“Storm” Thoughts

November 29, 2006

As I mentioned earlier, National Weather Service has a Winter Storm Warning out for the metro area for late tonight through tomorrow morning.  My thoughts haven’t really changed much since last night’s post.

1.  Nice cold air over us, but not as dry as with our last few ice storms, so evaporational cooling won’t have as strong an effect (temps won’t dip much with start of precip).
2.  That dip in temp with precip may easily be countered by increasing southerly wind by midnight.  This is very different than when a normal ice storm starts and 70 mph wind are gusting out of the Gorge.  For example, we usually get gusty east wind here at the station at the beginning of ice storms.  That’s due to the easterlies coming down off the West Hills.  It’s calm here right now.  So I wouldn’t be surprised if at 6pm we are at our lows for the night.
3.  If so, then only spots close to the Gorge or in the hills will get freezing rain at the beginning.  Forget west of the West Hills, north of Hazel Dell, or south of a Downtown PDX-Gresham line.
4.  Cold front passes through around 4am.  A sharp switch to SW wind with this EVEN IN THE GORGE means a jump to a well-mixed atmosphere with a 4,000′ snow level.  That pushes metro temps into the low-mid 40′s by daybreak!  Hard to believe with this cold of air over us now, but I see no good reason to doubt it.
5.  As for snow…maybe briefly at the beginning, but seems unlikely with such strong warm advection above.

I don’t think this warrants a Winter Storm Warning in the metro area, but maybe in the Gorge.  I’m not sure why the NWS doesn’t just issue a Freezing Rain Advisory instead.  Maybe because snow or sleet could be mixed in?

Very simple forecast beyond tonight’s…ridging or split-flow through the foreseeable future.  What a dramatic change that follows the calendar….wet November to dry December?  Mark

UPDATE 11PM:  Moving in quickly now…but the Portland Radar died at 9pm!  Nice timing.  Looks like temps will stay about where they are.  If you are 32 or below, you’ve got ice in the next hour.  Drive carefully!  By the way…excellent timing by models…10pm to 1am arrival time right on.


Brief Comments

November 29, 2006

Just a brief post at mid-afternoon.  I’m going to wait until after 5pm to post again (after my busy schedule dies down and the early evening TV forecasts are already made).  I see the NWS has issued a Winter Storm Warning now for the metro area…isn’t that interesting…

Nice chilly airmass overhead.  PDX is only 34, easterly flow coming out of the Gorge at Corbett is only 27 degrees at about 700′.  Dewpoints are way down too.  Unfortunately PDX-DLS gradient is a weak 4 millibars, hardly ice storm material there.  This would seem to support snow or freezing rain tonight when precip begins sometime between 10pm and 1am.  But maybe not?  I’m making the forecast right now and I’ll be back in a couple hours.
I DO notice that Salem and Kelso are probably already out of the "danger zone" with 36 and a dewpoint of 29 (or 28 at KLS).  That is too warm for freezing rain even with evaporative cooling when the precip starts.  That doesn’t even account for warming from a south wind by midnight in both spots.   And forget about cooling tonight, we’ve got solid cloud cover overhead…Mark


Ice Storm or No Big Deal?

November 28, 2006

Chilly evening in progress out there with clear spots already in the mid 20′s.  Cold & dry air finally pouring through the Gorge this evening too with dropping dewpoints at Troutdale with a light east wind.  I’m a bit concerned about the low clouds lingering around the metro area at 10pm, but I’m assuming the drier air moving in from the east will evaporate that in the next few hours.  Otherwise low temp forecasts will be in jeopardy.
Now as for tomorrow night’s system…some common themes from 00z mesoscale models.
1.  Precip on all models begins around midnight.
2.  Strong gorge outflow never really develops, probably because the system is approaching from the northwest.  It’s those lows coming up from the southwest that really suck out the cold air and wind.  I wouldn’t be surprised if we dont’ see 8 millibars from DLS-PDX.  In each of our last 3 ice storms that gradient exceeded 12 millibars.
3.  Low pressure tracking by to the north is also very unlike the last 3 ice/snow storms.  This pattern produces a gusty south wind west of the Cascades and kills eastside high pressur quickly too.

This all adds up (in my mind) to a very brief freezing rain event.  I’m amazed how quickly the MM5 and our RPM model warm things up, much faster than they did with the last few storms.  I believe it with this pattern.  Temps tomorrow night may only briefly touch 32 degrees as a result and then quickly jump to 40 by daybreak in most of the metro area.  In fact wind turns westerly in the Gorge (or at least calm) by 10am Thursday!
Snow is unlikely because there is just too much warm air above from what I see, of course a few flakes are possible at first, but that shouldn’t last long.

11pm Extra:   After looking at 00z Canadian, 00z GFS, 00z ECMWF I took the rain out of the 7 Day forecast.  It’ll show up within the hour online…looks like a long term ridge coming up.    But, there may be a silver lining.  Just like last year in early December, chilly air will probably get trapped below the ridge east of the Cascades, especially since there won’t be much chance to melt the snow cover over about half of the Columbia Basin right now.  So I lowered high temps for early next week too…Mark


Cool Afternoon

November 28, 2006

A brief post for now, then more detail on tomorrow night’s system later when I have more time.

As expected, arctic front has just turned into more of a "northerly drift" across the metro area.  At 2pm temps range from 32 up at Kelso to 40 or so down towards Aurora/Salem.  Visible satellite picture shows the edge of the northerly breezes moving down into the Willamette Valley.  And it really is a "drift", very little movement out there.  But mesoscale models never showed a good push from the north.  What they do continue to show though is a good easterly gradient developing through the Gorge overnight.  Of course as that begins we have calm winds here in the metro area for a very cold night, then the Gorge wind spreads across the city tomorrow.  35 seems like a good high since The Dalles should stay around 28.  I am impressed by how cold that arctic airmass was.  12 at Bellingham with the northeast wind raging this morning, and widespread dewpoints below zero in north central Washingotn.  Too bad it isn’t like previous ice storms where that cold air really gets sucked down through the Gorge.  On that note…I have lots of thoughts about tomorrow night, but it’s off to a news meeting because all the newsies want to know the scoop…Mark


4:00pm Update

November 27, 2006

Just a brief bit of time to blog so I can get things done quickly in the next hour.  Always better to have forecast pages all finished just in case something hits the fan between 5-9pm.

Cold showers moving through this afternoon.  That heavy band produced snow to near sea level around 2:30pm as it crossed the metro area.  More showers moving over the Coast Range now.  Notice how cold the airmass is…we didn’t even have an hourly report to 40 degrees in the metro area. So I think we’ll drop close to or under freezing in all of the metro area by 8pm at the latest.  Of course any light showers then will accumulate.

Of greater interest is the continued insistence of mesoscale models (MM5 & our RPM) dropping some sort of surface boundary/dying arctic front/surface trough feature down the coastline overnight tonight.  That will mark the end of the snow showers, but there may be a brief burst of widespread light snow with it.  Both models have it through the metro area around midnight, then all dry by daybreak Tuesday.  I assume this is related to an obvious surface low off the NW Washington Coast this afternoon.  Clouds are streaming onshore looking quite a bit more solid ahead of it…just about reaching down to Astoria at 4pm.  Looks to me that there could be a good 1-3" at the beaches on the North Oregon Coast tonight!  Even a better chance there than here!

Cold and dry then tomorrow/Wednesday.  Dry east wind arrives tomorrow night.  System Thursday looks like a quick warmup (all the way to 38-40), but I bet we get a nice burst of snow/freezing rain Thursday morning…Mark


Sunday Evening Update

November 26, 2006

Well, isn’t this getting interesting?  At 8pm we can see snow coming down at 1,800′ on our KPTV tower and snowflakes mixed in with the rain on our Fox Tower Camera (450′).  Temperatures are falling steadily due not to radiational cooling, but cold air pouring inland.  But first a scolding about "wishcasting".  In forecasting class up at the UW we were taught to be very careful about this…you really want something to happen (snow) so your forecast basically veers towards what you want to see.  There was a ton of this on the comments the last 24 hours.  Many of you were convinced the low was headed into Oregon even though EVERY SINGLE model showed it moving inland somewhere either in Western Washington (GFS, ECMWF, MM5) or Vancouver Island (NAM).  In the end the GFS-MM5 from the UW once again NAILED it with a passage around Hoquiam then inland to Seattle.  I for one am amazed the model can do so well.  If 2 weeks ago before the windstorm I would have suggested the low was heading into Newport instead of Hoquiam, I would have been laughed off the blog.  My forecast would have been for light north wind instead of gusty east wind.  So why was the "snow low" different?  It wasn’t  and the main reason many were expecting it to go south was because they (I too) would really like to see snow.  So my point is to watch out in the future, but there’s nothing wrong with speculating on possibilities either.

Forecast tonight/tomorrow is pretty simple…very cold onshore flow continues until late tomorrow evening.  Snow levels are down to around 1000′ right now and will hover between sea level and 1000′ through tomorrow.  That means anyone COULD see snow, but it’s far more likely as you go up in elevation.  There doesn’t appear to be a ton of moisture, but 2-4" at/above 1000′ seems safe.  A heavy shower band could dump 1" even to sea level but that’ll be tough to do during the daytime.  Some sort of dying "arctic front" seems to come through here tomorrow evening, then it’s on to dry offshore flow for Wednesday.  Gorge winds kick in Tuesday night.  That gives us a chilly 35 or so (with sunshine) Wednesday. 

00z GFS is insistent on busting a weak system inland Wednesday night or Thursday morning.  Obviously that’s either a snow or freezing rain producer.  It won’t be a prolonged event because the low is going by to the north as it passes through Canada, unlike our ice storms where a low swings by to the west and then moves offshore again…Mark


Sunday Morning Weather Quickie

November 26, 2006

Not a lot of time this morning, but I woke up at 3am, 5am, 7am and now I’m really awake checking out the windows just like the rest of you.   I had snow at some point (1000′ near Corbett) because there is slush on the deck even though it’s raining.

I just checked the KPTV tower temps at 1400 & 1800′ and they have both jumped into the mid-upper 30′s so the pre-frontal snow threat is over. 

As you folks below have discussed in GREAT detail, the low is about to move onshore.  Maybe between Astoria & Hoquiam.  GFS once again was much better than NAM on low placement.

Cold air  behind the front sure looks impressive doesn’t it?  That will arrive late this afternoon and overnight.   In a way that’s easier forecasting than snow levels ahead of the low.  It’ll be a well-mixed airmass since onshore flow will be strong.  I see buoy #46036 is 39 degrees and dropping about 300 miles  west of Forks, WA.  pretty chilly temp for sitting just 10 feet above a 50 degree ocean.

I didn’t look at a ton of maps this morning, but I feel generally pretty good about the forecast for tonight/Monday.  It’ll be the snow shower pattern through tomorrow.  Hard to get sticking snow into the city with breezy south wind continuing, but easy to get lots of snow as you head up towards 1000′.   NAM-WRF says freezing level is down to 1800′ by 10pm tonight and stays below 1000′ from then through Wednesday.  I don’t think actual modified arctic air (switch to easterly Gorge wind) arrives until late Tuesday.  So we’ll stay pretty moist through Tuesday morning.

I’ll be in with Drew at 5pm tonight and again at 10pm.  Another busy weather day…Enjoy!  Mark


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