Monday, March 9, 2015

Staying Wet Most of This Week

By Jay Grymes & Steve Caparotta

WAFB First Alert Quickcast:

- mild temperatures but staying wet this week
- rain totals in excess of 3” to 4” possible between now and mid-day Friday

Let’s start with the good news: after a string of freezes during the first week of March -- including a rather uncommon March dip into the 20°s -- we’re on track for near-normal temperatures this week as a mild Gulf air mass dominates our weather. 

The trade-off for our milder temperatures this week will be a run of rainy days, and that is where the not-so-good news comes in.  Based on the extended guidance and a sense of the projected upper-air pattern, many WAFB neighborhoods could see 2” to 4” of rain this week with some locally-higher totals.  Standing water and nuisance street flooding could be an issue for many of us and we’ll need to keep watch on our area rivers too should some of the bigger rain totals become more widespread.

It’s a bit coincidental, but the NWS Climate Prediction Center (CPC) has also declared that a “weak” El Niño has finally become established.  While our expected run of rain this week cannot be directly attributed to the CPC’s El Niño declaration, history tells us that El Niños tend to make for wetter-than-normal winters and springs for our viewing area.  In fact, El Niños are linked to wet winters and springs around the entire Gulf rim.  For southeast Louisiana, rainfall for roughly 7-in-10 winter/spring periods averages above-normal when an El Niño is present.

(As a quick reminder, El Niño is represented by warmer-than-normal waters over the central and eastern equatorial Pacific Ocean.  These warmer waters interact with the atmosphere and affect the atmosphere’s circulation pattern.  For the Gulf of Mexico region, the presence of an El Niño is associated with an increase in the development of non-tropical lows over the Gulf of Mexico.  More lows in the Gulf translates into more raindays for the region.)

A key factor in our “wet” forecast for this week is the anticipated upper-air pattern to our west and southwest through the coming days.  Our forecast models show a persistent, deep trough extending down over Old Mexico through the middle to latter part of the week.  This pattern will provide a steady southwesterly flow over the Gulf Coast states.  Such a set-up tends to keep us under the influence of mild Gulf temperatures but it also pulls Gulf moisture inland while providing mid/upper-level lift to encourage rain and t-storm development.  The pattern also drives rain-making disturbances along the southwesterly flow and over the region, adding to the rain-making mechanisms.

We expect this pattern to persist through Thursday, at least.  Look for daily lows in the 60°s for the next three days with daily high temperatures in the upper 60°s to low 70°s.  the air will remain muggy throughout the next several days, and periods of low clouds and patchy fog will make for less-than-attractive days.

Heading into Friday, the upper-level pattern will begin to modify, delivering a drier weather pattern for the weekend along with cooler morning starts.

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