Marine Weather and Tide Forecast for Irmo, SC

KEY MESSAGES
1) A rain/snow mix is expected to start Sunday morning, possibly with a band of enhanced rates. Significant accumulation probabilities remain lower, but there has been a uptick is possible amounts toward the CSRA. Patchy black ice is still possible early Monday morning.

2) Arctic air mass moves into the Southeast early next week with temperatures well below normal through Wednesday.

DISCUSSION
Key Message 1: A rain/snow mix is expected to start Sunday morning, possibly with a band of enhanced rates. Significant accumulation probabilities remain lower, but there has been a uptick is possible amounts toward the CSRA. Patchy black ice is still possible early Monday morning.

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: 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 /18Z SATURDAY THROUGH THURSDAY/
An active period is expected for the TAFs as clouds increase this evening, followed by rainfall developing. MVFR and IFR cigs are expected to develop Sunday morning.

Clouds that developed earlier this morning have tended to dissipate over the last few hours, with sunshine present at all sites currently. The increase in temperatures across the region have led to strong low level winds mixing down, with consistent gusts of 18- 22 knots noted at the TAF sites. Clouds are forecast to begin increasing this afternoon and evening as lift and moisture increase ahead of our next system. This is expected to be fairly quick this evening as the lift will increase rapidly. As early as 03z-05z, showers are expected to develop across the CSRA and northern Midlands as a cold front sinks towards and through the forecast area. This rain will quickly become more widespread after 06z, overspreading all of the TAF sites. As cooler air filters in behind the surface front, low clouds are forecast to develop across the area thereafter. MVFR and IFR cigs are likely from 08z-10z through the end of the period at all sites as moderate rain overspreads the sites. The weather then gets quite interesting, especially at the Augusta sites. There is expected to be a period of rain/snow mix at these sites from 13z-18z as cooler air filters in behind the front.
Uncertainty exists as to when rain will change over to snow and how much will actually fall. However, confidence is high enough in the mix to put it in the TAFs at this point. It is unlikely that OGB will see any snow, and the probabilities at CAE/CUB are low enough right now to preclude any mention. But we'll need to keep an eye on it today and tonight. By the very end of this period, precip is expected to slowly push to the northeast, with restrictions likely to end quickly after 18z on Sunday.

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

CAE WATCHES/WARNINGS/ADVISORIES
SC...None.
GA...None.