Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 282334 AFDILN AREA FORECAST DISCUSSION National Weather Service Wilmington OH 734 PM EDT Wed Oct 28 2020 .SYNOPSIS... Widespread rain is expected tonight into Thursday before drier and cooler conditions return to the region Friday into this upcoming weekend. A reinforcing shot of seasonably chilly air will arrive Sunday into early next week, with mainly dry conditions past the next 36 hours. && .NEAR TERM /THROUGH THURSDAY/... Quite a lot to unpack with the forecast here, so let`s get right to it. This afternoon, the Ohio Valley finds itself in a quasi-zonal midlevel flow regime with the impressively and anomalously deep/stacked low over the south-central plains into north Texas and Hurricane Zeta coming onshore the northern Gulf. With the broad and seasonably strong southwesterly midlevel flow between these two features, moisture transport aloft from the northern Gulf into the TN/southern OH Valleys will only increase as we progress further into the evening. Midlevel energy pivoting N/NE into the region late this evening will allow for a blossoming of light rain progressively from south to north late this evening, with a subtle signal for some light rain developing near the I-70 corridor late this evening (past 00z) along a lingering and weakening H8 convergent axis stretched west to east near I-70 across the region during this time period. Attention will quickly turn to a much stronger elevated frontal boundary pivoting in from the SW past midnight. This pivoting elevated (H8-H9) baroclinic zone will undergo frontogenesis/tightening tonight as a seasonably potent LLJ streams northward into the WSW-ENE oriented boundary. This will create tremendous moisture and mass convergence in the H8-H9 layer beginning in the predawn hours initially to the SW of the local area before working its way ENE into the far SW part of the ILN FA -- perhaps southeast IN into southwest OH -- by sunrise. This will be particularly stout across south-central IN Thursday morning as the H8 low tracks from western TN into NW KY, maximizing convergence in this layer and creating ample lift for heavier rain rates to develop. Although rain will undoubtedly overspread the entire ILN FA tonight -- warranting 100 PoPs for the entirety of the CWA -- there will likely be an enhanced axis of heavier rates coincident with the H8 front developing and pivoting into the Tri- State area beginning around 09z. This will mark the beginning of a 6-8 hour period where rain rates will be the most robust, with pockets up to 0.5 in/hour possible in SE IN/northern KY/SW OH until late morning when the H8 low begins to jog east, allowing the convergent axis to subtly shift to the south and decrease in magnitude the amount of convergence/lift over the immediate ILN FA. Lows tonight will range from the lower 40s in the NW to the mid 50s in the SE with the overcast conditions and widespread rain. Winds will gradually go northeasterly and pick up toward daybreak. Sustained winds of 15-20 MPH will be possible Thursday morning. && .SHORT TERM /THURSDAY NIGHT/... Widespread rain -- with an enhanced axis of heavier rates from SE Indiana into southwestern/south-central OH -- will be ongoing at daybreak Thursday. Even by 12z, we may see some spots nearing 1" in total rainfall already, especially near/south of the Ohio River. This steady/heavy rain will continue through the morning hours before the magnitude of LL convergence/ascent begins to gradually decrease by the afternoon (as the LLJ pulls to the east), yielding a trend toward lighter rainfall rates for the afternoon. This is not to say it is going to clear up by the afternoon -- because it won`t. We will, however, see a trend toward lighter rainfall rates as the LL convergence decreases and begins to pivot a bit further to the east/southeast. For the afternoon hours, we will see a deformation axis of rain linger back well to the west, with steady /but lighter/ rain continuing well into the evening before tapering off late evening from west to east. With all of this in mind, quite a bit of consideration was given to issuance of a Flood Watch for at least a part of the ILN FA. Although for the month thus far many spots are fairly close to normal for rainfall, it has been a bit wetter-than-normal over the past 10 days, with multiple heavy rain events moving through the region during this span. Additionally, the 12z guidance came in a bit more aggressive/further north with the strip of heavier QPF (aligning with that potent/tight H8 baroclinic/convergent axis), with spots near the Tri-State area most likely to receive in excess of 2 inches of rain by the time the rain tapers off late in the evening Thursday. This uptick in QPF was particularly noticeable amongst the hi-res CAMS as well as a subtle shift further north in the GEFS and GEPS QPF means -- with both ensemble suites showing 2 inches of rain into the southeast IN/northern KY/southwest OH by the time rain ends early Thursday night. With all of this being said, there are several factors that will work against widespread flooding issues, most notably the extended timeline for which we are expecting the rain to fall. It will be raining for the better part of 18-20 straight hours for many spots, with about half of the rainfall coming in just a 4-hour span or so centered around daybreak. So even with this, our soils should be able to handle 1-1.25" of rain over a 4-hour span. That would equate to about 1/3" of rain per hour, which is well below prevailing FFG. If we see a setup favoring this rate to continue for most than just a handful of hours over any one location, it is feasible that a short-term flood advisory will be required to handle lowland flooding and higher water in "typical" trouble spots. But anticipate that this will be the exception rather than the rule. There is no doubt that it will be soggy on Thursday. There will be standing water in fields and in low-lying spots in yards by the afternoon. But it appears unlikely at this juncture that there will be any widespread threat of flooding to inundate structures or do anything more than just fill up local creeks/waterways. The biggest threat to flooded streets will be the potential for leaf debris to temporarily clog typical drainage spots, delaying and/or slowing the drainage of water in some isolated spots. We will certainly be monitoring the evolution of the rain rates Thursday morning and will be watching for any training scenario where those 1/3"-1/2" rates train over the same locales for any extended period of time. But right now, will continue to highlight the potential for isolated flood issues in the HWO. Storm total rainfall will range from less than 1 inch in west- central Ohio to 1-1.5" near the I-70 corridor to 1.5-2.5" near the Tri-State area and near the Ohio River. Rainfall of 1.5-2" in the far south is expected. Rain will end from west-to-east late Thursday evening, with sustained NE winds of 15-20 MPH during the morning and afternoon tapering slightly by Thursday night as much colder air begins to filter in from the NW. Lows Thursday night will range from the mid 30s in west-central OH to the mid 40s in the lower Scioto Valley and NE KY. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... West-northwest flow aloft with surface high pressure building into the area Friday. Low level CAA along with flow off the lakes may result in a few showers into central Ohio on Friday. Have elected to go with dry fcst with any impacts very low due to limited coverage and probability. Temperatures look to be 10 to 15 degrees below normal with highs on Friday generally from 45 north to 50 south. With the sfc high pressure building across the area Friday night expect mainly clear skies and light winds. This will lead to Saturday morning temperatures at or below freezing, with frost likely. Will continue this mention of freezing in the HWO product. Mid level flow backs westerly with sfc high pressure moving off to the east on Saturday. After a cold start -- a low level southerly flow will only bring modest warming to temperatures. Expect highs on Saturday of 50 to 55. Shortwave and sfc low to track thru the Great lakes with an associated sfc cold front to sweep east across the region on Sunday morning. A few showers are possible, mainly acrs northern and eastern counties. In the wake of the cold front, winds will become gusty Sunday afternoon into Sunday evening. Wind gusts of 30 to 35 kts will be possible. Northwest flow aloft develops with another surface high to build across the area Monday and Tuesday. Dry conditions to continue with below normal temperatures early in the week. Highs look to be around 15 degrees below normal Monday in the lower and middle 40s. Some moderation Tuesday with fcst highs of 50 to 55. Southerly flow looks to bring temperatures close to normal at midweek with Wednesday`s fcst highs in the upper 50s to lower 60s. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... Aviation will be affected by plentiful moisture ahead of the remnants of Hurricane Zeta. MVFR ceilings to start will drop to IFR tonight as rain begins. Rain will become heavy Thursday while the remnants track across Kentucky, causing IFR visibilities. Wind gusts over 20 knots out of the northeast will be common on Thursday in the rather tight pressure gradient on the north side of the system. Some improvement back to MVFR along with subsiding winds may be observed near the end of the forecast as the remnant low moves east of the Appalachians. OUTLOOK... IFR ceilings possible Thursday night through Friday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...KC NEAR TERM...KC SHORT TERM...KC LONG TERM...AR AVIATION...Coniglio

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