Western Showers, Eastern Deluge?

Changes are afoot that will result in a shift in the weather for Utah and a significant rainfall event in the eastern United States.

The pattern currently developing over North America will give weather forecasters headaches from coast to coast.  It features multiple interactions between upper-level closed lows and high-amplitude ridges, with a tropical cyclone thrown in just for good measure.  The interactions between these features are notoriously difficult to nail down and this leads to considerable forecast uncertainty.

The low hanging fruit is in Utah.  Our run of well-above-average temperatures will come to an end on Friday as an upper-level trough from the Pacific swings into the Intermountain West, bringing cooler air and showery weather.

After that, it's tough to say.  The models have been all over the place dealing with the interaction of Friday's trough with another upper-level trough dropping down from British Columbia over the weekend (such an interaction between two troughs is called the Fujiwara effect).  I'm giving up on details for the weekend except to say that temperatures will thankfully be more seasonable.

The real forecast challenge in the east.  A significant rain system is sweeping through the northeast currently, then it looks like a prolonged rain event will develop along the mid Atlantic and southern New England coast through Friday night in advance of tropical cyclone Joaquin.  Such rain events are sometimes called predecessor rain events or PREs.

After that, we have diverging solutions from the various members of the GEFS and Euro ensembles regarding the track and intensity of Joaquin ranging from a direct track into the mid atlantic states (see GFS above) to a track that keeps it well offshore.  The Euro ensemble has more members keeping Joaquin or its remnants offshore, whereas the GFS has more members calling for something moving into the mid atlantic or at least along the coast (see below).

Source: Penn State e-Wall

Put them together and you have a huge range of possibilities.  A good summary is provided by the National Weather Service Sterling, VA area forecast discussion:

"FORECAST CONFIDENCE SIGNIFICANTLY DECREASES OVER THE WEEKEND AS MODELS CONTINUE TO STRUGGLE TO RESOLVE THE BLOCKING PATTERN (CLOSED LOW TO OUR SW...HIGH OVER NORTH ATLANTIC) AND HOW TC JOAQUIN INTERACTS WITH IT. SOLUTIONS RANGE FROM A COASTAL IMPACT TO THE TC REMAINING WELL OFFSHORE. PLEASE REFER TO NHC BULLETINS FOR THE LATEST DETAILS AND BE AWARE THERE IS AT LEAST POTENTIAL FOR HEAVY RAIN AND FLOODING (THE LARGEST THREAT GIVEN THE TWO PREDECESSOR RAIN EVENTS)...WINDY CONDITIONS...AND COASTAL FLOODING OVER THE WEEKEND."

Overall, this is a great example of a low predictability pattern, but one with the possibility of high impact weather.  Rainfall associated with the PREs is likely, but precisely where and how much.  Then, will that be followed by a further deluge?  From a scientific perspective, the challenge is developing ensembles that provide reliable estimates of event probability.  From a communications perspective, the challenge is how to effectively inform the public and decision makers when the range of possibilities is very large.