Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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254 FXUS61 KPHI 252028 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 428 PM EDT Sat Mar 25 2017 .SYNOPSIS... A backdoor cold front will push southward through the region overnight, allowing Canadian high pressure to briefly move into our area on Sunday. Low pressure moving from the Mississippi Valley into the Great Lakes will allow a warm front to move northward through the region on Monday, followed by a cold frontal passage Tuesday night. Canadian high pressure will build into our area Wednesday through Friday. By next weekend, low pressure moving into the Ohio Valley may send a warm front toward the region. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Temps have really warmed up this afternoon to near record levels over southern NJ and Delmarva, showing the power of the late March sun where skies have remained clear except for cirrus. This warmth constrast with mid-40s temps in far northern NJ where much cooler air is funneling down from the lower Hudson Valley, and cloud cover has limited the heating. Radar continues to show bands of light precip, mainly aloft, altho a few spots have reported .01 inch. A more solid band of precip is noted over NY state but it seems to be shrinking with time. Still see pressure rises just to our north even through early/mid afternoon, and thus expect high pressure to continue building in. This will push the cold front south across NJ and eastern PA late this afternoon/evening and across Delmarva later this evening. Temps will fall steadily overnight behind the front and reach mins in the 30s north to 40s south by sunrise. Temps in the higher Poconos and adjacent NW NJ may reach freezing for a couple of hours, which has implications for p-type. Low-level moisture and clouds will increase as winds veer to the NE behind the front. Also, winds above the lowest few thousand feet will be SWly setting up a pattern for overrunning. Otherwise there is now clear mechanism for lift so have kept PoPs in the chance range tonight. Pops increase from evening into overnight and are higher north of PHL. As noted above, there could be some freezing rain or drizzle at higher elevations far north NJ and Poconos, but confidence in both temps and precip is not very high, so no WSW will be issued at this time. Following shifts will monitor the situations. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... For Sunday the forecast area will remain in the cool moist air north of the front. A widespread overcast and winds off the ocean will limit heating and temps will struggle to rise. Daytime max temps will generally be aboutg 20 deg colder than today. The overrunning pattern will continue along with chances for occasional light rain, again with the chances increasing from Delmarva northward thru NJ and eastern PA. Any rain amounts should be limited to a few hundreths of an inch. The GFS is quite a bit more bullish on QPF for the next 24 hours or so but it seems to be overdone. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Unsettled weather will continue through Tuesday night with a return to fair weather Wednesday and Thursday. Some uncertainty remains with regard to freezing rain chances on Sunday night. Then, considerable uncertainty with the next system Friday into Saturday. Temperature-wise, around 10 to 15 degrees above normal from Monday thru Wednesday, falling to around 5 degrees above normal Thursday into Saturday. In terms hydrology, given the above normal temperatures through the long term, the snow pack north of I-78 will continue to melt. The model consensus brings additional rainfall amounts of up to 1.5 inches across this area through Tuesday night with most of this spread out between more substantial precip Sunday night into Monday morning and Tuesday night. We expect rises on area streams and rivers, but no flooding concerns at this time. There remains the potential for spotty minor coastal flooding along the ocean front for the high tides centered on Sunday evening and again near sunrise on Monday. See: Tides/Coastal Flooding section. As low pressure moves through the Ohio Valley into the Great Lakes from Sunday night into Monday, overrunning precip is likely. For the most part, the models remain consistently too warm for freezing rain Sunday night into Monday morning. In addition, boundary layer winds are sufficiently strong enough to advect warm air northward through the low-level column, despite somewhat of an in-situ cold air damming signature. In addition, the lack of low-level dry air will limit evaporational cooling. Given the proximity of the cold air, we cannot rule out freezing rain, but the probability of it occurring is low. Nevertheless, we maintain the wording from the previous shift with regard to freezing rain potential in the HWO. Please note that the most likely ice glaze map on our website only reflects amounts tonight into early Sunday afternoon. Expect showers to continue Monday afternoon into Wednesday, with another round of more substantial precip likely centered on Tuesday night, associated with a cold frontal passage. We do not anticipate any p-type issues during this period. There remains uncertainty with the timing of this front, particularly given the time range, with the GFS being the most progressive. For now, we expect a gradual return to fair weather on Wednesday, continuing into Thursday. There is considerable model uncertainty beyond day five, as the GFS and ECMWF diverge in their handling of low pressure exiting the Mississippi Valley. Initially a southern stream system, the ECMWF phases it with a northern stream system, bringing a period of precip to the entire area Friday night into Saturday. The GFS keeps both streams separate, and takes a southern stream system to our south, but close enough for precip across Delmarva and southern NJ. The GFS does not have support from the UKMET or Canadian, both of which are more similar to the ECMWF. For now, the forecast brings in chance PoPs Friday into Saturday. Also, given the orientation of the surface high, cold air damming is possible. Thickness value indicate the cold air is right on our door step, so p-type could be an issue. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... As of mid/late afternoon all our TAF sites have VFR condition with just a high or mid-level overcast. However, things will be going downhill later this afternoon/evening as a cold front pushes north to south across the region. By 00z winds are forecast to shift to the NE and lowering ceiling will follow as moisture comes in off the Atlantic. MVFR cigs are expected to develop by mid to late evening and IFR cigs are possible by late tonight. Timing is a bit uncertain but confidence in the trend is high. Generally poor conditions will continue through the day Sunday. OUTLOOK... Sunday night thru Monday night...Periods of MVFR/IFR possible in low clouds and fog. Restrictions will be more widespread Sunday and Monday night, while VFR develops on Monday from around I-95 and points southeast. Tuesday thru Thursday...MVFR possible Tuesday morning northwest of I-95, otherwise, VFR thru thru the period. There is the potential for wind gusts up to around 25 knots, out of the south-southwest on Monday, then northwest on Wednesday. && .MARINE... Although sub-SCA conditions currently prevail over the forecast waters, the SCA already in place for later tonight still looks good. A cold front will cross the area north to south this evening, ushering in stronger NE winds. Some wind gust may reach 25 kt tonight and Sunday, but higher confidence is in seas building to 5-6 feet over the SCA area. OUTLOOK... Sunday night...Small Craft Advisory in effect for the NJ waters for combined seas increasing to 5 or 6 feet and scattered gusts 25 kt. Monday thru Wednesday...Seas may remain elevated Monday into Tuesday, and the SCA may need to extended into this period. Quite a bit of uncertainty with regards to winds and seas on Wednesday, which may be near SCA thresholds. Thursday...Sub-SCA. && .TIDES/COASTAL FLOODING... In the wake of a backdoor frontal passage, a 36 hour period of onshore flow is expected through Monday morning, with easterly wind gusts up to 25 MPH at times. Astronomical tides are also increasing, coincident with the New Moon this Monday, March 27. The ESTOFS remains most aggressive out of the guidance suite, with near minor flooding for the sunrise Monday high tide along the DE and NJ oceanfront, and it has has outperformed the other guidance recently. Both the NOS and ETSS are more conservative. The wind will also be trending more parallel (southerly) to the shore by sunrise Monday, so there remains uncertainty with regard to water levels reaching the minor flooding threshold. && .CLIMATE... March monthly average temp: While the negative departures will diminish considerably the next few days...the month as a whole for PHl is currently expected to end up about one half to 1 degree below normal. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM Sunday to 2 AM EDT Monday for ANZ452-453. Small Craft Advisory from 8 PM this evening to 2 AM EDT Monday for ANZ450-451. && $$ Synopsis...Franck Near Term...AMC Short Term...AMC Long Term...Franck Aviation...AMC/Franck Marine...AMC/Franck Tides/Coastal Flooding...Franck Climate...Drag

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