Scientific Forecaster Discussion
000 FXUS63 KPAH 122030 AFDPAH Area Forecast Discussion National Weather Service Paducah KY 230 PM CST Thu Dec 12 2019 .SHORT TERM...(Tonight through Saturday) Issued at 200 PM CST Thu Dec 12 2019 In the immediate term, models were having difficulty handling the dry air at the surface this pm. Dew points remain largely in the 20s and there's even some upper teens around, which may be suspect. The models appear better synchronized with clouds that are largely mid and high variety and winds, with some gustiness, that are higher closer to the tighter gradient in our northwest tier of counties. Beyond the immediate term, gridded time/height cross sections show we should see a continuation of the very dry air in the lower trop thru the day Friday. This should work to preclude (most) precipitation chances from slinging much back into the FA, as moisture overruns a boundary that is well to our south and east. But it looks like we may carry a collaborative small pop in our far east, into Friday evening. Pops there (east) should diminish late Friday night, as upper trof energy swings thru. However, a 2ndary punch of clipper wave energy will bring its own sling of moisture down the nwly pike, and the blend has consistently picked up on smallish pops with its track, across basically our northeast half of the FA. We'll stay with these ensembled assembly of smallish pops for late Friday night thru Saturday, but overall QPF is scarce, and boundary layer temps just warm enough, so they should offer minimal impacts. .LONG TERM...(Saturday night through Thursday) Issued at 200 PM CST Thu Dec 12 2019 As a surface high builds into the Midwest and western Great Lakes region Saturday night/Sunday, isentropic lift should get going by Sunday afternoon east of a mid level trof out over the southern Rockies. This may be able to generate some light precipitation along and to the north of a west-east oriented sfc boundary located from the western Gulf Coast states ne into the lower TN River Valley. Precip chances will then increase Sunday night into Monday as a surface low develops on the frontal boundary and begins to strengthen and head northeast. It seems as though enough cold air will be in place on Sunday into Sunday night for any precipitation to fall in the form of snow or a rain/snow mix, especially north of the OH River. Would not be surprised to see some accumulations during the late afternoon and night-time hours, but the qpf forecast is a bit problematic and of lower confidence. For now, will compromise with a qpf close to WPC numbers, but it should be noted that there are some models, including the 12z ECMWF, that triple the WPC qpf. Of course, that would translate into higher snow totals than we currently have in the official forecast. In addition, there may well be a narrow band of mixed wintry precip along or somewhere near the OH River. Stay tuned. On Monday, surface low pressure will rise northeast along the frontal boundary. Though models have converged somewhat on the track of the low, there is still uncertainty as to whether the track is right up the OH River, or farther southeast toward the Nashville TN area. For now, looking like consensus is to lean more toward the farther south solution. This is important because if the low track were to end up farther nw, then we could be dealing with a convective risk too, as more instability would become available along/east of the low track. Thinking now is that just enough warming will take place during the daylight hours Monday across our northern counties to allow any snow to change over to a cold rain, which could dig into any snow totals. In addition, it is now looking as though any deformation snows wrapping around the back side of the system on Monday evening will miss our forecast area to the nw and north, but not by much. However, any lingering precip that does last into Monday night would likely turn back over to a little light snow or flurries before ending later in the evening. Another shot of very cold, Canadian air will spill into the region behind the system Tue/Wed, with high temps holding in the 30s to near 40 and overnight lows down into the teens to lower 20s. Some moderation should begin to take place by Thursday, but readings should still be below seasonal norms. && .AVIATION... Issued at 200 PM CST Thu Dec 12 2019 Time/height cross sections show dry air wedged below 700 MB, so cloud bases should remain mid and high VFR variety, even with cigs, thru the first half of the forecast at least. During the latter half, some saturation down begins to occur, so during the planning period and beyond, we may see lower but probably still VFR bases enter the forecast picture. Southerlies prevail thru the package. && .PAH WATCHES/WARNINGS/ADVISORIES... IL...None. MO...None. IN...None. KY...None.