Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Rounds of Showers, Storms to Precede Gorgeous Final Weekend of July *

12:30 p.m. Tuesday, July 23, 2024:

A vigorous disturbance tracking northeastward through the mid-Atlantic States triggered widespread showers and heavy, drenching thunderstorms across the Lower Susquehanna Valley from Monday afternoon into Monday night. Extensive cloud cover greatly limited instability and prevented any severe weather, but the storms tapped into the rich moisture supply and dumped up to 3-4" of rain across parts of northern MD, southern Adams County, and north-central PA. A general 1-2" of rain fell across central PA, mainly in areas west of the Susquehanna River. As evidenced by the measly 0.21" of rain at Millersville, amounts dropped off considerably farther east (see below). The U.S. Drought Monitor from July 16th showed moderate-to-severe drought covering most of southern PA and northern MD (see below). Thus, the rain was primarily a good thing with the exception of localized flash flooding near Hagerstown, MD, Monday evening. I expect the area of drought to shrink on this week's Drought Monitor update, with the biggest changes across northern MD and the western half of the Valley.

Most of the Commonwealth will be caught between upper-level disturbances this afternoon. Thus, no showers or storms are expected despite partly-to-mostly cloudy, warm, and somewhat humid conditions. Highs will be in the mid-to-upper 80s with dewpoints in the mid-to-upper 60s.. not that uncommon for late July (see below). Showers will return tonight in advance of the next disturbance, but more widespread thunderstorms should hold off until Wednesday afternoon. Compared to Monday, Wednesday's activity will be more scattered, lighter, and faster-moving, so rainfall amounts will also be much lower. Warm and muggy conditions will continue to "rule the roost" tonight and tomorrow with dewpoints in the upper 60s to low 70s and lows/highs in the low 70s and mid-to-upper 80s, respectively.

Coverage of showers and storms will likely decrease Wednesday night before increasing again on Thursday. A cold front traversing the mid-Atlantic States from northwest-to-southeast will be the triggering mechanism for the storms. However, extensive cloud cover ahead of the front will limit instability, and most of the upper-level energy associated with it will track from southeastern Canada into northern New England. As a result, any severe weather across northern MD, southeastern PA, and the Lower Susquehanna Valley should be highly-localized and in the form of isolated damaging wind gusts up to 60 mph. Torrential downpours and lightning may also accompany the most intense storms, but they'll be moving fast enough for stream and river flooding to not be a concern. Despite Monday's onslaught of heavy rain, 6-hour flash flood guidance across the region still ranges from 2-4". Additional rainfall amounts through Thursday will average 0.25-0.50" or less, but as much as 1-2" may fall in the hardest-hit locations. Localized flash flooding of low-lying, poor-drainage, and urban areas is, therefore, possible; but that can occur during almost any thunderstorm. Much like Wednesday, coverage of showers and storms should be most widespread Thursday afternoon. By the early-evening hours, the front should be near or along the I-95 corridor. Thus, storms will probably be south/east of the Lower Susquehanna Valley by 6-7 PM and the major metros by 8-9 PM and off the mid-Atlantic coastline by or shortly before midnight. Obviously, you'll want to keep an umbrella handy from tonight through Thursday and be prepared for areas of slow travel on the roads. Weather-related travel concerns will be greatest during the afternoon hours but primarily in the form of standing water in spots. If you encounter a flooded roadway, always turn around and seek an alternate route. High temperatures and humidity levels on Thursday will be similar to tomorrow.

With the upper-level, Jet Stream ridge, or "heat dome," centered over the western United States through Thursday, a compensating trough, or dip in the Jet Stream, will sag into the Eastern States late this week. The trough's axis will be along the Eastern Seaboard from Friday into Saturday and continue to thwart 90-degree heat through the start of the weekend. Behind the aforementioned cold front, winds will turn north-to-northwesterly later Thursday night into Friday and usher drier and less humid air into the mid-Atlantic States. Dewpoints should fall through the 60s Thursday night and end up in the comfortable mid-to-upper 50s from Friday through Saturday. Gradual clearing will occur Thursday night, and there should then be very few clouds in the sky from Friday through at least Sunday. Lows will be in the more comfortable mid 60s Thursday night, perhaps dip into the upper 50s to low 60s Friday night, and still drop into the low-to-mid 60s Saturday night. After our brutal stretch of oppressive heat and unbearable humidity, it will undoubtedly feel great to sleep with the windows wide open! Due to a sprawling high pressure system overhead, the weather will also be ideal for pretty much any outdoor activity on Friday and Saturday afternoons with highs in the seasonably warm mid-to-upper 80s (see below). Just remember to lather on the sunscreen! 

By the second half of the weekend and early next week, the Jet Stream pattern across the nation will reverse. The western U.S. ridge will be replaced by a trough, and the opposite effect will occur in the East. As a result, 90-degree heat could make a comeback as early as Sunday afternoon, and we'll likely be in the midst of the year's 4th heatwave by the final day or two of July (see below). Fortunately, humidity should remain quite low on Sunday and Monday, so heat indices will be similar to the actual air temperature and in the low-to-mid 90s. Mainly clear skies and light winds both nights should also allow temperatures to bottom out in the mid-to-upper 60s, offering relief from the daytime heat.

That should, however, change by the middle of next week with the return of southwesterly flow. So, my primary focus could, yet again, be on extreme heat and humidity and the dangers that come with them by next Tuesday or Wednesday. And don't be fooled.. I expect more of the same in August.. 🥵 -- 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