Wednesday, November 28, 2012

Cold Start on Thursday Morning!

By Jay Grymes & Steve Caparotta

A stubborn cloud deck not only generated a damp mist for some areas late into Tuesday night, but was persistent enough to extend past sunrise for many of us, keeping temps across metro Baton Rouge and much of the northern WAFB communities in the 40s.  If you were with us during the 10PM weathercast on Tuesday, you saw that the cloud line extended from western Wilkinson County towards Lafayette with clear skies to the west.  Apparently, the cloud line stalled through the night and into the morning before finally eroding with the morning sunshine. 

The clouds over the Capital City quickly parted after 8AM, leaving a sunny sky for the remainder of the day.  Skies will remain mainly clear through the evening and overnight tonight, and that should allow Thursday’s early morning lows to drop into the 30s for just about everyone along and north of the I-10/12 corridor.

High pressure currently centered over the mid-Mississippi Valley will slide eastward on Thursday and Friday, establishing “return flow” (SE to southerly winds) that brings a little Gulf warmth and low-level moisture into the WAFB viewing area.  So after Thursday’s chilly start, temps will start a slow but steady climb into the weekend.

Expect highs in the upper 60s for Thursday afternoon under mostly sunny skies.  Lows will only drop into the mid to upper 40s for Friday morning, with Friday afternoon highs in the low 70s for most WAFB neighborhoods under fair to partly cloudy skies.  And the way it looks now, the Red Stick should plan on highs in the mid 70s on Saturday and mid to upper 70s on Sunday.

By Friday, mornings may be accompanied by patchy fog thanks to a rise in dewpoints associated with the return flow.  The extended outlook for the weekend includes little more than a very slight chance for rain -- less than 20% for both Saturday and Sunday.  Based on the current guidance, our next rain-producing cold front is scheduled for something only the lines of late Tuesday into early Wednesday.

Here’s a quick look at the NWS Climate Prediction Center’s (CPC) 3-month temperature and precipitation forecasts - - and remember, interpreting these maps must be done with care.  The initial assumption is that there are “equal chances” (EC) for temps to average “near normal,””significantly above-normal,” or “significantly below-normal.”  In other words, CPC starts with a 33.3% chance for each of the three possible outcomes, then adjusts those percentages based on their long-range guidance.

For the WAFB viewing area, the “seasonal average” temperature outlook for the 3-month period of December, January and February (DJF) leans slightly towards a warmer-than-normal season, with roughly a 35% to 37% chance for “above-normal” temps.  Another way to look at this is that there is roughly a  70% chance for temps to be “near-normal to above-normal” for the WAFB viewing area.

  You may already be thinking: that is not a substantial change from the starting point of 33.3% + 33.3% for “near-normal” plus “above-normal.”  And you would be correct, which means that CPC does not  have any strong indicators suggesting a large departure for winter temperatures for our area, especially when compared to the strong signal for “warmer-than-normal” weather over the U.S. Southwest.

The 3-month outlook for rainfall across the WAFB viewing area is even less insightful.  The CPC forecast for our viewing area is “EC,” which means that there is no clear guidance for any kind of trend for noteworthy rainfall departures this winter.  That lack of guidance comes as no surprise: the winter/spring El Niño that was anticipated by many experts during the summer and early fall has failed to develop, leaving the El Niño/Southern Oscillation (ENSO) in a “neutral” state.

  During El Niños, our viewing area tends to receive above-normal rain during the winter and spring; during La Niñas -- the “flip side” of El Niño -- we tend to be dry.  But during “neutral” ENSOs (sometimes called La Nada), all bets are off.  During past La Nadas, winter and springs rains for our area have ranged from some of the lowest to some of the highest seasonal totals, with all options in between.  In other words, there is no clear long-range guidance for winter rainfall in our area when ENSO is in the La Nada phase. 

Yes, one might question the value of these three-month “forecasts.”  The shifts in percentages from “EC” for the coming winter temperatures are limited at best, and there is no information added with the winter rainfall projection.  But these “seasonal forecasts” (better labeled as “outlooks”) are honest assessments based on the current state of the science for long-range predictions.

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