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.

Weather for May 10, 2010; Less windy tomorrow?

It appears that the strong winds will begin to diminish by tomorrow morning as the pressure gradient between high pressure building eastward and low pressure slowly lifting through the Canadian Maritimes begins to relax. The flow tomorrows turns anti-cyclonic. However, it will still be breezy but under mostly sunny skies as there will be some low level mixing. Temperatures will struggle into the lower 60's, so it will be another cool, but dry May day.

A colder regime is setting up into early next week

That's right folks, a colder regime is setting up and will presist into early next week. A potent ULL is currently tracking eastward from out of the Plains states, as its associated surface low, at 997mb at 00Z and its cold front is treking eastward into the OH Valley.

The 12Z GFS/NAM have trended drier with the frontal passage. Why? The upper level forcing will be north of the NYC area. A warm front, clearly shown on the WV imagery at 850mb in the mid levels is trying to trek to the NE at this hour. That will bring an increase in clouds in SW low level flow. A couple of things play against the warm front clearing the NYC area to the north: the surface low is still well to the west, chilly ocean waters, and a developing LLJ at 850mb not overspreading the region until morning. The front should clear southern portions of the NYC area. However, the best lifting and convergence is to the north.

However, there is a chance for thunderstorms. First, theta e ridge will advect in to create marginal to moderate levels of instability, however elevated in nature because the warm front should not make much progress northward and those north of the front will be stuck in a stable, cool airmass. Remember, the airmass is not all that humid and moist. Shear is impressive in the low levels as well as deep layer shear is impressive. Could see some convection fire by daybreak in the vincinity of the warm front. Then the warm front tries to lift through by 12Z.

Looking for an afternoon frontal passage after blending the ECMWF/GFS/NAM. Should see plenty of cloud cover, and that will inhibit destablization of the atmosphere. However, if skies clear faster, then watch for a line of low topped thunderstorms moving W to E with the cold front with strong mid level forcing, strong deep layer shear, and enough of a mid level wind profile. Damaging winds will be the main threat if those low topped thunderstorms develop.

After the frontal passage, strong CAA will commence, and winds will increase as the stacked low intensifies in the Gulf of Maine and Candian Maritimes and High Pressure builds out of Central Canada eastward to the Mid-Atlantic. Winds could advisory level criteria tomorrow evening on Long Island. Temps will rapidly drop into the 40's most areas tomorrow night. Sunday will be a cool day with cyclonic flow aloft. Temperatures will struggle to get to 60 degrees. The high continues to build overhead on Monday, and the low turns more anti-cyclonic. Still, temperatures will struggle into the lower 60's. Tuesday and Wednesday, a warm front tries to approach from the southwest, bringing a chance of showers, with moderating temperatues to near normal levels.

Thunderstorms today?

Currently, we have a shortwave approaching from the G/L treking eastward with an associated cold front. Most of the convection so far has been confined to PA and western NJ this afternoon. As this convection heads east, it will encounter very dry air in the low levels, as dewpoints are in the 40's and 50's. That will help to inhibit the convection as well as the intensity of the convection. Soundings indicate that the storms could produce microburst wind  gusts and hail. Shear looks great, with about 60 knots of bulk shear, but very little instability due to the dry air and low moisture content. So straight line winds can be expected with the strongest storms as the front marches eastward.

Severe Wx tomorrow?

The pressing question for tomorrow besides the heat is going to be whether we see severe wx or not. So, let's get to it.

Let's put up the WV imagery first:

The cold front is well off to the west of the area, as it slowly crawls eastward through the MW states. The upper level ridge is clearly in place in the east, and by later on tonight and tomorrow, the ridge axis will move off the coast. For most of the day, the axis was over New Jersey. SW flow aloft is clearly in place, and more moist humid air will advect in tomorrow from the Eastern G/L states. High cirrus clouds will move in the ridge, a "dirty ridge" to be exact for tomorrow as convective debris attempts to move in from the Tenessee Valley.

For tommorow to start the day, we will be dealing with some mid level and high level clouds as a shortwave trough tranverses the region to the NE. There could be some advection fog and stratus clouds across Long Island and SE CT, which will burn off during the morning under low level SW flow over still colder Atlantic waters. Then partly sunny conditions will be found and surface heating will commence over interior areas allowing for temperatures to once again get up into the 80's and lower 90's, even as 850mb temperatures are a tad bit higher tomorrow than today. Strong SW flow in the lower levels will allow for seabreezes at the coast, making for cooler conditions there. However, amny areas on Long Island did hit 80 degrees earlier on before the seabreezes kicked in. It will be more humid tomorrow, as dewpoints climb into the 60's.

So, severe wx tomorrow? I'd think so. The ingredients will be there for afternoon convection. First trigger is the approaching cold front. The cold front will approach very slowly eastward as it alligns with the SW upper flow, running paraellel to the front. Problem here is there is not a ton of upper level support to kick the front through so the front will weaken with time. MLCAPES will top off around 1,000 K/KG, more low level moisture as we see a rise in dewpoints. So the front will run into a moderately unstable airmass. We were capped for most of the day today, but that won't be the case tomorrow as a mid level shortwave treks through, and it will provide lift, so little to no capping. Another trigger will be the pre-frontal trough that will develop to the west of the NYC area. Soundings show pretty decent low level shear and deep layer shear and a 50 knot SW LLJ will develop at 850mb, that will be a huge key as well. Strong mid level winds, strong deep layer and low level shear will be conducive for supercells and bowing segments to form, as well as a isolated tornado or two. Large hail and damaging winds can also be found. Further east, the marine layer will inhibit surface instability, thus convection as well. Upper jet support won't arrive until later at night for those areas, but loss of heating will prevent any severe wx, but weaker convective activity can still be found. Showers will continue tomorrow night as the front slowly sinks southward. Models show some elevated marginal instability, and with enough forcing from the front, thunder is possible.

Not a pretty day expected tomorrow.