For those of you just wanting the basics:
1. Today will be the last warm day. Most, or all, of the day will be dry and mostly sunny.
2. Scattered thunderstorms COULD develop anywhere in our viewing area (least likely at Coast) after mid-afternoon. The chance will continue through the overnight hours.
3. They may only be scattered, which means some of you may not see anything.
4. Thunderstorms are almost always spotty and randomly placed, which means we can’t say “Gresham gets one tonight but Camas does not”. You’ve heard forecasting snow is tough? We just plain can’t forecast exactly where thunderstorms will pop up.
5. We are not in a Severe Thunderstorm Watch box.
Today will be our last warm day; it’s back to the usual April programming tomorrow, but it’s been a nice ride. I really enjoyed waking up to warm temps outside this morning instead of the heater kicking on.
As mentioned in last night’s post, thunder is possible just about anywhere this afternoon through the overnight hours. In fact a thunderstorm woke many of you up in the Battle Ground area around 1:30am…if you look closely you can see the strikes on the metro lightning map here:
We are in moist southerly upper-level flow right now and that continues through Wednesday morning. This pattern often DOES produce some sort of thunderstorm activity west of the Cascades, although much of the time you need some sort of real “trigger” to allow thunder to develop (not including last night!). A very warm day helps of course, but as of this morning there doesn’t appear to be a good vort max or upper-level disturbance coming north to set off a widespread event. That, along with looking at model depictions of activity, makes me think it’ll be randomly scattered this afternoon/evening.
We will have a marine push (cooler ocean air) coming inland this evening, and there has been a time or two where the convergence of northwest winds coming through the Columbia River gap and southwest wind into the Willamette Valley has been able to develop a large area of metro-wide thunderstorms. I clearly remember the first day of May ratings one year when it went on all evening. Maybe in the last days of April 2003 or 2004? The WRF-GFS shows a bit of that low-level convergence after 8pm tonight.
Any storms that develop will be moving south to north or SW to NE.
Once we get the marine layer in here after daybreak Tuesday it’s over. The atmosphere will be more stable and we’ll be only 60-65 for a high temp tomorrow. Wednesday could be interesting with a cold front coming in and southerly flow above still in progress…something to keep an eye on. And Thursday/Friday sure look chilly with snow levels possibly briefly down around 2,000′ in the mornings.
I’ll update this afternoon when action starts to pop.
Chief Meteorologist Mark Nelsen