Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Oppressively Humid with Showers, T-Storms This Weekend *

1:00 p.m. Friday, June 28, 2024, Update:

Wednesday's cold front was responsible for more than 250 reports of wind damage across southern New England and the mid-Atlantic States. Locally, the southern-tier counties of PA were spared, but areas north of the Turnpike got walloped by the pre-frontal line of storms (see below). As is often the case in a convective environment, rainfall amounts varied widely from county-to-county. Up to 3" of rain fell in Cumberland and Perry Counties, while Millersville University picked up less than 0.15" (see below). Behind the front, northwesterly winds ushered cooler and less humid air into the Commonwealth on Thursday, but high temperatures still reached the mid-to-upper 80s.

An area of high pressure settled over the Northeast Thursday night and has since shifted to a position near the mid-Atlantic coastline. Under clear skies, temperatures bottomed out in the refreshing mid 50s (outlying areas) to low 60s (urban centers) early this morning. This afternoon will be equally as nice with highs in the low-to-mid 80s and dewpoints still only in the 50s (see below). Add in mostly sunny skies, and you really couldn't ask for a nicer end to the last week of June. Get outside and take advantage of the gorgeous weather! 

Unfortunately, the sunshine and low humidity won't stick around over the weekend. The high pressure system will shift into the western Atlantic tonight into Saturday. On its backside, southerly flow will draw warmer and "juicier" air back into the mid-Atlantic States this weekend. Dewpoints will rise into the 60s tonight and wind up around 70°F Saturday afternoon. They'll then peak in the low-to-mid 70s Saturday night and Sunday morning before falling back into the 60s later Sunday afternoon. Due to the significant uptick in moisture, skies will turn mostly cloudy later tonight and remain that way through Saturday night. Despite the cloud cover, there should still be enough peeks of sun for high temperatures to reach the mid-to-upper 80s Saturday afternoon. Factor in the high humidity, and heat indices will still likely reach the low-to-mid 90s for a few hours after midday. Temperatures won't fall much Saturday night and may remain in the mid-to-upper 70s in many areas. Thus, highs in the low 90s seem like a "good bet" Sunday afternoon, especially with more sunshine in the forecast. Heat indices will top out between 95-100°F Sunday afternoon, so drink plenty of non-alcoholic beverages to stay properly hydrated. Limit time in the sun; take frequent breaks from the heat in an air-conditioned room or building; and wear loose-fitting, light-colored clothing to mitigate the risk of heat-related illnesses. Never leave pets or children unattended in vehicles during the summer months! It only takes about 20 minutes for the internal temperature of a car to reach 120°F on a 90-degree day. 

As far as precipitation over the weekend, Saturday night should be the most active period across northern MD and the Lower Susquehanna Valley. A slow-moving cold front will sag southeastward from the Great Lakes region through the mid-Atlantic States from Saturday into Sunday. It will ultimately sweep off the Eastern Seaboard Sunday night. Ahead of the front, an upper-level disturbance moving through northern/western PA and New York will trigger the first round of showers and thunderstorms on Saturday. The storms will turn strong-to-severe across northern and western parts of the Commonwealth Saturday afternoon, but they probably won't reach the Lower Susquehanna Valley until Saturday evening. In addition, most of the disturbance's energy will stay north/west of the Valley, so the severe weather threat should remain low in areas south/east of I-81. The greatest threat for damaging wind gusts, flooding downpours, and a couple of tornadoes will be west of I-99 and along/north of I-80. However, I won't rule out an isolated wind damage report or two in parts of Perry, Franklin, or northern Dauphin Counties Saturday evening. With the loss of daytime heating and instability, a transition to a primarily heavy rain threat will occur Saturday night. Most if not all of the day will be dry to the south/east of I-81, but widespread showers and storms are expected overnight. A general 0.50-1.0" of rain is likely over the weekend with most of that coming Saturday night, but the nature of summertime convection could mean a large variation in amounts from county-to-county. Any flood threat will be of the low-lying, poor-drainage, and urban variety. Area streams and rivers are far too low for isolated 2-3" rainfall amounts to cause any issues.

The cold front will traverse northern MD, southeastern PA, and the Lower Susquehanna Valley from northwest-to-southeast on Sunday and trigger another round of showers and storms. However, the front will probably already be over or east of the Valley by the early-afternoon hours, so storms may not turn severe until reaching the I-95 corridor. I expect storms to be more isolated on Sunday, as well, with a good part of the day dry. I certainly wouldn't cancel any outdoor plans this weekend, but you may have to move them inside for a few hours Saturday evening and/or Sunday afternoon. Behind the front, another Canadian high pressure system will settle into the Great Lakes region on Monday and shift into the mid-Atlantic States by Tuesday. Winds will turn north-to-northwesterly on Sunday night and usher a much cooler and drier air mass back into the region early next week. Dewpoints will plummet into the low-to-mid 50s by Monday morning and remain below 60°F through Tuesday night. A moisture-starved, upper-level disturbance trailing behind the cold front will move through the Commonwealth on Monday and cause skies to turn partly-to-mostly cloudy by the late-morning hours. However, there will essentially be no chance for showers on Monday, and most of you will welcome the refreshing, 10-20 mph breeze and highs perhaps only in the upper 70s. Skies will rapidly clear late in the day, and Monday night should actually turn out quite cool with lows in the 50s. I highly recommend sleeping with the windows wide open and letting the fresh air circulate inside your home. Chances like these are limited in the summer months, so take full advantage of the opportunity! Tuesday will be even nicer than Monday with the high directly overhead. Light winds and mostly sunny skies will accompany low humidity and pleasantly warm conditions with highs in the low-to-mid 80s. After a remarkably warm June, July will definitely start on a cooler and more pleasant note.. but it won't last..

The high pressure system will shift into the western Atlantic during the middle of the week, and winds will turn south-to-southwesterly again by Wednesday. The southerly flow will direct hotter and more humid air back into the mid-Atlantic States just in time for the 4th of July holiday. After a comfortable Tuesday night with lows in the low-to-mid 60s, high temperatures should approach or exceed the 90-degree mark Wednesday afternoon under mostly sunny skies. Dewpoints will still only be in the low 60s on Wednesday but surge into the upper 60s or low 70s by the holiday itself. The hot and humid conditions will likely persist through most if not all of the holiday weekend. In fact, high temperatures could reach 90°F each afternoon from Thursday through Sunday. A large, Jet Stream ridge, or "heat dome," will develop over the Southern States late next week and hold its ground through the holiday weekend. We'll be along the northern periphery of the ridge during this time.. a zone notorious for "ridge riders," or disturbances that trigger scattered showers and thunderstorms. Thus, the potential exists for storms to spoil some 4th of July fireworks displays throughout the region, along with outdoor plans from Friday-Sunday. Timing each of the disturbances is impossible at this distance, so check back on Tuesday to find out which days might be the stormiest over the holiday weekend. Welcome to the "summer doldrums.." -- Elliott

11:30 a.m. Tuesday, June 25, 2024:

Phew! Last week's heatwave certainly lived up to expectations. For the first time in 58 years (1966), the mercury reached or exceeded the 90-degree mark on 6 consecutive days at Millersville in the month of June. While quite common in July and August, heatwaves of this magnitude and duration are quite rare in June. The heatwave peaked from Friday-Sunday with highs in the mid-to-upper 90s each afternoon, and Saturday's high temperature of 99°F was the highest since July 3, 2018. It was also a new record high for June 22, breaking the previous record of 98°F from 1921, and just one degree shy of the century mark.. a number not reached at Millersville since July 2012. Heat indices peaked between 100-105°F across all of northern MD and southeastern PA Saturday afternoon and were only a few degrees lower on Sunday. Saturday's mean daily temperature of 87°F tied the highest on record in June, and Sunday's low temperature of 77°F matched the highest minimum on record in June. A sprawling Jet Stream ridge, or "heat dome," over the eastern U.S. was responsible for the persistent heat and humidity. The ridge was finally suppressed by a potent cold frontal passage Sunday night into Monday morning. Behind the front, northwesterly winds of 15-30 mph ushered a cooler and much drier air mass into the Lower Susquehanna Valley by Monday afternoon. As a result, high temperatures failed to hit 90°F for a 7th-straight day, but I doubt anybody was complaining about that. Instead, highs were around average, or in the mid 80s, for most, and dewpoints plummeted from around 70°F at 3 AM Monday morning to the mid 50s by 10 AM (see below). Wind gusts actually reached 40 mph in spots Monday afternoon.. quite rare in the absence of thunderstorms during the summer months. However, the stiff breeze subsided quickly Monday evening, and last night featured perfect conditions for sleeping with the windows open. An area of high pressure settled into the central Appalachians and promoted clear skies, light winds and lows in the upper 50s to low 60s. 

Following this morning's refreshing start, it will turn hot again this afternoon with highs in the upper 80s to low 90s under mostly sunny skies. Contrary to yesterday, winds will turn southwesterly to the north of the high and usher the hotter air mass back into the region. Fortunately, humidity will still be low with dewpoints in the 50s, so heat indices should match or even be slightly below the actual air temperature. Don't get used to it, though. Dewpoints will climb into the 60s tonight and reach the upper 60s to low 70s Wednesday afternoon. To make matters worse, sweltering heat will also return on Wednesday with highs in the low-to-mid 90s and heat indices between 95-100°F ahead of a vigorous cold front. Temperatures will only bottom out in the low-to-mid 70s tonight and mid-to-upper 60s Wednesday night, so cooling demands will be "through the roof" again over the next 36-48 hours. In order to mitigate the risk of developing heat-related illnesses on Wednesday, avoid alcohol and drink plenty of water and other beverages high in electrolytes. Wear loose-fitting, light-colored clothing; take frequent breaks from the heat in an air-conditioned room or building; and limit exposure to direct sunlight. As far as diet, avoid salty/dehydrating foods. Instead, eat plenty of fruits and vegetables high in electrolytes, minerals and nutrients. Applying a cool compress around the neck can also help the body from overheating. Never leave pets or children unattended in a vehicle! On a 90-degree day, the internal temperature of a car can reach 120°F in just 20 minutes and 135°F in an hour. 

The extreme heat and humidity won't be the only weather-related concern tomorrow. In fact, the primary hazard will come in the form of scattered strong-to-severe thunderstorms from the afternoon into the overnight hours. The aforementioned cold front and a vigorous, upper-level disturbance along its tail end will be the triggering mechanisms for the violent storms. Morning sunshine will cause the atmosphere to destabilize, and there will be plenty of moisture available with dewpoints around 70°F. Convective Available Potential Energy, or "CAPE," is a severe weather parameter used to approximate updraft strength within thunderstorms. It is essentially a measure of atmospheric instability and depicts the "fuel" available for developing storms. CAPE values across southeastern PA, northern MD, and the Lower Susquehanna Valley may reach 3,000 J/kg tomorrow afternoon, indicating a highly-unstable atmosphere and heightened risk of damaging wind gusts up to 70 mph (see below). There may even be enough "shear".. or a change in wind speed and/or direction with height above the ground.. present to support a couple of tornadoes across southern/eastern PA and the Delmarva Region. As a result, all modes of severe weather.. including hail and localized flash flooding.. will be possible between 2 PM Wednesday and 2 AM Thursday.

A few discrete supercells may develop during the mid-to-late afternoon hours well ahead of the front, but much uncertainty surrounds this possibility. Those will be followed by more organized clusters or a line of storms along the front during the evening or overnight hours. Have multiple ways to receive severe weather alerts from the National Weather Service tomorrow. If a warning is issued for your location, seek shelter immediately in an interior room or basement. Tree, powerline and roof damage are all possible, along with power outages and flooded roadways. The storms may impact the Wednesday evening commute, so motorists should expect delays and stay alert for blowing objects and debris on roadways. Use low-beam headlights and/or hazard lights in blinding downpours and drive within the speed limit. If you can do so safely, exiting the highway is also a good option. Most areas will receive one half to one inch of rain from Wednesday's storms, but as much as 3" may fall in the hardest-hit locations. These higher totals are most likely north of the PA Turnpike and west of I-81 but could occur in more isolated form farther south and east. There will likely be a large variation in amounts from county-to-county due to the scattered nature of summertime convection (see below). With 6-hour flash flood guidance currently between 3-6" across the entire region, larger creeks and rivers should not flood. The smallest streams could briefly approach or overflow their banks, but low-lying, poor-drainage, and urban flooding will be the primary concerns. As always, never attempt to drive through floodwaters! Instead, seek an alternate route.. "turn around, don't drown."

Behind the front, showers and storms should gradually taper off and push south/east of the I-95 corridor late Wednesday night, and skies should quickly turn mostly sunny Thursday morning. A refreshing, northwesterly breeze of 8-16 mph will usher a cooler and much drier air mass into the Commonwealth late this week. Dewpoints will fall back into the 50s by Thursday afternoon and remain there through Friday evening. High temperatures will still be in the mid-to-upper 80s on Thursday and only a degree or two lower on Friday, but that should still feel awesome compared to Wednesday and this past weekend. A large high pressure system will settle over the Northeast on Friday, as well, and promote a continuation of mostly sunny skies. Much like last night, Thursday night should be perfect for sleeping with the windows wide open due to lows in the upper 50s to low 60s. Unfortunately, the sunshine and low humidity won't stick around for the upcoming weekend. The high pressure system will shift into the western Atlantic Friday night, and southerly flow on its backside will draw warmer and "juicier" air back into the mid-Atlantic States over the weekend. Dewpoints should quickly rise through the 60s Friday night and may wind up in the low-to-mid 70s for most of the weekend. Widespread cloud cover should prevent temperatures from exceeding the mid-to-upper 80s on Saturday, but heat indices could still top out in the mid 90s due to the oppressive humidity. Lows on Friday and Saturday nights will likely be in the upper 60s and low-to-mid 70s, respectively, with highs on Sunday similar to or just above Saturday. If you have prolonged outdoor plans this weekend, follow the heat safety tips provided in paragraph #2 above. 

A slow-moving cold front will sag southeastward from the Great Lakes region into the Commonwealth from Saturday into Sunday. Compared to tomorrow's front, this one should be much weaker and less organized. Additionally, the upper-level energy supporting it should track through New England instead of along the Mason-Dixon Line. Thus, any severe weather should be highly localized, but showers and thunderstorms will essentially be possible anytime from Saturday morning through Sunday evening. I certainly wouldn't cancel any outdoor plans, but you may have to move them inside for a few hours on one or both days. The front should move through northern MD and southeastern PA later Sunday or Sunday night, and another Canadian high pressure system will then settle over the Northeast early next week. Monday and Tuesday could be a virtual repeat of this Thursday and Friday with temperatures perhaps even a few degrees lower. Highs may only be in the low 80s on Monday and mid 80s Tuesday with lows in the 50s Monday night. Of course, the pleasant conditions will probably vanish just in time for the 4th of July holiday. Given the progressive pattern and timing of systems, odds favor a return of southwesterly flow, oppressive humidity and 90-degree heat late next week. Whether or not storms will threaten fireworks celebrations next Thursday evening remains to be seen, so you'll have to check back on Friday and again early next week for updates! -- 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