textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Cold Weather Advisory extended for Carbon and Monroe counties until 10 AM Monday for wind chills as low as 15 to 20 below zero.

Latest KDOX radar imagery shows the precipitation shield of the coastal system struggling to advance northward, likely being impeded by dry air. As a result, little to no snow is expected for southern Delaware and extreme southern New Jersey.

Temperatures mid to late week trended down slightly for several areas. Consequently, Tuesday is the best chance that many areas have to get above freezing.

Many models had changes in how the mid week system was depicted (most notably with the surface low being less well defined). Consequently, this is a lower confidence forecast through this period.

KEY MESSAGES

1. Cold temperatures combined with gusty winds lead to another morning of dangerously low wind chill values.

2. A coastal storm sliding to the south and east brings in gusty winds and tidal flooding impacts through Sunday.

3. Below freezing temperatures are expected to persist across parts of the area through the week.

4. An active weather pattern will persist with two chances for some snowfall mid to late week.

DISCUSSION

KEY MESSAGE 1...Cold temperatures combined with gusty winds lead to another morning of dangerously low wind chill values.

The dangerous cold continues this morning with wake-up temperatures near 0 in the Poconos and NW NJ to the low to mid teens along the coast in southern Delmarva. Combined with a gusty northwest wind, wind chills will be below zero for nearly the entire area and getting as low as 15 to 19 below zero in the Poconos and higher elevations of northern NJ. The Cold Weather Advisory remains in effect through 10 AM Sunday Morning.

The cold snap continues for Sunday with highs only getting in the low to mid 20s, with mid to upper teens in the Poconos.

Lows Sunday night will be a degree or two warmer, but still in the single digits across the Lehigh Valley, Poconos, and northwest New Jersey and low teens everywhere else. The only saving grace here is that winds will be diminishing across most of the region. As a result, wind chills will not be as severe for most of the region, hovering right around 0. The only exception to this would be Carbon and Monroe counties where wind gusts will remain elevated through the overnight, as high as 40 mph, resulting in wind chills as low as -15 below 0. Therefore, the Cold Weather Advisory was extended for Carbon and Monroe counties only until 10 AM Monday morning. For the rest of the region, given the improvement of wind chills expected Monday morning compared to the last several days and the overall marginal temperatures, opted not to extend the Cold Weather Advisories.

KEY MESSAGE 2...A coastal storm sliding to the south and east brings in gusty winds and tidal flooding impacts through Sunday.

An impactful storm that is bringing significant snow to North Carolina and southern Virginia will just brush our area as it moves out to sea to our south and east on Sunday. The main concerns with the off shore storm for our region are strong winds and coastal flooding.

For the NJ and Delaware coastline, winds will continue to ramp up through the morning and early afternoon as an area of low pressure deepens rapidly off the coast of the Outer Banks and moves ENE. The pressure gradient will be the tightest over the coast, due to the proximity to the low and the incoming area of high pressure coming in from the central US. Northwest winds are forecast to gust as high as 45-50 mph, mainly across Sussex County, DE and Cape May County, NJ. As a result, no changes to the Wind Advisory, which remains in effect from midnight tonight until 4 PM Sunday. Lesser gusts expected further up the coast in NJ, where gusts generally hover between 35 to 45 MPH. Further inland (outside of the Poconos - more on that below), gusts drop off even more dramatically, coming in between 25 to 35 MPH. Highest wind gusts will come from daybreak through the early to mid afternoon.

For the higher terrain of the Poconos (Carbon and Monroe Counties) the wind risk is mostly from a low to mid level jet that begins to develop by mid day Sunday. This will result in the strongest winds being more northwesterly through the afternoon, and aided by some downsloping off the higher terrain. This area will see the highest gusts from the late afternoon through the late evening, where gusts over 50 MPH are possible. Impacts may be exacerbated in this area as well since snow and ice may still remain on trees and wires.

With coastal/tidal flooding, minor tidal flooding is expected with the Sunday morning high tide cycle from Barnegat Bay southward in New Jersey and Atlantic coastal Delaware. A Coastal Flood Advisory remains in effect for the highest confidence areas to receive impacts during the Sunday morning high tide. Areas further north, like Manasquan and Sandy Hook could get very close to coastal flood advisory thresholds, though guidance keeps it just shy. The wind direction now looks to be more out of the north or even north/northwest and that usually does not result in significant tidal flooding north of Manasquan Inlet. At this point, tidal flooding is unlikely for the northern shores of the Delaware Bay, the tidal Delaware River and northeastern portions of the Chesapeake Bay. In these areas, the wind direction is also unfavorable for tidal flooding. Minor flooding may continue near the times of high tide through Monday, however the highest water levels and greatest impacts are anticipated to occur during the Sunday morning high tide. The wind direction being parallel and then switching to mostly off shore will help limit coastal/tidal flooding impacts. However, it will be working against higher astronomical tides with the full moon on Monday, and increased swells from the off shore system. Further complicating the impacts of tidal flooding impacts is ice that is already developing on bays could be pushed onshore during high tide, and flood waters could freeze on roadways.

Finally in terms of snowfall, dry air continues to impede on precipitation advancing far enough north and west to provide a robust chance for any snowfall. Some flurries to a light dusting remains possible for southern Delaware and extreme southern New Jersey, but the chance for snow continues to diminish.

KEY MESSAGE 3...Below freezing temperatures are expected to persist across parts of the area through the week.

The longwave mid and upper level trough axis will shift east, further away from our region Monday, and by Tuesday, flow aloft will be mostly zonal (allowing for the most "mild" day of the week, but still 5 to 10 degrees below normal, and still likely at or below freezing for most areas north and west of the I-95 corridor. However, this is short-lived as a series of troughs digging southeast will bring reinforcing shots of cold air. Areas that don't get above freezing on Tuesday, are likely to stay below freezing through the rest of the week. This current stretch of below freezing temperatures began last weekend (Jan 23 through 26 - varies depending on the site), which means those areas will be on track to be at or below freezing for at least 2 weeks. The longer this stretch continues, the higher the risk for impacts to people and infrastructure including frozen pipes and widespread ice on waterways (more information on implications for marine interests in the marine section). This would be a top five longest stretch of below freezing temperatures for many of our climate sites.

KEY MESSAGE 4...An active weather pattern will persist with two chances for some snowfall mid to late week.

The first system we are watching is forecast to slide across the eastern seaboard Tuesday into Wednesday. Interestingly, the lee-side surface low is less defined on this morning's model runs, but the northern stream trough is depicted as more intense (as compared to model runs this time yesterday. Of note, through this period, it is likely to be a split flow pattern, which are notoriously hard to model, so it is a lower confidence forecast. As far as impacts to our region, the chance for snow is 10 to 40 % (with higher chances over Delmarva). Even if the area sees snow, the window of opportunity for any snow is brief, so at this point any impacts are expected to be limited.

Chances for snow are higher with the second system, a clipper system, moving through Friday into Friday night. Chances for snow in this periods are 30 to 50%. In general, clipper systems tend to be fast moving, cold systems with the speed and lack of moisture limiting how much snow occurs. It is notable, however, that guidance currently has an upper level trough associated with the subtropical jet pivoting across the Southeastern US Thursday, just out of sync with the trough associated with this clipper system Friday-Friday night. Should the timing of either trough shift closer to the other and end up phasing, a more robust system could be in the works. The confidence this far out is naturally very low, but something we will continue to monitor.

AVIATION /08Z SUNDAY THROUGH THURSDAY/

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

Sunday...VFR. Northwest winds 15-20 knots with gusts 25-35 knots. Moderate confidence.

Sunday night...VFR. Northwest winds around 10 kts with gusts 15-20 knots, diminishing to 5-10 kts with gusts 10-15 knots after 05Z.

Outlook...

Monday and Tuesday...VFR with no significant weather.

Tuesday Night and Wednesday...Sub VFR conditions possible with the chance for some snow (20-30 percent for KPHL, KPNE, KTTN, KILG, KMIV, and KACY).

Thursday...VFR with no significant weather.

MARINE

A Gale Warning is in place for all marine zones through late Sunday afternoon. Winds out of the north/northwest 25-35 kt with gusts around 40 kt. Seas of 6 to 10 feet on the ocean with 3 to 5 feet on the lower Delaware Bay. Winds diminish to SCA criteria Sunday night.

With gusty winds, high seas, and very cold air over the water, a Heavy Freezing Spray Warning is in effect for all ocean zones for tonight through Sunday Night. With lesser waves but still cold air and wind over Delaware Bay, a Freezing Spray Advisory remains in effect for the Bay tonight through Sunday Night.

Regarding the river ice threat...Ice has formed across area bay waters and inland estuaries. This includes, but is not limited to the Delaware estuary. On rivers, the ice has grown in thickness enough to start causing restrictions in flow. An example of this can be seen at the Trenton River gauge. Per satellite imagery, the Delaware River is pretty much ice covered from just south of Trenton upstream to about Washington Crossing.

The most common ways river ice can break up is either through a thermal or mechanical means. We would like to see temperatures rise into the 40s for a few days, but that's not in the offing through next weekend. Rainfall, producing river rises, can also break up ice. But again, that's not in the forecast either this coming week.

As a result, we expect the ice we're seeing to expand before it contracts. Even though there may be a pause in the growth during the daylight hours this coming week, temperatures at night will support expansion.

Outlook...

Monday...winds and seas will gradually diminish, but SCA conditions are likely to continue for much of the day time hours. Freezing spray possible.

Tuesday through Thursday...winds and seas should stay below SCA criteria. Chance for freezing spray decreases.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Cold Weather Advisory until 10 AM EST this morning for PAZ060>062-070-071-101>106. Cold Weather Advisory until 10 AM EST Monday for PAZ054-055. Wind Advisory until 1 AM EST Monday for PAZ054-055. NJ...Cold Weather Advisory until 10 AM EST this morning for NJZ001- 007>010-012>027. Wind Advisory until 4 PM EST this afternoon for NJZ023-024. Coastal Flood Advisory until noon EST today for NJZ020-022>027. DE...Cold Weather Advisory until 10 AM EST this morning for DEZ001>004. Wind Advisory until 4 PM EST this afternoon for DEZ003-004. Coastal Flood Advisory until noon EST today for DEZ003-004. MD...Cold Weather Advisory until 10 AM EST this morning for MDZ012- 015-019-020. MARINE...Gale Warning until 4 PM EST this afternoon for ANZ430-431. Freezing Spray Advisory until 6 AM EST Monday for ANZ430-431. Heavy Freezing Spray Warning until 6 AM EST Monday for ANZ450>455. Gale Warning until 6 PM EST this evening for ANZ450>455.