Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 220148 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 948 PM EDT Fri Jul 21 2017 .SYNOPSIS... Multiple disturbances are expected to bring several rounds of showers and thunderstorms to the region through the weekend and into early Monday. High pressure and dry conditions are then expected for Tuesday. More disturbances in northwesterly flow aloft will cross the region later in the week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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A very humid air mass remains in place over the ILN CWA, with the 00Z KILN sounding indicating an unstable and uncapped environment. Forcing has been the missing factor through most of the day, though a few isolated storm clusters developed near Cincinnati, and a weak boundary (part of a tailing boundary from an earlier MCV) is now starting to ignite near Wilmington. Nonetheless, with overall support for organized ascent rather weak, and no shear to speak of, activity through the first half of the overnight is not expected to be more than intermittently strong (and unlikely to be severe). A much different scenario is expected for the second half of the overnight period, as an environment much more favorable for organized storms gradually advects to the ESE out of northern Illinois. The thermodynamic environment over the ILN CWA (and downstream of existing convection) will be entering its usual min, which may lead to some storms becoming elevated with time, as indicated by RAP/HRRR soundings. What these storms will bring is their own source of forcing (low level convergence ahead of another developing MCV) and shear (increasing 30-35 knot westerly 850mb flow). These factors should help the storms stay strong to severe, at least for portions of the line/MCS/cluster, even as the storms make it into the ILN CWA somewhere in the 3AM-6AM time frame. Current storm trajectories, storm-level flow, and recent HRRR runs would suggest the greatest threat for strong storms will be in the northern third of the forecast area. Nonetheless, eventual propagation toward instability will likely lead to more of the area being impacted in some way. PoPs have been increased and re-focused in terms of timing and placement, based on this latest forecast information. Finally, added some fog to the forecast for the areas where heavy rains occurred from Cincinnati southeastward. Previous discussion > Apparent temperatures were expected to remain at or over 100 in the southwest, which had me extend the current advisory into the overnight hours as the eastern portion of it drops off later today. Upstream thunderstorm complex west of Indianapolis and a shower complex underneath the MCV northeast of Indy will work into the CWA later this afternoon into this evening. Expect convection to be possible almost anywhere this evening but with a higher propensity southwest of a Richmond/Lebanon/Georgetown line. Overnight should see convection from this evening waning, but another upper level disturbance will ride southeast in the mean flow and cross northern Ohio. Storms from this will primarily affect the I-70 corridor and northward, with isolated activity possible elsewhere late overnight. Confidence in the occurrence and/or placement of these storm clusters is low. Forecast will be dynamically updated as storms fire and a better timing and areal extent of them are able to be extrapolated in both time and space. With this being said, forecast is probably too low on pops overnight in the northern CWA where it was limited to chance category.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday had me bring another tier of counties into the heat advisory fold as unusually warm dewpoints solidly in the middle 70s combine with temperatures in the low 90s to push apparent temperatures over the 100 degree mark. Storms that are present in the morning should be focused north of and along the I-70 corridor, but a few models are indicating some isolated activity popping south of it just before daybreak. The outflow boundary that lays out along the I-70 corridor later in the day is expected to be a focus for a second round of development, given an added boost by a continued feed of moist and unstable air on southerly winds. Storms will fire along a w-e oriented line and then move south in the evening. Any storm in the next few days will have a higher potential for damaging winds and especially very heavy rainfall. The tornado and large hail threat appear to be negligible at this moment in time. Storms will weaken and diminish as they move south to the Ohio River on Saturday night. Dewpoints will remain in the middle 70s until the airmass changes beyond the short term period. && .LONG TERM /SUNDAY THROUGH FRIDAY/... A weak trof/boundary is forecast to push through the region on Sunday. This feature may be entering our far eastern/southeast zones by peak heating which would bring the highest PoPs to these locations. All in all, just have slight chance/chance of showers/storms. SPC has a slight risk for severe storms. There could be an isolated severe during the afternoon due to diurnal heating. For the period Sunday night into Monday night, mid level trough will dig southeast across the Great Lakes/eastern Canada. As this occurs, a cold front will move slowly southeast through our region. With the lack of strong low level forcing and upper level support, the chances for showers/storms appears low and should affect our eastern/southern zones in the afternoon on Monday. Skies should become at least partly cloudy Monday night as high pressure builds into the region from the north with a drying and stabilizing air mass. High pressure should remain over the region through at least Tuesday into Tuesday night with dry weather expected. As the high moves east on Wednesday, return flow and increasing moisture may lead to a small chance of showers and thunderstorms. Models indicate that the next frontal boundary to affect our region will be during the Thursday/Friday time frame. Differences are seen in the timing, so a broad brush forecast has been used with a chance of showers and thunderstorms. Temperatures will cool back down to more July normals Tuesday through Friday. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... While VFR conditions will prevail through the evening, an area of thunderstorms will move through the Cincinnati area over the next hour or two, possibly bringing MVFR/IFR TSRA conditions to the CVG/LUK TAF sites. Some MVFR visibilities are possible with an area of rain moving into CMH/LCK as well. Later in the overnight period, a larger complex of thunderstorms is expected to move into the region, affecting the northern four TAF sites near daybreak. These storms have now been specifically included in the TAFs, and future forecasts may need to forecast further reduced conditions (lower ceilings / higher winds) with the storms. It is most likely that these storms will remain north of Cincinnati. Outside of the storms, some MVFR visibilities are also possible at the Columbus airports. After a break in the activity through late morning and early afternoon, additional thunderstorm development will be possible going into the afternoon and evening. Southwest winds will increase to around 10-15 knots. OUTLOOK...Thunderstorms will be possible again from Saturday night through Sunday, and then again on Wednesday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...Heat Advisory from 1 PM to 7 PM EDT Saturday for OHZ070-071. Heat Advisory until 7 PM EDT Saturday for OHZ077-078. KY...Heat Advisory until 7 PM EDT Saturday for KYZ089>096. IN...Heat Advisory from 1 PM to 7 PM EDT Saturday for INZ066. Heat Advisory until 7 PM EDT Saturday for INZ073>075-080. && $$ SYNOPSIS...Franks NEAR TERM...Franks/Hatzos SHORT TERM...Franks LONG TERM...Hickman AVIATION...Hatzos is the U.S. government's official web portal to all federal, state and local government web resources and services.