Tuesday, November 25, 2014

Know the ground, know the weather

"Know the ground, know the weather
and then total will be your victory."
- Sun Tzu, The Art of War, c.400-320 b.c



8:45 AM EST 11/26 - TEMPERATURES ACROSS MID-ATLANTIC DROPPING AS EXPECTED. STORM HAS A MOISTURE PLUME THAT EXTENDS TO SOUTH AMERICA.
  • SNOW WILL MIX WITH RAIN FROM NORTHWEST TO SOUTHEAST THIS MORNING. EXCEPT FOR AREAS ALONG AND EAST OF I-95, WHICH STAYS RAIN/SLEET UNTIL 1-2 PM. 
  • NORTH AND WEST OF I-95, EXPECT HEAVY WET SNOW STARTING NO LATER THAN 11 AM, CREATING DANGEROUS VISIBILITY ISSUES, EVEN IF ACCUMULATION IS SLOW.
  • PARENTS OF STUDENTS IN SCHOOL DISTRICTS CLOSING EARLY ARE REMINDED THAT DANGEROUS DRIVING CONDITIONS MAY EXIST DURING DISMISSAL AND TRANSPORT HOME. PLEASE MAKE EVERY ATTEMPT TO SEE THAT YOU ON TIME FOR PICKUP, OR ARE HOME BEFORE ELEMENTARY-AGE CHILDREN ARRIVE. 
  • WE HAD HOPED TO AVOID REPEATING THE FEB 2007 DISASTER. Perhaps next time.

7:30 PM EST 11/25 - COASTAL LOW FORMING OFF GEORGIA, RADAR SHOWS EXTENSIVE MOISTURE ACROSS THE SOUTHEAST SUGGESTING FORMATION OF AN "ATMOSPHERIC RIVER."
  • SCROLL BELOW FOR FINAL SNOWFALL MAP | "STORM GRADE" TOTALS in progress
For those still unconvinced this system will surprise many of us with significant heavy snow, take note of the point of origin for much of the moisture heading our way:
- From the Atlantic? Some, not all.
- From the Gulf of Mexico. Sort of.

- How about the East Pacific? 

The real answer: ALL THREE. This system is tapping into moisture sources from three bodies of water, starting in the East Pacific, then scooping up more in the Gulf, and finally tapping Atlantic sources once the winds shift northeast.

TAKE A LOOK FOR YOURSELF! Below is a NOAA Water Vapor image for the Caribbean, E Pacific and western Atlantic, showing an unbroken tropical feed of moisture 2,000 miles long originating from the West coast of Mexico-- all the way across the Gulf of Mexico, and all the way north to your door. THAT's why we're confident this system has potential to be significant.



WHAT TO LOOK FOR: This is, as they would say, a "fluid situation." We present several look-fors that will help you detect in the AM hours which direction this storm forecast will go-- either BOOM or BUST. Our thanks to Forecaster Troy from the University of Maryland, College Park for this assessment:

1. If temperatures are still well above freezing by noon time, snowfall accumulation could be less than expected.  
2. If you see snowing coming down heavily in the morning hours near I-95, this indicates the "bust scenario" of colder temperatures is playing out. Thus, the storm is likely to over-perform, exceeding current accumulation forecasts.
3. If the storm is stronger than forecasted, higher snowfall rates would also produce a more rapid drop in temperatures. This is known as "evaporative cooling" in which the formation of snowflakes from water vapor actually chills the surrounding air-- leading to more snow, falling faster and sticking easier because there is more of it to overcome warm surfaces.
FINAL SNOWFALL MAP 



BUST SCENARIO 1: TOO WARM! (75% chance) With so much warm air in place ahead of this storm, it is "on its own" to cool down the atmosphere enough to have "snow producing "temperatures. 
  • In this scenario, those in the C-2” zone would see nothing, the 2-4” zone would struggle to get a coating, and the 3-6” zone could only pick up a couple of inches. There are two sub-possibilities here:
A) Surface temperatures don’t cool down below the mid 30s. We end up with a “white rain” of snow that can’t stick, or if the warm layer is deep enough, just rain. This could move across much of the metro areas during the heaviest precipitation, preventing accumulating snow.
B) Warm air noses in in the mid-troposphere during the height of the storm. This flips precipitation along I-95 to all rain instead of a mix.
BUST SCENARIO 2: OVER-PERFORMING COLD! (25% chance) On the flip side, we could have abundant evaporational cooling that puts temperatures at 31 or 32° instead of a few degrees higher. 
  • That small difference means that much more snow can stick. In this scenario, western areas could see up to 8” , the 2-4” zone could be more like 4-6” , and the C-2” zone sees up to 4”
BOTTOM LINE? Sun Tzu was right, if you know the ground, and know the weather, then your victory will be total. In less than 12 hours we all shall find out who shall be victorious in the call for this first winter storm of our 11th season of forecasting!


Contributors: 
Forecasters Mike Natoli and Troy Arcomano, Univ of MD. Advisor R. Foot, Forecasters Jason Mitchell, Connor Meehan

27 comments:

kyleZ said...

Hoping for the "Houston we have a problem" definitely colder than I anticapated at this point 42 in Harford county.

Westsidehideaway said...

Looking up through the Westside hood night sky, while putting out my recycle bin a few minutes ago, I saw clouds and a faint star or two. I can only hope that by morning I am dragging the bin back in cause the city truck won't come due to the snow coating the streets. Hey. It is fun to think about. Just like hitting the lotto.

Jeremy S. said...

Okay, Andy. I'm hearing that temps are lower than projected. Currently 40-41.5-ish at my place. Dose this foreshadow higher totals?

Westsidehideaway said...

Yes. Andy. Inquiring Minds Want To Know !

ravensbbr said...

That Sun Tzu dude rocks...

Andy, Southern York County Pa said...

It may. Radiational cooling has been very efficient. The main thing to look for are last minute model trends concerning track and intensity. In the scheme of things 1.5 to 2 degrees is nothing. In terms of major snow it is everything. Models are not that great from 24 to 36 hours at perfectly predicting surface temps. Those on the margins will benefit greatly from ANY cooling when it comes to sticking and impact. This storm will surprise those on the margins. Tonight's new data will be interesting for the fringe areas.

weed said...

this is so exciting....when's the next update????

BioPat said...

Hi all, just checking in to see when the next set of data will be available. Clouds definitely in place and the grocery store shelves are emptied Thanksgiving and a snowstorm, thank goodness we just needed carrots and onions.
I'm still looking for morning rain with a mid-morning turnover to mixed slush then finally all snow. It will be interesting to see how long it takes for any "stickage" to occur.

Westsidehideaway said...

Yep. Clouds are now in place here in West Bmore. Hitting the pillow and will check with y'all in the AM. Gobble gobble

Andy, Southern York County Pa said...

Remember models never handle deformation zones, convective banding, and surface temps very well 24 hours plus out. This storm is apt to surprise the people in the fringe zone.

kyleZ said...

Models trending colder so far?

ravensbbr said...

Temps here in NE CC slooowly falling...down to 40 from 42 earlier...couldn't go hunt this AM...sucks...consolation prize of whiteout conditions maybe later? We'll see... :) #powderhoundlogic

ravensbbr said...

And...big fat flakes falling. No stickage. 37 degrees, wind NNE @ 7

ravensbbr said...

And...stickage. Anyone else?

kyleZ said...

A lot of stickage here but the rain variety hoping for atleast some snow.

Mike Cheuvront said...

Major Stickage here at the PA line in Northen Manchester!

Noah Jaques said...

The rain just changed over to snow here in Woodstock and the flakes are so big! It's only been snowing for 15 minutes but you can already see a dusting of snow on every surface except roads and sidewalks.

Leslie said...

Beautiful in New Market. Big fluffy wet flakes, stickage on my neighborhood streets. Happy Thanksgiving, be safe everybody!

kyleZ said...

Snow in Harford finally and 34 degrees.

Amy said...

Wet sloppy snow in pasadena. Definitely not sticking.

NeedaSnowday said...

We have thunder snow!

Butch Dynomite said...

THUNDER SNOW Parkville!!

kyleZ said...

Thundersnow!!! Saw a bolt in Harford county was outside with my sister and dog and it was right in front of us amazing and very loud.

Butch Dynomite said...

Not really sticking as much as falling faster than it melts

ravensbbr said...

Solid 3"+ here in NE CC, compaction playing in some, temps down to 32. Secondary streets covered.

Mike Cheuvront said...

Great to kick off the 2014-15 Snow-a-thon with a nice storm at Thanksgiving. Solid 3 or so in Manchester. The storm seems to be performing on the lighter side of accumulations. Cold air was not entrenched in time. Probably for the best since so many need to travel. Beautiful outside. -Andy- Again-amazing call from so many weeks back. I remember your post and salute you. You are the bossman.

ravensbbr said...

"Dang dry slots."

-said all powderhounds, always.

Andy, any chance of this wrapping back in as the low moves up the coast, or not so much?