textproduct: Philadelphia/Mt Holly

SYNOPSIS

An expansive area of high pressure centered to our north will continue to dominate our weather through Monday. Low pressure develops off the coast of the Carolinas on Monday before tracking out to sea through Tuesday. Another low then tracks across the Tennessee Valley Wednesday into Thursday before moving across the eastern Great Lakes region Thursday night into Friday. A secondary low may develop near our area later Thursday. The entire system moves to our north during Friday into the weekend, then a cold front may approach later Sunday.

NEAR TERM /THROUGH MONDAY/

High pressure centered over the Great Lakes and eastern Canada remains in control across the region. Scattered deck of stratocumulus clouds has developed this afternoon, providing a mix of sun and clouds. Winds remain light out of the northerly direction. A weak sea breeze has apparently developed along the coast, shifting winds to more of an NE-ENE direction.

Tonight, decreasing clouds overall with mostly clear skies North/West and partly cloudy for Delmarva and south NJ. Temperatures will be seasonably cool with upper 20s/low 30s N/W and upper 30s/low 40s S/E. Light NE winds overnight. Frost advisories were issued across areas of southeast PA, central NJ, and southern NJ for zones where the growing season remains. Overall a tricky forecast once again gauging the risk of widespread frost, but light winds and mostly clear skies will contribute to the potential once again tonight in certain zones. The biggest questions at this time is how extensive will the cloud coverage be across the Delmarva and Southern NJ as well as how far north the extent of the clouds will be. At this time, cloud coverage across the Delmarva looks like it will be sufficient to help slow radiational cooling tonight, thus were left out of the Frost Advisory. Cloud coverage across the pinelands of New Jersey look to be clear enough to allow for at least some decent radiational cooling. Temperatures around Philly metro did get rather chilly last night, with several locations touching the upper 30s. Should they get clear enough skies, Philly metro suburbs and some outskirts of Philadelphia County proper could see temperatures cool even lower than last night, warranting a Frost Advisory for those locations.

High pressure to the north will remain in control for Monday with mostly clear to partly sunny skies. Winds shift more to the northeast around 5-10 mph.

SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/

A cut-off low slides across the Southeast states late Monday into Tuesday and merges with the flow of a more broad upper low across the northeastern US, resulting in a surface low pressure developing off the coast of the Carolinas. This feature is mostly expected to miss our area as it quickly tracks out to sea with the stronger flow aloft on the south side of a trough near the Canadian Maritimes. It is possible that within the onshore flow that some light showers, sprinkles or drizzle develops along the coast later Monday night into Tuesday before spreading inland some during Tuesday. It may take some time though for the air mass to saturate with some initial stratus development, and therefore the shower/rain chances are of low confidence during this time frame. Northeast winds will also strengthen into Tuesday due to the gradient between the high to our north and the low off the coast. Along the coast winds should be 15 to 20 gusting 30 to 35 mph at times. Farther inland the winds will be more like 10 to 15 gusting to around 20 mph. Temperatures will continue to be on the cooler side of average with highs Tuesday generally in the 50s. Lows will continue to be mostly in the 30s to low 40s, a few degrees more mild thanks to the increasing clouds at night.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Summary...Low pressure affects the area Wednesday night through Thursday night; Chilly conditions continuing.

Synoptic Overview...A dynamic setup for the mid to late week time frame as a strong upper-level through closes off as it slides across the Ohio and Tennessee Valleys Wednesday into Thursday. This then ends up across the Mid-Atlantic and Northeast later Thursday before gradually weakening and shifting north Friday and into the start of the weekend. At the surface, strong high pressure centered across much of eastern Canada Wednesday into Thursday shifts east. Low pressure sliding across the Tennessee Valley may remain the main low as it lifts into the eastern Great Lakes Thursday night then into adjacent Canada during Friday. Another low may develop along the occluding front near our area. The setup favors a storm, however the details regarding its strength and exact track remain uncertain. A continued onshore wind will impact the coastal/back bay areas that had significant impacts from the coastal storm a few weeks ago.

For Wednesday and Thursday...There still is some uncertainty regarding how low pressure evolves, with more of the guidance showing the parent low into the upper Ohio Valley and eastern Great Lakes remaining the stronger one. The upper-level flow is forecast to support a storm and while timing still needs to be settled on, the main focus for rain and stronger winds looks to be late Wednesday night through Thursday night. A strong upper-level trough that closes off is forecast to slide across the Tennessee and Ohio Valleys, which then translates into the Mid-Atlantic and Northeast through Thursday night. Stronger onshore flow is forecast to occur during this time frame as a robust low-level jet crosses our area. As the closed low approaches, surface low pressure tied with it strengthens toward the eastern Great Lakes. An area of stronger divergence aloft should also support a surface low developing across our area along the occluding front. The strength of this second low is in question given the varying solutions in some of the ensemble guidance. Given the onshore flow which increases to gale force for a period of time, already sensitive coastal/back bay communities due to the coastal storm a few weeks ago may experience strong winds, coastal flooding and potentially significant beach erosion. The rainfall forecast is challenging as well as this will depend on the track and forward movement of the system. The bulk of the rain looks to be tied to strong forcing for ascent which is in conjunction with a robust low-level jet. We need the rain so this is something of interest for our entire region. Given the uncertainty with the timing/details, the only main change from the National Blend of Models (NBM) output once again was to increase the wind and wind gusts on land given the low-level jet and enough vertical mixing. Some guidance hints at some instability developing later Thursday, which could result in some embedded stronger convective elements however it is not clear at this time if it will be enough to get thunder/lightning to occur. As a result, kept a mention of thunder out of the forecast for now.

For Friday and Saturday...Based on the consensus timing at this point, low pressure should be exiting to our north Friday morning. This occurs as the parent closed low pivots northward and gradually weakens. Remaining rain or showers then should be ending from south to north Friday. Cyclonic flow lingers however and with some cold air advection in the wake of the storm will keep temperatures on the chilly side. A tightened pressure gradient in place with cold air advection should result in decent mixing Friday and therefore a gusty wind. Some pressure gradient remains in place Saturday, therefore a breeze looks to remain, however this should diminish with time especially during Saturday evening.

For Sunday...An upper-level trough sliding across the Midwest and Great Lakes will have weak low pressure with and also a cold front. This cold front may not make it to our area until at night or just beyond. There should be less wind given a much weaker pressure gradient that is currently forecast to be in place ahead of the aformentioned cold front. Temperatures still look to be on the chilly side.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

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

Rest of today...VFR expected. There will be some mid level clouds this afternoon. Light North winds 5 to 10 knots much of the day. High confidence.

Tonight...VFR expected. Mostly high clouds expected. Light North to Northeast winds. High confidence.

Monday...VFR expected. High clouds with north to northeast winds around 5-10 kts. High confidence.

Outlook...

Monday night...Remaining VFR. Northeast winds begin to increase to 10-10 kts. Increasing mid-level clouds.

Tuesday...Increasing clouds with MVFR cigs possible, especially for MIV and ACY.

Wednesday and Thursday...Sub-VFR probable as rain develops, especially on Thursday. Gusty northeasterly winds probable, especially closer to the coast.

Friday...Sub-VFR conditions should occur at least in the morning as rain/showers taper off. Gusty winds become southwest to west.

MARINE

Sub-SCA conditions through Monday with light north winds around 10 kts or less Sunday night, turning northeast around 10-15 kts Monday. Seas remain 2-3 feet.

Outlook...

Monday night through Tuesday night...SCA conditions develop late Monday with Gales possible Tuesday and Tuesday night. Northeast winds ramp up to 20-25 kts. Gusts of 30 to 35 kts possible. Seas build from 5-8 feet Tuesday to 8-10 feet Tuesday night.

Wednesday and Thursday...Small Craft Advisory conditions likely. Gale force wind gusts probable mostly on Thursday. Dangerous seas on the ocean building to 8-12 feet.

Friday...Gale force wind gusts probable. Seas up to 10 feet on the ocean zones gradually subsiding.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Frost Advisory from 1 AM to 9 AM EDT Monday for PAZ070-071-104- 106. NJ...Frost Advisory from 1 AM to 9 AM EDT Monday for NJZ012-013- 015>023-027. DE...None. MD...None. MARINE...Gale Watch from late Monday night through late Tuesday night for ANZ431-450>455.