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