05/16/10  11:05 AM

Upper level disturbances can be a real pain in the next.  For example, this morning a very weak upper level disturbance was able to push just far enough north to bring a few widely scattered sprinkles and variable clouds to the region.  Those clouds are breaking up now, but they can create havoc with a forecast.  Another rather annoying type of disturbance is the upper low that gets cut off and trapped under a building ridge.  These types of disturbances are usually poorly forecasted by model guidance and typically have a couple of tricks to pull on your forecaster.

To the left is the culprit that will bring a rather unsettled periods of weather for the northern Mid Atlantic.  This upper level disturbance over the Plains this morning will march towards the Ohio Valley by Monday night and slowly move through the northern Mid Atlantic on Tuesday and Wednesday.  Now, there are some issues here that are bothering me with the way that the model guidance is handling this disturbance.  I’ll name them in a minute, but I’m having a hard time buying into the complete wash out idea on the variety of model guidance, and I have a pretty good idea why the model guidance in some example is pounding locations with heavy rainfall.

When I look at the whole nature of this disturbance in the Plains, the facts come clear to me that this disturbance is not the primary “pattern driver”.  What I mean is that this disturbance is moving within the steering currents, not driving them.  That’s an important distinction to me.  What is happening this week in the grander scheme is that the pattern is shift just as I thought it would to a much warmer pattern for the end of May and for likely much of June.  As a result, a large ridge is building over the eastern two-thirds of the United States as the Polar Vortex lifts north and the negative NAO pattern shifts east towards Iceland (not like they don’t have enough problems there with those Volcanoes).  Now, what’s happening is that this disturbance is being cut off under this ridge from tonight through Monday night, meandering and WEAKENING as the disturbance drifts towards the Ohio Valley.  Why the Ohio Valley?  Because the upper level ridge building towards the Great Lakes is forcing the disturbance in that direction!

Okay, so what does this all mean for the up coming week?  Well, what this tells me is that the disturbance is nothing more than a weakening connection to the Polar Vortex lifting north, which means that this disturbance will likely end up further north than even currently forecasted.  I should note that previous guidance had this disturbance moving through through southern Mid Atlantic and Southeast, and now the trough axis is further north towards New England.  So that means the moisture advection from the Atlantic will not be as impressive as shown in guidance as we move forward because all the dynamics will be drawn towards the lowest pressure/heights in Canada.

So let’s piece this all together!  I expect tranquil conditions through Monday with variable clouds at times.  Moisture in the atmosphere will increase due to the growing moisture advection from the Gulf of Mexico right up the coast.  This disturbance will interact with this moisture and create areas of scattered showers and a few embedded thunderstorms Tuesday through Wednesday.  However, this is not a disturbance driven on isentropic lifting, but rather increased instability from the mid levels to the upper levels.  So, I expect the showers to increase in coverage in the afternoon and weaken in the evening and overnight on both days.  The basic rule for both Tuesday and Wednesday is that showers will be an on and off threat.  Due to the nature of the atmosphere, these showers will create heavy downpours at times and will be slow to move.  Thus while one location remains dry all day, another will have a rather wet day.  Again, this isn’t a region wide wash out situation to me, but a day much like what is being seen in the Tennessee River Valley with some locations under the influence of showers and a period of heavy rain while others remaining dry or clearing out fast, and remember this is when the disturbance is much strong than what I expect later on in the week.

Once this disturbance exits, the weather conditions rapidly improve for the end of the week and into the weekend.  High pressure will take hold with clear skies and increasing humidity.  High temperatures will be in the 70′s to mid 80′s.  Still waiting on some data, but I would not be surprised if I have to bump up the high temperatures for the end of the week into the upper 80′s for the Delaware River Valley, which will depend on some wind orientation and subsidence.

Overall though, after dodging some showers this week, the weather pattern will transition to a very pleasant pattern indeed!

For details for your location, check out the seven day forecast!