textproduct: Charleston

WHAT HAS CHANGED

Snowfall amounts have been increased slightly and p-type change over timing has been narrowed down a bit more.

KEY MESSAGES

- 1) Increased risk for a rain/snow mix with minor accumulations possible across far inland areas Sunday morning as a coastal low develops over the Atlantic. - 2) Cold temperatures in the low to mid 20s (upper 20s to lower 30s along the coast) possible Monday, Tuesday, and Wednesday morning.

DISCUSSION

KEY MESSAGE 1: Increased risk for a rain/snow mix with minor accumulations possible across far inland areas Sunday morning as a coastal low develops over the Atlantic.

Showers will slowly increase in coverage overnight and especially Sunday morning as a cold front pushes offshore with the approach of a large upper trough that is forecast is dig across the eastern CONUS on Sunday. The core of the strongest deep-layered forcing owing to increased DPVA ahead of a secondary shortwave that will round the base of the longer wave trough and ageostrophic contributions from the right entrance region of a 170 kt jet streak embedded in the polar jet is forecast to traverse the area from mid-morning through mid-afternoon helping to induce cyclogenesis offshore of the Southeast U.S. coast (Miller-A pattern). Expect widespread rains to impact Southeast South Carolina and Southeast Georgia during the morning into the early afternoon hours with precipitation tapering off from west-east as colder and much drier air filters into the region. Some much needed rainfall will occur with most areas averaging near 0.50". While these amounts are not overly heavy, any rainfall will help with the ongoing drought situation.

The remains a concern for p-type issues across far interior areas Sunday morning as the column cools with strengthening post-frontal cold air advection behind the cold front that will push offshore prior to daybreak. As is typically the case in winter Miller-A cyclogenesis patterns, the p-type forecast is highly complex and sensitive to timing as the cold air will be chasing the exiting rain shield. 18/12-18z model soundings still suggest interior Southeast Georgia favors the best juxtaposition of lingering precipitation with thermal profiles supportive of snow with soundings at KAQX, K2J5, KMHP and KRVJ all showing cloud temperatures of at least -20C (sufficient for 100% ice nucleation) with sub-freezing profiles all the way down to just above the boundary layer. Farther to the east, guidance suggest conditions will become increasingly less favorable for cloud ice nucleation as the column begins to dry from the top-down. While a brief rain/snow mix may occur as far east as the I-95 corridor, the primary p-type east of the Reidsville-Allendale corridor should be all rain.

Scenarios: Similar to yesterday, the most likely scenario is for rain expanding across the area overnight, then transitioning to a rain/snow mix in the Reidsville-Allendale corridor by mid- morning, possibly changing to all snow before ending during the early afternoon hours. There is about a 2-4 hour period where some minor accumulations could occur and snow could become the dominant p-type. Warm ground temperatures and light-moderate precip rates should limit any brief accumulations to grassy areas, elevated surfaces and possibly some bridges, but significant impacts are unlikely at this time. There are still signals in the morning's guidance that suggest some convective enhancement could occur where bands of strong 850-500 hPa frontogenesis align with pockets of negative EPV, thus locally augmenting both thermal and UVV profiles. This could yield corridors of localized, heavier snowfall rates even with surface temperatures above freezing. There are signals of this enhancement occurring in both the H3R and RAP simulated reflectivity products which highlight the formation of several convective bands as the sharp tail end of the negatively-tiled shortwave rounding the base of the mean upper trough pivots through. Should this enhancement be realized, the resulting heavier snowfall rates could support some localized amounts as high as 0.5-1" before rapid melting occurs. This should be treated as a reasonable worse case scenario for planning purposes.

Snow Amounts/Impacts/Winter Headlines: There is low confidence for snow amounts given the uncertainties noted above. Snow amounts of 0.1-0.4" where introduced for parts of interior Southeast Georgia up into Allendale County, SC, given the latest deterministic, ensemble and statistical guidance trends. These accumulations should be largely limited to grassy areas and possibly some elevated bridges/overpasses if freezing surface temperatures can be realized. While these amounts are just below Winter Weather Advisory criteria, a few slick spots can not be ruled out so people are encouraged to use extra caution while traveling Sunday morning. It should be noted that the early morning run of the Probabilistic Winter Storm Severity Index (WSSI-P) showed a 30-40% chance for minor impacts and a 5-10% for moderate impacts, mostly for driving conditions. These probabilities may increase later this afternoon with the arrival of the afternoon WFO forecast. The need for a Winter Weather Advisory will be reassessed this evening and again overnight when shorter term thermal profile trends can be more readily identified.

KEY MESSAGE 2: Cold temperatures in the low to mid 20s (upper 20s to lower 30s along the coast) possible Monday, Tuesday, and Wednesday morning.

On the backside of the departing surface low and exiting shortwave aloft, cooler temperatures filter down into the region Sunday night into Monday. Weak cold air advection (CAA) from a surface high pressure moving into the Gulf of America will aid in temperatures falling down into the lower to mid 20s, and into the upper 20s to lower 30s along the coast. Wind speeds are a bit uncertain given the weaker CAA, but there appears to be two ways we may end up needing a cold weather advisory. The first being winds remain fairly steady (though still on the weaker side) with temperatures in the lower to mid 20s, resulting in wind chills in the mid teens to lower 20s, which is currently the favored scenario. The second scenario would be winds further weakening, which would then allow for increased radiational cooling potential bringing temperatures down into the upper teens to lower 20s.

For both scenario's, inland southeast Georgia and inland southeast South Carolina have the highest potential of either temperatures or wind chills at or below 20 degrees F, in the 40-60% range. While temperatures won't be as cold as you head towards the coast, temperatures in the mid/upper 20s combined with increased wind speeds may also result in wind chills at or below 20 degrees F, with probabilities 20-30%. A Cold Weather Advisory may be needed.

Ahead of our next cold front, highs on Monday are only expected to reach into the upper 40s to lower 50s. The front pushes through during the late evening hours, with a surface high pressure moving into the southeast region from the central CONUS. This will bring another round of overnight lows in the mid 20s inland to lower 30s along the coast, with wind chills in the upper teens to upper 20s. Probabilities for wind chills at or below 20 degrees F are highest for areas west of the I-95 corridor, ranging from 30 to 50%, which may prompt another Cold Weather Advisory.

Highs on Tuesday are again expected to be in the mid 40s to lower 50s. Our final cold overnight period from this event will occur as the aforementioned surface high pressure slides by just to the north of the local area. Similar to the Monday morning set up, we could see Cold Weather Advisory criteria (20 degrees F or lower) across inland areas from either temperatures alone if there is no wind (40- 70% chance), or from wind chills if winds are a touch stronger (30- 40% chance).

AVIATION /16Z SATURDAY THROUGH WEDNESDAY/

17/18z TAF Discussion: KCHS/KJZI/KSAV: VFR conditions this afternoon/evening will give way to lower cigs and vsbys Sunday morning as widespread light to moderate rains spread into the area. The risk for IFR conditions is high with cigs expected to drop into IFR thresholds shortly after daybreak at all terminals. There is moderate confidence that cigs at the terminals will drop below alternate minimum thresholds.

Extended Aviation Outlook: Rain/cig impacts will diminish Sunday afternoon. There are no other concerns at this time.

MARINE

Through Sunday: Southwest winds remain on the breezier side out ahead of an approaching cold front today, though winds and waves remain well below Small Craft criteria. Winds decrease overnight, but will again rise Sunday morning behind the cold frontal passage which swings the winds around to become out of the northeast, with scattered to areas of showers continuing throughout the day. Wind gusts into the lower to mid 20s will overspread the waters throughout the morning hours, strongest across our near and offshore Georgia waters from Savannah down to the Altamaha Sound. 6 foot waves look to remain confined close to the 60NM boundary, decreasing to 3 foot waves along the coast. A Small Craft Advisory has been issued for the waters from Savannah to Altamaha Sound out to 60NM. Rain exits the area Sunday afternoon/evening, with winds subsiding into the overnight hours.

Monday to Wednesday: No high confidence marine concerns in the extended. Northwesterly winds become breezy on Tuesday, with gusts up to 20 kts throughout the afternoon hours. Wave heights begin to trend upwards on Wednesday as swell moves in from the Atlantic.

CLIMATE

Snowfall Records for January 18: KCHS: 0.4/1977 KSAV: 1.0/1893

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from 7 AM to 4 PM EST Sunday for AMZ354. Small Craft Advisory from 7 AM Sunday to 1 AM EST Monday for AMZ374.