Special Weather Discussion

Latest Weather Discussion by Kyle Elliott

* Heavy but Beneficial Rain, Falling Temperatures Wednesday *

12:15 p.m. Tuesday, December 10, 2024:

Monday's fast-moving disturbance produced several tenths of an inch of rain across most of the Lower Susquehanna Valley. Amounts ranged from less than 0.10" along the I-95 corridor to around 0.50-0.60" in most of Franklin County (see below). Behind the disturbance, the combination of high humidity in the low-levels of the atmosphere and warmer/saturated air moving over top of the cold, wet ground caused widespread, dense fog to develop Monday night. The fog reduced visibility to near-zero in spots this morning and was responsible for difficult travel conditions during the AM commute (see below). The fog has since burned off, but cloudy skies will linger throughout the rest of the day. However, rain showers won't return until late this afternoon or evening, and highs should reach the low-to-mid 50s.. about 10°F above normal.. despite the overcast conditions. 

A substantial storm system will develop across the mid-Atlantic States tonight into tomorrow and rapidly intensify as it zips northward into eastern Canada by Wednesday night. As mentioned above, showers will overspread northern MD and southeastern PA from southwest-to-northeast this evening. Periods of moderate-to-heavy rain will then develop after midnight and persist through Wednesday afternoon. A few rumbles of thunder may accompany the heaviest downpours, as well, but any stronger thunderstorms should remain along or east of the I-95 corridor. The steadiest rain will occur during the daylight hours on Wednesday and then gradually taper off from southwest-to-northeast between 5-10 PM. A widespread 1-3" of rain is expected through Wednesday evening, and it will be largely beneficial due to the ongoing, moderate-to-severe drought across the mid-Atlantic States. Motorists are advised to travel within the speed limit, increase following distance on the wet roads, and allow extra time to get to-and-from work or school tomorrow. Water levels on area streams and rivers are running well below normal for mid-December, and the rain will fall in an ~24-hour period. Thus, flooding of low-lying, poor-drainage and urban areas will be about the only real concern (see below). If you encounter a roadway covered by water, turn around and seek an alternate route!

A cold front extending southwestward from the storm system will move through southeastern PA and the Delmarva Region Wednesday afternoon, and temperatures will fall rapidly in its wake. In fact, they may only be in the upper 30s to low 40s by 7 PM after a midday high near 60°F (see below). Ahead of the front, southerly flow will continue to direct increasingly warm and humid air into the region tonight. After falling a few degrees this evening, temperatures will slowly rise throughout the rest of the night. Dewpoints will also rise into the mid-to-upper 50s tomorrow morning, indicating the presence of a moisture-laden air mass. However, winds will turn west-to-northwesterly behind the front and usher much colder and drier air back into the Commonwealth. Dewpoints will plummet by some 40-50°F in just 24 hours, ultimately ending up in the high single digits or low teens by Thursday afternoon. A few wet snowflakes may mix in with the rain before ending Wednesday evening, especially north/west of Route 11/15, but the atmosphere will dry out quickly due to the downsloping flow off the Appalachians. Keep an umbrella and rain gear handy over the next 30-36 hours, and don't forget to grab a winter coat again Thursday morning! Temperatures will only be in the upper 20s to low 30s by daybreak, and wind chills should be around 10°F lower. In stark contrast to Wednesday, highs will be around 5-10°F below average Thursday afternoon, and wind chills won't get out of the 20s despite partly-to-mostly sunny skies. Westerly winds may gust up to 35 mph anytime from late Wednesday afternoon to sundown on Thursday, so enjoy the above-normal warmth and light winds while they last..

A Canadian high pressure system will settle into the mid-Atlantic States on Thursday night and Friday and shift into New England over the weekend. With the high directly overhead, winds will be much lighter on Friday and only around 5 mph. However, the combination of clear skies and diminishing winds will promote efficient, radiational cooling Thursday night. As a result, Friday morning may be the coldest since last January with lows in the mid teens (outlying areas) to low 20s (urban centers). Despite abundant sunshine, high temperatures should only be a few degrees above Thursday's numbers and in the mid-to-upper 30s.. but at least wind chills will be much higher than Thursday on account of the lighter winds. The second week of December could certainly be ending on a much colder and more unsettled note than that! Friday night will be another mainly clear and cold one with lows around 20°F, but the frigid air won't have the staying power of last week's icy blast. Instead, the pocket of Arctic air will lift back northward into Canada this weekend, and southeasterly flow on the backside of the aforementioned high pressure system should direct somewhat milder air back into the region. As a result, both Saturday and Sunday will likely turn out more seasonable with highs in the 40s. I'll be monitoring a storm system taking shape over the Central States from Friday night into Saturday but don't anticipate any precipitation reaching areas east of the Appalachians until at least Sunday. A thick veil of cirrus clouds streaking eastward well out ahead of the system will likely dim the sun on Saturday, but clouds probably won't lower and thicken until late Saturday night or Sunday morning. The track and intensity of the system are quite uncertain at the present time, but odds favor an "all-rain event" due to southerly flow and a complete lack of Arctic air in advance of the system. Check back Friday for the fine-tuned details!   

Overall, I still expect a rather progressive and temperature weather pattern through the end of December with very few opportunities for measurable snow across northern MD and southeastern PA. The continued lack of high-latitude blocking over northern Canada and Greenland will hinder the development of "big storms" along the East Coast and reduce the likelihood of measurable snow south/east of the Appalachians. Instead, weaker systems and "minor" precipitation events.. with the exception of tomorrow.. will remain the norm in the weeks leading up to Christmas. Sorry, snow-lovers, but the prospects of a White Christmas look bleak yet again this year. Time will tell.. -- Elliott