Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 201935 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 235 PM EST Mon Feb 20 2017 .SYNOPSIS...
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High pressure will build over the Mid Atlantic through tonight before shifting offshore Tuesday. A warm front will slide through the region Tuesday night. We may then see a cold front nudge into eastern Pennsylvania and northern New Jersey Thursday night, but it should quickly lift back north as a warm front. As low pressure lifts into eastern Canada this weekend, the associated cold front will sweep through our region on Saturday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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While cooler than the weekend, another mild February afternoon in progress with temperatures running warmer than guidance. Boundary layer warmth continues to win out. As a sharp upper-level trough/closed low continues to exit the New England coast through early this evening, a narrow but high amplitude ridge will approach our area through tonight. This will drive strong surface high pressure over our area during the course of tonight, bringing with it subsidence. The flow aloft will start to switch from north-northwest to southwest toward daybreak as the aforementioned ridge axis nears our region. This will result in increasing warm air advection up the Ohio Valley. The ridge axis is very pronounced, which may limit the amount of high level clouds making it to our area through the overnight within the warm air advection pattern aloft to our west. As a result, we slowed down the gradual increase in some of the high clouds through the night. This introduces more of a challenge regarding the low temperature forecast. The dew points have been dropping this afternoon from north to south due to subsidence and more efficient vertical mixing. This should relax some this evening as the mixing diminishes, however the northerly flow should drop the dew points a bit more especially across the northern areas. For now we stayed closer to a MOS blend, which results in no major changes from continuity. Temperatures may drop quicker this evening as the winds diminish more combined with lower dew points, especially for inland and the more sheltered areas.
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&& .SHORT TERM /TUESDAY/...
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A high amplitude ridge will slide across our area through early afternoon Tuesday, before shifting offshore. This will push surface high pressure offshore, allowing for a southeasterly surface flow to develop. Low pressure up near Hudson Bay will have a trailing cold front southward to the Tennessee Valley, with a closed low tracking across the Gulf Coast states. The flow turns more southwesterly aloft ahead of these features across our region, which will contribute to some increasing warm air advection aloft. An upper-level jet streak is forecast to be moving across the Great Lakes Tuesday, which may help contribute to some mid to high level clouds across our area through the day. Overall, some increase in clouds are expected as we go through the day ahead of a developing warm front. Despite some warming aloft Tuesday, the low-level flow is expected to be out of the southeast which will help keep temperatures cooler. This will keep the immediate coastal locales even cooler given the wind off the chilly ocean. High temperatures were mostly a blend of MOS and continuity, with some tweaks especially along the coast.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
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For a relatively quiet pattern, the long term is shaping up to have quite a bit to discuss. Tuesday night...main question through this period will be how quickly will the surface high and upper level ridge shift off shore. As once that happens, the warm front and rain showers will move into our region. Wednesday and Thursday...Big story through this period will be the warmth! Significant warm air advection pattern is expected to develop. Stayed close to the previous forecast with highs generally in the 60s on Wednesday and in the 60s and 70s on Thursday. This is several degrees higher than any guidance, but with our most recent warm air advection event (Saturday and yesterday), all guidance showed a cold bias anywhere from 5 to 10 degrees for the MOS guidance and up to 15 degrees for the operational runs of the GFS. It looks likely that some more records will be broken, especially on Thursday (climate section below has the record highs for both dates for our climate sites). As for precip through this period, GFS continues to show some QPF over the region on Thursday, but this seems unlikely as we will be at or near the saddle point between three surface lows (one in eastern Canada, one off the coast of FL, and one over the high plains). For now, just have a slight chance across Delmarva. Friday...There remains poor agreement between models and poor run to run consistency with how far south the cold front will get on Thursday night/early Friday. With the 00Z runs, the GFS shows the front briefly sinking into E Central PA and NW NJ Thursday night, before lifting north of the region by mid day Friday. ECMWF shows the front staying well north of the region. This looks to be primarily related to differences in the tracks of the previously mentioned surface low which will be lifting from the High Plains to the Great Lakes region through this time. ECMWF keeps the low on a more progressive and northerly track versus the GFS. Hesitate to make many changes to the forecast given the uncertainty, but if models trend closer to the ECMWF solution, this could mean that the current forecast is too cool (temps could be close to Thursday highs with the ECMWF solution) and too wet (with a further north track, our region would stay in the dry slot through the day on Friday and Friday evening). Will have to continue to watch this as we get closer. Saturday...Regardless of the previously mentioned differences between the models, it looks like the prime period for precip with this low will be Saturday morning and afternoon in the warm sector just ahead of the cold front. The cold front looks to sweep through our region later in the day (meaning we should have warm air advection through much of the day time hours). In the upper levels, it looks like we will have a jet over our region through this period. Looking at the GFS model soundings through this period, instability is lacking, but there are large bulk shear values. As mentioned previously, the operational runs of the GFS have had a very strong cold bias, so accounting for that, we could see modest instability during the day on Saturday. If all of this timing holds, and if we do warm more than what the GFS is depicting, then it could set the stage for strong thunderstorms. There are still too many what ifs to include in the hazardous weather outlook at this time, but we will continue to monitor as we get closer. Sunday...strong cold air advection behind the cold front could mean the first day in over a week of near normal temperatures. Could see lake effect snow showers propagate into the Poconos while we have strong northwesterly flow, but that doesn`t look to last too long as a surface high should build over the region late in the day resulting in light boundary layer winds.
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&& .AVIATION /20Z MONDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of this afternoon...VFR. North-northwest winds 10-15 knots with some local gusts to 20 knots at times, diminishing toward early evening. Tonight...VFR. North to northeast winds 5 knots or less, or becoming locally light and variable. Tuesday...VFR with some increase in clouds mainly above 15000 feet. Winds becoming southeasterly during the morning at 10 knots or less. Outlook... Tuesday night...Ceilings could lower to MVFR with some rain showers at night. Wednesday and Thursday...Mostly VFR conditions are expected. Friday...Starting VFR, however could lower to MVFR if rain showers move in earlier than forecast.
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&& .MARINE...
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The conditions are expected to remain below Small Craft Advisory criteria through Tuesday. High pressure will slide across our area tonight before moving offshore during Tuesday morning. This will result in northwest to north winds with gusts up to 20 knots to diminish, and turn northeast to east before becoming southeasterly Tuesday. There could be somewhat of an enhancement to the southeast flow Tuesday nearshore and up Delaware Bay, however winds are expected to remain under 20 knots. Outlook... Tuesday night through Friday...Winds and seas should stay below SCA criteria. A gradual shift from southeasterly to southwesterly is expected Tuesday night. By Thursday night, winds could be light and variable shifting to southeasterly on Friday.
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&& .CLIMATE... Another significant warm-up expected later this week. The record high temperatures for Wednesday and Thursday are listed below. Wednesday 2/22 Thursday 2/23 ACY 68-1991 72-1985 PHL 68-1997/1974 75-1874 ILG 69-1997/1974 72-1985 ABE 68-1974 71-1985 TTN 66-1997 74-1874 GED 72-1997 no data RDG 70-1974 72/1932/1922 MPO 56-1997 60-1977 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse/Johnson Near Term...Gorse Short Term...Gorse Long Term...Johnson Aviation...Gorse/Johnson Marine...Gorse/Johnson Climate...

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