A winterlike storm that continues to unload snow and ice over the southern Rockies and Plains and Zeta, a tropical storm currently spinning over the south-central Gulf of Mexico, are forecast to combine forces and bring the northeastern United States drenching rain and even accumulating snow in some locations before the week draws to a close.
Zeta slammed into the Yucatan Peninsula of Mexico as a hurricane on Monday night, and is now set to charge across the Gulf of Mexico and make landfall in Louisiana late Wednesday, likely as a strong tropical storm or low-end Category 1 hurricane on the Saffir-Simpson Hurricane Wind Scale. Forecasters are calling for Zeta to then move inland and take a curved path over the South Central and Eastern states late this week. The storm will weaken and transition to a non-tropical system, but still pack a punch.
The storm that continues to bring cold air and snow and heavy ice to several Southwest states is also forecast to lose intensity as it moves northeastward, but both systems may aid each other, reorganize and merge in such a way as to generate drenching rain, gusty winds and heavy mountain snow in the Northeast.
It's rare for tropical systems to help trigger winter weather -- but not unprecedented. October is certainly the time of the year for this to take place. Superstorm Sandy tapped into cold air on its western flank and produced up to a couple of feet of snow on the central and southern Appalachians during late October 2012.
After moving off the east coast of Florida in October 2005, Hurricane Wilma's moisture fed into a coastal storm, which caused high waves and beach erosion along the northeastern U.S. coast, along with high winds and snow from Pennsylvania up to New England, where more than 20 inches of snow fell.
Unlike Sandy, which moved westward from the Atlantic toward the East Coast, and Wilma, which emerged over the western Atlantic after pushing across Florida, Zeta is predicted to spend many hours over the interior southern U.S. before it meets up with enough cold air to produce snow.
Only fast forward motion will allow Zeta to retain some of its wind field and produce gusty winds from the southern Appalachians to the mid-Atlantic coast.
"The setup for the Northeast will involve a fresh injection of cold air as high pressure will build eastward from the Great Lakes region spanning Thursday and Friday," AccuWeather Senior Meteorologist Mike Doll said.
Without the fresh cold air, only rain would fall or most rain and just a few wet snowflakes at the tail end of the storm.
Regardless of the fresh cold air ingredient, the storm will start off as rain spreading over the central Appalachians and mid-Atlantic on Thursday then reaching New England Thursday night.
During Thursday night to Friday, as colder air is drawn in, rain is forecast to become mixed with and change to wet snow over the mountains of central and northern Pennsylvania, the southern tier of New York state and western and central New England.
With a full changeover to snow now anticipated over the mountains, a moderate to heavy accumulation is appearing more likely over the higher terrain into Friday. Such a scenario could weigh down trees that still have a canopy of leaves and lead to power outages.
"Snow could be seen even at some of the valley locations in western and central Pennsylvania, the southern tier of New York state and interior western New England as well, should enough cold air be drawn in," Doll said.
In the mid-Atlantic and along the southern coast of New England and eastern Massachusetts, the storm duo will bring drenching rain and windy conditions.
The zone of precipitation is not likely to expand into Canada, and the northern edge of the rain and snow will be rather sharp as an area of high pressure to the north will be accompanied by dry air across the Great Lakes and northern tier of the U.S.
Any shift in the press of dry and cold air could have an impact not only on the northern extent of the rain and snow but also the entire notion of a change to snow and its intensity in the first place, according to forecasters. Should the dry air press in more forcefully, then it could limit most of the rain and any snow farther to the south across Pennsylvania and West Virginia, northern New Jersey and the Connecticut hills.
On the other hand, if the push of dry air is much weaker, then rain could reach all the way to southern Ontario and Quebec. In the latter scenario, snow may be less widespread if cold air is kept at bay and remains locked up to the north.
AccuWeather's current forecast represents a consensus among its dozens of meteorologists and an average of the possibilities at this point.
Closer to where Zeta will track from the Ohio Valley to the mid-Atlantic coast during Thursday and Thursday night, the rain could become heavy enough to cause flooding in poor drainage areas. Flash flooding could threaten communities from Pittsburgh, Philadelphia, Baltimore, New York City, Boston, Richmond, Virginia and Washington, D.C.
Flooding could be enhanced where fallen leaves block storm drains. Fallen leaves can also add to the dangers of the rainfall by making highways and streets in wooded areas extra slick.
Additional hazards will accompany the heavy precipitation, making travel even more difficult. Fog will shroud the ridges a few hours after the rain gets underway, and along the coast, a stiff wind is forecast to develop. The combination of rain and wind could make driving on highways and walking with an umbrella a challenge. Winds could be strong enough to break weak tree limbs and knock over poorly-rooted trees.
Despite the problems rain and wet snow may cause for travel, the rain will be beneficial in areas of the region that have been experiencing abnormally dry to drought conditions since the summer.
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