Saturday through Tuesday Hybrid Lake-effect/Upslope Snow Event

Sunday 9:30 AM Update:
It snowed last night but then it all melted because we were 3 degrees above freezing. Areas just to the East of us will do much better due to elevation. Also there seems to be something about the topography that doesnt like Ithaca. Look at the Ithaca split on radar.

Still going with 3-6 inches total accumulation. Hopefully the bands will move around once we transition completely to Lake Effect Snow.

Saturday 7:44 AM Update:

We are still a go for snow. Models have generally converged to us getting around a half a foot of snow. Some will be lost to melting so accumulations will be lower. We are now in the range of multiple high resolution short range models. However, the event duration is too long for these models to cover the entire event. So we shall look at the 4 km resolution NAM again. You can see the lower totals over the finger lakes. This is both due to lower elevation and the moderating effect of the lake. Even then, Ithaca proper is forecasted to get 5 inches by the model. Let's see how much being 500ft above the lake helps us. It'll be a beautiful day for a bit. Enjoy it.

ORIGINAL TEXT
It might hit 70 tomorrow. And then 6 hours later it could be snowing. Time to bring up the memes again.

Tomorrow will be a nice late spring day. Temperatures will hit the mid to upper 60s by mid afternoo, possibly 70 thanks to a nice southerly breeze. By mid afternoon (3-4 pm), a strong cold front will be knocking on our doorstep. As the rain starts, temperatures will rapidly drop, as much as 20 degrees during the first hour as temperatures aloft crash. Depending on how fast the surface cools, rain could change over to a period of wet snow, after 9pm Saturday and continuing into the early morning hours.

4km Resolution NAM depiction of temperatures across the Northeast at 2PM on Saturday. The front is about an hour out from Ithaca (black star) By 4PM, it is near 40.

Then a lull is expected as the synoptic scale activity and the lake effect/upslope snow machine cranks up Sunday morning. This situation is unique in that there is a large pool of cold air aloft associated with a strong upper level low (ULL) passing overhead Sunday. The cold air aloft makes it easier for the relatively warm air near the surface to rise (instability). In addition, the lower levels of the atmosphere will be moistened by relatively warm air being injected by the surface low pressure to our NE. This creates a ripe environment for multiple lake effect bands to develop unusually far away from the lake.

4km NAM showing a changeover to snow around 8PM. Typically, temperatures lag a bit compared to models so the changeover probably will be closer to 9 PM. This is still subject to change.

Since we are dealing with smaller scale bands, exact snow amounts are hard to predict and can range a lot over short distances but snowfall will be more widespread than usual due to the instability and moisture. Snowfall amounts will be greatest oh the hills directly to the south of Lake Ontario and Erie, with 8-12 inches of snow expected. In Ithaca proper, 2-3 inches of snow is forecasted by the NWS. On Cornell's campus, I expect 4-6 inches of snow since we are at a higher elevation. Snow amounts can easily exceed that if we are stuck under a snow band for a couple of hours, which I think is a possibility. It can also be under the forecast if the bands miss us. At that point it's not forecasting, it's nowcasting.

If you zoom in reeeeeally close, you can see the tight gradient between downtown and the hills we are on just east of downtown. Goes from 2-3 to 4-6 inches.

Winds after the front will initially be out of the NW at 20-25mph with gusts to 35mph Sunday dropping to the 10-15 mph range on Monday and Tuesday. Temperatures will be around freezing Sunday morning before dropping into the upper 20s Sunday night and mid 20s Monday night. Windchills could feel like the upper teens at times. Most of the snow should fall Sunday afternoon into Monday morning before diminishing into flurries by Tuesday morning. After Tuesday, it will be sunny and temperatures will begin a slow rebound before another system approaches late week. The impacts of that storm remains to be seen.

4km NAM showing lake effect bands with areas of upslope/ ULL enhanced snow.

This is just a summary of the situation for this weekend. There are too many things to write about here in detail-how topography affects snow amounts, why the lake effect bands even make it here, thundersnow potential, snow ratios,ect. In any case, if you like snow, yay! join the club. If not, at least it's not very cold and break is soon. Updates will be posted to this post so check back for them.

Thanks for reading!

-Alex

Saturday-Tuesday Rain to Snow Event

The somewhat decent warmth that we have been experiencing this week will come to an end Saturday as a cold front sweeps through. Consequently, it's going to snow Sunday and Monday. How much is still up in the air. It probably will be under 6 inches and that's probably too high.

Blizzard winding up over the Northern Plains

A storm is currently winding up over Nebraska, Wyoming, and Minnesota and is expected to track into SE Canada and cut off from the jet stream. Ahead of the storm, strong winds out of the south will drive temperatures into the mid 60s in the Ithaca region. This will be followed by a strong cold front ushering in subfreezing air as the storm moves to our east and cuts off. Then, it will meander around Canada for a few days. Since we are on the backside of the low pressure system, the atmosphere will be cold enough for snow. The meandering storm will throw lobes of energy around the storm. In addition, strong low level winds will promote upslope and lake effect snowbands. Let me explain what that means

4km resolution NAM model predicts temperatures in the 60s ahead of the cold front on Saturday. Temperatures will rapidly drop into the 30s behind the front, with rain showers expected, turning to snow.

Synoptic snows
Widespread snowfalls are easier for global models like the GFS and the Euro to predict from longer lead times because of the large scale nature. Global models have lower resolution than short term mesoscale models. Examples would be well defined low pressure systems like the one that is in the plains right now. Lead times for approximate track can be as long as a week, as is the case with this event. Details are better left to a blend of mesoscale and global models a day or two out.

Mesoscale snows

Mesoscale snow comes largely from two phenomena around here, Lake effect and Upslope. The mechanisms are similar. Moisture is forced upwards by topography and snow falls but the source of the moisture is different. The moisture in lake effect snow is from a lake. Air humidifies and warms as it travels over warm water before being cooled over land, forcing condensation and the formation of clouds. Moisture for upslope snows are generally synoptic. In this case, the cut off low ingests relatively warm, moist air from the Atlantic, which is cooled as the air is forced to rise over high terrain and it snows. In a way, lake effect snow can be considered a form of upslope snow.  The potential for these events can be detected in global models but specific bands are better resolved with high resolution mesoscale models like the HRRR, NAM, and RGEM. Predicting these bands are hard. It's basically nowcasting. The HRRR is run every hour out to 18 hrs. Just extrapolating based off of radar can be pretty useful. Or you could look out the window.

Back to the forecast: Snow
Since we are far away from the lakes, lake effect snows should be limited to a few passing snowshowers. Our elevation is also a bit too low for upslope snows. However, there will be a bit of synoptic scale influence before the storm moves too far away. I would expect a period of light snow Sunday giving way to snow showers. Total accumulation will probably stay under 3 inches although upside is still possible depending on the exact track. We can defininitely also end up with no snow.  Mesoscale features will be determined by where exactly synoptic scale features end up. It is still too far out to nail down exactly where the storm is going let alone where a snowband will set up. By no means will everyone get their fair share of snow. Some areas will jackpot while some areas will get close to nothing. People with experience with lake effect will know how isolated snow can be in that situation.

Lake effect snowbands behind the cold front moving through the Northeast, also predicted by the 4km NAM. The snow bands are being enhanced by an upper level low that has cut off from the jet stream. The potential is there, the question is will it be realized.

As we approach the expect time of the storm, updates will be provided. As always, thanks for reading.