Sunday, October 9, 2011

Its All in the Timing

How many times does this happen?  The National Weather Service forecasts rain coming in at a certain time and the forecast is off by a few hours, either early or late.  The truth is that this happens all the time and such timing error increase with the length of a forecast.  And variations in timing of consecutive forecasts provide valuable information on forecast uncertainty.  Consider the rain over Saturday night.

Here is a radar image from the new Langley radar at 7 PM on Saturday night. 


A frontal band is moving in, crossing the north-central WA Coast. Now here is the 74 forecast valid this time (1-h precipitation at ending 7 PM).  Got the right idea of frontal system...but too far south.

 Here is a forecast for the same time from the next cycle (62h forecast).  Whoa!  The frontal band has shifted way north...too north.

Next forecast cycle and the 50 hr forecast...the band is shifted south...not too bad.

The 38 hr forecast?  Pretty similar but a tad stronger and farther south.


The 26hr forecast? Similar, but slightly weakened.

And finally the 14h forecast. Very much the same, but perhaps a bit more diffuse.

These are very typical forecast sequences.  Go far enough out in time and the forecasts are shifting in a big way, particularly in the time that features pass a location.  As you get closer in time, the solutions tend to stabilize.  On occasions when the solution stabilizes late or not at all, forecast confidence is low, and vice versa.    But just because a solution stabilizes does not mean it HAS to be right, and sometimes the forecast models equilibrate on  a solution that is not correct.

Forecasters have a special name for the variation in forecast for the same time (e.g., 10 PM August 5) but for different forecast intervals (12, 24, 36 hr forecast, etc):  dmodel/dt.  If any of you have calculus you know why.  For those who haven't, this translates as the change in model solution over a specific time interval... a.k.a. the time  derivative.   Large dmodel/dt--low confidence in the forecast, small dmodel/dt--more confidence.

Ten years I did a paper with Brian Colle and David Ovens that showed that models  were typically fast by an hour or so.  I haven't repeated this study, but I suspect that although the errors are probably less now, errors in timing are not usual.  That is why the coastal radar is so important.  When our models are off by a few hours--too fast or slow, the radar will give us a heads up when weather systems are 6-9 hrs out.

Dog Alert
There have been repeated sightings of our dog in and near Terrace Creek Park in Mountlake Terrace.  If you are in that area, please let us know if you see her.

7 comments:

Tony said...

Cliff, Why do the Olympic mountain tops often show no rain in the UW radar composite? Radar-Shadow but there is actual rain? Also will the http://www.atmos.washington.edu/cgi-bin/latest.cgi?atx_n0r+-notitle get updated to integrate the Langley radar as well

lowell said...

Love the new radar. In any case since you are able to see the heuristic data from your forecasts, is there a way to add another factor to your formula to account for the slippage? Sounds like it would be fun to model that.

Otherwhise, I hope you find your dog soon, it must be very stressful for you and your family. Best of luck on that.

smokejumper said...

You've said this many times before, but with the new radar, we'll be able to see if precip starting is on schedule.

But just as important, when will it end. Always seems with our pacific fronts, the mid level stratiform precipitation really outruns the front itself? The new radar has already shown examples when the true surface front or trough, actually makes landfall. Very vital information.

I can't wait for the first cyclone bomb off the coast.

Discover Paragliding said...

Here's another north Oregon coastal resident that is LOVING the new Langley radar. I can't say how many times it's been raining here when the previous radar data showed clear skies.

Just check the new radar this morning to plan our day. Nice. :-)

Brad

Olga Owens said...

we actually live two blocks from that park. will keep an eye out!

Rrrnay said...

Cliff, I so hope you find your little girl soon. I'm wondering if you've tried setting one of the Hav-a-heart traps for her in the park?

She's probably skittish now, and food may not be enough to entice her into it...but if you put a blanket in it with the scent of her family on it, that might help her feel at ease enough to go in.

Just a thought.

Cliff Mass Weather Blog said...

Olga...can you send me an email? thanks...cliff

cliff --at-- atmos.washington.edu