"But seeing they could not See; hearing they could not Hear"
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"From its chamber comes the whirlwind, and cold from the scattering winds." - Job 37:9.

"The wind blows to the south and turns to the north; round and round it goes, ever returning on its course".

Thursday, February 10, 2011

Surface Cold Front Heading Into South Florida

Images: 1) Surface front location at 8:15AM this Thursday morning, very weak surface low pressure bubbles based on pressure readings, with no associated cyclonic circulations 2) surface boundary location around 10:15am right as the front crossed the Beach Line in East Central Florida

RECAP CENTRAL: Cold front crossed the Beachline at 10:15am this morning accompanied by some moderate, spotty rain showers and low cloud ceilings as well as some big temperature drops. Shortly after passing (for example)...the Canaveral AFS was 55F degrees whereas Melbourne to the south was 75F. Of course behind the boundary winds shifted immediately to the NNW-N. Front is now located across South Brevard as of 1pm and treading south toward South Florida where temperatures are in the low-mid 80s, and last I looked Ft Lauderdale was reading 87F and West Palm was 86F... with dewpoints around 70F all across SE Florida.

TODAY: For North Central and Central expecting mostly cloudy skies this afternoon. Where temperatures dropped initially they will likely rebound a bit this afternoon with peak heating yet to arrive. Rain showers are quickly disappearing from the radar scope behind the boundary and the sun is even trying to poke out here and there. Looks like it will be mostly cloudy this afternoon for a brief time, becoming predominately overcast once again once the sunshine works on the moist atmosphere still in place this afternoon. In fact, it seems to be getting cloudier even now as I type this! Can't write off a rain shower along and south of the Beachline entirely either through the remainder of the day, mostly later this afternoon and early evening hours.

The mid-upper level troughs associated with this weather complex have yet to pass...and as such the true cold air advection lags behind the surface front by over 36 hours. Where temperatures dropped initially I'm expecting a small rebound into the mid 50s (further north) to near 60 the remainder of the afternoon. Further south along and ahead of the boundary the change in temperatures will not be as drastic, but there will be one to a smaller degree.

SOUTH FLORIDA: As noted above, very warm to almost hot (for this time of year) down there early this afternoon, as well as muggy. The atmosphere has been quick to respond in the pre-frontal environment with some rain showers popping. Latest SPC mesoanalysis does show some moderately unstable air down that way, being somewhat capped by warm air aloft. A little convergence along the front proper could instigate some good lightning strikes here and there, but wind profiles are almost unidirectional...with no sea breeze convergence in play to aid in low level convergence and weaker winds aloft as those which are flowing overhead across Central and North Florida. Would expect the best chance of heaviest rain/possible lightning over eastern Palm Beach and Broward Counties...possibly Dade as well but it gets sketchy further south. In short, no severe or overly strong storms, but thunderstorms nonetheless.

TONIGHT: All model guidance is showing a resurgence in mid level moisture by tonight and into Friday ahead of the mid-level troughs, and cloud cover depictions are filled in across Central and South Florida through Friday in accordance with the aforementioned. With the sun poking through nicely as I type it is tempting to ignore what guidance portrays, but the fact is definite that those troughs are a long way off still as a more zonal flow across the SE states develops. It's clear as mud until sometime mid-morning Saturday when we are clear of any mid-level influences with yet the upper level 500mb trough to clear on Sunday.

With that said, believe it will remain mostly cloudy this afternoon despite the peak of sunshine now being observed here with temperatures otherwise noted above. Not much of a cool off tonight with cloud cover and higher dewpoint air still in place. Looks like the low tonight will fall to about what it got down to right after the front passed, if even that. Additionally, expecting winds to become more northeasterly later today (off the water) which will modify the overnight low temperature near the coast.

FRIDAY: Mostly cloudy through the day with a high in the low to mid 60s. Looks like the front will hang up for a bit over far southern Dade County tonight with a weak surface low forming offshore which will race off to the NE across the Bahamas which will aid in dragging the front completely through the state and the Keys. Locally, NE winds tonight over Central might back more toward the north early tomorrow...then resume a NE component by late morning as the low pulls away and high pressure tries to exert a greater influence from the north.

SATURDAY: As written yesterday, the NAM and GFS greatly diverge in regards to cloud cover and even a rain chances over Central and South this day. The later two runs of the NAM continue to show that trend, whereas the GFS shows nothing like this. Both models are consistent in their disagreeing outlook for Saturday. At least they are consistent, but regrettably different.

With that said, in this post I'm running optimistically and siding with the GFS, given the tendency for the NAM to run on "5-hour Energy" drinks at times as well as with the developing zonal flow pattern now in the works which should stretch out all the dynamics which would otherwise support rain chances or significant cloud cover on Saturday (as shown by the NAM). But to blend the two a bit (which is probably unrealistic) I'll give the NAM a little benefit of the doubt and place some clouds in for the early morning of Saturday...clearing within 2-3 hours after sunrise once heating commences. It's very hard to pick out a time of solid cold air advection, but it currently appears that it will be once the low that develops off the SE Florida Coast moves NE and develops into a closed 925mb low overnight into Saturday..another reason to clear out the clouds (if there are any) Saturday morning, which would be a bit in response to the final 500mb trough axis sinking through on going into Sunday...per the stretched out vorticity fields in the both the NAM and GFS.

SUNDAY/MONDAY: By Sunday morning the show is over from the synoptic scale meteorological aspect at least in regards to the SE United States. Coolest mornings these two days with a mainly north wind. Immediate A1A will be spared the worst of the cool spell (note, I didn't write 'cold') given the wind component with the interior and west side of the state getting the coolest to almost cold, but not freezing cold by any means. Lows along A1A will be in the upper 40s, whereas outside of the Barrier Islands it will be in the low-mid 40s. Monday morning will be the transition morning in regards to the cold, looks like a pool of cold air will reside on the north and west sides of Lake Okeechobee far away from the area of interest in this post, as well as over North Florida where it goes without saying it will be colder all mornings for a few more days. Otherwise, other areas will be similar to that felt on Sunday morning.

TUESDAY/WEDNESDAY: Afternoon highs transition from 60s to low-mid 70s with the biggest transition on Wednesday when higher dewpoint air overspreads the entire peninsula and North Florida. Morning lows will only be in the 50s to even low 60s A1A corridor on Wednesday morning with afternoon highs in the 70s.

BEYOND: Not much too write as it stands now for a good week. Focus will be on where high pressure that moves in will have originated from. Some periods of stratocumulus decks, and maybe a back door cold front set up around late Tuesday or Wednesday with little affect other than a coastal wind surge and a line of stratocumulus clouds, all of which is well beyond the scope and resolution (with any accuracy) of model data given the number of days between now and then.

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