Winter Weather Forecasting
- Hello everyone ,
One of the most dynamic , thrilling ,and frustrating part of forecasting weather in the northeast is predicting nor’easters or major winter storms. Where will the snow-rain line be and how much?. Just when you think you have it nailed a wrench gets thrown in and changes the whole forecast. What the models had down as 1-2 feet of snow from DC to BOSTON 7 days ago ends up with 1-2″ of rain or a complete miss. The following blog is designed to help you (the weather enthusiast , or future forecaster) in your winter storm tracking, and to gain some very important information on what to look for. So lets dive in , we have lots to discuss.
The morning comes and you decide to check the latest 0Z GFS guidance , to your excitement it is showing a major winter storm to impact the east coast in the next 7 days. Are the models right? , maybe – maybe not , so what do you look for? , well , lets start out west and i mean way west like the eastern pacific. There are three aspects to look at with the pacific pattern. The state of ENSO , strength of sub tropical jet stream, and the strength and orientation of the polar/arctic jet streams.The el nino southern oscillation can have three states defined as cold(LA_NINA) warm (EL-NINO) or neutral. Many make the mistake in thinking that the strength of enso is most important. While the strength of enso is important , the orientation of enso IMHO is far more important.The reason really is because the location of rising and sinking air enhances or limits thunderstorm development around the date line. These thunderstorms alter the pattern in the pacific. When there is thunderstorm development around the date line , the potential for a major winter storm on the east coast goes up drastically. So one of the first things you should check out if models go crazy with blizzards is if the date line has developing thunderstorms. Next , you need to head north up to alaska. When a deep trough is present over the gulf of alaska , the polar jet stream under strong influence from a pacific air mass will dive into the west coast of north america. This type of pattern leads to a very progressive polar jet stream that would force the entire pattern set up along the east coast to shift into the atlantic. This would result in a major winter storm for the coast much less likely or not at all. When a trough is in place around the aleutions of alaska , the ridge/trough axis alignment sets up perfectly for a winter storm to develop. A ridge axis is able to build over the western coast of north america and a trough is more likely along the east coast. This type of pattern introduces polar and arctic air masses into the east coast and also sets up the upper level pattern over the atlantic to support high latitude blocking. Lets go back south to the sub tropical jet stream. The STJ sets up from the tropical convergence zone along the tropical pacific towards the southern california coast. The jet stream moves through the SW US towards the SE US coastal waters. The more active the jet stream is , the more disturbances are present to produce a winter storm. However , the more active the STJ is , the higher the error is present in the models because we don’t have enough observations off the west coast of mexico to get a good handle of the disturbances several days out from event. When the STJ is active that is when the probability of Miller B or nor’easters is far greater.
One very important feature that needs to be in place at 500mb is a deep trough or ULL around 50N / 50W with a ridge extending over greenland. This is called a west based negative NAO , without this feature the likelihood of a major winter storm is diminished. A negative NAO pattern sets up several important features for a storm to develop. One feature is the ULL forces disturbances in the polar and STJ to move much slower. This feature called blocking is what causes disturbances to interact , phase and track in a northeast or north northeast path instead of a east or east-northeast path. Another feature it provides is the upper level convergence at 500mb leads to a strong polar high pressure to remain in place over new england. The position of the high pressure is important for keeping low level cold air in place along the coast. When the NAO is positive there is no mechanism to keep high pressure locked over new england and therefore the low level cold gets pushed away or exits ahead of the low pressure. The result of this leads to winds to veer to the east off the atlantic , which in turn drives in relatively warm air off the atlantic ocean. With high pressure locked in the winds instead are from the north to northeast and lead to the low level cold locked in to the coast. Another important aspect is the cold air from the high pressure is much drier than the air moving in from the low pressure which enhances ratios and potential for heavy snow due to the moisture gradient. Sometimes the high pressure to the north is very strong and that leads to a suppressed storm track which leads to the low pressure sliding well south of the northern mid atlantic region.
A strong knowledge of sea surface temperatures is very important. How many times have we seen snow that is falling change over to sleet or rain?. This is caused by whats called a coastal front. A coastal front is warmer air roughly about 10meters above the atlantic ocean trying to invade the coast. The warmer air at first invades the mid levels of the atmosphere causing the snow to mix with sleet and rain. To understand the sea surface temp anomalies is important in figuring out the thermal gradient along the coast. The stronger the thermal gradient between land and ocean the more powerful the storm can become. The source region for cold air in north america is over northern and northwestern canada. When strong polar and arctic air masses develop those air masses race south towards the canadian border. The key is to strike a balance so that the cold air does not overwhelm the pattern and force the storm track to the south. The arctic oscilllation is a measure of pressure and heights between the north pole and the mid latitudes. When the arctic oscillation is negative lower heights are found closer to the mid latitudes. When this occurs mostly the polar vortex is located around southern ontario or quebec. Much like a very strong negative NAO a very strong negative AO will lead to a suppressed storm track. The flip side is if the AO is positive this usually means the PV is closer to the pole , and keeps the coldest air masses over canada leading to a warm pattern for much of the US. A positive AO also leads to progressive low pressure and tracks to the north of the northern mid atlantic.
Now lets go back to our ridge/trough axis placement discussed earlier , The area to focus on first is the ridge axis over the western US. The position of the ridge axis in this location is essential in determining the track of the upper level disturbance downstream over the east coast. The ideal sweet spot is central montana , the trough axis over the east sets up over the mississippi valley. The orientation sets up a storm track for a winter storm to either form over the gulf coast and track towards the mid atlantic coast or for a new low pressure to develop around north carolina and track towards the coastal waters. The ridge axis over the western US is why the pattern in the pacific is so crucial. If a trough or ULL is too strong and is orientated close to the west coast , the ridge axis will either be too far east or too flat , leading to a very progressive pattern over the east. If the trough axis over the pacific is too far west then frequently the ridge axis is along the west coast which leads to a trough axis over the western gulf coast or plains and that leads to a storm track towards the great lakes.
A major factor for a winter storm is the boundary layer air mass. When looking at the boundary layer which is the ground to 10meters off the ground , you should take into consideration the time of year and current temperature and dew point range. When temperatures are near freezing in air temperature , the higher sun angle in the afternoon will warm the pavement above freezing up to 33-36 degrees , which means you have snow forecast but instead of 3-6″ only a wet road surface prevails. This issue is a factor in late february or march as a stronger influence from the sun is present. Another factor in the time of year is the thermal state of the atlantic ocean. The atlantic ocean cools far slower than the land, which is why coastal storms become so intense. However , in early december the sea surface temperatures off the immediate coast are still above 40 degrees and that warm air tends to influence surface temperatures along the immediate coast. What would be a all snow event due to 850mb temperatures being below zero degrees celsius ends up being a rain/snow mix due to surface temperatures around 34/35 degrees. If the air mass moving into the forecast area is very cold and dry , this can lead to a prolonged period of virga , which is snow or rain evaporating before reaching the surface. If the air mass is not being reinforced then warmer air masses from the south and east will be capable of eroding the low level cold air much easier as the cold air mass moderates. The key to making a forecast is to be sure to include all factors of the environment you are forecasting for. Having a understanding of boundary layer , are temps along the coast below freezing? , where is the coastal front located? , are the winds from the northeast or east northeast? , whats the dew point and dry bulb temperature?
Probably the most important part of all of this hard work in tracking the winter storm comes down to current observations day off storm. First , look at your dew points and the amount of dry air. The amount of dry air is key in determining the start time of precip. When air is drier than forecasted a lower snow total may be needed. Second , the saturation process of the atmosphere can help to cool the atmosphere. This process called evaporational cooling can cause temperatures to fall rapidly below freezing and lead to a colder environment than forecasted for. Breaking down the thermal boundary layer also provides you a clear view of the coastal front in place. The stronger the thermal gradient or difference between temperatures over the coastal plain versus the ocean itself , the stronger the low level lifting along the coast. This will lead to banding of precipitation within the precipitation shield of storm. The development of a dry slot ( which is when dry air is drawn to the northeast on the eastern side of the 700mb low pressure system) can be nowcasted using the water vapor satellite. The infrared satellite image can be used to detect the beginning stages of a coastal low forming as cloud tops will cool rapidly. The cooling of the cloud tops typically seen in blues and oranges signify how high the cloud tops are in the atmosphere. the colder the cloud top , the higher the cloud top is in the atmosphere , and therefore how much lifting is being produced under those clouds to lift the water vapor. When you see yellows and oranges develop along the coastal plain , you should be aware that low pressure is not only developing but intensifying because air is rising rapidly. The observation of convection within the warm sector (south of warm front & east of the cold front) is important because the additional latent heat release can intensify the low pressure at the lower & mid levels. The intensification of the pressure gradient not only can intensify precip rates but can also shift storm track depending on how strong and where the convection develops.
i hear this a lot with winter storms , ” the storm is becoming stronger and will produce its own cold air and change rain to snow along the coast”…..hmmmm produce its own cold air….oh you mean adiabatic cooling….the idea with adiabatic cooling is the storm gets so strong at the lower and mid levels that a void in the atmosphere is created. To balance out this void (conservation of mass) cold air is drawn into the center of the low pressure from aloft and at the surface. Cold dense air at 500mb is drawn to the surface to balance out the mass loss. This process is more advertised for precipitation intensity rather than precipitation type change over. Think about this….the air warms adiabatically when pulled down right? so the atmosphere must be pretty unstable for this process to cool the lower atmosphere. You must have enough instability to produce a rapid cooling process but keep warming from sinking limited. Very rare , it can happen but this theory holds more water with precip intensity not changeover.
LETS PUT SOME BASIC STUFF ALL TOGETHER
THE 500MB PATTERN
PNA positive…good sign , RIDGE AXIS OVER MONTANA….good sign , TROUGH AXIS OVER CENTRAL GULF COAST….good sign , NAO NEGATIVE….good sign , POSITIVE VORTICITY ADVECTION FOCUS TOWARD COAST….good sign
THE 700MB PATTERN
700MB LOW TRACK EAST OF LOCATION….steady , heavy precipitation , no break in precip expected.TRACK OVER LOCATION….steady precip , threat for dry slot , WEST OF LOCATION….dry slot influence likely.
TEMPERATURES BELOW FREEZING….good sign , ABOVE FREEZING…rain or rain / snow mix. 850MB TEMPS -3TO-6 , HEAVY SNOW LIKELY FOR AREAS BETWEEN THESE RANGES.850 MB TRACK EAST OF LOCATION….frozen precip likely , WEST OF LOCATION….rain likely
950MB TO 925MB TEMPERATURES BELOW FREEZING….snow likely. ABOVE FREEZING…rain or mix.
HIGH PRESSURE LOCATION OVER ONTARIO TO NEW ENGLAND….snow likely. EAST OF LOCATION….change over to rain likely
LOW PRESSURE LOCATION INLAND…snow to rain or all rain. ALONG COAST…snow, mix to rain. TRACK 150-200 MILES OFF COAST….snow / mix
LET THE GAMES BEGIN AND THE GOOD TIMES ROLL!!!!!
Meteorologist Steve Garry