Saturday, August 17, 2019

The First Autumn Storm Makes Landfall on Wednesday

It is very typical for first "autumn-like" storm to reach our region during the third week of August....and this year will not be an exception.  In fact, we have two systems that will be making landfall this week, with the first one being the strongest.

You can see the impact of this "first autumn storm" in the precipitation climatology Seattle.  Below is the  daily  climatological probability of enjoying at least .01 inches at Sea-Tac.  At the end of August the probability jumps to roughly 25%! 


Rain comes in on Wednesday, associated with a robust Pacific front, with the 24 hr total ending 5 PM Wednesday being substantial over the Olympic Peninsula and north Cascades.  On the coast, some locations could get several inches.


The Wednesday system is associated with a front connected with a very intense low center moving into southeast Alaska (see surface weather map at 2 AM Wednesday). Let's say that some of the Alaska cruises will be enjoying some rock and roll that has nothing to do with music.


Another system comes in late Friday and Saturday, with substantial amounts shown in the 24h total ending 5 AM Saturday.


All this rain will be associated with a massive reconfiguration of the atmosphere to a more fall-like pattern.  To illustrate, here is the upper level map for around 30,000 ft ASL, showing the jet stream winds in colors.   At 2 PM today, there is a ridge (higher pressure and heights) stretching from the Aleutians to California, and the jet stream (yellows and light green colors) is to our north.


But by 11 PM next Saturday everything is different, with a fairly strong jet stream heading right into our region.   You won't have to worry about smoke in the Northwest.  You will have to worry about finding your umbrella.


This pattern change will have a huge impact on wildfire potential, greatly reducing the changes of new fire initiation or the growth of old, smoldering burns.


23 comments:

  1. Hope this prediction is correct and that far north (Whatcom county) gets some hugely needed rain.

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    1. Pretty much a bust here. Some showers, lots of drizzle. Soil moistened only on top. Still droughty. Hardly the hyped Big Change. And next week continues dry, dry, dry.

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    1. Interesting. The NWS forecast definitely predicts (and has been predicting) the Wednesday system, but nothing after that for the foreseeable future. In fact, the current AFD has this to say: "
      000
      FXUS66 KSEW 192145
      AFDSEW

      Area Forecast Discussion
      National Weather Service Seattle WA
      245 PM PDT Mon Aug 19 2019

      .SYNOPSIS...
      A warming trend will take place through Tuesday across the region as a ridge of high pressure builds in. This will be brief as a trough of low pressure pushes in from the Pacific into central BC and provides cooler temperatures and rain across the region through Wednesday night. A weak high pressure will build back into the region for the late week with temperatures near normal. There may be a few showers around on Saturday, but otherwise it will be a mainly dry stretch of weather to close out this week.

      &&

      .SHORT TERM /TONIGHT THROUGH THURSDAY/...
      Clouds have been slow and stubborn to fully dissipate, although most areas have at least developed sun breaks at this time and many areas have become mostly sunny. Temperatures are running from near to slightly below normal at this hour with 60s and lower 70s across the area. A mid level trough of low pressure will slide northeast out of the Pacific from later tonight into Tuesday. Ahead of the system, increasing 500mb heights across Western Washington will set the stage for a warmup tomorrow. Highs will be a good 5-10 degrees warmer than today, with many low 80s developing. Skies will be sunnier as well, but should steadily cloud up from west to east in advance of this approaching system. The system will bring rain to the coast by early Wednesday morning and spread steadily east through the afternoon hours. Heaviest rain totals will be on the Olympic peninsula to the coast with lighter amounts farther east. Temperatures will remain mainly in the 60s on Wednesday with the clouds and showers. While this system has plenty of moisture, it looks to remain progressive as the trough axis crosses the area by late Wednesday afternoon and conditions become just showery overnight Wednesday with chances diminishing by daybreak Thursday.


      Heights remain relatively high on Thursday and even begin to increase again as a strong ridge over the eastern Pacific builds south of the area. The ridge will not make too much northern progression this week however as a trough well to the north will hold it at bay and keep more of a flat ridge or zonal flow in place. This will mean Thursday should be near normal with decreasing cloudiness.

      .LONG TERM /FRIDAY THROUGH MONDAY/...
      For the extended portion of the forecast, the zonal flow will continue with a strong mid level ridge staying well south of the area. Some indication of possible showers on Saturday but the bulk of the system should remain north and so will include only small chances of rain on Saturday. Either way, temperatures will linger around normal through the weekend, in the upper 60s and lower 70s. By early next week the focus will begin to shift on northward expansion of the strong ridge to our south. This may set the stage for an eventual return to hot weather to close out August. Although this is a little past the current forecast period, it is worth noting that CPC highlights the area for a risk of excessive heat the last few days of the month." The CPC's 6-10 and 8-14 day outlooks (e.g., https://www.cpc.ncep.noaa.gov/products/predictions/610day/), also predict warm and dry weather for our region's immediate future.

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  3. Agreed. This has been an amazing summer!

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  4. This has been a horrible summer

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  5. @perfectlyimperfect - yep, reminds me of one in the 80's that followed a couple of hot days in May.

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    1. I think I remember the summer, we had invited friends over for a July 4th party. It was not summer weather so we planned to eat indoors. Late in the morning my wife came outside where I was getting the grill ready and said "It's too cold in here! I'm turning on the heat" OK. She turned on the heat.

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  6. Rainy weather is among my faves, even on the Olympic Peninsula when it sometimes comes down in buckets.

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  7. Best classic NW summer in decades!!

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  8. I'm not quite ready for this yet! I love fall....but.....

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  9. Can't please everyone. Good thing the weather is not obligated to please anyone. For those who think fewer 90+ degree days is a bad trade for less wildfires and smoke....really?

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  10. Horrible summer? What summer? I'm still waiting for summer...

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  11. wa rocks!
    So happy fall is on the way! Buying the brightest/ funnest umbrellas I can find!!

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  12. Born and raised near Mt Shasta. Snow first touches the mountain in August, maybe by Thursday I'll see a dusting on the live cam. Ps. Great blog

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  13. I, personally, I'm a fan of the cooler weather. So this was an incredible summer in my opinion. Even had a fair amount of rainfall. And, thank God, very few wildfires in the region. We can only be so blessed for that last one. I am looking forward to the increased rainfall, a cooler fall, and the chill of the holiday seasons in the coming months. Thank you summer 2019!

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