|FORECAST helicity of over 450 m2/s2 Advecting Toward East Central Florida late morning through early afternoon|
TODAY: Fairly complex situation evolving this morning as expected. For this morning so far, tossing the GFS since it has quick evolution of a surface low, whereas the trends of the short term RUC and the latest NAM hold off on that formation until later.
NOW: Appears a weak warm front just above the surface is stretched from off shore Ft. Myers toward Melbourne. Strong NE wind at Canaveral north of this feature back more toward the east and decreasing further south.
Instability from the Gulf Stream advecting on shore from Ft. Pierce toward MLB and south to Miami, and expect this could work on shore toward the Tip of the Cape by mid-late morning. The RUC and to some degree the NAM shows a variety of surface lows to form from near Miami to offshore Ft. Pierce to offshore the Cape with just above the deck a more concentric and larger area of low pressure forms along the warm front elongated as stated above through late morning all within the base of a 700mb trough. The warm front should be able to get as far west and north as Port St. John and NNE toward the North side of the Cape by noon as a slightly negatively tilted upper level trough moves through the Deep South and crosses the North Half of Florida.
Increasing Bulk Shear from the surface to 500mb, Helicity of 400-500 m2/s2 (very high), and CAPE of 400-750 (which is weak) combined with continued on shore flow in the "northeast quadrant' of a developing 925-850mb low centered near the NW side of Lake Okeechobee resulting in onshore flow, with winds veering to S to SSW-SW- W aloft and vorticity moving in with the upper level trough could set up a coastal rain event, all combined with the fact that all of the south half of the state will be within the left exit region of a strengthen upper level jet resulting in increasing upper level divergence and increasing lift could rationalize this rainfall and possibly some elevated rotating storms...but tornadoes do not look likely.
Worst case would be heavy rainfall along coastal Brevard to Vero Beach, but on the other-hand...if the factors all add up just a few miles further to the east and more quickly (per the GFS)...then light to moderate rain will be the rule ...with the better chance of moderate rainfall nearest the coast.
An urban flood statement was issued earlier for MLB and south, and with sunrise and greater progression/formation of mid level features with the approach of the upper level trough rain chances/amounts could increase especially near the coast where the low level winds meet upper level SW winds. There could be thunder by mid morning where all the ingredients meet and rotation in the mid-levels results in heavy down pours. It would not surprise me if additional flood statements are issued, and maybe a tornado warning..but believe any rotation to reach the ground would occur offshore.
The big questions are
1. Where will the 925mb low form and equally as important, when?
2. How far north will the 70F degree dew-points get. All models agree the Beach Line zone toward Tampa, the "Magic Dividing Line". Highest helicity will be right along the 70F dew-point line dividing the state in half..again along the "Magic Dividing line".
Easterly winds are resulting in good coastal convergence, so additional rainfall could occur all the way toward JAX near and east of I-95, but instability will be near zero north of Port Canaveral.
Vort maxes are still on the approach in the mid and upper levels with the heavier rain already occurring despite the fact that this energy and colder air aloft has not yet arrived.
The other big question is depending on where the mid level and surface features align, how far north will the heaviest coastal rain get? Guidance is indicating any mid-level convection to make it as far north as KSC with better flat out rain further north. It showed a brief lull in the activity early this morning as a ripple in the 700mb flow might be creating the rains currently occurring, but as the main mid-upper level trough approaches, this area lifts further north toward KSC to the beach line and west toward MCO (Orlando) with the best convergence at the beaches over the Barrier Islands of Brevard from 10am-2pm.
FURTHER SOUTH: It appears the rain has ended for South Florida for the time being, perhaps being 'capped' by warm air aloft over running an otherwise more unstable atmosphere. Whether any storms will impact this area later is a bit questionable, dependent on where the surface low develops, or more appropriately the low just above the surface. If it forms offshore, the resultant pseudo-cold frontal trough will form off shore, and this area will already by behind the front, thus...very little more rain. On the other hand, if the warm front turns into a cold front (rather than a front forming along the Gulf Stream), then South Florida could have another round of heavier rain and thunder after 3pm until around 6pm. South Florida will need to be monitored during the course of the afternoon to see how things evolve for their second shot of rainfall.
IT'S HALLOWEEN, WHEN DOES IT END?: Looks like rainfall will taper off and end with the setting sun everywhere, with some light rain possibly continuing from time to time until 8pm near the coast somewhere from Brevard to Miami contingent upon where the surface low strengthens to the east of the coastline (and back wash moisture wraps around the north side of it)...This day is standing up to its "Trick or Treating" by Mother Nature reputation. A rotating storm passed off shore north of the Port (Canaveral) on this day in 1997.
BEYOND: Briefly, continued NE winds rule the roost with another frontal boundary to cross near Friday but have no impacts but to enhance NE flow once it passes. Coastal showers (sprinkles) possible by Wednesday. There are TWO opportunities setting up for a Nor'easter like set up along the SE U.S> coast in the first 10 days of November which would be mainly an erosion issue, but the details are yet to be ironed out.