Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Bleak, Cloudy, and Relatively Dreary Pattern Through the Weekend *

2:00 p.m. Friday, May 17, 2024, Update:

After a brief respite in the unsettled weather pattern over the past 48 hours, a disorganized storm system tracking from the Tennessee River Valley to the southern mid-Atlantic coastline will bring showers back to the Lower Susquehanna Valley tonight. Prior to the showers' arrival, the week is ending on a cloudy note due to a continuation of easterly flow off the Atlantic Ocean. Despite the clouds, temperatures are only around 5°F below normal this afternoon, or generally in the mid-to-upper 60s, and winds are light. So, it won't be a bad afternoon for outdoor activities such as fishing, hiking, biking, running, or golfing. At the time of this writing, showers are currently impacting eastern OH and western PA but are only creeping eastward. Thus, most of the afternoon will be dry across south-central PA. In fact, showers should not even reach Franklin County until at least 5-6 PM, and most areas east of Route 11/I-83 will not receive any rain until after midnight. As has been the theme since March, yet another damp, dreary, and cool start to the weekend is then on tap tomorrow. Showers or even a period of steadier, light rain will persist through the early-to-mid afternoon hours before tapering off during the late-afternoon and early-evening hours. I don't expect any of the rain to be particularly heavy, but it could certainly spoil outdoor plans or force them to be moved inside for several hours. Due to the low clouds and wet weather, highs will only top out in the low-to-mid 60s for most, or around 10°F below average for mid-May. Rainfall amounts will generally increase from northeast-to-southwest across the region and range from a mere 0.10-0.20" or less near the PA/NJ border to as much as 1.0" along and/or west of the South Mountain Range (see below). Six-hour flash flood guidance values currently range from 1.5-4.0" across most of southern PA and the Delmarva Region, so flooding of streams and creeks won't be an issue. However, there may be areas of standing water in low-lying and poor-drainage areas, especially west of Route 11/15. Keep this in mind if you have travel plans on the roads. 

In stark contrast to my expectations on Tuesday, the second half of the weekend now appears drier, warmer, brighter, and quite pleasant. Fortunately, the storm system will move farther south and out to sea from Saturday night into Sunday. In its wake, an area of high pressure will begin to build into the Northeast and promote partial clearing during this time. Any lingering showers will end by 7-9 PM Saturday evening, and the rest of the night should turn out dry and comfortable with lows in the 50s. Speaking of overnight lows, they'll be in the 50s through at least Tuesday night on account of dewpoints hovering in the mid-to-upper 50s over the next 4-5 days. Anyhow, the weather should do a "complete 180" on Sunday with partial sunshine or continued clearing accompanying high temperatures in the low-to-mid 70s. After another lousy Saturday, you'll be able to venture outside without an umbrella or gain gear on Sunday and partake in the outdoor activity of your choice. I'll take it!

Looking ahead into next week, an upper-level ridge, or northward bulge in the Jet Stream, will return to the eastern United States. The subsidence, or sinking motion, beneath the ridge will promote lots of sunshine and a continued warming trend from Monday-Thursday (see below). The aforementioned high pressure system will ultimately settle over the Delmarva region by Monday and Tuesday and shift into the western Atlantic later in the week. With the high pressure in control of our weather, shower and thunderstorm chances will essentially be "nil" from Monday-Wednesday. Instead, mostly sunny skies and light winds will accompany highs in the mid-to-upper 70s on Monday, upper 70s to perhaps 80°F on Tuesday, and low-to-mid 80s Wednesday/Thursday. Given our recent stretch of cloudy, unsettled, and dreary weather, the upcoming stretch of dry and tranquil conditions will be most welcome and much needed! Soil moisture is currently well above normal along most of the Eastern Seaboard, so an extended period of dry weather is definitely a good thing.

Not until Thursday will the upper-level ridge begin to break down and a cold front bring the next round of showers and/or thunderstorms to the Lower Susquehanna Valley. Most of the energy associated with the front should lift into southeastern Canada and northern New England, so the probability of severe weather appears minimal at this time despite afternoon/evening timing of the cold frontal passage. Behind the front, winds will likely shift to the northwest for the start of Memorial Day Weekend and usher slightly cooler but very pleasant air (70s) into the region. The forecast thereafter is rife with uncertainty, so check back on Tuesday for an update. As always, specific details are nearly impossible to finesse more than 4-5 days in advance. Thus, I would be foolish to give an early projection for the holiday weekend or Memorial Day itself. It would be sure to bust, and I never enjoy the smell of a poor forecast.. -- Elliott

12:30 p.m. Tuesday, May 14, 2024:

As expected, Mother's Day Weekend turned out to be mostly cloudy, cool, and rather unsettled across the mid-Atlantic States. A slow-moving, upper-level low pressure system sagged southeastward across the region over the weekend and was responsible for frequent bouts of showers and embedded thunderstorms. The weekend certainly wasn't a washout, but the weather could have definitely been better for outdoor activities. South-to-southwesterly flow finally returned on Monday and ushered a warmer and drier air mass into the Lower Susquehanna Valley. Morning clouds gave way to afternoon sunshine, and high temperatures topped out in the comfortable mid-to-upper 70s. Unfortunately, the gorgeous weather was short-lived and will continue to be a fleeting phenomenon over the next week. A slow-moving storm system will track from the mid-Mississippi Valley to the southern mid-Atlantic coastline spanning this afternoon through Wednesday evening. Ahead of the system, skies turned mostly cloudy last night, and the sun won't make an appearance again until Thursday. Scattered showers will impact northern MD and southeastern PA throughout the day, but steadier rain and drizzle won't arrive until after sunset. Due to the clouds and showers, high temperatures this afternoon will be a few to as much as 10°F below average and range from the low-to-mid 60s across central PA to the low 70s around Philadelphia (see below).

As mentioned above, periods of steadier and generally light-to-moderate rain will develop tonight and continue through tomorrow afternoon. Even during the steadiest rain, roadway visibility should remain on the order of 2-3 miles or more. However, most areas will receive a general one half to one inch of rain through Wednesday evening with locally higher amounts up to 1.50 inches (see below). Rainfall totals will only be this high on account of the event's longer (18-24 hour) duration. Six-hour Flash Flood Guidance (FFG) values currently range from 1.50-4.0 inches across the region, so stream and river flooding won't be a concern. However, expect areas of slow travel on the roads through the Wednesday evening commute, and be alert for standing water in low-lying, poor-drainage, and urban areas. Keep an umbrella handy at all times over the next 36 hours.. you'll make good use of it. To nobody's surprise, high temperatures will only be in the 60s again tomorrow with the absence of any sunshine. So, you might want to put a jacket or sweatshirt on before heading out the door to work or school, as well.

The storm system will move farther out to sea Wednesday night into Thursday, causing any lingering showers to exit southeastern PA by or shortly after midnight. In the wake of the storm system, partial sunshine will return on Thursday and accompany warmer and more pleasant conditions with highs around average (70s) for mid-May. A 10-20 mph north-to-northeasterly breeze will make the outdoor air feel a bit cooler, but Thursday's weather will still be a vast improvement from that of the next 36 hours. A weak bubble of high pressure should settle over southern New England and the Commonwealth from Thursday night into Friday and promote dry conditions during this time. However, skies will probably turn mostly cloudy again on Friday in advance of the next storm system following a partly cloudy or even mainly clear Thursday night. Dewpoints will generally be in the 50s to around 60°F at times through the weekend, so overnight lows should consistently be slightly above normal and mainly in the 50s. Despite the cloud cover, it will still be a rather pleasant end to the week with light winds and highs in the 70s. Both Thursday and Friday should provide opportunities for outdoor activities such as fishing, hiking, biking, golfing, or running. Unfortunately, the same cannot be said for Saturday and Sunday. As has been the theme since March, yet another storm system will set its sights on the mid-Atlantic States just in time for the weekend. The track and intensity of the weekend system are uncertain at this distance, but they could be quite similar to the current one. If so, then showers would likely return later Friday night or Saturday with periods of steadier rain and drizzle slated for Saturday night and Sunday. This would equate to another damp, dreary, bleak, and chilly weekend with highs most likely in the 60s.. at best.. both days. Another 0.75-1.50 inches of rain is a real possibility barring a significant shift in the storm's track. Regardless, you may want to consider rescheduling outdoor activities or moving them inside this weekend. Since the start of spring, we've been stuck in a rut with the worst weather taking place on weekends. Let's hope Memorial Day Weekend.. the unofficial start of summer.. brings an end to that streak. Check back on Friday for an update! 

Part of the reason the storm track is so far south by mid-May standards is due to a strongly negative North Atlantic Oscillation (NAO). A large ridge, or northward bulge in the Jet Stream, has developed over east-central Canada.. to the south and west of Greenland.. and will only amplify through the upcoming weekend. In this type of large-scale pattern, storm systems are forced to track "underneath" the blocking high pressure system over Canada and instead through the mid-latitudes. Unfortunately, a blocking pattern of this magnitude often results in storm systems passing south of the Mason-Dixon Line or Interstate 70. To their north, easterly flow off the Atlantic draws moisture westward up against the eastern slopes of the Appalachians, causing low clouds and unseasonably cool/wet weather to plague most of MD, PA, and NJ. The upcoming weekend appears to be no exception, so we'll have to wait until next week for a multi-day stretch of drier and brighter conditions. Sure, this weather pattern is downright depressing, but it comes with the territory of "spring" in the mid-Atlantic States. Summer can't come soon enough.. -- Elliott