Cold Weekend

March 30, 2007

A brief post tonight since I’m a bit behind, plus it’s 11:05pm on a Friday night.

Models are hanging onto a very cold airmass moving in over the Northwest tomorrow evening.  Our RPM model shows snow supposedly possible down to 1000′ Sunday morning.  There is no meteorological reason that couldn’t happen with 850mb temps down to -7 or -8 deg.C.  BUT, showers look awfully weak until we get to the midday and afternoon heating Sunday.  At that time the snow level would be up to 2000′ or so.  So a very chilly day coming up for Sunday with scattered showers and sunbreaks, and the foothills could easily see a late-season dusting of snow…maybe an inch or two above 1,500′.

Warming next week is still on track, although 00z GFS wouldn’t support temps much above 65 degrees…Enjoy the chilly weekend!  Mark


Warm Day and Cold Ahead

March 29, 2007

SnapshotA very nice day today…apparently even my "wild" forecast of 66 last night for this afternoon was too conservative.  I notice the 850mb. temp at Salem this afternoon was +10 C.  That’s 2-3 degrees warmer than models had forecast.  According to the "Mark Nelsen 850mb chart" for March, a +10 on average will produce a high near 66.  With a good easterly flow, up to 72 or 73.  So we could have been even warmer!  We did have a nice east wind through the Gorge and over the Cascades, but barely enough to make it into the city.  Looks like the east wind just touched the airport this evening, but blew at Troutdale all day.
A nice combination of southern stream energy and a cold northern stream system offshore.  It’s moving pretty quickly so I’m a bit worried that we could see completely cloudy skies with light showers before the more widespread light rain arrives in the evening tomorrow.  Tough call.
Weekend forecast continues to be a challenge.  The big picture shows a deep and cold upper-level trough developing off the NW coastline Saturday and Sunday.  If you look at the 3 day forecasts of 500mb. vorticity, you’ll notice energy drops into this trough from the Arctic Ocean and northern Alaska!  If it was winter we’d be talking possibly snow with the showers Sunday.  Since it’ll be April 1st and the showers don’t look very heavy, I doubt we will see sticking snow below 2000′.  A cold Sunday for sure…hopefully the payoff Easter Sunday (the following week) will be a warm and sunny day.  One can always hope right?  Mark


Wet Winter?

March 28, 2007

SnapshotSo after the landslide today, I wondered how wet our "very wet winter" really was.  So I checked the last 5 years of October-March rainfall.  Looks like only slightly above average.  The last two winters have definitely been a bit on the wet side.  As always with weather…it’ll be interesting to see if we continue the wet trend next winter.  Moving on to tomorrow’s weather…a warmup definitely in store.  MM5 shows a good 10-12 degrees of warming, which puts us at least into the mid 60’s.   A good easterly flow sets up all day tomorrow through the Gorge, up to 4 millibars, which is pretty strong for the last few days of March.  This will be a mild downslope/offshore wind, so I wouldn’t be surprised to see a 70 degree temperature here in the metro area by Thursday afternoon.  Much cooler airmass and trough moves inland this weekend, causing havoc in the 7 Day forecast.  Upper-level maps would seem to indicate a cold, showery weekend.  But surface progs don’t show much, if any, precip for Saturday.  So maybe just a chilly weekend with showers returning Sunday?  Anyone else care to take a guess?  By the way, I’ve left the live, streaming radar centered on the severe weather outbreak in progress across the Great Plains tonight.  It’ll stay there until 3am or so.  Fun to watch to the lightning strikes too…Mark


Change in Forecast

March 27, 2007

SnapshotA challenging forecast season continues the next few days…ugh!  The rain and clouds were slow to leave today, although in the end the gusty northwest wind and partly cloudy skies arrived, just a bit late.
The cold trough is shifting off to the east tonight, leaving us waiting for another chilly trough to arrive late this week.  Two changes to the forecast for the next 5 days.  One is the surprisingly dry and warm weather the next two days, especially Thursday.  I notice our model shows some very weak convection over the Coast Range early tomorrow afternoon, although it does not show it drifting out over the valleys.  Hopefully it stays that way. 
Since the next system doesn’t move inland now until Friday afternoon, that leaves ridging ahead of it and brief warm easterly downslope wind Thursday.  All models have boosted 850mb. temps up to +6 or higher (Celsius).  This easily supports mid 60’s this time of year…so the forecast high Thursday just jumped about 10 degrees!
Showers arrives sometime Friday, but then move out quickly by Saturday morning.  The 2nd significant change in the forecast is for Sunday.  00z models continue a trend of bringing a weak system through N. California and up into Oregon on Sunday.  If so, we may see rain showers Sunday.  Longer range models are all over the place after that time.  I DO notice the GFS doesn’t have much rainfall in the next 7 days…it’s looking more like spring…Mark


Chilly and Wet

March 26, 2007

SnapshotI haven’t paid all that close attention to the rafinfall totals this year (since the New Year).  We definitely calmed down rainfall-wise after early December didn’t we?  Hard to believe we’ve had 28" of rain at PDX since the wet season began.  But even tougher to believe that 28" of rain fell at Bonneville Dam in just ONE MONTH (November).  They have had about 6 feet of rain this wet season!  You didn’t see the graphic to the left at the beginning of the 10pm show because I pressed the button too fast.  It happens…
So it’s good to know that climatologically we are just about done with the steady rainfall.  Generally April and May are lighter rainfall months with rainy periods mixed in with increasingly warmer sunny periods.  Yes, yes, I know that some years we can have 2 weeks of straight rain in either month, but other years (last year) we can have a sustained warm/sunny period.  The 2nd half of April and 1st half of May were mild, dry, and mostly sunny last year.
Of course our 7 day forecast now takes us through the beginning of April, and it doesn’t look soaking wet, just wet at times.  Models are all over the place after Friday, so hopefully the sunny forecast will hold…Mark


Slow Friday Night

March 23, 2007

SnapshotThe rain showed up a bit earlier than expected tonight; that along with the cloud cover soon after the noon hour kept our highs below 60 degrees.  Oh well, the one thing I like about weather forecasting is that you start over each day…a brand new forecast.  So hopefully this weekend’s forecast will work out a bit better.  A classic orographic lifting rainfall pattern this evening.  Notice only a trace or so west side, but up to 1/3rd inch on the east side.  The westerly flow runs into the Cascade foothills and drops some of it’s moisture as it begins rising.  That’s why the Coast Range and west slopes of the Cascades are considered a temperature rain forest.  Another reason ferns grow ON the moss that grows ON the trees up there.
Most rainfall tomorrow should be with the cold front in the late afternoon and evening.  Most notable for people like me that want to get outside at some point is the quick cutoff of the rainfall behind the front.  Other than a possible "pop-up" shower Sunday afternoon, it should be dry from around midnight Saturday night to sometime Monday night or Tuesday…Mark


On the Line

March 21, 2007

Snapshot2SnapshotA tough forecast again for the next two days as the wet westerly jet stream sets up over Washington and southern British Columbia.  As you can see to the right, the forecasts for Seattle and Eugene are quite simple…either warm and dry or wet and cool.  From Longview down to Portland we are on the line though.  Both the 00z MM5 and our own RPM model continue to show the rainfall line staying just barely north of Portland Thursday and just about down to Portland later Friday.  Whew…it’s going to be close.
Easier forecasting Saturday and Sunday;  a cold front sweeps inland late Saturday with plenty of pre-frontal rain.  Then showers taper off during the day Sunday.  We may get a brief break Monday before more "troughiness"  arrives Tuesday-Thursday.  Oregon spring break is looking very wet.  Maybe at least you Washingtonians can get nice weather the following week for your spring break.


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