"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".

Wednesday, April 18, 2012

Evening Showers/Rumbles Spread East Today - Severe Threat Increases For Weekendr

Latest mid-afternoon radar. Showers could spread east later today and to the east coast well past dark as impulse passes over head, Little chance of thunder East of the State's Spine tonight. Details below.

TODAY: Frontal boundary and weak low pressure circulation is in the Western Panhandle extending north and east. The boundary can be seen by the extent of rain showers and thunder into the Gulf . There is heavier activity even further south in the Gulf, and storm tops are being sheared off, appearing across the Central portions of the state as cirrus clouds, whereas south Florida and other areas are seeing some harbingers of summer like cumulus clouds today.  Instability is weak across the peninsula though. Thus, in sticking with yesterday's post, believe most rain showers after dark will fizzle as they move east to west due to lack of mid -level moisture per forecast soundings and weak upward forcing combined with the meager instability. Forecasts I see are showing thunder, but if so, I'd think it would have to be very isolated. There has been consistency that showers and maybe thunder could persist until after sunrise along the east coast just offshore from JAX toward MLB or TIX..although this looks a bit questionable. This could be a back building type situation though from off the Gulf Stream waters, since some energy remains aloft.

THURSDAY/FRIDAY: As noted yesterday, it continues to appear that although the surface boundary near the Panhandle will begin to pull out, it merely gets replaced with mid-upper level waves of energy ejecting eastward from the gathering upper level dynamics still well west of the state. Otherwise, no changes from yesterday's reasoning of the Baja upper low merging with the upper level storm system moving into the US NW Coast. 

The timing could be critical, but as conspired toward the other day, rain/thunder chances look to be a good bet both days, and some could be strong in a few cases due to cold air aloft and the east coast sea breeze merger east of Orlando, up and down the state late day one or both days. 

Also, on one or both days activity could might be more restricted to the north or south half of the state. On a side bar, it is noted that the GFS seems to having a typical bias love affair with far East Central in the rainfall department with totals toward 2" by the end of Friday. This can mean though some areas receive little to none, whereas other areas could receive more than 2.00" if a heavy storm or two passes of the same region. I suspect these days will start with activity mainly toward the west side and far interior but spread rather quickly east  now that sufficient (but not impressive) instability develops as opposed to today, little capping or convective inhibition in place, and cold air aloft conducive for strong storms, possibly more likely toward and east of I-95. This will NOT be like summer conditions though when  instability and sea breezes are  the sole reason for activity. Upper level energy should be at play coupled with cold air aloft. If activity gets started too soon though we 'could' end up with a lot of rain showers by late day, although rain is shown to begin and last well into both evenings.

As advertised, the time is drawing closer to the weekend now, and details although varying from yesterday are following a trend. Latest guidance indicates per the GFS (last night's ECMWF was close in line)...that an upper level trough will dig yet deeper than previous forecast. Likewise, jet stream winds aloft approach the state toward South Florida, with Central located toward the right entrance region of the jet core. This is important because that is the area where the greatest upper level divergence occurs above low level convegence within an atmosphere of veering wind profiles. 

There is one fly in the ointment! If storms form to the west over the Loop Current, they could   spread high clouds across the state, greatly dampening any severe threat on Saturday, although rain and thunder remains quite possible to likely state wide. The other variable to down play a severe threat would be TOO MUCH moisture stacked up through the atmosphere. Severe weather loves 'dry slots or layers' which aid in storm strength (lift).

ON THE OTHERHAND: Latest GFS seems to indicate a late day pre-frontal trough  to approach the West Coast mid-afternoon or earlier to spread eastward during the mid-late afternoon with respectful low level southerly winds and increasing jet stream winds aloft as shown below: In this case, regardless of cloud cover, instability might not matter all so much anyway. If so, storms could be hard to see on the approach due to clouds preceding their approach. 

Saturday afternoon shows trough and low in Gulf.   Note the SSE winds along the east coast (orange arrow). Purple arrow  is the jet stream winds. The low will track along the red line (warm front) into Sunday. As we can see, the state is in the 'warm sector' south of the warm front with veering wind profiles as we go up into the higher atmospheric levels. Note how the orange arrow indicating surface winds is CROSSED with the jet stream winds (purple). Storms growing upward into the atmosphere will be forced to rotate with greater upward lift, resulting in rotating thunderstorms and/or Supercells . Stronger winds, hail, and tornadoes can manifest from Supercell Thunderstorms, although not all supercells generate tornadoes. Do note once again, despite these favorable wind profiles for weak -mdt rotation, a storm must be able to generate first. If there is too many high clouds in place, that chance the absence of   strong  or sufficient vorticity (wind energy) passing overhead.

SUNDAY: With the prefrontal trough passing through, next comes the cold front. It still looks like this will be a squall line type situation with embedded bow echos and small cells possibly forming ahead of the line, although not as much so as yesterday. Actually, it looks more like a continuation, or resurgence of similar activity of Saturday beginning after sunrise and spreading east . 

**Note the close proximity of the surface feature below by sunrise Sunday.

AGAIN we see conflicting wind directions with height... with the white this time showing surface winds and purple showing jet stream winds..criss-crossing. South Florida really looks to be under the gun being located under the stronger jet stream winds, and Central to North is located along the greater divergence which aids in lift. This is definitely going to be 'play it as it comes situation', so preliminarily it would be considered  wise to heed statements issued by the National Weather Service or News Channels or other media outlets. 
 OVERALL: Granted, this blog is not an official relay station, but in a worst case scenario for  both late Saturday into Sunday it is STRONGLY advised as a baseline to remain abreast, attuned, advised, aware, etc.. to future outlooks via reliable outlets. I do not believe I've seen Florida in a DAY4 (4 days in advance) OUTLOOK before from the Storm Prediction Center .  

This means confidence is high or relatively enough-so that the chance of a severe weather event or events is increasing but not yet pegged down to specifics regarding just HOW severe, and what form 'the severe' will take..timing remains a major issue as well in regard to how various parameters will meld together.  

Lastly, experience says these upcoming events seldom unfold as expected, so we will need to watch for the 'windows of opportunity" (time frames).of greater severe time draws closer. 

Worst case scenario would be, I expect,  a tornado watch for most of the state for early to mid-day Saturday until the entire event is deemed to conclude, or separate watches for both events. The other option contingent upon the vast array of factors involved is that a Severe Thunderstorm Watch (s)will fit the given scenario (s). In this particular situation as posed before us. Activity should conclude by sunset Sunday based on morning model runs.with lingering showers and clouds into Monday. 

*Note: given the latest guidance , and this "quite the unofficial post" any tornadoes will not be Garden Variety Florida spin ups, but the real thing. 

BEYOND THE WEEKEND: A few cool mornings as high pressure tracks across the state behind the system, mainly away from the coasts. Lows in the mid 50s- 40Fs far north, with coasts in the 60Fs. Dry. The threat of strong onshore winds  went to the way side in later runs after the post was put on line yesterday, as expected might be the case. In other words, pleasant for sometime to come.

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