Mainly Dry Pattern With Dorian Subsidence
Dorian is approaching the theoretical upper limit of its wind speed based upon sea surface temperatures, despite a barometric pressure higher than would be expected. This is partly due to its compact, small size and the conservation of angular momentum. As sea surface temperatures continue to warm with changing climate, the potential wind speed of such storms will tend to increase over time (at least, theoretical limits of wind speed will rise).
Although much more complicated, of course, since temperature differences force pressure differences that generate pressure gradients which drive the wind, the atmospheric area in which Dorian will tend to move can be seen on this extracted example (above) from a point in space and time. The best chance, as has been forecast from the MEAN of the 51-member ensemble group of the European Model, of a mainland landfall will be along the coastline of North Carolina. Note this would be an official landfall, as due to expansion of the wind field over time the impacts will be felt all along the coast even without official landfall.
If you have been following this in previous days you observed a trough in the flow field in advance of Dorian, stretched along and just east of the Atlantic Coastline (the suggested pathway for Dorian to follow).
As the storm circulation of Dorian lifts northward the flow field of the storm overwhelms this visible trough, although, an area of lower pressure often remains (actually develops) in advance of low pressure systems (relative to the surrounding atmosphere). Such changes, and associated height falls, along with axes of theta-e (representing available potential energy) are used in combination with many other parameters to help forecast future storm movements (both tropical cyclones as well as mid-latitude cyclones). Improved forecasting over time being largely driven by vast increases in computing power, via supercomputers, and a vast increase in the amount of data available to be put into models. Due to the non-linear nature of fluxes and feedbacks between interconnected components of the climate system, the challenge of accurately forecasting these events remains significant, to say the least, and no one should get overly confident that the next big storm will also follow the predicted pathway as closely as Dorian up to this point (the storm event is NOT over by a long-shot at the time of this writing for the Carolinas, on 5 September 2019 at 0730z, with many long hours remaining).
This is merely for educational purposes, as this pathway is NOT set in stone and only small westward shifts could greatly change impacts along the Atlantic Coast. It should also be noted that the future storm track may come into alignment with the Gulfstream Current, such that future intensity changes become more uncertain.
Gulf of Mexico-Atlantic Gulfstream Current
Updated_6 PM_4 September 2019: Observe that a recent increase in the strength of Dorian is occurring as the storm moves more in alignment with the high energy content of the Gulf -Stream Current, as I suggested a couple days ago.
Although some local showers and downpours in thunderstorms will be possible Monday afternoon, and possibly on other days, mainly dry conditions are currently expected across most of the mountain area for most of the time through this week as potent Hurricane Dorian moves along the Atlantic Coast, with increasing subsidence around its periphery. Building heights and warmth is expected in Dorian’s wake by this weekend into next week, which will tend to prolong the general dryness and promote late season heat across the Tennessee Valley and southern Appalachians.
The NAM Model suggests some localized, hit-miss activity will be possible, especially during the next few days ahead of main subsidence (sinking air) influences of Dorian.
A cold front and initial indirect influences from Dorian will turn low-level winds northerly, with significantly cooler air, Thursday into Friday, especially in locations along and north-northwest of the Cumberland-Allegheny Front where adiabatic upslope flow cooling will be enhanced. As Dorian pulls away, subsidence will begin to rule the vertical motion field by this weekend into next week as discussed later in this section.
Dorian’s impressive satellite presentation continued into the early hours of Labor Day, with only limited signs of change.
A hurricane is a thermal heat-engine with strengthening, when all else is neglected, directly proportional to the rate of energy transfer across the air-sea surface interface. Surface and sub-surface ocean temperatures are therefore critical to development via strengthening of the latent heat flux and a vertical transfer of energy stored in the ocean to the atmosphere. This creates formation of a temperature gradient which supports a pressure gradient that drives horizontal and vertical air flow into and out of the system. Maximums of theta-e and vorticity become subsequently concentrated within the eyewall of tropical cyclones able to utilize available energy to reach hurricane status (wind speeds of 74 mph or higher).
At just past midnight, September 2, the position of Dorian was nearly identical to where the mean of the 51-member European Ensembles predicted it to be from the 1200z (8 AM) model run of September 1. The real test, however, will come on this Labor Day when Dorian is predicted to slow down and “briefly” stall by Monday afternoon before making a NW turn.
Water vapor imagery (below) reveals strongly sinking air within the eye and a beautiful, feathery array of gravity waves associated with anticyclonic outflow aloft. Gravity waves are also generated by mountains (like the High Knob Massif) and function to transfer mass, energy and momentum through space and time (spacetime). Like ocean waves rolling into a beach, feeling the frictional drag of the bottom, they eventually break and dissipate, fully releasing their transported mass, energy and momentum to the surrounding atmosphere.
UPDATE_The new Labor Day morning update of the European Model remains very consistent with above noted trends, with a slow wobble ongoing before a turn and increase in forward speed develops into Tuesday (September 3). The new track forecast keeps the eyewall of Dorian offshore and close to or barely crossing the Outer Banks of North Carolina.
NOTE: I am not allowed to show the detailed 51-member European Model (ECMWF) ensemble model cluster, or the individual member solutions, without a license from the European Center for Medium Range Weather Forecasting. The cluster below includes many different models generated by different agencies.
Late Season HEAT Builds Into Next Week
Late Summer 2019_High Knob Massif
A short-lived push of cooler air during September 5-6 will give way to warming conditions, at least by day, during this weekend through much of next week as upper air ridging develops across the mountain region.
It is not surprising that the core of highest upper-level heights into this weekend and next week will be located over the same region where heights will be lowest this week with the movement of hurricane Dorian, as a huge amount of air being ejected aloft in gravity wave filled upper-level outflow will be sinking in its wake.
Recall the atmosphere expands vertically as it warms, such that the height of any given level rises. A vertical column from the surface to the top of the troposphere expands such that a 500 MB forecast chart can be used to predict mean temperature in addition to other parameters. The 591 decameter value on the above forecast chart is equivalent to 19,378 feet (meaning that the 500 MB level will expand vertically to reside approximately 3.7 miles above the smoothed surface). That is well above the climatological mean height of the 500 MB surface for this time in September, thus forming a positive height anomaly over the SE USA. Observe that a stronger (+) height anomaly at 500 MB is forecast over NW Canada, where the actual 500 MB height will be lower than over the SE USA but more anomalous relative to climatology given the typically colder, more contracted vertical nature of the atmosphere above the Northwestern Territories at this time of year.
Subsidence (sinking air aloft) in the wake of Dorian will act to enhance a synoptic-scale pattern supportive of late season heat next week as the hurricane moves across the North Atlantic.
Anomalous to record level heat for September will be building over much of the southern-central portions of the USA, especially to the east of the Rockies centered across the middle Mississippi, Tennessee-Ohio valleys and southern Appalachians.
