textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Confidence continues to increase regarding showers to rain Friday PM through Saturday PM, with temperatures above normal through Sunday.

KEY MESSAGES

- Showers to rain Friday afternoon through Saturday night with above normal temperatures.

DISCUSSION

KEY MESSAGE 1... - Showers to rain Friday afternoon through Saturday night with above normal temperatures.

High pressure centered over the Mid-Atlantic region today will shift eastward and off the coast on Friday as a low pressure system tracks eastward across the Great Lakes and eventually into portions of Ontario and Quebec Friday afternoon. A subtle warm front associated with this low will lift northward through the region tonight, and southerly flow will increase into the day Friday. With limited cloud cover initially and warm air advection taking place, much above normal temperatures are anticipated across the entire region, generally ranging from 10-15 degrees above normal during the afternoon.

By late afternoon, the first round of anticipated rain chances will begin to arrive ahead of an approaching weak cold front. As of right now, the best chance for showers (40-60%) appears to be from late afternoon into the early nighttime hours. Forecast soundings across the area do indicate some very modest instability (MUCAPE up to around 1000 J/kg), so a rumble of thunder or two cannot be ruled out. No severe weather or flooding concerns are anticipated with this initial wave of showers. The cold front will pass through the region during the overnight hours. It appears increasingly likely that there will be a lull in rainfall activity once this first round of showers moves out, and until more widespread rainfall moves in on Saturday.

Ahead of an upper-level trough this weekend, low pressure develops on a weak front and tracks across our area Saturday and Saturday night. As this occurs, a surface warm front will gradually shift northward, however with a surface low tracking across our area the main warm sector may not shift all that far northward. The northward movement of this warm front will determine high temperatures Saturday and also temperature trends (may have a more non-diurnal temperature curve with high temperatures occurring Saturday evening). The guidance overall seems to be showing more of a cool wedge holding across much of our area, especially along and north/west of the fall line. An area of large scale forcing for ascent arrives during the day Saturday, especially in the afternoon and evening, resulting in rain which should become heavier at times Saturday afternoon and/or Saturday evening. The precipitable water values are forecast to increase up to 1.5 inches Saturday evening. Model forecast soundings overall show even less elevated instability especially given more of a cool wedge setup. While a few rumbles of thunder cannot be ruled out, the risk of thunder looks rather low at this point. The initial cold front moves offshore by later Saturday night and some showers may linger until a secondary cold front arrives Sunday morning. In the wake of the secondary front, stronger cold air advection occurs and the low-level lapse rates steepen. This will result in a breezy Sunday with west to northwest wind gusts peaking in the 30-40 mph range during the day Sunday before diminishing at night. It will turn colder during Sunday but especially Sunday night as low temperatures drop into the 20s for most of the area.

Overall, there is high confidence in showers to rain occurring Friday afternoon through Saturday night. The confidence is lower though with the specific rainfall amounts. Overall, around an inch of rain looks to occur across the region through Saturday night and the flooding risk is minimal to none. Given the ongoing stretch of mild temperatures, ice jams or any creek/river ice should be gone, so runoff should not be hindered in any way.

Outside of the weekend wet weather, no significant weather is expected through next Thursday.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

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

Rest of today...VFR. Wind generally out of the west-northwest at 5- 10 kt, though some variability in direction is expected. High confidence.

Tonight...VFR. Wind quickly becomes lgt/vrb with calm periods likely. High confidence.

Friday...VFR initially, with ceilings trending towards MVFR after 15- 17Z. Approaching 00Z, ceilings are expected to decrease further towards IFR. Showers move in from west to east after 21-23Z. Moderate confidence.

Outlook...

Friday night...MVFR/IFR conditions with low clouds and some showers.

Saturday ..MVFR/IFR conditions with a period of rain, some of which will be heavy at times. Improvement in visibilities and ceilings should occur by later Saturday night.

Sunday...Ceilings improve to VFR through the morning. Some lingering showers could result in some visibility restrictions in the morning. West-northwest winds increasing during the day with gusts up to 30 knots, then diminishing some at night.

Monday and Tuesday...VFR.

MARINE

No marine headlines are currently anticipated through Friday. Tonight, southeast winds around 10 kt are anticipated with seas 1-3 feet. Friday, south-southeast winds will increase to 10-15 kt with gusts around 20 kt. Seas increase to 2-4 feet. There is a slight (around 20%) chance for Small Craft Advisory conditions, but confidence is not high enough to warrant advisory issuance at this time.

Outlook...

Friday night...No marine headlines expected.

Saturday and Sunday...Small Craft Advisory conditions probable. Winds become east to southeast Saturday and increase with gusts to around 25 knots mostly Saturday night. Winds increase some more during Sunday out of the west-northwest with gusts up to 30 knots. There is some potential for a period of low-end gale-force gusts (35 knots) late Sunday afternoon/evening. Seas peak at 4 to 7 feet.

Monday...Small Craft Advisory conditions possible, especially through the morning.

Tuesday...No marine headlines expected.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.