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FXUS63 KJKL 071745 AFDJKL

AREA FORECAST DISCUSSION National Weather Service Jackson KY 1245 PM EST Thu Dec 7 2023

UPDATE
Issued at 1059 AM EST THU DEC 7 2023

A sunny and mild early December day is unfolding across eastern Kentucky. Going forecast remains on track other than a few tweaks to bring observations into line with the forecast grids.

UPDATE Issued at 645 AM EST Thu Dec 7 2023

There is a band of clouds drifting in from the northwest at around 8,000 feet AGL that will affect the area for a time this morning. Have updated the forecast mainly to move this band through before dissipating. Did also add in the latest obs and trends for the T/Td grids with this update. These tweaks have been sent to the NDFD and web servers.

SHORT TERM /THROUGH FRIDAY/
Issued at 420 AM EST Thu Dec 7 2023

Key Messages:

* Warmer and drier today and Friday. RH falling to near 30 percent in a few places each afternoon.

* Breezy conditions from southwest winds gusting up to 25 mph this afternoon. Bluegrass area will see the highest winds.

09Z sfc analysis shows high pressure nosing into eastern Kentucky from the south. This has helped to settle the winds and clear the skies for much of the night. Accordingly, good radiational cooling and near calm winds have led to very chilly temperatures in the sheltered valleys - down to the low 20s while ridges have managed to slip to the lower 30s where light southwest winds are noted. Dewpoints, meanwhile, are holding in the low to mid 20s. The small dewpoint depression is likely prompting some river valley fog development but not much is showing up on satellite or in the obs at this time.

The models, and their individual ensemble suites, are in excellent agreement aloft through the short term portion of the forecast.
They all depict 5h heights rebounding today in the wake of a deep trough departing to the east and as a low amplitude ridge passes through the Southeast into Friday. This will in turn switch fairly fast northwest flow to a more southwesterly tilt tonight continuing into Friday. Only a few weak 5h impulses will slip over the area in this transitional flow. Given the small spread among the model suites, preferred the blended NBM as the starting point for the forecast grids, though with some adjustments drier for dewpoints and RH each afternoon - as well as colder in the valleys at night.

Sensible weather will feature a nice warm up today under mostly sunny skies, but with dry and breezy conditions this afternoon.
A few locations may see their RH fall to near 30 percent this afternoon coincident with southwest breezes of 10 to 15 mph and higher gusts - mainly for locations northwest of I-64. For tonight, another night of mostly clear skies and light winds will set up another (but milder) ridge to valley temperature split and less threat of fog. Even warmer conditions follow on Friday with the driest air shifting off to the far east where RH may approach 25 percent for a time in the afternoon, but the winds will be lighter than those of this afternoon.

The main adjustments to the NBM starting point were to go with drier dewpoints this afternoon and for Friday afternoon. Did also add in the NBM 75th percentile for high temps today and Friday. PoPs were kept to a minimum through the period - in line with all guidance.

LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/
(Friday night through Wednesday)
Issued at 330 AM EST THU DEC 7 2023

Key Points:

* Strong low pressure system to bring widespread rain, thunderstorm chances, and breezy conditions this weekend.
* Though the threat is low, isolated flash flooding can't be ruled out within heavier rainfall rates/convection.
* Rain will mix with or changeover to snow Sunday into Monday morning, with light accumulations possible across the higher SE terrain. Little to no accumulations expected elsewhere.
* Precipitation-free weather expected by Monday night and through the end of the period.

The period will begin on the backside of a departing ridge, where an active set-up then takes shape through the weekend. Aloft, a deepening positively tilted trough will start out across the Intermountain West to Northern Plains, deepening and diving southeastward through the weekend before taking on a negative tilt through the Ohio Valley and into the Mid Atlantic. At the surface, high pressure will be sliding off the East Coast, where deep southerly return flow will advect in seasonably high PWATs ahead of a strengthening 996 mb low and associated strong cold front. In terms of climatological norms, PWATs will be near the 90th percentile, with low pressure around two SD's below that of early to mid December norms. Surface low pressure will deepen, bombing out across the Upper Great Lakes, before its upper support phases with northern stream Canadian energy late weekend into Monday. A second low will develop along the boundary into New England early next week. With a strengthening gradient across the region, winds will be on the increase through Saturday, peaking with the front Saturday night into Sunday morning. Ensemble probabilistic guidance favors sustained winds between 10 and 20 mph, with gusts between 25-35 mph.
Higher end gusts can't be ruled out on the highest and most exposed terrain, but there is minimal support for gusts any greater than 40 mph within deterministic and ensemble guidance, and only a low-end "unusual event" in accordance with the Extreme Forecast Index.
Nonetheless, it will be gusty Saturday into Sunday with the passing front.

Widespread showers are expected along and ahead of the baroclinic zone, with a few hundred Joules of MUCAPE making isolated to scattered thunderstorms possible as well. Aside from gusty winds, there will be a dual threat of heavy rainfall, followed by a low- end/low confidence winter weather threat. Anomalously high moisture, paired with the strong front/upper support and residency time, could make for localized heavy rainfall/ rates and a low confidence threat of flash flooding and higher running water bodies- particularly within better convection. Fortunately, we have been on the drier side of averages in recent weeks and we will have dry antecedent conditions coming into the event. Nonetheless, the probability for exceeding 1" in 24-hours is generally greater than 50%, with forecast QPF between 1.5 inches and 2.5 inches. These higher end amounts will favor locations closer to the Tennessee Valley (i.e the Cumberland Basin), which tend to be lesser flood prone areas overall.

As the parent low and attendant cold front depart east of the area through Sunday, strong CAA will allow for a change over to frozen precip with lingering moisture and higher lapse rates. Confidence in the timing and finer details remains limited at this time, but a brief period of rain mixing with or changing over to snow will be possible area-wide with little (dusting to couple of tenths on grassy surfaces) to no accumulations expected for the bulk of the area. That said, favorable NW/upslope flow could contribute to light snow accumulations (up to 1") across the higher terrain, generally above 2,000 feet near the Virginia border, later Sunday into Monday.
Heights will then rebound through early next week, as shortwave ridging to zonal flow fills in across the Ohio Valley. High pressure will accompany at the surface, meandering across the Deep South and allowing for rain-free weather through the end of the period.

AVIATION
(For the 18Z TAFS through 18Z Friday afternoon)
ISSUED AT 1245 PM EST THU DEC 7 2023

VFR conditions prevail through the period with some passing high and mid-level clouds at times. Southwest winds will average near 10 kts with gusts of 15 to 25 kts this afternoon, before diminishing later this evening. However, a low-level jet over the Lower Ohio Valley will lead to some LLWS just of the off the surface tonight, most notably at KSYM.

JKL WATCHES/WARNINGS/ADVISORIES
NONE.

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