Some Observations and Forecasts of Wind and Wind Gusts Associated with Isaac as it Made its Second Landfall

Early Wednesday morning, 29 August 2012, Hurricane Isaac made its second landfall at around 0700 UTC per a National Hurricane Center Advisory. While more complete data is unavailable, and specific observational tools may not tell the whole story, I thought I would share a few of the analysis products for Isaac that the Tropical Cyclone Winds CSTAR project has examined with other storms.

The Hurricane Research Division (HRD) continues to participate in the H*Wind Project whose purpose is to develop an integrated tropical cyclone observing system in which wind measurements from a variety of observation platforms could be used to develop an objective analysis of the distribution of wind speeds in a hurricane. The HRD wind analysis requires the input of all available surface weather observations (e.g., ships, buoys, coastal platforms, surface aviation reports, reconnaissance aircraft data adjusted to the surface, etc.). More information on the H*WIND data is available in a previous blog post – HRD Real-time Hurricane Wind Analysis System (H*WIND. The H*WIND data is available every 3 hours and an analysis was available just before Isaac made landfall at 0430 UTC, and is shown above.  Archived H*WIND data for Isaac is available at this link.

The Multi-Platform Tropical Cyclone Surface Wind Analysis (MPTCSWA) from CIRA for 0600 UTC 29 August is shown to the right. This product is intended to provide an “estimate” of the 1-minute maximum winds at 10-m elevation. Note that there are several limitations with this data set and more information on the MPTCSWA is available from a previous CIMMSE blog post – Hurricane Rina, Irene, and the Multi-Platform Tropical Cyclone Surface Wind Analysis. Archived MPTCSWA data for Isaac is available at this link.

Short term wind forecasts from the NDFD valid at 0300 UTC (left) and 0600 UTC (right) on 29 August are shown below. The forecasts appear well collaborated and depict a symmetric storm system with a sharp gradient in wind speeds on the perimeter of the storm.

A quantitative verification of wind and wind gust forecasts is not available at this time but some general comments can be made. The figure below shows the short term wind forecasts from the NDFD valid at 0300 UTC (left) and 0600 UTC (center) on 29 August with the 0430 UTC H*WIND analysis (right). The H*WIND analysis is preferred over the MPTCSWA because of the greater number of observations the H*WIND uses and some studies note that the MPTCSWA has a high bias compared to the H*WIND. A recent study notes that for well sampled, stable, TCs over open water, the H*WIND itself has a general positive bias to the wind speed as noted in a blog post – H*Wind Uncertainty Estimates. When comparing the H*WIND analysis with the NDFD wind forecasts, the NDFD winds appear to have some positive bias, especially at longer distances from the storm center. In addition, the over-land wind speed reduction might be under represented.  The positive NDFD wind bias and an insufficient over-land reduction of winds are consistent with a verification of NDFD forecasts for tropical cyclones in the Southeast, completed as a part of the TC Wind CSTAR project. More information on the NDFD verification is available in a previous CIMMSE blog  post – NDFD Final Verification.

Short term wind gust forecasts from the NDFD valid at 0300 UTC (left) and 0600 UTC (right) on 29 August are shown below. Some minor collaboration issues are noted, especially on the perimeter of the storm. A very large area of wind gusts in excess of 50 kts is shown in the dark red but it is difficult to note whether the wind gust speeds are too strong.

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4 Responses to Some Observations and Forecasts of Wind and Wind Gusts Associated with Isaac as it Made its Second Landfall

  1. bptyner says:

    Thanks for sharing this, Jonathan. Very good analysis and I agree with many of your conclusions. Does anyone know off hand if the effort to add battery backup to many of the ASOS stations in our study region this year was also done in the Gulf of Mexico region? Also, is there a good source where I can acquire this near real-time ASOS 1 min wind data? NCDC only has data on its website through July 2012. I would like to do a quick analysis and compare the forecasts to NDFD data if possible.

  2. Jonathan Blaes @ WFO RAH says:

    Hi Bryce,

    Thanks for the note. I believe there were ~65 southern region ASOS locations that were to be upgraded or reconfigured which covered most of the ASOS near and within 100-200 km of the coast. AWOS locations are vulnerable due to power and communications issues while ASOS units should be primarily susceptible to communications problems.

    The only source of one-minute ASOS data that I am aware of is from the URL below (five minute data is available too). The ftp site currently only has data through July but I think they dump a whole months worth of data in their at one time at the beginning of the next month. Since Monday was a holiday and today is the first work day of the month, it may take a few more days but probably will be there soon.
    ftp://ftp.ncdc.noaa.gov/pub/data/asos-onemin/
    ftp://ftp.ncdc.noaa.gov/pub/data/asos-fivemin/

    A comparison of the forecasts to observations for this storm would be interesting. I’d enjoy seeing it.

    JB

  3. Very interesting post, JB, thanks. It does look like the WFO forecasts (the source of NDFD) at 03z and 06z overdid winds on the west side of the storm. In comparison to the H*Wind analysis, the peak magnitude on the east side wasn’t too bad, although obviously they were more spatially extensive than what actually occurred.

    The fact that the H*Wind picks up on the eye of the storm brings up an interesting point. The NDFD did not depict the calmer eye winds, since NWS CONUS offices do not, according to guidelines, use eye diameter when creating TC winds using the TCM wind tool. (This has been the general rule for several years, and I believe that it remains in effect, at least for ER offices; please do correct me if I’m wrong.) While our grids used to be created and sent at 5 km resolution, they are now done at 2.5 km resolution, which is within the scope of most TC eyes. Is it now time to revisit the idea of not including the eye in our wind forecasts, at least for coastal offices? Just a thought.

    -Gail @ RAH

  4. hurricanebob says:

    Good point about the eye diameter Gail. I don’t see why we shouldn’t since the eye is depicted based on the wind directions. Sounds like a good topic for the NOAA Hurricane Conf. Thoughts?

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