An East Coast rainmaker with tropical pedigree, coming midweek

A wet and windy rainstorm will be moving northward along the East Coast this week, giving some highly populated areas from south Florida to Downeast Maine gusty winds and very heavy rains. Already, one of the pieces of this emerging weather system produced 10 inches of rain on Key West, Fl. over the weekend.

Current satellite image with fronts and weather systems superimposed. Still in its formative stages, the storm will be largely powered by a strong jet stream/cold front combination that will swoop equatorward toward the Southeast coast from the Northern Plains (shown by the blue contouring in the image to the right). It will receive additional support via its merger with a tropical system currently trying to organize over the southeastern Gulf of Mexico (shown by the red L) that brought all the rain to the Keys.

With very strong winds and dry air aloft associated with the jet stream already approaching the Deep South, and a very chilly airmass expected to reach the Gulf Coast by Tuesday night, the tropical low (AL95) has just a handful of hours to develop before it will be overwhelmed by the midlatitude system. Given the presence of high wind shear over AL95 right now, it has little hope of becoming a named system in the short time it has alone before the merger.

In the unlikely event it were named, it would become Rina. But to focus on whether or not AL95 becomes the 17th named system of the year misses the real weather story unfolding this week.

Weather map forecast for Wednesday morning (NOAA) All of the global weather models indicate that a strong low pressure system will move northward near or along the East Coast, beginning Tuesday near Florida and winding up over the Canadian Maritimes by Friday. A picture of the surface fronts and pressure field expected on Wednesday is shown in the schematic to the right. The low pressures expected to develop with this system will bring gusty winds in the 25-35 mph category (with higher gusts) over much of the eastern third of the country.

As suggested by some of the forecasts, high rainfall totals are possible with this system along the entire East Coast. Flood watches are already in effect for the Miami area. Heavy rain is right now falling throughout the southern third of the Florida Peninsula, with several inches of rain likely in this region through Tuesday.

The picture to the right is a prediction of accumulated rainfall through Thursday night from NOAA’s Hydrometeorological Prediction center.

The dual nature of this system, with the details of AL95’s capture by the midlatitude jet still somewhat uncertain, is apparently splitting the rain into two distinct swaths. The one along the immediate Coast from the Florida Panhandle to Coastal Maine is tied to a mild and humid airmass that will be drawn northward from the Gulf of Mexico and Caribbean Sea ahead of the cold front. Thunderstorms will likely accompany the rain, with the possibility of severe weather in the southerly flow in the warm sector (See the Storm Prediction Center for details).

The second forecasted rain swath over the Ohio and northern Tennessee Valleys will be associated with a chilly north wind, as the coldest air of the season gets drawn into the western side of the circulation. Highs on Wednesday will only be in the 50s (at best) in Nashville, TN and Birmingham, AL. I wouldn’t be surprised if snowflakes were mixed with the rain in the coldest spots on the western to northwestern edge of the precipitation envelope.

In a larger context, this kind of tropical/extratropical development is not unusual this time of year. Ocean waters are still warm enough to support hurricanes in the Gulf of Mexico and Caribbean Sea, and the jet stream is now quite capable of triggering non-tropical cyclone development at these latitudes. With this upcoming storm, the global dynamics are positioned in precisely the way that will allow a tropical system (see my last post) to help create Fall’s version of a nor’easter.

(On Tuesday, Jason will take a closer look at what exactly to expect around Washington, D.C. and the mid-Atlantic. At this point, due to uncertainty in the system’s development and track, making regionally specific predictions is a stretch)