Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* "All Systems Go" for an Early Spring This Year *

2:00 p.m. Wednesday, February 28, 2024:

Meteorological Spring officially arrives this Friday, March 1, but it has already begun to feel like the season across the mid-Atlantic States. On Monday, the mercury hit 60°F at Millersville for the first time since November 17, 2023 (see below). High temperatures also reached or exceeded the 60-degree mark in Lancaster, Harrisburg, and York. Despite the return of clouds and a few showers Tuesday afternoon, the feat was repeated with highs of 61°F in Harrisburg, 62°F in York and Millersville, and 63°F at Lancaster Airport.

A large ridge, or northward bulge in the Jet Stream, developed over the eastern United States early this week in response to a deep trough across the Intermountain West. Strengthening, south-to-southwesterly flow on the backside of a high pressure system over the western Atlantic has pumped unseasonably warm and increasingly moist air into the region since Monday. A warm front lifting northward through the mid-Atlantic States was responsible for Tuesday afternoon's showers. As expected, this warmup has been a "dirty" one.. i.e. muted by clouds and precipitation.. since Tuesday afternoon. More showers and thunderstorms impacted the Lower Susquehanna Valley Tuesday night due to strong rising motion well in advance of an approaching cold front. Thermodynamically-speaking, the atmosphere is relatively stable today. However, a vigorous, upper-level disturbance and "base" of the aforementioned trough will move directly overhead later this afternoon and evening. Additionally, a massive temperature gradient, or difference in temperature, exists from one side of the cold front to the other. These two features are indicators of dynamic lift over the region, converging air in the low-levels of the atmosphere, and strong, rising motion. Thus, widespread showers and thunderstorms will continue to impact the region this afternoon prior to the cold frontal passage.

Speaking of the front, it will traverse northern MD, southeastern PA, and the Lower Susquehanna Valley from west-to-east between 3-8 p.m. A narrow band of stronger and gustier showers and thunderstorms may develop along the front this afternoon. Regardless of whether or not this occurs, west-to-northwesterly wind gusts of 45-55 mph are anticipated along and immediately behind the front. Along these lines, a Wind Advisory is in effect from 3 p.m. this afternoon through 7 a.m. Thursday. The highest gusts should occur before midnight, but they may still be on the order of 35-40 mph through midday Thursday. Unsecured objects will be blown around by the strong winds. Downed tree limbs and some power outages are also expected. Be alert for locally heavy downpours, as well as reduced visibility, standing water, and debris on roadways. There won't be enough rain to cause flooding of streams and creeks, but motorists should still not drive through roadways covered by standing water. Maintain a tight grip on the steering wheel, as well, especially on north-south oriented highways, due to the dangerous crosswinds. Most areas will receive 0.5-1.0" of rain through the early-evening hours, but up to 1.5" is possible in the hardest-hit spots (see below). Showers and storms will push east of the I-95 corridor by 8-9 p.m.

Behind the potent cold front, temperatures will fall by 20-25°F in a span of just 6 hours or less. They'll be in the upper 50s to low 60s immediately ahead of the front but plummet into the upper 20s to low 30s by daybreak (see below). Despite the sub-freezing temperatures early Thursday morning, I am not concerned about a "flash freeze." Pavement temperatures will lag actual air temperatures due to the recent warmth, and the strong winds should cause roadways to dry rather quickly overnight. Skies will gradually clear after midnight and should be mostly sunny tomorrow. However, a pocket of colder air over southeastern Canada and the northeastern United States will prevent high temperatures from exceeding the low 40s. Wind chills will be stuck in the 20s to low 30s throughout the day on account of the blustery conditions. Thus, you'll definitely need to grab a winter coat and gloves before heading out the door to work or school.

Fortunately, an area of high pressure will settle over the Commonwealth Thursday night and cause winds to fully subside. The combination of clear skies and light winds will provide the ideal setting for efficient, radiational cooling, and low temperatures should bottom out in the low-to-mid 20s. In stark contrast to the last day of Meteorological Winter, another moderating trend will already be underway for the first day of Meteorological Spring. Tomorrow's cold shot could indeed be winter's "last gasp." Quite frankly, there are simply no other "cold days" on the horizon through at least the first 7-10 days of March. Just like Punxsutawney Phil predicted and I alluded to since mid-February, it's still clearly "all systems go" for an early spring this year! The window for accumulating snow has essentially been slammed shut across northern MD, southeastern PA, and the Lower Susquehanna Valley.

The aforementioned high pressure system will shift into the western Atlantic on Friday, and southerly flow on its backside will pump milder air back into the mid-Atlantic States. After a chilly start, temperatures should peak in the upper 40s to perhaps 50°F Friday afternoon. High-level cirrus clouds will stream back into the region during the afternoon hours, but mostly sunny skies should be the "theme" of the day. A weak low pressure system over the Lower Mississippi Valley on Friday will zip northeastward into the Carolinas by Friday night and exit the Virginia coastline sometime on Saturday. A storm track of this nature is often favorable for wintry precipitation in northern MD and southern PA, but that won't be the case this time due to a complete lack of cold air. Instead, showers or steadier light rain and drizzle will overspread northern MD and south-central PA from southwest-to-northeast Friday evening and continue through at least Saturday morning. Rain may taper off Saturday afternoon, but low clouds and drizzle will likely linger into the evening hours. Rainfall amounts from Friday night through Saturday afternoon should average 0.25-0.50", but higher amounts (0.50-1.0") are possible along the I-95 corridor. High temperatures may fail to reach 50°F Saturday afternoon on account of the clouds, rain, and an 8-16 mph easterly breeze. Many colleges and universities, including Millersville, have their "spring breaks" between March 2-10 this year. The weather won't be ideal for outdoor activities on Saturday, but that should change in a big way from Sunday into early next week! Behind Saturday's rainmaker, a sprawling high pressure system will take control of our weather for a few days. Sunday will likely turn out partly-to-mostly sunny and warmer with highs in the upper 50s to perhaps low 60s. Monday and Tuesday's weather could be quite similar, but that depends on the exact location of the high pressure system and extent of any returning east-to-southeasterly flow off the Atlantic. Regardless of the exact outcome, temperatures will be at least 5-10°F above normal during the first week of March, and there won't be any threat of rain (after Saturday) until the middle of next week. 

The Madden-Julian Oscillation (MJO).. or an eastward-moving wave of convection between the Indian and western Pacific Oceans.. is currently inactive but will emerge into phase 3 tomorrow. However, the convection should exit the Indian Ocean by March 3rd or 4th and shift into Indonesia and the Philippines. By mid-March, it may reach the western Pacific. These locations correspond to MJO phases 4-7, and all of these are "warm phases" for the eastern United States during an El Niño March. Essentially, the location of the tropical convection can modulate the Jet Stream pattern across North America to either dip southward or bulge northward over the Eastern States. In the coming weeks, the MJO will clearly be a big help to warm-weather lovers like myself. Furthermore, the Stratospheric Polar Vortex (SPV), or band of westerly winds enclosing a large pool of extremely cold air that develops 10-30 miles above the ground over the Arctic Circle every winter, is currently located on the Eurasian side of the globe and will remain there for the foreseeable future. The SPV is rather weak and expected to remain so through March, but this could simply be its natural breakdown, or "final warming," that occurs every spring. In either case, an SPV displacement toward the opposite side of the globe is usually a "kiss of death" for winter-weather lovers in the mid-Atlantic States. In the low-levels of the atmosphere, the coldest air often follows the SPV with a lag time of 1-2 weeks. A similar displacement late last February had little impact on the weather pattern across the United States. Instead, Arctic air ultimately followed the SPV and dumped into Eurasia during March. I would be foolish to think this case an exception. As the old adage goes, "History repeats itself."

Finally, any North Atlantic blocking will remain "transient," or short-lived, through at least mid-March and centered over Iceland or northern Greenland. This is what meteorologists refer to as an "east-based negative North Atlantic Oscillation (-NAO)," and it typically has little influence on weather patterns along the Eastern Seaboard. When combined with a mean trough over western North America, an east-based -NAO can actually cause Jet Stream ridging to "lock in" over the eastern U.S. for an extended period of time. To make a long story short, there are no signs of "winter's return" in March. Instead, unseasonably mild conditions should "rule the roost" with widespread.. and impressive.. positive temperature anomalies. There will undoubtedly be brief shots of below-normal temperatures, but the number of "warm" days should far outweigh the number of "chilly" ones. Obviously, you'll never hear me complain about that. Bring on spring! 🔥 -- Elliott