"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, October 13, 2011

Scattered Showers / Slightly Better Chance of Thunder Through 11pm

Going back and forth today in regard to where there is a better chance of rumbles, with two regions (North vs. South) with the dividing line running from near Sebastian to South Tampa Bay... later North of the line and earlier onset south of that line.

TODAY OVERALL: Low pressure is becoming more focused now at all atmospheric levels from Central Wisconsin to the arrow head of Minnesota. A much larger trough encompasses the Eastern Third of the U.S. with what appears to be 3 distinct vorticity lobes rotating through Mississippi, Alabama, and Georgia per satellite imagery.  Only surface observations reveal what appears to be the true cold front by wind fields and dew point temperatures. A weaker surface low reflection which is barely closed is rotating up the east coast and is now located just off NYC. There are two other small indiscernible weak circulations scattered about within the large scale upper level trough, which is slightly negatively tilted.

Over Florida, the pre-frontal cold front is located as shown above per observations and satellite imagery. There is another boundary running from South Tampa Bay toward Sebastian Inlet, with a weak ridge running just north of that boundary to Southern Volusia County. Thus, sinking air with this feature should hold off  showers until much later today as it slowly erodes and the prefrontal trough approaches across Dead Central.

Further South, much less cloud cover (and daytime heating) could lead to showers almost anywhere, but the most favored area should be close to the east coast as peak heating ensues toward 1-2pm  from just south of Vero to Northern Dade County.

Short term models, namely the RUC and High Resolution Rapid Refresh are split with where the better thunder chance will be. The HRRR favors the southern zone , whereas the RUC favors the more northern zone toward sunset with the prefrontal trough. This post thus will account for both, since each area has both pros and cons  for the rumble department.  Otherwise, showers could easily occur in both.

BEST GUESS: Sea breeze development could easily occur today, but is not expected to be a big player today since it will be quite light and remain close to the coast (within 5 miles, maybe a bit more toward SE Florida).  The RUC has a big CAPE (instability) hole over East Central near the coast, but that might be because of earlier cloud cover which is breaking up, so disregarding. Either way, instability is not all so tremendous. The driving factors today appear will be respectable mid-level lapse rates as the 700 -500mb heights fall with the incoming trough during peak heating. Heating being, where there is cloud breaks. This is another factor to consider.

The other factor will be some lower to mid-level vorticity rotating through the much broader scale synoptic features. The earlier NAM and the GFS favored dead Central (namely Brevard and Indian River counties) late with these features, showing good vertical velocity providing lift.  Not expecting strong storms as of yet due to lack of surface based instability ..but as we work toward dark strong wind fields in the 20,000 ft level to the jet stream could create more than one active thunderstorm from Ormond beach toward Port St. John as the prefrontal trough moves in. But at this time appears to sketchy to highlight.

The prefrontal trough is generally 10-14 hours ahead of the actual front, and it should be across Dead Central between the hours of 7pm -11pm. The actual front will be sliding across the Panhandle tonight and a few showers and thunder could occur with that feature where better moisture ahead of it exists further north.

TOMORROW: By sunrise the prefrontal trough will be near Southern Dade County with the actual front across Dead Center, give or take 2-3 hours. The front is then expected to reach the pre frontal trough location by late day. Moisture with the latter feature is lacking, so no rain is expected with it. 


WEEKEND: Both features should align along the north Florida Straits. The latest 2AM GFS has rethought its position, and now has these boundaries back further north to where it first had figured they'd be 2 runs earlier.  Thus, the Keys and perhaps including the Key Largo area will be in and out of the rain chances over the weekend (at least as far as the local forecast goes). Elsewhere, surface winds will be quick to shift onshore behind the cold front, which really is hardly  even a cool front. Temperatures are not expected to fall as much as previously ascertained, thus the coastal areas will hardly notice a difference south of Cape Canaveral on Sunday morning, with dew points evening out across the South Half of the state by the end of the day. Overnight lows and daytime highs perhaps 2-4 degrees cooler than the past two days at best, if even that from Lake Okeechobee region and south. Most notable away from the coast is where the cooling will be recognized. It will be brief though.

BEYOND: Another frontal boundary and upper level trough will enter the Deep South working into Monday/Tuesday time-frame. Ahead of this feature the  earlier front of tonight/tomorrow will be drawn back north as a modified, subtropical warm front (not an officially recognized term) and moist air (with PWAT 2" +) in its wake as it lifts toward Georgia. Rain chances increase all areas during the day Tuesday, so that by evening most any where could receive rain. Timing , yet to be fully determined..

Meanwhile, a large and slightly organized area of low pressure is visible over the Yucatan. Between this broad feature which extends into the Southern Gulf and most of the Caribbean as well as what will become two frontal boundaries from Central Florida (by next weekend) it gets very messy. The GFS has been consistent for the most part for an inverted warm frontal like trough to extend from the region near the Yucatan to extend NEward across at least the South Half of the state if not further north, with rain chances re-emerging along and south of the boundary through  next weekend and into  the following week. 

It appears this could all culminate in a rain event for South Florida, namely from West Palm Beach to Ft Myers and South, but not of the magnitude of the previous event that occurred north of this region last week. Even so, some areas near Cutler Ridge can flood easily, and the area is already saturated from the previous event of late.

There might also come a time in association with the second front mid-next week the chance for stronger storms will need to be looked into. With this features comes Deep SSE-South  moist flow and divergent jet stream level winds hedging on departing left exit region winds, aiding in lift. Not sure which. This is one week out already though, so more to determine surely lurks.   

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