Monthly Archives: April 2014

NYC Weather Update – Apr 30, 2014

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Are you wondering why today is so cool with highs barely touching 50, but tomorrow is forecast to have high temperatures in the low 70s? It’s because there’s a warm front currently situated to our south. Let me clarify the two most common types of frontal boundaries:

  1. A warm front delineates a boundary where the airmass behind the front is significantly warmer than the airmass ahead of it.
  2. A cold front, not surprisingly, applies to the reverse situation and delineates a boundary where the airmass behind the front is significantly colder than the airmass ahead of it.

Currently, a warm front sits south of us, causing an onshore flow (easterly winds) off the frigid Atlantic Ocean. Why the easterly winds? Looking at the image below, you’ll see yellow contour lines that demarcate isobars of pressure. In the Northern Hemisphere, air flows counterclockwise around a low pressure center and clockwise around a high pressure center. The counterclockwise flow also applies to the frontal boundaries attached to a low pressure center.

In our present situation, the air flows towards the east (counterclockwise) south of the west-east oriented warm front (in red), and then from the east to west north of it. Hence, we are experience very cool conditions, and the moisture being funneled off the Atlantic Ocean into our region is fueling these steady showers.

Current surface analysis from the WPC with my overlay showing movement of the warm front to our south.
Current surface analysis from the WPC with my overlay showing movement of the warm front to our south.

We should actually see temperatures begin to rise overnight as the warm front pushes through the area. Once this occurs, we will be in what is referred to as the “warm sector”, which is an area that sits behind a warm front and ahead of an advancing cold front. This type of setup is favorable for the development of thunderstorms.

Therefore, Thursday will be a warm, cloudy day with high temperatures in the low 70s and a chance of showers and thunderstorms in the morning.

Friday, with the cold front having passed through, conditions will clear up and we’ll have a nice, sunny day, with temperatures near 70. There will be a chance for afternoon showers.

Saturday continues the trend of nice weather with another day in the upper 60s and mostly sunny skies. Again, there will be a chance for afternoon showers, especially inland where daytime heating can produce enough energy (and instability) to induce those showers.

One other note about today and tomorrow, and that’s the whopping headline numbers for how much rain is forecast to fall, with totals ranging from as high as 3″+ in the western portions of the Tristate to 1.5″+ in the east. This could easily cause flooding in poor drainage areas, small streams, and urban areas.Screen Shot 2014-04-30 at 1.50.54 PM

 

 

Deep South Severe Weather Outbreak – Apr 28, 2014

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A major severe weather outbreak is currently underway for portions of the Deep South from Louisiana up through Tennessee, extending east from the Mississippi to western Alabama. As you can see in this satellite image, there are numerous supercell thunderstorms with high cloud tops piercing into the tropopause, indicative of extremely strong updrafts. Low level helicity (a measure of the vorticity and spin imparted on the air) is also high, along with abundant moisture and convective available potential energy (CAPE) will continue to fuel dangerous storms capable of spawning multiple tornadoes, damaging wind gusts, and severe hail.

Major severe weather outbreak over the Deep South viewed from space
Major severe weather outbreak over the Deep South viewed from space

NYC Weather Update – Apr 28, 2014

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After a beautiful start to the work week, we are looking at a wet and cool mid-week before things clear up this coming weekend.

A low pressure center that is currently pounding the Midwest and Lower Mississippi Valley with hail, tornadoes, and severe storms will slowly move into the Great Lakes region. Once it does so, a strong area of high pressure situated over Quebec will prevent the low from making steady progress.

Therefore, Tuesday-Thursday will each see a chance for rain showers, in particular on Wednesday when a warm front associated with the aforementioned low pressure center makes its approach towards our region. Due to the long duration of the precipitation event and the possibility of heavy downpours Wednesday, there may be some localized small stream and urban street flash flooding.

Temperatures Tuesday and Wednesday will be quite cool with highs only in the mid 50s caused by onshore winds from the cool Atlantic waters flowing from the southeast and east ahead of the warm front. Once the warm front passes through on Wednesday night, temperatures will jump dramatically as we enter the warm sector behind the warm front and ahead of an advancing cold front. High temperatures Thursday could easily top 70.

A persistent low pressure center is forecast to track very slowly across the Great Lakes this week.
A persistent low pressure center is forecast to track very slowly across the Great Lakes this week.

Friday should see an end to this period of active weather as the high pressure over Canada weakens and finally allows this low to lift out of the area. Highs will be pleasant in the mid to upper 60s.

NYC Weekend Update & Severe Weather in the Plains

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The atmosphere over the area the past couple days has been both exceptionally dry and abnormally cool. This trend will end Friday with the approach of a cold front and low pressure system. Because the air mass currently in place is so dry, it will be difficult for the air to moisten at the outset Friday. Thus, forecasters are calling for a mostly dry day with increasing cloud cover, and rain showers moving in over the course of evening and overnight hours. High temperatures Friday will remain in the low 60s.

North American Model simulated radar image for Saturday @ 2PM EST
North American Model simulated radar image for Saturday @ 2PM EST

Saturday will remain a cloudy day with the chance for showers diminishing but still present. As the simulated radar image above suggests, there should be a break in precipitation between Friday evening and Saturday afternoon. However, it does appear that there is enough upper air support for afternoon showers and even some scattered thundershowers are possible Saturday afternoon. High temperatures will be warmer in the upper 60s. Skies should gradually clear once showers pass through, so the latter half of the day will still be sunny.

Sunday temperatures will cool off again with the passage of the cold front Saturday, however, sunny skies will return with high pressure building. Temperatures should be pleasant in the low 60s. Monday looks at this point to be a clone of Sunday.

 

Severe Weather in the Great Plains Sunday

It’s definitely severe weather season at this point in the Plains States and Tornado Alley. These areas have been repeatedly pummeled the past few days by severe hail storms. This weekend promises to be no different. As you can see below, a strong low pressure center is forecast to develop in the lee side of the Rockies Sunday with central pressure as low as 988mb.

Meanwhile, in the upper atmosphere, a vorticity maximum (see below) is forecast to form, with an area of strong divergence just to its east near the forecast center of the surface low. Upper level winds will be largely unidirectional. These are all ideal conditions for a severe weather outbreak. As a result, a wide swath of the Plains States from central Texas through Oklahoma, Kansas, and into Southeastern Nebraska have been placed under a Slight Risk area for severe weather by the Storm Prediction Center.

Note the strong low pressure center forecast to develop over the lee side of the Rockies - central pressure of 988mb
Note the strong low pressure center forecast to develop over the lee side of the Rockies – central pressure of 988mb
Upper air charts for Sunday afternoon. Note the strong vorticity maximum (area of dark red) over eastern Colorado.
Upper air charts for Sunday afternoon. Note the strong vorticity maximum (area of dark red) over eastern Colorado.

 

NYC Midweek Weather Update – Apr 23, 2014

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You can feel that cold front has passed, can’t you? A stiff northwest wind is ushering in an unseasonably cool air mass, and temperatures will be quite a bit lower than yesterday, with high temps about 10 degrees less than yesterday in the mid-upper 50s. Cloud cover should continue to diminish throughout the day.

Windy and cool weather persists Thursday – with high temperatures again struggling to hit 60. The breezy conditions and very dry air mass in place will lead to conditions conducive to the spread of wildfires.

GFS model's output for Friday
GFS model’s output for Friday

Our next shot of rain comes Friday, with a weak low pressure system moving over the Great Lakes and a trailing cold front. Forecast models suggest the system weakens as it approaches, hence lowering the chances for rain. If it rains, only light, showery precipitation is expected. High temperatures Friday will be slightly warmer, in the low 60s.

Saturday & Sunday – So far, we have a nice weekend on tap with mostly sunny skies and temperatures slightly below average in the low-mid 60s both Saturday and Sunday.

NYC Afternoon Weather Update – Apr 22, 2014

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Altocumulus clouds over the Empire State Building
Altocumulus clouds, preceded by cirrus clouds (earlier today and yesterday) are indicative of approaching frontal boundaries

Just a quick afternoon update for you all. It’s been quite a warm afternoon as warm air continues streaming into the area ahead of an approaching cold front. The frontal boundary is still quite a ways off, however, you can already see increasing cloudiness in the form of these altocumulus over the city. Some precipitation is beginning to show up over portions of western NJ. The atmosphere right over NYC is still on the dry side, with humidity recorded at LaGuardia Airport recently only at 47%, and dewpoint temperatures only 44 degrees. This should serve to limit the potential for convective activity and thunderstorms later in the day. In fact, as the area of precipitation to our west enters the region, showers may break up or scatter upon entering the relatively drier airmass.

NYC Weather Update – Apr 21, 2014

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Brilliant start to the week this Monday with comfortable and seasonable high temperatures in the low-mid 60s.

HPC forecast

Tuesday – A cold front will rapidly approach the region from the west. Ahead of this cold front, warm, moist Gulf air will flow in, allowing for high temperatures to edge close to 70. Given the moisture content and warm temps, there is some potential for enough instability in the atmosphere to support the growth of thunderstorms later in the afternoon hours. Some of these storms could produce small hail. Thunderstorms, should they form, would be mostly limited to inland areas west of NYC as the marine layer near the coast tends to exert a stabilizing influence.

Wednesday and Thursday will be similar weather-wise with high temperatures suppressed in the upper 50s to low 60s due to the influence of strong winds from the north and northwest in the wake of the departing cold front. We should see clear and sunny skies both days. Excellent conditions for radiational cooling on both nights will allow lows to drop into the upper 30s and low 40s across the region.

The next chance for rain will be this Friday into the weekend with another cold front moving in from the middle of the country.

Increased Pollution in Asia = Stronger Pacific Storms

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pollution in china
Buildings in Lianyungang, China, are shrouded in smog on December 8, 2013. Aerosol pollution from Asia is likely leading to stronger cyclones in the Pacific, more precipitation, and warming temperatures at the North Pole. – National Geographic

A recently published study a by a Texas A&M team led by professor of atmospheric sciences Renyi Zhang points to a possible link between increased air pollution from Asian sources and stronger Pacific storms. There are implications for global climate, including impacts on El Niño. Stronger Pacific storms would lead to increased precipitation in places like the Pacific Northwest.

At its most basic level, the theory behind this study is a fundamental to atmospheric science and is well understood. Water vapor that makes up clouds first need condensation nuclei, such as salt, dust, or in this case, pollutants in order to coalesce around and form clouds. An increase in the number of available condensation nuclei should in theory lead to more robust cloud growth, which allows for more energy to be released into the atmosphere through the latent heat of condensation. This in turn will allow for stronger convective clouds, thunderstorms, and cyclonic storms.

NYC Weekend Weather Update – Apr 18, 2014

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SunnySkies

Sunny skies are your headline for weather this weekend. After what’s been a rudely cold interruption to spring, we’re returning to more seasonable temperatures and a calm weather pattern.

High pressure will dominate the area Saturday leading to sunny conditions with temperatures rising into the mid-upper 60s.

Sunday – high temperatures will be somewhat inhibited by onshore winds off the ocean as the high pressure center moves to our northeast and winds turn to the northeast and east.

Monday – the streak of pleasant weather continues after a cold start to the morning with highs again reaching the low 60s under sunny skies.

Next chance of precipitation will be Tuesday with a weak low pressure system and cold front moving in from the Great Lakes.

NYC Weather Update – Apr 15, 2014

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Wet, windy, and cold conditions will predominate our weather today. We’ve already hit our high temperature for the day and will see temperatures continue to plummet throughout the course of the day as rain and the attendant strong cold front moves in later this afternoon. Gusty winds in excess of 40mph, and some rumbles of thunder are possible in the stronger rain showers.

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The heaviest rain will be this afternoon/evening, ending gradually overnight. Behind the cold front responsible for this heavy rain, a biting northwest wind will drop temperatures down into the low-mid 30s! Again, there is some small potential for snow flurries across NYC during the tail end of this storm.

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We could be looking at rainfall totals up to 2″ in the area, crazy wet for this time of year. Now, Wednesday, you’ll feel the impact of the passage of that cold front, as northwest winds will persist around 15-20mph, suppressing high temperatures in the mid-40s, much below the seasonal average.

Thursday and Friday will both be dry days with high pressure moving over the region, but high temperatures will remain below normal with an onshore breeze blowing in from the east and northeast. Overnight lows both days will remain quite cold in the mid-upper 30s. You thought you didn’t need that winter weather gear anymore, eh?