Archive for negative position
Afternoon thoughts- heavy rain on the way
Posted by: | Comments1:20 PM
An area of steady rain has started to develop over the southern Mid Atlantic back through western New York this afternoon. The band of moderate to heavy rain over the eastern Great Lakes through New England is associated with the stalled warm front and will not impact the region. However, as the upper low approaches from the southwest, mid and upper level winds will pivot from southeast to northwest due to the upper level trough tilting to a negative position. As a result, a strong 850 MB low level jet stream will drive northwest from the Atlantic into the northern Mid Atlantic, which will produce a band of moderate to heavy rain this afternoon and continuing on through early Tuesday morning. This area of moderate to heavy rain, which is starting to consolidate to the southwest of Philadelphia, will produce 1 to 3 inches of rainfall by tomorrow morning with the heaviest rainfall focused over central and northern New Jersey, eastern New York, and Connecticut.
The area of low pressure will move through New Jersey tomorrow morning with a dry slot over the region on Tuesday morning followed by another round of scattered showers in the early afternoon. The heavy rainfall will be capable of producing flash flooding and significantly reduce visibility at times for this evening’s rush hour and tomorrow morning’s rush hour.
-- Weather When Posted --
- Temperature: 57°F;
- Humidity: 82%;
- Heat Index: 57°F;
- Wind Chill: 55°F;
- Pressure: 29.95 in.;
Wet rush hour marks the start of soaked weekend
Posted by: | Comments3:50 PM
The rain is here and it is not going to be stopping any time soon through this weekend, which will be a perfect time to catch up on some movies, a good book, or perhaps finally conquer a very difficult video game. What ever you do this weekend, it likely will not be done outdoors.
Light to moderate rain continues to develop and expand north and west over much of the Philadelphia and New York City metropolitan area. This rainfall is not directly connected to what will be an impressive coastal storm tomorrow morning, but is a result of the developing moisture advection at the mid levels off the Atlantic and a stationary front positioned along the New Jersey coast. The isentropic lifting produced by this frontal boundary is supporting the development of this rainfall, which will have breaks in the steady rain from time to time. However, for this rush hour, steady rain and increasing winds from 15 to 20 mph with gusts over 25 mph can be expected for the New York City metropolitan area down through the northern Philadelphia metropolitan area. More scattered rainfall can be expected further south and west, over the Hudson Valley, and Connecticut.
The upper level dynamics for this storm, namely the 500 MB low over the Mississippi Valley is developing as expected. Note the spreading out of the moisture off the Southeast coast. This is a calling card for divergence aloft and means that lifting will continue to intensify as the evening continues. This fanning out of the moisture plume from the Sub Tropical jet stream has been expanding north and west through the day in response to the Polar jet stream and upper low interacting with the Sub Tropical jet stream and tilting the long wave trough axis into a negative position. Currently, the two jet streams and associated disturbances remain separate, as we can see with the area of dry, sinking air between the two areas of lifting (moisture). However, the development of mid level moisture and weak lifting over central and eastern South and North Carolina means that the phase of the two jet streams is on the way.
When the phase occurs this evening, the primary low over the Tennessee Valley will weaken rapidly and the coastal low over North Carolina will become the new primary low pressure system. Strong upper level divergence at 500 MB on up to 200 MB will support strong lifting throughout the Mid Atlantic and allow the surface low to deepen into a strong coastal storm by tomorrow morning. With the trough tilting into a negative position, the upper, mid, and low level jet streams will all set up in an orientation from southeast to northwest, which will drive plenty of Atlantic moisture right into the northern Mid Atlantic and support a prolonged heavy rainfall event. The final piece of the puzzle is the high pressure system over Quebec (note the dry air seen in black over Maine) that will enhance the pressure gradient and support a strong easterly wind sustained at 20 to 40 mph throughout the region with gusts over 60 mph possible along the coast and over the coastal waters.
Because the upper low captures the surface low on Saturday evening and causes the storm to become vertically stacked, the low pressure system will be slow to exit, which will keep rain in the forecast through at least Monday afternoon. However, the worst conditions are expected on Saturday afternoon through early Sunday morning where very heavy rain will develop and sustained winds will be at their peak.
The rainfall map to the left includes today’s rainfall on through Monday morning, which will be rather impressive for many locations. The potential for over 4 inches of rain is very real and given the mesoscale forcing that will develop due to the stationary front just off the coast, the threat needs to be addressed.
This storm will be capable of flash flooding of rivers, streams, and urban locations throughout the region, thus the Flash Flood Watch that has been issued for the region. Due to the persistent easterly wind, a Coastal Flood Warning, as has been warned about by me for several days, has also been issued for much of the New Jersey coast. High Wind and Gale Warnings have also been issued to address the strong winds that are expected. Those that may be boating in these conditions should also take head of the Small Craft Advisory for this weekend that has been issued as well.
The good news is that after this storm, the weather pattern improves significantly with high pressure dominating the northern Mid Atlantic from Tuesday on through next weekend with dry conditions, clear skies, and temperatures in the 50′s and possibly even 60′s for the Delaware River Valley.
So my advise for this weekend is stay home or at least indoors. Hey, at least it’s not snow!
-- Weather When Posted --
- Temperature: 41°F;
- Humidity: 86%;
- Heat Index: 41°F;
- Wind Chill: 32°F;
- Pressure: 30.03 in.;
Elevation snow threat collapses, heavy rain along the coast
Posted by: | Comments8:57 AM
Those that had sporting events last night, like the Yankees last night, mainly were spared the worst conditions due to high pressure strengthening over New England. The high pressure strengthening combined with the low progression of the coastal low kept much of the heavy rain south of the Bronx, which allowed the Yankees to escape a rather complicated schedule if a rain out was the result.
As for the snow threat, which was a complete bust by everyone from myself to the National Weather Service, the reason for the lack of snow for most locations comes down to two point. One, the heavy precipitation shield was just not as expansive as forecasted. Two, the marine air mass continued to overwhelm the cold air at the mid and upper levels as the 850 MB low remained south of the region, which prevented the air mass at the higher elevations to cool. Therefore, the higher elevation snows never materialized.
Surface Map at 8 AM- Penn State E-Wall
This morning, the coastal low is moving past the Maryland/Delaware coastal waters and is slowly moving towards the New England coastal waters.
The majority of the rain with this coastal low is focused along the coast with periods of moderate to heavy downpours. The upper level trough that is supporting the development of the coastal low is expected to slowly tilt towards a negative position, which will force the coastal low to stall just east of Long Island through tonight and into tomorrow morning. As a result, rain will continue to move in from the Atlantic towards the Coastal Plain over the next 24 hours.
There will be a sharp cut off in the precipitation shield that will develop through the afternoon today. Basically if you follow these two rules, you’ll know what to expect. If you are east of the Delaware River then periods of moderate to heavy rain can be expected. If you are to the west of the Delaware River by 50 miles, expected light to moderate scattered showers. If you are over 50 miles west of the Delaware River, a few passing showers, overcast skies, and an annoying northerly wind.
The rainfall amounts through tomorrow morning will still be impressive, however the focus now a bit further north and east at this point. The heaviest rainfall will be found over much of central and southern Connecticut, Long Island, the southern Hudson Valley, the fiver boroughs of New York City, northeastern New Jersey, and down through northern Monmouth County, New Jersey with 1 to 2 inches of rain expected. To the west of this area including much of central New Jersey and the northern Delaware Valley, rainfall amounts of 0.50″ to 1.00″ can be expected with an increasingly more scattered nature to the rainfall towards the west. Finally, southern New Jersey and much of the western suburbs of Philadelphia can expect less than a half inch of rain by Monday evening from this event going forward.
Winds will remain an issue with a tight pressure gradient in place. Winds along the coast and coastal waters will continue to remain around 15 to 30 mph with higher gusts to 45 mph while away from the coast, winds will range from 10 to 20 mph. Winds will continue to veer from the northeast to northwest as the low pressure system slowly moves past the New Jersey coastal waters.
Once this low pressure system exit the region by tomorrow afternoon, a nice stretch of weather conditions can be expected as high pressure takes hold of the region. Clear skies and dry conditions can be expected through Friday with temperatures rising to average to slightly above average levels. In short, the 60′s return!
500 MB on Saturday Afternoon
A strong ridge will build over much of the East coast through this week, however a strong cold front with several waves of low pressure along the frontal boundary will move through the region on Saturday with periods of heavy rain. There is some debate on the strength of the Polar air mass behind this cold front between the GFS and ECMWF, however one aspect that is not in contention is the slow nature of the cold front and the threat for yet another wet weekend for the northern Mid Atlantic.
Finally, I saw an interesting feature on the ECMWF this morning that gave me pause. The ECWMF develops a strong TROPICAL low pressure, likely a hurricane on this guidance, that moves from the Caribbean Sea towards the eastern Gulf of Mexico. Besides the serious
ECMWF Hurricane Threat?
nature of such an event along the Gulf Coast (especially the Panhandle of Florida), this tropical system would have a few important influence on the evolution of the pattern towards the end of this month in terms of heavy rain fall and potential enhancement of any disturbance that happens to be dropping into the eastern trough at that time. I’ll keep tabs on this potential and we’ll see what develops.
-- Weather When Posted --
- Temperature: 44°F;
- Humidity: 93%;
- Heat Index: 44°F;
- Wind Chill: 36°F;
- Pressure: 29.97 in.;
Evening Thoughts- Heavy rain and maybe more
Posted by: | Comments9:58 PM
Once again, there is a lot to talk about this evening in terms of the approaching cold front, the associated upper level trough, and we can’t forget Hurricane Bill.
Hurricane Bill as of 5 PM this afternoon has weakened into a category 3 hurricane, however this is in relative terms compared to earlier strength. Hurricane Bill still had dangerous sustained winds of 125 mph. In fact, I think in the next update Hurricane Bill will strengthen into a category 4 or close to it once again. The eye wall has rebuilt and the shear has slightly decreased. Hurricane Bill is looking much healthier than a few hours ago. As I said this morning, expect Bill to go through periods of weakening and strengthening as the eye wall is replaced. This is normal for hurricanes, especially intense ones.
The question of course is where exactly is Hurricane Bill going. Over the past few days I’ve been keying on the upper level trough that is moving into position over the Great Lakes to the Gulf Coast. The key here is what happens when an intensifying disturbance that dives into the Tennessee Valley on Friday night and Saturday morning. Does the upper level trough remain neutral, which produces a southwesterly upper level wind over the coastal waters OR does the disturbance causes the long wave trough to shift into a negative position, which leads to a southeasterly wind aloft over the Mid Atlantic coastal waters, and therefore draws Bill in towards the coast. There is a third possibility as well, that I think we need to dive into the possibility of Hurricane Bill creating a separate upper level dynamic that may influence this trough as well.
This evening’s water vapor image has a few details that pertain to the different possibilities discussed above. The upper level trough currently has a weakly positive to neutral orientation. There is clearly a strengthening jet streak diving through the base of this trough. We can tell this by the rapidly sinking air over eastern Kanas, Oklahoma, and Missouri. The air is sinking because the air mass directly ahead of this jet streak is rapidly rising. Another mature jet streak can be found over the Mississippi Valley into the Ohio Valley where showers and thunderstorms are igniting this evening. It is this jet streak that will be a major determining factor in how the entire trough axis will orientate itself over the next three days and will be a major influencing factor on the Mid Atlantic as a result.
This jet streak is expected to intensify the upper low over the Great Lakes, which may cause the trough to tilt towards a negative axis. As a result, this shift in the axis would pull Hurricane Bill towards the coast rather than away from the coast. This has been seen in several model guidances in the SREF and GFS ensembles.
Of course, there are other factors to consider as well and one of those factors is what if Hurricane Bill simply becomes strong enough to force the upper level trough to slow down. This has happened in some intense hurricanes and Hurricane Bill certainly has the capability to reach a category 4 strength once again if not stronger. When hurricanes become this intense, they produce a strong upper level high pressure system due to air sinking around the edge of the storm. Note on the water vapor image tonight that the high cirrus is fanning out in a clockwise manner. This is an indication of a strong anticyclonic rotation or high pressure in the upper levels. One can suggest a hypothesis as I am tonight that the upper level structure of Bill may be strong enough at times to slow the trough over the East cost from shifting towards the coast, and thereby slowing the progression of the surface cold front. I think in some respect the model guidance tries to illustrate that influence on the precipitation field for Friday afternoon through Sunday morning as showers and thunderstorms train over a stalling cold front over the forecast area. So, if Hurricane Bill does become strong enough to influence the progression of the cold front and upper level trough, then we have a complete different type of ball game here, one in which Hurricane Bill is in control and not the other way around.
Regardless of the exact track of Hurricane Bill, I think it is safe to say that as the cold front stalls and the mid level moisture from Hurricane Bill interacts with the cold front that showers and thunderstorms will train over the forecast area on Friday and Saturday. Not every location will have constant heavy downpours, however where the showers and thunderstorms eventually set up, rainfall amounts of 2 to even 4 inches are not out of the question.
I plan on doing a live chat session tomorrow to cover this event as it unfolds until this cold front and hurricane are off the coast.
-- Weather When Posted --
- Temperature: 78°F;
- Humidity: 88%;
- Heat Index: 80°F;
- Wind Chill: 78°F;
- Pressure: 29.94 in.;
A Roller Coast For Temperatures And Snow For The Interior
Posted by: | CommentsMost of you are outside right now, and if you are in southern New Jersey you may be confused as to whether we are in April or December as temperatures soar into the 60′s. Well the party is about to come to an end as cold air breaks into the forecast area, but not before two separate storms impact the region first.
The best dynamics currently are lifting to the north of the forecast area, however heavier precipitation is redeveloping over Virginia and the Carolinas. As we move forward into tonight and Thursday morning, I think the GFS and NAM QPF guidance is a bit over done. I just don’t see the forecast area getting over 3 inches of rain over the next three days combined. There may be some isolated locations, but not wide spread.
So tonight the cold front presses south with colder air slowly bleeding into the forecast area. However, the coldest air is not directed towards the forecast area, but south towards the Mississippi Valley. This is due to the influence of the clipper that remains separate from the Gulf of Mexico low develop on Thursday. As a result, the cold air will slowly build into the forecast area at the surface, which will still drop temperatures roughly 20 degrees between now and tomorrow morning.
By Thursday morning, areas of rain will move through the forecast area as disturbances round the trough that is deepening. The Gulf of Mexico low will be gaining moisture and strength as the thermal gradient tights over the East coast. Now here is where the northern branch interacts with the Gulf Low in a negative aspect for snow lovers along the coastal plain. The northern branch slightly pulls the upper low associated with the Gulf Low to the Northwest as the whole trough begins to tilt into a negative position. This slight influence slows the progression of the CAA towards the coast and keeps the thermal gradient strong yet stagnant.
By Thursday night, the rain/snow line will be located northwest of Philadelphia towards the NY/NJ/PA boarder and towards Poughkeepsie. This is a fast moving storm due to the lack of blocking over the Canadian Maritimes, thus the surface low and 850 MB low will move quickly through the forecast area on Thursday night. Most model guidance agrees with taking the 850 MB low over western New Jersey to Poughkeepsie, New York, and then over southern New England. This track will all but guarentee an all rain solution for the majority of the forecast area.
However, the interior is another matter. The cold air will not give much at the surface over northeastern Pennsylvania through the central Hudson Valley. However the mid levels will warm to slightly above freezing. The latest soundings forecast for these locations suggest a long period of sleet mixing in with the snow over many of these locations. Where the mixing will occur the least is where I have the highest snowfall totals and then small accumulations as you move south and east.
On Friday, the low pressure system will quickly move towards southern New England. There is going to be a lot of mid level energy around the forecast area, so I kept lingering precipitation over the forecast area. Temperatures will naturally fall behind the storm with a rather chilly day on Friday. The precipitation will change over to all snow over the entire forecast area with possibly some lingering flurry action towards the coast. I am concerned that a stronger disturbance behind this storm will touch off an area of light to moderate steady precipitation, thus the idea of an isolated inch over the coastal plain and parts of northwestern New Jersey and the southern Hudson Valley.
-- Weather When Posted --
- Temperature: 51°F;
- Humidity: 87%;
- Heat Index: 51°F;
- Wind Chill: 48°F;
- Pressure: 29.98 in.;
Freehold
57°
Philadelphia
62°
Albany
59°
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