Flash-flooding thunderstorms in parts of Northeast precede cooldown
Updated Jul 16, 2021 11:33 PM EDT
Very hot and humid conditions across much of the Northeast will be replaced from north to south by somewhat cooler air during the weekend. Once the heat is gone, no quick return is likely. Often, the price of more comfort temperature-wise is the threat of drenching thunderstorms. Of course, as with many generalizations, everything is relative. For example, a cold front breaking the heat during a drought offers a positive opportunity for much-needed rain. However, if rainfall has been above average, new rainfall can cause life-threatening, damaging flash floods.
In the middle of Pennsylvania, widely separated thunderstorms developed at midday, then more groups and lines of storms developed farther west during the afternoon. In the following photograph, you can see the swelling cumulus clouds and also some evidence the updrafts were affecting thinner clouds just above (note the thinner clouds in an arc above the large cloud dominating the left half of the picture).
The upper air ridge that promoted the heat wave in the Northeast is drifting off the New England coast as a fairly weak trough approaches from the Great Lakes. The map below is a GFS prediction for 8 p.m. Friday.
The next map, for Sunday evening, shows that the eastern ridge has been replaced by a weak trough. Despite its weakness, it has the effect of trimming the heat.
This map for Tuesday evening shows the trough strengthening over the Great Lakes and Northeast.
The satellite radar composite shows extensive cloudiness with showers and thunderstorms from Louisiana to Michigan and over to New York state. The presence of abundant moisture and slow-moving upper air features means there could be enough time for some clusters of thunderstorms to linger over the same areas long enough to produce flash flooding. Unfortunately, the nature of this type of situation means the arrangement of thunderstorms is constantly changing. This means that while a few places could experience flooding at any given time, the majority of the region will not. If you are planning outdoor activities during the next couple of days, it's a good idea to keep checking the radar on your handheld device.
The GFS forecast map for Friday evening shows extensive rainfall during the six-hour period ending at 8 p.m.
The next map for Saturday evening shows the redevelopment of heavy showers and thunderstorms farther east than on Friday. I am not showing you the interim maps, but typically in thunderstorm situations, the bulk of the action will be in the afternoon and early evening. In addition to heavy rain, there can be strong gusty winds. As a shaft of rain falls through the thunderstorm cloud, the air below is called dramatically in comparison to the air not affected by the falling rain. Cool air is more dense than warm here, so we can often see a burst of wind descending from the cloud. When this burst reaches the ground, it cannot keep going, so it spreads out as a gust front.
The next map shows that by late Sunday, there is much less shower activity across the Northeast than earlier in the weekend. The extreme heat is gone.
The next two maps for Monday and Tuesday reflect the second trough dropping in from the northwest and strengthening over the Northeast. With cooling aloft, there can be additional showers and thunderstorms even though the overall air mass will not be as moist as what the region has recently experienced.
The map for Wednesday shows a batch of showers getting ready to move off the New England coast with dry, cool air following.
If Thursday's map is correct, it is dry and rather comfortable across the middle and north Atlantic states, but it looks like another cold front is about to cross the lower Great Lakes.
The following sample forecast shows that the greatest cooling will be across New England, but even in Philadelphia, there are no predicted temperatures of 90 degrees Fahrenheit or higher after tomorrow. Judging from predicted nighttime low temperatures, the evenings will be rather comfortable in the Middle Atlantic states, but air conditioners in most buildings are unlikely to take a vacation.
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Weather Blogs / Northeast US weather
Flash-flooding thunderstorms in parts of Northeast precede cooldown
Updated Jul 16, 2021 11:33 PM EDT
Very hot and humid conditions across much of the Northeast will be replaced from north to south by somewhat cooler air during the weekend. Once the heat is gone, no quick return is likely. Often, the price of more comfort temperature-wise is the threat of drenching thunderstorms. Of course, as with many generalizations, everything is relative. For example, a cold front breaking the heat during a drought offers a positive opportunity for much-needed rain. However, if rainfall has been above average, new rainfall can cause life-threatening, damaging flash floods.
In the middle of Pennsylvania, widely separated thunderstorms developed at midday, then more groups and lines of storms developed farther west during the afternoon. In the following photograph, you can see the swelling cumulus clouds and also some evidence the updrafts were affecting thinner clouds just above (note the thinner clouds in an arc above the large cloud dominating the left half of the picture).
The upper air ridge that promoted the heat wave in the Northeast is drifting off the New England coast as a fairly weak trough approaches from the Great Lakes. The map below is a GFS prediction for 8 p.m. Friday.
The next map, for Sunday evening, shows that the eastern ridge has been replaced by a weak trough. Despite its weakness, it has the effect of trimming the heat.
This map for Tuesday evening shows the trough strengthening over the Great Lakes and Northeast.
The satellite radar composite shows extensive cloudiness with showers and thunderstorms from Louisiana to Michigan and over to New York state. The presence of abundant moisture and slow-moving upper air features means there could be enough time for some clusters of thunderstorms to linger over the same areas long enough to produce flash flooding. Unfortunately, the nature of this type of situation means the arrangement of thunderstorms is constantly changing. This means that while a few places could experience flooding at any given time, the majority of the region will not. If you are planning outdoor activities during the next couple of days, it's a good idea to keep checking the radar on your handheld device.
The GFS forecast map for Friday evening shows extensive rainfall during the six-hour period ending at 8 p.m.
The next map for Saturday evening shows the redevelopment of heavy showers and thunderstorms farther east than on Friday. I am not showing you the interim maps, but typically in thunderstorm situations, the bulk of the action will be in the afternoon and early evening. In addition to heavy rain, there can be strong gusty winds. As a shaft of rain falls through the thunderstorm cloud, the air below is called dramatically in comparison to the air not affected by the falling rain. Cool air is more dense than warm here, so we can often see a burst of wind descending from the cloud. When this burst reaches the ground, it cannot keep going, so it spreads out as a gust front.
The next map shows that by late Sunday, there is much less shower activity across the Northeast than earlier in the weekend. The extreme heat is gone.
The next two maps for Monday and Tuesday reflect the second trough dropping in from the northwest and strengthening over the Northeast. With cooling aloft, there can be additional showers and thunderstorms even though the overall air mass will not be as moist as what the region has recently experienced.
The map for Wednesday shows a batch of showers getting ready to move off the New England coast with dry, cool air following.
If Thursday's map is correct, it is dry and rather comfortable across the middle and north Atlantic states, but it looks like another cold front is about to cross the lower Great Lakes.
The following sample forecast shows that the greatest cooling will be across New England, but even in Philadelphia, there are no predicted temperatures of 90 degrees Fahrenheit or higher after tomorrow. Judging from predicted nighttime low temperatures, the evenings will be rather comfortable in the Middle Atlantic states, but air conditioners in most buildings are unlikely to take a vacation.