New York City Metro Weather

First nor’easter of the season develops tomorrow

By: NYCvort, 8:17 PM GMT on October 13, 2010

Northern branch shortwave ridging to our north and the southern jet cruising through the southeast results in little flow over the northern Mid Atlantic today along with weak surface high pressure in control. This “calm before the storm” is deceiving. The MJO is currently moving into the strong phase 6 position that I forecasted last week. As I alluded, this is an El Nino-like phase, where nor’easter development would be possible given the right ingredients. The ingredients are there and they are gradually coming into position. Strong energy is beginning to separate from the flow over the northern Plains. There is also additional energy back over northern Saskatchewan. Note the upper ridge/high over the west coast—all of the energy is pressed to come right down the slide on the eastern flank of the ridge. All of these features are visible on this afternoon’s upper level pattern analysis:

Together, the two intense pieces of energy will carve out an anamolously deep cut off low over the northeast on Friday. In the meantime, the first piece will develop an area of low pressure near the Delmarva region late tomorrow. As a result, rain will overspread the NYC metro area. Then as the upper low forms on Friday, the surface low will intensify over New England, with a minimum surface pressure in the 980s. This will result in strong NW winds on the backside of the low:

Temperatures at 850mb could drop below the freezing mark for the first time this season. Despite good mixing and partial sunshine on Saturday, highs will only be in the 50s. Below is an image showing the system becoming vertically stacked over New England. This is the most impressive image of the nor’easter and its associated upper level feature that I could find:

As far as upper lows are concerned, this one will be on the progressive-side. As the HPC noted earlier this week, additional energy will already be breaking down the western ridge and a trough will be approaching the west coast all during the first half of the weekend. You can see both of these features on the image above. This will keep the upper low moving along rather than stalling out like the previous ones.

Updated: 8:20 PM GMT on October 13, 2010


MJO expected to strengthen & influence our weather pattern

By: NYCvort, 4:42 AM GMT on October 06, 2010

One of the interesting things that really caught my eye when I was looking at the weather data yesterday is the MJO (Madden Julian Oscillation) forecast by both the UKMET and GFS guidance. The MJO phase basically depicts where convection is located over the Pacific. The reason why this affects our weather is that a trough deepens in response to the energy associated with this convection, and teleconnections downstream relative to this trough will influence our weather pattern. Over the past couple of months, the MJO has not been all that impressive. However, this is all expected to change over the next couple of weeks as the MJO strengthens and orbits into phases 5 and 6. Below is the current MJO forecast according to the UKMET guidance:

The GFS is not quite as impressive, but it too depicts a similar idea with slightly less amplification. In October, phase 6 could be indicative of a rather impressive trough in the east, as indicated by Allan Huffman’s MJO phase composite study:

Consequently, the GFS has been bringing a nor’easter up the coast in its long range forecast, which could form in the kind of pattern that is expected to evolve. Of course you can’t follow the models verbatim that far out, but the MJO is a good indicator of where we’re going with the pattern. The increased convection associated with the strengthening MJO will cause an amplification of the upper level pattern downstream of the convection. This is evident on the GFS ensemble forecast for the middle of next week, shown below. Take a look at the image on the left hand side. Note the blue shading over eastern Asia. This is the upper level trough associated with a phase 6 MJO. Then continue to follow the pattern downstream of this trough. Note how wavy it is, with strong positive height anomalies over the central Pacific, and strong negative anomalies in the Gulf of Alaska. The wavy pattern continues right through the US:

Teleconnections relative to anomalies in the locations mentioned above would put a trough over the east. This is why after a brief warm up late this week through the middle of next week, I would expect to see temperatures drop to near if not slightly below normal levels by the mid to late part of the month.

Updated: 4:50 AM GMT on October 06, 2010


Cold advection continues to change our weather tonight

By: NYCvort, 3:43 AM GMT on October 02, 2010

One of the things that I like most about the weather is how the effects of the data and graphics that you analyze on a computer screen can be visualized by watching the sky. Walking up my block yesterday evening, the low clouds were racing from south to north. This evening, there were low clouds, and they were racing, but in the opposite direction, from north to south. This simple observed change tells the story of why yesterday it was so warm and humid, while today it is much cooler. The 850mb temperature map from this morning is a good depiction of the big mid level temperature contrast from ahead of the front where there were strong southerly winds and warm temperatures, while behind the front is a strong northerly flow and much cooler temperatures:

In fact, 850mb temperatures were above 60 degrees early this morning, but they have plummeted into the mid 40s under the strong northerly flow. There is not much to contend with as the weakening shortwave (this morning’s heavy rain producer) lifts up and out of the area and so does the moisture. Under the continuing cold advection scheme in the low-mid levels, lows tonight will be 50-55 in and around town. Despite the full sunshine associated with the dry southwest upper level flow, less than ideal mixing along with 850mb temperatures getting down close to 40 in the afternoon will cap highs tomorrow in the mid 60s.

On tonight’s water vapor, note the departing moisture plume and the dry weather building in from the west. Also note the white “comma” extending southwest from Hudson Bay—this is the strong shortwave that will help to dig the northern stream trough into the east and will likely eventually spawn a closed low late this weekend:

Updated: 5:40 PM GMT on October 02, 2010


Heavy rain on the way, then get ready for a transition day

By: NYCvort, 4:23 AM GMT on October 01, 2010

An upper level low over the southeast partially phased with a northern stream trough over the eastern Great Lakes will open early tomorrow and race northeast. This feature is currently creating a remarkable feed of moisture out of the Atlantic, which is visible on the water vapor satellite image below:

A swath of heavy rain currently to our west will pass overhead late tonight and early tomorrow. This rain is the result of the Atlantic moisture feed combined with the impressive dynamics of the lifting open shortwave. The surface low providing the low level convergence is associated with the remnants of Tropical Storm Nicole.

I’m sure everyone is ready for a change in the weather pattern, and tomorrow will be our transition day. Mid and upper level heights will fall tonight through tomorrow. 1000-500mb thicknesses are above 576dm tonight, but they will be sub-564dm by late tomorrow. 570dm is usually considered the cut off for comfortable humidity levels, so obviously our very humid air mass tonight will turn into a cool dry one by tomorrow evening. Look for temperatures to fall from the low 70s very early tomorrow morning into the upper 50s by late in the evening.


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.