Wednesday, July 3, 2013

Isolated T-Storms for the 4th

By Jay Grymes & Steve Caparotta

Based on the afternoon radar trends, today’s rain coverage was a little bit more widespread than we expected.  But you may have noticed that the rains were focused along a narrow SW-to-NE band.  What was a bit of a bigger surprise was the “Slight Risk” for severe weather posted by the NWS Storm Prediction Center (SPC) for Louisiana southeastern parishes for today and tonight.

If you were watching the regional radar trends through the day, you were seeing the showers and t-storms riding from SW to NE along the stalled frontal boundary draped over SE Louisiana, being supported by mid-/upper-level southwesterly flow.  We expect that front to continue to linger over the region into Thursday before fizzling out.  That SW-to-NE transport is being generated at the base of the upper-level trough (a southward dip in the jet stream) that we’ve talked about the past few days.  Keep in mind that the location and southward extent of this trough -- and its upper-level “closed low” over the Missouri Valley -- are rather unusual for this time of year.

With Louisiana near the “bottom” (southern end) of the trough, we’re in position for mid-/upper-level southwesterly flow associated with the jet stream flow around the trough.  That upper-level transport coupled with low-level moisture coming off the Gulf is fueling today’s rains.  In addition, that combination is aided further by the lift created along the weak stationary boundary, provides a set-up for a few stronger t-storms -- that’s why the SPC has flagged SE Louisiana for the “Slight Risk.” 

Although the SPC’s risk advisory is in effect until early Thursday, we suspect that the severe storm threat will decline after this evening’s sunset. However, the southern parishes should be ready for off-and-on passing showers throughout the night.  So we’ll go with spotty-to-isolated showers through the overnight, with the better chances for showers and storms closer to the coast. 

Our forecast for Independence Day celebrations remains essentially the same as we’ve said through the week.  After a mainly-dry start to the day, we’ll go with a 20% to 30% chance of mostly afternoon and early evening showers with an afternoon high around 90° or so.  You may notice that the humidity on Thursday will still be a little on the low side for July -- we think we’ll enjoy at least one more day with dew point temperatures lingering in the 60°s. 

We can’t rule out a spotty shower or two during the later evening hours on Thursday -- around “fireworks” time -- but most of the rain should be out of the region soon after sunset.

Also, as we’ve been mentioning the past few days, the outlook gets notably wetter for Friday and into the weekend.  A broad area of low pressure (an easterly wave headed to the west)  currently extends over the eastern and southeastern Gulf: that feature will come into play for the central Gulf Coast’s weather by Friday and Saturday.

In fact, we have a pair of weak low pressure areas in the Gulf region -- the one over the eastern and southeastern Gulf and another over the southern Gulf and the Yucatan.  That pair could make for an interesting upcoming run of days.  For the time being, there is no real indication of significant tropical development from either of these two features, but anything that close to home at this time of the year bears watching.  The National Hurricane center (NHC) currently clumps the pair together into a single large area that they post with a 10% chance of tropical development over the next 48 hours.  (Of these two areas, we’re thinking that the more southern one has the better chance of spinning-up into something down the road.)

What we do expect are scattered-to-numerous showers and t-storms by Friday with rain likely for Saturday and Sunday; rain chances will be greater closer to the coast on all three days.  We’re going with a 40% rain chance for Baton Rouge on Friday, with rain chances in the 60% to 70% range for Saturday and Sunday.  Don’t be surprised to see rain accumulations of 1” to 2” or more along and north of the I-10/12 corridor by the end of the weekend, with totals running as much as 2” to 4” with locally-higher totals closer to the coast.

We’ll carry “scattered” rains into the first half of the work week too.

No comments:

Post a Comment