Rain Chances Rise
Southeast winds have set up across the Finger Lakes between high pressure off the coast of New England and low pressure over Michigan.
A frontal boundary extends east from this low, separating dry air to the northeast and increasingly humid air over the Finger Lakes.
The southeast winds will keep much of the region in the 70s, but as the winds come off the higher elevations of Central New York and the Southern Tier, the northern Finger Lakes will warm due to the “downsloping” wind. Rochester could reach the mid 80s with highs near 80 degrees extending to Geneva and Syracuse.
Especially in these areas, the atmosphere will become unstable enough for a few scattered showers and thunderstorms as a disturbance rides along the frontal boundary.
Some showers are possible this morning with a few more showers and thunderstorms developing in the late afternoon.
Overall, the coverage of the rain should be spotty, but there will be some potential for heavier downpours where it does rain.
Once the sun sets, the atmosphere will become stable and precipitation should quickly dissipate.
Flash Flood Threat Friday
Friday will have a more significant chance of rain and thunder as a cold front moves into the region.
Winds will be more southerly or even southwesterly, leading to more instability.
As the cold front moves in, showers and thunderstorms should develop during the afternoon. Winds aloft will be light, so thunderstorms will be slow moving and prone to redeveloping over the same areas.
With copious amounts of moisture in the atmosphere, this could lead to isolated, but significant flash flooding issue. In other words, 98% of the region will probably not see flooding on Friday, but the 2% that does could see moderate to major impacts.
As usual, urbanized area, and steep terrain and gorges will be especially prone to the flooding.
The severe threat will be limited due to the light winds aloft, but a brief gusty wind cannot be totally ruled out.
High temperatures should rise to around 80 degrees before the thunderstorms initiate and the atmosphere cools.
Areas of fog will be possible Friday evening in areas that see rainfall during the afternoon.
Cooler Weekend
Behind the cold front, north winds will set up for Saturday. Not only will these north winds bring cooler air into the region, but Lake Ontario will have a role in modifying the air further.
High temperatures will only reach low 60s along the lakeshore, with mid 60s for most of the Finger Lakes. The Southern Tier should reach the upper 60s as the influence of Lake Ontario disappears.
The amount of sunshine on Saturday is a bit uncertain. Clouds will be likely in the morning, but sunshine will attempt to come out during the day. The clouds may ultimately win out though, with just scattered appearances of the sun, especially during the late afternoon.
Sunday is not looking much sunnier. Winds will turn back to the southeast, tapping into a combination of moisture from the Atlantic Ocean and moisture left behind by Friday’s front.
Like today, temperatures will be warmest in the northern Finger Lakes due to the downsloping of the wind off of the higher elevations. Mid 70s will be possible there, with upper 60s and low 70s further south.
Models have trended dryer for the daytime hours Sunday, trading the scattered showers that were expected for a steady rain Sunday night.
The first half of next week looks quite chilly with highs threatening to not reach 60 degrees. Scattered showers will remain a possibility as well. With plenty of cloud cover in place, frost is not expected to be a concern, though the potential will be monitored.
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Drew, At some point would you explain the use and meaning of dew point vs humidity?
Meteorologist Drew Montreuil
Humidity, or the moisture in the atmosphere, is measured in a number of ways.
I typically use dewpoint. The dewpoint is the temperature at which moisture in the air will condense into water. So, if the dewpoint and temperature are the same, expect fog or rain. The higher the dew point, the more moisture there is because warm air can hold more water than cold air.
Relative Humidity, which I very rarely use, is how much moisture is in the atmosphere relative to how much it can hold. 80% humidity with a temperature of 40º is very different than 80% humidity with a temperature of 75º. People misuse relative humidity all the time, saying things like “It was 90º with 100% humidity!” That is saying the temperature and dewpoint were both 90º which never happens.
Another measure I use heavily but don’t report on is Precipitable Water. This one is a lot more complex and confusing to the general public, but is a great forecasting tool, especially in these high-moisture, flash-flooding situations. Our PWAT values over the next two days will be on the order of 2-3 standard deviations above normal.