some info on reading the graph
Posted by
cypresstx on 8/27/2011, 11:18 pm
from their manual, since there are probably people who usually don't read them also on the board:
The graph combines several sources of data to produce a total water level prediction. To do so, it graphs the observed water levels in comparison to the predicted tide and predicted surge before the current time. This allows it to compute the "Anomaly". The "Anomaly" is the amount of water that was not predicted by either the tide or the storm surge model. This "Anomaly" is averaged over 5 days, and is then added to the future predictions of the tide and storm surge to predict the Total Water Level.
The first thing one notices is that there are two magenta vertical lines. The earlier one is when the storm surge model was run. It is run at 0Z and 12Z every day and the text form is available at: http://www.nws.noaa.gov/mdl/marine/etsurge.htm. The later magenta line is when the graph was generated. It is currently being generated 15 minutes after the top of every hour. (This is also the date that follows the label.)
The next thing one notices are the horizontal lines labeled MLLW, MSL, MHHW, and MAT. These stand for the Mean Lower Low Water, Mean Sea Level, Mean Higher High Water, and Maximum Astronomical Tide. MAT was computed using our tide model, by computing the maximum of the predicted value for every hour (on the hour) for 19 years. The thought is that there is probably flooding if the total water level crosses MAT. The other datums came from http://www.co-ops.nos.noaa.gov/data_res.html.
One might next notice the red observation line. This is based on data attained from Tides Online . Please see their Disclaimer for information as to the quality of these observations. If there is no red line, then either Tides Online does not have data for that station, or there has been a communications break down.
The next thing of interest is the green Tide line. This is the astronomical tide at every hour. The Harmonic Constants used were obtained from http://www.co-ops.nos.noaa.gov/data_res.html.
We then note the gold storm surge curve, which is created by "pasting" one 48 hour prediction to the next 48 hour prediction. That is, using 12 hours from each prediction until the last prediction where we use 48 hours. The result is that we may generate kinks in the curve every 12 hours, where the model adjusted its prediction based on new data from the GFS wind model.
Next we note the blue curve, which is the "Anomaly" referred to above. This is simply the observation - (tide + storm surge). Preferably it is constant. The amount of deviation from a constant is an approximation of our error. Since we add the 5 day average of this value to our prediction, the perfect forecast does not have to have a zero Anomaly.
Finally we see the black forecast curve. This is what we are really interested in, which is the total water level created by adding the 5 day average anomaly to the predicted tide, and the predicted storm surge. |
134
In this thread:
Storm Surge in NYC/Long Island -
Gianmarc,
8/27/2011, 10:40 pm Post A Reply
This thread has been archived and can no longer receive replies.