New York City Metro Weather

Earl now a cat 4, but is he a threat to the east coast?

By: NYCvort, 4:45 AM GMT on August 31, 2010

There has been a lot of talk in the media about the potential for Hurricane Earl to affect the eastern seaboard late this week. This morning’s increased coverage of Earl’s track was sparked by a significant westward shift in the models last night. The models settled a bit eastward this morning. I would like to take the time out to give my own opinion on Earl. First of all, I want to briefly say that the way in which tropical systems can threaten the northeastern United States is how the system, which is initially moving through a belt of easterlies, interacts with the westerlies cutting across the country. Danielle posed no threat to us because she was steered away by the trough that deposited cool dry weather late last week. We were also protected by the cut off high that still remains in place. The difference with Earl is that the trough which is expected to interact with this hurricane is still going to be to our west as Earl approaches. We will no longer be protected by the cut off high as the trough should have already begun to break down the eastern US ridge.

Earl will ride around the Bermuda high and eventually get suctioned by the trough approaching from the west. In order for Earl to have a chance of a direct impact along the east coast, the trough would have to be both delayed and deeper.

The GFS model shows Earl miss the east coast. Take a look at the dip in the heights (depicted by the colored contours) over the western Great Lakes. The trough is far enough west, but most of the dark green contour remains over Canada:



The GGEM (Canadian) shows Earl in a similar spot as the GFS, but upon closer observation it is for a different reason: the dip on the GGEM is farther east, encompassing the entire Great Lakes region. However, look at the very close spacing of the contour lines, and the bright green dipping into the Lakes. This model shows a trough that is significantly deeper than the GFS:



The ECMWF (European) model is the farthest west with Earl. Take a look at the dip in the heights over the western Great Lakes. The dark green is firmly established across much of the western Lakes, signifying the sufficient depth of the trough (though not quite as deep as the GGEM, it is deep enough). However, the difference with the ECMWF is that the trough is far enough west. Thus, the trough is both far enough west and deep enough:


Images courtesy of RaleighWx

When I first looked at the set up where Earl was expected to make a run toward us but then be jutted out to sea, what immediately caught my attention was my concern about the trough being delayed. I felt that this could very easily occur because it has been the rule for much of the summer. Troughs are delayed and the heat lasts longer than we anticipate. The tendency for the long wave weather pattern to move slower along has been supported by a +AMO, which causes a -NAO pattern to be established in the North Atlantic much of the time, hindering the progression of weather systems. So when it was obvious that delaying the approaching trough would put the east coast at a greater risk, I was immediately concerned. A delayed trough would allow the storm to come closer to the coast before recurving, similar to the ECMWF.

Now on the idea of the trough being deeper, based on the trends this summer, the ridge has almost always won out and the troughs have been shallower than predicted. However, I have noticed in the past couple of weeks that troughs have been verifying as deep as depicted by the models showing troughs with greater depth. This was one of the reasons why Danielle was easily steered away by the trough last week—it was as deep as the models were forecasting. We did have much cooler weather, and the lows at night even in the city were actually a little lower than I expected. So whereas earlier in the summer the troughs were shallower, I think that now as we are heading toward the fall and sun angle is lower and the days are longer across North America, the support for deeper troughs has returned. A deeper trough would create more force to pull Earl upward toward the northeastern US coast, like the ECMWF. So putting together the idea of a deeper delayed trough, this has caused me to side with the models which are farther west with Earl.

The one thing that continues to bother me about forecasting Earl to track near the coast is how far out to the east the consensus remains amongst the tropical cyclone models. The reason for this may be that they do not have as good a handle on the trough which is currently out west because they are just that—tropical models. I would tend to believe that synoptic models, like the ECMWF and GFS, would do better on the overall pattern than models developed specifically for tropical systems. The ECMWF did turn out to be right with Danielle when some of the other models, including many tropical cyclone models, were forecasting a direct Bermuda hit (which turned out to be wrong). The ECMWF has been persistent on the farther west idea of Earl for a while now, and the other synoptic models have joined in somewhat during the past day or so. Also, some of the tropical cyclone models have trended a little closer to the coast as well. Some of the new model runs give the approaching trough a slight negative tilt, which could help to pull the storm inward.

It is still too early to tell whether or not the eastern seaboard will be directly affected by Hurricane Earl. The question for me is not so much how intense Earl will be, as he is likely to be a hurricane due to the warm ocean water and initial lack of wind shear. The question remains where he will go, which will not be determined by what is going on in the tropics, but rather by the trough that will be approaching from the mainland US. Even though my gut is still questioning the likelihood of Earl tracking closer to the coast, scientifically I am putting my token on the western edge of the guidance. However, this will need to be monitored, and the only thing that is certain right now is that Earl will cause another round of high surf and rip currents at the ocean beaches. Watch the trough—this will be the key to Earl’s movement later in the week. A deeper, slower trough would increase the chance for Earl to affect the coast. Just as an aside, if the deeper trough does verify, then we could be in for some very pleasantly cool weather for Labor Day weekend.

Updated: 4:56 AM GMT on August 31, 2010

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Trough delivers cool air mass that slowly modifies as heights build

By: NYCvort, 9:55 PM GMT on August 26, 2010

There is an expansive ridge over the Rockies providing fair weather across much of the country. This same ridge will become a major player in our weather this weekend and next week as it migrates eastward. The upper low that kept cool and unsettled weather in our area for the past three days was picked up last night by an approaching trough. This left us with mid level temperatures in the mid 50s overhead, but with the flow now turned around to a drier westerly direction. Sunshine and an offshore flow have boosted highs into the low 80s after a bit of an early morning struggle to overcome the lingering ground level moisture. A cold front swept through earlier this afternoon, and dewpoints will continue to drop through the 50s and into the upper 40s by tomorrow. A weak cap associated with mid level ridging has prevented convection from developing today ahead of the front. A shortwave partially attached to an upper low near Hudson Bay will move through this evening, and this will drive in cooler mid level air, with 850mb temperatures dropping into the upper 40s by early tomorrow. A very comfortable night is on tap with lows dropping into the upper 50s in the suburbs and lower 60s in the city. A substantial surface high will quickly build in behind the front, with clear skies and lightening winds. As a result of 850mb temps only in the upper 40s combined with full sunshine and lighter winds, highs tomorrow will reach the upper 70s. With mid level temperatures this low and still some damp ground, I would expect a cool morning with temperatures rising only slowly. A deep trough balancing out the western ridge will control our weather through Friday, with the coolest air mass overhead that we've had since early in the summer.

Energy rounding an eastern Pacific ridge will cause a trough to dig into the west coast and deepen. This will force the western ridge to spill quickly into the east. A shot of energy well to our north will help send a weak second shortwave through Friday night, but heights will still be rising fast because of the shortwave’s focus being so far to the north and the ridge building in. The more significant effect of this second shortwave is that as the energy well to our north slowly drops south as the shortwave lifts out, it is expected to deepen slightly over the Atlantic and help to capture Danielle. For us, this will allow for one last bout of remaining slightly cooler mid level air to be forced through, with 850mb temps staying close to 50 degrees Friday night. Under a nearly calm wind, lows will once again drop into the upper 50s in the suburbs but into the low to mid 60s within the city’s urban heat island. The upper level ridge remaining centered just east of the Ohio Valley through the weekend will position the temperature-dependent 850mb high southwest of us over the central mid-Atlantic. This means that a northerly flow around the high will help keep us shielded from the heat overriding the high to the north until it makes its way all the way around and the air mass begins modifying. This will result in only a slow rise in temperatures this weekend despite heights surpassing 588dm at 500mb on Saturday. 850mb temps will still be in the 50s, and with the surface high just to our west, winds will be even lighter than Friday making for less than ideal mixing. Highs will rise into the low 80s with mid 80s in the city. Before the air mass has a chance to modify, nighttime lows will remain comfortable under a continuation of low dewpoints/humidity, with 60s expected in the city one more time Saturday night.

High pressure will remain in place through early next week. This will allow for nearly full sunshine and clear skies right through the period with no rain chances. An impressive +594dm upper high is expected to build over the northern mid-Atlantic as the ridge becomes sandwiched between Danielle to the east and a trough over the west coast:

Image courtesy of RaleighWx


Temperatures will be on the increase each day. As the once dry air mass gradually modifies, dewpoints will be on the rise as well but remaining capped in the 60s. A late August heat wave could be upon us, with 850mb temperatures well into the 60s, possibly approaching 70 at times. This will send highs soaring into the 90s, with the hottest day appearing to be Monday. One thing that I think could prevent temperatures from skyrocketing is that high pressure over the area will keep winds light and may prevent the atmosphere from becoming fully mixed. Lows at night will continue to be on the cool side, especially outside of the city, and in order for us to be affected by the warm mid level air it must get mixed down to the surface. The light winds will also prevent any kind of significant downsloping effect, thus limiting highs. These two factors will help to counteract the potentially very hot effects of a strong upper high over the region.

Updated: 12:47 AM GMT on August 27, 2010

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Southern branch disturbance stalls and cuts off from flow

By: NYCvort, 12:20 AM GMT on August 22, 2010

A very interesting weather pattern is setting up across the mid-Atlantic region late this weekend into early next week. As the aforementioned compact upper low skirts off into the Atlantic, an upper high over the Canadian Maritimes is expected to open up tomorrow while a southern stream disturbance approaches our area. There is broad troughiness over western Canada, resulting from a series of small upper lows, and a compensating ridge building up through the Canadian Maritimes. As the shortwave travels eastward across the country, it is moving further and further away from the northern jet stream flow, which is up across central Canada (as a result of the Canadian Maritime ridge sending it northward). This will allow the shortwave disturbance to close off as it becomes detached from the flow and dig southward, eventually either resulting in a weakness or stark separation in the persistent ridge across the southern US and western Atlantic. The feature will not be able to continue moving along as a result of the strong western Atlantic ridge, so it will become sandwiched between pieces of the ridge over the southern Plains and western Atlantic. In order to exit our area, it needs to either (a) be able to move forward or (b) get picked up by a northern branch trough. The western Atlantic high will not allow A, and B can not occur until the trough shown in the picture over the west coast breaks down the northern Plains extension of the ridge. Below is the 500mb pattern off of the NAM for tomorrow night:





The NAM is much stronger with the cut off low than the GFS. One thing that I was concerned about is convective feedback error, but I feel that there is synoptic support for such a feature to develop based on what I discussed above. Convective feedback error could still be the reason why the upper low is stronger on the NAM because my reasoning was for location and not strength. You can tell by looking at the 500mb vorticity charts that the NAM adds quite a bit of energy to the system tonight and tomorrow over upstate New York while the GFS is more modest. This added energy may very well be the result of convective feedback error. Therefore, I am not ready to give the NAM solution my full support just yet.

The weather conditions for Sunday will not be significantly affected by the NAM/GFS difference. Rain will develop tomorrow morning ahead of the shortwave and continue into tomorrow night. This rain will be feeded by the dynamic lifting ahead of the wave as well as moisture coming in from the Atlantic. The stronger NAM solution does wrap some more tropical air into the system from the southeast, so it would end up being a bit more humid tomorrow if this plays out, with dewpoints in the mid 70s as opposed to low 70s. It will still be very humid either way.

It gets a little more dicey as we head into Monday. The NAM statistical guidance shows that most of Monday will be dry, while the GFS suggests that rain is likely much of the day. Let me discuss why this variation exists in the models. The NAM begins to close off the shortwave tomorrow over western NY, while the GFS waits to do it until Tuesday over the Delmarva Peninsula. This difference will affect where the greatest amount of divergence will be focused at the upper levels of the atmosphere. The NAM closed low solution causes the focal point to be overhead, while the GFS open wave solution makes it just to our south. This determines where the surface low will be positioned. It also subsequently changes the position of the mid level low, around which there is a dry slot (this wreaks havoc with winter storm forecasts because in the wintertime this can mean the difference between needing snow boots or city plows, rather than who needs to carry an umbrella). This all becomes important for us on Monday, as the NAM causes us to get dry slotted, as it wraps dry mid level air around the 700mb low centered off to our west. The GFS on the other hand is much weaker, broader, and slightly further east with the mid level low, resulting in a broader dry slot that is just to our south and east rather than overhead. This is why the GFS keeps showers in the area on Monday, while the NAM is dry. It is all dependent on whether the NAM’s addition of energy is because it is a higher resolution model able to pick up on it and not the result of convective feedback error. The temperature forecast would also be affected, with low to mid 70s under the deep moisture if the GFS is right, but low 80s with the NAM solution.

After having discussed the possible NAM/GFS scenarios, I just wanted to note that I personally like the ECMWF solution, which is basically a cross between the two. I like it for that reason and also because the ECMWF is generally the best performer of the models. It shows a closed low begin to form over central NY on Monday, but it suggests that the best divergence will still be focused out ahead of the trough, so the low placement is more like the GFS. For this reason, I feel that showers will be possible on Monday. The ECMWF then proceeds to deepen the upper low and dig it further south and keeps it to our west through Tuesday, this aspect very similar to the NAM. We should have a better idea tomorrow on what is actually going to take place after seeing whether or not the energy gets added into the disturbance’s equation.

The low will then open as it is picked up by the approaching northern trough on Wednesday. Unsettled weather with clouds and scattered shower threats will continue until then under an easterly component upper level fetch throwing back Atlantic moisture. The NAM/ECMWF deeper low solution would pull in greater amounts of moisture, but it would also increase the chance for dry slotting. Moderate humidity and cooler temperatures can be expected Tuesday/Wednesday with highs in the 70s and lows in the 60s under a northeast flow as the surface front remains offshore. It looks like it will be pleasant with low humidity levels late in the week in association with the strong northern branch trough.

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A cooler weekend in store but with increasing humidity

By: NYCvort, 4:28 AM GMT on August 21, 2010

To follow up on my compact closed low discussion, the upper low actually did form like the NAM/ECMWF models predicted, but it was too far to the north to affect our weather as far south as New York City. As a result, today was a hot day for some but with low humidity levels following the passage of a weak cold front this morning. Temperatures across many of the urban areas approached or surpassed the 90 degree mark.

Tonight will be comfortable with lows in the 60s and low humidity. The weak mid level front associated with what is left of the compact upper low finally passed through this evening. 850mb temperatures started out above 60 degrees this morning (supporting today’s hot temperatures), and will be in the mid 50s tonight through early tomorrow. This, combined with a developing onshore flow, will keep highs in the low 80s tomorrow, along increasing humidity.

A southern stream disturbance will approach from the midwest and Great Lakes on Sunday. This could bring some much needed rain to the metro area. More details to follow.

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Compact cut off makes tomorrow’s high temperature questionable

By: NYCvort, 5:24 PM GMT on August 19, 2010

An upper low continues to spin over Hudson Bay before retrograding into western Canada and remaining in place through the weekend. Last night’s NAM and GFS models were quite different in their forecast high temperatures for Friday. The root cause of this variation is the difference in the way that they handle a strong shortwave cutting across eastern Ontario this morning. This shortwave is currently in the slow process of separating itself from the upper low over Hudson Bay. Last night’s NAM cuts it off and spins the new compact closed low eastward tonight as it detaches from the flow and revolves around northern New England tomorrow into early Saturday. On the other hand, the GFS keeps an open shortwave which it lifts out tonight. The former results in a sharp mid level thermal gradient in our vicinity as colder air gets pulled down around the backside of the low, while the latter keeps the cold air locked up well to the north. The NAM solution brings a mid level front through and drops 850mb temperatures down close to 50 degrees late tomorrow, while the GFS keeps them in the upper 50s. The difference would result in highs either making it into the upper 80s, or remaining in the upper 70s to low 80s with very low humidity. This would also affect tomorrow night’s low temperatures, for with the much lower dewpoints under clear skies/light winds, temperatures could fall into the upper 50s suburbs/low 60s city (NAM) vs. mid 60s suburbs/upper 60s city (GFS). Even looking as far back as yesterday’s early model runs, the ECMWF featured the idea of a cut off while the GFS and GGEM kept it open. The ECMWF and NAM are often similar in the short range while the GFS sometimes has its own depiction which at times actually turns out to be more correct in this type of scenario.

Looking toward the weekend, the question of this upper low formation will actually affect Saturday’s weather, but not in the way you might expect. The NAM solution keeps us less humid, but it won’t keep us much cooler despite the 50 vs. 55 degree 850mb temperature difference. The deep low just departing northern New England would keep the ridge axis just a tad further west and more defined, thus leading to a slightly stronger surface high that is further west and south. This would keep the high near our area, rather than to the north and northeast which would otherwise provide an onshore flow (GFS solution). This results in highs of 80-85 either way, with the only real difference being in the humidity levels. The NAM solution would keep the humidity very low, while the GFS would bring up the dewpoints substantially with the onshore flow, especially by Saturday evening as mixing lessens.

This morning’s NAM has kept the cut off low idea, but now it waits until tomorrow morning to close it off, and this lagged timing doesn’t allow the cold mid level air to get nearly as far south. Thus, 850mb temperatures remain in the mid 50s and highs would be in the mid 80s with low humidity. I think this middle of the road solution is the most likely one, but if the GFS (which still keeps its open shortwave solution as of this morning) is right and the shortwave never cuts off, then tomorrow would be an even warmer day.

So our weather through Saturday will be guided by the possible development of a very small area of low pressure in the upper levels of the atmosphere. It’s currently looking like the influence of this low, if it develops, will not be too significant, with lower humidity and only slightly cooler temperatures expected.

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Weak cold front will usher in warm and drier air

By: NYCvort, 4:10 PM GMT on August 16, 2010

I was expecting to see temperatures rise much faster today, but with the extensive cloud cover and wet ground, temperatures will struggle a bit more to go up. This added moisture at the surface level has also caused it to be slightly more humid, with dewpoints in the low 70s throughout the metro area.

There is an upper level low centered over south central Canada and an upper ridge across the southern US. The low will pull energy from northern Canada into it, and as a result, it will strengthen and become rather strong. Unfortunately for those of us looking for cooler weather, we will be on the southeastern periphery of this fairly stationary low.

A spoke of energy will ride around the low and push a cold front through the region tonight. This will generate a threat for showers and thunderstorms. With close to 2500 J/KG of mid level CAPE, 2000 J/KG of surface based CAPE, and an LI of –5 (as a result of warm/humid surface and cooler/much drier mid levels), this would ordinarily spell strong thunderstorms. However, the main limiting factor will be the wind profiles, with barely 30 knots of 0-6 km shear expected. Also, the cold front that is approaching is weak and not well defined this far south. With precipitable water values of around 1.75 in, I would expect that heavy rain would be the main threat. This morning’s NAM is showing a bullseye of heavy rain develop over the metro area this evening. The potential is there, but it may be overdone. In this type of scenario some areas could get poured on while others don’t get anything. Some storms have just fired up over north central Pennsylvania.

Behind the front it will be drier with dewpoints dropping off late tonight and lower humidity levels as we head into the mid-week period. However, the front is oriented such that it will actually pull up warmer 850mb heights with an upper high building over the western Atlantic. As a result, temperatures at 850 will cool only slightly following the passage of the front. Since we will be on the southeastern periphery of the upper low’s influence, most of the cooler air will be located over south central Canada, the Great Lakes, and eventually into northern New England. 850mb temperatures will spike into the low 60s early this week. With drier air and better mixing tomorrow, temperatures will approach 90 in the urban areas. A southern stream disturbance currently over the west coast will approach our area on Wednesday with a chance of rain Wednesday night. I am siding with the slower timing of the NAM because of its support from the ECMWF. The GFS is likely too progressive in bringing the system through given the weaker flow with most of the jet energy well to the north. The models currently depict a heavy rain scenario. With over 2 inches of precipitable water and a developing surface low, this may be possible.

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Backdoor cold front pushes in maritime polar air mass

By: NYCvort, 6:28 PM GMT on August 11, 2010

While we often look to the west or northwest for a cool down and relief from the heat, this time around we will be looking to the north and northeast. A backdoor cold front pushed through last night, with a secondary front coming down tonight. This will usher in a cooler and just moderately humid air mass. After what we have been experiencing for the past few days, it will feel like a relief. 850mb temperatures will continue drop off slowly, from the mid 60s yesterday to the low 60s today, and into the upper 50s tomorrow. High pressure over southeastern Canada will slowly translate off the northern New England/Canadian coast through the end of the week. An onshore flow will develop tonight and continue into the weekend. The onshore flow will be the main factor that will keep us cooler during the day when coupled with the slightly lower 850s. Highs will struggle to make it up to 80 degrees tomorrow. Nighttime lows should make it down close to 70 tonight but into the mid to upper 60s starting tomorrow night and through the weekend. We won’t even get out of the 70s on Friday. An upper level disturbance sliding down will interact with the surface front and trigger some showers tomorrow.

Looking at the upper levels: The jet stream pattern across North America has turned into somewhat of a mess, and this will actually help to keep us in the maritime polar air mass for an extended period of time. An amplified northern stream ridge extends up into Hudson Bay, Canada, while the southern stream representation of the ridge is much flatter and broken down across the northern US. With not much flow associated with it, this ridge will very slowly translate eastward. The upper low over the Canadian Maritimes will be slowly lifting out; however, a piece of the trough will remain and strengthen into its own new vortex in order to counteract the stubborn ridge. What this lingering trough will basically do is, when combined with a weakening upper level air flow, ensure that the air mass will remain largely stationary once delivered. The upper level winds are expected to be remarkably light this weekend, only around 5 to 10 knots at 500mb much of the time, so I really can’t see the cooler air going away anytime fast as it gets locked in. This will also shield us from any major disturbances, so I wouldn’t expect any rain this weekend.

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More relief is on the way just in time for the weekend

By: NYCvort, 11:04 PM GMT on August 05, 2010

An impressive polar vortex will continue to break off over Hudson Bay through the end of the week. There will be a significant amount of jet energy associated with this feature as a result of a very strong upper level pressure gradient in connection with a high amplitude ridge over the western US and Canada. A strong upper level disturbance at the base of the trough associated with the polar vortex will combine with this jet energy and allow it to close off into its own cyclonic circulation. The fast jet will allow the feature to race down across the Great Lakes and carve out a broad upper level trough over the northeast tomorrow. As the reformed vortex lifts out this weekend, the initial piece will pinwheel around while losing some of its energy. This will have a small influence on our weather pattern as the disturbance, combined with a ridge just east of the Rockies, will allow for a troughy pattern to continue through Sunday.

The compact reformed vortex will help to press a series of weak cold frontal boundaries through, with a strong cold front finally pushing through late tomorrow morning. A weaker cold front will break the oppressive heat tonight, with lows in the lower 70s but it will still be humid. After the strong cold front pushes through tomorrow, the key word for this air mass will be dry. It will also be refreshingly cooler. The dryness of the air mass was already felt to some degree today because the mid-level dry air and shortwave ridging just to our west prevented an outbreak of showers and thunderstorms despite the warm and humid low levels. The mid-level cold front will move overhead early tomorrow, and 850mb temperatures will drop off significantly during the day, from the mid 60s early tomorrow morning into the mid 50s by tomorrow evening. Highs tomorrow will be in the mid to upper 80s under a well-mixed atmosphere. With the mid-level front passing through in the morning, I would expect temperatures to rise rather quickly, but then level off and begin to fall in the afternoon with a northwest flow in the low-mid levels ushering in cooler air. Then with the cooler, drier air mass already in place by tomorrow night, descent radiational cooling conditions will develop and lows will drop into the mid 60s in the city and to around 60 in many suburbs. 1000-500mb thicknesses will crash to around 5600m by early Saturday. New York City will actually be on the southern end of a strong thermal gradient fostered by the polar vortex over eastern Canada, with much cooler weather just to our north.

While looking at the upper level pattern forecasted for late this weekend, I observed a lot of upper level blocking across the high-latitudes. There are expected to be three closed upper level lows: one just west of Alaska, a second over northern British Columbia, and a third over eastern Canada. An upper high will be centered right in the middle, over north central Canada, with a ridge over Greenland. This is a result of the pattern change that I discussed last week. The influence of the strong –QBO pattern seems to be reasserting itself once again, with a return of high-latitude blocking. This may very well have an influence on our local weather pattern in the coming days.

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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.