textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Aviation section for 00Z TAFs.

A Flood Watch remains in effect for the I-95 corridor, northern NJ, and the Lehigh Valley until 11 PM Monday.

Increasing confidence in an unsettled pattern beyond Tuesday.

KEY MESSAGES

1. Showers and thunderstorms increase in coverage tonight into Monday with the potential for areas of heavy rain leading to a risk of flash flooding. There is also some risk for strong to severe thunderstorms.

2. A warm front is expected to lift through the region Tuesday Night into Wednesday, bringing the next chance for showers and storms.

3. For the later half of the week, we will have a warm, humid, and at times stormy pattern in place.

DISCUSSION

KEY MESSAGE 1...Showers and thunderstorms increase in coverage tonight into Monday with the potential for areas of heavy rain leading to a risk of flash flooding. There is also some risk for strong to severe thunderstorms.

An upper-level trough is moving into the Northeast through Monday. As this trough moves into the Northeast, there will be plenty of upper-level energy moving through the region. There is also a warm air advection set up into tonight. For Monday, there will be a jet streak overhead with our location in the right entrance region. All of these factors will lead to a period of unsettled weather in the near term.

As we head into tonight, an increasing south to southwest flow will continue to bring in increasing moisture with PWAT values tonight surging to 2+ inches tonight. The coverage of showers and thunderstorms will also be increasing with the warm air advection regime allowing for an increase in ascent. Overnight, instability will also be increasing, but the timing is not very supportive of severe weather. Through tonight, there is a Marginal (level 1 out of 4) risk for excessive rainfall. Model soundings continue to show this risk into tonight as well with the high PWATs and warm cloud layer processes.

On Monday, a cold front will be approaching the region. There continues to be a signal for ripples of energy along the front. Also, in the upper-levels, a jet streak will be overhead with our area in the right entrance region which allows for additional forcing and ascent for our area. Showers and thunderstorms look to be ongoing into tomorrow morning with an increase in the coverage again during the afternoon and evening.

As PWATs continue to remain above 2 inches ahead of the front, the risk for heavy rain continues. The overall flow will be weaker tomorrow with training and slow moving storms being a concern leading to the heavy rainfall risk. There is a slight (level 2 out of 4) for excessive rainfall for tomorrow for areas near and east of the I-95 corridor. The remainder of the area is in a Maringal (level 1 out of 4) risk for excessive rainfall. Due to the risk for heavy rainfall tonight and into tomorrow, a Flood Watch has been issued for the I-95 corridor, northern NJ, and the Lehigh Valley for the potential of rainfall rates of 1-3 inches per hour. Rainfall amounts of 1-2 inches are possible with localized amounts to 3-5 inches. This Flood Watch is in effect from 8 PM this evening until 11 PM Monday.

In terms of the severe weather risk tomorrow, uncertainty remains around the exact extent of instability present due to cloud cover and the round of showers/thunderstorms around tomorrow morning. If enough heating develops during the course of the day, then this combined with some increase in shear ahead of the front could lead to at least some severe thunderstorm risk. The Storm Prediction Center has highlighted areas near and northwest of the I-95 corridor in a Marginal risk (level 1 out of 5) for isolated severe thunderstorms with the primary risk being damaging wind gusts.

KEY MESSAGE 2...A warm front is expected to lift through the region Tuesday Night into Wednesday, bringing the next chance for showers and storms.

Once the current system gets east of our region, the next thing to watch is a warm front slowly lifting north over our region. The most likely time period for this frontal passage is Tuesday night into Wednesday, leaving the day time on Tuesday relatively dry. However, at least one model is showing a very fast warm frontal passage with the front north of our region by late Wednesday Night. In this fast solution, the pattern is odd, depicting a very pronounced mid and upper level short wave ridge in place throughout the day. This pattern isn't favorable for fast frontal passages. Therefore, have favored the blend of guidance, which favors the slower solution.

As far as hazards, the primary hazard is likely to be heavy rain leading to flash flooding. There will likely be significant low level moisture advection behind the front, increasing the risk for heavy rain. That combined with the fact that storm motions are likely to be quite slow (less than 20 mph), increases the risk for flash flooding. One factor which we won't know until late Monday is how much antecedent conditions will play a role in the flash flooding threat - areas that get heavy rain through the day on Monday will have little opportunity for the water to drain before this next round of rain chances.

The severe threat is less clear. This will be dependent on the timing of the front - if the frontal passage, and thus prime period for any showers and storms is during the daylight hours when instability is maximized, there will be some threat for damaging winds. Some model soundings show a very dry mid layer, increasing the threat for downbursts.

KEY MESSAGE 3...For the later half of the week, we will have a warm, humid, and at times stormy pattern in place.

Once the mid week warm front moves north of our region, we will likely be sandwiched between a Bermuda centered well to our east, and a nearly stationary mid level trough axis centered over the Western Great Lakes region.

The net result of this is that our region will have warm and moist southwesterly flow in place for multiple days. As such, expect hot and humid conditions. Additionally, there could be daily rain chances, from two different sources. First, if there is any shower or storm development on the sea breeze in the afternoon. Secondly, there could be some embedded very small short wave troughs that propagate northeastward over our region during this time.

AVIATION /00Z MONDAY THROUGH FRIDAY/

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

Tonight...MVFR to IFR. Ceilings continue to lower overnight leading to all terminals dropping to MVFR and then gradually to IFR. The coverage of showers and thunderstorms increases overnight and has been handled by SHRA with some TEMPO groups focusing in on thunderstorms. Visibility restrictions are also expected with any showers or thunderstorms impacting the terminals. Low confidence for the convection with medium confidence on the ceiling restrictions.

Monday...IFR to MVFR. During the morning, ceilings look to lift a bit to MVFR. There is VCSH at the Lehigh Valley terminals given shower coverage. The remainder of the terminals have prevailing SHRA and some PROB30 groups, mainly 18Z-21Z for potential thunderstorms. Visibility restrictions are also expected with any showers or thunderstorms impacting the terminals. Low confidence for the convection with medium confidence on the ceiling restrictions.

Outlook...

Monday Night and Tuesday...Prevailing VFR. Some lingering showers and storms Monday Night may briefly lower conditions to MVFR.

Tuesday Night through Friday...Periods of MVFR or lower conditions possible, primarily in the afternoon and evening periods each day, due to showers and storms.

MARINE

A Small Craft Advisory remains in effect for all of the nearshore Atlantic coastal waters from 8 PM this evening through 6 AM Tuesday. South flow increases tonight into tomorrow with wind gusts up to 30 kt. Seas also increase to 5-8 feet. Some gusty thunderstorms are possible tomorrow, particularly in the afternoon and evening.

Outlook...

Monday Night...lingering elevated seas are possible through the evening hours, but should eventually drop off below 5 FT.

Tuesday and Wednesday...winds and seas should stay below SCA criteria.

Thursday and Friday...Currently forecasting winds and seas to stay below SCA criteria, but wind gusts near 25 kt are possible both days.

Rip Currents...

There is a HIGH risk for the formation of dangerous rip currents along the NJ coast on Tuesday due to long period swells with surf heights near or above 3 feet. For the Delaware Beaches, there is a MODERATE risk for the formation of dangerous rip currents.

Looking ahead to Tuesday, surf heights should decrease, so the risk for the formation of dangerous rip currents is expected to be MODERATE along the NJ coast, and LOW for the Delaware Beaches.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch through Monday evening for PAZ060>062-070-071- 101>106. NJ...Flood Watch through Monday evening for NJZ001-007>010-012>019. High Risk for Rip Currents from Monday morning through Monday evening for NJZ014-024>026. DE...Flood Watch through Monday evening for DEZ001. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ450>455.