Quiet Monday before rain ahead of Isaias clips region

Isaias Predecessor Rain Event

The Finger Lakes will have a quiet Monday in a narrow gap between three weather systems.

Some clouds exist this morning in the wake of low pressure lifting north into eastern Canada. Trailing behind this system is a front draping southwest across the Great Lakes and Ohio Valley.

The cloud cover should gradually be reduced through the morning for at least a partly sunny afternoon. However, some new clouds far ahead of the third system, Tropical Storm Isaias, will push in late.

Temperatures today will reach the low 80s for most of the region. Winds will be much lighter than yesterday as they turn from the west to the south. A stray shower cannot be totally ruled out late this afternoon, but most areas will stay dry.

Tonight into Tuesday, a somewhat rare meteorological phenomenon will begin to unfold just east of our region. Called a Predecessor Rain Event, they occur well ahead of an incoming tropical system.

The presence of the aforementioned frontal boundary, a favorable jet stream aloft, and a surge of tropical moisture associated with Isaias will lead to an outbreak of rain to the north and northeast of Isaias.

The core of the heavy rain is highly likely to stay to our east, impacting the Poconos and Catskills with the potential for flash flooding. How far west the rain comes will be a major question for our region and is one that is unlikely to have much of an answer ahead of time.

Given the uncommon nature of these systems, the small scale influences that affect them, and the partial reliance on the track and strength of the parent tropical system, weather models can struggle with these set ups. The uncertainty is therefore inherently higher than normal.

That said, the highest chance for rain amounts of an inch or more from about midnight tonight through Tuesday evening will be for areas east of Cayuga Lake, and especially east of I-81. Rainfall will also be heavier to the southern portions of the regions instead of the north.

Further west, rain amounts will quickly decline and areas west of Canandaigua Lake may see little to no rain at all.

Further complicating the rainfall forecast will be the possibility for some thundery downpours to develop along the edge of this rain event. These could lead to locally higher amounts within the main area of rain or could even develop outside of the main area of rain in the otherwise dry western regions.

To wrap all of that up, the further southeast in the region you are, the better your chance for a steady, meaningful rain tomorrow. Western areas will have to rely on isolated thunderstorms for any rain. The heaviest, flood producing rains should stay well to the east, but will have to be watched. And, forecast confidence is low because of the nature of this event.

The rain will taper off Tuesday evening, though a few showers may linger into Wednesday morning. The rest of the week looks dry with temperatures mainly in the 70s.