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Heavy Rain/Flooding 4/30-5/1


bluewave

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The models are indicating that we will see a deep cutoff low get stuck under a strong block

from 4/30 into early 5/1. The heavy rains should slowly train over interior sections and 

eventually make to the coast under the very strong LLJ with the potential for flooding.

 

 

 

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Timing of the rain per 18z GFS.

 

Light spotty activity, especially over the I-95 corridor and points NW beginning around dawn on Tuesday morning. Staying spotty and light to moderate throughout the day. Rainfall totals generally ~0.25" through 06z Wednesday. More SW and less NE.

 

Moderate rain comes in early on Wednesday morning, in time to affect the morning rush. Looks like spotty activity overnight into early Wednesday. Slight break around mid-afternoon on Wednesday. Very heavy rain moving in between 6 and 9PM, especially NW of NYC. Heaviest rains overnight on Wednesday into Thursday morning. Heaviest over NNJ and the LHV. 18z GFS showing roughly 3.75" in about 6 hours falling over central northern NNJ. Moves out early Thursday morning.

 

Thursday - Heavy rain early, then spotty light rain and drizzle all day. Thursday night, clearing out. Total QPF in the 2.5-4.00" range. More NW, less SE.

 

The general conclusion with all of this is that the heaviest axis of rain will fall over the most flood prone areas of NNJ and NE PA.

 

HPC has the area blanketed in 3-5" totals.

 

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Here's what I'm thinking.

 

Tuesday: Rain will start to come up early in the morning. I see the rain really starting to pick up in the overnight hours.

 

Wednesday: Even more rain. Quite heavy during the day, and huge amounts during the evening.

 

Thursday: Heavy at midnight, but will gradually wind down. Day should be mostly dry with a couple of showers.

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Heaviest rains come in Wednesday evening, with the warm front/triple-point low producing a 50kt+ LLJ and Pwats 1.50"+. Might also be enough elevated instability for some embedded thunder:

 

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The steadiest and heaviest rain looks to be west of the city, as these setups often favor. There will certainly be heavy rain for a time in the city and along the coast, but it might be shorter duration as the front passes.

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The steadiest and heaviest rain looks to be west of the city, as these setups often favor. There will certainly be heavy rain for a time in the city and along the coast, but it might be shorter duration as the front passes.

They city and the coast don't have the same river flooding concerns as inland areas.

 

While it's supposed to rain for ~60 hours about 85% of what falls is expected to do so in about 12 hours or less.

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The steadiest and heaviest rain looks to be west of the city, as these setups often favor. There will certainly be heavy rain for a time in the city and along the coast, but it might be shorter duration as the front passes.

 

It usually depends over the best convergence sets up. Which is always hard for the models to pinpoint. The last few rainfall events, have had higher rainfall totals over the coastal plain.

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The SREF's trended quite a bit wetter this run as they start to come into better agreement with the rest of the modeling. 09z SREF mean has 2.00"+ for KNYC which is pretty impressive for a low resolution ensemble mean. SREF is also showing a pocket of very strong 0-3km helicity crossing the region with the warm front on Wednesday. A brief spin up or two is not out of the question if enough SBCAPE can be generated.

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One pretty certain thing that's showing up on the higher res models is that the initial band of rain that attempts to push in here around midnight tonight isn't going to make it. Initially a narrow ridge will keep us in the clear but most of us should be raining tomorrow by 12z, especially west of the city.

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