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A ribbon of deep tropical moisture is forecast to sweep across the east coast for the next week, leading to the potential for major rainfall totals and the potential for flash flooding for much of the east coast.

Precipitable water (PWAT) is a measure of how much rain would fall (in inches) if all the moisture in the atmosphere fell as rain all at once. The higher the PWAT value, the more moisture in the atmosphere, and the more rain would fall. PWAT values reaching 2.0" is indicative of the presence of deep tropical moisture. These maps are a good way to predict how heavy rain will be in a thunderstorm. Higher PWAT values indicate the potential for torrential downpours, especially since the thunderstorms will be slow moving or stalled.

A deep ridge of high pressure over the Atlantic Ocean will serve as an atmospheric sump pump for Central America, sucking deep tropical moisture from near the Equator straight up to the northern Atlantic, as witnessed in the images below.

These are forecast precipitable water (PWAT) maps from this morning's run of the GFS model, valid for later this afternoon and tomorrow afternoon, respectively. These maps will look pretty much the same straight through at least Wednesday.

This afternoon:

Tomorrow afternoon:

As the high over the Atlantic strengthens throughout the week, the ribbon of tropical moisture will tilt a bit, spreading into the Deep South and covering more of New England.

Here's the forecast PWAT map for Friday morning:

 

Here's a GFS forecast sounding from eastern North Carolina very early Tuesday morning. This is a SKEW-T/Log-P chart (a sounding) showing the temperature, dew point, and wind through a slice of the atmosphere. This is the same type of chart produced from the data collected by weather balloons, only this is a model forecast.

The red line is the air temperature, and the green line is the dew point. The fact that you can't tell them apart (the dew point is more or less equal to the temperature) means that the atmosphere is 100% saturated -- a testament to the amount of moisture in the atmosphere.

This deep tropical moisture mixed with the typical summertime pop-up thunderstorm regime will lead to potential flash flooding for everyone from Florida to Maine. The Weather Prediction Center/WPC (formerly the Hydrometeorological Prediction Center/HPC) issues quantitative precipitation forecasts (QPF) for up to a week out, showing how much rain they expect to fall across the country.

Their QPF (rainfall forecast) for the next 7 days reflects the deep tropical moisture -- showing 8" of rain in the Big Bend of Florida, with pockets of 5" of rain over parts of the southeastern United States. The WPC mentions in their discussion that even these high rainfall totals could be a "conservative estimate" given the ample moisture in the region.

The National Weather Service uses flash flood guidance maps to determine the risk for flash flooding in each county/parish in the United States. By determining the moisture content of the soil in a certain area, the NWS can determine how much rain needs to fall over a 1, 3, 6, 12, or 24 hour period to create a flash flood. The more moisture present in the soil, the less rain will absorb into the ground, and the more will run off. If enough rain falls fast enough and runs off without absorbing, it could result in a flash flood.

Take the current flash flood guidance for the DC area (shown above) as an example. The colors in each county corresponds to the amount of rain that needs to fall in one hour to produce flash flooding. DC proper and Arlington County only need to see 1"/hour of rain to see flash flooding, mainly because they're densely populated urban areas. They have more concrete/asphalt than soil, so it's much easier for the rain to run off instead of absorb into the ground. Fairfax County needs to see 2.0"/hour for potential flash flooding. Prince William needs 2.25"/hour. Places like Fauquier and Culpeper Counties to the west of DC need extremely heavy rainfall -- 2.75"/hour -- since they're relatively rural counties with extensive farmland and fields to absorb the rain.

If thunderstorms stall over an area or start training (one thunderstorm after the other moves over the same areas, like a train on tracks) over a longer period of time, it would require more rain to produce a potential flash flood. Thus, the 3 hour flash flood guidance is higher than the 1 hour, the 6 hour higher than the 3 hour, and so on.

Here's an example of thunderstorms training over the same area. This is southwest of Charlottesville, VA.

It's worth noting that it will require much less rainfall over places like Pennsylvania and New England to produce flash flooding due to the copious amounts of rain these areas have seen in recent days. For instance, in almost all of Vermont, it will only take about 1.5" of rain falling in a 3 hour time frame to produce flash flooding.

You can check out flash flood guidance for every state in the country at this link.

Almost half of all flash flood deaths occur in vehicles. Cars can be swept away by less than two feet of moving flood water. People can be swept away by just 6 inches of moving water. You can't tell how deep the water is, even if you think you know how deep it is. Heed the NWS' saying: "turn around, don't drown." It's your own damn fault if you do. Don't risk the lives of the people who have to swim out there to rescue you.

It's summer. Torrential summertime downpours are common. But this is expected to be a prolonged event with lots of very high rainfall totals. These storms will also be slow-moving or stalled over one area. Flash flooding is likely, especially where the storms set up and train or stall, and even more so in New England where the soil is already saturated from recent heavy rains. Pay attention to your local National Weather Service office for potential flash flood alerts.

UPDATE: In the time that's elapsed since I posted this diary, the National Weather Service has issued flash flood watches for counties shaded in dark green. The dark red shading is an active flash flood warning. Expect more flash flood watches to be issued as the week progresses.

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