|
<br />,4,
<br />
<br />John Henz, HDR - we maximize all storms used in this study, He mentioned that not all storms that are the
<br />'worst case' have been generated yet (ex: Big Thompson where surface dew points were extremely high),
<br />The point was that the PMP employed a maximization of surface dew points that have been observed in the
<br />recorded history of Eastcm Colorado, He mentioned that the numerical forecast models can handle 'general
<br />precipitation' but there are difficulties encountered when thunderstorm-related precipitation is involved, Henz
<br />also acknowledged that some improvements have been made in large storm modeling,
<br />
<br />Ed Tomlinson, A W A ' In response to John Liou stated that the definition is not theoretical for PMP studies,
<br />We need to go from a definition to a value to give to the engineers to get a PMF output. There is no model to
<br />compute the theoretical PMP number, Assumptions need to be made and are made by the A W A, the NWS
<br />and the World Meteorological Organization alike, If there are good storms on record for a region we can
<br />analyze those storms, maximize those storms with more moisture available, We are conservative and we make
<br />the worst assumptions with dew points; then we take that to increase the rainfall. We have to stick with the
<br />accepted methodologies, If the procedure works and we get a bigger storm next week to add to the historical
<br />list; we will most likely get a value near the theoretical PMP value; if we have a bigger storm tomorrow, the
<br />previous maximum factor may be lower, but we may still get a number similar to the calculated PMP,
<br />
<br />Lou Schreiner, BOR - The NWS follows what Tomlinson is saying, but the NWS goes beyond to include all
<br />generalized storms, We feel this is appropriate and we are probably more conservative, Both methodologies
<br />are still storm based with storms that have occurred in the past. There are limits to what storms we use, like
<br />A W A using local storms and the NWS using storms from across the eastem US, In the late 40s, the NWS was
<br />commissioned to develop numbers that hydrologic engineers could use, Over the years, the NWS believes
<br />that the best methodology is to go to the generalized reports, The Bureau, in the 50s - 70s, we went back to a
<br />procedure similar to the A W A method, but in the 80s, we adopted the generalize reports because we thought
<br />they were superior to the site,specific studies; the generalized report will give a more conservative values in
<br />the end, Mr, Schreiner added comments that all storms (spatially) applied by the NWS also goes through a
<br />theoretical adjustment as well, In addition, he also mentions 'how much' data is 'realistic' in arriving at a
<br />theoretical value as well. Over the years he believes that the best and most consistent methodology is to go
<br />through 'general reports' ,
<br />
<br />Ed Tomlinson, A W A - there are models like the MM5 and the RAMs model that can produce rain on the
<br />ground, but they are not certified and not totally consistent yet.
<br />
<br />John Henz, HDR - We find the highest dew point values for a storm and don't assume another storm is
<br />feeding off of that dew point between the storm and the dew point location, We use the highest dew points to
<br />get the most maximum rainfall on the ground, We can use higher dew points and higher feeding winds;
<br />
<br />Jack Byers, DWR - You mentioned other models that are being use; how do those compare and how far are
<br />these models from being useful in this study? How do the 'new' models that simulate weather compare with
<br />reality and how far are they from being useful?
<br />
<br />Ed Tomlinson, A W A ' The models aren't being run for this purpose at this time because there is no funding
<br />right now; we would like to make that comparison, In Iowa some MM5 runs ha ve shown that their procedures
<br />might not be working well, but is that due to the model not being perfected yet.
<br />
<br />Larry Lang, CWCB - The NWS has said they could start working on a theoretical model in about ten years
<br />if they had the funding and the personnel.
<br />
<br />John Henz, HDR - with this last snow storm, it was forecasted very well by the models, three days in
<br />advance, we knew that we were going to get hit with heavy storm, but it was a general storm, local storms
<br />cannot be modeled as well; the new models can get within 10% of reality for general storms;
<br />
<br />Flood Protection. Water Project Planning and Finance. Stream and Lake Protection
<br />Water Supply Protection -Conservation Planning
<br />
|