"But seeing they could not See; hearing they could not Hear"
“The views expressed herein are those of the author and do not necessarily represent those of the National Weather Service or affiliate/related organizations. Please consult .gov sites for official information”

"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, November 17, 2011

Dis-solving/De-Volving Frontal Boundary Making for Low Confidence Forecast

Radar Shortly after sunrise shows band of convection with lightning/thunder involved across North Central/West Central along a pre-frontal trough, with the actual cold front lagging by a good 3-6 hours.
CURRENTLY AT POSTING TIME 845AM EST: Low clouds/ceilings and some fog exist across the peninsula from along the leading edge of the pre-frontal trough (noted by the rain/thunder on radar above) and South, with the densest fog being officially report near West Palm, Winter Haven, and Ft. Myers. Elsewhere, skies vary from cloudy to nearly clear. ..with some sprinkles near the Keys and showers off shore South and South Central along the west edge of the Gulf Stream and/or the western periphery of high pressure pushing offshore the East Coast in the lower portions of the mid-levels.

Yesterday, a record high was recorded at JAX, and a record warm morning  low temperature was measured at Melbourne (72F). 

TODAY: Low confidence forecast in the rain department today. Based on latest RUC and very early morning GFS/NAM, what is left of the pre-frontal trough will press toward a North Brevard to Sarasota line by noon or shortly there-after. The actual cold front remains several hours behind though. During the course of the later morning to early-mid afternoon expect that some diurnal effects could allow a bit of a 'catch up game' to commence between those two features, although for the most part the boundary denoting the prefrontal trough never really clears much further south than the CDL (Central Dividing Line) tilted toward Sarastoa to Titusville/Sebastian (somewhere in there).

 Cloud cover will also be tricky since the rain/thunder forecast (particularly from Central portions) will be at least partially reliant on 'lack thereof clouds' to destabilize the atmosphere. Additionally, during the course of the day Deep Layer Shear (through all portions of the atmosphere) and increasingly weaker lapse rates (temperatures not decreasing as much with height) will add to the uncertainty on how unstable the atmosphere will become through a deep enough layer to generate thunder. 

As can be seen on the earliest available visible satellite image, high clouds could impact Central late morning through afternoon. Whether those will erode and/or press off more toward the NE is uncertain, but so far that does not look to be the case.
In this satellite presentation, the band of clouds/rain can be seen across the North Central Peninsula in relation to the cold front. It does not appear that they will be eroding, and could begin to blanket Central Florida as far South as Melbourne by late morning, however, some of what is near East Central in this image is actually only  low clouds / fog, which will dissipate by mid-morning.
     Time-phasing just exactly 'when the front will be where' becomes more difficult when looking through various levels of the atmosphere. High pressure is also building ENE from the SW Gulf, so the upper level winds could become WNW-NW prior to the surface wind shift related to the front proper. 

When all is said and done, and to keep it at a less confusing level for reading purposes, Dead Central will be within the confounds of the frontal boundary at some level or another (surface to 5000 ft) from where the pre-frontal boundary is now located and south toward a Vero-Charlotte (west coast) until late afternoon. South Florida might never see a solid SW wind ahead of the front, but could still see showers and maybe even some thunder either way though.

EARLY MORNING FIRST BEST "WILD  GUESS" for rain and thunder chances. Do not expect a broad coverage at any one time, with rains confined to either Central (especially near the east side) and South of Lake Okeechobee after 1pm and beyond. Dashed orange indicates where thunder could occur, but as noted above, at this time of day (9AM) it's as clear as Mississippi Mud just exactly where thunder is most likely to occur.
IN SUMMARY FOR TODAY: I'd anticipate that it will become much more clear where thunder/showers could or will occur by 11AM -1PM once diurnal heating has commenced and the cloud pattern trend becomes apparent. Latest RUC/NAM and the GFS (which has been consistent) that there will be a frontal 'hang-up of sorts" during the diurnal cycle along a tilted CDL as noted previously (above). Showers/thunder could occur here near that line from just after noon time toward 4pm concurrent with other showers further south of Lake Okeechobee (mostly over the Everglades)..SW of Lake Okeechobee.  Chances are, that once the sun gets within about 30 degrees of the horizon there will be a rapid transition with the frontal boundary re-emerging on paper over Southern Dade county (around 7pm)..after which point it progresses toward Central Cuba by Friday afternoon.

LATER TODAY/TONIGHT: Rain chances end for those locations currently receiving rain now by late morning, with the better chance toward Southern Volusia to Southern Tampa Bay and south across the Space Coast / Orlando. (as noted) well as South Florida mainly near the East Coast and SW of Lake Okeechobee from 1pm and beyond.

Winds today will be mostly SW and become nearly west behind the decaying prefrontal trough. Frontal passage will be non-eventful with a wind shift to WNW eventually NW and then North, working down the coast from Central to South Florida between 6pm to mid-evening, and increase in speed as winds become Northerly and eventually NNE-NE overnight to early morning in the 20-30mph range, especially near the coast.

TEMPERATURES: As of this morning, temperatures behind the front are in the mid-upper 50Fs in the western Panhandle, and even still in the 60Fs behind the front after immediate passage. 70Fs remain toward JAX and South. Post frontal temperatures late tonight through Friday will resolve with lows mainly in the low-mid 60Fs from Ormond and South, and rising toward the mid-upper 60Fs Space Coast and low 70Fs Treasure Coast and south. Afternoon highs ranging from the 60Fs NE Florida toward 70F mid-lower Space Coast and above Treasure and south. Ongoing NE-ENE winds of 12-25mph off the ocean (the quick recovery in onshore winds) will preclude significant low level atmospheric drying, so this day could also be accompanied by scattered to broken stratocumulus clouds and rain showers/sprinkles (mainly South Central/South Florida) providing for further reason to temper down afternoon highs as well as preventing overnight lows Friday night from falling little if at all South of DAB after sunset Friday night.

WEEKEND: GFS continues with the trend of high pressure rolling quickly from Western Kansas eastward to Arkansas/Tennessee and off the Carolinas by Later Saturday. Winds become Easterly and gradually decrease on Saturday afternnon . Eventually, winds above the deck become almost ESE and rotate a portion of the mid-level front back north toward South Central. Rain showers and continued cloud cover at times during this transition (especially Sunday) could result in sprinkles and measurable but small amounts over South Florida and Coastal South Central, with lesser amounts to barely a trace north to JAX...although sprinkles could occur toward JAX as early as Later Friday as well.

MONDAY: By Monday it all becomes a moot point as winds decrease and high pressure in the mid levels builds directly over the state ahead of the next large upper level trough over the Western US. Still though, there could be some annoying Stratocu almost anywhere through Monday/Tuesday, with a re-emergence of the early morning/overnight low cloud/fog issue in extended forecasts under the high pressure since low level moisture remains, especially Tuesday/Wednesday.

WEDNESDAY/THANKSGIVING: As noted in previous posts, we've been essentially on a "one front per week" schedule/cycle over the past month, and the next front enters the picture unfalteringly (unfaultering?) so per guidelines. This has been on the platter for several model runs, but just exactly what transformation /weather conditions this front will entail is only slowly coming to light. The trend has been, or appears to be heading toward, a cloudy and possibly more wet Thanksgiving and into the weekend..with comfortable temperatures: aka - the round the clock 70Fs, with some lower 80Fs scattered asunder.

However, on the other hand, the GFS is screaming "PATTERN CHANGE" as well. This will be a change in the hemispheric (northern) realm more than likely, with the next front forecast to come in quickly only 4 days later as opposed to 7. This will be very close to the last day of the month, eerily on the cusp of calendar/meteorological winter, December 1. Could be a big drying trend in the future with another overall lowering in morning lows and afternoon highs the majority of the time, or rather, toward norms. In short, winter could be knocking on our chamber doors...Get Back, Jack!...(need we say, frost?).  

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