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Cold air mass brings cruel beginning to May

Don Paul

Don’t say I didn’t warn you. Back on April 6 I wrote an article advising gardeners it was “too early to plant.” It still is, particularly THIS May. Here is the range of average last frost dates for New York State, with data supplied via Cornell Cooperative Extension.

Following a much cooler Monday with daytime highs in the upper 40s-50, skies will be clearing for Monday evening. With a dry, cold air mass in place along with light winds, clear skies would be a key ingredient in the development of widespread frost and freeze conditions to develop before Tuesday morning. While leeks, onion sets and potatoes can withstand such conditions, tender plants will need to be covered tonight. The threat of a true freeze near the lakeshores will be lower, due to the relative warmth of the water compared to the air. But it will be cold enough for some scattered frost even close to the lakes, if we keep our clear skies. Farther inland, morning low temps will reach the upper 20s-low 30s, allowing at least more widespread and more damaging light freeze conditions. Here are forecast predawn lows from a high-resolution model.

For such low temperatures to be realized, it would be important to keep clear skies for virtually the entire night. Another high-resolution model depicts the mainly clear skies in the evening hours.

The one low-probability saving grace could be some cloud cover moving in from the southwest, as seen in the predawn depiction.

If such partial cloud cover moved in, it could provide a little insulation and hold near surface temperatures up a bit. My advice: don’t count on the cloud cover for protection. For one thing, there may be ample time for frost and freeze conditions to develop before any clouds show up, if they show up. The modeled temperature forecast above takes into account such a possible development, and still forecasts subfreezing temps inland. Tuesday night will still be fairly cold, but with more protective cloud cover. If your plants survive Monday night, they will be able to handle Tuesday night.

Measurable precipitation this week will be spotty and light. Although chilly temperatures retard evaporation, there will be enough drying to mitigate the rash of muddy paws for dog owners. We may see a few light showers develop Thursday ahead of a cold front that will usher in the unseasonably cold temperatures you’ve been hearing about. Any lingering showers will mix with at least a bit of wet snow by late Thursday night and Friday. Daytime highs at the end of the week will be around 20 degrees below average:

As for precipitation, the European/ECMWF is more aggressive on the appearance of any snow or a mix on Friday. It has a weak wave of low pressure passing just to our south, placing us in its colder northeast circulation. This would bring more widespread showers of snow and rain, and some accumulation at higher elevations would be a real possibility.

The American GFS and Canadian GEM models are in some disagreement with the ECMWF, showing sparse coverage and insignificant amounts on Friday. On Saturday, there is better model agreement on spotty light snow or mixed showers across the region from time to time in a cold northwest flow, with readings in the low-mid 40s. By Mother's Day, there is good model agreement on dry conditions, with a gain of a few degrees to the upper 40s to near 50. There is no current indication of a potential repeat of our Mother's Day weekend storm of 1989, when 7.9 inches of slushy snow fell at the airport, though we’ll be keeping an eye on that weak low going by to our south on Friday, at least for the hills.  When all is said and done, here are total accumulations forecast by the ECMWF, the most aggressive model. Keep in mind, the model does not take into account inevitable daytime melting in May from marginal temperatures and a high sun angle.

In other words, for now I think the accumulations are overdone, but I will be updating this forecast in the comments section below the article the next few days as necessary.

As for the longevity of the cold pattern, the ECMWF ensemble mean (the mean of 51 separate model runs) still shows a cold upper level low close to us by the evening of May 13. So, sadly, my original forecast of a colder than average first half to the month still holds. However, I also had written of hints of this pattern giving way to moderation in the second half of the month. This far out in time, confidence has to be lower, but the American GFS ensemble mean shows a warming ridge setting up over the eastern United States toward May 19-20, ridding us of the accursed upper low if the GFS verifies. There is also agreement in a longer range model called the CFS toward the end of the month into early June.

Such a pattern would actually give us a shot at ABOVE average temperatures. The CFS is far from the most reliable model, but we have to take what we can get to find some optimism around here.

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