Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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345 FXUS61 KPHI 220530 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1230 AM EST Mon Jan 22 2018 .SYNOPSIS... A low pressure system will propagate from the southern Plains to the Great Lakes region into Tuesday. A warm front will be gradually moving north across our area later tonight and Monday. As the low moves into southeastern Canada, a cold front is expected across our area Tuesday. High pressure will then build across the region for the end of the week. Another low pressure system may move into the area late next weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 1230 am update: Made quite a few changes to the forecast early this morning. Temperatures were running about 4-8 degrees too low in Delmarva, so bumped these up substantially for the rest of the night. Current thinking is that the low temperatures are probably too low across the whole area tonight, but am not confident enough to increase them at this time. I also expanded the region of patchy fog/drizzle to include most of the area to the northwest of I-95 based on recent surface obs and radar trends. Starting to see visibilities go down at the especially susceptible valley sites northwest of the Fall Line, and sprinkles/drizzle have been reported occasionally at Morristown, Mount Pocono, and Allentown. See no reason to think this will change much overnight; if anything, conditions will deteriorate somewhat by daybreak. Will keep an eye on surface obs to determine if locally denser fog develops. Fortunately, temperatures look much too warm for any freezing fog/drizzle to develop. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/... 1230 am update: Increased temperatures across the area tomorrow based on latest statistical guidance, and generally reduced PoPs for much of the area during the day. Primary sensible weather in the northern CWA will likely be patchy fog or drizzle, and this area should retreat northward somewhat as the warm front moves a little poleward during the day. Previous discussion... An potent closed low gradually moves toward the Great Lakes during Monday. This will continue to drive strengthening low pressure also closer to the Great Lakes toward late Monday afternoon. This system will also be driving a warm front northward. The eastward extent of the warm front is forecast to be shifting northward during the day Monday, and with a lack of true cold air damming this boundary should be able to steadily shift northward. In addition, flow above the surface will be southerly and strengthen through the day also helping to push the warm front north. There should be low clouds and some fog north of the surface warm front, and this however could slow its northward progress to some extent. Any light rain and/or drizzle north of the warm front should be ending in the morning. The warm front though may actually be more pronounced aloft given the current airmass in place is not all that cold. The movement of the warm front and also some clearing will determine the amount of warming that is realized. For now, we used a model blend which placed the warmest temperatures near and south/east of I- 95. && .LONG TERM /TONIGHT THROUGH SUNDAY/... Monday night and Tuesday...The main changes through this period were a slightly later arrival of the front and a further east placement of the southerly jet ahead of the front. This southerly jet should be propagating east through the morning, off shore by late morning. With the later arrival of the cold front, expect a longer period of low level warm air advection in the morning, allowing for a brief period of elevated instability to develop for much of the area. Have expanded the mention of thunderstorms across more of the area. There remains uncertainty if instability will develop before the jet is further off shore. If we do have convection at the same time we have the low level jet, gusty winds are possible with any storms that develop. However, it would have to be very efficient mixing to get severe gusts at this surface. This seems very unlikely to happen, so will not mention any potential for severe storms in the HWO at this time. Precipitable water values could be over one inch, which is certainly above normal for late January, but still considerably less than the 1.46 inches which was observed in the KIAD sounding on the 12th. Additionally, very fast storm motions should limit any flooding threat. The low (and consequently the warm conveyor belt) is expected to be far enough to the NW that we shouldn`t see any precipitation behind the front. Wednesday through Saturday...A large surface high slowly builds east through this period, over our region and eventually off shore. We should start the period below normal thanks to the Tuesday cold front, but could see temperatures moderate as low level flow becomes southwesterly late in the week. Sunday...Another low pressure system lifting into the great lakes region looks to bring the next chance for rain and next cold front either Sunday or Monday. Through this time, it looks like most, if not all of the precip should be ahead of or with the front, so expect all rain at this time. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Current thinking is that CIGs will predominantly remain VFR through the night, but VSBYs may lower to MVFR at RDG/ABE with patchy fog/drizzle. Remaining sites will likely continue to see VFR, though cannot rule out transient sub-VFR VSBYs at TTN and ILG. Winds nearly calm. Overall confidence is below average. Monday...Any sub-VFR conditions should gradually become VFR across the area by 15Z. Winds light/variable becoming predominantly south or southeast during the afternoon (but remaining below 10 kts). Overall confidence is average. Outlook... Monday night...conditions lowering to IFR or less with low ceilings and visibility restrictions due to rain. Southerly winds of 10 to 15 kt are expected. A period of low level wind shear (LLWS) is likely to develop late Monday night as a strong low level southerly jet develops, though exact timing and location of the jet is uncertain for now. Tuesday...Starting IFR or lower Tuesday morning, but improving to VFR conditions behind a cold front expected during the day Tuesday. Also expect an abrupt wind shift from southerly to westerly winds with the cold front. Wind speeds of 10 to 20 kt with higher gusts will be possible especially just behind the cold front. LLWS conditions may linger into Tuesday morning along the coastal plains. Timing of the front, and consequently improving conditions is uncertain at this time. Tuesday night through Friday...VFR conditions expected. Westerly or northwesterly winds less than 15 kt. Gusts above 20 kt possible during the day Wednesday and Thursday. High confidence. && .MARINE... The conditions are expected to remain below Small Craft Advisory criteria through Monday. The winds continue to shift from a more southerly direction this afternoon and this will continue through Monday. This southerly flow will increase some though later tonight and Monday from south to north, however any gusts are expected to remain below 25 knots. Outlook... Monday night into Tuesday morning...Southerly winds will be increasing late Monday night. By sunrise, expect southerly winds gusting to at least 30 KT. Gale force winds are likely on the New Jersey and Delaware coastal waters primarily after sunrise Tuesday morning. Late Tuesday through Thursday...An abrupt shift to westerly winds with gusts above 25 kt is expected behind a cold front moving through late Tuesday. There is a small chance that gale force conditions develop again on Wednesday on the coastal waters. SCA conditions are likely to continue into Thursday. Friday...Winds and seas are expected to stay below SCA criteria. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Tuesday morning through Tuesday afternoon for ANZ450>455. Small Craft Advisory from 4 AM to 1 PM EST Tuesday for ANZ430- 431. && $$ Synopsis...Johnson Near Term...CMS Short Term...CMS/Gorse Long Term...Johnson Aviation...CMS/Johnson Marine...Gorse/Johnson

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