377 FXUS61 KILN 212316 AFDILN AREA FORECAST DISCUSSION National Weather Service Wilmington OH 716 PM EDT Sat Aug 21 2021 .SYNOPSIS... A weak trough of low pressure will approach the area on Sunday and then dissipate. High pressure will follow and prevail for the early part of the week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Enhanced cumulus across northern counties has resulted in a couple of showers developing. Isolated convection will be possible through the rest of the afternoon. Cumulus will dissipate with loss of heating. With light and variable winds, patchy fog will develop across parts of the area, except in river valleys where fog will be more extensive. Lows will be in the mid to upper 60s. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Weak surface trough will move towards the forecast area Sunday afternoon. Not much forcing with this, but could see a few more showers and thunderstorms in northwest counties late in the day. The trough will wash out Sunday night and clouds will diminish again. Dew points will increase a bit during the period with temperatures a few degrees warmer than persistence. && .LONG TERM /MONDAY THROUGH SATURDAY/... Summary: Mid-level ridge building into the Ohio Valley early next week supports mostly dry conditions and increasing warmth to start. Thunderstorm chances increase Wed-Fri as sufficient heat/moisture combine with an upper level trough and surface front. While instability should help sustain thunderstorm growth during the afternoon and evening, marginal wind shear should limit the potential for widespread severe weather activity. As a result, the primary concerns during the week will be elevated heat indices and isolated severe storms. Heat details: Ensemble model guidance continues to suggest a deepening mid-level ridge over the Great Lakes and Ohio Valley regions to start the week. Positive height anomalies throughout the week indicate above normal temperatures with Tuesday through Thursday looking to be the warmest. With a stalled surface boundary settling into the central Great Lakes by midweek, persistent dewpoint temperatures in the low 70s would enhance heat indices into the upper 90s and perhaps the lower 100s with no potential of relief from a frontal passage. As is typically the case, thunderstorms/cloud debris could inhibit warming for parts of the forecast area with the most likely days Thursday and Friday. For now, Tuesday-Thursday appear to be the most concerning with regards to heat related hazards. Thunderstorm details: With the mid-level ridge building into the region, overall wind flow is weak and primarily westerly. Afternoon forecast soundings for Monday show a strong cap around 800 mb, and deep moisture is lacking. This results in a dry forecast for Monday. For Tuesday, capping inversion is still in place, however, deep moisture has increased slightly and a mid-level disturbance is expected across the southern Great Lakes. Thunderstorms will develop across the unstable air mass to the northwest, with some potential for lingering convection to move from the northwest late afternoon before weakening. On Wednesday, the forecast area is expected to be well within the warm sector. Surface moisture and deeper atmospheric moisture have improved, increasing instability values across the area. The increasing surface moisture will help to overcome the cap with higher chances for thunderstorm Wednesday afternoon and evening. However, when it comes to severe weather potential, the shear values are still meager. The potential for severe weather would be similar to situations earlier this August. Given the instability, isolated severe storms are certainly possible. Further north and closer to the stronger wind flow would be the favored location for more widespread severe activity Wednesday afternoon/evening. For Thursday-Saturday, some uncertainty begins to increase with regards the exact position of the mid-level ridge and surface front. Euro ensemble suggests a further south position, leading to more clouds and a more uncertain thunderstorm forecast. GEFS keeps a stronger ridge, leading to a quick decay of the front. Thunderstorm chances are still forecast regardless of the exact outcome, however, heat and thunderstorm details do deviate depending on which solution is chosen. Given this, see no reason to deviate from the NBM solution with thunderstorm potential continuing through the end of the forecast period. Throughout the week, isolated severe events are possible with the most likely threat being damaging winds. The lack of wind shear/steep mid-level lapse rates suggest tornado/large hail potential would be very low. Weak flow and positive PWAT anomalies suggest locally heavy rain events will also be possible. These would be similar to some local events from earlier this month near London, OH and the east side of Columbus, OH. && .AVIATION /23Z SATURDAY THROUGH THURSDAY/... Isolated cumulus and a few showers across our north should continue to dissipate through early evening. Otherwise, expect to see some higher level clouds continue to stream down across at least western portions of our area into the overnight hours. In areas that remain mostly clear, light winds overnight will result in some MVFR to locally IFR fog development later tonight into Sunday morning, especially in the river valleys. Any fog will burn off through mid morning with some scattered cu possible again heading into Sunday afternoon. OUTLOOK...Thunderstorms are possible Wednesday and Thursday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS... NEAR TERM... SHORT TERM... LONG TERM...McGinnis AVIATION...JGL