textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated aviation section.

KEY MESSAGES

1. Weak low pressure passes through the region tonight with some showers.

2. An off shore low Saturday into Saturday night may bring a little rain to coastal areas.

3. Freezing conditions possible in the Poconos on Saturday night.

4. Elevated fire weather conditions are possible on Sunday.

5. A cold front Wednesday into Thursday could bring the next chance for widespread precipitation.

DISCUSSION

KEY MESSAGE 1...Weak low pressure passes through the region tonight with some showers.

Weak low pressure will move through the region overnight tonight, bringing increased clouds along with some showers. Overall rainfall amounts are still expected to be quite light, with generally less than 1/10 inch. Showers will likely become a bit more sparse in coverage as they approach the coast overnight, and diminish by early Saturday morning.

KEY MESSAGE 2...An off shore low Saturday into Saturday night may bring a little rain to coastal areas.

The slight eastward trend in the track of the off shore low with the 00Z guidance run continued with the 12Z guidance. Rain with this system looks likely to be confined to the coastal counties, and might not reach the shore at all. Nudged POPS down across the board for Saturday. If this trend continues, rain chances could decrease further for coastal areas.

The trend also favors a less breezy day overall during the day on Saturday, as the tightest pressure gradient with this system will be well off shore.

KEY MESSAGE 3...Freezing conditions possible in the Poconos on Saturday night.

Behind tonight's system and in the wake of the coastal low passing offshore Saturday, another chilly air mass will move into the region. This will help bring temps down near freezing in the Poconos, so have issued a freeze watch for these areas. Elsewhere, there appears to be just enough wind to preclude any significant frost or freezing conditions, but will keep watching trends.

KEY MESSAGE 4...Elevated fire weather conditions are possible on Sunday.

As the offshore low slides further away from our region, a surface high is expected to build over the southeastern U.S. on Sunday. This will result in an increasing pressure gradient and northwest flow for our region. This type of pattern suggests a very deep mixed layer and dry air advection. Some of the latest model soundings continue to show a mixed layer all the way to 775 mb!

Models (and consequently blends of models) tend to have a high Dewpoint bias with such patterns, especially in the Spring. Consequently, have included dewpoints that are on the lower side of guidance during the day on Sunday. Even with temperatures which will be 5 to 10 degrees below normal, this would still result in relative humidity values at or below 30 percent.

With the deep mixed layer and the pressure gradient, breezy conditions (with wind gusts at or above 20 mph) are likely. The combination of the low relative humidity and gusty winds could result in rapid fire spread, if fuels are dry enough.

KEY MESSAGE 5...A cold front Wednesday into Thursday could bring the next chance for widespread precipitation.

The next chance for widespread precipitation across our region is Wednesday into Thursday as a cold front approaches the northeastern U.S.. Latest model guidance is hedging towards a cold front approaching on Wednesday, then stalling near or west of our region, before a secondary cold front finally crosses through our region on Thursday.

On the synoptic scale though, this appears to be a split flow pattern setting up, with the subtropical jet potentially as far north as our latitude. Given that, and the fact that this is still in the day 6 and 7 time frame, there is low confidence in any one solution. However, regardless of the evolution of the cold front, there should at least be a brief window for widespread rain as it crosses through our region.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

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

Tonight...Mainly VFR. Scattered showers may briefly result in MVFR cigs or vsby, most likely for KRDG/KABE. Winds generally southwest at 5-10 kt. Moderate confidence.

Saturday...Mainly VFR after any patchy low cigs from the overnight showers break. A few showers can't be ruled out during the afternoon hours for KMIV/KACY, but the chance appears to be diminishing. Winds generally northwest around 10 kt, with a few gusts 15-20 kt possible in the afternoon. Moderate confidence.

Outlook...

Saturday Night through Monday Night...VFR conditions expected with significant weather.

Tuesday...VFR with some wind gusts of 25-30 kts possible.

Wednesday...Potential for sub-VFR conditions with showers possible.

MARINE

Sub-Small Craft Advisory conditions through Saturday night. South to southwest winds this afternoon and tonight shifting northwest Saturday and Saturday night, but gusts should remain below 25 kts. Seas will average 3 to 4 ft.

Outlook...

Sunday...Winds above 20 kt likely, but confidence is low that winds will reach 25 kt.

Monday through Wednesday...Prolonged and widespread SCA conditions (due to both wind and waves) are likely. There is a chance (20-30%) for winds to get to gale force, primarily late Tuesday-Tuesday Night.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Freeze Watch from Saturday evening through Sunday morning for PAZ054-055. NJ...None. DE...None. MD...None. MARINE...None.