textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Confidence continues to ramp up that the area will be affected by a significant storm system beginning Saturday night and potentially continuing all the way into the day Monday. Forecast models continue to trend farther north with the track of this low increasing the probability of the area seeing significant precipitation amounts. While the probability of at least moderate snow amounts is still high for much of the area, if this trend north continues any further southern parts of the County Warning area over portions of our Delmarva zones into southern NJ may even see a wintry mix at the height of the system late Sunday. Therefor it's still uncertain where the heaviest snow amounts will fall. See below for more details.

KEY MESSAGES

1. Bitterly cold conditions will continue this morning.

2. Artic air with bitterly cold temperatures and wind chills return to the area late Friday through Saturday and beyond following a brief warm up this afternoon through Thursday.

3. Confidence continues to increase the area will be affected by an impactful winter storm beginning Saturday night and potentially lasting all the way into Monday. Portions of the area are likely to see moderate to heavy snowfall but there remains uncertainty on where the heaviest amounts will fall.

DISCUSSION

KEY MESSAGE 1....Bitterly cold conditions will continue this morning.

Arctic high pressure is currently centered over the Mid-Atlantic. Skies are clear and winds are nearly calm across the entire area. The result has been ideal conditions for radiational cooling. Temperatures may drop a few degrees further still through daybreak, with most locations ranging from near 0 to around 10 degrees, and slightly warmer in the urban corridor.

The good news is that the high pressure will begin to slide east by this afternoon, allowing for southerly flow to begin increasing across the area. This will bring the bitterly cold temperatures to an end for now.

A weak disturbance does look to pass through the region this afternoon and into this evening, and some model guidance is hinting at the development of a few associated snow showers across our northern zones. Slight chance (20%) PoPs have been introduced for portions of the Poconos, Lehigh Valley, and adjacent portions of far northern NJ. No accumulation is expected.

KEY MESSAGE 2...Artic air with bitterly cold temperatures and wind chills return to the area late Friday through Saturday and beyond following a brief warm up this afternoon through Thursday.

Following briefly milder weather Wednesday and Thursday, arctic air will build back into the region by late Friday through Saturday as a strong and large area of artic high pressure moves in across the Great Lakes. Lows Friday night will be back into the single digits to low teens with highs Saturday only in the teens to around 20. Minimum wind chills during this period will likely get down to the low single digits to as low as the -10s (Poconos) Friday night into Saturday morning meaning cold weather advisories are likely to be needed. This overall very cold weather pattern is likely to continue into next week and potentially beyond.

KEY MESSAGE 3...Confidence continues to increase the area will be affected by an impactful winter storm beginning Saturday night and potentially lasting all the way into Monday. Portions of the area are likely to see moderate to heavy snowfall but there remains uncertainty on where the heaviest amounts will fall.

Low pressure will begin organizing itself near the Texas Gulf coast Saturday. This will occur as several pieces of upper level energy dive southward into the base of the persistent long wave trough dominating over the CONUS. Saturday night into Sunday these pieces of upper level energy will to at least some degree "phase" with one another helping to drive deepening surface low pressure through the southeastern states then north and east towards roughly around the Virginia coast by late Sunday. Forecast guidance has continued to trend northward with the placement and the track of the low due to better phasing of upper level energy as just noted. However there continues to be some uncertainty regarding more specific details and this will ultimately affect the storms track, speed, and strength. This will impact where the heaviest precip falls along with precip types. Nevertheless, confidence has grown that much of the area has a good chance of seeing moderate to even heavy snowfall totals with this system. The NBM probabilities of 6+ inches with this storm now range from around 60 percent over the southern Poconos to the 80 to 90 percent range near and south of the urban corridor. Guidance is also in good general agreement that total QPF amounts with the system have the potential to be 1+ inches in many areas. The main challenge is again, the track of the low which determines who gets the heaviest precip and if we actually get into any mixing limiting snow amounts. The GFS has trended north putting much of the area in the "sweet spot" for very heavy snow amounts. However the GEM and ECMWF have trended north as well and now track the low close enough to Delmarva that mixing may occur at the height of the storm late Sunday. Regardless, expect snow to most likely start breaking out over the area Saturday night into Sunday morning. All areas look to certainly be cold enough initially to see all snow. In fact lows Saturday night look to once again be down into the single numbers north with teens to around 20 farther south.

Heading into the day Sunday, expect snow will be continuing across the area and may fall heavy at times along with NE winds starting to increase, especially along the coast. Late day Sunday into Sunday night is when southern portions of the CWA (mainly Delmarva into adjacent portion of south Jersey but potentially as far north as Philly) could see some mixing if the low takes a close enough track to the area. However given the strong artic high and antecedent cold airmass in place, plain rain looks less likely than a mix with or change to sleet and freezing rain if any mixing does in fact occur. If this mixing does occur for a time, a change back to all snow is likely later Sunday night into early Monday before precip winds down across the area into Monday.

As already mentioned, a lot of uncertainty remains regarding total snow amounts. 24 hours ago this looked like more of a question of a near "miss" to the south vs. a big snowstorm. Now it's looking like a more or less a lock that this storm will affect the area with significant precip with the question then shifting to whether the track actually gets too far north to bring a mix or changeover into the area. If this occurs, the very heaviest snow amounts may end up near or even north of the urban corridor vs coastal and southern portions of the area. We say this while stressing the fact that the very heaviest snow amounts within these type of large systems are still often only 100 miles wide or less. S/E of the snowfall max you get ice or rain vs. N/W precip amounts are lighter. For this reason, it will still take at least a couple more forecast cycles to have a clearer idea on the details of where any "blockbuster" snow amounts could occur within our County Warning area.

AVIATION /08Z WEDNESDAY THROUGH SUNDAY/

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

Through 12Z...VFR. Wind light and variable with calm periods likely. High confidence.

Today...VFR. Light and variable to calm wind becoming south around 5 kt during the morning, then 10-15 kt with gusts around 20 kt in the afternoon. High confidence.

Tonight...VFR. South-southwest wind 5-10 kt with low- level wind shear up to 40 kts. Moderate confidence.

Outlook...

Thursday through Friday night...VFR. No significant weather expected.

Saturday through Sunday...VFR conditions early on Saturday, likely to become sub-VFR by Saturday night and into Sunday. Periods of snow likely.

MARINE

A Small Craft Advisory has been issued for all NJ and DE Atlantic Coastal Waters from 1 PM this afternoon until 7 AM Thursday.

Light wind this morning will become southerly this afternoon and increase to 15-25 kt with gusts as high as 30 kt. Winds will shift more southwesterly overnight tonight but remain elevated. Seas 1-3 feet this morning increasing to 4-6 feet by late this afternoon and continuing into the night before beginning to diminish.

Outlook...

Thursday through Friday night...VFR. No significant weather expected.

Saturday through Sunday...VFR conditions early on Saturday, likely to become sub-VFR by Saturday night and into Sunday. Periods of snow likely.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 1 PM this afternoon to 7 AM EST Thursday for ANZ450>455.