Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Tranquil and Pleasant Pattern Through Mid-September *

Brief 2:00 p.m. Friday, September 6, 2024, Update:

This week's weather has been absolutely spectacular and quite reminiscent of early fall! Cool nights and warm afternoons have been accompanied by light winds, low humidity and lots of sunshine. Low clouds and areas of fog developed east of the Appalachians late last night but have since burned off and given way to partial sunshine this afternoon (see below). High temperatures will be similar to Thursday, or generally in the mid-to-upper 70s, with only a slight uptick in humidity. As a result, the weather will remain fantastic for outdoor activities, so I hope you can get outside and soak up some sunshine after work or school!

From tonight through Sunday, an upper-level low pressure system will close off from the main branch of the Jet Stream and meander from the western Great Lakes region into southeastern Canada. By Monday, it will depart into the Canadian Maritimes and be replaced by another sprawling high pressure system. A cold front trailing southward from the upper low will move through the mid-Atlantic States on Saturday. South-to-southeasterly flow ahead of the front will draw somewhat more humid air back into the region. Dewpoints will peak in the mid 60s tonight and on Saturday morning but then tumble back through the 50s Saturday afternoon and evening behind the front. After a mainly clear start this evening, clouds will increase through the rest of the night, and a band of showers will impact northern MD, southeastern PA and the Lower Susquehanna Valley on Saturday. In the last 24-48 hours, the timing of tomorrow's cold front has sped up a bit. Thus, showers are now expected to traverse the region from northwest-to-southeast between 8 a.m. and 5 p.m., ultimately clearing the I-95 corridor by dinnertime. Due to a relative lack of moisture ahead of the front, unfavorable timing and virtually no instability, embedded rumbles of thunder are no longer in the cards. Additionally, wind gusts within the cold frontal band of showers should not exceed 35 mph. The strongest, upper-level dynamics will be well to our north and extend from southern NY to northern New England. Thus, tomorrow's showers will be light and intermittent and only last 1-3 hours in a given location. As a whole, this will be a "nuisance event" with rainfall amounts of 0.10" or less in most spots (see below). Slightly higher (0.10-0.25") amounts may occur north/west of Interstates 78/81, but even that is a pittance compared to August's onslaught of drenching storms. I certainly wouldn't cancel outdoor plans tomorrow, but keep an umbrella handy and be prepared to move them inside for a bit. Due to the clouds and showers, tonight's lows and tomorrow's highs are forecast to be in the low-to-mid 60s and low-to-mid 70s, respectively.

Behind the front, skies will rapidly clear Saturday evening, and northwesterly winds will usher cooler and less humid conditions into the mid-Atlantic States. Sunshine will then "rule the roost" on Sunday with dewpoints only in the 40s. Despite relatively cool conditions by early-September standards, t-shirts and jeans will still be the attire of choice for most Sunday afternoon with highs in the low 70s (see below). You may need to grab a jacket or sweater before heading out the door to Sunday morning church services but should be able to shed them by or shortly before lunchtime. Temperatures will bottom out in the upper 40s (outlying areas) to mid 50s (urban centers) early Sunday morning, but lows of this magnitude are still only around 5°F below average.

The forecast for next week looks very much on track, so refer to Wednesday's discussion below for more details. I only see one minor change, and that is with respect to the warming trend. High temperatures could now be back in the mid-to-upper 70s by Monday, creep into the low 80s on Tuesday, and reach the mid 80s Wednesday. By next Friday and/or Saturday, highs could be around 10°F or more above normal and in the upper 80s to perhaps 90°F. Average highs in mid-September drop into the upper 70s, but we'll experience the opposite effect next week with a "summer comeback" in the offing. Bring it on! -- Elliott  

11:45 a.m. Wednesday, September 4, 2024:

Meteorological fall officially began on September 1st, and it only took an extra day to feel like it. In the wake of a secondary cold frontal passage, northwesterly winds ushered much cooler and less humid air into the Commonwealth early this week. Under mostly sunny skies, high temperatures reached the comfortable upper 70s on Monday and mid 70s yesterday. Even better, dewpoints tumbled into the 40s by the end of the holiday, and they won't climb back into the 60s until Friday. A sprawling high pressure system over the Northeast and mid-Atlantic States will continue to promote dry and pleasant conditions through week's end, and the tranquil weather pattern should be a common theme through at least mid-September. Due to large-scale subsidence, or sinking motion, beneath the high, there will also be plenty of sunshine over the next two days and again next week. High temperatures will only be a few degrees below average this afternoon and tomorrow, or generally in the upper 70s to around 80°F (see below). For the third night in a row, lows will bottom out in the crisp mid/upper 40s (outlying areas) to low 50s (urban centers) tonight. Despite an uptick in cloud cover Thursday night and partial cloudiness on Friday, it will still be a beautiful end to the week with lows in the mid-to-upper 50s and highs around 80°F, respectively. Overall, the weather looks fantastic for outdoor activities and sleeping with the windows open through Friday. Lather on the sunscreen, and enjoy the extended taste of fall! 

From Friday night through Sunday night, an upper-level low pressure system will close off from the main branch of the Jet Stream and meander from the western Great Lakes region into southeastern Canada. By early next week, it will depart into the Canadian Maritimes and be replaced by another sprawling high pressure system. Thus, the only meaningful chance for rain across northern MD, southeastern PA and the Lower Susquehanna Valley in the next 10 days will come on Saturday. A cold front trailing southward from the upper low will traverse the mid-Atlantic States from northwest-to-southeast spanning Saturday through Saturday night (see below). Winds will turn south-to-southeasterly later Friday into Friday night and draw more humid air back into the region. Dewpoints should reach the upper 50s or low 60s Friday afternoon and peak in the mid 60s from Friday night into Saturday prior to the frontal passage. Skies will turn mostly cloudy Friday night, and a band of showers will then likely dampen the region on Saturday. Timing of the showers is currently uncertain, but odds favor the afternoon and evening hours from the Lower Susquehanna Valley points south and east. Due to strong, upper-level dynamics associated with the system, some of the showers could produce locally heavy downpours and wind gusts up to 45 mph. A few rumbles of thunder cannot be ruled out, as well, but the low-levels of the atmosphere should be quite stable due to extensive cloud cover ahead of the front. I certainly wouldn't cancel outdoor plans on Saturday, but keep an umbrella handy and be prepared to move them inside for a few hours. Most areas will receive 0.25-0.50" of rain or less from Saturday's showers, so there won't be any flooding concerns. Lows will be back in the 60s Friday night with highs once again in the 70s to perhaps 80°F along the I-95 corridor Saturday. 

Behind the front, skies will clear Saturday night, and winds will shift back to the northwest. This light, northwesterly flow from Canada should then persist through most of next week and usher yet another refreshing air mass into the mid-Atlantic States. Dewpoints should tumble into the 40s by Sunday morning and remain in the 40s and 50s through most if not all of next week. As mentioned in the paragraph above, a sprawling high pressure system will build into the Central States on Sunday and then expand across the eastern one-half to two-thirds of the nation next week. The large-scale subsidence beneath the high and dry, northwesterly flow from Canada will keep precipitation at bay, promote lots of sunshine and lead to large diurnal variations.. i.e. the difference between the daily high and low temperatures. A gradual warming trend will take place from Sunday through the middle of the week, but clear and generally calm conditions at night should promote more chilly mornings with lows in the mid 40s to low 50s. However, the sun angle in early-to-mid September is still equivalent to that in late March and early April, so temperatures will rebound quickly and peak in the low 70s Sunday afternoon, mid 70s on Monday, upper 70s to perhaps 80°F on Tuesday and the low 80s by Wednesday. Quite frankly, next week's weather looks very similar to this week's and will probably be considered "perfect" by many of you. I prefer it a bit hotter but certainly won't complain about our extended "fall preview" and absence of unsettled weather prior to the official start of the season on September 22nd. Barring heavy rain and wind from a tropical cyclone or its remnants, the weather in September and October tends to be drier, quieter and more pleasant than the summer months across PA and MD. You can usually engage in outdoor activities without fear of heat-related illnesses but also dress in shorts and t-shirts most afternoons. This year looks to be no exception, and that can only be considered a "win!" -- Elliott

Friday, May 31, 2024:

2024 Atlantic Hurricane season outlook:

Meteorological spring is wrapping up on a sunny and pleasant note today, but I expect plenty of heat and tropical activity this summer. Not only does June 1st mark the beginning of meteorological summer, but it's also the start of the 2024 Atlantic Hurricane Season. El Niño, or the phase of ENSO defined by above-average sea surface temperatures (SSTs) in the equatorial Pacific, has been present since June 2023. However, its death is imminent, and La Niña conditions (below-average SSTs in the equatorial Pacific) should return by July or August. Due to this rapid transition from El Niño to La Niña, wind shear.. or the change in wind direction and speed with height above the ground.. should be lower than normal in both the Atlantic and Pacific Basins this season. During El Niño, the southern branch of the Jet Stream typically becomes dominant over the northern branch. As a result, easterly trade winds in the tropics weaken and can even become westerly in the mid- and upper-levels of the atmosphere. During La Niña, the opposite is true, with an enhancement in easterly trade winds and weakening of the Jet Stream's southern branch. Unlike severe thunderstorms, wind shear is detrimental to tropical storm and hurricane formation. So, less shear and stronger easterly trade winds support more tropical activity and an above-average number of named tropical cyclones.

The atmospheric response to El Niño and La Niña is often delayed by several months, but that may not be the case this go around. Over the last year, the atmospheric response to El Niño was relatively weak, perhaps on account of 3-straight years of La Niña conditions prior to summer 2023. Thus, the atmosphere could exhibit an almost-immediate response to the developing La Niña. Things may get off to a relatively slow start through the July 4th holiday due to the lingering effects of El Niño (more wind shear) and dry air from the Sahara Desert over the main development region (MDL) of the tropical Atlantic. However, I expect that to change in a big way during the second half of summer, and the Atlantic Basin will likely be littered with tropical cyclones during the season's peak (September-October). La Niña is expected to strengthen throughout the fall, so I anticipate at least a few more named storms in November as well. In addition, SSTs as of May 31st were well above normal across the Gulf of Mexico, Caribbean Sea, and MDL (see below). Hurricanes behave like giant heat engines that use warm, moist air as fuel. Warmer-than-normal SSTs provide the additional fuel necessary for frequent storms and rapid intensification.

SSTs May 31 2024

Due to the factors outlined above, I expect the 2024 Atlantic Hurricane Season to be incredibly active and perhaps challenge the record for most named storms in a year (28 in 2005). Of no coincidence is that 2005 is one of my "analog years" for this summer. Anyhow, I am predicting:

  • 21-27 NAMED STORMS
  • 10-14 HURRICANES
  • 4-7 MAJOR HURRICANES (Category 3 or higher)

Unfortunately, I expect at least several of those storms to make a direct landfall in the United States with a higher-than-normal chance of catastrophic impacts. Residents along the Gulf Coast from Texas to Florida appear most at risk this season and should prepare now for the worst-case scenario of a direct impact. For information on hurricane preparedness, visit the National Hurricane Center's website, your local National Weather Service Forecast Office's website, or other emergency management resources. I don't have a good feeling about this one.. 

2024 summer outlook:

As for my 2024 summer outlook, searing heat could become quite common across the Northeast and mid-Atlantic States, especially in July and August. Historical analogs.. or years with similar, large-scale patterns or pattern transitions.. support the idea of a "backloaded summer". In the past 25 years, 1998, 2005, 2007, 2010, and 2016 featured a transition from El Niño to La Niña conditions between the spring and autumn months. Thus, those 5 summers are my "analog years" for summer 2024. Much like the Atlantic Hurricane Season, things may get off to a slow start (in terms of heat) in June. There is often a correlation between drought/soil moisture and extreme heat in the summer months. As of May 28th, the only parts of the nation experiencing significant drought were the Florida Peninsula, northern Rockies, southern Plains, and New Mexico. However, soil moisture was most below normal across the Midwest, Central States, Ohio Valley, and South Texas as of May 30th (see below). Thus, I expect the core of the heat in June to be across the southern Plains and northern Rockies with "near-normal" temperatures across the center of the nation, Great Lakes region, Northeast, and mid-Atlantic States.

soil moisture and drought

However, I expect things to change around or shortly after July 4th. As La Niña develops and the southern branch of the Jet Stream weakens, storms systems and cold frontal passages should become more infrequent. After an active start to the summer in terms of showers, thunderstorms, and severe weather, I expect drier-than-normal conditions to develop in July and persist through August and perhaps September. Thus, the core of the heat should progressively shift farther east each month from June-August. Both branches of the Jet Stream might be very weak from mid-July through at least mid-September, allowing a semi-permanent "heat dome" to develop across the Central and Eastern States. If so, then air conditioners could be working overtime and cooling demands "through the roof" starting sometime in early-to-mid July. High humidity may very well accompany some of this summer's heatwaves, so follow heat preparedness tips and advice given in my Special Weather Discussions (SWDs) and on Twitter (@MUweather) throughout the summer. In terms of 90-degree days, I expect an above-average number (30-40) of them with the possibility of a 100-degree day or two during the second half of July or August. Relative to average, the "coolest and wettest" month of the summer should be June with August being the "hottest and driest." In aggregate, I expect temperatures from June through August to average 1-3°F above normal, but with a near-normal (-1 to +1) June and sweltering (+2 to +4) August. As a whole, rainfall might average "near- to slightly-below normal" for the summer on account of a relatively wet June and bone-dry August.

summer 2024 analog blend

As an FYI, I'll keep these outlooks available until November 1st below my bi-weekly SWDs. Let's see how I do! -- Elliott