New York City Metro Weather

Cold air advection continues tonight

By: NYCvort, 9:31 PM GMT on July 30, 2010

Additional cold air advection will occur this evening and overnight. It will be more upper level based, with winds at 500mb turning toward the WNW behind a departing shortwave. A ridge near the Rockies will allow for a trough in the northeast/mid-Atlantic to suppress the jet stream to the south of our area through the weekend. This means that for a change disturbances embedded in the jet will ride down the Great Lakes and pass in our vicinity. A duel jet structure is expected to develop late in the weekend, with a trough lingering in the northeast through early next week. This will ensure continued comfort with low humidity levels and cooler temperatures. A trough riding ashore the northwest US coast will push the ridge axis into the southern plains. However, the lingering northeast trough will prevent the warm air from building into our region until the middle of the week. With 850mb temperatures dropping down to 50 degrees tonight, along with light winds associated with weak high pressure, lows will fall into the 60s across the entire area, with low 60s in many suburbs. With the mid-level cold air remaining in place through tomorrow afternoon and a developing weak onshore flow, highs will only make it into the upper 70s with some low 80s possible in the city.

An upper level disturbance will approach for Sunday, and it will begin to slow as it gets detached from the main upper flow. With low level high pressure standing firm over northern New England and the Canadian Maritimes, this could spell periods of rain starting Sunday and wet weather continuing into Monday. The rain will be partially the result of weak dynamics but mostly isentropic lifting, as warm air rides up and over the cooler and more dense air mass in the low levels.

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Medium Range cooler Outlook: Pattern change looks imminent

By: NYCvort, 9:37 PM GMT on July 29, 2010

All of the data that I have looked at points to a major pattern change about to take place over the extended forecast period. The string of nearly oppressive heat and humidity that we have dealt with much of this summer is about to be broken. This certainly doesn’t mean that we won’t have hot days and humidity to deal with from time to time, but it won’t be persistent like it has been, and I also think that there will be more comfortable days than uncomfortable ones over the next couple of weeks. The change all starts tonight as a trough digs from the Great Lakes region and into the northeast. The ability for the trough to dig this far south is supported by the lower AO index, which is currently just hanging on as every so slightly positive.

What is most interesting to me is that all of the teleconnections that I am looking at support a pattern change during the two-week forecast period. The AO is forecast to go more significantly positive once again for the start of August as the anchor of our trough slowly recedes to the north and east. However, an impressively strong polar vortex (especially for the summer), depicted by both the European and GFS models, is pressed to move straight down from northern Canada next week. As such, the GFS ensemble mean AO forecast is showing the AO index dropping to a standard deviation of –1 below normal late in the period. If correct, which I feel confident about, this would actually be the first time the AO has gone negative since our significant cool down to start July, and it will rival how low it went at that time. It is not likely to get as cool, especially at night, however, because we are now in August and ocean water temperatures are warmer, and overall North America is warmer and more humid. At the same time, though, we also don’t have the same sun angle as we did in late June and early July, and the days are shorter, so we’ll have to see what reverse effect this might have on high temperatures.

The NAO index is forecast to go positive, with only one out of all of the ensembles showing a true negative NAO after the start of August. A mean positive NAO would support a more progressive long wave pattern across North America as a whole, with our weather being more changeable and not as “locked in.”

The PNA index is looking very interesting, with some ensembles actually showing a PNA of greater than 2 standard deviations above normal by late next week. This is only one possibility, but if true, it would be the highest the PNA has gone since April. Even the ensemble mean is showing at least a 1 standard deviation above normal PNA to start August, which would send some cooler air down from Canada as a ridge is positioned out west with a trough remaining in the east.

Finally, the EPO index, which is currently slightly negative, is forecast to drop into a more significant negative position by the beginning of August. A –EPO followed by a +PNA pattern would build a huge amplified ridge near the west coast that will move inland and further support the idea of energy continuing to slide down from Canada and affect the northeast with cooler weather. This big ridge is what will allow the strong polar vortex to come down.

Another interesting thing to note is that both the GFS and ensemble mean are showing the most El Nino influenced upper level weather pattern to start August that we have experienced since last winter, with an upper level pattern exhibiting over 1 standard deviation above normal El Nino characteristics. The GFS goes as far as to say that we get close to 3 standard deviations above normal, but I'm not sure it will get that high. Regardless, we are obviously entering an El Nino weather pattern, which will support cooler weather for the eastern US.

As I discussed above, all of the teleconnection indices that I looked at, including the AO, NAO, PNA, and EPO, as well as the El Nino influence on the pattern, are forecast to change over the next two weeks. Based on this, I believe that a moderately progressive cooler pattern will prevail over the next two weeks. Rounds of cooler air will be delivered tonight through the weekend, with its effects still felt into early next week. With the AO forecast to go more significantly positive, we will feel the effects of that by the middle of next week with a period of warmer temperatures and high humidity. The European and GFS both show opportunities for a brief period of less humid air to arrive late in the week with a shortwave/cold front. As the strong polar vortex starts to slide down late next week, the GFS is more progressive in passing it well to our north next weekend, while the European keeps it in central Canada. A solution somewhere in the middle is likely.

The tropics may bear some watching the following week as the European, GFS, and especially the Canadian all bring a tropical wave toward the east coast. This will all depend on how the potential storm develops, its speed, and the specific weather pattern that will be in place at that time.

Updated: 10:08 PM GMT on July 30, 2010

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A weekend cool down begins tonight

By: NYCvort, 9:21 PM GMT on July 29, 2010

A strong cold front pressed through this afternoon. With the center of the upper level ridge migrating west today/tonight and relocating near the Rockies, a trough will slide down into the northeast and mid-Atlantic. Spokes of energy rounding a polar vortex in northeastern Canada will send shots of cooler air down into our region through the weekend. The first and most significant round of cold advection will occur tonight ahead of a shortwave. A mid level cold front will move through this evening, and 850mb temperatures will plummet from close to 60 early in the evening to around 50 by early tomorrow morning. Thicknesses will drop below 5640m for the first time in quite a while. The cold advection tonight will lead to temperatures falling from the upper 80s late this afternoon into the mid 60s throughout the area by sunrise tomorrow. The humidity will drop off significantly this evening, and dewpoints in the 50s will feel great. Weak high pressure will build in from the Great Lakes, and with not very much wind tomorrow, even though 850s will be low 50s, highs will just barely make it into the low 80s along with plenty of sunshine and some fair weather clouds.

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Another round of cooler air set to arrive

By: NYCvort, 4:11 AM GMT on July 29, 2010

Just a quick update: A cool upper trough currently centered just southeast of Hudson Bay will usher through a strong cold front midday tomorrow. Energy will slide down on the backside of the trough and deliver refreshing cooler air tomorrow night. A few showers are expected to accompany the frontal passage tomorrow morning.

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Strong cold front ushers in a change in air mass tonight

By: NYCvort, 5:50 PM GMT on July 25, 2010

With a broad shortwave approaching and strong cold front moving through this afternoon, showers and thunderstorms are likely. With close to 2 inches of precipitable water and decent vertical wind shear, but a lack of mid level CAPE and 1000 J/KG of surface based CAPE yielding marginal instability (LI of just –1), I think the greatest threat from these storms will be heavy rain as opposed to severe weather. Dewpoints have already fallen into the upper 60s, and with temperatures close to 90 it is still very uncomfortable but not quite as bad as it was yesterday.

The 12z model runs from this morning lock in on the idea of much cooler weather arriving tonight. After temperatures failed to drop below 80 degrees last night with breezy and humid conditions, lows will drop into the upper 60s tonight and with dewpoints falling into the upper 50s it will feel great. With 850mb temperatures in the mid 50s tomorrow, highs will be in the mid 80s with sunshine and low humidity. Winds will go light tomorrow night, and with a dry air mass in place, there will be more of a range in low temperatures, with low to mid 60s possible in many suburbs and upper 60s to near 70 in the city.

The upper level pattern over North America looks remarkably different than it has for a while. One of the most recognizable changes is a train of upper lows from Alaska and across Canada. The upper high that was positioned just south of the Aleutian Islands is weaker and more suppressed. This represents a pattern change, and the effects of this change are about to take effect across our area, starting tonight. The persistent upper ridge across the southern states and especially the southeast will retrograde to a position near the Rockies, which is a feature that denotes a positive PNA pattern and leads to troughing to at least some degree across the northeast. A northern stream component of that ridge is expected to form as it jaggedly aligns and bulges upward into western Canada. This will continue to send the upper level circulations in our direction. The upper level ridge does remain persistent and elongated across much of the nation, so temperatures will still remain above normal much of the time. However, it will also mean that record-challenging heat is not expected through the beginning of August as upper level energy sliding downward continues to prevent the ridge from building into our area.

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Very hot and humid today, but relief is on its way

By: NYCvort, 4:59 PM GMT on July 24, 2010

I am very glad to continue the news that relief from the heat is well on its way! Two upper level features, one from southern Canada and the other from the northwestern US will essentially phase over the Great Lakes region this weekend. This will help to shrink the broad upper ridge centered near or just off the southeast coast. The trough combo, anchored by an upper low over southeastern Canada, will result in the most significant break in the heat/humidity since early July. However, out ahead of this feature we will be on the northern periphery of the ridge, ensuring one last hurrah for the hot and humid weather this weekend. I think if there was more wind today we could have gotten even hotter, but dewpoints would have probably been a little lower. Such as it is, highs will be in the 90s with dewpoints in the low to mid 70s. This will yield heat indices over 100 degrees, in the dangerous range. A prefrontal trough and mid-level shortwave will approach late today, which could touch off showers and thunderstorms. LIs will be hovering close to –4, along with mid level CAPE close to 2000 J/KG and low level CAPE around 2500 J/KG (very warm, humid, and sufficiently mixed boundary layer), so the atmosphere will be unstable, and I think we will see some action this evening. Better shear arrives tomorrow, and the models have slowed down the passage of the cold front a bit, so as such, there is a chance for more storms mid-day on Sunday ahead of another mid-level shortwave and the actual cold front. However, the instability will be decreasing fast, and the atmosphere will actually become quite stable by evening, so there may not be enough time for the low levels to become sufficiently unstable by the heating of the day before the passage of the front in the afternoon. Also, the approaching upper level trough will be positively tilted, so this may actually help to lower the storm chance. Although there will be more wind on Sunday, 850mb temps will be slightly lower and dewpoints will be a little lower, so it won’t be as bad, though highs will still break 90. There will be quite a drop in temperatures Sunday night under a cold air advection regime following the passage of a strong cold front, with the 850mb front passing through in the evening and mid-level temps drop off 10 degrees in 12 hours. We will experience a steady drop in temperatures, beginning as early as late afternoon and continuing overnight. Lows will drop into the upper 60s, possibly even in the urban areas. It will be much nicer on Monday, with highs in the 80s along with comfortable humidity levels. To be honest, the “cool” down that I am advertising will essentially bring us back down to normal temperature-wise if we’re lucky. But it will feel a lot better considering what we’ve become accustomed to so far this summer. And, like I said, temperatures and dewpoints shouldn’t be going up significantly in a hurry. Although it will get more uncomfortable by mid-week, it won’t be nearly as bad as it has been, and there may be some more relief coming soon after.

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Medium Range Forecast: Relief to start next week

By: NYCvort, 2:04 PM GMT on July 22, 2010

Despite a few painfully quick teasers, the heat and humidity has been a seemingly never-ending feature in our weather for most of July. I’ve actually lost track of how many heat waves we’ve had so far this year. Anyone who likes the heat/humidity has seen plenty of it throughout the NYC metro area and continues to do so. However, for those of us who are wishing for the heat and humidity to break, I have some data that points to the possibility of some much needed relief.

The PNA, which has been strongly negative according to ESRL/PSD and NCEP, is forecast to make a significant reversal and go slightly positive for the last week in July before returning to near neutral to start August. A positive PNA will support troughing in the northeast. However, the EPO, which has been a bit negative (thus the flatter/slightly troughy pattern currently in place), is expected to turn positive once again late this week as an upper low drops into the Gulf of Alaska. Teleconnections downstream relative to this feature support ridging in the west central US and a trough in the northeast. In addition, a combination +PNA +EPO pattern will likely result in shorter wavelengths, thus a shortwave ridge often centered toward the central western part of the US with a trough having the opportunity to affect the northeast.

As such, the European, GFS, and Canadian models all show a trough digging into the northeast late this weekend before lifting out in an NAO pattern which will have turned rather neutral. The AO oscillation, which really hasn’t been negative since the significant cool down to start July, is not forecast to go negative but rather to stay very weak positive. The AO ensemble mean is showing a more positive AO than I anticipate over the next couple of weeks. I am lowering the AO a bit in my forecast based on the ensemble mean’s trend to forecast a bit too high in the last couple of weeks. Still, I think the AO will remain at least slightly positive. Based on this, the first trough may not be able to dig quite as far south as the models are showing, and since this follows the models’ trend this year to dig troughs too far south in the eastern US, I am especially cautious in my prediction. But in a sense, the fast moving trough crossing through today will help carve out a path so that this more significant trough can dig farther south. The models are in disagreement in handling the upper low discussed above which will cut across southern Canada early next week, and depending on which is right there may be another opportunity to feel the effects of a trough by mid-week.

When I combine all of these ideas together, the teleconnections, the +PNA pattern, along with the models’ consistency of depicting this trough in every run, is enough for me to believe that we will lose the oppressive heat and humidity late this weekend and early next week. The initial trough will drag a cold front through, and after one more surge of heat/humidity along with a threat for strong storms, the humidity will lower and it won’t be as hot. The way in which our weather could remain more comfortable for a prolonged period is by way of troughiness persisting in the Gulf of Alaska, but I’m not locking in on how long I believe the “cool” down will last. However, I will say that with relatively good agreement on the PNA remaining neutral, after this Saturday I don’t foresee any record challenging heat through the first few days of August. If the GFS is correct in its depiction of troughiness remaining in the Gulf of Alaska, then teleconnections downstream would suggest continued comfort for the northeast. But this is still a long ways out.

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Severe thunderstorms possible this evening

By: NYCvort, 9:56 PM GMT on July 21, 2010

More on the thunderstorm threat: With a strong shortwave dropping down from western NY this evening, there is a threat for severe weather. A severe thunderstorm watch was posted for the entire area, and I believe this is quite warranted. Lifted indices dropping below –4, coupled with mid level CAPEs of 1250 to 1500 J/KG and surface based CAPEs close to 2000 J/KG, along with a fairly strong jet, will set the stage for showers and strong thunderstorms this evening, possibly severe. Much of the activity so far this afternoon has been just to our north, but we will likely get into the action this evening. The shortwave is strengthening as a result of left front quadrant jet divergence to our north, and it will be quite strong for this time of year. This strong shortwave will be interacting with a very warm, moist air mass at the surface to produce these storms.

Updated: 9:59 PM GMT on July 21, 2010

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Fast, wavy zonal flow pattern continues

By: NYCvort, 5:09 PM GMT on July 21, 2010

As expected in the recent fast zonal flow across the northern US, the models have been experiencing difficulty in timing the individual disturbances embedded within the flow, and just a small deviation in the intensity of each disturbance can mean a noticeable difference in air mass. With the low level thermal front near our area, the NAM and GFS have been waffling between when the front will be to our north or to our south. Dewpoints swayed back and forth from the 70s and 60s on Monday ahead of and behind thunderstorm cells, respectively. There may be some relief from the heat/humidity late this weekend, but in the meantime, take advantage whenever the surface air flow has a northerly component and lowers the humidity temporarily for a bit. Keep in mind that air temperatures in the low to mid 70s can feel either oppressive or quite comfortable depending on the moisture content of the air.

A strong shortwave and associated cold front will approach late today along with showers and thunderstorms. Thanks to the intensity of the shortwave, a trough will briefly dig in, and behind the front it will not be as hot and it will be much less humid for Thursday with a comfortable dry northwest flow. Thursday night will also be comfortable because although heights will be rising aloft, we won’t feel it at the surface with a very light to calm wind. However, the progressive nature of the flow will ensure that a ridge builds back in by Friday with heat and humidity increasing once again for the start of the weekend.

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Very unsettled weather in fast zonal flow

By: NYCvort, 3:04 PM GMT on July 19, 2010

A shortwave came through this morning and combined with a pre-frontal surface trough to produce a line of showers and thunderstorms. We will see a break in the action as we head into the afternoon; however, a second shortwave will approach in the fast zonal flow late in the day as the actual cold front nears. This will set the stage for more possible showers and thunderstorms. The atmosphere has now stabilized somewhat, so we will have to see if some afternoon sunshine combined with this feature will be enough to become unstable again. I think it will, but it will be a bit more difficult than if the line of storms hadn't moved through this morning.

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Slow moving cold front ushered in little change

By: NYCvort, 4:12 PM GMT on July 11, 2010

The ocean low was captured by the southern stream jet cutting across the country, and these two features helped to completely demolish the heat ridge that baked us with 100+ degree weather and a four day heat wave. A cold front slowly approached yesterday due to a great lack of mid level airflow. This lack of flow, when combined with close to 2 inches of precipitable water amounts, resulted in the development of showers/thunderstorms capable of producing heavy rain. The storms developed and hardly moved at all, seemingly until the atmosphere just couldn’t produce anymore rain! An upper level trough will approach before lifting tonight. This will keep our weather unsettled for the remainder of the weekend, even despite the passage of the cold front last night. With a surface trough, some leftover moisture, a good deal of upper level cyclonic curvature, and the upper trough/some energy remaining just to our west, I can’t completely rule out a shower or thunderstorm this afternoon. High temperatures will be in the upper 80s with moderate humidity. The lower humidity should last into Monday but that’s about it.

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NYC’s 1st heat wave of 2010: Losing heat but not steam

By: NYCvort, 4:54 PM GMT on July 07, 2010

The heat wave continues, but the worst of it is over. This is now the fourth consecutive day of 90+ degree temperatures, going above and beyond a classic heat wave. High temperatures yesterday broke 100 degrees across most of the area, with the only exception being across parts of northeastern Nassau County where highs were in the upper 90s. Yesterday’s high temperature in Central Park was 103 degrees! Records were broken at all of the major airports, as well as the Park. Temperatures remained in the 80s across much of the area last night. The upper level pattern will retrograde a bit through the end of the work week as a closed oceanic low over the western Atlantic moves back toward the coast. Temperatures at 850 mb will still be in the upper 60s today, supporting highs in the upper 90s to near 100. Winds at the upper levels are shifting northeast as the ocean low retrogrades, and as a result, a backdoor cold front will move through later this afternoon and early evening. Surface winds will shift southeast and break the oppressive heat tonight as lows will drop into the 70s for the first time in three days for the urban areas. 850 temps will be falling into the low 60s by tomorrow morning, so it won’t be as hot tomorrow, but it will remain humid as we will be entrenched in a maritime tropical air mass. There is a slight chance of a shower or thunderstorm behind the backdoor tonight.

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NYC heat wave expected to begin on Sunday

By: NYCvort, 3:52 AM GMT on July 03, 2010

As expected, the AO oscillation took a drop and when combined with the omega block across the country provided us with a break in the heat and humidity for three days now. The one thing that did surprise me was the ability for the surface to heat up even despite the cold temperatures throughout the atmosphere. It just goes to show what a relatively high sun angle combined with very long days can do. Yesterday’s observed morning mid-level temperatures in the low to mid 40s translated into afternoon highs in the mid 70s, with upper 70s to around 80 in the urban areas. This means that Kennedy Airport, which hit 80 degrees, actually attained an afternoon high that was more than 35 degrees higher than the 850 temperatures! This was because of the ideal northwesterly wind direction, which creates a downslope flow and blows right through much of Manhattan and Queens, before getting to Kennedy.

The omega block is only breaking down slowly due to additional energy dropping into the upper low just off the US/Canadian west coast as well as the very strong –QBO which has dominated our weather pattern since last winter and continues to remain a key player. One final push of cold advection came through last night. An upper ridge has already begun to build into the area today, but it will take another day before we start to feel the effects of that ridge here at the surface as relatively cold air still remains in the mid levels. The position of the ridge with an axis near the Ohio Valley will support high pressure around the southern mid-Atlantic/southeastern US. The ridge basically stalls out as the base of the trough that just brought us the pleasant weather closes off and gets caught between a western Atlantic ridge and the ridge stalling over the eastern US. I feel that this will create more of a westerly component to the low-level flow, thus keeping the humidity levels rather moderate for a while instead of the air becoming steamy right away like it did recently. However, as the ridge remains stalled out and energy is thrown at it from all sides, it will eventually weaken and the associated high will start to disappear and it will become oppressive.

Even though it won’t be oppressively humid until later next week, I do expect the first heat wave of 2010 to develop. Last night’s European model forecasted 850 mb temperatures in the mid 70s at the mid-levels of the atmosphere by the middle of next week! The GFS doesn’t show anything like that, though, so I wouldn’t take it as fact yet, but it’s certainly something to look at. Regardless of whether it’s 850 temperatures in the 70s or upper 60s, it really doesn’t matter much because it all suggests a heat wave will occur. All we need for a true heat wave is three days of consecutive 90+ degree high temperatures. Considering how the summer has gone so far, the good track record of the Euro, and the anticipated pattern setup, I believe a heat wave will take place early next week in NYC. Because both the ECMWF and ECMWF ensemble mean show much warmer mid-level temperatures than the GFS, I’m going to go above the GFSX MOS guidance and forecast upper 90s by mid next week.

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About NYCvort

I am a meteorologist from New York who has been studying and forecasting the local weather for years. I especially enjoy tracking winter storms.