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May 17th-20th 2018 Heavy Rain Discussion & Obs

Rtd208

By Rtd208,
in New York City Metro

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Rtd208

Rtd208

Posted
Posted

Mt.Holly: 

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... 
  High pressure over eastern Canada continues to build to the south  
  and east and moves into northern New England tonight. Meanwhile,  
  weak low pressure lifts to the north and east tonight, and begins to  
  push the stationary boundary north as a warm front. It will be hard  
  for showers to make it into the northern portions of the forecast  
  area, so will keep Pops confined to chance for the southern Poconos  
  and northern NJ. Otherwise, with increasing low level moisture, and  
  PWATs building to around 2", widespread moderate to heavy rain  
  showers will lift into the Delmarva, southeast PA, and southern NJ  
  through the night. For areas north of I-195, generally 1/10-1/4" QPF  
  expected. For areas where moderate to heavy rain will fall,  
  generally 1/2" to 3/4", with locally up to 1", QPF is expected. The  
  heaviest rain will move into extreme southern portions of the  
  forecast area prior to daybreak Friday. 
   
  && 
   
  .LONG TERM /FRIDAY THROUGH WEDNESDAY/... 
  The main focus in the long-term period remains the prolonged heavy  
  rain event expected through early this weekend. 
   
  The 00Z operational model suite is in fairly good agreement bringing  
  widespread 2-4 inch rainfall to areas south of I-78 through  
  Saturday, as a slow-moving upper low lifts from the Mid-South at 12Z  
  Friday to the Mid-Atlantic by 12Z Sunday, becoming absorbed and  
  sheared out by stronger northern-stream flow. Of particular  
  importance are three meteorological phenomena present during this  
  evolution: (1) a deep/southerly low-level fetch with backward  
  trajectories into the eastern tropical Pacific and the Caribbean,  
  which will provide highly anomalous (and deep-layer) moisture to the  
  region Friday and Saturday, (2) a quasi-zonal/quasi-stationary front  
  across the Mid-Atlantic, providing a low-level, subsynoptic-scale  
  source of lift, and (3) favorable positioning of a slow-moving upper- 
  level jet streak (extending from New England into the Canadian  
  Maritimes). Until the upper-trough passes the Mid-Atlantic Saturday  
  night and Sunday, the region will be in prime territory for periods  
  of heavy rainfall, aided by warm-rain convective processes owing to  
  the deep, nearly-saturated troposphere. 
   
  Model soundings exhibit relatively meager instability (generally  
  elevated, given the front`s position to the south of the area)  
  through Friday night. Given this lack of CAPE below freezing,  
  relatively little lightning is expected with the convective showers  
  that develop. However, the warm-rain processes at work will generate  
  highly efficient rain rates, and there is strong evidence that  
  training of convection is possible at times through the event. The  
  00Z NAM and ECMWF output show widespread 3-6 inch rains for areas  
  near and southeast of I-95, with the CMC and GFS a little bit on the  
  drier side. These differences may mean little, though, as antecedent  
  rainfall has made for saturated soils that may be able to handle  
  little additional rainfall, particularly if occurring with heavier  
  showers. Did not make changes to the flood watch with this forecast,  
  as there remains a little uncertainty with the heavy rainfall  
  potential on Saturday. Nevertheless, the output from operational  
  models suggests another round of heavy rainfall is likely before the  
  upper low progresses through the area. Additionally, the upper  
  levels will begin to cool/dry during this phase of the event, which  
  will likely mean greater coverage of lightning. 
   
  There appear to be two phases to the heavy precipitation Friday and  
  Saturday. The first occurs Friday and Friday evening as a weak  
  perturbation impinges on the quasi-stationary boundary. Increasing  
  low-level flow (850-mb winds 20-35 kts) will aid in pronounced  
  isentropic lift near and poleward of the boundary, placing the  
  southern CWA (generally I-76 southward) in a very favorable location  
  for heavy rainfall. NAM and ECMWF are suggesting 6-hour totals of 1- 
  2 inches will be common, with locally higher amounts possible where  
  the strongest convection develops. As the perturbation continues  
  northward, the strongest lift moves off with it. However, the front  
  will advance northward as well, and the region will become  
  warmer/moister by Saturday. Another perturbation will lift  
  northeastward during the day, and more convection should readily  
  develop in the Mid-Atlantic just downstream. Convection may train in  
  this environment as storm motions will be north-northeast with low- 
  level flow from the south-southwest. This provides a favorable  
  environment for convective regeneration nearly opposite to  
  individual cell motion. Combined with the anomalous moisture in  
  place, more heavy rainfall is expected Saturday, though the threat  
  may become more localized in this regime. 
   
  The upper low passes to the northeast Saturday night and Sunday, but  
  there are indications a surface trough will remain in place on  
  Sunday. With the origins of the system in the southern stream, it  
  will remain warm/humid on Sunday (likely warmer, actually), with an  
  environment still favorable for isolated/scattered convection. Kept  
  slight-chance to chance PoPs on Sunday to cover this threat, which  
  the 00Z ECMWF, in particular, is honing in on. PoPs may need to be  
  raised if a more pronounced source of larger-scale lift is depicted  
  in future simulations. Notably, a northern-stream trough passage  
  (well to the north...in Canada) may provide such a source should its  
  southern extension become stronger than currently advertised. 
   
  A cold front will approach the area early next week as a stronger  
  midlevel perturbation reaches the Northeast. An attendant surface  
  low will likely develop along the front. Combined with differential  
  cyclonic vorticity advection and associated midlevel cooling, more  
  convection is expected to develop along the front Monday and Monday  
  night. Increased PoPs slightly during this period, especially Monday  
  evening, given the somewhat slower timing depicted by some of the  
  guidance. This front will likely hang up again as it meets the  
  sturdy subtropical western Atlantic ridge. With more upstream  
  perturbations incoming, chances for convection continue into Tuesday  
  and possibly beyond if the front does not progress too far south of  
  the area. Models are depicting a drier period as a surface high  
  noses in from the northwest around midweek, but this is not certain  
  given the strong ridging to the southeast. Currently, there is no  
  choice but to leave PoPs in for much of the rest of the long term at  
  this time. I did trend temperatures to seasonal averages by midweek  
  after a rather warm/humid start. 
   
  &&
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Rtd208

Rtd208

Posted
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Posted

Mt.Holly: 

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... 
  Friday night through Sunday... 
   
  The main story during this period will be ongoing periods of heavy  
  rain with an associated flooding threat that will last at last  
  through Saturday as tropical moisture continues to stream north  
  across an oscillating front. Forecast models in good overall  
  agreement with this scenario and Flood Watch has been expanded  
  through the day Saturday.  
   
  To begin the period Friday evening, a warm front will be located  
  near the Delmarva with a deep layer southerly flow of tropical  
  moisture continuing to result in anomalously high PWAT values  
  streaming north over this boundary. Lift will be also be aided by  
  perturbations in the upper level flow moving north. Expect  
  rain, heavy at times, to be ongoing at this time but likely  
  heaviest from around Philly southward closer to the warm front.  
  Heading through Friday night, warm front at the surface won`t  
  make too much progress north however expect warm advection to be 
  quite strong in the 850- 950 mb layer with 35 knot LLJ from the 
  south bringing elevated warm front with some associated  
  instability north through the forecast area. As result, rain,  
  heavy at times will continue across southern parts of the  
  forecast area but also expand north to affect northern and NW NJ 
  into the southern Poconos. Some thunder will be possible,  
  mainly over far southern areas, with the limiting factors being  
  very warm temperatures aloft. However this same environment  
  factor will continue to result in very efficient warm rain  
  processes favorable for producing high rain rates. In addition,  
  as deep layer flow aligns mainly out of the S/SW profiles  
  continue indicate a favorable set up for backbuilding and  
  training cells which could also enhance flood threat. So this  
  all said, threat for widespread urban and small stream flooding  
  along with more localized flash flooding will continue across  
  most of the area Friday night. This could result in very poor  
  travel conditions due to the reduced visibilities in heavy rain  
  and any flooded roadways.  
   
  Heading into Saturday, aformentioned surface warm front lifts  
  north as weak low pressure moves from Ohio NE across the lower  
  Great Lakes. Southerly flow of rich, tropical moisture continues 
  across the area with lift also continuing to be aided by upper  
  level forcing as main upper level trough begins to approach from 
  the west. Not expecting any real sunshine due to continuing  
  saturated conditions but weak diurnal heating in warm sector  
  will nonetheless result in precip becoming more convective in  
  nature increasing the chance of thunder. As such, while the rain 
  likely won`t be quite as steady, expect that periods of rain  
  and showers will continue across the forecast area and that  
  these will continue to be quite heavy at times. Favorable  
  environment will continue to persist for some backbuilding and  
  training of cells continuing to exacerbate flooding threat area  
  wide. It should also be noted, as we head through time, the  
  flood threat on larger, mainstem rivers will increase as excess  
  run-off and stream flow makes its way into these. More on this  
  is detailed in the hydro section.  
   
  Saturday night into Sunday, some changes in the pattern finally  
  occur, at least temporarily. Upper level trough axis swings from  
  Ontario/Quebec east into Atlantic Canada and this will drag some  
  drier air in the mid and upper levels south. At the surface, cold  
  front will move south toward the region Saturday night but stall  
  across the region into Sunday. The upshot is that the more  
  widespread soaking rains will abate during this period but the  
  threat of more scattered showers and storms will continue. In and of  
  itself the rain in this period may not be that significant however  
  conditions will be so saturated that it won`t take much to  
  exacerbate any ongoing hydro issues. Additional rain amounts for the  
  weekend period (Friday night through Sunday) should generally be in  
  the 1 to 1.5 inch range but locally amounts will be higher due to  
  the convective nature of the system. Also, this is in addition to  
  what occurs through Friday so all told, a prolonged significant rain  
  event.

Upton: 

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Unsettled weather pattern continues through the period.

Polar trough axis pivots off the Canadian coast into Saturday,
allowing Bermuda high to exert more influence on the East coast.
Meanwhile, a broad, weak trough over the Central US begins to
shear NE ahead of the next northern stream shortwave digging
towards the Ontario/Quebec border.

At the surface, this will have southern low pressure lifting
northeast through the Ohio Valley into the Great Lakes into
Saturday, with its associated warm front slowly lifting north
towards the region. Progress of the warm front northward will
likely be slow, as strong Canadian high pressure only slowly
moves offshore through Saturday. A soaking stratiform rain
likely continues into Saturday ahead of the warm front. Rain
could be moderate to heavy at times with modest frontogenetic
forcing and a sub-tropical moisture feed up the coast into
Saturday morning. Easterly low-level flow would favor orographic
enhancement along hill terrain of SW CT, Lower Hud, and NE NJ.

Warm front (perhaps with weak wave) should be able to move north
through much of the region by Saturday night as low pressure
track northeast through the eastern Great lakes, and 40-50 kt
LLJ moves across coastal areas. Activity should become more
showery, but potential exists for a few thunderstorms with heavy
downpours late Sat into Sat eve as warm front moves north.

A weak cold front then will approach the region on Sunday ahead of
shortwave energy shearing into the NE. Uncertainty on how quickly
the front passes through Sunday into Monday, and how far south it
pushes thereafter, due to differences in amplitude of next couple of
polar shortwaves rotating around Hudson Bay. Sct shower/tstms
possible along the front Sun afternoon, but overall forcing and
instability look weak. 12z guidance is into better agreement
that the front stalls through the DelMarVa with a brief respite
from the wet weather Sun night into Mon. Another frontal system
may impact the area later Mon through Tue night with high
pressure returning for the second half of the week.

&&

 

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Stormlover74

Stormlover74

Posted
Posted
4 hours ago, Rtd208 said:

Precipitation currently hitting a brick wall in Southern PA/NJ, I think this will be a case of delayed but not denied. While we may not see the heaviest rains with this system we should still get a widespread 1-2" rainfall with locally higher amounts once the precipitation moves in.

The nam continues to give us only about a quarter inch

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Rtd208

Rtd208

Posted
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Posted
3 minutes ago, NJwx85 said:

Been pouring here for several hours

Looks like pretty decent rains moving up from the south and west. I think today and tomorrow may wind up being fairly wet especially with the warm front beginning to lift back north which should introduce some thunderstorms as well.

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