National Weather Service
Area Forecast Discussion

614 AM EST Mon Dec 10 2018
SYNOPSIS
Surface high pressure will extend across the region today. The high will move south of the Ohio River tonight and Tuesday while a low pressure system moves east across the northern Great Lakes. Another low pressure system will move into the region by mid week. Unsettled weather can then be expected toward the end of the week as a more substantial weather system affects the area. Below normal temperatures early this week will moderate through the rest of the week.



NEAR TERM
UNTIL 6 PM THIS EVENING
A surface ridge axis extended from the middle Mississippi River Valley to the southern Great Lakes early this morning. This ridge axis will gradually drift southward through the day.

Biggest forecast concern today will be the development of low clouds (stratus) and/or fog across the northern zones through sunrise. GOES-East has shown low clouds expanding across lower Michigan, northern Indiana, and northern Ohio over the last several hours. Models tend to struggle on the timing and extent of low level moisture due to their known biases, which make it difficult to know how much of our area will be affected today. The NAM is usually the most pessimistic/aggressive, the GFS too dry and optimistic, and the RAP somewhere in between. Have leaned more on RAP model soundings, but have also incorporated some blended models (NBM, HREF) to help determine how much and how far south clouds will expand south through the morning hours. It appears locations along and south of the Ohio River will remain safe and thus will experience a good deal of sunshine and warmer afternoon highs. Farther north, particularly from the I-70 corridor and points north, clouds may hold pretty tough today. Two reasons: 1) moisture will be trapped underneath a subsidence inversion. 2) We are at the lowest sun angle of the year and are experiencing shorter daylight hours, which does not bode well to fully erode/dissipate stratus. Thus, skies will range from mostly cloudy to cloudy north, with perhaps some erosion on the southern and western periphery this afternoon, with sunny skies south. Where cloud heights drop below 500 feet, some patchy fog may occur, particularly through the morning hours. Highs today will range from the upper 20s in the thickest clouds to the upper 30s along and south of the Ohio River. Will monitor temperature trends where clouds persist as highs may have to be trimmed some from current forecast.



SHORT TERM
6 PM THIS EVENING THROUGH 6 PM TUESDAY
Surface ridge axis will drift south of the Ohio River tonight and Tuesday while shortwave energy and a low pressure system move across the northern Great Lakes.

As mentioned in the near term, stratus could still be an issue across the north going into tonight. It is tough to know how much will have eroded by the end of the daylight hours and whether it will expand again overnight. For now, have allowed for clouds to gradually shift east/northeast as the flow backs to the southwest. Outside of the lower clouds, skies will be mainly clear elsewhere. Lows will generally range from the upper teens to the lower 20s.

On Tuesday, mostly sunny skies are expected. The tightening pressure gradient between the high to the south and the low to the north, combined with some diurnal mixing will, result in warmer temperatures, along with some locally gusty winds. Highs will range from the mid 30s north to the lower 40s south.



LONG TERM
TUESDAY NIGHT THROUGH SUNDAY
On Tuesday night, an expansive area of dry surface high pressure will be centered over the southeastern states, with a surface ridge extending northward into the Ohio Valley and western Great Lakes. A mid-level shortwave will be moving into the eastern Great Lakes, but should remain too far away from the Ohio Valley to have any impacts, outside of a slight increase in clouds across northern and central Ohio. A sharp mid-level ridge will be moving over the region by Wednesday, and as the surface high slides slightly to the east, this will place the ILN CWA within a regime of warm advection. With rising heights and southerly flow, temperatures will continue the warming trend, with highs expected to be in the upper 30s to lower 40s -- not quite to climatological normals, but getting very close. There should actually be some respectable forcing in the 925mb-850mb layer on Wednesday afternoon, associated with warm frontal processes just aloft, as a LLJ begins to kick in. However, with the very dry antecedent conditions, actually getting precipitation out of this airmass this early in the modification seems unlikely.

Instead, it will probably be on Wednesday night || early Thursday morning when the first chances for precipitation begin, associated more closely with a compact shortwave moving across the southern Great Lakes. With some differences in magnitude and placement still showing up between the GFS/ECMWF, and moisture still remaining somewhat a limited factor, PoPs for this feature will be limited to the 20-40 percent range. It might be cold enough for a brief mix with some snow, depending on how late into the Thursday morning period the precipitation chances persist.

Behind this wave on Thursday, there will be a very brief period of ridging in advance of the next push of warm and moist air from the south on Thursday night into Friday. This continued southerly flow will allow for the warming trend to continue, with temperatures rising another couple degrees each day through Friday. Regarding the large-scale pattern, it is interesting that the GFS has now become more closely aligned with the more southerly progression for the expected upper low development on Friday into Saturday. This helps to start to bring the forecast back toward medium confidence, after a day or two of wildly varying model solutions. To note -- the 00Z operational ECMWF keeps similar placement with the system, but on a slower forward speed. For now, as this solution seems to be on its own, a consensus of the 00Z GFS || 12Z ECMWF will be used as a basis for the forecast. Without being directly impacted by the upper low, this solution does limit the more significant potential for precipitation to just Friday, as the longitudinal feed of moisture runs into a SW-to-NE wall of deformation forcing from the lower Mississippi Valley through western Pennsylvania.

Dry high pressure is expected to move into the region for the weekend, as temperatures stabilize at values near normal.



AVIATION
11Z MONDAY THROUGH FRIDAY
A surface ridge axis located across the southern Great Lakes will move slowly south today. Despite this ridge, plenty of low level moisture is located in the northerly flow. This will result in IFR ceilings and MVFR visibilities at the northern terminals until about midday. Then, MVFR ceilings around 1500 feet will continue as the moisture will be trapped under a subsidence inversion. Farther south, it looks like the low clouds will not make it due to enough sunshine to occur to stop the forward progress. However, KILN is on the edge, and this will have to be watched.

For tonight, it is hard to determine at this time if low clouds will reform and/or expand, depending in part what happens today. Have allowed ceilings to scatter across the north. Otherwise, skies will be mostly clear. Surface ridge axis will drift south of the Ohio River while a low pressure system begins to move into the northern Great Lakes. This will allow winds to back with time to the southwest.



OUTLOOK

MVFR conditions possible Wednesday into Wednesday night and again on Friday.



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