N A T I O N A L    O C E A N I C    A N D    A T M O S P H E R I C    A D M I N I S T R A T I O N

Main Site
Mirror Site
Gulf of Alaska
Ketchikan, AK
Sitka, AK
Juneau, AK
Skagway, AK
Elfin Cove, AK
Yakutat, AK
Cordova, AK
Valdez, AK
Seward, AK
Seldovia, AK
Nikiski, AK
Anchorage, AK
Kodiak Island, AK
Alitak, AK
Sand Point, AK
King Cove, AK
Adak Island, AK
Atka, AK
Unalaska, AK
Surge List
East Coast
Gulf Coast
West Coast
Norton Sound
Arctic Alaska
Gulf of Alaska
NWS logo

Extratropical Water Level Guidance

About Datums
MDL Marine Team

What is a datum?

In the context of this site, datum refers to a vertical tidal datum. The four datums used by this site are defined as follows. For more on tidal datums, please see NOAA's National Ocean Service (NOS) tidal datum definition page.

Highest Astronomical Tide
"The [height] of the highest predicted astronomical tide expected to occur ... over the National Tidal Datum Epoch (NTDE). The present NTDE is 1983 through 2001." HAT is an estimate of the highest tide predictable strictly from the effects of gravity.
Mean Higher High Water
"The average of the higher high water height of each tidal day observed over the NTDE."
Mean Sea Level
"The arithmetic mean of the hourly water heights observed over the NTDE."
Mean Lower Low Water
"The average of the lower low water height of each tidal day observed over the NTDE."

What do the Datum buttons do?

The buttons allow the graphs or text to be displayed in other datums.
  • HAT provides an estimate of where the "grass line" is. Crossing HAT is an indication that flooding will occur as people tend to build to the grass line.

  • MHHW is an estimate of how high water gets each day; however it is exceeded by the tidal cycle alone for approximately half the month. This site uses it as a warning that waters are likely to be high, so please pay attention. In addition, NOS and NHC consider this to be the threshold for flooding. Please see their Memo as .pdf.

  • MSL is the average water surface and the most familiar to the general public. Deviations from MSL provide a precise description of unexpected amounts of water, but it is difficult to directly tie it to human impacts due to the variability of the tide range centered on MSL.

  • MLLW is an estimate of how low water gets each day and is the standard datum used by NOS tide stations (and earlier versions of this site). It is useful for mariners concerned with running aground.

NOTE: "Surge Guidance" and "Anomaly" values are changes in water level, so are "datumless". This means that while they appear to move on the graph when toggling datums, they actually continue to be centered on 0.