textproduct: Philadelphia/Mt Holly

SYNOPSIS

A cold front associated with a strong low pressure system moves through tonight and looks to stall near or over our area through the start of the weekend. The cold front moves through Sunday night followed by a stronger cold front Monday night into early Tuesday. High pressure then builds in later Wednesday.

NEAR TERM /THROUGH FRIDAY/

920 PM update... So far the evening has been quite quiet and tranquil, with warm and humid conditions prevailing. However, still expect widespread showers and scattered thunderstorms to move in overnight with wave of low pressure and cold front approaching, so have not altered the forecast significantly. In fact, have raised POPS overnight into early morning Friday. Otherwise, no major changes at this time.

3 PM discussion... Low pressure tracking across Canada will drag a cold front through the region late tonight and Friday morning. Showers and scattered thunderstorms will develop ahead of and with the passage of the cold front. There is the possibility for some severe thunderstorms with damaging wind gusts, but there are some limiting factors. Although surface based CAPE will be up around 300 J/kg, MUCAPE will be much higher, around 1000 or so J/kg. There will also be a 50 to 60 kt low level jet that will pass through southeast Pennsylvania and southern New Jersey, and 0-6 km Bulk Shear will be in the 45 to 55 kt range. Despite that, BUFKIT profile soundings indicate a capping inversion up to around 900 mb. SPC has most of the area in a Marginal Risk (1 out of 5), and has removed the Slight Risk (2 out of 5) for the eastern shores of Maryland.

There is, however, a risk for heavy rain due the high dew points, relatively low surface winds, and the aforementioned low level jet that could result in localized flash flooding. PWATs will average 1.5 to 1.7 inches, and there may be some training of heavy rain showers and possible thunderstorms across the southern Poconos, northern New Jersey, and the Lehigh Valley. Generally on a line north of Philadelphia, mainly from after midnight through daybreak Friday.

Cold front works its way through the region Friday morning, although will get hung up just south of Delmarva. The threat for convection comes to an end, and showers taper off during the morning hours.

Highs on Friday will in the low to mid 60s for most of the region, except in the upper 60s to around 70 in Delmarva. A much less humid air mass also spreads south with dew points in the 30s and 40s north of the Fall Line, and in the 40s and 50s south of the Fall Line.

SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/

As high pressure slided to our north Friday night into Saturday, low pressure tracks up the Ohio Valley and then moves mostly to our north through Saturday. This will pull the stalled boundary just south of Delmarva back north as a warm front. Some additional showers develop Friday night and Saturday, although the focus should become more focused farther northward with time. Onshore flow however will keep the area rather cloudy, and the location of the front and associated warm sector to its south will impact how warm the area gets. As of now, it should get into the upper 60s to low 70s across much of Delmarva with temperatures then quickly turning cooler northward. Again, these temperatures will be highly dependent on how far north the warm front gets. Some fog may begin to develop Saturday night mainly along the immediate coast and the adjacent marine area as dew points start to increase and rise above the chilly ocean water temperatures.

Heading into Sunday, as a significant upper-level trough slides across central to eastern Canada, it is forecast to amplify southward with its leading edge starting to move into the Northeast Sunday night. This trough will drive low pressure well to our north, however a trailing cold front will arrive in our area mostly Sunday night. The existing warm front across the area looks to surge northward early Sunday morning, and most of the area should get within the warm sector during the day as southerly flow increases. This could result in high temperatures getting into the 70s across much of the area with even low 80s possible across portions of Delmarva and the coastal plain. The warmth will also depend on how much sunshine occurs as well as the timing of the cold front and associated showers. It will feel more humid as dew points rise through the 50s and even the low 60s for many areas ahead of the cold front. This warm and more moist air mass may also result in marine fog that could impact the coastal areas as well, before it gets pushed out as the low- level flow veers more from the southwest. While the parent trough lags well behind the surface cold front, height falls arriving with a greater thickness packing also arriving from the west later in the day should result in enough forcing for ascent for showers to develop and move in from the west especially in the afternoon and more likely at night. The timing of the front looks to be mostly at night which is less ideal for stronger convection, however some thunder is certainly possible given the strength of the incoming trough. The intensity of any convection will depend on the available instability and shear magnitudes in combination of any stronger forcing for ascent.

LONG TERM /MONDAY THROUGH THURSDAY/

Summary...A stronger cold front moves through Monday night with high pressure moving in for the mid-week. High temperatures falling to several degrees below average through the middle of next week.

Synoptic Overview...An upper-level trough is forecast to amplify into and across the East during early next week, before starting to lift out on Wednesday. The next upper-level trough then looks to begin to amplify across the eastern CONUS for Wednesday night into Thursday. At the surface, a stronger cold front moves through Monday night. High pressure then builds in Tuesday night before shifting offshore Wednesday night. Another low pressure system may approach the region by Thursday.

For Monday and Tuesday...This time frame starts as more of a day in transition on Monday as the first cold front is offshore and we await a secondary cold front tied to the amplifying upper-level trough. This second front may end up moving through our area Monday night with little in the way of precipitation as the deeper moisture is shoved offshore with the earlier cold front. Given the strength of the incoming upper-level trough however, cannot ruled out some additional rain showers with the second cold front mainly Monday night or early Tuesday. This could also be in the form of some wet snow across the higher elevations of the Poconos if enough moisture remains. High temperatures Monday should be closer to average. The second cold front should be offshore to start Tuesday with strong cold air advection underway. There will be a tightened pressure gradient in place between departing low pressure well to our northeast and incoming high pressure from the Midwest to the Ohio Valley. This will translate to a gusty wind on Tuesday before diminishing some at night. High temperatures Tuesday look to be several degrees below average. It will be cold Tuesday night with most if not all areas dropping below freezing. There may be to much wind for frost formation, however a freeze warning may be needed where the growing season has started (most of our Delmarva zones).

For Wednesday...The strong upper-level trough is currently forecast to be lifting out with time. This will result in surface high pressure building into our area before it starts to shift offshore at night. There may still be a gusty wind in place before a tightened pressure gradient relaxes as the the center of the surface high builds closer. High temperatures will be several degrees below average once again, and the wind will determine how cold it gets at night despite the air mass especially aloft starting to modify. A clear sky with dew points at or below freezing would typically yield a cold night especially if the wind is able to completely decouple. As a result, there is some risk for frost and/or freeze (the growing season has started for most of our Delmarva zones).

For Thursday...The next upper-level trough looks to begin deepening across the Eastern US and a surface low may begin to approach the region as a result. However, this will be dependent on the strength of the developing low relative to the departing high, so much of Thursday may end up remaining dry. Temperatures do look to begin to moderate though as southerly flow returns.

AVIATION /01Z FRIDAY THROUGH TUESDAY/

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

Tonight...VFR to MVFR with brief periods of IFR possible. Scattered showers and isolated thunderstorms are possible between 03Z and 12Z. Expect mainly MVFR ceilings in this activity, but IFR visibilities will be possible in the heaviest storms. Some LLWS is possible between 03Z and 10Z at KACY and KMIV. Southwest wind around 10 kt, shifting to northwest after 06Z at 5-10 kt. Low confidence in the timing and placement of heaviest storms and lowest flight categories.

Friday...Showers and isolated thunderstorms taper off around 15Z and conditions improve. Ceilings lift to VFR, with perhaps some lingering MVFR ceilings at KMIV and KACY. Northwest wind 5-10 kt with a few gusts around 15 kt. Moderate confidence.

Outlook...

Friday night and Saturday...Sub-VFR conditions probable along with some possible showers.

Sunday...IFR possible in the morning due to low clouds and/or fog, then some improvement however sub-VFR probable at times along with showers and possibly a thunderstorm in the afternoon and especially at night. Southwesterly wind gusts up to 25 knots possible during the day.

Monday...Sub-VFR conditions possible with a slight chance for showers.

Tuesday...VFR with no significant weather.

MARINE

Winds have not been particularly strong over the waters, but seas are still expected to increase, so Small Craft Advisories remain in effect for the ocean waters. Latest guidance holds elevated seas even later into Friday, so extended the SCAs into the morning. Winds may be gusting to 25 kt or so for much of the night, and even when gusts are less than 25 kt, seas will be above 5 feet. Conditions subside to sub-SCA criteria on the ocean by midday Friday. For the bay, sub-SCA conditions should prevail.

SW winds 15 to 20 kt with gusts up to 25 kt will become NW 10 to 15 kt with gusts up to 20 kt Friday morning. Sub-SCA conditions expected after midday Friday.

VSBY restrictions in showers and scattered thunderstorms tonight. A few thunderstorms may be capable of wind gusts in excess of 40 kt. Fog will develop late tonight as well.

Outlook...

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

Saturday...The conditions should be mostly below Small Craft Advisory criteria. Some visibility restrictions possible due to some fog in the afternoon and especially at night.

Sunday...Small Craft Advisory conditions possible. Visibility restrictions possible due to some morning fog, then showers and isolated thunderstorms late in the afternoon and especially at night.

Monday and Tuesday...Small Craft Advisory conditions possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until noon EDT Friday for ANZ450>455.