Be Careful When Reading Modeled Rain Forecasts

October 26, 2015

I see this statement (or something like it) winter after winter…“the models were showing X rain this weekend and we only got X much…they suck!” .  More than 50% of the time I think it’s misinterpretation of the model output.

Take a look at the 00z WRF-GFS output this evening for total rain ending Sunday morning.  This is the 36km resolution model:

wrf_36km

Definitely a big soaker on the way!  Looks like Somewhere between 2.50-5.00″ at PDX right?  Wow, that could cause some flooding. Salem maybe 2.50″ as well since it’s right on the line between pink and black.  Likely over 5″ in the Southwest Washington Cascades.  Now take a look at the 12km version of the same model:

wrf_12km

What?  It says 2.50″ at PDX, less at Hillsboro, and a bit more at Troutdale.  Salem is about the same or a little less than 2.50″.  Now take a look at the GFS model output on our broadcast software:MarkRain_GFS_ECMWF_Compare

That is extreme!  Looks like PDX could easily see 6-7″ here, Maybe 4″ out in Hillsboro and a crazy 8″ at Troutdale and Battle Ground right?  Nope.  Here is what I estimate from looking at each of these 4 models this evening:

MarkRain_ModelForecasts

Why so low?  Two things are going on:

  1.  In a coarser resolution model, the space between data points is larger.  That means in areas where terrain changes quickly (PDX to Larch Mountain WA anyone???), the amounts are going to be unrealistically high near the rising terrain and often shifted a bit to the west of where it really is.  I’ve seen that on our RPM model many times.  So you generally want to find the highest resolution output you can.  That also applies to going to a place like IGES and looking only at output for PDX.  It’s always too wet in a strong orographic flow (like what is coming this weekend).
  2. Specifically in the case of the ECMWF and GFS we get on our system at FOX12, WSI (our vendor for wx graphics) only gives us grids spaced 100 kilometers apart!  Yikes.  That means there’s only a point to contour from every 60 miles…really bad.  There may be one grid point in the Cascades and another over the western valley for example.  The contouring program is just averaging between those two points…pure interpolation.  Thus the inaccurate extreme amounts seen over and east of I-5.  Our RPM that goes out to 72 hours is 12 km resolution so that’s much better, like the 12 km WRF-GFS run above.

So how did I get those numbers I used on-air?  I generally look for the driest part of the northern Willamette Valley , even if it appears to be way over at Hillsboro or Forest Grove on a map.  That works well in these orographic events.  Keep in mind this applies to meteograms produced by picking a specific point as well.  Here’s the PDX 00z GFS meteogram:

gfs_pdx

But Hillsboro is far more reasonable…this is what I would expect in most of the metro area:

gfs_hio

Of course almost nothing I’ve written applies in showery weather patterns or precipitation coming from a different direction, but our big heavy rain/flood events are almost always under westerly flow.

By the way, yes, we’re going to get a soaking this weekend, but I don’t think 6″ or more in the valleys will be happening.  The Cascades will get a real pounding with 5-10″ possible.  Luckily rivers are low and the ground is dry…the first 3-5″ should soak in nicely.

Chief Meteorologist Mark Nelsen


Blob Update

October 26, 2015

Lots of talk the past 2 years about unusually warm water in the northeast Pacific.  It has been named “The Blob”.  I’ve seen some discussion on different forums recently that it has gone away.  Yes and no.  The positioning of warmer than normal sea surface temps has changed, but there is still a LOT of “warm” water offshore.  Note a month ago:

sst_30back

Warmest water relative to normal is way out there…looks “blobish”.  But big picture is warmer than normal water across all areas to our west.

Now the current image:

sst now

It has cooled quite a bit way out there, but the anomaly has strengthened within the 500 miles of the coastline…more reds showing up.  If you want marginal snow events here this winter you’ll want that to go away!  Plus that has to have some effect on our weather as it has in the past two winters.  Very “Warm PDO” look.

That’s all.  Maybe a posting later on a soaking coming this weekend and first mountain snow (to the passes) about a week from now.

Chief Meteorologist Mark Nelsen