Pattern Change In The Next 7-10 Days?

Lately, we have been looking for a possible pattern change in the next 7-10 days.  One thing that has me hopeful is the MJO. It looks to go into phase 1.

Phase 1 is an excellent phase for the NE/MA if you're looking for cold/snowy weather. You can see the 500mb composite for Phase 1 for December below.

You can see there is troughing near the Aleutians in the NE Pacific. This usually teleconnects to a +PNA. The +PNA is a ridge over the western states. Over the east, there is a trough. You can also see a -NAO, with the block WEST based as it's over Southern Greenland. This pattern is usually good if you're looking for an east coast snowstorm.

We might be looking at a possible storm in the Dec 15-22 period. Whether it's an east coast snowstorm remains to be seen, but if this pattern actually verifies, the possibility increases. 

First Snowstorm Of The 2011/12 Winter Upcoming!

So far, the winter has been brutal. The last major snowstorm to strike was the storm around Halloween. But now, we have another snowstorm baring down on the NE/MA, including the I-95 Corridor.


I will stress this. This won't be a huge storm. The pattern isn't ideal for a big storm. We currently have a -AO/+NAO and a -PNA. Typically this combination isn't going to allow for huge Nor'Easters along the coast, but we should see a plow able snowstorm come Saturday.


Let's look at a few models, shall we?


First up is the 12Z GFS valid at 24 hrs.

It clearly shows a -PNA. The look here is flat to almost troughy. This is due to the vortex in the Gulf of Alaska which helps to pin the PAC Jet underneath it. This jet helps to knock down every attempt at a building ridge in the west, also known as the +PNA. Because the flow is flat, shortwaves will typically trek west to east instead of diving over the ridge. This also leads to a flat flow in the east. Shortwaves will usually then phase into a trough. Because there is no trough, the shortwaves in the northern stream and southern streams stay separate. So we'll have a storm that travels eastward. The progressive nature of the flow will keep the storm weak and moving. There is no negatively tilted trough to allow the surface low to explode. We do have the -AO which allows the colder air to spill southeastward. The surface highs to the north help to keep in a NE flow, as known as CAD. They squash the surface waves south enough so even the coast will stay cold enough especially up near NYC. The NAO is till positive, so that reassures us this won't be a big storm. 


We can see something similar on the 12Z Euro valid at 30 hrs.

With all said, I only do snow maps for the NYC Metro Area. But because I hate doing maps, I am going for a general 3-6 inches across the area. 

To Snow Or Not To Snow

That is the question as we head into December. Right now, we are dealing with a strong La Nina. So, are we going to get cold and is it going to snow?

The NAO is expected to go negative. Some of the Ensemble guidance takes the NAO down to around -3, -4, which is pretty strong. However, the NAO is east based. The NAO needs to retrograde westward, and bring the highest height anomalies over Eastern Canada. With the-NAO west based, the blocking would hold in troughs over the Eastern US. The AO is expected to go negative as well. 

Problem then becomes what becomes of the Pacific. The Pacific is going to be quite unfavorable over the next couple of weeks. Why? The PNA is expected to be negative. Because of the Pacific Jet crashing into the Pacific Northwest (NW Flow aloft)  any of those s/w's will knock down the Western Ridge or +PNA. The Pac Jet is a staple of La Nina however. Typically, you'd want troughing in the NE Pacific as that would help to build up the +PNA ridge. 

The s/w's that do come onshore will not be able to dig south enough. What happens is that the energy amplifies and phases west of the Mississippi River Valley due to the -PNA. So, those storms head into the Lakes, and brings SW flow aloft over the East which floods the East with warmth.

However, because of the -NAO, it leads to more of a zonal flow over the east. The SE ridge is very evident in La Nina years, and each time a storm moves out, the ridge builds again. Sometimes, those G/L Cutters run into strong blocking, and secondary off the coast. Typically, they redevelop too far north, and don't undergo bombogenesis until off the NE Coast. We call these storms Miller B's. 

If the NAO was positive, we'd be dealing with a blow torch and a lot of warmth. 

If the PNA was positive, those s/w's will be able to dig further south and east, and be able to phase east of the Mississippi. You won't have phased energy heading into the Lakes, but rather a Miller A which can run up the east coast.

What do we want to look for?

The Ensemble guidance as of today do have the PNA index rising which can correlate to big East Coast snowstorms. The AO and NAO are expected to remain negative for the time being, and we'd like to see the -NAO block retrograde into Eastern Canada. The NAO allows for troughing to stay over the east, as well as blocking to keep storms from being progressive, quick movers. It can also hold in surface high pressure to the NW to allow for low level cold air to bleed in. 

Summery Pattern to prevail for next week!!

Hi guys, sorry for the absence, there just hasn't been much to write home about. It looks like we're going into a much more summery pattern. Why? Simple. Let's look at the upper level pattern. Right now, a trough is centered along the west coast. At the same, a ULL can be found over the northern part of Hudson Bay. The Polar Jet, which recently has dipped into New England will be shunted north of the Great Lakes into Canada. An ULL can be found over Iceland. This correlates to a +AO. At the same, the -NAO pattern from the winter remains, but it's now focused over rhe northern latitudes and is east based. The cooler airmass is now centered over Northern New England. An impressive ridge will build from the SE states, westward into the Plains. Relatively fast, zonal flow will rule the rest of the country, with the ridge to the south, and the trougb over the western states. A surface high will set up shop off the SE coast, which means a southwesterly flow becomes established not only in the low levels, but in the mid levels as well. A hot and humid airmass will become established over the Northern Mid Atlantic, except for mesoscale features, such as thunderstorms or sea breezes. From time to time, weak mid level disturbances will invade from the G/L. Most of the period should be dry however. Some "relief" might be found with a cold frontal passage later next week. But get ready for 80's and 90's with high dewpoints. Summer is here folks!!!

Forecast Discussion for May 20, 2010

As you can see on the WV imagery, the lead s/w trough that brought us the rainy weather is now heading out, NEward as it treks through New England. Another one is dropping southward over the area this morning. This has kept the low clouds in the picture for most of the overnight, and into the early morning hours. Regional observations show a few showers, relatively weak and scattered about, so it clip the western portions of the forecast area. Latest surface analysis shows weak low pressure moving out over Maine, and a surface ridge over the OH Valley. Both these features are very weak, so there is no pressure gradient between the two. A cold front front is movig down from Canada as well.

We'll begin to lose the clouds throughout the morning, as the s/w trough treks out. I'll go for mostly sunny skies by the early afternoon. The cold front will continue to trek southward, and a pre-frontal trough will develop out ahead of it. They'll be a few showers/thunderstorms, as indicated on a few short-range models such as the WRF. These showers should die out with the loss of daytime heating as they head southward with the front. So pops around 20% will be put into the grids.

As the s/w trough moves out with its associated cool pool aloft, temps at 850mb will begin to rise. We'll see temps between 10 and 12'C, which correlates to temperatures around 80 degrees or so. Afternoon seabreezes will be had at the coast, with an onshore flow ahead of the cold front, so temps there will be cooler.

Any showers or isolated thunderstorms will die out with the loss of heating. The cold front will run into the surface high that is building to our west, and should wash out with time. Temps will fall into the mid 60's in the urban corridor, Winds decouple over the interior, with decent radiational cooling conditions expected, with lows in the 50's as the airmas behind the washing out front is not all that cooler.

Forecast Discussion for May 16, 2010

Hi guys. Gonna do a forecast discussion for the next several days, let's get started.

A weak shortwave trough pushed through the area today, but that did not deter the beautiful weather we've had since yesterday. Scattered clouds can be found across the area, but that will lead to clearing skies this evening, before mid and level high clouds move in overnight. I am expecting party cloudy conditions across the metro tonight. The southwest-west winds should begin to diminish and a NW wind should become established aloft and a pretty decent radiational cooling night can be expected, especially away from the coast. I expect upper 30's to low 40's in the interior, and upper 40's along the coastline. Looking at the MOS guidance, I cut a few degrees off. But it has a pretty good handle on this.

The upper level ridge axis (576dm) will pass south of the area on Monday morning. This means the beautiful weather we've experienced will begin to go downhill. A cut off upper level low (cut off from the northern stream) will develop over the Tennessee Valley and slowly trek northeastward towards the NYC metro area. At the same time, a surface low will develop in the OH Valley. Then that low will secondary to another surface low pressure system off the Mid-Atlantic coast. Strong PVA is forecast to develop, that will help to allow the secondary low to intensify a little bit as the ULL closes off.

I'll increase clouds tomorrow afternoon, but not go to high chance pops until tomorrow night. Why? precip may initally be slow to develop before the coastal storm forms, plus there is not a lot of model support with regards to a stronger LLJ and insentropic ascent. Hoever, with the presence of a developing h85 theta ridge moving towards the area, the forcing for ascent will begin to increase. I will increase pops to likely on Tuesday

The NAM was an initial outlier, before the other models trended more northwest with the surface low. Better agreement regarding the NAM with the ECMWF/GGEM/GFS models. Because of this, we might not see such a tight gradient (northwest to southeast) in regards to the precip shield located with the surface low. The 12Z GGEM gets the low down into the upper 990's, while the ECMWF/GFS/NAM is weaker. With the strong PVA, I'll take the middle of the approach between the stronger GGEM and the weaker GFS/ECMWF/NAM. Undercut temperatures by a lot as the GFS/NAM were too warm. Mid to upper 50's will be the norm under a east-northeast wind in the low levels and moderate to heavy rain under a thick cloud deck. The H5 trough will be slow to move out, as it meanders eastward. Showers will linger into Wednesday evening before the H5 trough is kicked out by approaching high pressure at the surface and a mid level ridge. Temps a little warmer on Wednesday. Temperatures moderate rapidly under a building southwest flow aloft. Very well could see 80 degree temperatures next Friday. Latest ECMWF/GFS guidance show another upper level cut-off for next weekend, with another development of a coastal storm. Stay tuned!

Evening Update for Severe Wx

Hi guys. Going to do a evening update here.

Looked at the afternoon mesoscale analysis from the SPC and looked at surface observations, I noted broken clusters of thunderstorms, a few of them strong to severe treking eastward from the Apps into the Mid-Atlantic states. These storms will be capable of producing damaging winds and large hail, especially as they continue to transverse eastward into a swath of greater instability values. Noting 35-50 knots of 0-6km of bulk shear and MULCAPE values around 1500 J/KG and higher.

Destablization has been slow today.....mainly because of warmer mid level temperatures and some subsidence after the morning showers treked through the area. I've noted this across New Jersey, Eastern PA and SE NY. But in those same areas, the latest mesoscale models show an increase in instability, which has occured, with some storms turning severe in Central and Eastern PA. Bulk Shear increases a bit as well, and this helps to convection becoming better organized. Threat still includes large hail and damaging wind gusts, especially in Western NJ. In areas closer to the east, instability is not as prevalent, but there is higher levels of low level shear closer to the warm front, which could also lead to damaging wind gusts, even as instability levels are not as high. The front should be offshore after midnight, with drier air gradually working in on NW winds later on tonight.

Severe Storms This Afternoon?

That is the question being asked this morning. Gonna put the entire CWA area in a slight risk for severe thunderstorms, except for Suffolk County in Eastern LI, as they will be under a general risk for thunderstorms. That area may not see much in the way of destablization as they continue to sit in the marine layer.  As of 12Z, the surface warm has not yet cleared the area and the surface low, at 1015mb is located in Western New York with the surface cold front driving through NW Ohio. Instability will increase after the passage of the cold front, as models show about 45 knots of 0-6km bulk shear, and forcing increases for ascent. This leads to an enviornment for updrafts, mainly after 18Z or 3PM this afternoon. Favorable wind fields and the presence of some low level shear could make for damaging wind gusts mainly for areas southwestern areas of the forecast area, including SW Jersey. I am still iffy on areas further to the north, including Long Island. It will depend on how much destablization occurs after the warm frontal passage. I think Long Island will stay in an onshore flow, which will limit the amount of destablization present, due to a southerly low level flow over the cooler waters. Here, they will stay in the marine layer, but western sections of Long Island were kept in the slight risk. NYC and points west will warm into the upper 70's-lower 80's, while Long Island stays in the upper 60's-lower 70's with the onshore flow.

Dewpoints should continue to rise into the mid 60's. If everthing goes as according to plan, the presence of a cold front..forcing..50kts of 0-6km bulk shear..and at least 1000j/kg of MLCAPE suggests the potential for at least isolated damaging wind gusts by 18-21z.

Forecast Discussion for Thursday, May 13, 2010

Hi guys. Today was a raw, wet, cool, disgusting day. Will tomorrow be better? Looking at surface obs and short term model soundings, there is plenty of low level moisture in place, thanks to low level easterly flow. That means areas of drizzle and the potential for patchy fog in a few spots.

Low pressure is currently passing southeastward of Long Island and will continue to trek into the Atlantic tonight. Then high pressure will build in from the north, and a weak upper level ridge will build in. The nicer weather will be short-lived. This is what happens in a progressive pattern like this. Once the high moves offshore, the next frontal boundary from the Mid-West will move through. The GFS model continues to be too quick with bringing in precipitation tomorrow night. Even with the progressive upper level flow in place, it's still too quick with shortwaves moving through the flow. But there could be some showers later tomorrow night as a warm front approaches from the southwest. So I included pops for the NYC metro area as the warm front surges northward, as the area becomes warm sectored on Friday. As mentioned, the GFS becomes too aggressive with precipitation, while the NAM is further north with the precip. The SREF model becomes the reasonable middle alternative between the GFS/NAM and wll be used.

Ugly. raw and disgusting: Is this May?

That's the question being asked today. With reports of light snow in PA, and a WAA out for Ulster County, NY calling for up to 5 inches, I am beginning to wonder what happened to the warm weather.

We're going to be stuck in this weather up until the weekend. Reason for this: the surface warm front. The ULL, which affected us with the unseasonably cold airmass of this past weekend is now a 50/50 low and for now, is not going anywhere. This can be blamed to the west based -NAO which again has set up in the North Atlantic. So the pattern up there is somewhat blocky. The west based -NAO does now allow the surface warm front to move northward over the NYC metro area. Instead, the warm front will stall to the south of New Jersey.

Let's put up the 12Z NAM as an example.

What's fascinating about this? Looks at the temperature gradient. The 10M winds show a strong onshore flow (southeasterly low level winds) north of the warm front, while places SW of the warm front are in a southwesterly low level wind flow. This is as the surface low is still transferring east-southeast of Long Island and there is a lower level circulation just off the Jersey coastline.

The surface warm front is still well back in the OH Valley, but the 700mb and 850mb warm front has surged north of the surface warm front. Very dry, stable air is sitting north of the surface warm front in the cool sector, with dewpoints in the 20's and 30's. This is due to presence of high pressure nestled over New Jersey. But over Central PA where rain has fallen, temperatures have actually fell, into the upper 30's. So if the surface warm is still back over West Virginia, what is causing the rain to fall? Temperature difference in the mid levels to the surface. This is creating pretty decent instentropic lifting. There's your rain.

Clouds did increase, due to the presence of a weakening shortwave, being sheared by a PV in Eastern Canada, and a ridge in the southern states. This shortwave is treking NE in fast upper level, relatively zonal flow. The SE ridge heights were a little bit over-forecasted by the models, and because of this, the s/w is treking further southward as the height field bent southward a bit. At the surface, low pressure is slowly weakening as it treks eastward through the OH Valley, and will transfer to a weak area of low pressure off the Long Island coast. The surface warm front will attempt to approach overnight, and this combined with insentropic lift and ascent (southerly to southeasterly moist inflow) will provide for a period of light to moderate overrunning rainfall. Not expecting much in the way of QPF, about .50 inches or so. Easterly onshore flow should continue tomorrow with temperatures hovering in the upper 40's or so. The surface low pulls out tomorrow evening, with clearing skies. Should be somewhat of a temperature gradient with cool, moist marine air to the north of the warm front, as it rides along the nose of the 500mb ridge axis, and a more pronounced southerly flow to the south of the warm front.