Strong high pressure and sunny weather will be the main theme this weekend. Despite the sun, we do have a chance to see the coldest overnight temperatures so far this season tonight into Saturday. High temperatures start to warm from the 60s to the low-70s by the second half of the weekend and to start next week as the high pressure progresses east and we get warmer southerly return flow before the next storm system approaches Monday.
Rest of today – sunny with high temperatures in the mid-60s. Breezy with a refreshing northerly wind, 15-20 mph with higher gusts due to the relatively tight pressure gradient between the high pressure building moving in from the west and the low that brought us rain yesterday continuing to move offshore. Cold overnight, with lows in the upper-40s. Frost advisories and freeze warnings are up for many areas of upstate New York not too far from us.
850 mb winds as indicated by GFS model for 8AM today, indicating the presence of strong winds of 30-40 knots just above the surface.As the day progresses, the near-surface layer up to 850 mb is forecast to become well-mixed, and this will allow the faster winds above to reach the surface.
Saturday – a sparkling, sunny fall day with high temperatures in the low-60s. Wins will calm also. Overnight lows in the mid-50s with increasing clouds.
Weather Prediction Center surface forecast for 8AM Sunday
Sunday – the cloud shield preceding an incoming storm system will continue moving in Sunday resulting in partly sunny skies. High temperatures will warm to around 70°F as winds turn to the south. This is due to the high pressure exiting east (clockwise flow around this) as well as winds ahead of an approaching cold front. Overnight lows in the low-60s.
Monday – mostly cloudy with chances for rain increasing later in the day. High temperatures in the mid-70s. Heavier rain accompanying the passage of a cold front overnight with lows dropping into the upper-50s.
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.
A warm start to fall yesterday gives way to cooler, drier weather that will actually feel like fall. Much as was the case last week, high pressure will be in control over the area. This should lead to generally fair weather. However, there is some chance for rain Thursday as a cold front pushes through.
Rest of today – mostly sunny with high temperatures in the upper-70s. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Wednesday
Wednesday – high temperatures again mild in the upper-70s with warming influence of downsloping northwest winds. Overnight lows in the mid-60s.
Thursday – increasing clouds with high temperatures in the low-80s and overnight lows in the upper-50s. A chance of showers later in the day as a cold front approaches.
GFS model output for 2 meter above ground level temperature and 10 meter above ground level winds at 8PM Friday
Friday – high temperatures in the mid-70s with sunny skies and overnight lows in the low-60s behind the cold front.
The story of the weekend will be a continued stretch of fair weather with high pressure remaining in control. The positioning and movement of the high pressure center will result in a gradual warming pattern as winds turn to the south with high temperatures returning to summer-like levels in the low-mid 80s by the end of the weekend. A cold front scheduled to move through beginning next week will bring cooler weather back but warmer temperatures could make another return later next week, despite the autumnal equinox taking place Monday and fall formally starting.
Rest of today – sunny with high temperatures around 80°F. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8PM Saturday
Saturday – warmer, with high temperatures in the low-80s and sunny skies. Overnight lows in the mid-60s.
Sunday – sunny with high temperatures in the mid-80s. Overnight lows in the upper-60s.
Monday – another sunny day with high temperatures in the mid-80s, overnight lows in the mid-60s.
Climate Prediction Center short-range outlook for temperatures the second half of next week.
A weak frontal boundary passes through today. Behind this, high pressure will build and extend into the weekend. This persistent high pressure will result in mostly sunny conditions. Below normal temperatures in the low-70s will accompany northeasterly flow but a warming trend is forecast for the weekend as the center of high pressure shifts to the east.
Rest of today – mostly cloudy with a cold front pushing south then stalling out. Chances for light rain linger through the day. High temperatures in the upper-70s. Overnight lows in the low-60s.
High resolution rapid refresh simulated radar for 5PM today, showing some scattered light showers
Tuesday – high temperatures cool off into the low-70s behind the front. Winds turn to the northeast as high pressure builds over central Quebec. Overnight lows around 60°F but could dip into the upper-50s.
Wednesday – mostly sunny with high temperatures in the low-70s and overnight lows in the upper-50s.
GFS model output for surface pressure and winds valid at 2PM Thursday. The area of high pressure over the Northeast will serve to keep our weather fair, while pushing Hurricane Humberto safely out to sea.
Thursday – high temperatures remain in the low-70s, overnight lows in the upper-50s. It will definitely feel like autumn has arrived a few days early!
Easterly onshore winds will keep things cool to start the weekend. Strong high pressure slowly gives way to a frontal system late tomorrow, bringing a chance for rain. Behind this system, high pressure should build in again and persist, giving us some nice weather Sunday into next week.
Rest of today – mostly sunny with strong high pressure north of the area. Cool with highs in the low-70s under the influence of easterly onshore winds. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Saturday
Saturday – clouds build ahead of an advancing warm front and the day should end up being mostly cloudy. High temperatures a bit warmer in the mid-70s with winds shifting to the south ahead of the warm front. Rain chances increase towards the evening and overnight hours, with lows in the upper-60s.
Sunday – a trailing cold front will pass through and conditions should improve with rain ending early and gradual clearing. Highs in the low-80s, with some assist from downsloping winds from the west and northwest. Overnight lows in the nid-60s.
GFS model output for 2 meter above ground level temperature and 10 meter above ground winds at 2PM Sunday
Monday – high temperatures in the low-80s with mostly sunny skies as high pressure continues to build. Overnight lows in the mid-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.
Hurricane Dorian’s center passes well southeast of the city to start the weekend. Because of its large and expanding size, it will still influence our weather. Once Dorian passes, we’ll see improving conditions though cooler temperatures will be the rule with Canadian high pressure taking over.
Rest of today – mostly cloudy with isolated showers, more likely during the late afternoon and evening hours. It’s difficult to say even now whether the outermost rain bands from Dorian actually manage to hit the city, and even if they do, it’s also unclear how intense they will be. High temperatures in the low-70s. Overnight lows in the low-60s with cool, northeasterly onshore flow due to the cyclonic circulation around Dorian. For more info, see this detailed forecast.
GOES satellite imagery of Hurricane Dorian
Saturday – much improved conditions as drier air works in behind Dorian. Mostly sunny with high temperatures in the upper-70s. Overnight lows in the low-60s.
Sunday – a touch cooler though still mostly sunny with high temperatures in the mid-70s as winds turn more to the north and Canadian high pressure builds. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Sunday
Monday – high temperatures continue a 5-6°F below average trend, with highs forecast to be in the low-70s and mostly sunny skies. It will definitely feel like fall. Overnight lows in the mid-60s.
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!
