The Weather Prediction Center issued a Mesoscale Discussion that includes parts of Louisiana and Mississippi highlighting a chance for up to four inches of rain during the next few hours.
From the WPC:
Surface observations from 05-06z indicate that a weak surface low has formed across LA near the intersection of two thermal/dew point gradients. Radar imagery shows that a weak disturbance aloft moved out of TX around 05z into northwest LA to initiate new thunderstorm activity across northeast LA and
west-central MS. Precipitable water values of 1.2-1.7″ lie across this region per GPS data.Correlating the precipitable water values to the 1000-500 hPa thickness values across the area (~5640 meters) yields a nearly saturated atmosphere. Inflow at 850 hPa across LA/MS is cyclonic at 20 kts per VAD wind profiles, with flow remaining anticyclonic/unfavorable across AL at the time of this discussion’s writing. MU CAPE of 500-1500 J/kg lies across southern and central LA as well as southern MS, which has trended noticeably upward over the past four hours per SPC mesoanalyses.
Effective bulk shear across the region is impressive — 40-70 knots — which combined with unidirectional flow with height above 850 hPa could lead to the formation of mesocyclones and training convective bands. RAP forecasts indicate increasingly cyclonic and broad 850 hPa inflow from LA ultimately into AL, which should lead to a lengthening warm advection heavy rain band which would aid cell training within the existing and forecast MU CAPE gradient.
The 00z HREF probabilities of 0.5″+ indicate a similar scenario which appears to mature lengthwise around 12z. Think the 00z NAM CONEST and 00z Canadian Regional models may have the best idea concerning the length of the convective band, which should stretch from LA into AL by 12z. Hourly rain totals to 1.75″ are expected.
The mesoscale guidance has a modest signal for local amounts in the 3-4″ range, which would be of concern where two week precipitation has been 100-200% of average over portions of this region. It is unclear how much overlap there will be between the oncoming rainfall and the more saturated soils, so left the category as possible.
for those curious about the severe weather threat during this same time period, the Storm Prediction Center has not released any new information specific to this event or for the next few hours.
