ITHACA, N.Y. — There’s warm, there’s hot, and then there’s extreme heat. The presence of a growing and eastward-shirting “heat dome” will bring dangerously hot and humid conditions into Tompkins County this week, with the worst of it on Thursday and Friday.

Graphic courtesy of the Northeast Regional Climate Center.

Weather Recap

Looking back on June 2020, the story isn’t so much the temperature as it was the precipitation, or more specifically the lack of precipitation. According to the Northeast Regional Climate Center, on the temperature side the average ran pretty close to normal: 64.9°F, a sliver above the normal of 64.6°F. In fact, if you break it down between highs and lows, low temperatures ran 0.8°F below normal, while high temperatures ran 1.2°F above normal. Usually, when you see cooler lows and warmer highs, it indicates conditions dominated by drier airmasses, because moisture in the slow has a higher heat capacity than latent air, meaning it takes longer to heat up, and longer to cool down. You get the same effect with pools – they feel cool during a mid-day dip, but the water feels comparatively warmer than the air above it after sunset.

Regionally, temperatures trended warmer than normal by the coast, and cooler than normal over the Appalachians, especially south of the Mason-Dixon line. All of the major weather stations were above normal, with remote Caribou, Maine reporting its warmest June ever (and warmest day ever, 96°F on June 19th), and New York City’s LaGuardia Airport recording its third-warmest June on record.

Graphic courtesy of the Northeast Regional Climate Center.

Those dry airmasses definitely made their presence known on the precipitation side of June’s record-keeping. With 2.04″ of rain recorded during the month of June, precipitation was only about half of the normal of 3.99″, and most of June’s total came in the last few days of the month as that coastal upper-level low conjured up some thunderstorms last weekend. That 2.04″ is good enough for a tie for 22nd driest in the 128 years of valid local records.

Regionally, the coast and practically anywhere north or east of Pittsburgh was drier than normal for the month. 24 of the 30 first-order weather stations in the Northeast and Mid-Atlantic recorded dry Junes. Caribou, Maine not only had its warmest June on record, it was also its driest, and LaGuardia Airport had its fourth-driest. As a result of the lack of rain, much of the Northeast, including the Finger Lakes, has entered the first stage of drought, “abnormally dry” as defined by the U.S. Drought Monitor based out of the Univ. of Nebraska-Lincoln. Meanwhile, a large swath of New England has advanced to the next stage, “moderate drought”.

Conditions during this week ahead don’t bode well for addressing the lack of rain – most days will have little if any. But the heat and humidity will certainly make it feel steamy enough in the coming days.

Your Weekly Weather

The defining feature of this week’s weather is going to be the high pressure “heat dome” that will dominate much of the country in the upcoming week. The jet stream pumps in moisture from a warm Pacific Ocean and is pushed northward by a very large ridge of high pressure fed by hot continental air over the Central United States. The ridge is so big that it tends to create a blocking effect, preventing storms from trekking across the continental U.S. (However, along its northern edges with the jet stream, the jet may provide enough instability to trigger pop-up severe thunderstorms). The jet pumps in moisture from the ocean, which gets picked up by the adjacent high, and wrapped within the high’s clockwise circulation. What that means is that much of the country will be experiencing a hot, humid, long-term heat wave.

At least to start, steering winds are light with a weak bubble of high pressure over the Ohio River Valley. This is gently steering in hot but fairly dry northwesterly air for today and the start of Monday, so although the airport is reporting temperatures in the upper 80s up in Lansing, the dewpoint is fairly comfortable, in the upper 50s. This dry air also means that conditions are fairly calm, with just some high clouds streaming aloft, and a few fair-weather cumulus in the hillier areas.

For the rest of your Sunday, it’ll be a fairly pleasant evening for your socially-distanced BBQ or other outdoor activities. Temperatures will steadily taper down to lows in the low 60s overnight, with mostly cloudy skies and comfortable humidity.

However, by Monday, the dewpoint will be rising steadily through the day as the bubble of high pressure shifts east of Tompkins County, and begins to channel in hotter, more humid air from the Deep South. By afternoon, it will be in the upper 60s, which will definitely create a sultry, muggy feel to the air as temperatures climb into the low and mid 90s. A few isolated pop-up showers and thunderstorms will be possible in the late afternoon and early evening, but most areas will remain partly cloudy through the day. Monday night will be humid and partly cloudy, with lows in the upper 60s.

Tuesday will feel even worse, as dewpoints climb into the low 70s with highs in the mid 90s. With those conditions, the heat index, the “real feel” effect, will likely approach 100°F, meaning heat advisories and warnings may be issued. As always, get prepared beforehand, take precaution if you’ll be outdoors for an extended period, and know the signs of heat exhaustion and heat stroke. As with Monday, a few storms may pop-up on this outer fringe of the heat dome, but generally, most areas will remain sunny with a few passing clouds. Tuesday night will be partly cloudy and humid with lows in the low 70s.

Wednesday will be much of the same, although the jet stream will meander close enough to this edge of the heat dome to set off some more numerous showers and thunderstorms during the afternoon and evening hours. The energy is there to make these severe, so keep an eye out for severe thunderstorm warnings. Otherwise, expect partly to mostly cloudy skies and very humid conditions with highs in the mid 90s and a heat index around 100°F. Wednesday night will see the storms wind down with the loss of diurnal heating, leaving partly cloudy skies and a low around 70°F.

Thursday and Friday are looking to be the worst of it, as a developing coastal low off the Carolinas helps channel more of the heat dome’s hot, humid air into the Northeast. It will be very hot and oppressively humid Thursday – upper 90s for highs, with low 70s for dewpoint. The heat index will likely be around 105°F. Use extreme caution if outdoors, this is not weather you want to be exerting yourself in. Skies will be mostly clear, with an isolated thunderstorm or two, but generally dry conditions prevailing across the Southern Tier. Thursday night will be humid and partly cloudy with lows in the mid 70s.

Friday is another day in the furnace, with upper 90s for high and a dewpoint in the low 70s. The heat index will once again be 105°F or more during the late afternoon and early evening hours. However, some slight relief is coming in the form of a weak shortwave (pulse of instability) around the heat dome, which will push into our region enough to make the dome recede a little. Friday night will have scattered thunderstorms and mostly cloudy skies otherwise with a low in the low 70s.

This shortwave won’t make it any less humid, but it will bring the overall conditions a little cooler for the weekend – not exactly good, just less awful. Saturday and Sunday will see hazy, hot and humid conditions, not more typical for July, with temperatures in the upper 80s to around 90°F. Partly cloudy skies will open up to scattered shower and thunderstorms during the later afternoon and evening hours on both days, with the storms dissipating after sunset to leave partly cloudy skies and lows in the upper 60s.

Graphic courtesy of the NOAA Climate Prediction Center.

Extended Outlook

Most of the models have the heat dome hanging around the country for at least two weeks. Which, frankly, given that studies suggest COVID transmission is enhanced by air conditioning, is pretty darn worrying. Only the Pacific Northwest will be north of the jet stream and likely to see and cooler than normal conditions. On the precipitation side, slightly above normal conditions for this week will give way to normally drier conditions, with widespread abnormal dryness throughout the rest of the country as that heat dome blocks storm systems from crossing the continental United States.

It’s going to be a hot month folks. Hope you have some cold drinks on tap.

Brian Crandall

Brian Crandall reports on housing and development for the Ithaca Voice. He can be reached at bcrandall@ithacavoice.com.