Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 181717 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 117 PM EDT Thu May 18 2017 .SYNOPSIS... Broad high pressure over the western Atlantic will continue to influence our area today. A cold front will progress southward through the Mid-Atlantic on Friday, with cooler Canadian high pressure settling into the Northeast this weekend. The boundary will return northward as a warm front Sunday night. A cold front will sweep through the Eastern Seaboard on Monday. Another system may affect the area by the middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 930 am update: Temperatures are warming quickly this morning, with most readings about ten degrees higher than this time yesterday, mainly owing to the warmer low temperatures we had this morning. Still expect max temperatures to be about a degree or two warmer than yesterday`s highs. Made a few tweaks to hourly temperature grids, mainly to account for this morning`s trends, but changed max temps very little across the area. Only exception was near the coast, where current temps are running much warmer than forecast. Cooled readings in these areas a few degrees, especially through the morning hours. Only other change was to modify sky cover grids this morning based on latest satellite trends and near-term extrapolation. Otherwise, forecast is in good shape. Previous discussion below... A Bermuda high will keep us in this summer-like pattern as broad southwesterly return flow around the northwestern side of the anticyclone continues to draw anomalously warm air up the eastern seaboard today. Without much change in low-level pattern (both thermal and mass fields), expect temperatures to be very similar to yesterday. A notable difference today compared to 24 hours ago is that the area is starting out 15-20 degrees warmer than yesterday. Assuming all other things being equal, this warm start should translate to slightly hotter conditions today than yesterday with less of hill to climb. Just about all of the model guidance indicates that high temperatures today will be a degree or two higher than yesterday. Dewpoints are currently in the mid 60s area wide. Dewpoints are expected to drop a few degrees during peak heating this afternoon as drier air aloft mixes down to the surface. Accordingly, peak heat indices in the 90-95F range across most of the area this afternoon will be right near the high temperature today. South to southwest winds 5-10 mph this morning will increase to 10- 15 mph late this morning and afternoon. Gusts of 20-25 mph can be expected during peak heating. The upper flow pattern over the region will flatten today as the ridge axis progresses off the coast and a shortwave trough moves into the Great Lakes. This will set the course for several (much weaker) shortwaves to traverse the northern Mid-Atlantic states today, resulting in periods of mid and high clouds throughout the day. Do not think these clouds should be dense or persistent enough to have a huge impact on temperatures though. Hi-res CAM guidance showing the potential for convection to develop during peak heating across the higher terrain out to our west across central PA and down into the Blue Ridge Mountains. Will continue to advertise slight chance PoPs for our far western zones late this afternoon as the upstream convection moves east. The environment should be slightly more favorable for convective initiation today than yesterday owing to a weakening mid-level cap and a potential for one of these mid- level shortwave disturbances to provide lift. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Another warm night is expected tonight with high pressure anchored over the western Atlantic and southwesterly return flow continuing across our area. Forecast low temperatures range from the mid 60s in northeastern PA and northwestern NJ to the lower 70s along the urban I-95 corridor. The risk of showers and storms will continue for areas mainly west of the Fall Line. Some of the hi-res CAM guidance is showing a more organized line of convection (likely tied to the development of a pre-frontal surface trough and the downstream progression of a mid- level shortwave trough) reaching the southern Poconos, Lehigh Valley, far western suburbs of Phila and northeastern MD between 8 and 10 PM. This activity should weaken with the loss of daytime heating before it makes it farther east into the DE Valley or I-95 corridor. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... The end to the summer-preview weather will be at hand Friday when a cold front will cross the area during the afternoon. The much above normal temperatures will be replaced by more seasonable readings which will last into next week. Highs Friday will be in in the upper 70s/low 80s across the north and mid/upper 80s elsewhere. The models are not showing a great deal of convection with the frontal passage, but nonetheless, chc pops for thunder are in the grids for the srn/ern area with slight chc pops most other areas. We will keep this as is for now. The weather for the weekend does not look too bad at this point. High pressure will build in from the north saturday and then move to the east Sunday. The best chc for any showers would be later Sunday afternoon across the N/W areas. But overall, a fairly decent weather weekend. Temperatures will be back to readings more typical for mid- May with highs mostly in the 70s. Sunday night and Monday will see the return of wetter weather as low pressure well to the west causes a slow moving front to cross the area. We will continue with the high chc /low categorical pops for both periods. Highs on Monday will range from the low 70s north to the upper 70s over Delmarva and metro Philadelphia. Conditions will be rather unsettled Tue thru Thu with chc for showers both Wed and Thu and slgt chc pops for Tue. An upper trough will be settling across the area and with sfc low and slow moving front accompanying the upper low, we foresee rather good chc for showers and some thunder too. Temperatures should be within a few degrees of seasonal norms. && .AVIATION /17Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 18Z TAFs...Generally VFR through the period. There are slight chances for a shower or storm, mainly west of KPHL, this evening, but coverage of any storms that develop is likely to be sparse. Patchy fog may develop late tonight in susceptible valley locations, including KRDG/KABE/KMIV, but confidence too low to include anything sub-VFR at the moment. Winds generally southwest through tonight with a few gusts to 20 kts this afternoon. Directions should veer more westerly tomorrow, with gusts to 20 kts again possible by late in the morning. OUTLOOK... Friday...Mostly VFR, but sct tstms may bring lower conditions. Winds shifting to N or NW by afternoon. Friday night thru Sunday...Mostly VFR. Sunday night and Monday...Lower CIGS/VSBYS with showers and sct tstms.
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&& .MARINE... Winds and seas below SCA criteria through this afternoon. Similar to yesterday, S-SWly wind will increase this afternoon across the coastal waters in response to a developing Ambrose jet. Winds may peak 20 kt with perhaps a few gusts near 25 kt this evening. Even though confidence in 25 kt gusts are not particularly high, a SCA was issued for tonight as seas are expected to increase to around 5 ft in the coastal waters of NJ. OUTLOOK... Friday/Fri night...SCA conditions early then sub-SCA. Sct tstms. Sat thru Sunday...Mostly sub-SCA. Fair weather. Sun night thru Mon night...SCA developing. Showers and tstms. && .CLIMATE... This table below contains the daily temperature records for today (5/18). Today (5/18) ACY 93-1987 PHL 94-1962 ILG 94-1962 ABE 96-1962 TTN 95-1911 GED 91-1974 RDG 96-1962 MPO 87-1962 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 PM this evening to noon EDT Friday for ANZ450>453. && $$ Synopsis...O`Hara Near Term...CMS/Klein Short Term...Klein Long Term...O`Hara Aviation...CMS/O`Hara Marine...Klein/O`Hara Climate...

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