The week starts off with warm temperatures for this time of year, followed by entry of a cooler airmass. A storm system is forecast to bring rain mid-week. Temperatures will cool even more behind this storm with high temperatures dropping to slightly below normal levels in the low-60s by the end of the week.
Rest of today – mostly sunny, with highs in the low-70s. Overnight lows in the low-50s.
Weather Prediction Center surface forecast for 8PM Tuesday
Tuesday – a dry cold front passes over the region later today, leading to cooler high temperatures in the low-60s for Tuesday on the back of colder northwest winds behind the front. It should be a sunny day. Overnight lows in the low-50s.
Wednesday – a low pressure system over the Great Lakes will track east towards the area. Mostly cloudy skies will prevail with highs warming into the mid-60s due to a shift in winds to the south and southeast. Rain should start to overspread the area later in the afternoon and continue overnight. Overnight lows in the low-50s and even cooler possibly in the upper-40s. A secondary coastal low forming off the Carolina coast will become the primary low as energy transfers from the inland storm.
GFS model output for 1000-500 mb thickness and precipitation valid 2PM Wednesday
GFS model output of the same metrics for 2AM Thursday
Thursday – rain should clear out before the start of the day but winds will be breezy from the northwest. This will bring in cooler air again with high temperatures in the upper-50s and partly sunny skies. Overnight lows around 50°F.
A distinct fall feel this week with grey skies the norm and spotty rain chances for many of the days. A coastal storm brewing off the Southeast coast is forecast to track north and then northeast. National Hurricane Center is tracking this system as it has some potential to acquire subtropical characteristics. This will be a slow moving storm that will bring a long period of northeasterly winds to the area.
Rest of today – mostly cloudy, with highs in the upper-60s. Overnight lows in the upper-50s.
Weather Prediction Center surface forecast for 8PM Wednesday
The developing coastal storm depicted above.
Showers and thunderstorms over the western Atlantic between the southeastern coast of the United States and Bermuda are associated with a broad, non-tropical area of low pressure. This system is forecast to move northward or northeastward and could acquire some subtropical characteristics off the east coast of the United States by the end of the week.
Wednesday – although the storm referenced is forecast to remain well offshore, outer rain bands from it could bring showers to the area along with persistent northeast winds. These wind are anticipated to be on the strong side, steadily in the mid-teens with gusts in the 25 mph range. Mostly cloudy with highs around 60°F. Overnight lows in the low-50s.
Thursday – rain chances continue as the storm develops and becomes a closed low. Mostly cloudy with highs in the low-60s. Winds will become blustery from the northeast around 15 mph with higher gusts. This will be due to the increasing pressure gradient between the deepening low and high pressure over eastern Quebec. Overnight lows in the low-50s.
GFS model output of 850 mb winds and relative humidity. The coastal storm continues to spin off the Northeast coast, although its center remains well offshore (just above the pivotalweather logo in this frame).
Friday – rain still lingers though the storm starts to finally make some eastward progress as a trough approaches from the west. High temperature remain in the low-60s with mostly cloudy skies. Overnight lows in the upper-50s as warmer southerly flow starts to work into the region.
This week will see a wild temperature swing that will take us into the upper-80s, giving us a last taste of summer, followed up by a plunge into decidedly fall-like temperatures barely in the upper-60s. Friday night in particular looks like it could be the coldest yet this season, with overnight lows possibly dipping below 50°F. Temperatures start to recover over the weekend to around normal for this time of year around 70°F.
Rest of today – partly sunny with high temperatures warming into the upper-70s, near 80°F. A warm front is forecast to push through and winds will be coming from the southwest, ushering in some renewed warmth. Overnight lows around 70°F.
Weather Prediction Center surface forecast for 8AM Wednesday
Wednesday – with a warm front passing Tuesday, and high pressure setting up over the Southeast US, warm air will be surging from the southwest towards the region. This should allow high temperatures to rise well into the upper-80s and possibly close to 90°F. A cold front will be approaching from the north Wednesday night, bringing along rain and thunderstorms. Temperatures will cool off quickly with the rain and the cold front, dropping into the upper-50s.
Thursday – much cooler with high temperatures only reaching into the low-60s. Rain chances linger during the day with the cold front stalling out south of us and another low pressure center pushing east. Overnight lows not much cooler than daytime highs, around 60°F as rain chances continue.
GFS model output for precipitation, 1000-500 mb thickness, valid for 10AM Friday. Note the relatively closely packed black lines (isobars) between the high pressure centered over Wisconsin and the Low off the coast of Nova Scotia. This pressure gradient will deliver some windy conditions Friday.
Friday – conditions should clear up as the low pressure responsible for rain on Thursday moves off to the east. A tight pressure gradient is forecast to form between this low and a high pressure center building over the Great Lakes. This should result in a windy day, with high temperatures bouncing back into the mid-60s. Overnight lows are forecast to drop to around 50°F with the influence of the much cooler continental polar airmass and north/northwesterly winds.
High pressure and fair weather will get punctuated with the passage of a cold front scheduled to push through overnight into Sunday. This front could touch off a few thunderstorms. Temperatures during this time will be slightly above average in the upper-70s. A cooler airmass should finally move in Monday with more easterly onshore flow.
Rest of today – sunny with high temperatures in the upper-70s. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8PM Saturday
Saturday – warmer, with high temperatures around 80°F and sunny skies. Overnight lows in the upper-60s. Scattered thunderstorms are possible overnight as a cold front pushes through
Sunday – sunny again with high temperatures in the upper-70s. Despite the cold front, we should see some warmth from downsloping northerly winds and subsidence in the wake of the frontal boundary causing compressional warming. Overnight lows in the low-60s.
NAM model output for 2PM Monday, mean sea level pressure and 10 m above ground level winds. Note the easterly flow in NYC imparted by the high pressure to our east-northeast
Monday – another sunny day with cooler high temperatures in the low-70s, overnight lows in the low-60s.
Temperatures are on an upward trend towards midweek. A cold front moving in from the north brings the chance for rain and thunderstorms tomorrow into Thursday. Behind the cold front, a brief cool down to below normal temperatures occurs before temperatures go back to above normal in the low-mid 80s over the weekend.
Rest of today – early clouds should give way to some partial clearing by the afternoon, high temperatures in the upper-70s. Overnight lows in the upper-60s.
GFS model output for 2 meter above ground level temperatures and 10 meter above ground winds at 11AM Wednesday. Noticeably warmer temperatures will accompany the shift to southwesterly winds.
Wednesday – high temperatures in the upper-80s, with a chance for scattered showers and thunderstorms later in the day as a prefrontal trough associated with a cold front approaches. Overnight lows in the low-70s with continued chances for showers. Overall, a much warmer and more humid feeling, summer-like day with southerly winds and the presence of a high pressure center south of the area.
Thursday – clearing skies with highs in the low-80s. Cooler and drier behind the passing cold front. Overnight lows in the low-60s.
GFS model output for surface pressure and 10 meter above ground level winds on 2AM Friday. Winds by this time are forecast to have shifted to the northeast and east. Onshore flow will keep temperatures cool on Friday.
Friday – high temperatures remain in the mid-70s, overnight lows in the mid-60s as high pressure builds in briefly to the north. This will bring a cooler, northeasterly onshore wind to the area.
Last Thursday, we saw a round of severe thunderstorms develop and roll through the NYC area in the afternoon hours. My instructor (Steve Corfidi) and TA (Phil Lutzak) from my Penn State World Campus Weather Forecasting Certificate program noticed an noteworthy feature in satellite images of the event.
GOES archived visible imagery satellite loop from 1:16 PM through 5:11 PM Thursday, Aug 21, 2019.
You can see that there’s an arcing, convex, wave-like feature oriented southwest-northeast that sweeps east across the Northeast in the visible satellite loop above. If you’re having trouble spotting it check out the series of annotated images below that marks the leading edge of this feature in different points along its progression.
Professor Corfidi noted that this feature seemed to line up well with an area of drier air at the mid-upper levels of the atmosphere, which he picked up in the infrared Channel 8 satellite images. For reference, I’ve superimposed the IR channel on the visible satellite channel from the same time, which is also the 3rd frame of the gallery above.
GOES East IR Channel 8 image at 3:36PM Thursday, Aug 21, 2019
GOES visible satellite image from the same time, annotated to indicate the position of the leading edge of the wave-like feature
GOES IR Channel 8 superimposed on the visible satellite image taken at the same time (3:36PM Thursday, Aug 21, 2019)
What’s more, referring back to the visible satellite loop above, it’s evident that this feature was also partially responsible for firing up strong to severe thunderstorms along the NJ/PA border that eventually tracked east over the NYC area. Storm reports from the day indicate that several of these storms produced damaging wind gusts.
Archived radar image from 3:35PM, Thursday, Aug 21, 2019. Note the positioning of the strong radar echoes and the leading edge of the wave-like feature at this same time.
It’s evident there’s some causative relationship between this wave-like feature and the eruption of afternoon thunderstorms along its leading edge, and this all raises the question: what was this phenomenon? I did some investigation of various upper air analyses from the Storm Prediction Center and found that this phenomenon correlated well with two features at the upper levels of the atmosphere.
Storm Prediction Center analysis of 300 mb divergence valid for 4PM Thursday, Aug 21, 2019
Storm Prediction Center analysis of 400-250 mb potential vorticity valid for 2PM Thursday, Aug 21, 2019
First, we can see that there’s a swath of increased divergence noted at 300 mb (areas outlined in pink) that correlates somewhat with this area of drier mid-upper level air. The second image is perhaps even more convincingly linked to this phenomenon – showing an area of increased potential vorticity. But what does potential vorticity indicate about the atmosphere? In this case, potential vorticity indicates a lowering of the local tropopause – the boundary between the troposphere, where all our weather takes place, and the stratosphere above it. The stratosphere, relative to the troposphere is much drier, and this explains the source of the clear drier region picked up in the GOES Channel 8 infrared images.
Colorado State University depiction of the relationship between the stratosphere and troposphere when there’s an increase in potential vorticity
In fact, there’s a known relationship between potential vorticity and water vapor satellite imagery:
There is a clear relation between PV (potential vorticity) and water vapour imagery. A low tropopause can be identified in the WV imagery as a dark zone. As a first approximation, the tropopause can be regarded as a layer with high relative humidity, whereas the stratosphere is very dry, with low values of relative humidity. The measured radiation temperature will increase if the tropopause lowers. This is because of the fact that the radiation, which is measured by the satellite, comes as a first approximation from the top of the moist troposphere. High radiation temperatures will result in dark areas in the WV imagery.
Potential vorticity in this case was an indicator of increased divergence at upper levels, and this helps explain why severe thunderstorms initiated on the afternoon of Aug 21, 2019, despite the lack of a strong surface boundary providing convergence. This is because divergence and vorticity aloft helps induce convergence at the surface (and may have helped generate a prefrontal trough that day). Divergence aloft is essentially removing air from the top of the column, and since the atmospheric system always attempts to maintain a balance in terms of conservation of mass, momentum, etc, this air leaving the top of the column gets replaced by air flowing in at the surface. This is inflow of air results in convergence, and enhanced lift, as this air rises to replace the air that continues to be evacuated aloft. A source of lift is always a critical ingredient to any severe thunderstorm!
A mid-week cold front brings a chance for rain as temperatures warm towards the end of the week. Tropical Depression Six (which could become Tropical Storm Erin) could impact local beaches with high surf for Labor Day weekend.
Rest of today – partly sunny with high temperatures in the mid-70s. Overnight lows in the mid-60s. Some chance for light rain in the overnight hours.
Weather Prediction Center surface forecast for 8PM Wednesday
Wednesday – high temperatures in the low-80s, with a chance for scattered showers and thunderstorms later in the day as a cold front approaches. Overnight lows in the upper-60s with continued chances for showers and thunderstorms.
Thursday – clearing skies with highs in the mid-80s. Overnight lows in the upper-60s.
Friday – high temperatures remain in the mid-80s, overnight lows in the upper-60s as high pressure builds in briefly.
A Note on the Tropics
A week ago, there was talk about how this month could play out to be one of the few Augusts on record that had no tropical cyclones form in the Atlantic. That calm has been quickly shattered by the development of two tropical systems: Tropical Storm Dorian and Tropical Depression Six, which may become Tropical Storm Erin. While TD Six won’t impact the US directly, it could bring large swells and waves to beaches along the Northeast this weekend. Dorian, on the other hand, is an imminent threat to Puerto Rico, Dominican Republic, and could cause problems for Florida this weekend into early next week.
GOES East satellite image of Tropical Depression Six
GOES East satellite image of Tropical Storm Dorian
We begin this week with some typical “dog days” of summer heat. Shower and thunderstorm chances stick around for the first half of the week with the heat and humidity providing necessary instability. A more organized threat for possibly severe thunderstorms emerges Wednesday into Thursday with a pronounced cold front forecast to move through. Behind this frontal boundary, a refreshingly cool and dry air mass is expected to take hold for the weekend.
Rest of today – hot, humid, with high temperatures in the low-90s. Heat index values in the upper-90s. Partly cloudy with a chance for scattered showers and thunderstorms later this afternoon around 5-6PM as a prefrontal trough nears. Some storms could produce heavy rain and gusty winds. Overnight lows in the mid-70s.
Tuesday – still hot but a touch drier, as a cold front pushes south overnight into Tuesday. High temperatures around 90°F. Could maybe see a scattered thunderstorm still pop up but chances are lower relative to today with less humidity and instability to work with. Overnight lows in the mid-70s.
Wednesday – the cold front passing through Tuesday lifts back north as a warm front on Wednesday. Temperatures remain warm in the upper-80s. Chance for strong to severe thunderstorms possible late in the day as a cold front pushes through. If this front pushes through faster than forecast, in the late afternoon or early evening, there’s better chances for strong storms. Otherwise, showers and thunderstorms could continue overnight with lows in the mid-70s.
Thursday – should be the last day of this spell of summer heat with high temperatures in the upper-80s possible before the cold front above clears the area. Showers and thunderstorms may linger until the front clears. Overnight lows noticeably cooler in the upper-60s behind the cold front as a cooler Canadian air mass moves through.
A stationary front stalled out southeast of the city over the Mid-Atlantic will be the primary agent responsible for an extended period of humid, cloudy, and unstable weather over the weekend. As was the case last weekend, temperatures will be on an upward trend from the low-80s into the low-90s by the start of next week.
Rest of today – mostly cloudy with isolated showers and thunderstorms particularly closer to higher terrain north and west of the city. High temperatures in the low-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8AM Saturday
Saturday – more clouds than sun with the stationary front just south of the area. Continued onshore easterly flow behind north of this stationary front will keep things cooler and humid. High temperatures in the low-80s. Chance for showers and thunderstorms to develop again in the afternoon hours with an unstable airmass (due to the high moisture content caused by the onshore flow) as heating increases. Overnight lows in the low-70s.
Sunday – temperatures begin to warm up into the upper-80s as winds shift towards the south. The stationary front attempts to move north as a warm front but high pressure is forecast to develop west of us and block this northwards advance. Scattered showers and thunderstorms are again possible in the afternoon hours. Overnight lows in the mid-70s.
GFS model 2 meter above ground level temperature, 10 meter above ground level wind forecast for 2PM Monday
Monday – with winds shifting to towards the southwest, much warmer air will advect into the region. High temperatures expected to top out in the low-90s with overnight lows in the upper-70s. Mostly sunny skies forecast with the high pressure building.
Unsettled weather takes hold during the mid-week period as a couple prefrontal disturbances affect the area. The focus for heavier rain will be later Wednesday as a cold front approaches. Behind this cold front, conditions should improve and result in a pleasant, dry weekend.
Rest of today – mostly cloudy with a chance of isolated showers and thunderstorms later in the day as a shortwave trough lingers around the area. Temperatures in the mid-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8PM TuesdayStorm Prediction Center severe weather outlook for Tuesday
Tuesday – shaping up to be an eventful day in regards to weather. Temperatures should rise into the low-80s with overnight lows in the low-70s. During the day, a mesoscale convective system (organized large cluster of thunderstorms) will be approaching the area from the Ohio Valley. Forecast is uncertain in terms of where the frontal boundaries in the vicinity of this MCS eventually set up. Current thinking is that we end up south of a warm front earlier in the day that will serve as the initial focus for shower and thunderstorm activity. This activity should continue and increase in intensity as the frontal boundary starts shifting south and encounters warm, moist and unstable air. There is potential for all severe hazards including hail, damaging winds, and an isolated tornado.
Wednesday – mostly cloudy with shower chances diminishing later in the day . High temperatures in the low-80s and lows in the upper-60s.
Thursday – still a possibility for lingering showers but overall a better day with mostly cloudy skies and high temperatures in the low-80s with overnight lows in the upper-60s.
