:








California – Feb to June
Upper Midwest – Sep, Nov, Dec
Northeast – Nov to Jan
Northern Plains – Oct, Nov
Pacific Northwest – Dec, Jan
Gulf – Oct to Jan
Lower Mississippi – Apr to Jun
Mid-Atlantic – Oct to Jan
Summary Observations

Plains /
Colorado

Upper
Midwest


West
Coast
MidAtlantic
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!
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.