Ag Weather Week


Forecast and Outlook accuracy:  This report provides an overview of observations and forecasts for recent and upcoming weather a supplement to real-time weather forecast updates.  Weather Forecast skill degrades with increasing range.  For temperature, accuracy of predictions for a specific time and location becomes not much better than using a climatic average value beyond about 12 days.  For precipitation, the range of useful skill is considerably shorter, and varies with the type of weather system that causes the precipitation.  It is easier to predict precipitation from the movement of large frontal air masses than from localized convective thunderstorms.

Long range weather Outlooks differ from forecasts in that they do not try to estimate the conditions at a specific location and point in time.  Instead, they estimate the average conditions over an area across a period of time.  Because they are less specific, outlooks can maintain skill above using the climatic average over a longer range.  As with forecasts, outlooks for a given range are more accurate for temperature than for precipitation.  Weather Outlooks may be used occasionally for special monthly reviews, but most of the Ag Weather Week newsletters will only show the past 7 days, and forecasts for the next 7 days, days 8-14, and days 15-28.

Presentation:  Observed and forecast temperature is shown as the average of the daily mean degrees Fahrenheit.  Precipitation and Evapotranspiration (ET) are shown as the weekly rate of water gained or lost.

Evapotranspiration (ET) measures water loss by combined action of evaporation from surfaces and active transpiration by water movement in plants.  ET losses increase with temperature, wind speed, intensity and duration of solar radiation, and decreases with relative humidity.  ET also depends on the type, density and size of plant cover.  A low-growing turf grass at 4 inches height is the standard reference for calculating ET.  ET losses increase through the season as annual plants grow from small seedlings to fully developed plants, and as perennial plants like trees progress from budbreak to fully developed foliage.  ET is graphed as positive values, i.e. the more ET the higher the value.  But ET essentially functions as negative precipitation.

Water Balance: Precipitation minus ET (Precip-ET) measures the net difference between soil moisture gains from precipitation minus the losses caused by ET.  This value can be positive if the amount of Precipitation gained is greater than the amount of water lost through Evapotranspiration.  Precip-ET is usually negative during the summer unless there is more than 1 to 1.5 inch of rain during a week.  Precip-ET is not a direct measure of soil moisture, but it summarizes two atmospheric factors that greatly affect soil moisture.  The data source for observed Precip-ET is not updated as frequently and reliably as the other measures.  If there is not a recent Precip-ET observation available, only the forecast values for Precip-ET will be shown.

Map scales. For consistency and simplicity, a single scale is used so that the colors on the maps for different time periods have the same meaning.  For the same reason, Precipitation, ET, and Precip-ET values are standardized as the amount per week.  The total for the two-week period of the 15-28 day forecast is simply 2X the weekly rate.

Special Cases:  If there are white areas on a Temperature, Precipitation, ET, Precip – ET map, that means the value exceeded the color scale range.
If a large portion of a map exceeds the standard color scale, a customized map range will be used and the map will be marked with this icon:

Soil Moisture indicators

The Variable Infiltration Capacity (VIC) soil water model accounts for the effects of precipitation additions, evapotranspiration (ET) losses, run-off, and drainage on the water content of the top meter of soil The VIC has been validated in the Northeast, but as with the Palmer Z Index, CMI and USDM, it does not account for variability between soil types, topography, tillage, and vegetative cover at the local scale.  The 1-month ahead VIC outlook does account for El Nino.  All of the soil moisture values shown in this report are just general area-wide estimates that will differ from site-specific soil moisture.  If the difference is consistent over time, then the general estimates may be of some use as predictive tools for your site.

The VIC soil moisture is shown as the 0-100 percentile rank that compares the current condition to the 1916-2004 records for the same day of the year.  If the current value is the lowest on record, then the percentile is 0.  If the estimate equals the average of previous years, then the percentile is 50.  If it is the highest value on record, the percentile is 100.  The USDM categories are also defined by percentile.

The Crop Moisture Index (CMI) indicates short-term (up to 4 weeks) dryness or wetness of the upper meter of soil.  It is similar to the Palmer Z Index and responds quickly to changing conditions and can vary from week to week.  The CMI may not apply to germinating seeds and shallow-rooted crops that are unable to extract deeper soil moisture, or to cool season crops when temperatures average below 55F.  For clarity, the rating scale intervals has been rounded to whole numbers.  The CMI does not account for small scale variation in soil type, topography, and the effect of plant cover on water balance.

The Palmer Z Index will only be used if there is a monthly review.  The Palmer Z is a commonly used drought index for short-term drought on a monthly scale.  It reflects the departure from normal monthly moisture conditions using precipitation and temperature to estimate a simple water balance for one month.  Limited to just one month, it is a short-term drought measure without memory from previous months.  For clarity the breakpoints between categories have been slightly adjusted from published values.

The US Drought Monitor (USDM) tracks cumulative drought severity over many months.  It will only be used for monthly reviews unless there is potential for a governmental drought disaster declaration.  The USDM is based on temperature, precipitation supplemented with other observations such as stream flow, ground water, and reservoir status. The USDM has a longer lag time to respond to recent precipitation and drying than the other soil moisture measures described above.  The USDM is shown because it provides long-term perspective for drought status and because it is used for drought disaster declarations and water access regulations.  The VIC, Palmer Z Index, and CMI are more suitable measures of agricultural soil status and crop water needs.


This report is for looking, not reading.

An Extension colleague once told me that the ideal newsletter was “Short enough to read between the mailbox and the front door.”  This report breaks that rule by summarizing a great deal of information. But once you get familiar with the format, I think you will find it easy to skim through the pages and get what you want in a few minutes.  Use what you want and skip the rest.  Suggestions sent to about what to add or subtract or on how to summarize and display values will be sincerely considered and appreciated.  And thanks for the food.



Observed and Forecast Temperature, Precipitation, Evapotranspiration (ET), and Cumulative Degree days are from, and  Values are updated daily, with a day or two lag for the most recent observations.

Variable Infiltration Capacity (VIC) soil moisture maps are updated daily at

NOAA 7 and 14 day soil moisture anomaly forecasts are based on the Global Ensemble Forecast System in comparison to a 1971-2000 climatic average reference period.  But that citation may be out of date.
Maps are updated daily at

Crop Moisture Index map is from NOAA (National Oceanographic and Atmospheric Administration) website at  The CMI map and data table are updated on Tuesdays value up to the previous Saturday.  The CMI data table is at

Palmer Z Index map is from Climate

US Drought Monitor maps of weekly change and long-term drought status are updated weekly on Thursday with data through the previous Tuesday at  &


Long-range Outlooks.  These values will only occasionally be used if there are monthly reviews:

Temperature and Precipitation Outlooks for the 1st, 2nd, and 3rd upcoming months are from Climate

NOAA 1st, 2nd, and 3rd month Soil moisture outlooks are updated daily at  The reference period is presumably the same 1971-2000 used for the NOAA 7 and 14 day soil moisture maps.

The 3-month Seasonal Temperature, Precipitation, and Drought Outlooks are updated on the on the 3rd  Thursday each month (i.e. 15th to 21st) at  This report uses the Northeast version of those maps posted by the Northeast Regional Climate Center at
Numerical values for the 3-month temperature and precipitation outlook anomalies are at  &
The seasonal drought outlook is at



These charts show estimates of temperature, precipitation, evapotranspiration, and soil moisture for the upcoming forecast periods.  The values used to make the charts are taken from the anomaly maps for each time period shown above in this report.  Values from several point locations within each climate region are used to estimate an area-weighted value for the region.  Values for the 3-month temperature and precipitation outlooks are from the NOAA data table, not from maps.

The temperature, precipitation, ET, Precip-ET, or soil moisture value for any single point within a region can (and indeed often will), be substantially different than the regional average anomaly.  In addition, site-specific soil moisture varies with soil type, topography, tillage, vegetative cover, and other local factors.