textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

With the 00Z guidance run, there was a slight eastward track trend for the Saturday off shore low system.

There is an emerging risk for elevated fire weather conditions on Sunday.

Mid week next week, a cold front could bring the next change for widesperad precipitation.

KEY MESSAGES

1. Patchy frost will develop in the southern Poconos early this morning.

2. High pressure passes south of the region today, then weak low pressure passes through the region tonight.

3. An off shore low Saturday into Saturday Night may bring a little rain to coastal areas.

4. Elevated fire weather conditions are possible on Sunday.

5. A cold front Wednesday into Thursday could bring the next chance for widespread precipitation.

DISCUSSION

KEY MESSAGE 1...Patchy frost will develop in the southern Poconos early this morning.

A cold front continues to move offshore. Skies will clear out and northwest winds currently around 5 to 10 mph will diminish to less than 5 mph through daybreak. Temperatures in the southern Poconos will drop into the lower 30s, and with dew points not much lower, relative humidity values will approach 100%. As a result, patchy frost is likely to develop. A Frost Advisory remains in effect for Carbon and Monroe counties. A light icing on sensitive vegetation and or some patchy ground fog is possible.

KEY MESSAGE 2...High pressure passes south of the region today, then weak low pressure passes through the region tonight.

Surface high pressure is centered over West Virginia and will move off the Mid-Atlantic coast later this afternoon. Mostly sunny conditions on tap for today with highs generally in the mid to upper 60s, which are a few degrees shy of normal for this time of year.

Weak low pressure approaches from the west, and clouds will increase and thicken late this afternoon and early this evening. Some light rain will develop with its passage late tonight, but rainfall will be minimal. Generally less than 1/10 inch.

KEY MESSAGE 3...An off shore low Saturday may bring a little rain to coastal areas.

The There was a slight eastward trend in the track of the off shore low with the 00Z guidance run. If this trend holds, then rain with this system may be confined to the coastal counties. Since this is a relatively new trend, stayed close to the previous forecast and the blend of guidance which has rain chances through the I-95 corridor. However, if this trend continues, rain chances will decrease significantly for inland areas.

The other implication is that if this trend continues, winds won't be as breezy as previously forecast (at least for the day on Saturday), as the tightest pressure gradient with this system will be well off shore.

KEY MESSAGE 4...Elevated fire weather conditions are possible on Sunday.

As the off shore low slides further away from our region, a surface high is expected to build over the southeastern U.S. on Sunday. This will result in an increasing pressure gradient and northwest flow for our region. This type of pattern suggests a very deep mixed layer and dry air advection. In fact some model soundings show a mixed layer all the way to 775 mb!

Models (and consequently blends of models) tend to have a high Dewpoint bias with such patterns, especially in the Spring. Consequently, have included dewpoints that are on the lower side of guidance during the day on Sunday. Even with temperatures which will be 5 to 10 degrees below normal, this would still result in relative humidity values at or below 30 percent.

With the deep mixed layer and the pressure gradient, breezy conditions (with wind gusts at or above 20 mph) are likely. The combination of the low relative humidity and gusty winds could result in rapid fire spread, if fuels are dry enough.

KEY MESSAGE 5...A cold front Wednesday into Thursday could bring the next chance for widespread precipitation.

The next chance for widespread precipitation across our region is Wednesday into Thursday as a cold front approaches the northeastern U.S.. Some models show a cold front approaching on Wednesday, then stalling near or west of our region, before a secondary cold front finally crosses through our region on Thursday, while other models depict the initial cold front sweeping through our region entirely Wednesday or Wednesday Night.

On the synoptic scale, this appears to be a split flow pattern setting up, with the subtropical jet potentially as far north as our latitude. Given that, and the fact that this is still in the day 6 and 7 time frame, there is low confidence in any one solution. However, regardless of the evolution of the cold front, there should at least be a brief window for widespread rain as it crosses through our region.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

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

Early this morning...VFR. Light NW winds. High confidence.

Today...VFR. NW winds 5 to 10 kt, become S-SW this afternoon. High confidence.

Tonight...VFR. Some light RA with minimal, if any, restrictions. Light S-SW winds. Moderate confidence.

Outlook...

Saturday and Saturday Night...prevailing VFR. A brief period of MVFR ceilings possible, primarily at KACY.

Sunday through Tuesday...VFR conditions expected.

MARINE

Sub-Small Craft Advisory conditions today and tonight. Northwest winds 10 to 15 kt this morning will become south to southwest late this afternoon and tonight. Seas will average 3 to 4 ft.

Outlook...

Saturday...Winds and seas should stay below SCA criteria

Sunday...Winds above 20 kt likely, but less confident that winds will reach 25 kt.

Monday through Tuesday...Prolonged and widespread SCA conditions (due to both wind and waves) are likely. There is a small chance (less than 20%) for winds to get to gale force, primarily on Monday Night.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Frost Advisory until 8 AM EDT this morning for PAZ054-055. NJ...None. DE...None. MD...None. MARINE...None.