ITHACA, N.Y. — It appears the only way we are going to escape the tendrils of winter is to pry ourselves from its cold, dying hands. Rain and chilly air will dominate the area for most of the next week.
— NWS (@NWS) April 15, 2018
We’re halfway through April 2018 at this point. It’s been a rough one. So far, April is trending about 8 °F below normal, the fourth coldest in the 110+ years of valid records (only 1975, 1943 and 1982 were worse, according to data from the Northeast Regional Climate Center).
Last week was not as warm as initially anticipated. Highs initially predicted in the 70s for Saturday early in the week did not pan out. Here’s why.
To our north the past few days has a been a very strong surface high pressure area – its clockwise flow has been drawing frigid polar air down from the Canadian article. To our southeast, meanwhile, has been a very strong low pressure area, and its counterclockwise flow has tapped into tropical air and moisture, bringing unseasonable warmth to much of the Eastern seaboard. The area where these two air masses clashed? You guess it – upstate New York.
The area they collided, however, shifted further south with each model run during the week – and eventually deprived most of Tompkins County of the warm sector’s heat. Temperatures did at least get into the low 60s Friday, so there was that consolation.
— NWS Binghamton (@NWSBinghamton) April 14, 2018
Now, this complex weather system has resulted in some phenomenal temperature gradients for the Southern Tier and upstate. For example, on Saturday afternoon, if you got in a car in Watertown, and drove the three hours south to Scranton, Pennsylvania, your air temperature changed from the mid 20s, to the mid 70s. Anywhere in-between was a wild swing depending on close the frontal boundary was, the cold air mass had more momentum and pushed the warm air back out.
Now, when one has huge temperature gradients, they also have very large changes in atmospheric pressure, a big pressure gradient. Since air flows from areas of high pressure (denser, colder air) to areas of low pressure (warmer, less dense air), they generate a powerful, widespread wind.
This is also very problematic from a precipitation perspective. When there is a mix of warm air and cold air, the denser cold air typically forces the warm and in this case moist air upward. The moist warm air, made more unstable in the uplift, starts to produce rain. This falls through the cold layer and, because temperatures are below 32 °F, begins to freeze (sleet), or hits the ground as rain and then freezes into ice (freezing rain). Areas north of Route 79 have had to contend with an icy glaze, making travel even more difficult.
Your Weekly Forecast
Most folks have woken up this morning to a light rain, even downpours or a clap of thunder in some places. That also has to do with the clashing of air masses – the cold air forcing the warm, moisture-heavy air upward effectively wrings it out like a sponge thanks to the enhanced instability. That means a damp day is in store.
Generally, the further north one is, the less rain there will be, because there was less warm air in the vicinity. However, there’s still plenty south of Ithaca, so rainfall amounts of up to 1.5″ south of the city are possible (Newfield, Danby, Caroline and counties south), with perhaps 0.5″ – 1″ from Ithaca north. Isolated higher amounts are possible, so be mindful of ponding or localized flooding in the typical problem spots. The heaviest rains will fall in the 8 AM – 2 PM timeframe, and should start breaking down into scattered showers by sunset.
The morning started off windy, but the worst of it is over. As the low-level jet of southerly air moves eastward, so will its higher winds, which have been mixing down towards surface level. That means that the southeasterly gusts will slacken and the winds will taper down. Through all this, temperatures will be chilly, if not “cold cold” – highs will be about 50 °F, though it should drop once the cold front passes through.
Tonight, as the system loses its southern moisture tap and the showers taper down, they may be replaced with a few snow showers during the overnight. Temperatures will steadily cool into the mid 30s. Winds will be out of the southwest at 5 to 10 MPH.
By Tuesday, the primary center of the low pressure will have shifted northward, leaving a deep, strong, meandering storm over southern Canada. Its enhanced instability and cold temperatures will define the local weather for the next couple of days, as moisture wraps around and the storm drags in cold air from the north and west, with the Great Lakes providing additional instability.
At this point, it’s looking to be mostly a rain event for the valley areas and Ithaca proper, though a light accumulation of snow (generally no more than 1-2″) can’t be ruled out in the cooler higher elevations. Tuesday will be cloudy, scattered rain showers, and highs in the mid 40s. Tuesday night will have a few light rain or snow showers, mostly cloudy skies, and lows in the mid 30s.
Wednesday will be something of an improvement, if not pleasant, as the meandering deep low moves east and away from the area. Mostly cloudy skies, calm winds and temperatures in the upper 40s will dominate the area, and Wednesday will be mostly cloudy with lows in the mid to upper 30s.
However, rain should be returning overnight thanks to a clipper-type low moving in from the Midwest – nothing extreme, but Thursday will be another grey, rain showery day with highs in the mid 40s, and mostly cloudy skies Thursday night with lows in the mid to upper 30s. Yes, it’s basically a parade of storms this week. At least Thursday’s will move quickly in and out of the region.
Things finally start to improve by the end of the week. Friday will be mostly cloudy, a few lingering showers, and highs in the upper 40s. But the cloudy finally start to break down Friday night, as temperatures cool off a little more to about 32 °F under dry conditions. Saturday and Sunday are both looking to be mostly sunny with highs in the low to mid 50s.
There may be some signs of spring on the horizon. As the jet stream readjusts to a northward-moving high over the Atlantic, we can expect the northern branch of the jet stream to shift northward here, which sets up for some weak atmospheric ridging and a tap to some milder air further south.
However, any additional warmth will be limited. Most of the eastern U.S. which actually be cooler than normal thanks to a southern branch of the jet stream pulling cold Canadian air into the Mississippi River Valley (it’s been pretty much the same story since late February – big warm, dry ridge in the West, big cool, stormy trough in the east). Conditions are expected to be near or slightly warmer than normal, which is about 60 °F at this time of the year, with normal precipitation amounts for late April.