Sunny Tuesday

September 23, 2008

Junk I ran across this while I was going through my afternoon forecast routine.  The 6-10 Day 500mb height outlook.  This should get a good discussion going.  I'll post later.  Okay, it's 10:24pm and I'm back.   What a nice day today…after the really chilly start.  I see Timber ODOT station hit 31 degrees early today, and HIO was 35.

Warmer temps tonight with the increasing cloud cover from the offshore cold front.  Some rain later tomorrow and then post-frontal showers are the story for Thursday.  Looks like dry Friday-Monday, but a front drags by just to the north Saturday.  That's why I put more cloud cover in and lowered temps over the weekend.  850mb temps peak out around +15 Sunday and Monday, although now I see the 00z GFS has a +19 deg Monday.  I trust the +15 ECMWF a bit more, it's been more consistent. 

Models have been hinting at a much stronger jet stream developing the middle of next week in the eastern Pacific.  The map above shows the 6-10 day deviation from normal 500mb. heights.  You can clearly see the big negative anomaly in the map.

The 00z GFS is a bit slower on development of westerly flow.  It has no rain Tuesday…so much for my wet Tuesday forecast…but there's always tomorrow…Mark Nelsen


First Freeze?

September 23, 2008

   Well it has been a few days since the last post so I thought I might put one up for all us weather nuts.  So After this mornings rain, PDX is at about .43" for the month.  Before the rain this morning we were about 1.3" below average for the month alone.  For the year we are about 5.3" short.  So lets hope for some more… plus it's nice to hear when I am trying to go to bed at 10 AM after the morning show.  As for the first freeze, on average it is around November 8th.  The earliest first freeze record that I could find was on October 5th, 1993, with a really old record on October 13th, 1881! (who knows how accurate that is…1881) 
    I don't think we will see any freezing temps here in Portland in the next few weeks but it is going to be cold for a few nights7day_640x480
.  The average minimum temperature for this time of year is 51F.  We will see some mornings colder then that with tomorrow morning being around 44F.  If you goto the Cascades and some Central OR areas they will be at or below freezing tomorrow morning.  The NWS put out a special weather statement this morning about that. 
   For the rest of the week, I see some more rain Wednesday evening into Thursday.  After that we will be warming up to above average temps for the weekend.  Have a good night everyone!

Rob Martin

*UPDATE*

Ok so in response to Eternal Yamcha’s question posted at 1:08AM: One easy,
one REALLY geeky/long.

 

For the easy answer, the overall pattern does not support
east to west movement for this specific Low. 
This means that it gets blocked so to speak.  In terms of precip, in washington,
from say Bellingham
to the Cascades on the East, the precip amounts will range from .2” to maybe 1”
towards the Mountains from this cycle. 
Storms like this can die off when it hits land just because the
“ingredients” to keep it going are not available on land anymore, where over
the oceans, it has an abundance of “ingredients” 
  

 

Really Geeky/Long (w/ help from my class notes in college)-

 There are many
different ways you can explain this depending on what methods you start
from.  I am going to use Rossby Wave
Thinking.  If you are not familiar with
it you can Google or Wiki it and get an idea who/what  Rossby was/is. 
To start easy, let’s think of a wave. 

 

The top of the wave is a HIGH and the bottom is a LOW.  A wave is Progressive a lot of the time (not
always), but the over all flow of the “weather” is generally, usually, most of
the time, progressive.  Progressive means
moving “forward” or in our case (N Hemisphere) from west to east.   Another term I am going to use is Zonal
Flow.  Zonal flow is the flow of the
“weather” moving DIRECTLY west to east(Progressive Flow) where the opposite is
Retrogressive, or moving “backwards”. 

 

Back to Rossby… I am going to take a Synoptic Scale approach
to the Rossby thinking.  The significance
is that as the mean Zonal Flow increases, the wave number of the stationary
wave decreases, there are fewer stationary or retrogressive waves, and the
overall pattern become Progressive.  

Experience:  In Strong Zonal Flow, embedded waves move
rapidly from west to east. With Weak Zonal Flow, we often have blocking
situations, as in its stalls.  So in this
case there is Weak Zonal Flow.  If you
look at the same time frame for the GFS at two levels (say 300mb and 850mb) you
will notice that as the “low” stalls, the upper levels have Weak Zonal Flow (
Over Vancouver Island at that particular time, the flow is strong SW, not
W).  This would lead to blocking/stalling
in Rossby Thinking. 

-RM