textproduct: Wakefield

WHAT HAS CHANGED

Confidence continues to increase with respect to the development of coastal low pressure Sunday into Monday, along with accumulating snow Sunday night into early Monday. However, confidence remains low with regard to where the greatest snow accumulations occur.

KEY MESSAGES

1) Improving conditions through tonight with a cold front dropping through the region. This front settles into the Carolinas Saturday as a wave of low pressure tracks along the boundary.

2) Strong coastal low pressure likely impacts the region Sunday into early Monday resulting in the potential for accumulating snow along with hazardous marine conditions.

DISCUSSION

As of 330 PM EST Friday... KEY MESSAGE 1...Improving conditions through tonight with a cold front dropping through the region. This front settles into the Carolinas Saturday as a wave of low pressure tracks along the boundary.

A vigorous trough lingers over the Great Lakes this afternoon, with zonal WSW flow across the Mid-Atlantic region. At the surface, a cold front is pushing across eastern VA and SE MD and this has resulted in rapidly improving conditions. Temperatures have moderated into the lower to mid 60s where low clouds have scattered. There are some lingering showers across NE NC, but this activity should move offshore by 4-5 PM. Beneficial rain of 0.5-0.75" and locally 1.0-1.5" fell across central VA and the Eastern Shore during the past 36 hours. However, only a 0.1" or less fell across far southern VA and NE NC. Drier air spreads across the region tonight with lows in the 40s. A wave of low pressure tracks along the front Saturday as it settles across the Carolinas. This will bring clouds, but only a 20-30% chc of light rain across far southern VA/NE NC. Any QPF will be light and only on the order of a few hundredths of an inch. Highs Saturday will mainly be in the 50s to around 60F.

KEY MESSAGE 2...Strong coastal low pressure likely impacts the region Sunday into early Monday resulting in the potential for accumulating snow along with hazardous marine conditions.

A strong upper trough dives SE through the Midwest Saturday night and digs into the Mid-South and Carolinas Sunday, before becoming a closed low off the Mid-Atlantic coast Sunday night. Strong surface low pressure quickly develops off the coast later Sunday afternoon into Sunday night. Confidence has increased. However, there are some differences in the deterministic models and ensemble spread with regard to how close the low develops to the coast. This casts some uncertainty with sensible weather impacts, especially where the heaviest snow band sets up Sunday night/early Monday and how much accumulating snow falls across primarily NE portions of the local area. The 20/12Z GFS/GEFS remain on the higher end of the guidance and are overall more amplified and push accumulating snow much farther W/SW into central VA. 12z EPS probs for >3" (10:1 SLR) of snow are generally 70-90% from the Northern Neck to the Eastern Shore, with the CMC GEPS 50-80%. For >6" probs, the EPS is mainly 40- 60% from the Northern Neck to the Eastern Shore, with the GEPS similar but primarily on the Eastern Shore. Of note, there was a westward shift and increase in the EPS probs. The 12z EPS 50th percentile is 4-5" the Northern Neck to the Eastern Shore, with the GEPS similar for the Northern Neck and 5-7" for the Eastern Shore. With the westward shift in the EPS, a 3-4" median shifts into the northern portion of the I-95 corridor.

All these probs assume a 10:1 SLR and accumulate snow as soon as the model changes precipitation to snow. The heaviest precipitation likely occurs Sunday evening into the early overnight hours, and this is when cold air is initially pulled into the system changing rain to snow. This will limit some accumulation at the onset with snow falling on relatively warm and wet ground. SLRs were nudged down to ~8:1 for this period, and then gradually trend upward as colder air is pulled into the storm. The latest forecast has 4-5" over SE MD, 2-3" from the Northern Neck to the VA Eastern Shore, and 0.5-2" back into central VA, and 0.5" or less elsewhere. Snow could accumulate on roads where snowfall rates and amounts are higher and accumulation will mainly be on the grass and elevated surfaces where rates/amounts are lower. With an event like this, it is possible or even likely that we see a sharp cut-off from several inches of snow to little to no snow. A Winter Storm Watch will not be issued at this time given enough uncertainty on exact details, and that the event is still beyond 48 hours. In addition rain and snow, strong gusty winds are likely, especially along the coast.

Temperatures will be below average to begin the week, with a moderating trend by midweek ahead of another cold front. There is a chance of rain with this cold front later next week.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

As of 1206 AM EST Saturday...

VFR conditions will prevail through tomorrow evening at all terminals. Winds continue out of the SW between 7-12 kts. By later this morning winds will lighten and become variable. Throughout today winds will increase between 5 to 10 kt out of the NNE under mostly clear skies. Confidence for light showers across the far SE has increased and a prob-30 for ECG has been added. There still remains some uncertainity for showers making their way up towards ORF and PHF.

Outlook: Strong low pressure develops off the coast Sunday into Monday and this has the potential to bring another period of rain, gusty winds, and flight restrictions to the region Sunday into early Monday. Precipitation could end as a rain/snow mix or all snow at SBY and RIC. Drier air and high pressure will bring a return to VFR conditions later Monday, which continues through Wednesday.

MARINE

As of 330 PM EST Friday...

Key Messages:

- Small Craft Advisories remain in effect until 7 PM for the coastal waters north of Cape Charles Light due to elevated seas to 5 feet.

- A stronger system impacts the waters Sunday into Monday. Confidence in gale conditions continues to increase, and Gale Watches have been issued for all of the coastal waters and Chesapeake Bay from Sunday evening-early Monday afternoon.

SW winds of 10-15 kt prevail over the waters this afternoon. Seas are ~4 ft, although 5 foot seas continue to linger across the northern coastal waters where SCAs remain in effect until 7 PM this evening. Sub-advisory winds are expected through Saturday night as weak high pressure briefly settles over the waters. Winds then become NE at 10-15 kt by late Sunday morning in advance of our next system.

Deteriorating marine conditions are expected later Sunday and especially Sunday night/early Monday morning as a strong low pressure system develops near the coast Sunday and moves out to sea. The low will likely deepen to 975-980mb by Monday. However, how quickly (and close to the coast) it deepens will determine exact impacts to our marine area (but degraded marine conditions are likely regardless). There are still some differences in the models and their respective ensembles, but the 12z/20 guidance has trended slightly towards a stronger system closer to the coast. The GFS/NAM remain the strongest and closest to the coast, but the ECMWF and Canadian have trended slightly upward with respect to winds. The current forecast shows winds becoming N at 20-25 kt (w/ gusts to 30 kt) by Sunday afternoon before becoming NW Sunday night and increasing to 25-35 kt with gusts to 35-45 kt (highest across the northern coastal waters). Peak winds are expected to occur between midnight and 9 AM Monday before diminishing to 20-25 kt with gusts to 30 kt Monday afternoon (with the wind direction remaining NW). Regardless of specific model details, local wind probabilities for 34+ knot winds are now 70-90% over the ocean and 50-80% over the Chesapeake Bay. If things continue to trend stronger, occasional storm force gusts are possible, especially over the coastal waters north of Cape Charles. Given all the above, have issued Gale Watches for the entire bay and coastal waters from Sunday evening-Monday afternoon. SCA winds are expected to continue through Tuesday morning before finally dropping below SCA levels during the day on Tue as high pressure returns to the area.

AKQ WATCHES/WARNINGS/ADVISORIES

MD...None. NC...None. VA...None. MARINE...Gale Watch from Sunday evening through Monday afternoon for ANZ630>632-634>638-656-658. Storm Watch from Sunday evening through Monday afternoon for ANZ650-652-654.