Stormy Weekend Ahead; 3 Storms

September 26, 2013

Is it November or is it the last weekend of September?  The weather maps say November!   We have one wet front coming through the Pacific Northwest tomorrow, then a much stronger (wetter and windier) storm forecast to move through the Pacific Northwest Saturday afternoon through Sunday morning.  A possible 3rd system could arrive Sunday evening.

Forecast models have come into very good agreement on what the ECMWF model started showing 2 or 3 days ago…an unusually wet and windy system for the last few days of September.

The Highlights

  1. Rain arrives by noon at the latest Friday…then it’ll be wet from that point through the weekend.  It’s an “indoor weekend”.
  2. Heaviest rain falls overnight Saturday night into Sunday morning.  There may be localized flooding (roads and creeks) during that time.  I expect 1.5 to 3″ of rain in the lowlands and 4-6″ in the foothills/mountains.  A few of the usual wet spots will likely see 6-10″!
  3. Large rivers probably won’t flood since the heaviest rain will last less than 24 hours and they are starting at their early Fall low levels.  Ground is also still (somewhat) dry and can absorb the first couple inches of rain.
  4. Strongest wind is Saturday evening and overnight Saturday night
  5. Wind gusts will be STRONGER than what we saw last weekend…60-70 mph on the Coast and 40-50 mph here in the valley.
  6. That is unusually strong for this time of year, especially since leaves are still on the trees to catch the wind; there will definitely be more power outages this weekend.
  7. No snow in Cascades, this is a warm fall system with snow levels up around 8-9,000′.

 

The Details:

Good to see all the models coming into nice agreement on timing and intensity of frontal features this evening.  I’m not too wound up about flooding for the reasons mentioned above and that this isn’t a perfect “atmospheric river” or “pineapple express” event.  Most of those go on for 2 days or so.  The real heavy rain really appears to be later Saturday through very early Sunday.  Check out our RPM for 1am Sunday.  The peak of the rain and wind action appears to be around midnight Saturday night on most models:

Cloud_Rain_1amSunday

What an intense line of rainfall with the front!  This will move down into the Albany-Roseburg area Sunday morning.    Now here is the wind gust forecast:

RPM_Wind9pm

Also surprisingly strong.  If it was December I’d think it was a very impressive wind field.  But it’s only September 28th.  Wow.  Note the 60+ mph gusts just ahead of frontal passage Saturday evening.  The strong wind will back off dramatically after the front moves to the south later in the night.  So if you want to see strong wind at the Coast…try 5pm-Midnight.  Here in the valley, we had some gusts in the 35-40 mph range with the system early this week with just a 7 millibar OLM-EUG gradient.  This time it looks more like 11 millibars.  With such a warm air mass it’s easier for the stronger wind to mix down this time of year, thus 40+ mph gusts are likely Saturday evening/night.  Get your generators ready if you live in an outlying area and have another 10 episodes of The Walking Dead to get through over the weekend (me).

After adding more rain during the day Sunday (not as intense), this is what our RPM shows for accumulation.  It includes Friday’s rain too…a 72 hour total:

RPM_12KM_Precip_NWOR

Note the 10″ spots showing up in the Cascades…the WRF-GFS shows the same thing.  I think 3″ might be a little high here in the valleys, maybe 2″ or so.  So far we’ve seen 2.24″ rain in Portland this month…if we add another 2″, that’ll put us up to 4.24″.  The all-time September record is 4.30″ in Portland.  Even at the wetter downtown location, where records go back to into the 1800s, we’ve never seen 6″.  But let’s not count the “rain eggs” before they hatch.  It will definitely be the wettest September in years here.  Look at the last 10 years:

MarkRain_September

It’s make up time!  The title refers to when I used the graphic LAST September.

 

What happens after Sunday morning?  An interesting wrinkle provided by both the 12z ECMWF and now the 00z GFS.  They both are strongly hinting that another low pressure area develops and moves somewhere into Cascadia (BC to S. Oregon) later Sunday or Sunday night.  The 12z ECMWF looks like this with a 988 low moving into the Olympic Mtns:

ecmwf_apcp_f90_nw

The 00z GFS goes nuts with a 985mb low moving right onto the Long Beach Peninsula!

slp.72.0000

That could be a big windstorm for September 29th; one should assume that solution will change though.  More fun ahead…

Chief Meteorologist Mark Nelsen