2:00am - Monday Morning
Chief Meteorologist Chris White
A lot of things taking shape over the next week, one of the two biggest is Michael’s potential impact on our area. The second will be the first taste of fall weather behind Michael. The main thing to remember with this forecast is this is not Florence from three weeks ago. This is a different storm, different overall setup, different trajectory, different strength, different everything.
Michael is a strengthening tropical storm tonight as it approaches the Yucatan Straight. Pressure is down to 997mb and falling. I like to describe the atmosphere using the earth surface. High pressure is a mountain. Low pressure is a valley often times filled with a body of water like a river or lake. Air moves from high pressure to low pressure. Say we were playing basket ball on a hill. If the ball gets away from us where does it go? The ball rolls downhill. That is a good way to remember how air in the atmosphere moves. So lets call Michael the basketball. Michael is going to move in the direction of least resistance.
Lets look at all the moving parts.
The upper level ridge of high pressure over the Atlantic has been a huge game player in our weather for the last several weeks. Clockwise flow around high pressure has been a game player for these nighttime rain events. It has pulled tropical air into the region and you have felt that very humid airmass. The high has been so strong though that since Florence, areas out side of the mountains have been largely dry. Air hasn’t been able to rise until it gets lifted up over the mountains. There can be all the moisture in the world available but unless there is something to force the air upward (lift), clear skies and dry condtions prevail. Over the last three weeks since Florence the eastern slope of the mountains have been inundated with showers and storms that sit, and do not move, largely because of this counterclockwise flow.
The jet stream (indicated with yellow line) is finally going to make a move into our area. Cool high pressure will build into the region behind the cold front that will push through the area behind Michael. See the trough over the West? Its been locked there. The cold front is developing near Alberta Canada and will drop south and East over the next two days. This will carve a trough (valley) out in the atmosphere over the center of the country. That trough will move the cold front East. Lets brake this down into 4 quick questions and answers.
Questions on the table:
1) How quick does the upper high ridge (atmospheric mountain) weaken and move East?
2) How quick does the trough and associated cold front (valley/weakness) move East?
3) How quick does that trough pick up Michael and advance it North and East?
4) Will Michael’s winds become elongated along the frontal boundary, keeping winds higher as it progresses through the Carolina’s?
Again, Michael has to find a weakness (trough/valley) in the atmosphere before it can advance North and East.
Answering questions on the table:
1) Models indicate the high pressure ridge moving East and weakening by Wednesday Morning.
2) This should allow the advancing trough and cold front to move into and through the area Wednesday Night and Thursday.
3) After Michael finds that trough it will pick up forward speed racing NNE. This should allow Michael to maintain tropical storm status until moving back off shore somewhere between NC and VA Sometime Thursday.
4) Michael’s winds will not likely have a lot of impact over Western NC but the gradient winds could still cause gusts to 30mph. Along and East of Charlotte, through Eastern NC sustained winds over 50 mph for 6-12 hours appear reasonable. The one saving grace with Michael is that it should be quick and progressive unlike Florence was. That will hopefully minimize the higher rainfall totals. Tuesday Morning we will start to narrow down impacts by county for all of our 8 county forecast area.
***BUT WAIT THERE IS MORE***
One thing to watch is this area of disturbed weather that is sandwiched between the ridge of high pressure and Michael. It is currently North of Puerto Rico and moving WNW around that ridge. That area of disturbed weather will arrive over our area Tuesday Night into Wednesday well ahead of any Michael impacts. Upslope flow rainfall should begin Tuesday and overspread the entire forecast area Tuesday Night into Wednesday Morning. In fact models produce widespread showers and storms this afternoon. Some of these may cluster and organize along the Blue Ridge. Rain rates today will be around 1.4 inches per hour but models indicate rain rates will rise to nearly 2 inches per hour by Tuesday Night. With the antecedent soil moisture along the immediate east slope of the Blue Ridge in Burke, Caldwell, McDowell, and Rutherford much above normal already, rain rates of this magnitude could mean localized flash flooding Tuesday and Wednesday ahead of Michael. Then if Michael does move closer to the forecast area in the western side of the cone of uncertainty a significant flash flood threat could develop Thursday. Fine details remain to be worked on out past three days.
HERE ARE THE GRAPHICS FOR EARLY MORNING FOR HIGH IMPACT WEATHER BRIEFING 2.0. VERSION 2.1 WILL BE UPDATED LATER THIS MORNING BY DAY SHIFT.