Strong to severe thunderstorms possible on Thursday
|By: NYCvort, 4:16 AM GMT on April 23, 2011||+2|
Wed Night Update: Tomorrow’s potential severe weather threat
I would like to provide a few details about the severe weather potential for tomorrow that I highlighted last weekend. With a strong jet moving overhead,
we will have close to 40-50 kts of 0-6km shear:
Mixed layer CAPE values may spike to over 500 J/kg:
We will also see some turning with height in the low levels, though helicity values will not be too great. LIs will be up to just below -2 by midday tomorrow, indicative of marginal instability.
Pwats will be over 1.5 in. As the cold front approaches, we will see a squall line of heavy showers and strong thunderstorms, potentially approaching severe limits, especially from the city on westward. The main threat will be strong winds and heavy rainfall. Southerly flow off the ocean over eastern sections will likely lead to stabilization and substantially limit the potential there.
Current Water Vapor:
Original Discussion: Warm Easter Sunday expected amidst unsettled pattern
Through next Thursday, we will experience above normal heights over the eastern US. Coincidentally, this morning most suburbs experienced lows of at least 10 degrees below normal, evidence of the cold air mass that we are coming out of. So, this warming trend will occur in two notable stages. First, winds in the mid levels of the atmosphere have turned to the southwest this evening and will continue to increase overnight. This will lead to overrunning of warm air over the denser cold air that will remain trapped in the low levels as winds there remain out of the southeast.
This will lead to rain developing late tonight and continuing into Saturday. Temperatures will remain chilly overnight into Saturday with the cold air mass remaining in place in the low levels. In addition, winds will increase in intensity Saturday morning. This is in response to a strong gradient between departing high pressure and deepening low pressure over the Lakes from intense upper level energy riding northeastward (this energy can be seen on the 500mb panel of the image above). Then we will see the second phase of our warming trend, this one more noticeable to us directly. Winds will shift around to the southwest down here at the surface late in the afternoon. At this point, we can begin to advect in milder air. Dewpoints will rise well into the 50s, and temperatures will continue rising through Saturday night. This will set the stage for a warm Easter Sunday, with temperatures rising up into the 70s (even for coastal areas under a good wind configuration) with clouds and some peeks of sunshine, with more sun expected over southern areas as a mid level frontal boundary sets up just to our north and a surface stationary front nearly overhead. This will lead to unsettled weather continuing from Sunday through early next week. Expect to see some showers coming in late in the day on Easter as a weak low rides up along the front. That stationary front then drifts to the south behind the low on Monday with cooler conditions.
In the meantime, a trough will be lifting across the midwest as additional troughing digs down into the south central US sending another low through the Lakes. This time, though, with no strong high to keep the low level cold air locked in, deep layered south-southwest flow will allow for temperatures to soar on Tuesday with our region well into the warm sector.
Some areas may break 80. As that second trough then starts to close off over the midwest by mid-week, a stronger low will come up west of the Appalachians and send a cold front approaching late Thursday with showers and thunderstorms. With the trough turning negative as it approaches, dewpoints in the 50s, and strong instability, will need to monitor this for possible severe weather in our vicinity. Take a look at the interesting set up below:
This will then be followed by a big change in air mass for next Friday into the start of the weekend, but a rebound will quickly follow. However, looking further into the extended, the teleconnections and MJO support the idea of cooler weather returning to the northeast to begin May.
The ensembles are in good agreement in keeping the AO, which was some 4 standard deviations above normal earlier this month, falling to a negative state by the beginning of May. In addition, there are some indications that a negative NAO may return at the end of the period as well. The PNA, which has been in a negative state since the beginning of this month, may also turn slightly positive.
These ideas are also supported by the GFS ensemble forecasts of the MJO moving into phase 7 during the first week of May:
Research from Allan Huffman’s MJO study (http://raleighwx.americanwx.com/MJO.html) would suggest that an MJO in phase 7 during the month of May would indicate the following general pattern:
Note the trough over the Gulf of Alaska, ridge over the western US (positive PNA), and another ridge in the vicinity of Greenland (negative NAO). This teleconnective pattern would allow for an opportunity for a trough to set up over the northeastern US.
Here is the GFS ensemble forecast near the end of the period (two weeks from now, around May 7th):
Compare this image to the one from the May MJO phase 7 composite. Note the similarities. We have only a slightly displaced negative anomaly in the Gulf of Alaska in each image, a western North American ridge, and a northeast trough. We also have two very similarly placed positive anomalies, one just southeast of Greenland and the other over north central Canada. Not to mention a similar pattern near the pole and over the eastern hemisphere. It’s something interesting to consider going forward into May. If this verifies, it would equate to slightly below normal temperatures during the first half of May. With the warm sun this time of year, it would probably be a very refreshing bout of weather before the coming heat of the summer months.
|Updated: 4:24 PM GMT on April 28, 2011||Permalink | A A A|
Central Park may break 80 degrees on Monday
|By: NYCvort, 11:22 PM GMT on April 09, 2011||+2|
Over the past couple of days, a powerful trough has been digging into the western US. This deep trough will continue to act to pump up the heights over the southeast. As the trough translates eastward in the continued fairly progressive pattern, the ridge will bulge out ahead of it and bring up some very warm air into the region.
There may be some showers around Sunday night, but after the warm front comes through early Monday morning, any shower activity will cease as the mid levels dry out quite well, and once the low levels begin to follow suit, at that point the sun will come out. There will be a strong inversion, but we will be able to utilize the strong late summer-like sun angle to help us mix out the column. High temperatures will be very highly dependent on this. Another factor to keep in mind is the fact that winds will be out of the southwest, which means that locations to the east of the city will have more difficulty with the cold ocean air streaming overhead. Coastal locations would be the most susceptible to this.
Let’s take a closer look. I’ve chosen to use this morning’s high resolution WRF 2m temps as a model because they look very reasonable to me given the set up (the newer WRF from this afternoon is not warm enough in my opinion). The red shading is where temperatures would be in the 80s. Note how to the east of the city, there is a sharp downward slope in temperatures:
The reason why we have such a sharp gradient can be found in this second image above, which shows the WRF projected 10m wind. We can see that there is 10-20 kts of southwest flow across the region. Under this wind configuration, the air coming over Manhattan would not be passing over the water, so our full temperature capacity could be reached. Sea surface temperatures will be in the 40s, so areas to the east will be under the influence of cooler marine air. Suffolk County, LI and eastern Connecticut will be most affected by this. A second thing to consider using this wind map is the fact that there is so much boundary layer flow, and this would argue for better mixing capability, especially once the sun comes out.
The NAM and GFS MOS guidance both show highs on Monday in the 70s, and the ECMWF MOS shows us just barely getting to 70. However, I don’t think this is right, and I want to go over some of the reasons why I feel this way. First of all, the models usually have some problems with realizing how warm the low levels can become in the warm sector. In fact, 1000-500mb thicknesses will be nearing 570dm, which is quite summerlike. So, I think it’s important to take a look at the forecast 850mb temperatures:
They are forecasted to be around 60 degrees across the area by both the NAM and GFS. Furthermore, the NAM/GFS MOS guidance shows ceiling heights going unlimited in the afternoon, along with scattered clouds. This, coupled with 15-20 mph winds, will in my opinion allow us to add more than 20 degrees onto that number. To illustrate this more graphically, even the NAM forecast soundings show us mixing higher than 900mb by mid-afternoon:
For a situation like this one, given the reasons above, I do believe that Central Park has a good shot at getting into the low 80s. For areas southwest of the city, that will be a slam-dunk, while adjacent eastern sections will rise into the 70s. Overall, temperatures on Monday will be some 10 to 20+ degrees above normal from east to west across the region:
Following all of this “heat,” a cold front will approach Monday night. While areas well to the north and west of our region may be in for some strong thunderstorms on Monday, by the time the front reaches us, it will be losing its punch as its support (the trough) lifts out. It will also be nighttime, so the low level instability will be significantly diminished. This will result in a chance of showers and possibly a weak thunderstorm overnight.
Earlier forecasts were showing the front coming through, but now it looks like it may try and stall out as the northern stream energy lifts and a southern stream low attempts to close off. With residual mild temperatures in the mid and low levels (temps Mon night will stay in the 50s area-wide) and a lack of any real cold advection as the northern portion of the trough lifts into Canada, it wouldn’t have taken much for us to have another mild day on Tuesday if the sun came out, but with the models now a bit more unsettled we may have to deal with some lingering showers.
High pressure then builds in from the south and west, and more trough energy will enter into the western US in a reinforcing negative PNA pattern:
As such, with still no blocking downstream, we will continue to experience above normal heights. However, with the general synoptic scale air flow diminishing, we will be relying mainly on the strong sun to create our diurnal temperature cycle. This will lead to slightly above average temperatures right through the end of the week, with highs generally in the lower 60s and lows ranging from the 30s in many suburbs to the 40s in the urban areas. More unsettled weather is possible by next weekend.
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Fast moving pattern will keep our weather active this week
|By: NYCvort, 7:43 PM GMT on April 03, 2011||+2|
Before we get to this week’s discussion, I would like to take some time to review the failures of the “major storm” that I expected would impact the NYC area late last week.
Review of April Fool’s Day Storm
I have to say, I was pretty surprised about how last week’s storm system panned out. Given the set up, I really expected to see a more significant storm form off of our coast. The main thing that ended up holding it back was the fact that everything came together just a little too late. Parts of New England got hit with some snowfall after the storm started to get its act together. The graphic that I was showing you last week depicted a low that was more “tucked in” to the coast, but also stronger, and I think that was significant. Here is the actual analysis from last Friday morning:
Let’s begin by looking at the top left panel, which shows the 500mb set up. You can see how everything isn’t quite yet in phase. This can most easily be observed by examining how the height line crossing over the northern mid-Atlantic is not quite yet “in sync” with the height lines over the southeast. Also, look at the resulting 700mb pattern on the bottom left panel and note that we have only a poorly organized, just forming low with one closed contour. That all led to a weaker surface low, as shown in the top right panel, where there is a 992mb low (as opposed to the sub-988mb low I expected) that is already exiting the area. The weaker, more sheared solution led to significantly less QPF than I anticipated. If you recall in my discussion last week, my main expectation was that we would encounter a “major storm,” featuring rain and the best chance of an accumulating snowfall on the backside over the interior. In fact, we only really received light precipitation totaling a mere quarter of an inch of rain, which certainly was not a “major storm,” so unfortunately I have to scream at myself: “BUST!” Since we only received light precipitation, that meant that there was less opportunity for the dynamic/evaporative cooling. Like I said last week, this would have been the only way the atmosphere would have had the means to cool because there was no low level cold air filtering in on the backside. So that never happened. There was also the idea of a lack of cold air via the later phase. The northern stream really wasn’t giving its all just yet, which led to a warmer solution despite the fact that the low was farther off the coast. It’s a little hard to see, but if you can, note on the bottom right panel of the image above that the 850mb freezing line (solid black line running up and down the coast) is still only just nearing our area at that time prog, also leading to a warmer solution overall. By the time temperatures aloft finally got cold enough to support frozen precipitation, the springtime induced boundary layer was already warming as the morning progressed and led to midday precipitation the form of rain showers. So a combination of factors contributed to a downright embarrassingly small nuisance event for NYC.
This Week’s Weather Discussion
Regardless, it’s time to look ahead and see what interesting weather awaits us for this coming week. The last of our upper level trough will lift out through this afternoon followed by a ridge building through early Monday. Lingering marginally cold air exists in the mid-levels, but excellent mixing under aligned northwest flow has allowed highs to climb well into the 50s, which is right around normal for this time of year. However, our pleasant stretch of weather will not last as rain will develop ahead of a warm front tonight. That warm front is then expected to pass through our area midday on Monday with a real change in air mass. Dewpoints will rise and temperatures will top out in the low 60s and then stay at least in the 50s right through Monday night and into Tuesday. A very powerful phased trough will send a strong cold front through the area early Tuesday, with a line of convective showers and possible thunderstorms expected just behind the front:
We will also see temperatures drop quickly Tuesday morning after the front/rainfall moves through with a very sharp thermal gradient (see bottom right panel on the image above). Lows Tuesday night will fall back into the 30s. But it’s important to note that once again this cold push will be the result of a negatively tilting trough, with no cold high coming in behind it. This colder air will be driven by low heights, and heights will be rising even during the day on Wednesday, with a quick recovery into the 50s as high pressure builds in from the south. We will briefly encounter zonal flow mid-week. A weak disturbance in the northern stream will ride to our north and push through a weak low which could bring a round of showers to our northern communities Wednesday night. High pressure then moves across southeastern Canada with some weak low level easterly flow. In the meantime, a trough will dig into the western US late in the week and release closed energy sitting off the coast across the country and into our region. Though it will be sheared, the moisture associated with it will likely override the colder low level air provided by the easterly flow leading to more rain chances on Friday.
Take a look at this ensemble image for Friday:
Note how we have a trough over eastern Asia, followed by a ridge, then another trough near the eastern Aleutian Islands, followed by another ridge, and then another trough over the western US. This is all indicative of the fact that upstream of our area, everything keeps on moving along rather quickly with numerous disturbances all contributing to a continued fast-moving pattern with active weather. So even though the western trough will temporarily pump up the heights in the east and lead to warm weather next weekend, the large scale pattern argues that even that deep trough will be slowly moving along with the next trough over the eastern Aleutians as of Friday pressed to approach the west coast by the middle of the following week. This should continue to keep the weather rather changeable at least for a time in the extended. We certainly won’t have any resistance downstream over the Atlantic, with the NAO now positive and generally expected to remain that way right on through the period:
Have a great week
|Updated: 7:46 PM GMT on April 03, 2011||Permalink | A A A|
I am a meteorologist from New York who has been studying and forecasting the local weather for years. I especially enjoy tracking winter storms.
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