Saltstalker wrote:Thank you all very much.
My problem i guess is , im mot seeing what is predicted.
Endo wrote:I've always struggled a bit to see/understand current water level vs "normal". For example, my favorite flats area sucks when the bay water is 1-2ft above normal. I'd like to reliably know if the water is high or low, before I go down, so can alter my plans if needed.
Below is a screenshot of NOAA water levels. Can someone clarify how I would interpret this. Do you look at the difference between the red line (observed) and the blue line (normal level) and this would tell you if the water level is up or down from "normal" . So i wold look at this as say it looks as if the water level is 1-1.5 feet above normal.
Also, anyone know a way to view more than 30 days history in water levels? NOAA site limits charting to 30 days max.
Endo wrote:Thanks Neumie, I get that it is the variance vs the predicted level, but what is "predicted". Can we consider predicted = normal/avg water level?
I get all the things that can effect and drive the difference, just trying to make sure i'm interpreting the graph correctly.
Can we assume Predicted equals Normal/Average water level?
Or, does NOAA actually try to "predict" all these other influences like wind, weather, rain run-off, etc.... Or, does NOAA only predict based off historical avg water levels, and thats all they are looking at.
NOAA wrote:NATIONAL TIDAL DATUM EPOCH—The specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values ( e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of periodic and apparent secular trends in sea level. The present National Tidal Datum Epoch is 1960 through 1978. It is reviewed annually for possible revision and must be actively considered for revision every 25 years.
NOAA wrote:For the most part, tide predictions for U.S. reference stations are based upon analyses of tide observations for periods of at least one year. Since the extremes of meteorological conditions have been excluded from the analyses and predictions, the predicted tidal heights should be considered as those expected under average weather conditions. During times when weather conditions differ from what is considered average for the area, the mariner must take note of the corresponding differences between predicted levels and those actually observed. Generally, prolonged onshore winds or a low barometric pressure can produce higher levels than predicted, while the opposite can result in lower levels than those predicted.
Exclusive of weather conditions, the astronomical tide is subject to range variations which should be noted. Decreased ranges may be expected near the times when the Moon is in apogee (apogean tides) or in quadrature (neap tides), and increased ranges may be expected when the Moon is in perigee (perigean tides) or in a new or full position (spring tides). A larger diurnal range may also result when the Moon is in its maximum declination (tropic tides). The actual range will depend upon the extent to which combinations of these positions reinforce or detract one from the other. The effect of these astronomical lineups is included in the predictions and may be apparent upon inspection.
The mariner may be kept aware of the times of these astronomical events by referring to the astronomical data listed in this book. He should realize, however, that there is generally a time lag from a few hours to several days from the time of the astronomical event to the time of the resultant tide. During times of storm surges or when extreme weather conditions are imminent, the mariner should closely follow local weather forecasts as they relate to the effects upon the tide levels.
NOAA wrote:Observations Supporting Predictions.– All tidal predictions made by the National Ocean Service are based upon observations taken at the location in question. For most reference stations these observations often are of a continuing nature. As such, they are used to quality control the predictions and to update the harmonic constants used in generating annual predictions. For subordinate stations, the age and duration of their observations vary from a few days of observation taken decades ago to the most recent survey data.
The precision with which the position, ranges and mean tide level are reported in Table 2 is an indication of the age and analytical history of the supporting observation. Stations whose position is reported to the nearest tenth minute of latitude and longitude and whose ranges and mean tide level are reported to the nearest hundredth foot are supported by the most recent observations, analyzed with regard to current chart datums and the 1983-2001 National Tidal Datum Epoch. Stations whose position is reported to the nearest tenth minute but whose ranges and mean tide level are reported to the nearest tenth foot are typically supported by observations taken in the 1960’s and 1970’s with analysis based upon the previous National Tidal Datum Epochs. Finally, stations whose positions are reported to the nearest minute and whose ranges and mean tide level are reported to the nearest tenth foot indicated either older supporting observations or simply data not yet reviewed and entered into the Tables with full published precision. NOS is in the continuous process of updating the Tables with all available data.
Old observations are not in and of themselves an indication of poor present predictions. Certain coastal areas do not undergo much human or natural modification while other coastal areas are subject to nearly constant modification by both agents. Local knowledge of conditions is still very important to the wise use of these astronomical predictions.
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