textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Confidence has increased for heavy snowfall and potential for blizzard conditions to impact the region.

Blizzard Warnings and Winter Storm Warnings issued for portions of the area, in effect Sunday through Monday. A Winter Storm Watch remains in effect for portions of eastern PA and northeastern MD.

The Gale Watch was upgraded to a Storm Watch for the Atlantic coastal waters and lower Delaware Bay.

KEY MESSAGES

1. A major winter storm Sunday through Monday is expected to bring heavy snowfall and potential for blizzard conditions to portions of the area.

2. Widespread coastal flooding is expected with Sunday night's high tide cycle. Moderate coastal flooding likely for much of the coast with near major flooding possible. Further tidal flooding may linger into Monday as well.

3. Remaining unsettled into the second half of next week.

DISCUSSION

KEY MESSAGE 1...A major winter storm Sunday through Monday is expected to bring heavy snowfall and potential for blizzard conditions to portions of the area.

A potent H5 trough centered over the Midwest today will shift eastward towards the region tonight into early Sunday morning. At the surface, low pressure will begin to develop over the southeastern US in the exit region of the trough. From there, the low is expected to deepen rapidly as it tracks northeastward off the Mid- Atlantic Coast through Sunday afternoon and into Sunday night.

Some discrepancies remain amongst model guidance, with the GFS and NAM maintaining a track closest to the coast, and most significant snowfall accumulations across the entire region. The ECM remains farther offshore and a bit weaker with the system overall, but there remains a northwestward shift over the past few model runs. Other deterministic guidance generally falls into one of these two camps, but both outcomes would result in significant accumulating snow across at least some portions of the area. Ensemble suites continue to feature a consensus in a low track that will be close enough to the coast to bring potentially significant impacts from snow, winds, and coastal flooding. In addition to the surface cyclone off the coast, an inverted trough is depicted in most guidance, though there is some variance in its placement and intensity. However, these features tend to increase snowfall amounts and rates where they set up.

The current expectation is for light precipitation to begin overspreading the region from southwest to northeast some time late tonight into early Sunday morning. As the low begins to approach from the southwest Sunday morning and into the afternoon, precipitation rates should begin to increase. Temperatures during the daytime hours on Sunday are expected to be fairly mild, so precipitation could end up being be rain or a rain and snow mix for areas near and south of the I-78 corridor through midday before precip becomes heavier and a change to snow occurs. The rain/snow mix line should move south toward the coast and Delmarva fairly quickly Sunday afternoon and evening, as colder air will begin to infiltrate the area in concert with the heaviest precipitation rates. Therefore, precipitation is likely to change to all snow, and be heavy at times. Snowfall rates could exceed 2 inches per hour where the heaviest accumulations are forecast. The heaviest snowfall is anticipated to occur mainly Sunday evening through Sunday night, then begin to taper off into the daytime hours Monday. Snow will likely come to an end from west to east sometime Monday afternoon.

Total forecast snow accumulations have increased substantially across the board. Generally forecasting a widespread 8-18", with the greatest amounts focused across New Jersey and adjacent areas of southeastern Pennsylvania (including the Philadelphia metro area). There will likely be a sharp cutoff in the highest amounts toward our western and southern most zones across eastern PA and Delmarva. This is where some guidance still indicates snowfall amounts could fall short of warning criteria. There could certainly be locally higher amounts than currently advertised where ever more persistent banding of snow occurs, which could push totals over 18".

In addition to the heavy snowfall, as the low deepens Sunday night into Monday, winds will begin to ramp up, with gusts 30-40 mph for inland areas, and up to 55 mph possible along the coast. The snow will be heavy, wet, and sticky until later in the event. However, blowing and drifting of snow is certainly a concern, especially closer toward the coast where winds will be strongest.

Given the increasing potential for significant snowfall amounts, we have upgraded the Winter Storm Watch to warnings, mainly for NJ, DE, and SE PA areas where near a foot or more of snow is currently forecast. Along the Atlantic coast, Blizzard Warnings were issued to highlight to potential for significant blowing and drifting of snow, whiteout conditions, and strong winds during the storm. True, verifiable blizzard conditions are historically difficult to achieve inland, but we'll continue to monitor the forecast trends.

For our far western counties in eastern PA and northeastern MD, Winter Storm Watches remain in effect. Forecast totals are lower in these areas, and confidence in reaching warning criteria is lower for those areas at the moment.

Stay tuned for updates as changes to the snowfall forecast occur today through tonight. This storm could cause major impacts to our region, and exact details still need some refinement.

KEY MESSAGE 2...Widespread coastal flooding is expected with Sunday night's high tide cycle. Moderate coastal flooding likely for much of the coast with near major flooding possible. Further tidal flooding may linger into Monday as well.

Strong onshore flow ramps up due to deepening low pressure off the Mid-Atlantic coast on Sunday. This will have direct impacts on tidal levels which are expected to rapidly rise, especially for Sunday night's high tide cycle. The onshore flow which will be quite strong will pile water toward the coast. Fortunately, we will be in between the full/new moon phase cycle which will lead to lower astronomical tide contributions. One caveat and most difficult part of the forecast at this time is what the exact wind direction will be at the time of high tide on Sunday night. A direct northeast flow would pile water up along the coastlines and back bays whereas if the winds settle more northerly, then lesser impacts may be felt to tidal communities except where shorelines are northerly facing. At this point, there is enough confidence that most of the ocean-front communities and back bays of New Jersey and Delaware should rise to moderate levels as even if winds end up more northerly and shore parallel, those winds would prevent water from draining out of the bays and away from the coast at low tide Sunday afternoon/evening, resulting in tidal piling when the high tide comes in Sunday night. Potential remains for some coastal areas to get close to major if the right ingredients all come together, though there is still some uncertainty as to where along the coast this would most likely occur.

With some uncertainty remaining with the track of the low and the wind direction, continuing with the Coastal Flood Watch at this time for Middlesex, Monmouth, Ocean, southeastern Burlington, Ocean, Atlantic and Cape May counties in New Jersey; and Kent and Sussex Counties in Delaware where confidence in moderate level impacts is greatest. Some minor tidal flooding may linger into Monday as well before winds sway offshore.

For the remainder of the Delaware Bay coasts, confidence is increasing in the potential for widespread minor flooding with the Sunday night high tide cycle. A Coastal Flood Advisory will likely be needed for some or all of this area as a result. Elsewhere, in the tidal Delaware River and tidal Maryland Eastern Shore, no tidal flooding is expected at this time.

KEY MESSAGE 3...Remaining unsettled into the second half of next week.

Beyond our winter storm Sunday into Monday, we stay unsettled for the second half of next week.

A brief area of high pressure will build in from the southwest Monday night into Tuesday leading to drier conditions. The pressure gradient will remain tightened Monday night but begin to lessen Tuesday. Winds may gust upwards of 20-30 mph with localized gusts to 35-40 mph in the Poconos. These winds will lessen through the day Tuesday with gusts by the evening around 10-20 mph.

Our next system arrives Tuesday night into Wednesday morning. There is a low pressure system near the Great Lakes region that drags a warm front northward and warm air advection begins. This is helped by our high pressure system moving offshore and sinking southeast during this timeframe which results in a south-southwest wind that will usher in warmer air. Ahead of the warm front, there is the chance for some light snow (mainly 20-30%). As the surface low near the Great Lakes region moves northeast into Canada, a cold front passes through Thursday leading to mainly rain. Depending on timing, there could be some light wintry precipitation in the higher elevations of our northern areas.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas.

Today...VFR. W-NW winds around 10 kt with occasional gusts to 20 kt in the afternoon. High confidence.

Tonight...VFR to start then MVFR conditions likely developing after 08Z with rain and snow moving into the region. Conditions may approach IFR in spots closer to 10-12Z. Winds around 5 knots or less. Moderate confidence overall, low confidence on timing and extent of sub-VFR conditions.

Outlook...

Sunday and Monday...Restrictions expected in periods of heavy precipitation. Wind gusts 30-40 kt are possible, especially Sunday night through Monday morning. Blowing snow possible Monday. Improving conditions Monday night, but gusty NW winds will likely persist.

Tuesday...VFR. Gusty northwest winds possible during the day.

Wednesday...Sub-VFR possible with light rain/snow and low clouds.

MARINE

Sub-SCA conditions continuing through tonight. Winds generally around 10 knots with some gusts up to 15-20 knots, and seas continuing to decrease throughout the period.

Regarding the river/marine ice, Delaware Bay continues to be mainly ice free outside of shallow near-shore areas. Ice coverage remains around 10-30% from the the Delaware Memorial Bridge up to the Commodore Barry Bridge. North of that up through Philadelphia, ice coverage is around 30-70%.

Ice coverage will continue to decline with warmer temperatures through Saturday.

Outlook...

Sunday through Monday...Strong coastal storm will develop across the coastal waters. East winds 15-20 kt with gusts to 25 kt early Sunday will increase to 30-45 kt with gusts up to 55 kt possible by Sunday evening. Winds will shift to northerly overnight Sunday night and eventually northwestward on Monday, before beginning to decrease by Monday afternoon and evening. Seas will increase to 5-9 feet Sunday afternoon and eventually build to 10-17 feet Sunday night into Monday morning before beginning to decrease. The Gale Watch was upgraded to a Storm Watch for all Atlantic coastal waters and lower Delaware Bay. The Gale Watch remains in effect for upper Delware Bay. Heavy snow could lead to near zero visibility at times Sunday night through Monday morning.

Tuesday...Small Craft Advisory conditions expected with winds around 20-25 knots and seas 5-7 feet.

Tuesday night...No marine hazards expected.

Wednesday...Small Craft Advisory conditions expected with winds around 20-25 knots and seas 5-7 feet.

CLIMATE

The daily snowfall records for February 22 and 23 from our climate sites listed below:

February 22 Site Record/Year Allentown (ABE) 8.6"/1948 AC Airport (ACY) 8.0"/2001 Philadelphia (PHL) 7.0"/2001 Reading (RDG) 6.5"/1948 Trenton (TTN) 6.3"/2001 Wilmington (ILG) 5.0"/1935

February 23 Site Record/Year Allentown (ABE) 6.5"/1987 AC Airport (ACY) 6.5"/1987 Philadelphia (PHL) 6.5"/1987 Reading (RDG) 6.5"/1994 Trenton (TTN) 11.4"/1987 Wilmington (ILG) 12.4"/1987

PHI WATCHES/WARNINGS/ADVISORIES

PA...Winter Storm Watch from late tonight through Monday afternoon for PAZ054-055-060>062. Winter Storm Warning from 7 AM Sunday to 6 PM EST Monday for PAZ070-071-101>106. NJ...Winter Storm Warning from 10 AM Sunday to 6 PM EST Monday for NJZ016>019-021. Coastal Flood Watch from Sunday evening through late Sunday night for NJZ012>014-020-022>027. Blizzard Warning from 1 PM Sunday to 6 PM EST Monday for NJZ022>025. Winter Storm Warning from 4 AM Sunday to 6 PM EST Monday for NJZ001-007>010-012-015. Blizzard Warning from 10 AM Sunday to 6 PM EST Monday for NJZ013-014-020-026-027. DE...Winter Storm Warning from 10 AM Sunday to 6 PM EST Monday for DEZ001-002. Coastal Flood Watch from Sunday evening through late Sunday night for DEZ002>004. Blizzard Warning from 1 PM Sunday to 6 PM EST Monday for DEZ003-004. MD...Winter Storm Watch from Sunday afternoon through Monday afternoon for MDZ012-015-019-020. MARINE...Gale Watch from Sunday afternoon through Monday afternoon for ANZ430. Storm Watch from Sunday afternoon through Monday afternoon for ANZ431-450>455.