Scientific Forecaster Discussion
000 FXUS64 KMEG 220441 AFDMEG Area Forecast Discussion National Weather Service Memphis TN 1141 PM CDT Sat Apr 21 2018 .UPDATE... Updated for the 06z Aviation Discussion. && .PREV DISCUSSION... /issued 831 PM CDT Sat Apr 21 2018/ UPDATE... Small adjustments to evening temperatures. DISCUSSION... Thick cloud cover with only spotty sprinkles has allowed for temperatures to remain warmer than forecast this evening. Have made slight adjustments to hourly readings through midnight. Shortly after midnight leading edge of main convective swath seen in central Arkansas should be crossing the Mississippi River...quickly cooling the boundary layer temperatures. All other forecast parameters are currently on track. JAB && .PREV DISCUSSION... /issued 336 PM CDT Sat Apr 21 2018/ At 3 PM, another gorgeous day of weather across the Mid-South. Temperatures are mainly in the low to mid 70s under mostly sunny skies and cool easterly winds. Latest GOES east water vapor imagery shows a large cutoff low pressure system nearly spinning in place over the Oklahoma Panhandle. At the surface, a 1008 mb low pressure center was analyzed near Abilene Texas at this hour, with a warm front extending south and a trailing dry line/cold front to the southwest. .Short Term...Tonight through Sunday night... Widespread showers and some thunderstorms are expected to develop quickly after sundown as the aforementioned system moves east southeast and into the Lower Mississippi Valley tonight. Simultaneously, the attendant surface low over Abilene, Texas is expected to slightly weaken and move east into Central Arkansas by early Sunday morning. This will bring a prolonged period of heavy rain, some thunderstorms, and cooler than normal temperatures to the region. The only major change made to the forecast was that QPF totals were slightly lowered to between 1 and 2 inches, as the surface low is expected to track a little further south. As the cold front slows down and moves into forecast area tomorrow morning, there may be some destabilization ahead of it. SPC has outlined much of north Mississippi for a Marginal Risk primarily for tomorrow afternoon. Short term models show some redevelopment along the front early tomorrow afternoon, with 500 to 1000 J/kg of MUCAPE and 60 knots of deep layer shear. Storms that do fire along the front and realize the instability, do have the ability to produce damaging winds and/or large hail before congealing into a line of showers and thunderstorms later in the day. This scenario is highly dependent on the thermodynamic environment materializing. At this point, confidence is quite low. Nonetheless, a few strong storms with marginally severe hail and gusty winds are most likely. Will leave the HWO wording as is. .Long term...Monday through Late Next Week... The large trough is expected to remain in close proximity to the Mid-South through at least Tuesday afternoon. This will us in a low POPS and low QPF regime over the region. The main forecast story will be dominated by cloudy conditions and the much cooler than normal temperatures. The driest period appears to be early Wednesday, before a quick moving compact shortwave drops down across the area. QPF totals look low with this system. The models are still showing another cold front moving into the region next Friday, but the GFS has backed off on the timing and precipitation for some reason. Went ahead and split the difference through this period, with chance POPS and cooler than normal temperatures. AC3 && .AVIATION.../06z TAFs/ MVFR cigs have been slow to build in due to the residual low level dry airmass in place. Moisture has been slowly increasing and MVFR cigs should eventually build into all of the terminals by Sunday morning, near or just after sunrise. Rain showers will also increase in coverage areawide over the next few hours. East winds will also continue but remain lighter than earlier anticipated. A dry slot rotating into the southern portions of the region should allow for improving cigs by afternoon at KMEM and KTUP along with increased chances for showers and a few thunderstorms. Some stronger storms tomorrow afternoon could pose a hail threat if enough instability can develop near KMEM and KTUP. Cigs will gradually lower again later Sunday night, with IFR cigs and vsbys expected through much of the night along with isolated showers. JLH && .MEG WATCHES/WARNINGS/ADVISORIES... AR...None. MO...None. MS...None.