"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".

Tuesday, April 17, 2012

Rain Chances Begin Wednesday Toward Possible Severe Storms: Weekend

Image for late today at 500mb (see lower left of image). Notes in Red squared off by purple relate to this weekend. Also note the two upper level troughs (which also show in the mid levels) to pass by Florida before this weekend. More information below for discussion purposes is in the post.

TODAY: "Another Beautiful Day in Paradise"

WEDNESDAY: The first of several disturbances will cross over or near Florida as the ebb and flow of the tide rises with a succession of waves coming ashore overhead and washing through. Meanwhile, high pressure in the lower levels to the east of Florida with its clockwise circulation will usher increasing moisture across the state under the influence of SE to South surface winds in coming days. Guidance is split on Wednesday on the rain chances, with the NAM showing thunder toward the East Coast from near Lake Okeechobee northward to Brevard. On the other-hand, although the GFS implies the same 'pattern' it is mostly non-rain producing. Hedging more on the GFS for possible showers inland west side spreading toward the east late afternoon and after dark but weaking to perhaps mere clouds after dark, lingering near the coast toward the north side of the state. 

Based on a variety of parameters not really amounting to much against the prevalent atmospheric conditions we have been under for quite some time, do not think the atmosphere will be 'turned over' quite enough yet to yield thunder. This will be re-evaluated tomorrow morning for thunder chances from near NE Okeechobee County to Titusville and offshore as the first upper waves passes across (as shown above). Instability looks too meager to be able to hold fast to deep convection in the face of convective inhibition and cool onshore flow, although if winds become side-shore late in the day, isolated inland thunder is not out of the question if not East Central near or after dark.

THURSDAY: Second wave will now have greater influence as the first one will have carved a clearing for further waves to move a set of waves in the ocean comes the waves (of uncertainty) from above in a stormy sea. By Thursday moisture increases across the state, sea breezes play a part combined with energy from the next wave in the set combined with increasing atmospheric instability. Consistency since yesterday shows early-mid afternoon showers and thunder with plentiful cumulus clouds by mid-morning or so and warm temperatures in the low-mid 80Fs. The gun is loaded and has a trigger now, so thunder looks to be a much better bet state wide. The GFS goes for good coverage but the NAM holds back or simply has poor aim. I'm siding more with the GFS though, as the NAM has a tendency to 'isolate' selective areas, good for short term but not so good for mid-range guidance. This line of reasoning could easily prove to be incorrect though, but bears watching. Watching to see if later runs follows suit with the GFS which has been showing a better to good chance of rain for several runs now on Thursday with 60% coverage, although the official forecast says 20% (which would be the NAM).

FRIDAY: For now, we can say carbon copy or 'repeat sign' in this medley of Thursday. Activity on either day could remain off the east coast well into the evening over or near the Gulf Stream waters. Note again, from the image above as well discussed next, we'll be watching the annotations along the U.S. West Coast.

SYNOPSIS FOR WEEKEND: Note in the image the upper low off the Pac NW and lingering low off Baja. Guidance is now consistent with a 'merger' of these two features while the high pressure to the east of Florida moves out an  as a result of the two previous shortwave troughs on Thursday and Friday. As a result, the Southern Branch jet Stream now running south of the state is lifted north in a bit of a medley concerto with a repeat sign and a coda thrown in, harmonizing and merging with a crescendo on Saturday and Sunday as a now strong upper low rolls across Southern Bama. The drummers and cymbals join the chorus.

The weekend is sketchy in the timing realm (given how far out in time it is)...  the growing grass is slow  ..but the seeds seem to be planted for a severe weather event on Saturday and/or Sunday.  Monitoring is like 'watching the graphs grow".  Stronger winds in the mid-upper levels, divergence in the jet stream winds, and convergence at the surface with the help of a side shore 'sea-breeze' along the east coast (more moisture added) with instability and cold air aloft in the midst of increasing veering wind profiles with altitude (clockwise as on goes in the vertical) rise to counterclockwise circulations at the surface becoming outwardly clockwise aloft creating a vacuum in upward rising air currents.

Red - surface wind  Purple/Lavender - Jet Stream winds; crossing  surface and veering mid level winds
Sunday morning depiction

 It appears that there will be a QLCS Squall Line on Sunday, but possibly preceded by a "POTENTIAL ONLY AT THIS POINT" severe weather event on Saturday of lesser proportions. The evening European Model (ECMWF) run comes in close, but given the lack of greater details in that model on the internet free-service are lacking it is hard to tell just exactly how well they come to agreement. At this point, would not be surprised to see a Severe Thunderstorm and/or Tornado Watch for Florida on Saturday and/or Sunday. So far, this is not showing to be an overnight event (s)...and much can change in the next several runs. Already, the offices are eyeing the potential though, and the Storm Prediction Center seems to be waiting for consistency as well before loaded the gun to make a call of caution. 

BEYOND?: Good question. Guidance shows the eventual frontal boundary to sweep through with a rapid transition to onshore flow, thus..very little cold air follows, at least at this stage. This would make sense considering much or part of this system stems from the Southern Branch Jet. Boundary should lie east west across the Straits area and possibly retrograde back north. Earlier run showed onshore flow with winds of 30G40 mph Sunday night or Monday...but that scenario fell to the way side in the morning run. Still cannot discount it yet though, but the ECMWF did not appear to show that possibility..and tend to agree.

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