National Weather Service
Area Forecast Discussion

806 AM EST Wed Jan 16 2019
SYNOPSIS
Areas of freezing drizzle will be possible this morning, with drier conditions expected by tonight, as a weak cold front moves into the region. A low pressure system will move through the area on Thursday, providing a mix of snow and rain. A more potent area of low pressure will move into the area Friday night into Saturday, with a mix of rain, ice, and snow. Very cold air is expected for Sunday and into next week.



NEAR TERM
UNTIL 6 PM THIS EVENING
Some convective elements have developed within a northeast to southwest oriented band crossing the forecast area. In these areas of stronger echoes precipitation has been changing over to snow or snow grains with some places getting at least a dusting. Have done a quick update to account for this. It appears that behind this linear feature that even drizzle/freezing drizzle is diminishing as visibilities improve. Although with some very light echoes still noted on radar, cannot rule out a bit of additional trace precipitation. For now have continued with winter weather advisory.

The drizzle is expected to gradually weaken with impacts also diminishing as temperatures warm. The cold front, which will move into the northern Ohio Valley later in the day, will not actually make full progress through the ILN forecast area. It will probably strengthen the gradient in temperatures somewhat, leading to max temps ranging from the lower 30s (far northern CWA) to upper 30s (northern Kentucky). These are somewhat conservative temperatures, with the expectation that the thick low level cloud cover will remain in place for the entire day. At best, a few transient breaks may develop during the afternoon.



SHORT TERM
6 PM THIS EVENING THROUGH THURSDAY NIGHT
As the mid-level flow over the Ohio Valley pivots into a pseudo-zonal orientation, attention will turn upstream to a fast-moving shortwave coming out of the central plains on Wednesday night. A weak surface low is expected to develop in Oklahoma, moving ENE across southern Missouri and into the Ohio Valley on Thursday. This system will bring the next change for precipitation to the area, mainly on Thursday, including a chance of some accumulating snow.

The precipitation is expected to begin very early Thursday morning, moving across Indiana and initially entering the southwestern corner of the ILN CWA. This activity appears initially to be forced well aloft, with vorticity advection ahead of the mid-level wave, and the first push of theta-e advection noted at 850mb and 700mb at this time. As the surface low gets a little closer, eventually the boundary layer flow will also switch to more of a southerly orientation, with temperatures warming through the saturated air mass below 10kft. This will also increase the strength of the forcing, with precipitation absolutely certain to occur during the 15Z-21Z time frame across the ILN forecast area. PoPs were increased to 100 percent CWA-wide.

The precipitation type question is a little bit trickier, though certainly not to the extent of the weekend system that will be discussed in the Long Term AFD section below. First, it should be noted that despite the warm advection aloft near the start of the event, model soundings do not really indicate the development of a strong near-surface inversion. It is true that there will be an inversion in place initially, but this inversion will weaken considerably as moisture advection occurs. Rather than an inversion, temperatures should still decrease with height through the saturated layer, though generally weakly. This would indicate very little chance of any mixed precipitation. Precipitation should generally begin as snow, gradually mixing with rain from south to north as the event progresses.

On a similar subject, the strength of the warm advection near the surface is not expected to be overly impressive, and the antecedent conditions will still be relatively cool with the presence of a snow pack. Because of this, there are concerns about just how warm it will get on Thursday, and how far north the mix with rain will eventually get. This uncertainty can be seen when looking not just at model surface temperatures, but also at the resultant GEFS snowfall plumes, which show quite a bit of spread for a lower-QPF event like this one. With concerns on the advection strength and the cool conditions initially in place, this forecast will err on the slightly cooler side of the spread. Snow forecast numbers have increased slightly from the previous forecast, with a swath of up to an inch and a half running just north of the rain-snow transition from Wayne County IN through Licking County OH. This is close to the operational GFS run at KDAY, for example, and a little above the ensemble mean. As precipitation gets lighter later in day, however, some mix with light rain or drizzle may get as far north as the northern extent of the ILN CWA, but this remains a slightly uncertain part of this forecast.

Temperatures should have a fairly small range through this period, with highs on Thursday in the lower to upper 30s, and lows on Thursday night in the upper 20s to mid 30s. In fact, given the location of the low, temperatures may not begin to fall for a while on Thursday evening -- especially in the southeastern sections of the forecast area.



LONG TERM
FRIDAY THROUGH TUESDAY
Friday is expected to be the quiet weather day of this week as a ridge of high pressure wedges in from the north across the region. However, it will still remain mostly cloudy. Highs will range from the lower 30s north to the lower 40s south.

Models continue to latch on the potential for a significant winter storm to affect the Great Lakes and Ohio Valley this weekend. One thing to note is that the 00Z operational ECMWF and GFS have come in a tad weaker with aforementioned deepening surface low to move through the Ohio Valley Saturday and Saturday night. This would have ptype implications as a strong low will pull more warmer air north and allow for more mixed pcpn across parts of the area as well as more of the area turning over to rain. Given that this is still Wednesday, there will be plenty of time between now and Friday for operational models to oscillate in the strength and exact track of the low. As a result, have relied on a blended model approach and the use of ensembles such as the GEFS and EPS. That being said, probabilities still remain likely for significant snowfall to occur over locations along and north of I-70 (6 inches or more) with a mixed pcpn transition zone occurring between I-70 and north of the Ohio River, with predominately rain occurring south of the Ohio River on Saturday. There could be some significant ice in the transition zone (i.e. at least 0.25 inches of planar flat ice or more). There also could be hydro issues along and south of the Ohio River given copious moisture and ensembles indicating the potential for over 2 inches of rain. Will mentioned all these hazardous in the HWO. As we head into Saturday night, a cold mid level trough will drop into the Great Lakes and Ohio Valley, ushering in an Arctic airmass, pcpn will transition over to a period of snow before tapering off from west to east late Saturday night into Sunday morning. Winds will become gusty, and may result in some blowing/drifting of snow where it occurs, and also could cause power issues where ice occurs. Also, the colder temperatures will eventually bring colder wind chill temperatures to the region. Lows Friday night will range from the mid 20s north to the lower 30s south. Highs on Saturday will range from the upper 20s north to the mid 40s along and south of the Ohio River. Lows Saturday night will range from near 10 northwest to the mid 20s southeast.

For Sunday into Sunday night as snow tapers off, Arctic high pressure will build into the region from the northwest. Temperatures will slowly fall during the day Sunday. Lows Sunday night will drop into the 0 to 5 below zero range across the north, to 0 to 10 degrees south. With wind still staying up a little, this will result in cold wind chill temperatures below zero, with wind chill advisory criteria likely being met across the northern half of the CWFA.

On Monday, Arctic high pressure will traverse our region. Highs will range from near 10 north to the lower 20s along and south of the Ohio River.

The Arctic high will move off to the east by Tuesday with some rebound in temperatures.



AVIATION
13Z WEDNESDAY THROUGH SUNDAY
Aviation conditions look rather poor for the foreseeable future. For the morning hours, generally IFR/LIFR ceilings will remain in place, Some patches of freezing drizzle will continue to be possible, capable of producing lowered visibilities and a light glaze of ice. However, as temperatures warm, this will turn to just regular drizzle within a couple hours. Winds will remain out of the southwest this morning, turning to the west later in the day.

A very gradual improvement in ceilings is expected during the daytime hours, though IFR ceilings should last well through the morning, and MVFR by later in the day. Winds will also begin to shift in a clockwise manner, becoming light enough to be characterized as light and variable heading into the overnight hours.

As winds shift to the east early Thursday morning, MVFR to IFR ceilings will continue. The next chance for precipitation will move into the area right at the end of the TAF period, but it looks like there will be a period of snow to start things off at the CVG/LUK sites, with lowered visibilities and a return to some IFR ceilings.



OUTLOOK

MVFR/IFR conditions are expected Thursday with a mix of precipitation types. MVFR/IFR conditions are possible Friday. IFR conditions are likely with precipitation on Saturday and Saturday night, with MVFR/IFR conditions and gusty winds on Sunday.



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