Easterly Flow Arriving

May 22, 2008

Snapshot A change in movement on the radar imagery the last few hours shows the change on the way the next few days.  We had "surprise" showers late this afternoon and evening that developed over the I-5 corridor and streamed down to the south and east.  These were caused by the moist marine atmosphere bubbling up with weak convection.  But between 10-11pm, showers have begun to approach from the northeast.  I see it's now raining and The Dalles and Hood River.  This is the return flow around the back side of an upper level low centered over Utah.

That upper level low retrogrades (backs up) over Oregon the next 24 hours.  This is a great setup for good soaking rain east of the Cascades.  Much better than any "wet" Pacific system in the winter.  Eastern Oregon grain growers (wheat, barley etc…) that depend on soil moisture alone…no irrigation call a good May rainfall "May Gold".  A good soaking now as the wheat is in it's most active growth is the best they can ask for.  Most years there is no significant rain in May.  We'll see how much they pick up over the next few days.

The biggest forecast problem the next few day is timing the impluses moving around the upper-level low.  One moves into Western Oregon tomorrow, giving us a good soaking.  Then another "wave" of rain/showers comes in Saturday night.  Assuming models are correct in timing, we get a break during the day Saturday.  If timing is off by 8 hours, Saturday will be a rainy day.  Obviously the forecasting is quite difficult for this holiday weekend!  Mark Nelsen