Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Severe Weather Threat Monday.. Then A Run at 90 Next Weekend? *

6:30 p.m. Friday, May 13, 2022:

This week's weather could be summarized in one word: perfection. For the first time in what feels like forever, we experienced a four-day stretch of mostly sunny, dry and pleasant weather with low humidity and highs in the 70s. A sprawling high pressure system over northern New England and the Canadian Maritimes remained in close enough proximity to the Commonwealth to keep clouds and precipitation at bay from Monday through Wednesday. Over the last 24-36 hours, the high pressure shifted farther south and east and is now centered over the open waters of the western Atlantic Ocean. As a result, our dry, northeasterly flow has been placed by a moist, east-to-southeasterly flow off the Atlantic. The large-scale clockwise flow around a high pressure system produces east-to-northeasterly winds on its southern periphery and south-to-southeasterly winds to its west. This southeasterly flow caused cloud cover to move back into the Lower Susquehanna Valley Thursday night and dewpoints to soar into the 60s. In addition, what was left of last weekend's Nor'easter was forced to "retrograde" back into the Carolinas Thursday night into today. Because the system is still "cut off" from the main Jet Stream over Canada, it will take until Saturday evening to finally exit to our north and east. In the meantime, smaller disturbances, or "impulses," embedded within the large-scale flow around the remnant low will rotate northward through the mid-Atlantic. The scattered showers and embedded thunderstorms that have and are continuing to rumble across parts of the state this afternoon and evening were triggered by one of these disturbance.

Unfortunately, we can expect a "repeat performance" on Saturday as another disturbance moves northward through eastern PA. After a cloudy, warm and muggy night with lows in the 60s, some breaks of sunshine will appear later Saturday morning. It won't take much sunshine to destabilize an atmosphere laden with this much moisture, so expect numerous showers and drenching thunderstorms to erupt around or shortly after midday. Not every location will get hit by the storms, but those that do may receive up to one inch of rain in just 30 minutes. Highs in the mid-70s to around 80 will be accompanied by dewpoints in the mid-to-upper 60s on Saturday, so the outdoor air will feel even warmer and more tropical than it does today. Showers and storms will diminish in coverage and taper off Saturday evening, but the muggy air mass will stick around on Sunday and Monday. Winds will become southwesterly on Sunday, scouring out the low clouds and directing even warmer air into the region. Another weak disturbance moving in the more traditional west-to-east direction across the state will be the focal point for another round of scattered showers and thunderstorms Sunday afternoon and evening. With highs in the low 80s and dewpoints still well into the 60s, lightning and locally heavy downpours will be the main threats from the storms. Coverage will likely be less than on Saturday and focused across the higher terrain north and west of I-81, so I certainly wouldn't change or cancel any outdoors plans. Just make sure to have an umbrella handy and move indoors if lightning is seen or thunder heard.

As I mentioned earlier this week, Monday stands out as the day to watch for severe weather. A strong cold front will move through the Commonwealth on Monday, and the air mass ahead of the front will be sufficiently warm and humid for severe thunderstorm development. In addition, a vigorous, upper-level disturbance supporting the front will only enhance the lift/rising motion along and ahead of the front (see below). Dewpoints will still be well into the 60s with temperatures in the upper 70s to around 80, so plenty of moisture and instability will be available for the storms to tap into. The one ingredient that may be lacking is "wind shear," or a term that meteorologists use to describe the change in wind speed and direction with height. In general, the faster the wind speed increases and direction changes with height above the ground, the better the chance for damaging winds and tornadoes. Fortunately, the wind direction should be southwesterly throughout the depth of the lower atmosphere on Monday, and wind speeds even several thousand feet above the ground will be 65 mph or less. Still, those stronger winds aloft can be dragged down to the surface in severe thunderstorm downdrafts, resulting in straight-line wind damage to trees, power lines, homes and property. At this point, these locally damaging wind gusts are what I'm most worried about on Monday, along with frequent lightning and locally heavy downpours. I certainly can't rule out isolated incidents of hail but don't expect tornadoes to be a concern in this type of setup. The most likely timing for the storms to move through the Lower Susquehanna Valley, eastern PA and northern MD is between 12 p.m. and 7 p.m. Monday, but that can speed up or slow down by a few hours over the weekend. I'll finesse the timing of the storms via Twitter on Sunday, so be sure to follow @MUweather for frequent updates.

In the wake of the front, mostly sunny skies, less humid and more comfortable conditions will return for Tuesday and Wednesday. Dewpoints will fall back into the 40s with high temperatures in the low-to-mid 70s Tuesday and upper 60s to low 70s Wednesday. Despite a northwesterly breeze of 10-20 mph on Tuesday, I'd imagine that shorts and t-shirts will still be the attire of choice for most people. Although uncertainty in the forecast increases late next week, it's a good possibility that a warm front brings the next chance for showers and thunderstorms on Thursday or Friday. The clouds and showers ahead of the front may keep temperatures near- or slightly-below normal either Thursday or Friday, but big changes lie ahead for next weekend after the warm front lifts into New England. In fact, I'm beginning to think that our first 90-degree day of the season may come sooner than expected.. "sooner" as in next weekend or the following Monday. A large ridge of high pressure moved into the Bering Sea on Mother's Day Weekend and won't break down until late next week. Last December, I discussed how a high pressure system in the Bering Sea can be used to accurately predict the development of a Jet Stream ridge across the eastern U.S. some 10-14 days later. Using this rule, I certainly see plenty of evidence to support a multi-day stretch of heat and humidity beginning sometime next weekend and continuing into the start of the following week (May 21-24). If my thinking is correct, high temperatures could soar well into the 80s or even lower 90s during this stretch. Factor in 60- or 70-degree dewpoints, and I might be talking about the year's first heat-related concerns just one week from today. Then again, this is just conjecture and speculation at this distance, and other factors could cause the forecast to change. So, you'll have to wait until Tuesday to find out whether I'm still "bullish" on the potential for 90-degree heat next weekend. But, with the unofficial start of summer just a few weeks away, I'm sure many of you are happy to hear me talking about heat rather than a repeat of the 50-degree rain last Memorial Day weekend! 😊 -- Elliott