New York City Metro Weather

Cool start to the week turns very warm, with one string attached
Posted by: NYCvort, 3:55 AM GMT on September 20, 2010 +0
The jet stream will remain near the US/Canadian border through the week, with relatively low amplification of the features embedded in the flow. A weak shortwave will pass overhead late this evening as Hurricane Igor begins to temporarily enhance the mid-latitude flow pattern and develop a progressive trough over the western Atlantic. Heights will not come down significantly in our area as this trough is focused to the east, but with a mid level high pressure ridge near the Tennessee Valley, and Igor in the vicinity of Bermuda, cooler air will get funneled down from the north tonight through tomorrow:


This means that 850mb temperatures that were in the mid 50s today (which resulted in highs above 80) will fall into the mid 40s by late tomorrow. A weak cold front will move through this evening, followed by high pressure building over the Great Lakes region on Monday. The gradient between the high and Igor passing well to the east will make a for a breezy day, and with good mixing highs will still reach the mid 70s despite the cooler mid level temperatures, but with significantly lower humidity it will feel more comfortable. The high will move overhead early Tuesday, and as it gets very close Monday night, winds will lighten and with the low dewpoints, temperatures will drop off nicely. We should see lows by early Tuesday morning ranging from around 50 in the nearby suburbs to the mid 50s in the city. The upper high pressure ridge builds over the southeast during the day on Tuesday, with rising heights over our area. However, a lack of good mixing with the high in the vicinity will keep temperatures from getting above the low 70s. Good warm advection will send 850mb temperatures from the upper 40s early Tuesday morning into the low 60s by late in the day. This will set the stage for temperatures to rise well into the 80s on Wednesday, and with increasing humidity levels as the high moves farther offshore. In fact, mid level temperatures remain high right through the end of the week. Take a look at the upper level pattern for Thursday and note the red shading that depicts the strong ridging in our vicinity:



However, something important happens Wednesday night to help counteract this warmth (the “one string attached”). A very weak mean trough over the northern Great Lakes gets replaced with a building ridge. Combine this with an upper level confluence zone from converging split polar and Pacific jet streams over southeastern Canada, and we get a strengthening area of surface high pressure cutting across to our north. Take a look at the top left panel in the image below. Note how the black height contour lines coming from both the western US and Canada come together over Quebec. This is the confluence zone. Then take a look at the top right hand panel and note the surface high that consequently forms in the vicinity:


This feature will be significant to us because it will send a backdoor cold front through, and the easterly component to the flow will keep high temperatures in check, and the humidity should lower a bit in the modified air mass. There could be a shower associated with the front, but a lack of moisture in the atmosphere may very well keep us dry. The ridge axis will pass overhead Thursday night, the high will slide offshore, and temperatures will go back up again with increasing humidity once again for Friday. We will still have more of an onshore component to the flow on Friday than on Wednesday, which may keep highs at the city/coast a bit cooler than those inland.
Updated: 4:04 AM GMT on September 20, 2010   Permalink | A A A
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Severe storms rip through Brooklyn, Queens & Staten Island
Posted by: NYCvort, 5:25 PM GMT on September 17, 2010 +0
What started out as a calm cool morning turned into an unexpected nightmare for many New Yorkers. Showers and thunderstorms approached as anticipated ahead of a warm front, but the storms unexpectedly bombed out right near New York City. This begs the question—what happened? I looked back at the mesoscale analyses between 5 and 6 pm yesterday.

The thermodynamics of the atmosphere at the time were supportive, but not all that impressive. There was a little over a 1000 J/KG of mid level CAPE. There was also a little over 40 knots of 0-6km wind shear, which is decent but not fantastic either. The severe storm development was a balancing act, as it often is. There was plenty of shear to the north and west, but the CAPE decreased substantially farther to the north. The Bulk Richardson Number (BRN) takes into account both CAPE and shear, thereby outputting the best areas for severe weather development. When you combine the thermodynamics and the wind shear, where the BRN was above 10 corresponded roughly accurately to where severe weather was reported. Take a look at the image below showing the BRN at 5 pm. Note how Staten Island, Brooklyn, and most of Queens are all in the purple shaded region, while much of Manhattan and The Bronx are not. This is why The Bronx and Manhattan were spared from the worst of the storms. There just wasn’t enough energy roughly north of that line for severe thunderstorm development:


What I found to be much more impressive were the helicity values. There was over 300 m2/s2 of effective storm-relative helicity in portions of New York City. With that much helicity, whatever severe thunderstorms developed had a good deal of rotation in them. The blue lines represent values of helicity:
There were signatures of rotation in the radar, and this is why the National Weather Service posted numerous tornado warnings. They will be inspecting the three worst affected boroughs later today to see if there actually were any tornado touchdowns.

The kinematics of this event were impressive as well. There was a strong shortwave approaching, leading to a substantial amount of divergence in the upper levels of the atmosphere:
This is what caused the lift in the atmosphere that led to the showers and thunderstorms developing in the first place. However, without the right thermodynamics in place, the shortwave alone would not have led to severe weather development.

One final thing of note is that there was a 35 knot low level jet, which helped to mix down the strong wind gusts and fed additional moisture into the storms.

After seeing the radar images last night, analyzing all of these factors, and watching a video someone took of a funnel cloud spotted in the area, I would not be surprised if some of the wind damage was caused by tornadic activity. We will find out for sure later today.

Feel free to post any of your own storm reports, observations, opinions, etc.

NYCvort
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Strong energy delivers early fall-like weather tonight
Posted by: NYCvort, 4:19 AM GMT on September 15, 2010 +0
A weakening upper low over Hudson Bay will spin off a new low over the Canadian maritimes late tomorrow. In the meantime, a strong cyclonically curved piece of energy (which will eventually become the new low) will send a shortwave through late tonight. You can see this energy as a swirl near the New York/Canadian border on the water vapor satellite image below. This is what will be delivering the early fall-like air to the region:


850mb temperatures hovered around 50 degrees today, so we were able to get into well into the 70s. Cold air advection associated with the shortwave will drop 850mb temperatures into the low 40s by early Wednesday. A cold front passed through earlier today, and dewpoints have dropped off significantly. Under the cold advection scheme, temperatures tonight will fall into the mid and upper 50s, with low 50s in the suburbs. The atmosphere will be well mixed tomorrow, but I still think the statistical guidance may be overdoing the high temperature potential. There will be plenty of sunshine with highs close to 70, and with breezy conditions and very low humidity, it will really feel like an early fall day. A southern stream disturbance currently over the west coast is expected to phase with the northern jet over the Great Lakes on Thursday. This clipper-like system will squeeze out every last bit of available moisture late Thursday in the form of rain. It will also drag a cold front through and then we will dry out once again on Friday.
Updated: 4:41 AM GMT on September 15, 2010   Permalink | A A A
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Upper low remains in control of our weather through tomorrow
Posted by: NYCvort, 5:09 AM GMT on September 10, 2010 +0
The sky had somewhat of a wintry look to it this evening, with dark clouds racing through from the north and some purply streaks. This is compliments of an upper low which has cut off from the main flow of air. The low will creep south across Maine through early tomorrow before getting picked up by an arctic jet trough. A trough over the west will cut across the country and partially break down the ridge currently positioned over the central US. Note the upper low over northern Maine. The yellow and brown coloring over New Jersey shows that areas from there on northward are under the influence of the low:



The axis of the upper level ridge will pass overhead Saturday night. The ridge will become squeezed between the approaching fast moving western trough and the aforementioned new arctic/polar trough combo. This will result in a shorter wavelength/higher amplitude upper level pattern across eastern North America. Follow the trough which extends from Greenland right down through Bermuda (the slow movement of this feature is the blocking mechanism that causes the ridge to amplify as the wavelength shortens):



The main effect that this will have on our weather across the northeast is that the surface/mid level high will remain to our north, ensuring that cool air will continue to stream down from Canada and precluding any kind of return flow of warm/humid air as the polar front remains just to our south. The high is over the Canadian maritimes Saturday night, with a surface flow out of the northeast:



Now let's step back to the present and look at the details. Lows tonight will drop down into the upper 50s as low level cold air advection continues and 850mb temperatures fall into the low 40s. There will still be plenty of clouds around tomorrow as the low slowly begins to open and pull away. A slight moderating trend will continue through Saturday but 850mb temperatures will stay in the 40s, thus ensuring that temperatures will be unable to get higher than the 70s. There will be a chance of showers on Sunday in association with the trough/weakening attendant surface cold front.
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Sharp temperature drop ushers in autumn air mass
Posted by: NYCvort, 4:40 AM GMT on September 09, 2010 +0
Today certainly featured a wide variety of weather conditions. This morning waking up it was muggy with showers. Then the sun came out, the humidity dropped significantly and it became very dry, but it got hot. This evening, the temperature dropped sharply after winds shifted to the northwest and the humidity rose to a more reasonable level. A breezy morning also turned quite gusty by nightfall.

This is all in some way related to a cold front that crossed through late this morning. Even though it got hot, dewpoints dropped off substantially into the low 40s as strong winds and subsidence mixed down very dry air from the mid levels. An upper low currently sliding east over Quebec will close off tomorrow. As a result, it will slow down tonight and turn southeast as it begins to cut off from the flow while a ridge slides over it. This will move the center of the low across northern New England tomorrow through tomorrow night. Heights will continue falling overnight and begin to level off during the day tomorrow. A sharp 850mb temperature gradient moved through this afternoon, and mid level temperatures in the low 60s this morning dropped to around 50 this evening, and will continue falling into the mid 40s by early tomorrow. The upper low will meander until it gets picked up by a high latitude trough on Friday. Until then, the northern stream trough will remain in control of our weather, with much cooler temperatures than we have been experiencing for a while now.

The Labor Day weekend cool down, which featured a stronger/broader low/trough combo and cool nighttime temperatures, still fostered warm daytime highs. This was the result of a strong westerly wind trajectory at the surface which resulted in significant downsloping. This was why my forecast highs for last weekend were on the upper end of the statistical model guidance. The center of the low remained over Canada, and we were positioned on the southeastern side of the low. This upper low on the other hand is weaker but more compact, and it will come farther south into northern New England. The main difference for us is that we will be on the southwestern side as the low digs east of our longitude, and this will make for a northwesterly wind trajectory at the surface as high pressure builds down from Canada rather than across from the midwest. This, combined with a lot of cloud cover and mid level temperatures in the mid 40s both Thursday and Friday, will result in cool daytime lows and highs this time around. Temperatures have dropped off very quickly this evening, and lows will be near 60 by daybreak. Highs tomorrow and Friday will be in the low 70s. There should be more sun on Friday as the low begins to pull away, but the wind will be stronger tomorrow and from a slightly better direction for some downsloping. This is why I believe that high temperatures will be similar both tomorrow and Friday.

I’m planning on updating again tomorrow in more detail with a look at the weekend as well.

Until then,
NYCvort
Updated: 5:04 AM GMT on September 09, 2010   Permalink | A A A
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Impressive upper low digs out a delightful Labor Day weekend
Posted by: NYCvort, 4:48 AM GMT on September 04, 2010 +0
Just by looking at the upper level weather maps, you can easily see that the seeds of autumn are upon us, as the low pressure systems are able to gain ground farther south and often win out over the summer’s persistent high. A developing impressive –2 standard deviation upper low responsible for kicking Earl away from the east coast will be digging into the northeast tomorrow. This will bring a significant change in air mass to our region for the holiday weekend. The trough axis associated with the low will become negatively tilted on Saturday and cause the trough to dig. Take a look at how much energy is circulating around the upper low. Some of this energy will be suctioned into the low and allow it to continue deepening through Sunday. This is significant because lately a lot of the upper lows that have affected us have been weakening as they approach, thus lessening their influence on our weather. This upper low is still young and growing in intensity. Pay special attention to the two X’s and red coloring over Pennsylvania. This strong jet energy will deliver the cool air into the northern mid-Atlantic and New England. Also note the small red “ball” near the Gulf of Maine. This is the upper level energy associated with Earl and will eventually add additional energy into the upper level system (discussed in more detail later on):



As a result of the jet energy discussed above, heights will be crashing at all levels of the atmosphere, with 500mb heights that have been well above 588dm from Sunday through Thursday falling to 564dm by late Saturday. 850mb temperatures starting out in the mid 60s this evening will plummet into the low 40s by early Sunday. This is a drop of over 20 degrees in less than 24 hours. The GFS continues to bring slightly lower mid level temperatures/heights down into our area than the NAM. I’m siding with the NAM because of its support from the ECMWF. Still, take a look at how cold the mid levels get over the northeast on Sunday morning according to the GFS:

Image courtesy of RaleighWx


At the surface, a strong cold front will barrel through this evening, followed by a secondary front tomorrow, and a very weak third front early Sunday. This series of cold fronts is the result of an interesting triple barrel low series up over southeastern Canada and the Canadian Maritimes. There is the surface low reflection of the strong upper level feature (#1), a new secondary low formation (#2), and extratropical low Earl (#3). It will become windy as a strong pressure gradient develops between these three surface lows and a high building over the midwest:



Now let’s step back to this evening and look at the details. The convection associated with the main front will die off tonight as it hits a mid level ridge developing between the trough and Earl. With the moisture pool associated with the upper low focused well to our north, we will be rain-free this weekend. Dewpoints will drop off tonight following the passage of the main cold front. Lows will fall into the upper 60s throughout the area. Since we will be starting out rather warm at the surface on Saturday and 850mb temperatures will be crashing at the same time, temperatures will rise gradually in the morning, level off by midday, and begin to fall late in the afternoon. Highs will be within a couple degrees of 80 along with windy conditions. Temperatures will plummet tomorrow night, dropping into the upper 50s, even in the city, and it will remain breezy into the evening. With plenty of sunshine and ideal mixing, temperatures on Sunday will recover into the mid and upper 70s after a cool morning. There should be more of a range in temperatures Sunday night as the winds finally die down. Even though temperatures will be able to warm during the day with full sunshine, good mixing, downsloping, and still a reasonable sun angle and day length, the humidity will be very low all weekend, with dewpoints in the low to mid 40s tomorrow afternoon through Sunday. With humidity this low, it will feel cool at night (good sleeping weather) and pleasantly warm during the day.

Hurricane Earl weakened substantially as it became sheared by the approaching trough. Earl didn’t directly impact New York City or the immediate suburbs. However, even though Earl wasn’t a major player in our weather directly, the remnants of Earl will be significant for all of us because Earl is expected to become entrained in the upper level low. Earl’s energy will actually form a new upper low Sunday night as it eventually combines with our initial cool air delivering energy cluster shown over Pennsylvania in the first picture. This will slow the trough from lifting out and continue the great weather right into the end of the holiday weekend. When you combine the already strong and deepening upper low with the remnants of a major hurricane, there will be a lot of energy spread out over a broad distance by Labor Day. Earl gives us roughly an extra day of nice weather as it causes the elongated low to spin over the Canadian Maritimes on Monday:

Image courtesy of RaleighWx


With the trough finally lifting out and our next weather maker still out over the northern Plains, a ridge will pop up over the southeast and mid-Atlantic early next week. This will bring warmer temperatures with highs making a run at 90 over the first couple days of fall. Go figure.

NYCvort
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About NYCvort
I am a native New Yorker who has been studying and forecasting the local NYC weather for years. I especially enjoy tracking winter storms.

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