THE 50-50 LOW....
What it is... why it is important... what it does
In my forecasts... discussions and essays that talk of the Risk threat or Likelihood of Significant winter weather for the eastern US in general and the Middle Atlantic/ Northeast in particular I often make references to something called the 50-50 LOW. This often draws a response from weather hobbyists and professional meteorologists alike.... What does the term "50/50 Low" refer to? WHY is it important? and What is its significance?
The term "50-50 Low" refers to the
development or appearance of a strong or intense Low-pressure area both at the surface and
in upper levels of the atmosphere -- usually at 500 MB -- over or near the positions of 50
degrees North Latitude and 50 degrees West Longitude. Hence the name "50-50 Low".
If you look at a map of North America you'll see that position of 50N Latitude and 50 West Longitude is also close to the province of the Newfoundland Canada. Therefore some Meteorologists will refer to the 50/50 Low as the "NF Low" but the terms mean the EXACT same thing .
As most snow lovers can tell you whether its the Middle Atlantic region or the New England a lot of snowstorms along East Coast and specifically in the I-95 corridor (from Richmond to Boston) feature either a changeover to Rain OR at least mixed precipitation. Inland of course the changeover often does not occur and this is reflected in the climatology which shows inland locations from Southwest Virginia all the way into central New Hampshire -- often referred to as the Interstate 81 or I-81 Corridor --- receive two to three times as much snow as coastal areas.
Occasionally however... the conditions and all the pieces will fall into place that will
support a Significant East Coast Snowstorm
(what I call "S.E.C.S")
or a Major East Coast Snowstorm (M.E.C.S.).
Many forecasters and weather hobbyists are familiar with terms such as the Negative NAO (-NAO) and the +PNA pattern. However the set up for SECS / MECS is far more complicated than just getting those two important teleconnections to set up in the right location and phase. In the general sense the appearance of a 50-50 Low is favorable for the pattern to develop a significant East Coast snowstorm. That being said of course it is also true that having a 50-50 Low does NOT guarantee a S.E.C.S. event anymore that having a -NAO does.
There are many many cases where having
a -NAO or 50-50 Low does NOT result in anything close to a significant East Coast
Because there is coastal Low .
However IF there is a coastal Low AND both of these features on present on the Upper air maps BEFORE the coastal Low Low forms THEN experienced forecasters KNOW that the pattern is setting up to become favorable for a SECS / MECS Event.
It is the relationship between
... the Polar Vortex (PV)...
and the 50/50 Low
that is critical in determining what the overall SYNOPTIC (large scale) setup is going to be and which areas of New England and / or the Middle Atlantic regions (or Both) are more likely to be affected by a SECS or MECS event IF one develops develop.
The appearance of a 50-50 Low affects the overall pattern across eastern North America in several ways.
1. THE NAO...
There are several of the various manifestations of the phenomena known as the Negative NAO:
So one of things a 50-50 Low does is it often enhances the intensity or amplitude of the -NAO in general and the Greenland Block -NAO in particular.
(You have to keep in mind these features are in a symbiotic relationship... what makes meteorology Glorious is that it reminds all of us that weather is a very connected dynamic type of science. We have the tendency to teach science in the Western world as compartmentalized areas of study. This often results in some rather unusual jealousy and rivalry -- mathematicians who will not talk to physicists... astronomers who will talk to those who are engaged in SETI research. )
At any rate sometimes the appearance and development of a 50-50 Low will shorten the wavelength in the PJ across Northwestern Atlantic to such a degree that it causes a Neutral NAO to "go Negative"...
... or sometimes a strong negative NAO will force a ordinary storm in the Canadian Maritimes to stall and deepen and become the new 50-50 Low.
2. COLD AIR SOURCE FOR THE EASTERN US
The ONLY way that the major cities of the I-95 corridor can maintain an all snow event is for the cold air Source to be of sufficient magnitude and location to allow the ageostrophic circulation to set up and keep the cold air flowing all the way to the coast during the snowstorm.
Without a 50-50 Low in some form or another... when cyclogenesis on the East Coast is occurring the cold air source ( the arctic or semi arctic High-pressure area) which was located over the eastern Great Lakes or Southwest Québec will continue to move at a steady pace to the East... driven by a strong PJ which allows the HIGH to slide off the coast... allowing low level winds to become Easterly or Southeasterly and force a rapid changeover to rain.
This the second way the 50-50 low is critical for SECS / MECS is in the synoptic setup. The 50/50 Low keeps the cold air source--- the Arctic or semi Arctic High over the Great Lakes or southeastern Canada --trapped and locked into place.
3.PREVENTS SYSTEMS IN PLAINS/MIDWEST FROM PASSING WEST OF THE MOUNTAINS
The third way the 50-50 low is critical for the development of East Coast snowstorms... is that it forces potential systems (i.e. short waves) that drop out of central Canada (Miller B systems) into the Upper Plains and track further to the South and East since they have to travel underneath and around the 50-50 Low. OR in the case of a Deep South Low pressure areas (Miller A systems) the 50-50 Low prevents Southern from tracking west of the the Appalachian mountains.
CASE 1 29-30 DEC 2000 "Millenium snowstorm" NJ/NYC M.E.C.S
CASE 2 25 NOVEMBER 1950 Great Appalachian storm