Quick Evening Update

An extremely busy evening both on TV and off means my email Inbox is jammed…I’ll get to those later this week.

As mentioned this morning, models have come into good agreement and it’s even better on the new 00z runs.  More on tomorrow night’s big system in a minute.

This evening everything is going according to plan, a deepening surface low to the north is giving us a gusty southerly wind.  Temps are near their daytime highs as a result.  We just hit 36 at the station in northern Beaverton, the high for the day.  Our KPTV Tower temps have been rising since early afternoon and mesoscale models show the low level atmosphere maybe warming another 2 degrees by daybreak.  That mostly eliminates the chance for sticking snow at the lowest elevations.  One hitch is much heavier precipitation forecast to arrive between 7am and 10am.  This will be similar to this afternoon; heavy wet flakes will stick lower than they will tonight.  That’s why I said Trace-1″ at the lowest elevations by morning and 1-3″ up around 1,000′ and above.  The southerly wind may die down right around that time as well.

The rest of tomorrow looks similar to today temp & precip-wise.  With a light south wind we rise up to around 40 degrees, or at least the upper 30s. 

Good news for tomorrow night and early Wednesday morning.  All models now move the surface low up to somewhere near Astoria or a little farther north.  As the steady precipitation settles in after sunset tomorrow, the sticking snow level will probably be up around 1,000′.  Then a switch to light easterly flow between 10pm and 4am could allow the column of air overhead to drop to 32 degrees all the way down.  Model soundings are still showing this, just like I showed on the previous post.  It’s a very close call, but considering there will be no influx of cold/dry air from the Gorge, I’m not excited about a big metro-wide snowstorm.  I won’t be surprised if there is only a trace or less once again across the bulk of the lowest elevations from the Columbia River south.  I feel the best place to hold onto the cold air will be up against the Coast Range and then north of the Columbia River, up into Clark County.  Forget about it Salem…too mild down there (just barely).  The WRF-GFS and our RPM do not show accumulating snow in Portland, but it’s quite close!  Check out the WRF-GFS snowfall accumulation graphic valid from tomorrow at 4am to Wednesday 4am:

Once again, best chance north and west of Portland. 

Here’s the forecast graphic I used at 10pm for that period:

A southerly wind should move into here Wednesday midday at the latest.  Depending on the depth of the surface low, we could either have just southerly breezes or a significant wind event.  We can worry about that tomorrow.

Winter Storm Watch Thoughts

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After all these days, we finally have an agreement in the models for the critical Tuesday night through Wednesday forecast.  I’ve put in a slide show which takes the ECMWF, GEM, GFS, & NAM for 18z (10am) Wednesday on the same map projection.  Note that they all are much closer on a surface low moving up against the Oregon/Washington Coastline by Wednesday morning.  The NAM has finally turned warmer with 850mb temps in the -7 to -8 range tomorrow, although that’s still slightly colder than the GFS/ECMWF.

So what does it mean?

1. The National Weather Service has issued a Winter Storm Watch for much of Northwest Oregon and Southwest Washington for this storm.  I think that is a little much, but they do need to err on the side of caution since that’s their job.  Probably a good move since  it’s only a WATCH, not a WARNING.  That means if it becomes obvious there is only rain coming in the lowlands, they can just change it to a Winter Weather Advisory for the hills.

2.  All of these models are very marginal for snow here in Portland, especially the GFS.  That said, even the ECMWF/GFS 850mb temps around -4 to -5 with 6-10 hours of easterly flow could change us over to snow.  Note the 12z WRF-GFS sounding for 1am Wednesday morning.  That’s REALLY close to snow.  Most important, I told myself I wouldn’t make the “December 29th mistake” again.  When it’s marginal, I’m going to tell the public so instead of just taking them along with my “weather gambling forecast” they can be prepared in case I’m slightly off.

3.  Due to that, I think it will probably not stick Tuesday night and Wednesday morning for most of the metro area, but it’s going to be very close!  Luckily it should happen mainly at night too.  And this forecast is subject to change the next 24 hours if models come any farther south with the surface low.

Where could we get significant accumulation of snow?  The central/eastern Columbia River Gorge (10″+ possible).  Longview and northward in Southwest Washington (4″+), and the western parts of Washington/Columbia counties, and maybe Scappoose/St. Helens too.  A little bit of the “Forest Grove Effect”, where cooler air pools up against the east side of the Coast Range.  Those spots could see 4″+ as well.

Quick warming on all models Wednesday afternoon.  A south wind with 850mb temps above zero is 50 degrees easy.   Gusts could be over 40 mph depending on the low placement.  Interesting to note the NAM-GFS from UW is much stronger and farther south with the low than NOAA’s version of the NAM.

Oh yeah, as for the next 24 hours, I haven’t changed my thinking much:

REST OF TODAY:  Snow showers pick up later…increasing southerly wind lifts sticking snow level up around 1,000′.  Highs near 40

TONIGHT & TUESDAY A.M. COMMUTE:  Snow showers, snow level doesn’t lower much below 1,000′, so 1-2″ new on the hills closer to 1,000′ and above.  No freezing in the lower elevations.  That’s because of the mild southerly wind continuing to blow.

TUESDAY:  Mixed rain/snow showers all day, sticking snow only up around 1,000′ or even a little higher by afternoon…1-3″ on the higher hills only.

Chief Meteorologist Mark Nelsen