Heavy rains a comin'

[weatherwiz]

I know there has been some some about this in the SNE May obs thread but I figure I'd start a separate thread for this since that one is getting fairly long. I was going to start a thread on this last weekend but didn't get around to doing so.

A deep trough will develop over the next few days and position itself just to the west of southern New England with the trough axis located from the Ohio Valley all the way down into the southeastern states. With a mean trough axis in this position much of the eastern states from the gulf coast up through the mid-Atlantic will be engulfed in a very rich moist southerly flow which will pump a great deal of moisture northward from the Gulf of Mexico as well from the Atlantic Ocean, especially in the lower levels where the wind direction may be more of a southeasterly direction at times.

Along with the development and deepening of the trough a cut-off low will also develop and slowly propagate northward from the south towards the northeast. With a flux of moisture being transported northward and a fairly strong LLJ at times pumping in even more low-level moisture this cut-off low will be a focal point in providing a good deal of lift which will really help to generate precipitation each and every day. While there won't be a great deal of sunshine at all during the week there still will be solar heating occurring and this solar heating will also provide some extra lift and help to destabilize the atmosphere some.

The period from Sunday to about Friday is looking very unsettled. Fortunately it will not be raining all the time as the rain looks to be more periodic, however, when it does rain out given the high moisture content and how saturated the airmass will be the rain may be very heavy at times. While the airmass will not be very unstable due to lack of surface heating and poor lapse rates fairly high dewpoints coupled with some heating should yield to at least a weakly unstable airmass. With some decent jet energy at times, especially with the LLJ it isn't out of the question that some convection occurs as well. This would be important b/c any convection could lead to enhanced rainfall rates. This could lead to areas of flash flooding and will have to be watched.

One of the most difficult aspects of this forecast is determining where the heaviest rainfall will setup and quite frankly this may be rather impossible. Each and every day the axis of the heaviest rainfall may be quite different and considering we are not looking at a large and widespread rain shield some areas are going to get hit fairly hard while some areas really don't get hit hard at all....some locations could end up with 5-6''+ of rainfall total while other areas see as little as 1-3'' total.

By mid-week and especially towards the end of the week we finally start seeing some changes within the upper air pattern as the blocking begins to break down. A trough will finally enter the United States along the west coast and slowly begin to work eastward. As this occurs the ridge that will be over the central United States will also slowly push eastward, pushing the trough that will be positioned to our west through the area and eventually off the coast. What does this mean for us?

Eventually, for a short-period of time at least we should actually get a taste of early summer-time weather with very warm temps and high dewpoints. The trough working through the west and into the central US will begin to draw moisture northward and we should see another active period of severe weather across the US, from the Plains and eventually portions of the east coast. I suspect we see our first decent severe weather shot sometime in the final week of May or first few days of June.

[Dark Energy]

Wiz, trying to time any s/w trof and it's influence on any +RA is impossible at this point, like you said. It's basically hit or miss until we see a good LLJ set up over a certain region (for consecutive model runs). I love how the guidance can change a high precip axis within 12 hrs. Any met going with likely pops on one model run will likely bust. All that can really be said is what you outlined. Potential is there for high PW and downpours, but where is anyone's guess this far out. Good post!

[weatherwiz]

Wiz, trying to time any s/w trof and it's influence on any +RA is impossible at this point, like you said. It's basically hit or miss until we see a good LLJ set up over a certain region (for consecutive model runs). I love how the guidance can change a high precip axis within 12 hrs. Any met going with likely pops on one model run will likely bust. All that can really be said is what you outlined. Potential is there for high PW and downpours, but where is anyone's guess this far out. Good post!

Thanks!

Yeah this is extremely difficult to really pinpoint. One thing I think is possible is we see two periods each day with increased potential. One period during the overnight/early morning hours when the LLJ is usually the strongest and the other period during the mid to late afternoon hours when afternoon heating will provide some extra boost for to get the air parcels to rise.

One other factor that will make this even more difficult will be the mesoscale components. Low-level wind trajectory will be huge, especially for interior southern New England where if the low-level wind trajectory aligns the right way we could see some areas of precipitation enhancement due to upslope. Then you have areas near the coast that could see enhancement from any land/sea convergence and any type of coastal fronts.

[HoarfrostHubb]

Very well done Paul. You highlighted the setup and the difficulties associated with it.

Like you said, some folks will get poured on while others get mundane rain

[weatherwiz]

This is going to be one pretty interesting week as far as the rain and heavy rain potential goes. Some spots already saw a pretty nice soaking overnight but there is a lull in activity right now. However, this may change as the morning and afternoon goes on. Not only will this week be rather interesting but the forecasts each and every day will be highly complex as there will be lots of factors at play. Where the warm front sets up and who becomes warm sectored, how strong will the s/w's be rotating around the cutoff low and trough that deepens to our west, where does the strongest LLJ develop, etc.

As far as today is concerned as we move through the morning and into the afternoon hours showers should once again be on the increase as some s/w energy works towards the area. PWATS are going to be very high all day long and both the NAM/GFS develop a strengthening LLJ later on in the morning through the afternoon hours. With a wind flow off the ocean leading to the high PWATS this should provide enough lift/forcing for the redevelopment of showers. The atmosphere is pretty darn saturated so any showers could be fairly heavy at times.

As we move through the evening into overnight hours the coverage and intensity of the showers/rain should decrease as we lose some s/w energy and the LLJ that develops this morning/afternoon kind of fizzles out.

Tomorrow actually really shouldn't be all that bad...it doesn't appear any significant s/w's will be pivoting through and PWATS actually decrease for tomorrow...even some hints at a belt of dry air aloft. A developing LLJ to our south overnight off the mid-Atlantic coast works northward tomorrow morning/afternoon and this might be enough to spark off some on and off showers but tomorrow may actually be one of the drier days this week.

Moving into Tuesday/Wednesday is when things could become much more wet and perhaps even more in the way of widespread rainfall. By Tuesday the trough t our west will be much more deeper and the trough axis will still be located to our west, this will continue the very rich southerly wind flow which will continue to advect moisture northward up the eastern sea board.

Not only does the trough strengthen/deepen but the cut-off low strengthens a bit as well increasing the amount of lift just to the east of it. With the deepening of the trough to our west this should also yield to some height rises over our region, this could mean we actually see the warm front get pushed northward a bit and this is when at least parts of southern New England get involved in the warm sector.

What's also very interesting is how some models are hinting at the possible development of some sort of coastal storm. As the trough continues to deepen, this is looking like a fairly decent trough for this time of year, and the sub-tropical jet increases and with increased divergence aloft and convergence at the surface we actually see a coastal low start to develop and actually strengthen as it works up the coast. As this occurs we start to see a dramatic increase with the winds aloft.

By Tuesday/Wednesday as this system slowly works northward the winds aloft are going to increase, especially in the lower-levels of the atmosphere and this is going to throw even more moisture into our region. With the trough axis finally slowly beginning to shift eastward, the cut-off low slowly working towards the northeast, the developing sfc-low moving northward as well, the strong LLJ, and warm front in the area there is going to be a great deal of lift over southern New England.

I would think late Tuesday and Wednesday will be the timeframe of the heaviest rainfall and this is where a major chunk of the rainfall totals this week will occur. At this point it's extremely difficult to near impossible to pinpoint exactly where the axis of the heaviest rainfall will setup but some things to watch which could locally enhance rainfall are areas that are prone to enhancement from upslope. I would think though as much as 2-4'' of rain could be possible between Tuesday and Wednesday with some higher amounts possible as well.

With increasing dewpojnts and low-level warm-air advection there also may be a decent amount of elevated instability that works in aloft. If this occurs it could increase the potential for some elevated convection which could even further enhance rainfall rates and rainfall totals, especially locally. While there will likely be an inversion just above the sfc winds could be fairly gusty at times, especially along the coastal plain.

After Wednesday things should remain unsettled through Friday with mainly periods of showers but the upper-level trough finally begins to work towards and eventually off the coast.

[weatherwiz]

Well as thought it's looking like the Tuesday/Wednesday period will be the period of extremely heavy rainfall and this should lead to some flooding issues, flood watches already up for parts of the region and we may see the flood watches further expand throughout the morning and early afternoon hours.

A very strong s/w is rotating around the bases of the upper level low as we speak and some weak sfc low development is ongoing as well. These features are going to slowly work northeastward throughout the morning and afternoon hours. A LLJ will also develop and strengthen as well, anywhere between 35-45 knots. This is going to advect a great deal of Atlantic moisture into the region and further saturate the air. PWAT values are going to be around 1.5'' to 2'' which is pretty impressive for this time of year.

There should be several periods of very heavy rainfall throughout the region today and some of the rainfall rates could exceed 1'' to 1.5'' per hour at times. Lift should further be enhanced by a warm front which will be located just to the south of our region.

With some weak WAA occurring in the lower levels of the atmosphere and some decent lift there may be just enough elevated instability for some thunderstorms as well. Given how stable the boundary layer will be and how there will be no surface-based instability these storms will remain elevated. Besides the threat of lightning any storms will be capable of producing extremely heavy rainfall and this is where rainfall rates could approach 1.5''/HR.

There should be a bit of a lull in the widespread coverage of the precip tomorrow evening and through a portion of the overnight hours as the first s/w rotating through weakens. However, the lull will be rather brief as yet another potent s/w rotates around the closed upper-level low. By the early AM hours of Wednesday and through the morning and early afternoon hours of Wednesday there will once again be several periods of extremely heavy rainfall. Flooding could be an issue once again, especially if we see any training. With some cooling occurring aloft in the mid levels we will again see some elevated instability develop. This could lead to the development of some elevated convection and once again the threat here would be for some very heavy rainfall rates locally.

Finally after Wednesday things should SLOWLY begin to improve over the following few days.

Some locations though between today and tomorrow could easily pick up anywhere from 2-4'' of rain with isolated amounts higher, especially if we see more in the way of embedded thunderstorms.