Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 212237 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 637 PM EDT Tue Mar 21 2017 .SYNOPSIS... A cold front will slide through the area tonight. A large area of high pressure then builds in from the northwest and stays over our region through Thursday before shifting off shore. A warm front is forecast to lift through our region on Friday. Low pressure is expected to approach from the west over the weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 630 pm update: Next in the series of northwest-flow systems is set to move through the Northeast during the next 24 hours with a weak predecessor perturbation ejecting eastward through the Ohio Valley into the Mid-Atlantic region by late this evening. Associated large-scale lift should generally remain south of the CWA, but high-resolution guidance is suggesting the potential for a sprinkle or two in southern Delmarva around or after midnight. Not nearly enough confidence at this point to raise PoPs, nor would any precip amounts justify raising them anyway. Main effect in our area is a noticeable increase in clouds this evening, especially south of I-76. This will likely have some impact on the temperature forecast, lowering confidence in projections this evening somewhat. The main vort max digs through the Great Lakes overnight into the Northeast on Wednesday, with a strong cold front sweeping through the region late tonight. Winds will pick up noticeably after passage, and that will be the cue marking the end of our brief taste of spring early this week. Made some minor changes to the hourly temperature/dew point forecast through the evening hours based on latest trends. Otherwise, going forecast is in good shape. Previous discussion... Mostly quiet conditions much of tonight before a strong cold front crosses the area. The front should cross the northern areas 3 - 5 a.m. and reach the southern parts of the forecast areas 5 - 7 a.m. No precipitation and little in the way of cloudiness is expected. Temperatures will drop into the 20s up across the north and dip into the low/mid 30s over south NJ and se PA. The front will not reach Delmarva before morning, so readings there will only drop into the mid/upper 30s. Winds will be light before the front and then gust to 20-30 mph from the NW behind it. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Strong high pressure will build towards the area Wednesday. A strong and gusty NW wind with dry air will be across the area through the day. Skies will be clear and temperatures will be below normal. Highs will only be in the low/mid 40s over Delmarva and the 30s elsewhere. Wind chill values will will only be in the 20s in many areas for the afternoon. Winds will gust 30-40 mph at times. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... The long term will start off tranquil, but get complicated quickly! Thursday...With the center of the high passing almost directly over the region, Thursday should be a quiet and tranquil day albeit colder than normal. Temperatures will generally be 10 to 20 degrees below normal, with some record lows possible Thursday morning (see climate section below). Thursday night into Friday...This is where the forecast starts getting complicated. A warm front is expected to lift through the region during the day on Friday (a bit slower than what most models were depicting yesterday). At the same time, a broad upper level ridge will be approaching from the west. So we could potentially have mesoscale lift (associated with the front) in the lower levels, while some subsidence is possible in the upper levels. At this point, do not expect that as the ridge appears to be trailing far enough behind the front that the two will not be juxtaposed. Further complicating matters is the fact that the boundary layer will be quite dry leading into this event, with dew points likely starting in the teens and lower 20s across much of the region early Friday morning. If we see enough lift out ahead of the front, we could see a brief period of freezing rain (and possibly some sleet) generally north and west of the I95 corridor. Having said that, there have been a few events this winter where models were too fast with warm air advection across the Poconos and NW NJ, so they have the highest risk for multiple hours of freezing/frozen precipitation, but still considerable uncertainty. Saturday...Warm air advection continues. The one possible negating factor could be persistent cloud cover through the day. However, will continue to go on the high side of guidance as most guidance has had a consistent cold bias with southerly flow patterns. Saturday night into Monday...Good agreement that a low pressure system will be sliding northeast out of the southern Plains into the Great Lakes region through this time. That is about the extent of what I have high confidence in. Less confidence, though there appears to be better model agreement today, that the low will be weakening/filling as it propagates northeast. What I have very low confidence in, a front is expected to slide south as a cold front Saturday night, then stall, and lift north as a warm front Sunday night. How far south this front will get before stalling could have a big impact on temperatures (especially with the highs on Sunday) and precip type Sunday night into Monday morning. At this point, most model solutions show the front stalling somewhere between Philly and the Delaware Water Gap. For locations that stay south of the front through the entire event, expect another very warm day on Sunday, and all rain. Locations north of the front can expect temperatures as much as 15 degrees below normal on Sunday, and in the higher terrain, a wintry mix is possible Sunday night into Monday morning. For now have stayed close to a mean of model solutions given the high uncertainty. Tuesday...Another low may develop along what is left of the previously mentioned front. Models depict little upper level support and it should fill and weaken as well as it propagates east. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Clearing has continued to progress across the terminals this afternoon. Skies will only feature some high level clouds this evening and much of the overnight. A few low clouds with a sharp cold front which will cross the area towards dawn. I have only put few-sct coverage for any low clouds overnight. Winds will be light W/NW into the overnight, then winds will sharply increase after fropa. The timing of the front is roughly 09z West and 11Z east. Winds will become NW at 15 to 20 knots with gusts 25-30 knots. These winds will continue into Wed with mostly SKC. Outlook... Thursday...Mostly VFR conditions are expected. Friday...MVFR or lower conditions are possible with precipitation generally along and northwest of the Delaware Valley (including PHL, ILG, PNE, TTN, ABE, and RDG). For ABE, RDG, and TTN, there may be a brief period of a wintry mix Friday morning. Friday night through Saturday...mostly VFR conditions are expected. Saturday night and Sunday...MVFR or lower conditions are possible with rain across the region. && .MARINE... Nice conditions on the waters this afternoon will continue through much of the overnight. A sharp cold front will cross the waters well after midnight and cause winds to shift to NW and increase. Gale warnings are in effect for the period after this frontal passage as winds will probably gust 35 to 40 knots at times. We expanded the Gale to cover Delaware Bay and the Delaware coastal waters. Cold air and decent flow aloft should have no problem mixing down. No precipitation is expected tonight or Wednesday. Outlook... Wednesday night...winds and seas will diminish through the overnight hours. Thursday...Once wind gusts drop below 25 kt early in the day, sub-SCA conditions are expected. Friday...winds may approach 25 kt leading to SCA conditions Saturday...sub-SCA conditions expected. Sunday...there is a chance wind gusts could build above 25 kt late in the day. && .FIRE WEATHER... Relative humidity levels will drop behind the front Wednesday. Gusty winds will cause near Red Flag conditions during the afternoon. After calling the fire weather partners, it seems that soil moisture is still rather high in many areas. The Delmarva is the only exception where conditions may be rather marginal for RFW. We will probably issue an SPS overnight to highlight these dry windy conditions. && .CLIMATE... More record events possible this week. One or 2 of the following are vulnerable record equaling lows Thursday morning March 23, primarily ABE. Others are listed for reference ACY-13 1875 PHL-14 1885, 1875 ILG-14 1934 ABE-13 1934 TTN-13 1934 GED-16 1959 RDG-14 1906 MPO- minus 8 1912 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning from 6 AM to 11 PM EDT Wednesday for ANZ430-431- 450>455. && $$ Synopsis...Johnson Near Term...CMS/O`Hara Short Term...O`Hara Long Term...Johnson Aviation...Johnson/O`Hara Marine...Johnson/O`Hara Fire Weather... Climate...

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