A User's Guide to the UW BMA page

Using the UW BMA Page involves exploring the probabilstic nature of the weather forecast by changing any of the following controls: forecast valid time, weather parameter (temperature, precipitation, etc), probabalistic feature (deterministic forecast, upper bound, lower bound, uncertainty, or probability of exceeding threshold), and point location.

Changing the Date and Time

The current forecast valid time is displayed in the upper-left corner of the Web page. Clicking on the yellow triangular arrow pointing to the right will advance the date by one day. Clicking on the arrow pointing to the left will move the date back by one day. Although the default surface temperature is valid at one time only (4 or 5pm in the afternoon local time), the other parameters apply to a range of time. Information about this time span should be advertised above the date itself.

Changing the Weather Parameter

The weather parameter can be changed using the drop down menu located on the very top left of the page. Currently available options are surface temperature, daytime high temperature ("Min 2m Temp (24-48 hrs)"), daytime high temperature ("Max 2m Temp (24-48 hrs)"), and 12-hour accumulated precipitation ("12-hour Precip. Accum.") Selecting a new parameter and hitting the "Retrieve Data" button will reload the page with the new information.

Changing the Probabilistic Feature

A radio menu located within the menu of the left of the page controls the currently loaded 'probabilistic feature'. The UW BMA page provides a probabilistic forecast, which essentially adds another dimension to the forecast. The user can select between a deterministic forecast (a best guess or most likely outcome), an upper or lower bound (possible but unlikely extreme outcomes), a predictive interval half-width (a measure of forecast uncertainty), or the probability that a given value is exceeded (such as the low temperature will fall below freezing or that there will be a measurable amount of precipitation). Some of these probabilistic features depend on further values, which the user can set by using the entry boxes to the right of the radio items. These include the percentiles for the upper and lower bound (defaulted to 90% and 10% accordingly) and the threshold value.

For simplicity, certain combinations of these controls that are considered of principal interest are located across the top of the page. These include probability of freezing, probability of precip greater than 0 hundredths of an inch, probability of precip greater than 1/4 of an inch, and probability of precip greater than one inch. Although manipulating the left-hand menu can provide all of these, these "quick links" are included for convenience.

Changing the Point Location

While the map plot always displays for the entire model domain, the 'box and whisker' plot above the map applies to a single point location. In addition, the map display can be replaced by a plot of the PDF for the same point by selecting the "Probability Distribution" radio button (as oppsed to "Grid Forecast") from the menu on the left. The current point coordinates are given in the latitude / longitude boxes in the menu, and is represnted on the map by a gold star.

The user can change the point location by one of two methods; the first is simply by left-clicking on the desired location on the map. Alternatively, the new coordinates can be enterred into the lat/lon boxes and will be loaded when the user presses the "Retrieve Data" button.

Verification

The UW BMA page includes a companion verification website. This is accessible from the BMA page by clicking on either the pie chart on the lower left or the "Forecast Error" field also in the lower left, or by visiting bma.apl.washington.edu/verify.jsp

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