More Warm Weather

May 30, 2007

SnapshotIt would have been nice to break the record today after sweating out in the garden this morning, but at least we did restart the 90 degree counter for this season…it was Day #1.   A minor marine push  has occurred over the last few hours which means a cooler day tomorrow.  The upper-level ridge isn’t budging much through the weekend, so no big cooling trend for now.  These "northwesterly" pushes often don’t get much cloud cover into the interior in the mornings, so still mostly sunny skies the next 3 days.  We stay mild Friday, but then Saturday an approaching strong trough allows the ridge to strengthen ahead of it.  That pulls the warm air back to the west, so we should be back in the 85-90 degree range then.  One change in the 00z models is a slowing of the progression of the trough inland.  It probably won’t move in until Monday, so Sunday should still be quite warm.  With straight southerly upper-level flow both Saturday and Sunday, we’ll have to be on the watch for possible storms over or even west of the Cascades.  Then a strong marine push and weak front drop temps about 20 degrees for Monday.  So enjoy 4 more days of summer, then it’s back to May-like weather next week…Mark


Hot Day Ahead

May 29, 2007

SnapshotA nice little warmup today.  Some spots in the metro area jumped more than 40 degrees from sunrise until this afternoon.  The east wind always does it in the warm season, especially after a cool period (such as this weekend).  Notice how the east wind was strong enough to "cap off" temperatures just a bit.  I expected that, so I didn’t go for 90 today…tomorrow will be a different story. 
At 11pm we have a thermal trough just inland from the coastline.  It is created when easterly wind higher up in the atmosphere (4,000′ or higher) blows over the Cascades and/or Coast Range.  That lowers pressure in the lee (west side) of the mountains, creating an area of low pressure we call a thermal trough or "heat low".  It moves into the Willamette Valley throughout the entire day tomorrow.  So the sequence of weather events is for easterly flow to continue through the early morning hours, then we go calm in the valleys through tomorrow evening.  Then after 5pm or so cooler marine air pushes into the interior.  This is a perfect setup for record heat tomorrow.  A very similar event occurred two weeks ago on May 14th-15th.  Temperatures ended up much warmer than "expected".  The June version of my little "Mark Nelsen magic chart" still says 91-93 tomorrow, so no reason to stray from the 92 degree forecast.  That will be a record for the date.  All models show a shallow marine push tomorrow night, continuing into Friday.  Saturday should be just a bit warmer as a strong trough offshore develops brief ridging again over eastern Oregon/Washington.  So you can count on a summery Saturday.  Sunday is up in the air since some models bring that trough inland by Sunday afternoon…others don’t.  I assume slower is the way to go for now.  A sharp cooling on the way for Monday either way though…Mark

Just for fun, here is a little bit of "inside" gossip about TV stations and the competitive pressures of forecasting nowdays in Long Range Forecasts.



First Hot Weather

May 28, 2007

SnapshotIt’s very gratifying to take 3 days off work, come back, and find almost the exact same forecast you left the week before.  That’s the case tonight.  The cold trough that over the weekend moved through the Northwest is now into the Northern Rockies.  An upper-level ridge of high pressure is developing just to our west and it moves directly over the West Coast over the next few days.  This gives us another round of downslope easterly wind.  Of course in the warm season, this is a hot wind so today’s 68 degree high will be just a memory by tomorrow afternoon.  Remember that a couple weeks ago we went from 59 to 81 in just one day in the same quick switch from onshore to offshore windflow.  It appears that a thermal trough develops over the coastline by daybreak, then shifts over the Coast Range later tomorrow afternoon.  That gives a brief period of offshore wind even to the beaches in the morning.  That’s why I had such high temperatures in the Coastal forecast tonight.  Inland temperature forecasting is a bit simpler.  850 mb temp of +15 to +16 with "perfect" offshore flow (tomorrow), can give us as high as 90-91 at PDX.  However, I think the wind may be quite breezy even all the way to the airport, which can reduce heating, so I left it at 88.  Wednesday should be warmer since we still have easterly flow in the morning, then it goes calm in the afternoon.  But still no onshore flow at that point, so 91-94 is a good bet then.  A decent, but not major, marine push for Thursday drops us down into the mid 80s.  Same thing for Friday with a few degrees more cooling.
The weekend looks interesting with a sharp trough developing way offshore Saturday, then swinging onshore later Sunday.  That setup will pull the thermal trough back over the westside or at least over the Cascades for a warmer day.  Then when the trough arrives sometime later Sunday or Monday, we get a major marine push west of the mountains.  This will probably be one of those 20-25 degree one-day temperature drops.  Also, it can be a severe thunderstorm pattern east of the Cascades too.  Enjoy the warmer temperatures the next few days…I know my freshly sprouted corn and cucumbers will!  Mark


A Dry Weekend?

May 24, 2007

SnapshotSunshine all day long here in the western valleys…temperatures warmed nicely into the upper 70′s.  A weak push of northerly wind has been moving south through Eastern Washington tonight, much like an arctic front in winter.  Models show that turning flow easterly through the  Gorge by daybreak.  With a warm atmosphere still overhead, that should push high temperatures up a few degrees tomorrow.  My forecast could easily get screwed up by some high cloud cover keeping temps too low tomorrow.
Nice sharp trough swings through Saturday night through Sunday night.  This sets up a massive marine surge along with a cold upper-level atmosphere surging inland.  NAM showers showers in the chilly air Sunday afternoon, GFS does not.  I generally put more faith into the GFS, but a strong trough could give anyone a shower west of the Cascades.  I will be camping the first 2 days of the weekend in the Cascades…Sunday morning’s showers/drizzle at 40-45 degrees should be enough to chase me back home.
I’m much more confident on "the big warmup" beginning Tuesday.  All models (GFS, ECMWF, CANADIAN) have sharp ridging developing behind the cool trough.  A very nice thermal trough develops west of the Cascades Tuesday and continues at least through Wednesday.  East wind and 850mb. temps of +15 to +20 deg.C. will push temperatures up to or over 90 degrees in PDX-town in late May.  So IF the models don’t suddenly back off, we have quite a warm spell coming…Mark


Unexpected Rain

May 23, 2007

SnapshotI think I’ve said this several times in the last couple of months…I detest spring forecasting here in the Northwest.  Temperature was just right today, but a weak system moving down the coastline spread light rain inland this evening.  As you can see to the right, amounts were not very impressive.  But Shauna’s new deck treatment was messed up and probably more than a few mowing plans were changed too.  The showers are gone now and that system should be out our area well before daybreak.  A warm night and then clouds breaking up to partly sunny should push temperatures into the mid-upper 70′s tomorrow.  No changes for Friday, but then an approaching trough will give us a strong marine push Saturday.  That plus a cooler airmass overhead Sunday gives us a  chilly, but dry day Sunday.  I should point out (as I did at 10pm), that our RPM model shows some weak convection the next 2 afternoons over the Cascades and Coast Range.  I’m assuming this is overdone…but we saw what happened today!
Long range is a tough forecast as well.  If I believe the 00z Canadian, GFS, and 12z ECMWF (all the latest runs), I should go 90-95 next Wednesday and Thursday.  A strong thermal trough, east wind, and 850mb. temps around +20.  This would be our first hot spell of the season.  However, models have been consistently too strong on ridging for the last week or so.  I’ll leave big changes to tomorrow’s forecast…Mark
Long range forecasting continues to be messy too.


On to Warmer Weather

May 21, 2007

SnapshotAfter a wet two days (for May at least), it’s back to our drier May weather.  Looks like "storm" totals were anywhere from .25" westside of the metro area to almost 2" on the west slopes of the Cascades.  How’s that for orographic flow?  The weather pattern will be quite stable and, to be honest, a bit slow the next 7 days.  Onshore flow eases the next two days as the atmosphere itself warms.
Okay, I admit I did find something interesting that changes the forecast a bit for Friday.  A cold shot of air drops down into the Northern Plains later Thursday, which is accompanied by an area of high pressure too.  Both the 00z NAM & GFS show a sudden drop in dewpoints here in Portland by Friday morning along with an easterly wind, especially at 850mb.  So a brief, but strong offshore flow setup for us.  That along with 850mb. temps by Friday afternoon in the +12 to +14 range could easily push us into the 82-87 degree range.  ECMWF agrees, so that’s why our 7 Day forecast is significantly warmer than any other one in town.
Beyond Friday, as we head into the Memorial Day weekend onshore flow gradually increases.  So those of us that have camping plans early (me) will find the warmest temps the first half of the weekend.  But the big story in the Northwest of course will be the DRY weather for the weekend.  Locals know that Memorial Day weekend can easily be wet and chilly…Mark


Somewhat Wet Weekend

May 18, 2007

SnapshotA tough weekend forecast tonight…again.  Looks like the main cold pool stays off to the northwest of us tomorrow.  If so, once the steady rain this evening moves out of the area we should be mainly dry until Sunday.  But scattered, cold showers return on Sunday.  850mb. temps drop to around zero Sunday, which is pretty chilly this time of year.  As a result we’ll stay in the 50′s most, if not all, of the day Sunday.
00z runs of GFS and Canadian confirm flatter ridging for next week.  Still mainly, or all, dry from Monday through Friday, but not too warm…maybe only into the 70′s.  I have camping reservations next Friday and Saturday night, so it had better not turn cold and rainy in the Cascades…or I’m coming home…In the very near term, nice line of thunderstorms moving through southcentral Washington at 10:50pm.  Lots of lightning strikes just north of Hermiston and Umatilla…Mark


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