October 26-28, 2016: Wednesday night-Friday morning snow to rain event

Thursday 10/27/2016 6:44AM Update:
The snow/sleet/rain line has reached Ithaca. Thus ends the frozen precipitation portion of the storm. Surface temperatures are just above freezing so freezing rain will not be an issue.

Summary
It is going to snow late tonight into early tomorrow morning before changing to sleet and then finally to a cold rain. Lows will be near 30 tonight and highs tomorrow (Thursday) will only reach around 45 in the late afternoon. Around 0.25-0.5 inches of rain is expected and little frozen precipitation accumulation is expected. Precipitation could end as snow showers as the storm moves out Thursday night into early Friday morning.

12z (8AM) Wednesday 4km resolution NAM model forecast from hr 10 (6PM Thursday)  to hr 36 (4PM Friday) This is a projected radar reflectivity. Precip will arrive later than the radar suggests.

Timing
   Precipitation
Snow should begin to move in between 12AM and 3AM Thursday. Precipitation will start out light before becoming moderate around 5 AM. As winds from the south higher up in the atmosphere erode the cold air away, the snow will turn to sleet. The transition to sleet should begin between 6 and 8AM before changing to rain about 30 minutes to an hour after the sleet begins. Moderate to heavy rain will continue until the early afternoon. Light to moderate rain will continue through Thursday night.

   Temperatures
Temperatures should start around 32 when the snow starts, dropping a degree or two through evaporative cooling. Then they will hold steady until the southerly breeze starts to bring in warmer air. Temperatures will slowly climb into the mid 40s by the afternoon.

Precipitation amounts
   Rain:
The main precipitation type of the storm is expected to be rain. A quarter to a half an inch is expected so nothing too extreme like last week. Rain will be the heaviest after the transition as the warm front moves through and in the late afternoon as the cold front swings around.
   Snow:
Little to no accumulation is expected. Air temperatures are expected to be near freezing but ground temperatures are well above freezing, so any accumulation will be confined to slowed melting on elevated surfaces.
   Sleet: 
Little sleet accumulation is expected. A coating at most is expected before the changeover to rain.
   Freezing rain:
At the moment, it seems that temperatures will be just warm enough to keep freezing rain away. In order to get freezing rain, surface temperatures need to be below freezing but temperatures should rise as the changeover from snow to rain occurs. It is something to watch to see if cold air remains more entrenched than expected.


Updates will come as we get closer to precipitation onset. Nowcasting of precipitation changeovers will be done unless I happen to be in class.

Thanks for reading!
Alex

Winter has arrived in Ithaca: Forecast for week of October 25 2016

Minor edit: replaced Ned Stark with a Whitewalker

The cold promised by a northeast winter is finally here. The storm that dropped 2-4 inches of rain on Thursday night has exploded into a large midlatitude cyclone and is still slowly meandering to the NE over NE Canada. Cold air will continue flowing in behind the storm, dropping temperatures below normal.

Infrared satellite imagery of the Northwestern Atlantic showing a large low pressure system over Eastern Canada with cold front associated convection (thunderstorms extending into the Caribbean

This is our best shot at a freeze so far this season. Model projections predict that lows will dip into the mid 30s tonight and just below freezing by Tuesday night into Wednesday morning. Highs on Tuesday and Wednesday will be  in the mid 40s. Lows Wednesday night into Thursday will be slightly above freezing as cloud cover ahead of a approaching storm traps heat radiated by the Earth and winds switch to the SE.  Thursday looks to be wet and raw, with a high in the upper 40s. We should dry out and warm up slightly just in time for the weekend.

So about that snow:
The upper levels of the atmosphere are well cold enough for snow. The problem is available moisture and lift. Both are lacking on the back side of a low pressure system, where dry continental winds push out of the NW. So how are we going to get moisture? It lies with the Great Lakes and its famous lake effect snow.

Wind and water are important to lake effect. A cooler, ice covered lake in the winter will require air to travel over it for a longer time to pick up the same amount of moisture as wind traveling over a warm lake. Wind that is perpendicular to the lake will move air over the lake for too little time to pick up enough moisture but strong flow parallel to lake length is the best setup for lake effect. That is why cities at the eastern ends of the lakes, like Buffalo and Watertown, NY are such good places for lake effect.  In cold winters, when the lake finally freezes over, the lake effect tap is turned off for the rest of the season.

How lake effect works

This week, hit or miss lake effect streamers will continue for the next two days before diminishing as the winds relax. Temperatures aloft are well cold enough for snow. It's the surface temperatures that are an issue, thus the highest chance for snow will be at night and the early morning hours, before it warms up. The chance for seeing flakes at any one place is low, around 30% so we have to get lucky. The one thing that is working for us is timing. Since it is still early in the season, the Great lakes are still very warm.The dry northwest winds carry moisture off the lake surface and carries it towards us. As it is forced upwards by topography, condenses and forms lake effect streamers. Although streamers are not long, the warm waters will "supercharge" the bands this time around. Any precipitation that falls will originate from lake Huron and Superior as opposed to Lake Erie or Lake Ontario, which are closer. The problem is that air temperatures are just a bit too warm for snow.

NAM model at 1AM Tuesday depicting two lake effect precipitation bands (mostly rain) coming off Lake Huron with some enhancement by lake Ontario.

The next chance for snow will come as the cold air mass retreats ahead of the next storm early Thursday morning. Moisture will overrun the cold airmass. There is a good chance of some snow showers ahead of the main slug of moisture warms the atmosphere enough to change over to rain. Unfortunately, (or fortunately, I'm a snow nut) the approach of the low will direct southerly winds at us, warming us well above freezing, but not past 50 on Thursday. Slight changes in moisture transport and storm track could have visible effects on precipitation types. In my experience, (though confined to DC area snowstorms) precip tends to start a few hours earlier than predicted. However, since it is still early in the season, it's going to be hard to get substantial snow. Even then, some good snowTV would be pretty to go with the cold.

GFS model prediction for 8AM Thursday showing a bit of snow before changing over to rain

Thus concludes my first forecast for Ithaca. Please feel free to point out anything that seems out of place (especially geography) and ask any questions you have. Thanks for reading!

-Alex

Snow this week?

For those who are wondering about this week's weather in Ithaca, NY, I'll have a post up tonight.

A brief summary is that it will be cold and raw this week as a powerful midlatitude storm scoots off to the NE and cold high pressure builds in behind it. The storm is close enough to provide some showers and another storm could approach this week. Both have the potential to give us a couple of flakes. Nothing much, but a reminder that winter is here.