Scientific Forecaster Discussion
000 FXUS64 KMEG 130344 AAA AFDMEG Area Forecast Discussion...UPDATED National Weather Service Memphis TN 944 PM CST Tue Dec 12 2017 .UPDATE... Clear, calm and cold conditions prevailed across the Midsouth at midevening. 9 PM CST temps had cooled into the mid 20s near the TN River, while mid 30s prevailed over east AR. Latest HRRR and other short term guidance shows winds may edge up a bit over east AR prior to sunrise, which should be enough to level off the cooldown during the overnight. Forecast was recently updated to lower overnight lows over most of west TN and northeast MS, inclusive of the outlying Memphis suburbs. PWB && .PREV DISCUSSION... /issued 549 PM CST Tue Dec 12 2017/ UPDATE... Updated for 00z TAF discussion PREV DISCUSSION... /issued 334 PM CST Tue Dec 12 2017/ DISCUSSION... A deep trough continues to affect the eastern CONUS with an upstream Rex Block in place across the west. This pattern will persist for a few more days but the ridge will gradually shift west over the Pacific, signaling a synoptic pattern change. Until we see this shift occur, we'll continue to see fast-moving, moisture-starved weather systems. Fortunately, this change looks to occur by the end of the work week with rain chances appearing more likely during the latter half of the weekend, potentially lingering into early next week. Looking back at the short-term, winds today were a bit stronger than suggested by NWP but are finally beginning to decrease. With relative humidity falling into the range of 30-40%, fire danger was elevated but Red Flag criteria were never threatened. Wind speeds will quickly decrease after sunset, setting the stage for a cold night across the Mid-South. Dewpoints are currently in the teens and lower 20s and skies are mostly clear. This will promote strong radiational cooling. The caveat is that southerly winds will begin to increase late which may prevent temperatures from dropping too low, especially along/west of the Mississippi River. Winds will continue to increase on Wednesday as a deepening surface low tracks ESE near the western Great Lakes. Southwest winds are forecast to increase to 15-25 mph, with gusts potentially exceeding 30 mph at times, especially across the northwest half of the CWA. Temperatures will rebound into the 50s, pushing relative humidity down to 35% or less by peak heating. Should dewpoints mix more readily than anticipated, relative humidity could approach Red Flag criteria. It's worth noting that the Energy Release Component is in the 97th percentile across west TN and above the 90th percentile in east AR and north MS. This suggests a good amount of dry fuel in an area that has seen well below normal rainfall the past 30-60 days. In summary, fire danger will be elevated area wide and may approach critical thresholds in some areas. Another cold front will move across the area Wednesday evening, ushering a cooler Continental Polar air mass back into the region for Thursday and Friday. Highs will largely be in the 40s with overnight lows in the upper 20s to lower 40s. A positively-tilted shortwave trough will traverse the area Thursday night into early Friday, resulting in increasing cloud cover. While mid-level moisture will increase, it remains scant in both the low-levels and aloft (specifically in the dendritic growth zone). This indicates a low potential for rain/snow with this passing trough. While a few sprinkles and/or flurries cannot be ruled out, the potential really isn't worth mentioning in the 5th/6th periods as no impacts are anticipated. Southerly winds are expected to resume on Saturday, concurrent with the shifting synoptic pattern. The eastern CONUS trough is progged to move east with a flat shortwave ridge over the Ohio Valley (and Mid-South). The GFS and ECMWF are in fair agreement that a southern stream shortwave trough will lift northeast across TX Saturday night into Sunday morning. This will be the trigger for the enhanced rain chances through the day on Sunday. However, the models quickly diverge for early next week. The GFS is slow in departing this trough, keeping it west of the CWA through Monday night while the ECMWF is about 24 hours quicker. Should the ECMWF verify, the low PoPs for Monday may need to be removed. Regardless, with this being a Pacific trough, we're not anticipating much of a cool-down in its immediate wake. Johnson && .AVIATION... 00z TAFs VFR conditions to prevail, with light and variable winds expected at all TAF sites overnight. Southwest winds will pick up quickly by late morning with a surface low pressure system dropping out of the northern plains. Sustained winds will approach 20 kts with gusts close to 30 kts at all sites but KTUP where the pressure gradient will be a bit weaker. WLC && .MEG WATCHES/WARNINGS/ADVISORIES... AR...None. MO...None. MS...None.