textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Small Craft Advisory issued.

KEY MESSAGES

1. Showers and potentially some severe thunderstorms, later today into this evening, then turning much cooler with gusty winds on Monday.

2. Fair and seasonable for the middle of this coming week.

DISCUSSION

KEY MESSAGE 1...Showers and potentially some severe thunderstorms, later today into this evening, then turning much cooler with gusty winds on Monday.

An upper-level trough across central Canada is forecast to amplify some today into Monday on its south side from the Great Lakes and Northeast. This will drive surface low pressure eastward from the Great Lakes which tracks near our area later today. However, another weak low develops along the front and within the elongated pressure trough. This secondary weak low may become convectively enhanced as it slides east-southeast along the front late this afternoon into tonight. A frontal zone will be to our north and this will keep our area within the warm sector with southwesterly winds during most of the day today. However, a strong cold front will move southeastward across the area tonight bringing colder air across our area Monday through Tuesday. Ahead of the front, high temperatures today are forecast to reach the 60s to low/mid 70s for most of the region. A more southerly wind component though will keep it cooler along the coast due to the wind off the cold ocean.

A ribbon of stronger forcing for ascent ahead of and with the cold front and weak surface low results in an area of showers and storms arriving in our northwest zones around mid to late this afternoon then spreading southeastward through this evening. The model forecast soundings indicate instability developing within the warm sector, especially west of our area heading into central PA. It's there where storms are likely to initially fire this afternoon with a good chance of at least some being severe as lapse rates are steep through the low and mid levels and the shear and wind fields are very strong. These showers/storms should then move east and southeastward into the area through the late afternoon and evening, likely as a line or one or more line segments. The forecast gets a bit trickier for our CWA in terms of storm intensity as there will be less instability to work with...especially surface based instability as the low levels stabilizes some into this evening. That said, it should take some time for storms to weaken so it's likely there will be at least some scattered instances of severe weather in our eastern PA zones into northern Delaware where the Storm Prediction Center has outlined in a SLIGHT risk for severe storms. Elsewhere, across most of the rest of our Delmarva zones and NJ the threat level from SPC remains MARGINAL as storms should be weaker by the the time they get there. This means any instances of severe weather should be isolated for these latter areas. In terms of hazards, the steep lapse rates through the mid levels along with the strong deep layer shear indicate severe hail will be a potential hazard. But with the strong wind field damaging winds will certainly be on the table as well in terms of threats. The tornado risk looks rather low as of now unless the low-level moisture (surface dew points) ends up increasing more than currently forecast. While still some uncertainty with the exact timing, the main severe thunderstorm threat still looks to be between 4 PM and 10 PM.

The precipitable water is forecast to increase to 1+ inches as we get into this evening. This along with some potential for a time of training convection could result in some corridors of heavier rainfall (1+ inches). If heavy rainfall rates occur over the more urban areas, localized flash flooding is possible. Otherwise, the rainfall is much needed. Once the front starts to push south of the area overnight this isn't a situation where the precipitation will quickly come to an end. This reason for this is that with the new low developing along the front near the area, additional rainfall is likely north of the boundary across our area with the precip becoming more stratiform in nature. The severe weather threat will be over by this point but rain, potentially moderate at times, will continue for several more hours.

For Monday, the cold advection pattern behind the front will result in highs for the day likely occuring in the predawn hours with temperatures then dropping early before holding fairly steady at generally in the upper 40s to low 50s for most of the day. Rain should taper off west to east around or shortly after sunrise for most areas and it's worth noting it could end as a little wet snow or a wintry mix across the higher elevations of the Poconos. Little to no accumulation is expected though. Also for Monday, the pressure gradient tightens in the wake of the departing low to our east and northeast as high pressure builds toward the Great Lakes. This combined with stronger cold air advection will steepen the low-level lapse rates and result in deeper mixing. So expect a gusty northwest wind resulting during Monday especially in the afternoon into the evening, with peak wind gusts of 25-35 mph.

KEY MESSAGE 2...Fair and seasonable for the middle of this coming week.

High pressure builds in Tuesday and dominates our weather through Wednesday bringing tranquil conditions. Tuesday should be mainly sunny with more in the way of cloud cover for Wednesday as the high starts to weaken and a warm front approaches. As a result, Tuesday will remain on the cooler side of average with highs mainly in the upper 40s to low 50s. Temperatures then moderate some for Wednesday with the approach of the warm front with highs for most areas generally in the middle to upper 50s.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

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

Today...A mix of MVFR and VFR ceilings to start, with patchy fog still a possibility at MIV and ACY. Conditions will improve to VFR by 15-17Z. Showers and a few storms, perhaps strong, may begin to impact RDG and ABE as early as 20-22Z, but will likely hold off through 00Z for all other terminals. South- southwesterly winds generally 5-10 kt. Moderate confidence.

Tonight...MVFR ceilings at all terminals, with IFR possible late. Showers and a few storms, some perhaps strong, are expected to move through the terminals through 03-05Z. Thereafter, lingering showers are expected through most of the night. Winds becoming northerly at 5-10 kt with the passage of a cold front between 03-06Z. Moderate confidence.

Outlook...

Monday..Improving conditions, likely to VFR. Northwest wind gusts 20-30 kt on Monday.

Monday night through Wednesday...VFR. No significant weather.

Wednesday night through Thursday...Periods of sub-VFR conditions possible with a chance of showers.

MARINE

Conditions over the waters are expected to stay below Small Craft Advisory levels through this afternoon. However seas start to come up into this evening and for this reason a Small Craft Advisory is in effect for our NJ coastal waters zones beginning at 6 PM.

By Monday morning, expect northerly winds increasing to 15 to 20 gusting to around 25 knots for all zones so the Small Craft Advisory is expanded to include all waters beginning 8 AM Monday. These conditions should persist through Monday before diminishing later Monday night. Also expect some rain/showers and potentially a few storms over the waters Sunday night.

Outlook...

Tuesday and Wednesday...The conditions are anticipated to be below Small Craft Advisory criteria.

Thursday...Small Craft Advisory conditions may develop, mainly at night.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 8 AM Monday to 3 AM EDT Tuesday for ANZ430-431. Small Craft Advisory from 6 PM this evening to 6 AM EDT Tuesday for ANZ450>453. Small Craft Advisory from 8 AM Monday to 6 AM EDT Tuesday for ANZ454-455.