1/3 – Brittany’s “Diminishing Threat” Tuesday Evening Forecast
Conditions remain favorable for locally heavy rainfall and isolated flash flooding events to impact portions of the forecast area tonight. These conditions are driven by a combination of several factors. The first of these factors is a weak frontal boundary that is slowly pushing to the east across western Louisiana this afternoon. This front is expected to gradually stall over the forecast area this evening as the parent trough axis driving the front shifts further to the northeast, and a secondary impulse and jet max rounding the base of a longwave trough axis approaches from the west. The second factor is the orientation of the frontal boundary. The front is expected to be oriented in a southwest to northeast fashion, and this lines up directly with the overall deep layer flow regime and expected storm motion. The third factor is the anomalously high precipitable water values ranging between 1.5 and 1.75 inches that will still be in place tonight. These values are about as high as can be climatologically achieved in early January, and are considered a rather extreme event. The combination of increased deep layer forcing along the frontal axis and thunderstorms producing heavy rainfall training over the same locations will favor a few locations seeing 3 to 5 inches of rainfall on average. The biggest forecast challenge has been determining where that heavy rainfall axis will develop through the use of high resolution short term models, and the current thinking places that axis somewhere in the vicinity of metro New Orleans the Northshore. A flash flood watch is in effect for these areas through the early morning hours tomorrow. After the secondary impulse moves through, the front will pull to the east, and a drier and more stable airmass will begin to advect in from the west. This air mass will be Pacific based in origin, so no significant drops in temperatures are expected behind the front. Overall, a very pleasant period of weather is expected starting tomorrow and lasting through Thursday night with temperatures near to slightly warmer than average beneath clear skies.
A largely zonal flow regime will be the dominant feature through the long term period. Embedded within this zonal flow regime, a weak and fast moving shortwave trough axis will slide through the Lower Mississippi Valley Sunday into Monday. This fast moving upper level feature will likely send a very a weak frontal boundary into the region, and this front will stall and dissipate over the forecast area as winds turn parallel to the frontal boundary. At this time, heavy rainfall and thunderstorm activity is not expected, but scattered showers and overcast skies are expected to linger from Sunday into Monday. Given the zonal flow pattern aloft, temperatures are expected to be near average with highs in the 60s and lows in the 40s.