End of Winter Snow Rumors

February 20, 2012

Have you heard rumors of possible snow this weekend?  Even without being at work today, I know regular folks must be hearing things because hits to this blog just about doubled since yesterday, to the highest level since the end of January’s snow & flooding almost a month ago.  That’s with no interesting weather occurring right now!  Here’s the scoop…

1. Over the past 5 days, computer model simulations of the weather patterns (moisture, temps, weather systems etc…) have been showing some variation or another of a cool or cold spell coming somewhere this Friday or beyond.  We get several runs from each different model each day, and each goes out to at least 15 days.  You can imagine the amount of information we have available nowadays!  Our job in the past 15 years has morphed from one of forecasting to one of forecast model babysitting!  Which one is correct or which two are most likely seeing something the others aren’t?  You get the idea.
2.  Some models at varying times have shown a historic late season cold spell and decent chance for snow to sea level, while at times others have not.
3.  They ALL agree on AT LEAST cooler than average weather with snow down into the higher hills again Saturday through sometime next week.
4. They do NOT agree that snow will be showing up in the lowlands.
5.  As of right now (10:45pm Monday), the best guess is marginal hilltop snow showers Saturday night and/or Sunday and no snow in the lowest elevations, and no blast of arctic air either.
6.  The main issue right now is models are all over the place on details except for #3 above.

WEATHER GEEK DISCUSSION FOLLOWS:

Main issue this evening is models appearing to have no clue what is going to occur beyond Friday (other than the general trend of colder).  I have a feeling this is why local meteorologists are playing it quite safe with snow talk tonight.  One could hardly reconcile the excitement and wishcasting going on in weather forums/blogs and Facebook sites around the region right now with what is seen on TV and official forecasts.

The 12z/18z GFS and 12z ECMWF were all pretty reasonable with a decent chance for lowland snow beginning Saturday evening once 850mb temps dropped to -7 or so.  But they were also very dry, or at least just flurry-ish.  I’m not working (just today), but if I was doing the 7 Day forecast, I would have gone maybe 40 both Sunday and Monday and lows 25-28.  Just a few flurries on Sunday’s forecast too based on these runs.  If one of those earlier and colder models was correct, maybe only mid-upper 30s for a high Monday?

But now the 00z GFS comes in and produces far more of a splitting system Saturday, and then a return to more westerly flow Sunday and beyond.  The 00z GFS barely gives the hills a chance for snow…game over on that one.  However, the GFS has been really bad lately, so let’s throw that one out.  Note the huge variability in the 00z GFS ensemble chart after Sunday morning (just before the “27th”):

The new 00z ECMWF is in now in…slightly different, but quite a bit warmer.  It has a first wave coming through Saturday as all the model do, but warmer with thicknesses barely supporting hilltop snow too in strong onshore flow.  Then a 2nd wave comes down from the north late Sunday with a better chance for snow to lower elevations; although still marginal.  The ECMWF now has only -7 or -8 at 850mb as it bottoms out Monday morning as it too shows colder air digging farther south offshore with a very slight “splitty” look to that trough:

If you take both 00z runs at face value:
1. We don’t have any arctic air on the way  (although only a few runs/models have shown that the last few days anyway)
2. Snow to the lowest elevations could be very spotty Sunday if at all.

I don’t think one can read too much into any one model run or any one time period of runs, but it’s fair to say there is a lot of uncertainty to this weekend’s forecast.  It is possible we get out of this with very little interesting weather, or this evening’s run are just having issues with shortwaves.  Brian MacMillan’s 7 Day forecast sure looks good now…

Chief Meteorologist Mark Nelsen


12z ECMWF Ensemble Chart

February 20, 2012

Good agreement on it’s members through Sunday afternoon now (the “27th” line).  That’s 00z on the 27th which would be 4pm Sunday the 26th.  After that there are some members that bring real arctic air south, and others that warm it up.  Interesting that the very warm members are gone and every single one is below 0 deg. through Tuesday the 28th. 

I would say the operational 12z ECMWF is very similar to last year’s late February event, but without the easterly flow?  Although last year we didn’t get a big blast from the east…gust to 30 mph at TTD & PDX.  Real dry after Saturday night too.  It has less than .05″ from Sunday 4am to Tuesday afternoon.  Sunny and cold if it comes to fruition.  Lows in the 20s and highs near 40 Sunday-Tuesday.   Then a system sliding by just to the north Tuesday-Thursday for marginal (520+ thickness) snow chances again but onshore flow.


For Fun, the “Outlier” of the Day

February 20, 2012

For fun, take a look at the 6z GFS run. It’s the blue line, probably the coldest airmass ever seen in early March here just 13 days away! This is why it’s so nice to have access to ensembles of forecast models; it’s quite clear that this run was way “out of whack” with all the others, but it sure was eye candy.

Here is the fresh 12z version, much more reasonable.  Still huge disagreement beyond Saturday afternoon (26th and beyond):

The main message for later this week is that snow levels are going to come down quite low over the weekend, but we don’t know how low and who exactly could see snow. At this point it looks very similar to what we had last February when we had a couple mornings of wet snow that melted in the afternoons with little impact on traffic. The lowest elevations of the metro area had a trace to 3″ at that time. We don’t see a cold arctic blast coming in, just really cold showers (of snow) off the Pacific.

I’m not working today, just puttering around home (maybe clean the garage out if it rains too much). I’ll post this evening after the 00z GFS comes in.