textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Some spotty light snow and light freezing rain may impact the Southern Poconos, Lehigh Valley, and far northwest New Jersey late tonight through Tuesday morning.

Above normal temperatures continue to be forecast for much of this week. As we head towards Friday and Saturday, there is moderate confidence in above normal temperatures as temperatures are a bit lower than earlier in the week. Right now, highs look to be near or slightly above normal for that timeframe. Another potential component to watch is a backdoor cold front towards the second half of this week which would impact temperatures depending on how far south it reaches.

The bulk of the rain for the wednesday into early Wednesday night system looks to be mainly near and north of I-78. Totals look to be around a quarter of an inch or less. As you head south of I-78, amounts drop off quickly.

KEY MESSAGES

1. Low pressure passing north of the area may touch off some spotty light snow and light freezing rain may impact the southern Poconos, Lehigh Valley, and far northwest New Jersey late tonight through Tuesday morning.

2. Above normal temperatures are forecast for most of this week and may continue into the beginning of this weekend.

3.An unsettled pattern takes hold for this week with multiple chances for rain, and potentially some mixed precipitation in the higher elevations of our northern areas.

DISCUSSION

KEY MESSAGE 1...Low pressure passing north of the area may touch off some spotty light snow and light freezing rain may impact the southern Poconos, Lehigh Valley, and far northwest New Jersey late tonight through Tuesday morning.

High pressure will establish itself over the southern Appalachians tonight, then slides off the Southeast and Mid-Atlantic coasts tonight through Tuesday. Return flow sets up, and warm air advection will begin late tonight. Meanwhile, weak low pressure currently over the Great Lakes will track east and pass north of the area early Tuesday morning. Although the bulk of the precipitation will stay north of the area, cannot rule out some spotty light snow or light freezing rain, mainly across the southern Poconos, Lehigh Valley, and far northwest New Jersey. Will keep the slight chance PoPs that was previously in the forecast, as current NBM forecast is dry. Not expecting much, if any, measurable precipitation, and as a result, not expecting measurable snow or ice accretion. However, it may be enough to result in locally slick spots on untreated roads.

For the rest of the area, with increasing low level moisture, patchy fog will develop. For areas where temperatures drop below freezing, some patchy freezing fog is possible as well.

Skies remain cloudy through Tuesday morning, then clearing will develop by early afternoon. Much warmer compared to today (Monday) with highs in the mid to upper 40s for most of the area, and in the low 50s in southern Delmarva.

KEY MESSAGE 2...Above normal temperatures are forecast for most of this week and may continue into the beginning of this weekend.

A high pressure system that is moving offshore tomorrow leads to a southwest wind with its return flow and remains offshore into Wednesday. This leads to warm air advection and temperatures trending above normal for this timeframe. Beyond this, multiple systems move through the region allowing for more opportunities for warm air advection keeping above normal temperatures going even into potentially next weekend. One key component that will need future monitoring is the potential for a backdoor cold front for the second half of this week. There will be a high pressure system well to the north that will be advecting colder air and it is uncertain how far south this cold air can make it. The exact location of this backdoor cold front will have impacts on temperatures.

For the specifics on temperatures, highs range from the mid 40s to low 50s for much of the area Tuesday through Thursday. For the southern portions of Delmarva, highs on Wednesday through Friday get into the mid to upper 50s with some spots potentially getting to 60.

In terms of overnight lows, after tonight, most locations will stay above freezing. Lows range from the mid to upper 30s to low 40s for most locations Tuesday night through Friday night. Temperatures look to get back below freezing on Saturday night and especially Sunday night.

After over two weeks now with snow on the ground, these warmer temperatures will allow the snowpack to melt quicker.

Temperatures look to get closer to normal Friday and Saturday but may still remain slightly above normal. Sunday and Monday trend colder with highs starting to get near normal or below normal.

KEY MESSAGE 3...An unsettled pattern takes hold for this week with multiple chances for rain, and potentially some mixed precipitation in the higher elevations of our northern areas.

During this week and into next weekend, there will be multiple areas of low pressure moving through the region. This will lead to multiple rounds of precipitation mainly in the form of rain but some wintry precipitation is possible in the higher elevations of our northern areas at times.

For Wednesday into Wednesday night, a shortwave moves through. At the surface, there will be a low pressure system near the Midwest that drags a warm front northward on Wednesday which will result in some rain for the area Wednesday into early Wednesday night. Model guidance continues to lean in the direction of this being an all rain event for our area. Guidance has also trended a bit north with QPF amounts, with the highest rain totals north of I-78. However, QPF amounts across the board look rather light with the shift in higher rainfall amounts being more to the north. NBM probability over 1 inch is less than 5%. Probability of rainfall amounts over half an inch are around 15% or less north of I-78 with less than 5% south. Our area could certainly use some rain and this is expected to be at least some beneficial rain for our area. Thursday is a bit more interesting with the potential for a backdoor cold front sinking south and our southern areas may only remain in the warm sector which will lead to some isolated showers. As our next system approaches, we see warm air advection increase Thursday night and so does the precipitation as our front moves northward as a warm front.

Our next system arrives towards the end of the week for the Friday into Saturday timeframe. This is another low pressure system originating from the Midwest and moving towards our area. Just like the midweek system, this looks to be more of a rain event for most of the area. However, given slightly lower temperatures now forecast for the Friday/Saturday timeframe, there is the potential for some wintry precipitation mainly for the higher elevations of northern NJ and the Poconos. There is some potential that wintry precipitation could make it as far south as the I-78 corridor Friday night. NBM probability of QPF over 1 inch is around 20-40%. The details will be ironed out over the next couple days, especially once the timing comes more into focus.

Beyond this, there is the potential for yet another system towards the end of the weekend but there remains uncertainty with the details of this system as it is at the end of the long term period.

Overall, these systems generally look to bring primarily rain to most of the area, with the potential for wintry precipitation in far northern New Jersey and the southern Poconos depending on timing. One key note with these systems, we are not currently expecting moderate or major impacts. In fact, for most of the region, it will be more of a beneficial rain.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

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

Tonight...MVFR with some breaks to VFR, lowering back to MVFR, then IFR after 06Z. CIGs may get as low as LIFR prior to 12Z. Patchy fog will result in MVFR VSBYs. Some spotty light SN and light FZRA may impact KRDG/KABE after 06Z. LGT/VRB winds, becoming SW less than 5 kt. Moderate confidence.

Tuesday...IFR in the morning, lifting to MVFR by 18Z, then VFR thereafter. SW winds 5 to 10 kt. Moderate confidence overall.

Outlook...

Tuesday Night...VFR. No significant weather.

Wednesday through Wednesday night...MVFR/IFR conditions expected with low clouds and light to moderate rain moving through. Worst conditions will be near KRDG/KABE.

Thursday through Friday night...Sub-VFR likely due to low clouds and rain.

Saturday...Primarily VFR. A slight chance for rain.

MARINE

Northeast winds 15 to 20 kt will diminish to 10 to 15 kt this evening, then will turn southwest around 10 kt late tonight through Tuesday. Seas, however, will remain elevated at 6 to 8 feet, and will remain above 5 feet through Tuesday. As such, the Small Craft Advisory remains in effect for the ocean through at least Tuesday. Sub-SCA conditions for Delaware Bay.

VSBY restrictions possible in fog late tonight through Tuesday morning.

Regarding the river/bay ice...Ice cover continues across many area bay waters and inland estuaries. This includes, but is not limited to the Delaware estuary. On rivers, the ice thickness is enough to cause some restrictions in the flow. Per satellite imagery and ice analysis data, the Delaware River is mostly ice covered from the Delaware Memorial Bridge upstream, to at least Washington Crossing, though we are seeing improvement with warmer temperatures that will only get better with warmer days ahead.

According to latest analysis from the National Ice Center, Delaware Bay is actually mainly ice-free with less than 10% coverage of ice.

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, which parts of the region are forecast to continue to get to this week with some areas getting into the 50s early this week. Significant rainfall, producing river rises, can also break up ice but that is not in the forecast this week.

As a result, ice cover will gradually decline over the next week or so and we are already seeing major improvement in Delaware Bay.

Outlook...

Tuesday night...Lingering 5 foot seas possible which could result in an extension to the Small Craft Advisory on the ocean. Sub-SCA conditions expected on Delaware Bay.

Wednesday through Wednesday night...SCA conditions probable (50-70%) with seas 5-6 feet and wind gusts near 25 kt.

Thursday through Saturday...No marine headlines expected.

EQUIPMENT

KDIX will remain out of service through at least Monday and possibly Tuesday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Tuesday for ANZ450>455.