textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

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

Blizzard Warnings have been expanded across portions of the Maryland Eastern Shore, Delaware, and much of New Jersey. Winter Storm Warnings are in effect for the remainder of the area.

The Storm Watch for the Atlantic coastal waters and lower Delaware Bay has been upgraded to a Storm Warning, and the Gale Watch for the upper Delaware Bay has been upgraded to a Gale Warning.

Coastal Flood Warnings have been issued for the NJ and southern DE coastal areas.

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 is likely for many ocean-front and back bay communities along the Atlantic coast with 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, subtle pressure falls are being noted over a broad portion of the southeastern US, marking the beginning of surface cyclogenesis in the exit region of the trough. As the low begins to track northeastward towards our region, it is expected to deepen rapidly, particularly as it tracks off the Mid- Atlantic Coast through Sunday afternoon and into Sunday night.

While modest discrepancies remain amongst model guidance, there is increasing agreement today that the surface low will track close enough to the coast to bring significant impacts to most, if not all, of the area. The GFS and NAM remain the most aggressive, and feature a deep cyclone in the neighborhood of 970 mb tracking close to the coast, leading to significant snowfall accumulations across the entire region. The ECM remains a bit farther offshore, and features a more easterly trajectory of the low, but it too features warning level snow across the entire region. Most other deterministic guidance has trended north as well. Ensemble suites continue to feature a consensus in a low track that will be close enough to the coast to bring 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, extending northwestward across portions of eastern Pennsylvania. These features tend to increase snowfall amounts and rates where they set up, and this increases confidence in significant snowfall occurring across the entire area, even though the highest amounts are expected to be near the NJ and DE coasts.

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 continued to increase this afternoon. For most of eastern PA, northern NJ, and the Delmarva, widespread amounts of 10 to 18 inches are expected. East of the I-95 corridor across much of central and southern NJ, amounts exceeding 18" are expected, and isolated totals as high as 24" are possible. In addition to the heavy snowfall, as the low deepens Sunday night into Monday, winds will begin to ramp up, with gusts 35-45 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 model trends as of this afternoon towards a solution featuring a low track closer to the coast, and potentially deepening to around 970 mb, a very tight pressure gradient will set up over much of the area. As noted above, breezy conditions are expected areawide. However, confidence in frequent gusts above 35 mph has increased farther inland. This has prompted an upgrade to Blizzard Warning across southern portions of the Maryland Eastern Shore, central portions of Delaware, and much of New Jersey. It is possible that additional expansions could become necessary if expected winds increase. It is important to note that regardless of whether blizzard criteria is met for any given location, the combination of strong winds and heavy snow will lead to extremely dangerous or impossible travel conditions, as well as the potential for scattered power outages.

While additional refinements to the snowfall forecast will likely be needed tonight, it should be noted that confidence is now very high in major impacts to our region.

KEY MESSAGE 2...Widespread coastal flooding is expected with Sunday night's high tide cycle. Moderate coastal flooding is likely for many ocean-front and back bay communities along the Atlantic coast with major flooding possible. Further tidal flooding may linger into Monday as well.

Strong onshore flow ramps up due to bombing out low pressure off the Mid-Atlantic coast on Sunday into Monday. 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 coinciding with the time of high tide. Fortunately, we will be in between the full/new moon phase cycle which will lead to lower astronomical tide contributions.

At this point in time, there is rather high confidence that most of the ocean-front communities and back bays of New Jersey and Delaware will rise well into Moderate flood stage due to the strong background wind field and proximity of the deepening low off the coast. Even if winds do end up being more northerly than northeasterly, these 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 areas to reach Major flood stage if the right ingredients all come together. The most likely locations of possibly Major flood stage will be from Ocean County NJ southward to Sussex County DE, especially in the back bays where water piling will be enhanced.

As a result, have opted to upgrade the Coastal Flood Watches to Coastal Flood Warnings for the Sunday night high tide cycle for Middlesex, Monmouth, Ocean, Southeastern Burlington, Atlantic, and Cape May Counties in NJ, as well as Kent and Sussex Counties in DE where moderate to potentially major tidal flooding may occur. Minor tidal flooding is also possible Cumberland County in NJ and for the remainder of the Upper Delaware Bay coasts as well, but confidence in reaching minor thresholds is not great enough at this time, so have held off on any advisories for now.

For the tidal Delaware River and tidal Maryland Eastern Shore, no tidal flooding is expected.

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 35-45 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 and rain. Areas south and east of I-95 have PoPs of 20-30% with more of a rain/snow mix. North and west of I-95 has the better potential for precipitation with 30-50% PoPs that starts as snow later Tuesday night and turns to a rain/snow mix into Wednesday morning. As the surface low near the Great Lakes region moves eastward towards the Northeast, there will be a secondary low that approaches the region leading to more widespread precipitation that looks to be generally in the form of rain. Uncertainty remains with this low pressure system though, and depending on timing, there could be some wintry precipitation potential for areas near and north of I-78 on Thursday night. A high pressure system then approaches for Friday into Saturday leading to drier conditions.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

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

Tonight...VFR initially, then MVFR CIGs and VSBYs in RA after 09Z. Potential for IFR conds prior to 12Z. LGT/VRB winds, becoming E 5 to 10 kt. Moderate confidence overall, low confidence on timing and extent of sub- VFR conditions.

Sunday...IFR in RASN, changing to SN and lowering to LIFR from west to east from 18Z to 21Z. E to NE winds increasing from 5 to 10 kt in the morning to 15 to 20 kt with 25 to 35 kt gusts after 21Z. Moderate confidence overall, low confidence on the timing that precip becomes all SN.

Outlook...

Sunday night 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.

Thursday...Sub-VFR possible due to light rain and low clouds.

MARINE

Sub-SCA conditions continuing through tonight. Winds generally around 10 knots and seas of 2-3 feet.

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...A strong coastal storm will develop across the coastal waters Sunday into Sunday night. 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 12-18 feet Sunday night into Monday morning before beginning to decrease. The Storm Watch for all Atlantic coastal waters and lower Delaware Bay has been upgraded to a Storm Warning for Sunday afternoon into Monday afternoon. The Gale Watch for the upper Delaware Bay has been upgraded to a Gale Warning for Sunday afternoon into Monday afternoon. Heavy snow could lead to near zero visibility at times Sunday night through Monday morning.

Tuesday...Small Craft Advisory conditions expected with winds 15-20 kt and gusts 25-30 kt and seas 5-7 feet. Conditions are expected to improve during the afternoon hours.

Tuesday night...No marine hazards expected.

Wednesday through Thursday...Small Craft Advisory conditions possible with periods of wind gusts around 25 kt and seas building to 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 Warning from 7 AM this morning to 6 PM EST Monday for PAZ054-055-060>062-070-071-101>106. NJ...Winter Storm Warning from 10 AM this morning to 6 PM EST Monday for NJZ001-007-009-016. Blizzard Warning from 10 AM this morning to 6 PM EST Monday for NJZ008-010-012>015-017>021-026-027. Coastal Flood Warning from 9 PM this evening to 5 AM EST Monday for NJZ012>014-020-022>027. Blizzard Warning from 1 PM this afternoon to 6 PM EST Monday for NJZ022>025. DE...Winter Storm Warning from 10 AM this morning to 6 PM EST Monday for DEZ001. Blizzard Warning from 10 AM this morning to 6 PM EST Monday for DEZ002. Coastal Flood Warning from 9 PM this evening to 5 AM EST Monday for DEZ002>004. Blizzard Warning from 1 PM this afternoon to 6 PM EST Monday for DEZ003-004. MD...Blizzard Warning from 10 AM this morning to 6 PM EST Monday for MDZ019-020. Winter Storm Warning from 1 PM this afternoon to 6 PM EST Monday for MDZ012-015. MARINE...Gale Warning from 4 PM this afternoon to 1 PM EST Monday for ANZ430. Storm Warning from 4 PM this afternoon to 1 PM EST Monday for ANZ431-450>455.