An Elf’s Mischief: Rain to Winter Weather. Forecast Uncertainty

Above: Flood Watch through Wednesday Morning (Green)

I hope you had a wonderful and blessed Christmas with your family! Although the first part of the day was dry, it has been raining consistently since then, and the weather is expected to remain the same throughout the night and into Tuesday. There may be a few rumbles of thunder late tonight in Polk, Cleveland, Rutherford, and Gaston Counties and then along the I-40 Corridor Tuesday Morning. A Flood Watch remains in effect for the area through Wednesday morning. Be prepared for colder weather as a pattern change is coming Wednesday Night, with an upper-level low-pressure system forecasted to move across the area on Thursday night and Friday. That upper low may result in a brief and sporadic rain/snow mix in the region.

The National Weather Service’s Flood Watch is in effect until Wednesday morning (see above). The watch is due to prolonged upslope flow rainfall over the eastern escarpment on both Monday and Tuesday. However, there is some uncertainty regarding the rainfall amount as the HRRR model may be too aggressive. Some convection along the coast Tuesday Morning may reduce moisture transport upstream. The boundary could push east on Tuesday Afternoon instead of sticking around into Tuesday Night. However, the NWS is taking necessary precautions as significant localized flash flooding could occur if heavy rainfall materializes along the eastern slopes.

An upper low-pressure system deepens over the Central Plains and will cut off from the Northern stream flow. As a result, the frontal boundary will occlude, move east, and wash out. The HRRR appears too slow with this progress, but heavy rainfall totals will still pile up along the western foothills. Southerly flow has already strengthened to 40-50 knots ahead of that frontal boundary, pushing PWATS (Precipitable Water Values) to 1 – 1.5 inches.

Late tonight, the moist flow out of the south will strengthen and create an area of 400 to 900 j/kg of elevated CAPE, which is thunderstorm fuel. With the introduction of elevated CAPE into the region, we are introducing the chance for thunder across the South Carolina border counties early in the morning. Some of that convection may make it as far North as the I-40 corridor by daybreak or just after. These elevated thunderstorms will produce exceptionally high rainfall rates, but the individual cells should be progressive and not anchor. Hopefully, heavy rainfall can run off before more heavy cells arrive. Rainfall rates of 1 – 1.5 inches per hour will be possible tonight and Tuesday morning.

While some guidance projects 4 – 8 inches of rainfall along the eastern slopes, 2 – 5 inches is much more reasonable. Away from the mountains across the remainder of the foothills, 1 – 3 inches of rain seem likely. Flash flood guidance suggests that it will only take 1.5 to 2 inches of rain in an hour to begin seeing some flash flooding along the western foothills. It would take 3 – 4 inches in 6 hours to see flash flooding issues.

With mild temperatures in place, this will all be rain with temperatures in the upper 50s and maybe a few 60s along the South Carolina border counties.

Dry air will move in on Tuesday evening and Wednesday after the weather system passes. The dry weather will persist until Wednesday night, after which more moisture will arrive with a low-pressure system on Thursday. As the surface low passes beneath the cutoff low, there could be a period of northwest flow snow starting on Thursday afternoon and continuing into Friday.

Above: GFS Projected Weather Pattern 8am Friday December 29th

The surface low moves along the coast turns positive and enhances the dynamics aloft. That dynamic forcing could set up a deformation band of precipitation on the backside of the departing surface low. With enhanced dynamics aloft, the deformation band of precipitation could break out into the foothills on Thursday night or Friday morning.

However, there are still some questions that need to be resolved. Will the closed upper low remain closed and more robust, or will it open up and create fewer dynamics to work with? What is the timing of that deformation band, and can it fall hard and fast enough to bring the cold air from aloft to the surface? What will that surface low along the coast do as far as its track?

There are many variabilities, but experience tells us these upper-level systems can be unpredictable until they move into the area. Sometimes, they overproduce; sometimes, they create a forecasting nightmare and do not produce anything. We will go with a 30% chance of precipitation on Friday.

Expect temperatures in the 60s Wednesday, 50s Thursday, and 40s Friday. Friday’s temperatures may change.

After all that volatility, the pattern will become flat and uneventful through the weekend. The mountains could continue to pick up on that northwest flow snow through the early weekend before it tapers off Sunday. Due to the calm and clear weather expected outside of the mountains, prime radiational cooling will likely occur. Expect lows below freezing but daytime highs to moderate into the low 50s.

By New Year’s Day, precipitation will return to the area, and temperatures will decrease. According to global models, some energy will move into the region, and a low-pressure system could develop off the southern jet stream and move up the East Coast. There are some discrepancies regarding the amount of cold air that could enter the region from the North, which will depend on the position of a high-pressure system over the Northeast. Ensemble models suggest that some cold air could join a glancing shot of precipitation.

Above: GFS Weather Pattern New Years Day into January 2nd, 2024

This winter weather pattern is the best we’ve had since last Christmas. Let’s see if it produces this time around.