Your Tri-State forecast: fire and ice

 If you keep a keen eye on the daily forecast in the Tri-State area, you probably saw at least one of the alerts today, which are depicted in the colorful map below: 

Watches, Warnings, and Advisories valid 12pm ET on May 17th, 2023.

Image from the National Weather Service

Did the top two alerts in the column on the right catch your attention? A Freeze Warning (dark purple) has been issued north of the city for tonight, and a Red Flag Warning (in pink) is in effect for most of the state of Connecticut. In the city, the "Special Weather Statement", shaded in beige, gives information about the potential for fire weather through Wednesday evening. This begs the question: why do we have both fire and ice in the forecast?

First, let's look at the weather setup for today. The image below from NOAA's Weather Prediction Center shows the main weather features in place at 11am ET on Wednesday. The cold front (blue line with triangles) swept through yesterday, bringing barely a drop of rain, is now offshore. In its place, high pressure is nudging in from Quebec, just north of the Great Lakes. 

Surface analysis, valid 15Z on May 17th, 2023.
Image from the WPC

Cold fronts don't always bring cold air; instead, they are marked by a drop in dewpoint after the frontal passage. In simple terms, the air behind a cold front is less humid than the air preceding it. Yesterday's dewpoints were in the upper 40s to low 50s. This afternoon, they are falling into the 20s. The relative humidity has dropped below 30% across much of the Tri-State. Also notice on the map above the isobars, or lines of constant pressure, draped from north to south across New England into the Tri-State. Those lines indicate winds from the north, bringing in drier continental air from Canada. The lines are also fairly close together in relation to the other isobars on the map. This reflects the gusty breezes we've seen today, which have topped 30mph at times. The National Weather Service issued a Red Flag Warning in Connecticut because of this combination of dry air and gusty winds. 

Fire weather is not as common in May as it is earlier in the spring because as new growth develops on our trees and shrubs, it introduces moisture into the air (the process is called transpiration). In mid-May, this process is usually robust enough to keep humidity a little higher at ground level. But it hasn't rained in the Tri-State in about 10 days, and surface water is in short supply. You may have even noticed that there wasn't much dew on the grass this morning; dewy grass is usually the most obvious sign of transpiration. The newly-sprouted leaves also provide a canopy to shade the forest floor from the strongest rays of the sun. If any wildfires do happen today, they will most likely happen in spots that are not shaded.

Wildfires start more readily when a canopy of trees is not present.

Image courtesy of Drought.gov

As it turns out, the same weather setup that produces today's elevated fire danger is also responsible for the Freeze Warning in tonight's forecast. Temperatures could drop into the 20s north and northwest of the city, despite high temperatures in the 60s across most of the area. Dry air heats and cools more readily than humid air, so once the sun sets this evening, the heat that accumulated during the day will quickly escape into the upper atmosphere. Windy conditions can help impede the cooling process by keeping the atmosphere "mixed"; that is, not allowing for a sharp vertical temperature difference. However, our winds are expected to settle down after sunset as the center of high pressure slides closer to the Tri-State, allowing temperatures to drop as much as 30° or greater overnight. The cooldown will be especially pronounced inland, away from the tempering influence of the Atlantic Ocean, and at higher elevations, where the air is less dense and even more susceptible to nighttime heat loss. 

USDA plant zone hardiness map. Farmers and gardeners use their hardiness zone as guidance for planting and harvesting.
Image available for download at usda.gov

As you may know, Frost and Freeze alerts are only issued during the growing season; that's why we don't see these warnings in the winter. For more information on your growing zone, and how it has shifted in recent decades, check out this research by Climate Central.