A warm and potentially stormy start to the weekend on tap with Storm Prediction Center placing the area under slight risk for severe weather Saturday. Temperatures during this period in the upper-80s. Weather improves on Sunday behind a cold front. Next week, a lingering upper low may bring daily chances for rain in the afternoons.
This afternoon – mostly sunny with a high in the mid-80s. Overnight lows in the low-70s with increasing clouds towards daybreak.
Weather Prediction Center surface forecast for 8PM Saturday
Storm Prediction Center convective outlook for Saturday, showing the region under slight risk for severe weather.
Saturday – a warm front pushes through early in the morning and some showers could accompany this. Mostly cloudy with high temperatures in the upper-80s. A cold front is forecast to push through later, with a prefrontal trough ahead of it. These will serve as the loci for more organized convective activity. Thunderstorms forming in the warm sector behind the early warm front passage may be able to attain severe limits. The main threat is from damaging winds. Overnight lows in the mid-70s with rain chances diminishing early.
Sunday – despite the cold front passage, temperatures stay largely similar to Saturday in the upper-80s due to downsloping winds from the west. With an upper low lingering, isolated, diurnally driven thunderstorms may pop up in the late afternoon, mostly sunny otherwise. Overnight lows in the low-70s.
Monday– mostly sunny though a chance for afternoon pop up showers or thunderstorms persists. High temperatures remain warm in the upper-80s. Overnight lows in the low-70s. Below, you’ll see GFS forecasts for this upper low continuing to influence our weather going into next week. The tangible impacts for this will be continued chances of scattered showers and thunderstorms for much of the week until this upper low finally gets picked back up by primary westerly steering currents and moves offshore.
GFS model 500 mb height and wind. You can see evidence of a persistent upper low that drifts south, then gets cutoff from main steering currents, and remains largely stationary, even retrograding west during next week over the Northeast.
We have a stormy start to the forecast period with the Storm Prediction Center putting us under a slight risk for severe weather this afternoon. Behind this storm, windy conditions will prevail Friday with the pressure gradient tightening between an incoming high pressure and the departing low. This pressure gradient eases Saturday as high pressure moves more directly overhead. Decent weather continues Sunday before another storm rolls in Monday. Above normal temperatures for Thursday in the low-60s. Temperatures drop below normal in low-50s this weekend before picking up Monday into the upper-60s.
Rest of today – lighter stratiform rain in the morning hours with a warm front moving through. This will be followed up by a possible round of strong to severe thunderstorms in the afternoon hours (most likely around 1-2PM). The primary risk for severe weather comes from the potential for damaging wind gusts, with very fast winds aloft potentially mixing down to the surface. High temperatures in the low-60s. Overnight lows around 40ºF as gusty winds bring in strong cold air advection. See this detailed forecast for more info about today.
Storm Prediction Center convective outlook for Thursday, showing us in a slight risk area for severe weather later today.
Friday – despite a high in the low-50s, strong winds around a deepening low will make it feel significantly cooler. Clouds should gradually diminish during the day. Overnight lows cool, in the upper-30s.
Weather Prediction center surface forecast for 8AM Friday. A strong low pressure cetner is indicated tracking northeast out of Maine.
Saturday – winds ease as an area of high pressure moves more directly over us. High temperatures should range in the mid-50s with plenty of sun. Overnight lows will be in the mid-40s.
Sunday– clouds increasing with high temperatures in the upper-50s. Overnight lows will warm into the low-50s.
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.
Last Thursday afternoon, April 26, 2019, a line of severe thunderstorms produced potent, damaging winds, some in excess of hurricane force that caused disruptions to regional transportation networks in the DC, Baltimore, and Philadelphia areas. These storms provide an instructive example of what ingredients are required for severe thunderstorms, and how quickly everything can come together on a given day.
Synoptic Set Up (The Big Picture)
On Thursday morning, a low centered over the Great Lakes was progressing north and east. A warm front extended south and east from this low and was moving north, with a noticeable “kink” where there was colder air at higher altitudes along the Appalachians and related foothills. South of this warm front, southerly winds were helping temperatures rise well into the upper-60s and low-70s. A cold front was located a further back and was advancing across Pennsylvania, and the Virginias. This cold front would provide the focus for lift and thunderstorms later in the day, although some more isolated thunderstorms also accompanied the warm front.
Weather Prediction Center surface analysis of this storm at 11AM on April 26, 2019
Above the surface at 850 mb, evidence suggested an axis of relatively saturated air along with a low-level jet of 35-40 knots would develop, providing the moisture necessary for precipitation. Further up in the atmosphere, a negatively tilted 500 mb trough was evident upstream of the area with the Southeast PA region also appearing to be in the exit region of a 300 mb jet streak. Both of these would help enhance lift by providing divergence aloft in the atmosphere as air was removed from the column while decelerating out of the base of the 500 mb trough and 300 mb jet streak respectively.
Fig. 1
Fig.2
Fig. 1: GFS forecast model initialized at 7AM Thursday, April 26, 2019 depicting an axis/tongue of moisture (narrow area of blue) along the PA/NJ border around 5PM that day. Fig. 2: 300 mb analysis for 8PM on Thursday, April 26, 2019. Note the densely packed yellow contours close to the Southeast PA area at this time, indicating strong net divergence in the exit region of a curved jet streak at this level (blue shaded areas with wind barbs showing max winds of 80 knots slowing to 65 knots in the exit region).
Furthermore, winds throughout the atmosphere were strong, and increasing from 35 knots at 850 mb to 60 knots at 300 mb. Meanwhile winds at the surface were light, at 5 knots or so at the from the south. Winds aloft were more from the southwest. So, there was an element of both speed and directional wind shear in the atmosphere this day.
A Sunny Afternoon and Instability
From above, we see that we had several ingredients were taking shape last Thursday: a couple frontal boundaries providing focused lift, moisture at 850 mb, vorticity and net divergence at 500 mb and 300 mb enhancing lift, with strong winds at these levels enhancing wind shear. We still needed one more key component to truly set off some strong to severe thunderstorms: instability. How does instability build up in the atmosphere? The answer has to do with the daytime heating and the sun. That’s why thunderstorms often pop up later in the afternoon when daytime heating is maximized.
Fig. 3
Fig. 4
Fig. 6
Fig. 3: Storm Prediction Center mesoanalysis highlighting areas favorable for severe weather on the afternoon of April 26, 2019. Fig. 4: Storm Prediction Center analysis of 3-hour mixed layer CAPE (convective available potential energy, a measure of instability) change. Note that the pocket of a large increase in instability corresponds to the location of the pocket of clear skies below. Fig. 6: A marked up visible satellite image at 3:16 PM on Thursday, April 26, 2019 showing the approximate position of frontal boundaries extrapolated from the Storm Prediction Center analysis in the preceding image.
Why does daytime heating at the surface lead to destabilization of the atmosphere? This has to do with buoyancy and lapse rates. Lapse rate describes the change in temperature over a given altitude. As the sun heats the surface of the earth up, it shifts the environmental temperature line to the right on a skewT sounding as the one attached below, taken at 2PM on Thursday, April 26, 2019 at Washington Dulles International Airport (KIAD). This tends to increase instability because a warmer airmass above the surface will have greater buoyancy. A large lapse rate combined with enhanced buoyancy allows for air from the surface to rise, and keep rising forming towering cumulus clouds that can eventually build into thunderclouds. As long as a parcel rising from the surface stays warmer than the environmental temperature profile (red line), it will keep rising.
A skewT of a sounding taken at Washington Dulles International Airport (KIAD) at 2PM on Thursday, April 26, 2019. Refer to this post for how to interpret this skewT.
The Storm Prediction Center was well aware that the severe weather potential was maximized for areas that saw clearing skies in advance of the approaching cold front. They also picked up on tornado potential focused on the “kinked” warm front. This is due to the fact that such an orientation of a warm front leads to a situation where surface winds are locally backed, meaning they’re turning counterclockwise over time. This was also paired with a localized pressure fall of 3 mb over the two hours leading up to 3 PM on Thursday.
Storm Prediction Center mesoanalysis of 2 hour pressure tendencies. The area of Southeastern PA, northeast MD, and northern DE had seen a pressure fall of 3 mb leading up to 3PM Thursday.
As was the case with the Lee County Tornado that claimed 23 lives in Alabama on March 3, 2019, these locally backed winds due to the warm front and pressure falls (leading to some isallobaric winds) served to enhance storm relative helicity and create an environment favorable for storm rotation and the possibility for tornadoes. The backing winds also served to increase wind shear and the potential for severe weather. Luckily, in this case, other environmental factors weren’t supportive for a large, strong tornado.
Severe thunderstorms could impact the area later today ahead of an approaching cold front. Following the passage of this cold front, conditions will begin to improve, setting us up for a weekend with pleasant, cooler, classic autumn weather.
Rest of today – Storm Prediction Center in Norman, OK has placed parts of the NYC Tri-State (the city, north and west) under a slight risk of severe weather today. Warm, moist air (read: unstable, theta-e rich) is in place with the area in the warm sector of an advancing low pressure center located in Ontario/Quebec. Some rain/thunderstorm activity could take place along a weak warm front this morning. The main focus of activity will occur later today with a cold front approaching from the west.
This cold front will be accompanied by a streak of strong upper level winds (40-50 knots), which will be capable of providing the magnitude of vertical wind shear necessary for some strong to severe thunderstorms to organize. Given the direction of the winds at upper levels being largely parallel to the advancing frontal boundary, expect that the primary storm format will be a QLCS (quasi-lineary convective system) – a line or broken line of storms. High Resolution Rapid Refresh models depict the bulk of storm activity approaching the city around 8PM this evening. Given this storm format and synoptic set up, the most likely severe threat will be damaging wind gusts. Heavy rain could trigger flash flooding as well, since the ground is still quite saturated from yesterday’s rain.
The biggest limiting factor to storm strength today will be overcast skies limiting daytime destabilization from solar heating. As is often the case, I would expect the storms to be weakening somewhat as they approach a more stable marine air layer near NYC. Overall, expect a cloudy day with high temperatures in the low-80s and a muggy feel.
Storm Prediction Center Day 1 Convective Outlook
Weather Prediction Center forecast surface conditions at 8PM EDT
High Resolution Rapid Refresh simulated 1-km radar for 8PM EDT
NAM forecast for 500 mb winds and height valid 8PM EDT
Thursday – the cold front that brings us this possibly severe today will slow down as it pushes south of us, such that it remains close enough that a weak disturbance moving along it could bring us some additional showers late in the day. Skies will be partly sunny with much cooler high temperatures in the upper-60s.
Friday – easterly onshore flow on the back side of this cold front forecast to be to our south will keep skies cloudy with high temperatures around 70°F – chance for showers lingers.
Saturday – high pressure finally starts to take control of sensible weather and should produce a mostly sunny day with crisp conditions and high temperatures in the low-70s.
A wet start to the weekend will then give way to much warmer weather. We’ll get a break from the rain Sunday and Monday before the next shot for rain. Of note, a possible tornado may have impacted parts of northern Queens last night.
Rest of today – increasing chances for showers and thunderstorms especially in the early afternoon. Otherwise, mostly cloudy with high temperatures in the mid-80s.
Saturday – more widespread showers and thunderstorms in the morning, with some concern for flash flooding. The set up also could produce some isolated severe weather as a weak impulse of energy travels up the upper trough that’s been locked in over the Eastern US this week. Improving conditions later with highs in the mid-80s.
Sunday – the upper trough that’s been causing this week’s wet weather finally exits east. High pressure begins to build south of us, bringing warmer temperatures with highs in the low-90s and lots of sun.
Monday – more of the same as Sunday, sunny, hot and humid with highs in the low-90s.
Possible Tornado in Northern Queens Last Night
At 10:18 last night, residents in northern Queens, the eastern Bronx and northern Nassau received alerts from our local National Weather Service forecast office of a tornado warning. The damage was evident this morning, with many downed trees in College Point. A team from the NWS is assessing the damage to determine whether what hit this area last night was really a tornado, or if it was some other non-tornadic wind phenomena.
I was in a neighborhood that received this warning, but at first, I almost thought it was in error because there were no signs outside of anything awry, no thunder, no rain. However, a quick look at the base radial velocity imagery on the JFK high-definition terminal Doppler radar was revealing – there certainly was a storm with very strong rotation, hence why the warning notes “radar indicated tornado”. In the annotated base radial velocity image below, the white circle indicates a tight mesocyclonic circulation.
Without going too much into technical detail, the greens indicate inbound velocities relative to the radar station (air was flowing towards the radar), while oranges and reds indicate outbound velocities (where air was flowing away from the radar). When forecasters see a “velocity couplet” like this where inbound and outbound velocities are close together, they know this is a signature of strong storm rotation within a mesocyclone. If, within this couplet, you get a “gate to gate” (i.e. adjacent pixels) where the difference between inbound and outbound velocities exceed 90 knots, this is referred to as a “tornado vortex signature”. In this image, if you do the math, you get that the max difference between inbound and outbound velocities was 86 knots, and you an see what appears to be a tornado vortex signature.
We’ll see what the NWS’ assessment says later today, but it appears to me that there was a tornado at some point over northern Queens last night!
UPDATE (1:46PM August 3, 2018): The National Weather Service issued a report confirming that an EF0 tornado touched down in portions of College Point, Queens last night.
After two days of record-breaking heat in the low-90s, a pattern change is on tap for the NYC metro area going into the weekend. Cooler conditions will prevail in the wake of the passage of a cold front, which could also bring rain and thunderstorms (possibly severe). Saturday should shape up nicely but rain chances return Sunday before a cooler start to the work week.
Rest of today – the Storm Prediction Center has placed portions of the region under a marginal-slight risk of severe thunderstorm activity today. We continue to be an increasingly moist warm sector east of an advancing low pressure center. As the cold front associated with this low pushes east, storms will initiate first along a pre-frontal trough during the day today (late morning-early afternoon), then again along the actual frontal boundary itself later this evening. It is the latter round of these storms that could push severe limits. There are limiting factors to this activity especially near the coast with more stable air present. Cloud cover will limit day time heating, reducing overall instability. And the earlier round of activity should serve to actually mitigate the later round of storms. Aside from the rain, it should be mostly cloudy with some breaks in the sun (that would increase chances for storms), with high temperatures in the mid-80s.
Saturday – cooler air moves in behind the cold front tonight while a weak area of high pressure builds. High temperatures should top out in the mid-70s with partly sunny conditions. Rain chances increase towards the overnight hours.
Sunday – cooler still with mostly cloudy skies and chances for rain. High temperatures only in the mid-60s as a weak disturbance passes to our south.
Monday – skies finally clear up with high temperatures moderating and warming into the upper-60s.
The weather on Sunday turned out to be a real gift for fathers in the region, yielding a lot more clear skies than anticipated going into the weekend. We could have a rough start this week with possible severe weather, and another chance of rain mid-week. A tropical system forming over the Gulf of Mexico bears watching this coming weekend.
Rest of today – humid and warm with high temperatures in the low to mid-80s. Mostly sunny to start. All the ingredients for possibly severe thunderstorms will be in place this afternoon and evening. A flash flood watch is in effect for most of the area west of Long Island. The Storm Prediction Center has placed some of the region west of the Hudson River under an enhanced risk of experiencing severe weather with the city itself in a slight risk area.
A vigorous cold front will move east towards NYC today. As we can feel, the airmass ahead of the front is saturated and warm. The more sun we get before the front approaches, the greater the destabilization in the atmosphere. The cold front will line up with the flow of the jet stream, which will favor line segments and training of storm cells repeatedly hitting the same areas (flash flood warning is in place because of this).
Timing of the leading edge of the storms appears to be around 5pm. Any storms or substantial cloud cover ahead of this would decrease the severe potential. Primary threat along this leading line of storms would be damaging wind gusts with a small chance for hail and a couple isolated tornadoes.
Behind this leading line of storms, we could see a slight break around 8PM, but then more rounds of moderate to heavy rains that could lead to flash flooding.
Tuesday – severe weather and rain should dissipate fairly quickly after midnight. Decreasing clouds in the wake of the cold front though high temperatures remain similar to today due to the influence of southwest winds.
Wednesday – mostly sunny with high temperatures in the low-80s. A chance for rain in the afternoon, but much milder and less widespread than today.
Thursday – mostly sunny again, with highs in the low-80s again.
Potential Tropical Storm in the Gulf
Two possible tropical systems are churning, one east of the Windward Islands and the other crossing over the Yucatan into the Gulf of Mexico. The latter system may have a tangible impact in terms of bringing tropical moisture to the area this coming weekend.
We’ve been able to enjoy a very comfortable week of summer weather. High temperatures have been largely held in check in the low-mid 80s this week. This weekend, there will be a chance for some rain Saturday, but Sunday will be another nice day with mostly sunny skies and high temperatures in the mid-80s.
Rest of today – mostly sunny, high around 85ºF.
Saturday – watching for the risk of scattered strong to severe thunderstorms tomorrow. The degree of severity will hinge upon how much cloud cover there is during the first half of the day, and whether any storms pop up early. If there are more clouds early, and some storms show up early, the available energy for storms in the afternoon associated with a cold front will be limited. Ahead of this cold front, warm, humid air will flow in, and temperatures should peak near 90ºF.
Sunday – skies will clear quickly following the passage of the aforementioned cold front. High pressure begins to take over and will give us a nice, sunny day, with highs in the mid-80s.
Monday – almost a clone of Sunday with mostly sunny skies and highs in the mid-80s.
Tuesday – with high pressure still almost directly overhead, we will get another nice day with highs in the mid-80s.
Slight Improvement in Drought Conditions
With recent rounds of heavy rain and thunderstorms that have impacted the area, we’ve seen a slight improvement in drought conditions, but the situation is still far from desirable.
