Coastal Storm Observations/Discssions October 10th-13th

We can only hope Doc and I heard this on two other news stations - it fooled all the major mets! Another curveball from Mother Nature.

I'm disappointed in myself for not studying the upper level pattern closer with this storm. Clearly with a High to the north and all the dry air in the Atlantic this year, the rain would not develop as models showed because the 600-900mb levels are dry. Further, another storm in the Atlantic was robbing this storm of energy. Oh well, you take and learn from it. Off to the next one.

Although parts of SNJ have 3+ inches of rain.

Frank_Wx wrote:I'm disappointed in myself for not studying the upper level pattern closer with this storm. Clearly with a High to the north and all the dry air in the Atlantic this year, the rain would not develop as models showed because the 600-900mb levels are dry. Further, another storm in the Atlantic was robbing this storm of energy. Oh well, you take and learn from it. Off to the next one.

Although parts of SNJ have 3+ inches of rain.

We got caught model hugging badly with this one. As you said learn and move on

Through i have had pretty heavy rain all day today. Just close enough to the band of convection today.

Quietace wrote:Through i have had pretty heavy rain all day today. Just close enough to the band of convection today.

Barely a few sprinkles up here the last few days.Reminds me of that winter storm a few years ago that blasted Southern Jersey with 20 or more inches while we up here did not see a flake!

So huge bust along the northern half of NJ and all of LI right?  The precip maps on Wed for pretty much all the major models had as much as 1-5" of rain for these areas by this time.  So what exactly happened? Frank posted above stating that the HP to the North, the dry air, and the other storm in the Atlantic were to blame.  

As Thursday was unfolding and I woke up on Friday (the day the majority of moisture for me on LI was supposed to fall) it was clear to me on satellite imagery as to why coastal and northern areas busted. The HP to the north, and dry air were tiny players IMO.  The main driving force behind the physics of the bust was the second LP that developed well OTS east of the one we were focused on off the Delmarva coast.  

If you go back to Oct 6th in the Oct Observation thread I posted a blog discussing the known bias' of the different models and what they were showing as a soln at the time for this system.  Well low and behold the GFS actually had this thing pretty much pegged on Oct 6th.  Here is a quote from that blog.

" Here is hr 108.  Notice how the energy is trying to focus off the NC coast, but pay attention to the area of moisture to the NE of the focus of energy.  As time goes by the GFS eventually kicks out that piece of energy north and east and leaves behind a much weaker system off the coast.  As a result this weaker system gets influenced by the blocking to the north.  It actually retrogrades the energy back to the coast before actually heading south to just off the SC coast before eventually getting picked up by a front and NE OTS.  IMO the bias of this model makes this soln unlikely a this time, because a weaker system will be more heavily influenced by the blocking building back in effectively pushing it south like the model run shows, however, if you keep all the energy as one entity instead of kicking out a piece of it would lead to a stronger system which would come further north"

So the main reason there was this bust area was due to divergence.  Let me explain.  Look at the water vapor imagery from Friday morning.
 
HP to the north, LP off the Delmarva, and LP OTS creating a triangle if you will.  The X is the areas that busted.  Now as you all should know air moves from areas of HP to areas of LP.  Clockwise around HP and Counterclockwise around LP.  The developing LP OTS does two main things.  First which can be clearly seen on satellite is it diverts the tropical moisture feed OTS and into the LP that is OTS.  The northern edge of the cold front is more associated with the LP OTS rather than the one off the coast effectively cutting off the tropical moisture feed to the coast.  That being said; however, despite the fact that you can see dry air in the SE CONUS that looks like it is being pumped up the coast the precipitable water values, and relative humidity in the areas that "busted" were still plenty high to support steady rain Thursday through Sat both on the surface maps and at 700mb.  Unfort I did not save those maps. So the dry air really should not have been that big a player.  For those who do not know precipitable water values measure the total atmospheric water vapor contained in a vertical column of unit of air cross-sectional area extending between any two specified levels.

The second and main reason the LP OTS affected this system brings me back to the term divergence and back to the triangle formed by the two LP's and the HP to the north.  This next satellite image is the rainbow infrared imagery which shows convection.  Convection is where air converges and is forced upwards.  The darker the yellows, oranges, and reds on this image the more convection (rising air).  Remember when air rises it cools; when it cools the water contained within it condenses forming clouds and rain.  

Now look at the area I have circled.  Although the 2 areas of LP where relatively weak (1008-1010mb) the area between the two LP's has/is higher pressure relative to the centers of the two LP's.  The air in the center effectively diverges heading for the LP center off the Delmarva coast, and some of the air moving towards the LP center OTS.  As air diverges it is replaced with sinking air from above. The  divergence in the area I have circled effectively squashed any chance for the atmosphere to "ring out" despite high relative humidity and precipitable water values in that area.  

If there is no LP OTS the frontal boundary is attached to the LP off the Coast, the tropical moisture feed pumps in lots of moisture, and there is nothing but convergence in the area circled.  This type of "bust zone" is highly reminisant of all the Miller B type setups we had last winter where SE Pa area seemed to be surrounded by higher snow totals and they "busted".  As the LP from the southern and northern branches were beginning to phase the area between the two LP's had that same area of divergence.

Excellent analysis roc, very well stated in lay men's terms - easy to read and follow. Like you said on Oct 6th, the storm wouldn't be.
Off to pick apples and pumpkins in Warwick, NY!

Great stuff roc, I knew there had to be more to it, since the gfs is usually out to lunch and this time it was more accurate than the others...it saw something the other models didn't...good learning moment for me for sure...

Dunnzoo wrote:Great stuff roc, I knew there had to be more to it, since the gfs is usually out to lunch and this time it was more accurate than the others...it saw something the other models didn't...good learning moment for me for sure...

You and me both Dunn. Mugs I was fooled just like most everyone else. I thought what the gfs was showing last week was a product of it bias. I'll tell you what though watch for these divergence zones in between the phasing LPs during miller B setups this winter.