TODAY: Continuing on track today with the gradual evolution of events leading up to the first bonafide chance of measurable rainfall sometime toward sunrise toward noon Thursday along the east coast. Most likely, any measurable rainfall will occur between St. Augustine to Vero Beach, with trace amounts possible just about anywhere.
For today, similar weather to that which was experienced yesterday wherever one was. Another chance of skies becoming mostly cloudy at times south of Lake Okeechobee, initially near the coasts and working into the interior and west side later in the day. It appears the only chance of rain will be along the Keys, with a chance of thunder along the middle and lower Keys.
PHASE 2: Yesterday, very light NNE winds north of far SE Florida occurred as the winds at 2000 ft became NNE. Today, the low pressure area in the NE states is very close to the coast just south of Boston Harbor resulting in moderate to heavy rainfall across that area into the Southern 1/4 of New Hampshire and up along Coastal Maine. This low will move off to the ENE during the day very slowly. Meanwhile further south, the 850mb trough extending from this low (at 5000 ft) <associated with the large low pressure system> is working down the east coast. With its passage down the Florida east coast this afternoon, winds at that level will become NNE-NE as well, remaining very light. All in all, this means nothing to the observer, although surface winds this afternoon might become a tad stronger today making for a more notable light wind from the NNE-NE late in the day with a slightly better chance of stratocumulus clouds of greater vertical height, but still remaining widely dispersed/apart from each other. Toward South Florida overall moisture depth and extent is greater due to the circumstances surrounding the old frontal boundary over the Florida Straits.
As the mid-level trough works down the coast toward this evening associated with high pressure building toward the mid-Atlantic states, this boundary should be pushed south of the Keys even further and ending the chance of rain by nightfall.
On the flip side, the more the chances of rainfall decreases for the keys, the better the chances of clouds and eventually rainfall along the Florida East Coast increases going into overnight Wednesday night/Thursday.
Below is an image from the RUC (Rapid Update Cycle) model for 7am this morning. Also noted on this image is what is to evolve going into late Thursday/Friday out West.
WEDNESDAY- PHASE 3: As the low pressure off the NE states moves further away, phase 3 - The 700mb trough will swing down and pass across Florida. High pressure behind that boundary during the day will shift winds at that level to ENE-NE eventually as well (as opposed to the WNW direction they are today). Surface winds will remain NNE-NE round the clock at this point at 10-15mph and moisture depth will gradually increase as well, as such will the chance of more cloud cover but remaining mostly partly cloudy state wide as South Florida conditions improve. The old frontal boundary will now be down across Cuba.
THURSDAY/FRIDAY: Low pressure all across the Caribbean and SW Gulf toward the SW Atlantic, combined with high pressure building into the Mid-Atlantic States through a deep layer will increase moisture further with time across Florida. The pressure gradient will eventually increase with steady state 15 -20mph winds by early Friday. The first chance of measurable rainfall somewhere along the Florida east coast appears to come with and just ahead of the first wind surge early Thursday. During the day this chance spreads up and down the entire coast, but remains fairly light, but measurable EAST of I-95.
FRIDAY/SATURDAY: Additional wind surges in the 2-5000 ft levels increases low level speed shear and lift. With these winds comes vorticity streamers embedded in a bit of a baroclinic zone , with thermal winds aided in lower level wind speed strength, with pressure gradient winds at ground level. Rain showers increase in frequency and number and become heavier as moisture levels in the atmosphere above increase. Some thunder could occur by late Friday into Saturday. The GFS run of 2AM has an affinity for Indian River County north to Central Volusia counties on Saturday for bigger rainfall totals with potentially thunderstorms. At this time, there might be a slight chance of waterspouts mainly offshore, with perhaps a land-falling waterspout, although that chance will be very low and remote.
Throughout the duration of higher rain chances along the east coast much of the state beyond the spine of the state will likely remain rain free. In fact, most areas beyond 20-30 miles from the coast will be rain free through Friday.
The typical drive from further west, say for example from Orlando on Saturday, could consist of seeing increasing and lowering clouds as one approaches the coast, eventually running into some rain near I95 getting heavier crossing US1 and the rivers. Just an example, but the same general scenario about anywhere except perhaps South of Lake Okeechobee where there could be an 'Everglades Affect" along Alligator Alley heading toward Sunday. It will not be raining anywhere all the time, but an umbrella could become mandatory equipment when heading out; either that or simply wearing a bathing suit.
Late Saturday-Monday: The same old same old. Temperatures along the coast could possibly not vary more than 8 degrees if not less in areas of more consistent shower activity , remaining in the mid-upper 70Fs. Eventually almost every where along US1 and east will receive measurable rainfall by Friday Evening into Saturday. So far as of 2AM, the GFS is honing in on Saturday being particularly 'active' along the east coast from Brevard toward Martin Counties with stronger winds and possibly storms with thunder (but that could easily change with the next model run which will become early this afternoon).
It does not look like any stronger activity will move faster than 6-10mph since steering currents will remain around that speed. The strongest winds will be found 'just above the deck' around 2-8000ft, which is below the steering level, but fast moving low clouds are likely to be observed. Thus, the bigger rainfall totals of possibly over 4" by Tuesday over a 96 hour period will occur where the slower moving and heavier rain/thunderstorms occur.
The GFS still indicates totals of 9", namely just off shore from Port Canaveral to Central Palm Beach County, with actual land based estimates up to 4.5".
As suspected, the latest GFS is now extending the length of the REX blocking pattern to evolve as noted yesterday might be the case, from Monday to Wednesday, however surface winds will decrease in that case. And talking about winds, it looks like the strongest winds will remain just offshore, with coastal winds perhaps quite noticeably breezy to near windy on Saturday. Otherwise, it does not look like they will be anything unusual. The shallower shelf waters will aid in lift, and local geography could end up in higher rainfall totals though on land if all of the above evolves, but I will point out that none of the official resources (those being the Weather offices of the NWS) are talking big rainfall totals. In fact, only Miami mentioned them, and they as of early morning played them DOWN (might have been a mistake on their part though in regard to Palm Beach County).
BEYOND the TUESDAY/WEDNESDAY TIME FRAME: Models are in excellent agreement (of the 6 different ones I looked at including the Canadian, the European ECMWF, and the Navy NOGAPs...that we will have high pressure to the north and low pressure to the south in a modified REX block (modified in that the low is not completely closed in the mid-levels). The latest GFS (Global Forecast System) Model however eventually closes the low near Cuba to South Florida by Wednesday...and works it down to the surface just off the SE Coast of Florida. A Subtropical or a hybrid type low pressure system could form almost anywhere from off the Florida East Coast to off the West Coast into the SE Gulf, through the Yucatan Channel over into the Caribbean toward the Bahamas anytime and anywhere by early next week.
Models all seem to agree on such, but not on where or how. For example, the 2AM GFS shows a low to form off of SE Florida that gets wrapped NW ward , strengthening into a sort of hybrid tropical storm (of name) that cross NE and North Florida, then wrapping around the parent upper low over the Caribbean and moving the named system into the East Central Gulf of Mexico. I'm not buying it though. In any case though, the point is that although the prolonged wind/rain conditions of onshore winds developing Thursday will have by then been long gone, rain chances continue for a different reason.
Models seem to agree as well that heading toward Monday/Tuesday, low pressure over the Southern Gulf to the SW Atlantic will build northward across Florida as the high pressure to the north relaxes/realigns. This will end the stronger onshore flow as well as rain chances along the coast, but would also induce more of a modified afternoon thunderstorm regime.
It should be clear to realize from all that has been written above, that there is great uncertainty throughout all time frames beginning Friday (considering that is 3 days away already). But some heavy rainfall totals, if only just offshore, seems to be in the cards. The devil's in the details.