Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 290603 AFDILN AREA FORECAST DISCUSSION National Weather Service Wilmington OH 203 AM EDT Thu Oct 29 2020 .SYNOPSIS... Expect widespread rain tonight through Thursday as copious moisture enters the Ohio Valley ahead of Hurricane Zeta remnants. Cool and dry weather is forecast to accompany high pressure on Friday and Saturday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Rain will continue to overspread the region. As the surface low from remnants of Zeta translate northeast along the Appalachian`s spine, the pressure gradient over the Ohio Valley and much of forecast area will increase. The result of this will be strong northeast winds and periods of heavier rainfall beginning in the predawn hours. Steady temperatures in the upper 40s to lower 50s will be the rule through daybreak, with only a slight rise expected during the day. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... 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 NIGHT 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 /06Z THURSDAY THROUGH MONDAY/... Prevailing MVFR rain will be found for the bulk of the forecast with a slight improvement as rain ends from west-east at the tail end of the forecast. VFR cigs in northern TAF sites will quickly drop to the lower MVFR range and then drop below 1kft, lingering here for the majority of the day with a steady rain. Some heavier rain elements will bring IFR cigs and vsbys through the course of the day, particularly as winds gust to 20kt in the late morning/early afternoon. Conditions will improve later in the day but a light rain and MVFR cigs/vsbys will stick around until the tail end of the precip exits beginning in the west late in the evening. OUTLOOK... IFR ceilings possible Thursday night through Friday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Coniglio NEAR TERM...Franks SHORT TERM...KC LONG TERM...AR AVIATION...Franks is the U.S. government's official web portal to all federal, state and local government web resources and services.