Evolution of Events that could eventually unfold -
Light onshore flow will continue over Southeast Florida during the day time, possibly becoming a bit stronger Monday and Tueday, decreasing over night. During this time frame the low pressure areas at the surface and aloft over the NE U.S. will finally wrap up and move out through Tuesday.
During this time frame the key factors at work for Florida weather next week will happen in stages starting from the bottom (surface)..and up (to 15,000 ft). That main factor will be the depth of easterly flow which will come in stages. Stage 1 is shallow easterly flow working up the east coast beginning Monday afternoon when stratocumulus cloud chances will become more likely further north. The chance of showers remains mainly for Dade county and the middle Keys.
Stage 2 will begin Monday night into Tuesday when the depth of easterlies up and down the coast builds to 2000ft above ground level further increasing the chance of clouds with showers/and thunder chances remaining over South Florida but beginning to decrease.
The trend is for continuous surges of high pressure to move south for Southern Canada early in the week, each deeper and stronger than the previous. Eventually on Wednesday, the 850mb high pressure builds in with easterlies increasing to a depth over head of 5000 ft at which point coastal sprinkles and additional cloudiness ensues. By this time the low pressure area off the NE U.S. Coast will be displaced as the high pressure continues to build south to greater atmospheric depths reaching the 10 to 15,000 foot level by Thursday morning and increasing in strength.
Initially, this will be a cold core high pressure ALOFT, with colder air passing over the much warmer air above the ocean, near shore the Florida East Coast. It does not look like surface winds will be any stronger than 15-20mph, but eventually as the high pressure builds south and remains centered over the Mid Atlantic states (composed of cold air in the mid levels), this will generate a thermal wind just above the ground from 2,000 - 10,000 ft, blowing at a speed in the 30-35kt range, with winds right at the coast and perhaps along the west coast near 20mph (maybe stronger toward the west side of the state).
During the final stage which is to begin Friday thunderstorms are possible, and even heavier rain storms could generate wind gusts to 45mph if the forecast of deep 35 kts winds just above the ground comes to fruition. It is interesting the GFS is also showing temperatures around 4C at 700mb and temperatures as low as -11.5C at 500mb.
These are a good 5-10 degrees colder aloft than what was seen most of the summer, while surface temperatures will remain in the lower/mid 80Fs or upper 70Fs. Such cold air aloft, combined with 35 kts winds would result in strong surface wind gusts as down drafts in even only heavy rainshowers work to the surface. Wouldn't be surprised to hear a few tornado warnings for landfalling waterspouts, but lets first find out if even ANY OF THIS will evolve.
(The GFS during latter portions of the summer over estimated the temperatures aloft in regard to how cold they would be beyond 48 hours almost continuously, so such extreme temperatures aloft are highly questionable, and thus so is the expectation of high rainfall totals and stronger wind gusts).
Strictly based on the GFS at this point since I do not have access to the ECMWF model in detail:
The ingredients at time appear will all come together during the course of late Friday lasting through Sunday. By Friday evening or late afternoon, thunder could accompany onshore moving showers and fast moving low level clouds. Rain fall totals over a 6 hour period could add up to 2-3 inches if not more as hypothesized by the GFS runs which first showed this to unfold beginning nearly 36 hours ago. Actually, the GFS has shown rainfall totals just offshore if not onshore up to 10" over a 72-96 hour period between Thursday sunrise through Sunday night. ..with the first real rainfall commencing along coastal Brevard associated with this scenario occurring toward late morning to early afternoon on Thursday. If so, that would be just a precursor for what is to come later Friday through Sunday. The baroclinic zone is then forecast to gradually sink south toward Palm Beach County heading toward Sunday.
Rain could fall anywhere from Southeast Georgia to Key Largo, with some 'hot zones' for highest totals so far being indicated for JAX to SE Georgia, Central Volusia to Vero Beach, and Ft. Pierce to Southern Palm Beach County. The latter of which could end up being Ft Pierce and West Palm Beach toward Boynton Beach as the supposed event unfolds, with the Canaveral area being another 'hot spot' indicator. These last three spots would be due to geography for the Cape and proximity of the Bahamas further south where vorticity streamers are more likely to be induced upstream of them toward Palm Beach County.
Recall, it was two days ago that before the GFS showed this to occur it was mentioned in a post that a major rain event for the east coast could set up 'somewhere' along the East Coast, especially toward Ft. Pierce to West Palm Beach area. This model has now continued to show such an evolution since that time consistently for three full runs of the model so that chance of said event seems more likely.
The causes of the rains would be steep mid level lapse rates, low level speed shear, and vorticity streamers in the mid levels which form along what is referred to as a baroclinic zone, which as mentioned above.
The baroclinic zone is a region in which a temperature gradient exists on a constant pressure surface. That would be contained in the deep , unidirectional flow from the surface to 12,000 due to stacked high pressure over the mid-Atlantic states in which the temperature gradient aligns vertically along a long-fetch easterly flow at the surface.
As defined, baroclinic zones are favored areas for strengthening and weakening systems; Wind shear is characteristic of a baroclinic zone. The term "baroclinic" refers to the mechanism by which vorticity is generated. Vorticity is the curl of the wind velocity field. In general, the evolution of vorticity can be broken into rain contributions from advection (as vortex tubes move with the flow), stretching and twisting (as vortex tubes are pulled or twisted by the flow) and baroclinic vorticity generates.
In other words, key ingredients for generation of rain. During this time frame the surface front along the Florida Staits will sink into the Southern Caribbean toward the Yucatan and Central America. And as chances would have it, so will to the Southern Branch subtropical Jet stream. This leaves the area from the SE Gulf, Florida to the Bahamas, Cuba, and Jamaica in a more favorable environment for subtropical cyclone development either over or very close to Florida as the baroclinic zone components wane (Monday/Tuesday) or over the South Central/South West Caribbean along the old frontal boundary which will also be another but much weaker zone for tropical development.
MONDAY AND BEYOND: The pattern so far has been shown to break down toward Monday or Tuesday, at which point we are beyond one week from today. During this time frame much of the Plains States and the Ohio Valley into the Northern Plains in areas like Minnesota and the Dakotas could be experiencing an early Indian Summer.
Lastly, although not shown, we will have to watch for a blocking (Omega Block) to form. If so, changes from the pre-established pattern could be much longer lasting..but first, it has to happen.