465 FXUS64 KMEG 211746 AFDMEG Area Forecast Discussion National Weather Service Memphis TN 1246 PM CDT Sat Aug 21 2021 .UPDATE... Updated to include 18Z Aviation Discussion. && .PREV DISCUSSION... /issued 659 AM CDT Sat Aug 21 2021/ UPDATE... Updated for 12Z Aviation Forecast Discussion below. PREV DISCUSSION... /issued 339 AM CDT Sat Aug 21 2021/ DISCUSSION... Today looks to be the last day of significant rainfall potential for a while. A synoptic pattern chance is already underway with the subtropical ridge building over the Lower MS Valley. This ridge will become the dominant feature over the central CONUS by tomorrow, keeping the impressive cyclones near the US/Canadian border well to our north. This will usher in drier and hotter conditions through early next week. The moist, tropical air mass over the region will persist through today with precipitable water remaining over 2.1" in most areas. Instability will be slightly higher today and it looks like some dry air aloft may be trying to work into the area this afternoon. Deep-layer shear remains quite weak at <20 kts which should cause the approaching MCS over MO to lose steam as the outflow races out ahead. Even with this activity weakening, the outflow boundary it produces will be a focus for diurnal development. The strongest storms could result in an isolated damaging wind potential as highlighted in the SPC Day 1 Outlook though organization may be dependent on a cold pool developing. The latest CAMS indicate the potential for redevelopment behind the initial convection from this morning so we could see two waves today. Thus, PoPs will remain in the forecast into tonight, gradually ending this evening. Locally heavy rainfall will remain a possibility through this time. The other concern for today is the heat/humidity. Heat indices are forecast to top 105F this afternoon in the delta counties to the southwest of the Memphis metro area. While convection adds an extra layer of uncertainty in reaching criteria, confidence is high enough to support a Heat Advisory. If storms do not develop as currently expected, it's not out of the question that the advisory could need to be expanded somewhat. The ridge will be the main player tomorrow through midweek, limiting rain chances and pushing temperatures into the mid 90s. The wet period we've been in this week is setting the stage for some uncomfortable conditions over the next several days. Evapotranspiration should enhance boundary layer moisture, keeping dewpoints in the mid (or even upper) 70s through midweek, resulting in widespread heat indices between 105-110F. Some areas could see slightly higher heat index values. Thus, Heat Advisories are likely for all or most of the Mid-South over the next several days. Isolated thunderstorm chances will likely remain confined to northeast MS or parts of west TN Sunday with generally dry weather anticipated Monday through early Wednesday. The subtropical ridge begins to weekend during the latter half of the week as a fairly strong trough digs across the Pacific Northwest into the Northern Rockies. During this time, the medium range deterministic models are moving a weak, upper-level trough westward into the Carolinas. These solutions vary widely in how the steering flow will direct this system but the weakening ridge will signal temperatures moderating back to climatology and rain chances increasing to at least 20-30% each day Thursday into the weekend. MJ && .AVIATION... 18Z TAFs Thunderstorm location and timing continues to be a challenge in the 18Z TAF set and have opted for VCTS mention for now unless short term trends suggest otherwise. Any afternoon convection is anticipated to gradually diminish towards late evening. Patchy fog will be possible late tonight at MKL,JBR,TUP with light south winds becoming north tonight. CJC && .MEG WATCHES/WARNINGS/ADVISORIES... AR...Heat Advisory until 8 PM CDT this evening for Lee AR-Phillips- St. Francis. MO...None. MS...Heat Advisory until 8 PM CDT this evening for Coahoma-Quitman- Tallahatchie-Tunica. TN...None. && $$