All posts by Lei Zhao

NYC Weather Update – Jul 15, 2014

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A correction from yesterday – I had said today may see less potential for severe weather. In fact, the Storm Prediction Center once again has most of the NYC metro area under a slight risk for severe thunderstorms later today.

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Based on current satellite trends, it does appear there will be some breaks in the clouds this morning and afternoon. This will allow sufficient sunshine and daytime warming to fuel strong to severe storms across our region again later today. In the upper atmosphere, largely unidirectional winds from the south-southwest, a favorable jet stream set up, and approaching cold front suggest the potential for storms with damaging wind gusts and even some hail. Flash flooding is again a distinct possibility.

As you see below, portions of our area saw up to 1-1.5″ rain yesterday, and these same areas are in the target zone for severe storms today as well. With the upper level winds lining up from the south-southwest, training (where multiple storms hit the same location over a period of time) of storms is likely. This will enhance the risk for flash flooding.

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A note about the term unidirectional winds: Unlike a hurricane, severe thunderstorms thrive on upper atmospheric energy, and unidirectional winds at all levels are more conducive to imparting a rotation on a rising parcel of air (rotating thunderstorms can produce tornadoes).

Otherwise, expecting a warm, humid day, with highs in the mid 80s and some sun before storms arrive later in the afternoon. It will be cooler and more comfortable tomorrow with this stormy cold front having passed us!

NYC Weather Update – Jul 14, 2014

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The Storm Prediction Center has placed the entire NYC metro area in an area for a slight risk of severe weather today. A strong low pressure center over the Great Lakes and its associated cold front are slowly making their way towards the Eastern Seaboard. Despite the cloudiness today, forecasters are still fairly confident that atmospheric conditions will be favorable for the formation of severe thunderstorms in the region. Based on current radar trends, these storms would most likely not be arriving in the NYC area until later this afternoon. The primary threat to our area will be damaging winds and torrential rain capable of triggering flash floods.
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Tuesday will feature a similar set up for weather over the region, though there is less risk for severe weather tomorrow. However, strong storms are possible throughout the day and the risk of flash flooding is a distinct possibility. High temperatures will be in the mid 80s despite cloud cover, due to the influence of warm, humid air rushing in between the cold front and high pressure offshore over the Atlantic. This same warm, humid air mass is what will feed the storms for today and tomorrow. As you see below, the forecast rainfall total for today and tomorrow is as high as 3.6″ over Northeastern New Jersey, and ranges between 2.5-3″ for most parts of the Tri-State area.Untitled2

Wednesday – we get a reprieve from the stormy weather as the cold front and storm system finally move east of NYC. Expect a cloudy start to the day with gradual clearing and conditions improving markedly. The next weather story for Wednesday through the end of the week is the cool, polar air mass that will be a dominant feature. High pressure will build in behind this storm system and usher in much cooler temperatures, with highs in the low 80s Wednesday-Friday, and overnight lows in the low-mid 60s in the city, and in the 50s north and west!

Thursday & Friday – expecting these two days to be essentially the same weather-wise, with high temperatures in the low 80s in the city (and upper 70s further north). Sunny to mostly clear skies for both days.

NYC Weekend Weather – Return of the Polar Vortex? – Jul 11, 2014

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You may have heard from media reports that the dreaded polar vortex is set to make a return to the Eastern US during the middle of the coming week. I’ll discuss that in detail below. First, for the weekend forecast:

Today & Saturday – a very pleasant couple of days in the wake of the cold front that finally pushed through our area yesterday. You can feel a marked drop in humidity, and temperatures are much more comfortable. Saturday, high temperatures should top out in the mid 80s in NYC.

Sunday – a chance for afternoon thunderstorms and a high in the mid 80s, chance of rain increasing through out the day into the overnight hours.

Polar Vortex?

Forecast models are in good agreement that beginning Tuesday into most of the rest of next week, an upper air vortex (see below, the trough with an inclosed region surrounded by the blue arrows) will drop into the Midwest and Northeast from the northern regions of Canada. This vortex can be technically referred to as a polar vortex, a term that instantly revives media hype about how cold it will be. But keep in mind, during the summer, polar regions heat up as well, so even the coldest polar airmass and vortexes are much milder than their winter counterparts. Furthermore, the influence of the sun is quite strong as we just passed the summer solstice, so the sun is sitting at a very high angle each day and is highly effective at heating the air quickly. Hence, temperatures are expected to be much below normal for some areas, but we’re not talking about snow or ice.

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Upper air forecast from GFS for Monday, 8PM EDT, 500mb absolute vorticity.

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Next Week

There will be a good deal of energy traveling with the polar vortex, so we’re expecting a somewhat disturbed pattern of weather for the beginning of next week. Tuesday & Wednesday will see the best chances for rain and thunderstorms with a cold front approaching us from the west. Before the cold front, we’ll still see temperatures in the mid to upper 80s with a chance for thunderstorms Monday. Once the cold front passes, we’ll be right in the mix with the “polar” air mass, so our high temperatures will drop into the low 80s, and overnight lows will drop into the upper 60s for the latter part of the week.
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NYC Afternoon Weather Update – Jul 9, 2014

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The Storm Prediction Center has again placed western and interior portions of the NYC region under a slight risk for severe weather today. The atmospheric setup this afternoon is similar to yesterday, with plenty of solar heating and humid air leading to moderate instability. At this hour, CAPE values (a measure of instability) are already hitting 1500-2500 J/Kg across the area, with greater instability further south.

There are already a few scattered thunderstorms forming over Eastern Pennsylvania. These storms are not yet hitting severe limits, but conditions are favorable for further development and increase in areal coverage. With winds aloft mostly unidirectional from the southwest, damaging wind gusts would be the primary threat. Storms that form to the southwest of NYC near the Philadelphia metro area and Central New Jersey would be the ones to watch for immediate impacts in the city.

These storms are still several hours away, and it remains to be seen whether they will hold up upon encountering the cooler and stabilizing influence of marine air infiltrating along south-facing shores with the southwest winds and a sea breeze. Last night, the line of severe thunderstorms weakened just before it hit NYC, sparing us from some wicked weather for the night. The same case may unfold again today, but the situation bears monitoring.

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NYC Nowcast – Strong to Severe Thunderstorms Approaching

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Severe thunderstorms are approaching the area from our west. Several distinct bowing segments of severe thunderstorms have been making their way steadily towards our area from Central Pennsylvania since earlier this afternoon.

These storms have a history of producing damaging wind gusts and even a few tornadoes. Sundown over some of the areas under the current Severe Thunderstorm Watch areas in our region may weaken these storms, however, overall atmospheric conditions are quite favorable to the forward propagation of this line.
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NYC Weather Update – Jul 8, 2014

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We had a nice couple days of seasonable, dry, and sunny weather this past weekend in the wake of Hurricane Arthur’s passage, but that came to an end yesterday and today. The high pressure that gave us the nice conditions moved to the southeast and out over the open waters of the Atlantic, merging with the semi-stationary Bermuda high. As you may know, this set up favors hot, humid temperatures all along the east coast, and we are no exception. Expect high temperatures to top out in the low-mid 90s today, with uncomfortably high humidity.

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Wednesday – we may get some relief from the heat later today and overnight with a pre-frontal trough and scattered diurnal thunderstorms forming in the hot, unstable air. However, tomorrow will be more or less a repeat of today, with temperatures in the upper 80s to low 90s and muggy conditions.

Thursday – The cold front above won’t actually reach us until late in the day Wednesday, but when it does hit us Thursday, it will bring some chance for scattered thunderstorms and a slight drop in temperatures back down into the mid 80s.

Friday – amazingly, our streak of great weekend weather (with the exception of Arthur) is set to continue. Once the cold front clears us Thursday, we’ll see high pressure build in from the west Friday, which means some cooler winds and lower humidity airmass from Canada moving in. Temperatures should be milder, in the low-mid 80s Friday through the weekend.

Our next shot at precipitation will be late Sunday into the beginning of next week.

Typhoon Neoguri Image

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Despite the fact that Typhoon Neoguri shares a name with a well-known brand of packaged ramen, it certainly is no joking matter, as you can see from the impressive satellite image captured recently. Although the storm has weakened from a Category 4 Super Typhoon, it is still a powerful Category 3 storm currently churning up the East China Sea.

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This storm is bad news for Japan, especially the large populations living on the southernmost home island of Kyushu. Forecasters believe Neoguri will continue to weaken rapidly under the influence of increasing wind shear, but it is still set to make landfall on Kyushu some time Thursday as a Category 1 storm. The mountainous terrain of Kyushu will likely provide orographic enhancement for the rain bands coming off of Neoguri (the upslope side of the mountains facing Neoguri’s onslaught will have the tendency of squeezing out precipitation from the rain bands).

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Hurricane Arthur Update – Jul 4, 2014 @ 6PM EDT

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Hurricane Arthur is still being analyzed by National Hurricane Center as packing winds of 80mph. Although the inner core is still very symmetrical, you can see that the overall storm is beginning to lose some symmetry. In addition, central pressure has risen a couple millibar since earlier today, indicating the storm is continuing to weaken.

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There’s still plenty of rain in store for Southeastern New England, though, as a couple bands of heavy rain are still moving their way onshore. The worst of the storm is yet to come, with the center of Arthur forecast to make its closest pass to this portion of the country in the next couple hours. Overnight, tropical storm conditions are likely over portions of Eastern Cape Cod.

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Weekend Weather, Hurricane Arthur Update – Jul 4, 2014 @ 10AM EDT

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As of 9AM EDT, Hurricane Arthur has weakened somewhat from its peak intensity as a Category 2, and sustained winds have declined to 90mph. However, its eye is still clearly visible, and as you can see below, the storm is maintaining a symmetrical appearance with good outflow in all quadrants, meaning this is still quite a powerful storm. As Arthur continues moving northeast, it is expected to weaken rapidly upon entering much cooler waters and with increasing vertical shear from the codl front seen below.
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Track Forecast

Forecast models are in very close agreement about Arthur’s track over the next 12 hours or so. As you see below, they are tightly clustered around the 40N, 70W benchmark. Hence, forecast reasoning has not changed much since yesterday night. Arthur is expected to continue its northeast track and speed up under the influence of the cold front depicted above, moving offshore of New England and into the Canadian Maritimes as it makes extratropical transition.

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What This Means for Us

As expected, Arthur will be bringing multiple rounds of rain to the entire Northeast over the course of the day today. You can already see Arthur’s outer rain bands on our regional radar (you’d see it in the standard 124 nautical mile radar, but I’ve inserted an image of 248 nautical mile range radar image to see the storm better). As Arthur accelerates and makes its closest pass to this part of the country, we’ll see repeated bands of rain moving on shore. You can expect periods of heavy rain and thunderstorms, interspersed with some dry spells under cloudy skies.

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The Rest of the Weekend

Once Arthur and the cold front clear the Northeast, we will see a return to a spectacular weekend with high pressure building back in. Sunny skies, mild, and dry conditions will prevail across the entire region with high temperatures in the low-mid 80s on both Saturday and Sunday. As the high pressure moves to our southeast, we’ll see a return flow from the backside of the high pressure ushering in another warm, humid airmass for the beginning of next week.

 

Hurricane Arthur Update – Jul 3, 2014

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Hurricane Arthur has held steady with 90mph winds over the last 6 hours or so. The storm’s eye continues to grow more defined, with central pressure continuing to fall to 977mb. These signs point to further strengthening, and the National Hurricane Center is still forecasting Arthur to hit Category 2 strength as it makes its closest pass over North Carolina.

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Track Forecast

Over the course of the day, Arthur has moved further north than originally forecast, and is only slowly beginning to make its turn towards the northeast. As a result, most forecast models have been pushed a bit further to the west. Most of the models now have Arthur making landfall somewhere over coastal North Carolina, and then passing inside the 40N, 70W benchmark offshore of the Northeast. The bottom line is that the updated forecast track increases the likelihood of adverse impacts to coastal areas all along the East Coast. This includes high surf, beach erosion, and dangerous riptides even in areas that don’t see direct impacts. This new track also puts portions of Southeastern New England under risk of experiencing tropical storm conditions, and heavy rain bands from Arthur as it passes offshore.

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It’s a good thing that mandatory evacuations were placed for the Outer Banks and coastal North Carolina, as portions of these low-lying areas are forecast to experience storm surge inundation of greater than 3 feet during the peak of the storm.

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Blue shaded areas = 1-3′ of storm surge Yellow areas = between 3-6′ of storm surge