"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, October 27, 2011

Questions Regarding Second Half of Weekend's Wet Weather Chance

Latest satellite imagery shows weakened Hurricane Rita near the Yucatan. Yellow shows the moisture field in the Caribbean expanding north today. Ridge axis across Central Florida and cold front as shown from Virginia to South Central Texas. Note all the clouds (moisture) behind the front.

Very good weather forecast model agreement this morning with the evolution of events through Mid-Day Saturday. It is from there that the NAM/ECMWF vs. GFS differ in one (going on two) significant aspects which could greatly affect the umbrella requirement of the forecast from Central Floridians late Saturday and much of Sunday. For South Florida, there really is no impact (keep an umbrella handy for starters as it looks now). The overall differentiation between these models being the amount of moisture behind the surface cold front and how far south the front actually gets.  The other key aspect is how the GFS treats what is left of Rita in the Western Gulf. 

The GFS refuses to give up on the ship on the storm, and as such, the low pressure field surrounding the storm does not expand as deeply northward into the Central Gulf of Mexico but remains more compact Toward South Florida and the Caribbean. This in turn permits a clean frontal passage by mid-afternoon across Central.

On the other hand the NAM/ECMWF opens up the pressure field more which results in a discrepancy on how well the actua surface boundary through the bottom 3000 ft of the atmosphere can work down the peninsula on Saturday. The GFS takes the boundary clear through past the tip of the state, whereas the NAM hangs it up across Central for 18 hours, and continues to hang up the trough across South Central in the mid-levels even after passage of the front.

Otherwise, they are in close agreement. This disparity mainly only affects residents along Coastal Brevard/Indian River/St. Lucie and perhaps Martin in terms of whether it will rain late Saturday through Sunday. If the GFS is correct, no rain in those locations...on the other hand...say no more. 

MORNING Rapid Update Cycle (RUC) Model shows the Jet Stream. The southern branch jet is lifting north for the first time in over two weeks, in fact, it hasn't been this far north for I can't remember when by tonight. The northern branch is strongly across the NE States. Another slug of high wind speeds aloft await in Canada and are not shown. The upper level trough is shown across Texas. This configuration will slow down the southern end of the front from digging into the Gulf into Friday.

AREAS TO WATCH for what might directly or indirectly instigate rainfall through Saturday and possibly into Monday for parts of Central and South . Satellite derived imagery is showing the vorticity (wind energy in simplest terms) across the SW Gulf. The GFS did not pick this up very well. There is another area over the Bahamas that should eventually work north tomorrow once the ridge across Central Florida moves out to sea and drops further south. The ECMWF and FIM were really latching on to not only this vorticity, but also that associated with the frontal boundary last night. Note the big red area over SW Texas.
Some of this energy is expected to cross North and North Central on Saturday. ..and possibly again on Monday or maybe even Sunday (South Florida). But don't forget about that area over the Bahamas, with more entering the Eastern Caribbean  and working west to the Central Caribbean.

TODAY: Sunny to partly cloudy with more clouds over South Florida as deeper atmospheric moisture builds north. The deeper (but not ultimately the most moist air) will reach the Beachline corridor to Tampa by Sunset or sooner and stall there for the overnight, gradually working toward I-4 and perhaps I-10 by later Friday.  Best chance of rain today from West Palm and South, mainly showers but perhaps thunder over the Everglades to extreme SW Florida. Highs in the low 80Fs with pockets of mid 80Fs. Light wind.

FRIDAY: Showers remain over the South as frontal boundary approaches. Remember most of the moisture with the front is located behind it as shown in the first image (satellite), but moisture will have worked well far north from the Tropics as well by this time late Friday into Saturday. Rainfall limited by lack of dynamics as mid level ridge hangs on. Light wind.

OVERNIGHT FRIDAY/SATURDAY: Frontal boundary works down the state, possibly getting south of Dead Central by early -mid afternoon Saturday, Good chance of rain and possibly some isolated elevated thunder both ahead of the front and immediately behind it. Rain ending north Florida by noon. Chances of elevated thunder near the east coast. No severe weather expected as of yet but not quite 100% guaranteed. 

BEYOND: Stopping here for the forecast, but a discussion concerning 'why' seems warranted.

GFS MODEL: Front clears the state by late Saturday with strong NNE-NE winds beyind the boundary along the east coast. Winds 22G32 by Sunday morning throughout the day NE-East Central Florida, but dry Saturday night through the first half of Sunday north of Ft. Pierce.

NAM/EURO: Front gets hung up across Central  with rain chances AND wind near the coast. Likely induced by instability over the Gulf Stream waters and strong surface winds blowing across the low level instability. Front gets to far South Florida with the mid-level trough hanging up near Lake Okeechobee leaving Central (mainly SOUTH of Route 50) in a deformation zone. Showers right at the coast die off after reaching the coast (beyond I-95). Winds will be blowing down the east coast from off the coast of New England after frontal passage. This should allow ocean temperatures to cool to the north side of the Cape, but should be blocked from cooling on the south side of the CAPE this go around. If so, the NAM might have something going for it for rain chances from Port Canaveral and South along the Coast on Sunday...but such fine details at this point? Not so sure about that. I've noticed that The Weather Channel seems to be favoring the NAM/ECMWF solution of continued wet. Thinking that the more conservative and better bet for now is to run with the GFS.

Both models then bring another surge of moisture northward  from across the South Half of the state on Monday as the boundary buckles  with more rain chances mainly along and south of Route 46 toward Southern Volusia. Strong or breezy ENE winds Continue..perhaps veering to the ESE on Monday and weakening. Low pressure forms off East Central (somewhere) in the Atlantic downwind of the Gulf Stream and dragging the front back south and through the state Monday night.

BEYOND MONDAY: Will run with the majority that rain chances will end Monday night, although the FIM Experimental suggests a quick return to windy sprinkles along the East Coast by Tuesday. The GFS hedges more toward Wednesday and Beyond for this to occur.

TEMPERATURES: No cold air is expected. With N-NE  winds blowing across the Ocean South of the Cape with the coolest air in the 50Fs (at least) limited to NW Florida south toward Gainesville and the interior toward West Central north of Tampa. Highs and lows in the 70Fs, with 60Fs inland.

IN short, the big question is rainfall chances late Saturday through the first half of Sunday East Central. The other question is how quickly rain shower chances (likely only trace amounts) re-emerge early to mid week next week.

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