Posted: 12:15 pm Wednesday, June 6th, 2018
By Kirk Mellish
Meteorologists look at synoptic charts for the position of upper-level ridges of high pressure and troughs of low pressure in the jet stream, both analysis (current data) and forecast projections from models of the future.
Because upper-level ridges and troughs go a long way in determining wet/cool and warm/dry areas.
However, research has shown we don’t have to rely just on computers. Patterns of ocean sea surface temperatures can signal ahead of the models where troughs and ridges will be favored due to energy exchanges between ocean and atmosphere. The same is true of dry areas and wet areas on land due to the energy feedback loops between land and atmosphere.
They offer us clues without computers.
We often use these to estimate what kind of winter or summer we will have based on these known relationships.
We can also use them as guides for the average weather over the next few days to next few weeks.
In general high pressure is favored over or downwind (to the right) of abnormally warm sea-surface temperatures, with troughs of low pressure on either side. Conversely, unusually cool sea surface temperatures favor troughs of low pressure over or just downwind with ridges on either side.
Likewise, dry ground favors high pressure and wet ground favors low pressure with the opposite on either side of both just as with the warm and cold pools in the oceans.
A positive feedback loop means wet begets wet and dry begets more dry. Thus reinforcing the hot dry ridge or cool wet trough and vice versa. This is why wet spells and droughts tend to be self-perpetuating.
It takes less sun energy to heat dry soils (and the air above) and more sun energy to heat wet soils and the air above.
This is why dry winters can lead to dry springs and a dry spring strongly suggests a dry and likely hot summer and vice versa.
We had a lot of dry weather in winter and early spring then a tropical air mass and some upper level troughs came along bringing the long spell of somewhat cloudy and wet weather, that was even before Alberto came and turned it wet in the Southeast.
So no surprise that most long-range models continue to show an absence of long lasting extreme heat in the East or Southeast this summer.
The pattern matches show up shorter-term as well in the charts below:
The European Ensemble Model keeps the ocean patterns much the same through October:
U.S. NATIONAL DROUGHT MONITOR MAP (University of Nebraska):
See above forecast chart from the GFS Ensemble model. You can plainly see the central North American ridge aloft (500mb jet stream, about 19,000 feet) fits ocean and land indicators.
It all lines up nicely. And so far it fits my long-range summer outlook previously posted and some of the additional analog maps I tweeted June 3rd.
It’s this pattern that keeps us getting teased with hot weather but then it backs off again. That’s just fine with me. Although the tendency for cloudier and wetter than normal periods remains as well.
The updated European Seasonal (weathermodels.com) are showing much the same for the rest of summer on average:
A lot of other modeling is showing similar output.
We will still get our share of summer heat but I see no reason to change the summer outlook posted May 18th. The worst of the summers drought and heat still looks to be Central and West U.S. in the mean.
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