Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Much-Needed Rain Tonight; Wet Snow Friday? *

1:15 p.m. Wednesday, November 20, 2024:

After 50 days of generally quiet, mild and sunny weather, a complex and far-reaching storm system will bring howling winds, rain and even accumulating snow to parts of the Northeast and mid-Atlantic States late this week. Today could certainly be considered the "calm before the storm." Much like on Tuesday, skies across northern MD, southeastern PA and the Lower Susquehanna Valley will be partly-to-mostly cloudy this afternoon, and winds will be light with high temps in the low-to-mid 60s. However, the weather will quickly turn more volatile this evening. A vigorous cold front will traverse the mid-Atlantic States from west-to-east overnight and be accompanied by a line of heavy showers. Brief, torrential downpours and wind gusts up to 40 mph are expected within the line. Behind it, a 2-4-hour period of steady, light rain will follow. The gusty showers will race eastward across the region between 6 p.m. and midnight, and the lingering, light rain should then push east of I-95 between 4-5 a.m. Thursday. Through daybreak, rainfall amounts should range from 0.25-0.50" along I-81/Route 11 to around 1" near Philadelphia (see below). Given the ongoing, moderate-to-severe drought across the region, the rain will be largely beneficial and is much needed. Even better, the rain will not impact this evening's or tomorrow morning's commute, and wind gusts won't be high enough to down trees or cause power outages. Good news for all!

Behind the front, west-northwesterly winds of 12-25 mph will direct much cooler and drier air into the Lower Susquehanna Valley on Thursday. Skies will clear around or shortly before daybreak, and much of Thursday morning should be sunny before clouds return in the afternoon. Temperatures will still be around 40°F at sunrise but only peak in the upper 40s to low 50s during the afternoon hours. Factor in the wind, and the outdoor air will feel even chillier with jackets a necessity for outdoor activities. Tomorrow's forecast is fairly straightforward, but things get complicated and, quite frankly, very challenging from tomorrow night through Friday night. The primary low pressure system associated with tonight's cold front will develop over eastern Michigan and Lake Huron this evening. Thereafter, it will retrograde to the south and west, ultimately passing over Lake Michigan on Thursday and dying out in the process. As this transpires, a secondary low pressure system will form just off the New Jersey coastline and south of Long Island. From Thursday night through Friday evening, numerical computer models suggest a similar behavior to the primary low. In fact, the coastal low is expected to retrograde inland and ultimately "make a loop" over the Catskills, Poconos and eastern PA before exiting back off the New Jersey coastline Friday night. Much like the primary low, it will weaken while completing this loop after reaching maximum intensity over southeastern New York. A massive, blocking high pressure system centered south/west of Greenland and upper low centered east of Nova Scotia.. what meteorologists call a "50/50 low".. are to blame for the system's abnormal behavior. The circulation patterns around the Greenland block and "50/50 low" create an atmospheric traffic jam across southeastern Canada and the eastern United States. Instead of moving north- or northeastward into eastern Canada, storm systems are forced to cut underneath the massive block and, therefore, shunted farther south and east (see below). During these types of blocking patterns, primary low pressure system often weaken and give way to secondary redevelop several hundred miles to their south/east, and this case will be no exception. Unfortunately, the location of secondary redevelopment, strength of the secondary low and its eventual track are difficult to predict more than 1-2 days in advance and handled poorly by numerical computer models. Thus, the forecast late this week is a low-confidence one.. and one that can still change drastically. 

Unlike most storm systems, the coldest air will be on both the primary and secondary low's southern flanks. Extensive, east-to-southeasterly flow originating from the still-warm waters of the western Atlantic is drawing mild air northwestward into New England and southeastern Canada. As a result, the antecedent air mass ahead, or to the north and east, of both systems is anything but cold. Instead, high temperatures this afternoon will be in the 60s locally, 50s across southern New England and 40s the whole way up to Hudson Bay in Canada. There isn't a strong, Canadian high pressure system over northern New England to funnel cold air southward into the mid-Atlantic States. Instead, the two low pressure systems will essentially "manufacture their own cold air" as a result of strong, upper-level dynamics and evaporational cooling from precipitation. Relative to each surface low pressure system, the most upper-level energy and, therefore, coldest air in both the mid- and low-levels of the atmosphere will be on their southern and southwestern flanks. Thus, residents in the Catskills, Poconos, mountains of northern PA and upslope regions of the Alleghenies have the best chance of receiving a plowable snowfall Thursday night into Friday. Due to the persistence of above-normal temperatures since mid-September, ground temperatures are way above freezing and generally in the upper 40s to mid 50s across northern MD and southern PA. They're lower in the aforementioned, higher-terrain regions, and air temperatures will also be lower at higher elevations during the impending winter storm. Finally, the secondary low will pass over or near to those regions during its period of maximum intensity. As a result, dynamic cooling will cause rain to change over to heavy, wet snow in those regions Thursday evening, and up to 12" of snow could fall on the highest peaks. However, much less will fall in valley locations such as Williamsport, Scranton and Wilkes-Barre, and roadway accumulations will be significantly lower than those on non-paved surfaces. 

For northern MD, southeastern PA and the Lower Susquehanna Valley, the likelihood of accumulating snow will be MUCH lower and, with the exception of the highest peaks north/west of I-81, virtually non-existent. Barring a northeastward shift in the track of the secondary low, rain and snow showers should move back into south-central PA later Thursday night or Friday morning and then continue through Friday afternoon. However, temperatures will likely remain at least several degrees above freezing Thursday night, and they'll be in the upper 30s to low 40s Friday afternoon. Moreover, blustery conditions will persist on Friday with westerly wind gusts up to 30 mph, and the downsloping flow off the Appalachians will cause the air to "dry out" and precipitation to be relatively light across our region. Even if it snows lightly all day, these factors will make accumulation on roadways impossible and limit accumulation on grassy surfaces to a dusting or less. Thus, the biggest impacts to travel interests will be wet roadways and areas of reduced visibility in any heavier rain showers or brief bursts of moderate snow. You'll also want to grab an umbrella and winter coat before heading out the door to work or school Friday morning. Due to the combination of strong winds and temperatures around 10°F below normal, wind chills won't exceed the upper 20s to low 30s Friday afternoon. What a stark contrast to this afternoon's pleasantly mild conditions.. Yuck!

As mentioned previously, the secondary low will move back off the New Jersey coastline Friday night, so any lingering rain and snow showers should end quickly Friday evening. However, skies will remain mostly cloudy through the start of the weekend due to persistent, northwesterly flow over the still-warm waters of Lake Erie. In addition, a strong pressure gradient, or difference in pressure, between the offshore storm system and an area of high pressure over the Southeast will remain across the mid-Atlantic States from Friday night through Saturday night. As a result, windy conditions will prevail during this time with gusts as high as 35-40 mph on Saturday. Fortunately, a gradual moderating trend will take place over the weekend as the pocket of cold air in the mid-levels of the atmosphere lifts into northern New England and the Canadian Maritimes. After a brisk and cold Friday, Saturday will turn out more seasonable with highs in the upper 40s to low 50s. It will still feel much colder outside on account of the howling winds, but that persistent breeze will also prevent temperatures from dropping below the mid-to-upper 30s Friday night and upper 30s to low 40s Saturday night. The aforementioned high pressure system will then nudge northward into the Carolinas during the second half of the weekend and start of next week. It will be in close enough proximity to the Lower Susquehanna Valley to promote partly sunny skies, lighter winds and even higher temps (low-to-mid 50s) Sunday afternoon. Thanksgiving week should start on an even nicer note with highs in the upper 50s to perhaps low 60s ahead of the next cold front. That front and its associated storm system will be much weaker and more progressive than the impending one, so no more than a few brief showers should impact the Commonwealth Monday night. Behind the front, winds will turn northwesterly again and usher cooler air back into the region. As a result, next Tuesday will likely turn out partly sunny and blustery with highs around 10°F lower than Monday. Although still 8 days away, the weather on the busiest travel day of the year currently appears breezy and cool.. but dry.. across the mid-Atlantic States. If so, then conditions will be favorable for travel both on the roadways and in the sky. Fingers crossed! 🤞 There's a lot to sort out over the next few days, so check back Friday for a brief update! -- Elliott