Thursday, December 19, 2013

Rains Around the Corner

By Jay Grymes & Steve Caparotta

For the fourth consecutive night, temperatures dropped faster and further than the computer guidance suggested. Baton Rouge’s Metro Airport dipped to 43° right after midnight, a full 5° lower than most of our “reliable” models projected. However, clouds moved into the region more quickly than expected during the night, with that cloud ‘blanket’ helping temps rise through the remaining pre-dawn hours. By 6AM, Baton Rouge was back in the 50°s. 

Those early morning clouds and pre-dawn warm-up also limited the development of dense fog.

As we told you, it was back to the 70°s for today and we’re keeping highs in the 70°s right through the weekend. As we’ve shown you on-air over the past couple of days, an upper-air ridge currently over the central U.S. continues to warm us up. At the same time, an upper-air trough currently along the U.S. Pacific Coast is expected to swing eastward over the coming days. Guidance has the trough centered over the Southwest U.S. on Friday, moving into the Southern Plains by Sunday, and then lifting out to the northeast by Monday. At the lower-levels, surface high pressure that was over the Lower Mississippi Valley earlier this week has shifted east, promoting southerly flow that (1) is funneling more low-level Gulf moisture into the area and (2) will slowly destabilizing the local atmosphere in the coming days.


The forecast remains mainly-dry through tonight and into Friday morning. Friday starts off under the clouds and rather warm by December standards. Sun-up temps will be around 60° for the BR metro area -- that’s close to the ‘normal’ high for this time of year. Be ready for a return of patchy fog for Friday’s morning commute too, with some pockets of dense fog possible if not likely. We’ll go with scattered afternoon and early evening showers for Friday -- set rain chances at about 30% or so -- not quite as wet as we thought yesterday. So the good news is that it won’t be an all-afternoon, widespread rain on Friday. On the other hand, we could see just enough rain to make the Friday evening commute a real mess, especially along what is sure to be an extra-busy interstate corridor given the holiday weekend leading into Christmas.


Rains taper off Friday night into Saturday morning. Rain returns during the latter half of the day on Saturday: we’re posting rain chances at 60%. The key for Saturday shoppers will be the timing of those rains -- and a willingness to take on the frantic holiday shopping crowds!

We’re expecting only isolated to maybe scattered rains through the middle of the day, with coverage -- and intensities -- increasing during the latter half of the day and into the overnight. Timing of the front has come into better agreement with the American GFS model and the European ECMWF: these models show the heart of the storm energy arriving in the WAFB viewing area during Saturday’s evening hours. By comparison, our in-house RPM model appears to be much slower with the front’s arrival, holding off the main frontal energy until after midnight.

For now, we’ll go with the more traditional GFS and ‘Euro’ timing for the system’s arrival. 

While both models indicate rain continuing past midnight and into early Sunday morning, the GFS shows a faster clear-out following the front’s passage, with rains ending in the BR metro area well before lunchtime. The ‘Euro’ (ECMWF), however, keeps a chance of “backside” rains (clouds and rains behind the front) in the area well into Sunday afternoon. And as for our RPM, although it is slower with the arrival of the main energy, the latest run shows a relatively narrow band of rain that exits the viewing area by the mid-morning, if not sooner.

Severe Threat

More importantly, the latest guidance shows a better opportunity for active-to-severe weather from late Saturday into early Sunday -- the NWS Storm Prediction Center (SPC) has the entire Bayou State under a “Slight Risk” for severe weather during this time frame. While the greatest threats for severe weather will remain to our north, there is certainly a potential for a few strong-to-severe t-storms producing locally heavy rains and pockets of damaging winds in the WAFB viewing area. In addition, the set-up shows a potential for isolated tornadoes developing in the stronger storms ahead of the advancing front.

What’s more, the latest rain-total projections through the weekend from the NWS Weather Prediction Center (WPC) have increased and are now running from 1” to more than 2” for most WAFB communities, with isolated larger totals possible. That’s not enough to produce a serious flood threat but it is likely to create nuisance street flooding and standing water in the usual suspect spots.

Next Week
After that, it all looks good heading into Christmas. The air will turn much cooler, with highs only in the 50°s for Monday and Tuesday, with morning starts in the low to mid 30°s for Tuesday morning and Christmas morning.

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