textproduct: Charleston

WHAT HAS CHANGED

The aviation section has been updated for the 06Z TAF issuance. Confidence remains low in regard to a wintry mix west of I-95 Sunday while confidence is high for cold morning lows Monday, Tuesday and Wednesday morning.

KEY MESSAGES

- 1) Potential for a rain/snow mix Saturday night into Sunday as a coastal low develops over the Atlantic.

- 2) Cold temperatures in the low to mid 20s (upper 20s along the coast) possible Monday, Tuesday, and Wednesday morning.

DISCUSSION

KEY MESSAGE 1: Potential for a rain/snow mix Saturday night into Sunday as a coastal low develops over the Atlantic.

A high-amplitude trough is expected to dig across the Midwest and Deep South, then pivot across the East CONUS, helping force a strong cold front offshore Saturday evening, which then becomes the primary focal point of cyclogenesis off the Southeast and Mid-Atlantic Coast Sunday morning. Strong mid-upper lvl forcing associated with h5 vort energy and favorable divergence in the right-rear quadrant of an h25 jet streak along with ample low-mid lvl moisture advecting across the local area within a southwest flow will set the stage for precipitation across the region Saturday night into Sunday, before diminishing and/or shifting offshore Sunday afternoon. The main issue will continue to focus on a rain-snow mix possibility locally as strong cold air advection commences post FROPA Sunday morning, supporting below freezing temperatures off the sfc with thermal profiles increasingly favorable for rain mixing with and/or possibly changing to all snow as precipitation exits the area.

The main question continues to focus on the low-lvl thermal profile and ability to maintain snow prior to reaching the sfc and accumulation once setting on warmer grounds. Latest model soundings indicate near sfc temps are likely to remain above the freezing mark under widespread cloud cover through the entire event, even across far inland areas where snowfall accumulation is possible. Profiles suggest it will take ample moisture in the dendritic growth zone for the snow to accumulate across far inland zones well west of the I-95 corridor, mainly along a stretch from Allendale County, SC to Tattnall County, GA Sunday morning. However, the best combination of moisture and forcing appears limited to hours around daybreak Sunday into late morning Sunday, as soundings indicate moisture to become quickly removed from the -10 to -20C layer during daylight hours. Widespread clouds will attempt to keep sfc temps cooler through late morning hours, but a rain/snow mix event still appears limited given the lack of sub-freezing sfc temps and departing low-mid lvl moisture as daytime warming begins. By around noon, the chances for a rain/snow mix with any accumulation is likely coming to an end.

Although ground temperatures are likely to remain rather warm, localized, heavier snowfall rates could support a quick dusting to possibly as high as 0.5" before a quick melting occurs across the inland area noted above. NBM snowfall probabilities for 1" continue to run as high as 15-25% across the noted area, but this scenario looks unlikely given overall snowfall probabilities and appears to be a worst-case scenario for planning purposes. The rain/snow mix could reach as far east as the I-95 corridor, but an all rain event is favored along coastal areas.

As for impacts, winter weather impacts from this system still remain on the low side. The latest probabilistic Winter Storm Severity Index (WSSI-P) advertises a 15-30% chance of minor impacts, so exercise caution if travel is necessary as a few slick spots could occur, especially on bridges or elevated surfaces. A low-end risk for some patchy black ice could linger into Sunday night, but lingering winds and lowering dewpts should help dry roads out prior to the onset of freezing temps Sunday night.

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

Secondary upper level shortwave digs across the Ohio River Valley into Monday, dragging an attendant cold front toward the region. Strong high pressure over the Northern Plains then builds across the area in its wake, resulting in an influx of CAA. This will result in notably colder temperatures Monday, Tuesday, and Wednesday morning. Currently have morning lows in the low to mid 20s, with areas along the coast near/just below freezing. Despite winds remaining on the lighter side, will see wind chills plummet into teens to lower 20s across some of our interior counties Monday morning, which may prompt the need for a Cold Weather Advisory in the coming shifts. Similar conditions are expected Tuesday, with confidence in how widespread these values will be decreasing as we head into Wednesday. Otherwise, expect afternoon highs to remain seasonally cool, as temperatures only rise into the upper 40s to 50s.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

VFR conditions will prevail at CHS/SAV/JZI terminals through 06Z Sunday.

Extended Aviation Forecast: A period of flight restrictions in showers is likely Sunday morning at all three terminals.

MARINE

There are no concerns tonight through today, but winds are expected to increase Sunday in wake of a cold front shifting offshore and low pressure developing off the Southeast/Mid- Atlantic Coast. Winds gusts could approach 20-25 kt, highest across outer Georgia waters late Sunday morning into the afternoon. Additionally, seas will also build away from the coast, largest across outer Georgia waters (4-6ft) Sunday. A Small Craft Advisory could eventually be needed late Sunday morning and afternoon.

Monday Onward: High pressure gradually builds across our area heading into the new week, allowing quiet conditions to prevail across our waters. Northwesterly winds may become a tad breezy on Tuesday, with gusts up to 20 kts possible through the afternoon.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...None.