Happy Friday Everyone! I hope you've had a great weekend. Will it snow tonight? Maybe maybe not. My twin sister and I did a Facebook Live to address the possible late March snowfall.
I don't think we will see snow sticking down in the lower elevations, but I do think if you live around 1000 feet in elevation, it could easily accumulate. I hope everyone has a great weekend!
Day 328...
Happy Saturday Evening! For part of this post, we will be going back to last night and early this morning....
One of the most interesting things I love about weather is the technology that we have at our disposal when looking at a challenging forecast. I posted last night about the snow level being below the elevation of the radar. I need to clarify just a couple points here because I got a couple questions. Here is my post from Facebook.
Here is the a screenshot I took about 40 minutes after I had posted that. You can see the radar display is in "Hydrometeor Classification" mode. This tells people looking at the radar what type of precip is falling from the sky. Snow, rain, ice pellets, hail, etc. The radar can tell what is falling by how the radar beam bounces off the element falling.
Light blue is snow, pink is ice crystals
The radar returns were saying that precipitation falling was snow, however, ground observations were saying either rain or a rain/snow mix for most of the metro area. Let's do some digging into why the radar was showing one thing, but we weren't seeing snow in metro area at the lowest elevations. First off, you see the black circle in the picture above? That's the "cone of silence." It's the area directly above the radar. The WSR-88D Doppler Radar operates on a pivoting axis from 0 degrees up to 19.5 degrees above the horizon. Click here for a great introduction into the NWS Doppler Radar Program.
For our purposes today, we will be looking at the radar beam scan 0.5 degrees, the standard scan we all see on TV or basic radar apps. At that elevation of a half degree, we should be able to see what is happening just above the surface as the radar scans the horizon, but because KRTX (Portland NWS Radar) is located on Dixie Mountain to the NW of downtown Portland, near Scappoose, the elevation plays an important part of what is happening. The radar base is at 1,572 feet and sits atop a 100 foot tall tower. So let's say the radar is scanning at 1,672 feet at the horizon. By the time the radar beam reaches downtown Portland, the beam is now an additional 1000 feet higher, bringing our total to 2,672 feet above sea level.
Here is the Upper Air Data from Salem Friday Morning.
The number following SLE is 1200. That's the elevation of the freezing level. So the freezing level Friday Morning was at 1200 feet. The general rule of thumb for the Northwest is that the snow level is about 1000 feet below the freezing level.
So now that we've established the elevation of the radar, radar beam, and freezing level, let's put the pieces together as far as what we saw Friday night into Saturday morning. Showers started to increase Friday afternoon. The National Weather Service had already posted the SPS for possible low elevation snow. We talked about it in the Facebook LIVE video I did with my twin sister yesterday. There were some reports about low elevation snow around Eugene. As the precip arrived into the Portland area, more reports of snow were starting to pop up. Most people were seeing a snow/rain mix and no accumulation at the lowest elevations. As you got up to around 1000 feet, you really started to get some sticking.
Okay so if most of us weren't seeing all snow, why was the radar saying it was snow in the air?
Radar Elevation: 1672 feet at the tower (2672 feet over downtown Portland)
Freezing Level: 1200 feet above Salem (or Portland)
Snow Level: 200 feet (in theory) because of daytime heating, I would put the snow level really about 800-1000 feet.
So with the snow level between 800-1000 feet, that is clearly below the radar beam elevation. The radar was telling the truth, that at 2600 feet above Portland, it was clearly snowing. Temperatures around 1am, when I left the bar, were sitting in the mid 30's. Portland bottomed out at 36 around 2am. 36 is cold enough to support snow at the valley floor, but only under a heavy shower, strong enough to the push the snow level to the surface. We were a couple degrees too warm to sustain snow, however, had this been December or January, we would be looking at a very solid set up for a couple big days of snow. Thankfully or unfortunately for some of us, it didn't pan out that way.
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