**A prolonged and significant rain event coming to the Mid-Atlantic region...a classic “omega-block” weather pattern**

A classic “omega-block” weather pattern will develop by next week featuring two upper-level lows and an upper-level ridge sandwiched in between. This kind of a scenario can lead to several days of wetter, cooler weather conditions and that is on the table for the Mid-Atlantic region. Map courtesy NOAA, Pivotal Weather

Overview

The change of calendar to the month of May is bringing a changing weather pattern that is likely to result in a prolonged and significant rain event for the Mid-Atlantic region...something not seen in these parts in quite awhile. While there can be scattered showers and thunderstorms between tonight and late Saturday, the main event is likely to begin on Saturday night with widespread showers and thunderstorms and occasional rain will be a threat on Sunday, Monday, Tuesday and perhaps Wednesday as well in the Mid-Atlantic region to go along with cooler conditions. The upper part of the atmosphere is evolving into a classic “omega-block” pattern which will allow for cooler and rainy weather to stick around for several days in the Mid-Atlantic region, and this should alleviate drought and fire-hazard conditions endured during the past several weeks.

A few inches of rain are certainly on the table for the Mid-Atlantic region between now and the middle of next week. If this were to indeed take place then the drought and fire-hazard conditions of recent weeks in the Mid-Atlantic region would most certainly be alleviated. Map courtesy ECMWF, Pivotal Weather

Springtime weather phenomena

“Back-door” cool front

Springtime in the Mid-Atlantic region and Northeast US can bring about some weather events that are actually less likely to take place during other times of the year. One such atmospheric phenomenon that has an increased chance of occurring during the months of March, April, and May in this part of the country is the “back door” cool front which can flip temperatures dramatically in a short period of time. One such dramatic “back-door” cool frontal passage took place in late March with temperatures plunging a record-breaking 26 degrees during a one hour period in Central Park (New York City) making for a quick change of season from summer-to-winter. “Back door” cool fronts are more common in the spring across the northeastern states when western Atlantic Ocean water temperatures remain at very chilly levels and a low-level wind flow that shifts to an east or northeast direction from offshore can quickly change summer conditions to winter. (“Back door” reference comes from the fact that these kinds of fronts “sneak-in” from the northeast as compared to the more conventional west-to-east movement of cold fronts in this part of the nation).

A graphical representation of a classic “omega-block” weather pattern is shown here and it is very similar to what is likely to unfold across the nation next week. Credit “rochesterfirst.com”

“Omega-block”

Another weather pattern that has an increased chance of taking place in the springtime is known as the “omega-block” since it is characterized by a high-pressure system aloft that is sandwiched in between two upper-level low pressure systems, resembling the Greek letter omega (Ω) on a weather map. This kind of upper-level pattern is notorious for slowing down the movement of surface systems meaning if one is near or under and upper-level low, there can be a prolonged period of cooler, wetter conditions and that is on the table for the Mid-Atlantic region. The “omega-block” tends to have an increased chance of forming in the springtime due to the timing of the jet stream's seasonal shifts and the increased solar heating of the continents. Specifically, the jet stream tends to weaken in the springtime and shifts to the north and this can lead to the formation of stationary ridges and troughs.  With respect to the increased solar heating in the spring, the continents tend to warm up creating large areas of low pressure and this can disrupt the normal flow of the jet stream and contribute to an “omega-blocking” pattern.

Soil conditions have been quite dry during the past several weeks across much of the northeastern part of the nation with the classification of “severe drought” in some areas. A few inches of rain would certainly help to improve the situation and this is on the table for the Mid-Atlantic region between tonight and the middle of next week. Map courtesy NOAA

Main rain event time from late Saturday into the middle of next week

While there will be scattered shower and thunderstorm activity from tonight into Saturday afternoon, the main rain event likely gets underway late in the day on Saturday or early Saturday night. At this time, a cold front will slow down as it approaches the east coast, and low pressure is likely to form along the frontal boundary zone. Showers and thunderstorms are likely to become more numerous and widespread by Saturday evening...some of the rain can be heavy and there can be strong-to-severe thunderstorms as well.

The combination of a slow-moving upper-level low across the southwestern states and the Ohio Valley/Mid-Atlantic in coming days will lead to cooler-than-normal conditions. This map of 2-meter temperature anomalies for the period of 9 May to 16 May reflect those cooler-than-normal conditions from the southeastern states to the Mid-Atlantic region. Map courtesy ECMWF, Weather Bell Analytics (Meteorologist Joe Bastardi)

Once the wetter pattern gets established, it looks as though it’ll stick around on Sunday, Monday, Tuesday, and perhaps even on Wednesday as well with occasional rain as the deep upper-level low spins over the Ohio Valley. This should also become a cooler period in the Mid-Atlantic region as high pressure over southeastern Canada influences the weather around here...high temperatures generally confined to the 60’s along the DC-to-Philly-NYC corridor. The possibility exists for a few inches of rain between tonight and the middle of next week and this certainly would help to alleviate drought and fire-hazard conditions that have been experienced during the past several weeks.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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