textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Potentially historic winter storm to impact the region through Monday. Confidence is high for very heavy, crippling snowfall to impact the area. Areas of blizzard conditions likely.

Blizzard Warnings were expanded a few more counties inland, and now include Philadelphia, adjacent areas of SE PA, all of NJ, all of DE, Queen Annes County MD, and the southern Poconos. Winter Storm Warnings remain in effect for the remainder of the area.

The Storm Warning and Gale Warning start times were moved up earlier today to encompass deteriorating conditions, especially into the afternoon.

Coastal Flood Warnings have been issued for the NJ and southern DE coastal areas. A Coastal Flood Advisory has been issued for portions of the Delaware Bay coast in southern New Jersey.

KEY MESSAGES

1. A major, potentially historic winter storm is expected to bring very heavy, crippling snowfall and areas of blizzard conditions to portions of the area through Monday.

2. Widespread coastal flooding is expected with tonight'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 will likely linger into Monday as well.

3. Remaining unsettled through mid-to-late week.

DISCUSSION

KEY MESSAGE 1...A major, potentially historic winter storm is expected to bring very heavy, crippling snowfall and areas of blizzard conditions to portions of the area through Monday.

A potent H5 trough centered will continue to shift eastward towards the region this morning. At the surface, cyclogenesis is ongoing across the Carolinas early this morning 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 this afternoon and into tonight.

While minor discrepancies remain amongst model guidance, there remains good consensus 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 had trended a bit stronger and farther west, and also features warning level snow across the entire region. Most other deterministic guidance remains in general consensus with this evolution. 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. There is slightly lower confidence on exact snowfall amounts on our western fringe, but regardless, warning level snow is still expected everywhere.

Light precipitation is beginning to overspread the region from southwest to northeast early this morning. As the low begins to approach from the southwest into the afternoon, precipitation rates should begin to increase. Temperatures during the daytime hours will be mild, so precipitation will probably 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 this 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, which could end up being most of our area. The heaviest snowfall and strongest winds are anticipated to occur between this evening through tonight, then begin to taper off into Monday morning. Snow will likely come to an end from west to east sometime Monday afternoon.

Total forecast snow accumulations have increased a few inches across the board. For most of eastern PA and Delmarva, widespread amounts of 1 to 2 feet are expected. The heaviest snowfall should remain focused on New Jersey, with the greatest potential for 24"+ around Monmouth and Ocean Counties where coastal convergence is maximized and heaviest snowfall rates should occur. Farther inland, there will likely be some topographic effects on snowfall, with locally higher amounts possible on windward (north) sides of mountains and ridges.

In addition to the heavy snowfall, as the low deepens tonight into Monday, winds will begin to ramp up. Northeast winds sustained 20-35 mph will gradually shift north then northwest, with gusts 35-45 mph for inland areas, and up to 60 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 continuing towards a stronger 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, windy conditions are expected areawide. Confidence in frequent gusts above 35 mph has increased farther inland. This has prompted an upgrade to Blizzard Warning for Philadelphia, adjacent areas of SE PA, all of NJ, all of DE, Queen Annes County MD, and the southern Poconos. 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 life-threatening or impossible travel conditions, as well as the potential for scattered power outages.

Some additional refinements may occur to the snowfall forecast today, but regardless of exact amounts, confidence remains very high in major to extreme impacts to our region.

KEY MESSAGE 2...Widespread coastal flooding is expected with tonight'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 will likely linger into Monday as well.

Strong onshore flow ramps up due to bombing out low pressure off the Mid-Atlantic coast today into Monday. This will have direct impacts on tidal levels which are expected to rapidly rise, especially for tonight'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.

Confidence remains high 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 this evening, resulting in tidal piling when the high tide comes in tonight. 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.

Coastal Flood Warnings remain in place for tonight's 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 will occur. Confidence has increased in widespread minor tidal flooding occurring in Cumberland County in NJ as well so a Coastal Flood Advisory has been issued for this area as well. For the remainder of the Upper Delaware Bay coasts, confidence in reaching minor thresholds remains lower at this time, so have held off on any advisories for now.

While winds will shift offshore as we head into daytime Monday, the deepening offshore low will continue to keep ocean swells onshore through the day Monday. As a result, it is becoming increasingly likely that another round of widespread minor coastal flooding will occur with the Monday afternoon high tide within the area currently in a Coastal Flood Warning. Some of these areas will potentially approach moderate levels, particularly along the backbays in Ocean County, New Jersey. By Monday night, swells will be turning offshore, though another round of tidal flooding with the Monday night high tide remains possible.

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

KEY MESSAGE 3...Remaining unsettled through mid-to-late week.

Beyond our winter storm Sunday into Monday, we stay unsettled through mid-to-late 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 20-40% with more of a rain/snow mix. North and west of I-95 has the better potential for precipitation with 40-60% PoPs that starts as snow later Tuesday night and turns to a rain/snow mix into Wednesday morning.

Moving into later in the week, a surface low across the Ohio River Valley shifts eastward towards the Northeast, 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. We will pay close attention to the amount of precipitation that will fall from this warmer late week system. Heavier rainfall could lead to some decent melting of the expected deep snow pack from our Sunday-Monday snow storm. At this time, flooding concerns remain low, but our vigilance remains high.

A high pressure system then approaches for Friday into Saturday leading to drier conditions.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

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

Today...IFR conditions developing in RASN between 13 and 15Z, changing to SN and lowering to LIFR from west to east from 19Z to 22Z. 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.

Tonight through Monday...IFR. Periods of heavy snowfall through Monday morning resulting in VSBYs of 1/4SM or less at times. Northerly winds of 20-25 knots with gusts of 30-40 kts. Conditions may begin to improve by Monday afternoon as precipitation departs but strong winds will continue to result in areas of blowing snow. Moderate confidence overall, low confidence in timing of any improvements in conditions.

Outlook...

Monday Night...VFR. Gusty WNW of 20-25 knots likely continue.

Tuesday...VFR. Gusty northwest winds possible during the day. Sub- VFR conditions possible at night.

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

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

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

MARINE

A powerful coastal storm will develop across the coastal waters through tonight. East winds 15-20 kts with gusts to 25 kts developing by late this morning will increase and shift northeast then north to 35-50 kts with gusts up to 60 kts tonight. Winds will shift eventually northwestward on Monday, before beginning to decrease by Monday afternoon and evening. Seas will increase to 5-9 feet this afternoon and eventually build to 12-18 feet tonight into Monday morning before beginning to decrease.

The Storm Warning and Gale Warning start times were moved up earlier today to encompass deteriorating conditions, especially into the afternoon. The highest winds and seas will occur overnight tonight, with gale force winds likely lingering into Monday afternoon. Heavy snow could lead to near zero visibility at times Sunday night through Monday morning.

Small Craft Advisory conditions Monday night, with northwest wind gusts up to 30 kts.

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%.

Outlook...

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 Friday...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...Blizzard Warning from 10 AM this morning to 6 PM EST Monday for PAZ070-071-104>106. Blizzard Warning until 6 PM EST Monday for PAZ054-055. Winter Storm Warning until 6 PM EST Monday for PAZ060>062- 101>103. NJ...Blizzard Warning from 10 AM this morning to 6 PM EST Monday for NJZ013-014-016>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. Blizzard Warning until 6 PM EST Monday for NJZ001-007>010-012- 015. Coastal Flood Advisory from 9 PM this evening to 5 AM EST Monday for NJZ021. DE...Blizzard Warning from 10 AM this morning to 6 PM EST Monday for DEZ001-002. 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 MDZ015-019-020. Winter Storm Warning from 10 AM this morning to 6 PM EST Monday for MDZ012. MARINE...Gale Warning from 1 PM this afternoon to 6 PM EST Monday for ANZ430. Storm Warning from 10 AM this morning to 6 PM EST Monday for ANZ431-450>455.