textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A weak short wave trough could bring light snow showers to areas generally along and north of the I-78 corridor late Friday Night/early Saturday Morning.

There were minor changes in model depictions of the Sunday/Monday system. The trends have generally been for a more southerly track that does not phase with the northern stream trough (at least while it is near our region) resulting in less impacts for our region.

There is another chance for rain with a system Wednesday into Thursday.

KEY MESSAGES

1. Colder, below normal temperatures return through Friday night. Milder temperatures closer to normal expected this weekend, and especially next week.

2. Snow showers are possible late Friday Night into early Saturday Morning.

3. Another round of precipitation, primarily for the coastal plains, is possible Sunday into Monday with a low pressure system sliding south of our region.

4. The next chance of precipitation, primarily for areas north of the PA Turnpike/195 corridor, comes Wednesday into Thursday.

DISCUSSION

KEY MESSAGE 1...Colder, below normal temperatures return through Friday night. Milder temperatures closer to normal expected this weekend, and especially next week.

A gusty northwest wind will persist into the evening, as there's a fairly tight pressure gradient over the area in between low pressure off to our east and high pressure gradually approaching from the west. Temperatures have risen into the mid 30s this afternoon under partly to mostly cloudy skies. Scattered to broken cloud cover will persist through the evening, then gradually diminish overnight as cold advection wanes. Lows tonight will be on the order of 10 degrees colder tonight, mainly in the teens to low 20s with highs Friday once again mainly in the 30s, except 20s over the Poconos. Friday should also feature mainly sunny conditions.

The airmass over the area moderates this weekend as high pressure slowly shifts off to the east. High temperatures both Saturday and Sunday generally range from the upper 30s north to the mid/upper 40s south. Overnight lows, at least for Saturday night, will be below freezing (in the 20s). The overall moderating trend looks to continue into next week, although low pressure sliding by to our south later Sunday into Monday could result in some cooling on Sunday (see Key Message 3 below). Regardless of what happens with the storm, it will have departed by next Tuesday and Wednesday with highs by then looking to be well into the 40s for most areas with even some 50s over parts of the coastal plain. Dewpoints will also begin rising above freezing next week, which is anticipated to result in more substantial melting of the stubborn snowpack across the region.

KEY MESSAGE 2...Snow showers are possible late Friday Night into early Saturday Morning.

A fast moving mid level short wave trough will spark some snow showers which could progress as far southeast as northern New Jersey very late Friday Night into early Saturday morning. Initially dry air in the boundary layer could result in mostly virga. However, some model soundings depict that this will eventually be able to be overcome. That being said, it is a fast moving trough, so the window for accumulating snow from these showers is quite brief. Thus, any snow accumulations are likely to be a few tenths of an inch at most.

Overall, this is unlikely to produce widespread impacts, but with temperatures in the 20s and teens as the snow is falling, if there are any light accumulations then untreated surfaces could be slippery.

KEY MESSAGE 3...Another round of precipitation, primarily for the coastal plains, is possible Sunday into Monday with a low pressure system sliding south of our region.

There were minor changes in model depictions of the Sunday/Monday system. The trends have generally been for a more southerly track that does not phase with the northern stream trough (at least while it is near our region) resulting in less impacts for our region. Most notably, the main ECMWF, which was the northern outlier and depicted the low phasing with a northern stream trough in previous runs, trended closer to the consensus with other guidance which is further south and weaker. These solutions favor a warmer, faster system, with precipitation primarily confined to the coastal plains. This solution would result in little, if any, snow accumulations. The precipitation type, especially if it is confined only to the coastal plains would be primarily rain, but could start off as a rain snow mix. Further working against snow accumulations is the fact that the air mass in place ahead of this system is milder than what we have seen with the last few systems.

That being said, there are still a few models that depict the stronger and further north solution. Additionally, if the precipitation extends outside of the coastal plains, then precipitation in those areas is more likely to be snow for longer. Therefore, stayed with the blend of guidance for the forecast.

KEY MESSAGE 4...The next chance of precipitation, primarily for areas north of the PA Turnpike/195 corridor, comes Wednesday into Thursday.

The next system we are watching is a low pressure system which could be crossing the northeastern U.S. Wednesday into Thursday of next week. Unlike the late weekend system, the center of this low is currently depicted crossing north of our region. This would favor precipitation north of the PA turnpike/195 corridor. That being said, our region would be in the warm sector with this track, which would favor mostly, if not all, rain.

One caveat on this, there are some models are depicting the low staying so far north, that the precipitation will stay entirely north of our region.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

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

Today...VFR ceilings around 4-5kft. Northwest winds around 10-15 kts with gusts up to 20-25 kts. High confidence.

Tonight...VFR. Northwest winds remaining near 10 kts, with some occasional gusts up to 20 kts possible. Gusts were not included in the TAFs, but could be needed for some spots. High confidence.

Friday...VFR. Northwest winds 5-10 kts. High confidence.

Outlook...

Friday night and Saturday...VFR. 20% chance of a snow shower at ABE/TTN.

Sunday and Monday...Periods of MVFR or even IFR are possible with rain and snow. KMIV and KACY have the highest risk of sub- VFR conditions.

Tuesday...VFR expected.

MARINE

A Small Craft Advisory remains in effect for all marine zones through this afternoon. Winds have started to diminish, especially across the southern waters, but gusts up to 25 kts remain possible through 6 PM. Beyond this time, expect no marine hazards through Friday.

Regarding the river/bay ice...Significant 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 entrance to Delaware Bay northward, or upstream, to at least Washington Crossing. Delaware Bay has quite a bit of ice cover, however satellite imagery the last few days has shown icing flowing out of Delaware Bay into the Atlantic Ocean.

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 get to once again over the weekend with some areas getting into the 50s early next 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 continue. There may be a pause in the ice growth during the daylight hours as temperatures rise to and above freezing, however temperatures at night will continue to support some ice expansion.

Outlook...

Friday night and Saturday...The conditions are anticipated to be below Small Craft Advisory criteria.

Sunday through Tuesday...Winds and seas should stay below SCA criteria. However this will depend on the track and strength of low pressure sliding by to our south.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ430- 431-450>455.