textproduct: Columbia

WHAT HAS CHANGED

Winter Weather Advisory issued for Burke, Columbia, Lincoln, McDuffie, and Richmond Counties from 4 AM to 3 PM EST Sunday.

KEY MESSAGES

- 1) Snow may cause hazardous travel in the CSRA Sunday morning.

- 2) An arctic air mass arrives early next week.

DISCUSSION

Key Message 1: Snow may cause hazardous travel in the CSRA Sunday morning.

Evening Update:

Models have continued to show a sharper, more neutral or negatively tilted upper level trough compared to runs from previous days. Ultimately this has led to stronger synoptic support for precipitation with moisture further west and lingering over the region for longer through the day.

Widespread rain is expected to move into the area early Sunday morning. With temperatures mainly in the low or mid 40s through the overnight period, it would be unlikely to see anything but rain prior 6AM. It won't be until later in the morning when we see cold air rush in from the west that there is some potential for a rain-snow mix to develop into mid-day. It's worth noting that this setup where cold air comes in late in the event does not normally lead to significant snowfall accumulations for our region.

The main concern is if a heavier band of precip sets up Sunday morning. With greater dynamics in recent model runs we are more confident that there will be a heavier band that develops. This could cool the low levels dynamically and allow snow to reach the ground despite above freezing surface temperatures. Moderate snowfall rates would cause impacts to travel for our region even short lived or if it did not amount in significant accumulations. Although it's not possible to pin down exactly where this band will set up tomorrow, our confidence is highest in eastern GA. Therefore a Winter Weather Advisory has been issued for the GA counties in the CSRA for Sunday morning into mid-day.

Previous Discussion:

With the next round of guidance in, there has not been a significant change from a synoptic standpoint. The main noticeable change at this scale is there is better consensus on the sharp shortwave trough becoming a bit more negatively tilted as it nears the area with cyclogenesis ensuing along the coast. This has amped up the overall dynamic profile a bit more that in turn could alter the rather marginal thermodynamics. First, rain is expected to begin overspreading the region tonight as warm advection pushes in and greater upper support nears but a rain/snow mix is expected to begin forming toward early Sunday morning in parts of GA, spreading eastward.

Now for the complicated part. With this slightly more amplified shortwave, recent 12z guidance (mainly NAM and GFS) has shown sounding profiles that dynamically cool a couple degrees more than previous runs as a small pocket of warmer air at 850mb gets lifted. Surface temperatures and surface wet-bulb temperatures are still rather marginal in 12z runs, but rather than over 2000 ft of a transition zone from the below freezing layer aloft to the above freezing layer near the surface, new runs have this depth closer to 700-1100ft. High-res guidance and global models also continue to show the rather robust forcing in the saturated DGZ. Modeled cross sections should solid omega values in the DGZ with moderate forcing from FGEN also present. This is all to say, the recent 12z guidance indicates enough forcing to produce a band of moderate precip rates but a large degree of uncertainty remains. The main uncertainties are if the turnover from rain to a rain/snow mix will occur, when will the turnover from rain to a rain/snow mix occur, and if the rain/snow mix occurs, where will the band of moderate rates set up.

At this time, most guidance is on board with widespread rainfall across the FA Sunday with a rain/snow mix developing to our west and then pushing into the FA Sunday morning and into the early afternoon, especially toward the CSRA and parts of the Midlands with decreasing confidence in snow mixing in as you move toward the northeast. Next the most recent runs and even some of the ensemble and deterministic guidance last night is favoring the moderate band of precip rates to develop in parts of south central GA, possibly pushing into the CSRA and far southeastern Midlands.

All in all, the current outlook has seen a slight increase in chances for accumulating snow in the aforementioned area with probabilities for Winter Weather Advisory level accumulations currently between 25-35% in the CSRA especially (up from last nights guidance). Due to the high degree of uncertainty that remains in where exactly the moderate precip band sets up and if snow rates in this band would be enough to overcome surface temperatures above freezing, have decided to hold off on any issuance of a Winter Weather Advisory right now, but this will need to be monitored with the next run of guidance as reasonable worse case scenario snow amounts up to 0.5-1" could be possible toward the CSRA and parts of the southern Midlands if the moderate band sets up here. The next risk overnight and into Monday morning will be wet surfaces thats develop patchy black ice due to temperatures that quickly fall into the 20s.

Key Message 2: An arctic air mass arrives early next week.

Behind the storm system on Sunday arrives a very cold airmass. The synoptic scale pattern looks pretty similar to what it has been for a few days now with deep troughing favored over the eastern CONUS. GEFS 5 day 500 hPa height anomalies continue to look below normal, with the core of the upper level trough centered near the Great Lakes and lower Hudson Bay. On the southern side of this, strong convergence will favor persistent, strong surface high pressure across the eastern CONUS. The initial surge of this will be felt on Sunday night and Monday as cold advection ushers in temps in the 20s by Monday morning. Winds look like they'll offset whatever radiational cooling we could potentially have which should help keep us from seeing Cold Weather Advisory criteria. There will likely be some isolated spots that get near our 15F criteria but it shouldn't be widespread enough to justify a product. Temps will remain below normal both Monday and Tuesday as the surface high approaches from the west. By Tuesday night, the surface high is expected to pass just to our north with ideal radiational cooling conditions possible. This should result in the coolest night of the period, with lows dipping into the upper teens and low 20s. But in general, low temps are forecast to be 10F-15F below normal Sunday night through Tuesday night. Be sure to avoid frozen pipes by letting faucets drip each night! Precipitation is not expected given the low PW air in the region.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Conditions Deteriorate at the Terminals Tonight....

Satellite imagery this evening shows clear skies at OGB with scattered clouds at the other terminals. Thicker clouds should move into AGS/DNL shortly followed by CAE/CUB/OGB. No significant adjustments to the previous forecast were made with this update. Rain chances increase this evening with rain likely tonight and into Sunday morning. Ceilings should continue to fall as the night goes on, with MVFR to IFR cloud decks developing prior to daybreak Sunday. Uncertainty remains regarding the potential changeover to a rain/snow mix near the end of the event, but maintained the mix at AGS/DNL with this update. Would not be surprised if at least some mixing occurs during the late morning and early afternoon hours of Sunday at the other terminals, but snow related impacts are most likely to occur along and south of the Savannah River. Conditions should improve quickly late in the day, towards the end of the current TAF period.

EXTENDED AVIATION OUTLOOK...VFR expected beginning Sunday evening and lasting through at least mid week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...None. GA...Winter Weather Advisory from 4 AM to 3 PM EST Sunday for GAZ040-063>065-077.