It’s a new weekend and yet again we are discussing the risk for heavy rain and flash flooding across portions of the Mid-Atlantic region. A cold front will be slowly moving southward across the Northeast through early-next week. Given the slow nature of this front, it will practically be stationary over some areas, acting as a focal point for heavy precipitation. These cold fronts often enhance and allow for the development of heavy rain and thunderstorms. Because it will be oriented west to east, which is the direction the rain will move, there will likely be the development of training storms. These training storms can be prolific rain-producers, dropping several inches of rain in just a short period of time because they are moving over the same locations.
Another important ingredient for heavy rain is how much moisture will be present in the atmosphere. This weekend, precipitable water values, which measure the amount of available moisture in the atmosphere, will increase as the weekend progresses. The greatest flood threat may arise on Sunday. On Saturday, values will generally be 1 to 1.6 inches, indicating a moderately-moist atmosphere. On Sunday, values will likely exceed 2 inches, especially in coastal sections of the Mid-Atlantic. The greater amount of moisture, the greater the risk for heavy rain.
Sure, there will likely be heavy rain in some areas, but another area of significant concern to us is how saturated the soil is following weeks of heavy rainfall. Soil moisture is well above normal in the Mid-Atlantic region, with widespread percentiles of 80-100% based on GRACE (Gravity Recovery and Climate Experiment) satellite data from NASA.
This Friday, we aren’t concern about any flooding in the Mid-Atlantic. Other than a few hit or miss showers and thunderstorms that move from the central Appalachian Mountains into Maryland, Virginia, and the DelMarVa during the afternoon and evening hours, it will be a hot, dry day. Back toward the west, however, rounds of thunderstorms will move through the states just north of the Ohio River.
The flood threat will ramp up this weekend as multiple disturbances ride the jet stream draped over the Great Lakes and Mid-Atlantic. On Saturday, numerous rounds of showers and storms will ride the stationary front from the western Great Lakes and Ohio River Valley into the Mid-Atlantic. Most activity will take place during the afternoon and overnight hours thanks to the daytime heating. Some of these storms may even turn severe, bringing small hail or damaging winds. While the precipitation activity relaxes somewhat overnight, there will still be areas that experience rainfall. In particular, an area of training rain and thunderstorms may develop early Sunday morning near the Mason-Dixon Line. This is a feature which we will be watching closely throughout the day Sunday.
Speaking of Sunday, scattered thunderstorms, some of which will contain tropical downpours, will continue to affect the western Great Lakes and the Ohio River Valley — mainly north of the river — in the afternoon and evening hours. Meanwhile in the Mid-Atlantic, training heavy rain and thunderstorms will be a significant concern. This rain will begin north of the Mason-Dixon Line, and as the day progresses it will slowly shift to the south along with the cold front. By the overnight hours it will primarily be affecting the area south of the Mason-Dixon Line and the DelMarVa.
By Monday, the northern Mid-Atlantic will dry out for a brief time thanks to the progress of the cold front, but showers and thunderstorms will remain dominant across the Ohio River Valley and the southern Mid-Atlantic region. Flash flooding will remain a concern in these areas. Then as a new disturbance swings through on Tuesday, the flood risk may increase once again in a larger spacial area of the Mid-Atlantic. It’s too early to discuss details at this point in time, but additional thunderstorms will be possible through the end of next week, although these storms will be scattered in nature given the unsettled and unorganized weather pattern. There are some hints that maybe the following weekend will be dry thanks to a cold front passing through, which will clean out the air mass — at least for a brief time.
Rainfall through this weekend will be heavy in some areas. A widespread 1 to 2 inches is forecast for many areas, but localized areas will receive at least 4 inches between now and Sunday night. Some areas will experience flooding — hopefully not Ellicott City, Maryland again. If you experience flooding, be sure to turn around and don’t enter flood waters even if it seems safe. Be sure to listen to local officials for any information if flooding occurs.
Wet Weekends Have Dominated Since April:
We discussed how this will be another weekend featuring rain in the Mid-Atlantic, but we have some statistics to illustrate just how wet it really has been. In Philadelphia, it has rained the past 5 weekends; this weekend will likely make it the 6th one in a row. This wet stretch began during the weekend of May 5-6, although the rain didn’t amount to much of anything. Instead both days featured a non-measurable, brief shower. The rainiest weekend was the following one — May 12-13 — when a total of 1.98 inches of rainfall was measured, including 1.49 on May 12, making it the wettest day of the month. The city averages 3.71 inches of rain in May, but ended up with 5.21 inches. About half of the month (12 days) experienced measurable rain (at least 0.01 inches), and 16 days were cloudy.
To the south in Washington, DC, it has also been a rather ugly and wet stretch. Their streak of wet weekends began one weekend earlier (April 28-29) than Philadelphia’s, when a trace of rain was measured on one of the days due to a light shower. Every weekend through now has followed suit with a least a trace of rainfall. Their wettest weekend was the most recent one (June 2-3), when 1.7 inches fell in total. They also ended the month of May with 12 days of measurable rainfall, the same as Philadelphia.
One last city we want to highlight: New York City. The most populous city in the United States has also had their fair share of ugly weather. Similar to Washington, DC, the wet weekend streak began during the weekend of April 28-29, with both days featuring measurable rainfall, adding up to a total of 0.18 inches. Their wettest weekend was the weekend of May 19-20, when 0.65 inches of rain added up. Their most recent weekend (June 2-3) also featured rain, keeping the streak alive. This weekend, the streak may come to an end with the rain activity likely staying to the south.